Natural acquisition of immunity to Plasmodium vivax: epidemiological observations and potential targets

A hybrid transmission model for Plasmodium vivax accounting for superinfection, immunity and the hypnozoite reservoir

Malaria is a vector-borne disease that exacts a grave toll in the Global South. The epidemiology of Plasmodium vivax, the most geographically expansive agent of human malaria, is characterised by the accrual of a reservoir of dormant parasites known as hypnozoites. Relapses, arising from hypnozoite activation events, comprise the majority of the blood-stage infection burden, with implications for the acquisition of immunity and the distribution of superinfection. Here, we construct a novel model for the transmission of P. vivax that concurrently accounts for the accrual of the hypnozoite reservoir, (blood-stage) superinfection and the acquisition of immunity. We begin by using an infinite-server queueing network model to characterise the within-host dynamics as a function of mosquito-to-human transmission intensity, extending our previous model to capture a discretised immunity level. To model transmission-blocking and antidisease immunity, we allow for geometric decay in the respective probabilities of successful human-to-mosquito transmission and symptomatic blood-stage infection as a function of this immunity level. Under a hybrid approximation-whereby probabilistic within-host distributions are cast as expected population-level proportions-we couple host and vector dynamics to recover a deterministic compartmental model in line with Ross-Macdonald theory. We then perform a steady-state analysis for this compartmental model, informed by the (analytic) distributions derived at the within-host level. To characterise transient dynamics, we derive a reduced system of integrodifferential equations, likewise informed by our within-host queueing network, allowing us to recover population-level distributions for various quantities of epidemiological interest. In capturing the interplay between hypnozoite accrual, superinfection and acquired immunity-and providing, to the best of our knowledge, the most complete population-level distributions for a range of epidemiological values-our model provides insights into important, but poorly understood, epidemiological features of P. vivax.

Mathematical models of Plasmodium vivax transmission: A scoping review

Plasmodium vivax is one of the most geographically widespread malaria parasites in the world, primarily found across South-East Asia, Latin America, and parts of Africa. One of the significant characteristics of the P. vivax parasite is its ability to remain dormant in the human liver as hypnozoites and subsequently reactivate after the initial infection (i.e. relapse infections). Mathematical modelling approaches have been widely applied to understand P. vivax dynamics and predict the impact of intervention outcomes. Models that capture P. vivax dynamics differ from those that capture P. falciparum dynamics, as they must account for relapses caused by the activation of hypnozoites. In this article, we provide a scoping review of mathematical models that capture P. vivax transmission dynamics published between January 1988 and May 2023. The primary objective of this work is to provide a comprehensive summary of the mathematical models and techniques used to model P. vivax dynamics. In doing so, we aim to assist researchers working on mathematical epidemiology, disease transmission, and other aspects of P. vivax malaria by highlighting best practices in currently published models and highlighting where further model development is required. We categorise P. vivax models according to whether a deterministic or agent-based approach was used. We provide an overview of the different strategies used to incorporate the parasite's biology, use of multiple scales (within-host and population-level), superinfection, immunity, and treatment interventions. In most of the published literature, the rationale for different modelling approaches was driven by the research question at hand. Some models focus on the parasites' complicated biology, while others incorporate simplified assumptions to avoid model complexity. Overall, the existing literature on mathematical models for P. vivax encompasses various aspects of the parasite's dynamics. We recommend that future research should focus on refining how key aspects of P. vivax dynamics are modelled, including spatial heterogeneity in exposure risk and heterogeneity in susceptibility to infection, the accumulation of hypnozoite variation, the interaction between P. falciparum and P. vivax, acquisition of immunity, and recovery under superinfection.

Modeling resource allocation strategies for insecticide-treated bed nets to achieve malaria eradication

Large reductions in the global malaria burden have been achieved, but plateauing funding poses a challenge for progressing towards the ultimate goal of malaria eradication. Using previously published mathematical models of Plasmodium falciparum and Plasmodium vivax transmission incorporating insecticide-treated nets (ITNs) as an illustrative intervention, we sought to identify the global funding allocation that maximized impact under defined objectives and across a range of global funding budgets. The optimal strategy for case reduction mirrored an allocation framework that prioritizes funding for high-transmission settings, resulting in total case reductions of 76% and 66% at intermediate budget levels, respectively. Allocation strategies that had the greatest impact on case reductions were associated with lesser near-term impacts on the global population at risk. The optimal funding distribution prioritized high ITN coverage in high-transmission settings endemic for P. falciparum only, while maintaining lower levels in low-transmission settings. However, at high budgets, 62% of funding was targeted to low-transmission settings co-endemic for P. falciparum and P. vivax. These results support current global strategies to prioritize funding to high-burden P. falciparum-endemic settings in sub-Saharan Africa to minimize clinical malaria burden and progress towards elimination, but highlight a trade-off with 'shrinking the map' through a focus on near-elimination settings and addressing the burden of P. vivax.

Malaria

Malaria is resurging in many African and South American countries, exacerbated by COVID-19-related health service disruption. In 2021, there were an estimated 247 million malaria cases and 619 000 deaths in 84 endemic countries. Plasmodium falciparum strains partly resistant to artemisinins are entrenched in the Greater Mekong region and have emerged in Africa, while Anopheles mosquito vectors continue to evolve physiological and behavioural resistance to insecticides. Elimination of Plasmodium vivax malaria is hindered by impractical and potentially toxic antirelapse regimens. Parasitological diagnosis and treatment with oral or parenteral artemisinin-based therapy is the mainstay of patient management. Timely blood transfusion, renal replacement therapy, and restrictive fluid therapy can improve survival in severe malaria. Rigorous use of intermittent preventive treatment in pregnancy and infancy and seasonal chemoprevention, potentially combined with pre-erythrocytic vaccines endorsed by WHO in 2021 and 2023, can substantially reduce malaria morbidity. Improved surveillance, better access to effective treatment, more labour-efficient vector control, continued drug development, targeted mass drug administration, and sustained political commitment are required to achieve targets for malaria reduction by the end of this decade.

Plasmodium vivax infection alters the peripheral immunoregulatory network of CD4 T follicular cells and B cells

Regulatory and effector cell responses to Plasmodium vivax, the most common human malaria parasite outside Africa, remain understudied in naturally infected populations. Here, we describe peripheral CD4+ T- and B-cell populations during and shortly after an uncomplicated P. vivax infection in 38 continuously exposed adult Amazonians. Consistent with previous observations, we found an increased frequency in CD4+ CD45RA- CD25+ FoxP3+ T regulatory cells that express the inhibitory molecule CTLA-4 during the acute infection, with a sustained expansion of CD21- CD27- atypical memory cells within the CD19+ B-cell compartment. Both Th1- and Th2-type subsets of CXCR5+ ICOShi PD-1+ circulating T follicular helper (cTfh) cells, which are thought to contribute to antibody production, were induced during P. vivax infection, with a positive correlation between overall cTfh cell frequency and IgG antibody titers to the P. vivax blood-stage antigen MSP119 . We identified significant changes in cell populations that had not been described in human malaria, such as an increased frequency of CTLA-4+ T follicular regulatory cells that antagonize Tfh cells, and a decreased frequency of circulating CD24hi CD27+ B regulatory cells in response to acute infection. In conclusion, we disclose a complex immunoregulatory network that is critical to understand how naturally acquired immunity develops in P. vivax malaria.

Plasmodium vivax MSP1-42 kD Variant Proteins Detected Naturally Induced IgG Antibodies in Patients Regardless of the Infecting Parasite Phenotype in Mesoamerica

Background: The serological tests using blood stage antigens might be helpful for detecting recent exposure to Plasmodium parasites, and seroepidemiological studies would aid in the elimination of malaria. This work produced recombinant proteins of PvMSP142 variants and evaluated their capacity to detect IgG antibodies in symptomatic patients from Mesoamerica. Methods: Three variant Pvmsp142 genes were cloned in the pHL-sec plasmid, expressed in the Expi293F™ eukaryotic system, and the recombinant proteins were purified by affinity chromatography. Using an ELISA, 174 plasma or eluted samples from patients infected with different P. vivax haplotypes were evaluated against PvMSP142 proteins and to a native blood stage antigen (NBSA). Results: The antibody IgG OD values toward PvMSP142 variants (v88, v21, and v274) were heterogeneous (n = 178; median = 0.84 IQR 0.28–1.64). The correlation of IgG levels among all proteins was very high (spearman’s rho = 0.96–0.98; p < 0.0001), but was lower between them and the NBSA (rho = 0.771; p < 0.0001). In only a few samples, higher reactivity to the homologous protein was evident. Patients with a past infection who were seropositive had higher IgG levels and lower parasitemia levels than those who did not (p < 0.0001). Conclusions: The PvMSP142 variants were similarly efficient in detecting specific IgG antibodies in P. vivax patients from Mesoamerica, regardless of the infecting parasite’s haplotype, and might be good candidates for malaria surveillance and epidemiological studies in the region.

Longitudinal IgG antibody responses to Plasmodium vivax blood-stage antigens during and after acute vivax malaria in individuals living in the Brazilian Amazon

BACKGROUND

To make progress towards malaria elimination, a highly effective vaccine targeting Plasmodium vivax is urgently needed. Evaluating the kinetics of natural antibody responses to vaccine candidate antigens after acute vivax malaria can inform the design of serological markers of exposure and vaccines.

METHODOLOGY/PRINCIPAL FINDINGS

The responses of IgG antibodies to 9 P. vivax vaccine candidate antigens were evaluated in longitudinal serum samples from Brazilian individuals collected at the time of acute vivax malaria and 30, 60, and 180 days afterwards. Antigen-specific IgG correlations, seroprevalence, and half-lives were determined for each antigen using the longitudinal data. Antibody reactivities against Pv41 and PVX_081550 strongly correlated with each other at each of the four time points. The analysis identified robust responses in terms of magnitude and seroprevalence against Pv41 and PvGAMA at 30 and 60 days. Among the 8 P. vivax antigens demonstrating >50% seropositivity across all individuals, antibodies specific to PVX_081550 had the longest half-life (100 days; 95% CI, 83-130 days), followed by PvRBP2b (91 days; 95% CI, 76-110 days) and Pv12 (82 days; 95% CI, 64-110 days).

CONCLUSION/SIGNIFICANCE

This study provides an in-depth assessment of the kinetics of antibody responses to key vaccine candidate antigens in Brazilians with acute vivax malaria. Follow-up studies are needed to determine whether the longer-lived antibody responses induced by natural infection are effective in controlling blood-stage infection and mediating clinical protection.

Impact of Epstein-Barr virus co-infection on natural acquired Plasmodium vivax antibody response

Background

The simultaneous infection of Plasmodium falciparum and Epstein-Barr virus (EBV) could promote the development of the aggressive endemic Burkitt’s Lymphoma (eBL) in children living in P. falciparum holoendemic areas. While it is well-established that eBL is not related to other human malaria parasites, the impact of EBV infection on the generation of human malaria immunity remains largely unexplored. Considering that this highly prevalent herpesvirus establishes a lifelong persistent infection on B-cells with possible influence on malaria immunity, we hypothesized that EBV co-infection could have impact on the naturally acquired antibody responses to P. vivax, the most widespread human malaria parasite.

Methodology/Principal findings

The study design involved three cross-sectional surveys at six-month intervals (baseline, 6 and 12 months) among long-term P. vivax exposed individuals living in the Amazon rainforest. The approach focused on a group of malaria-exposed individuals whose EBV-DNA (amplification of balf-5 gene) was persistently detected in the peripheral blood (PersVDNA, n = 27), and an age-matched malaria-exposed group whose EBV-DNA could never be detected during the follow-up (NegVDNA, n = 29). During the follow-up period, the serological detection of EBV antibodies to lytic/ latent viral antigens showed that IgG antibodies to viral capsid antigen (VCA-p18) were significantly different between groups (PersVDNA > NegVDNA). A panel of blood-stage P. vivax antigens covering a wide range of immunogenicity confirmed that in general PersVDNA group showed low levels of antibodies as compared with NegVDNA. Interestingly, more significant differences were observed to a novel DBPII immunogen, named DEKnull-2, which has been associated with long-term neutralizing antibody response. Differences between groups were less pronounced with blood-stage antigens (such as MSP1-19) whose levels can fluctuate according to malaria transmission.

Conclusions/Significance

In a proof-of-concept study we provide evidence that a persistent detection of EBV-DNA in peripheral blood of adults in a P. vivax semi-immune population may impact the long-term immune response to major malaria vaccine candidates.

Systems biology of malaria explored with nonhuman primates

"The Primate Malarias" book has been a uniquely important resource for multiple generations of scientists, since its debut in 1971, and remains pertinent to the present day. Indeed, nonhuman primates (NHPs) have been instrumental for major breakthroughs in basic and pre-clinical research on malaria for over 50 years. Research involving NHPs have provided critical insights and data that have been essential for malaria research on many parasite species, drugs, vaccines, pathogenesis, and transmission, leading to improved clinical care and advancing research goals for malaria control, elimination, and eradication. Whilst most malaria scientists over the decades have been studying Plasmodium falciparum, with NHP infections, in clinical studies with humans, or using in vitro culture or rodent model systems, others have been dedicated to advancing research on Plasmodium vivax, as well as on phylogenetically related simian species, including Plasmodium cynomolgi, Plasmodium coatneyi, and Plasmodium knowlesi. In-depth study of these four phylogenetically related species over the years has spawned the design of NHP longitudinal infection strategies for gathering information about ongoing infections, which can be related to human infections. These Plasmodium-NHP infection model systems are reviewed here, with emphasis on modern systems biological approaches to studying longitudinal infections, pathogenesis, immunity, and vaccines. Recent discoveries capitalizing on NHP longitudinal infections include an advanced understanding of chronic infections, relapses, anaemia, and immune memory. With quickly emerging new technological advances, more in-depth research and mechanistic discoveries can be anticipated on these and additional critical topics, including hypnozoite biology, antigenic variation, gametocyte transmission, bone marrow dysfunction, and loss of uninfected RBCs. New strategies and insights published by the Malaria Host-Pathogen Interaction Center (MaHPIC) are recapped here along with a vision that stresses the importance of educating future experts well trained in utilizing NHP infection model systems for the pursuit of innovative, effective interventions against malaria.

Recurrent Plasmodium vivax Cases of Both Short and Long Latency Increased with Transmission Intensity and Were Distributed Year-Round in the Most Affected Municipalities of the RACCN, Nicaragua, 2013-2018

The characteristics of P. vivax recurrent episodes were examined using a centralized secondary source of malaria records in Nicaragua and in the two most affected municipalities in the RACCN. The study of 36,787 malaria cases due to P. vivax or P. falciparum revealed that, nationwide, 3624 patients had at least one recurrent infection. This was achieved by matching names, gender, age, community/municipality, ethnicity, etc. P. vivax was responsible for 88% of recurrent infections of 25-450 days of latency (51.9% were women and 48.1% were men), and these were assumed to be relapse episodes. Of them, 88.2% and 4.4% occurred in the municipalities of Puerto Cabezas and Rosita, respectively. The proportion of P. vivax patients having presumed relapse episodes rose with elevated transmission rates in both municipalities, reaching 7% in Rosita (2017) and 14.5% in Puerto Cabezas (2018). In both areas, relapse episodes were evident over time and were characterized by the production of a continuous stippling pattern with a slope evolving from one transmission peak to the next. During the dry season, short-latency relapse episodes were more robust, while long-latency ones increased just before the P. vivax transmission season began, with a high proportion of long-latency relapses during this period. The abundance of recurrent P. vivax infections, the wide range of relapse latency lengths, and temporal distribution tended to favor year-round transmission. It is necessary to evaluate compliance with and the effectiveness of primaquine treatment and contemplate the use of an alternative drug, among other actions.

Hypnozoite dynamics for Plasmodium vivax malaria: the epidemiological effects of radical cure

Malaria is a mosquito-borne disease with a devastating global impact. Plasmodium vivax is a major cause of human malaria beyond sub-Saharan Africa. Relapsing infections, driven by a reservoir of liver-stage parasites known as hypnozoites, present unique challenges for the control of P. vivax malaria. Following indeterminate dormancy periods, hypnozoites may activate to trigger relapses. Clearance of the hypnozoite reservoir through drug treatment (radical cure) has been proposed as a potential tool for the elimination of P. vivax malaria. Here, we introduce a stochastic, within-host model to jointly characterise hypnozoite and infection dynamics for an individual in a general transmission setting, allowing for radical cure. We begin by extending an existing activation-clearance model for a single hypnozoite, adapted to both short- and long-latency strains, to include drug treatment. We then embed this activation-clearance model in an epidemiological framework accounting for repeated mosquito inoculation and the administration of radical cure. By constructing an open network of infinite server queues, we derive analytic expressions for several quantities of epidemiological significance, including the size of the hypnozoite reservoir; the relapse rate; the relative contribution of relapses to the infection burden; the distribution of multiple infections; the cumulative number of recurrences over time, and the time to first recurrence following drug treatment. We derive from first principles the functional dependence between within-host and transmission parameters and patterns of blood- and liver-stage infection, whilst allowing for treatment under a mass drug administration regime. To yield population-level insights, our analytic within-host distributions can be embedded in multiscale models. Our work thus contributes to the epidemiological understanding of the effects of radical cure on P. vivax malaria.

Antibodies Against the Plasmodium vivax Apical Membrane Antigen 1 From the Belem Strain Share Common Epitopes Among Other Worldwide Variants

Malaria is a human parasitic disease distributed in many tropical countries and caused by various Plasmodium species. Plasmodium vivax has the largest geographical distribution of the Plasmodium species and is predominant in the Americas, including Brazil. Only a small number of P. vivax vaccine formulations have successfully reached clinical trials relative to their P. falciparum counterparts. One of the candidate antigens for a blood-stage P. vivax vaccine is apical membrane antigen 1 (PvAMA-1). Due to the worldwide distribution of Plasmodium parasites, a high degree of variability has been detected in this antigen sequence, representing a considerable challenge to the development of a universal vaccine against malaria. In this study, we evaluated how PvAMA-1 polymorphisms influence vaccine-derived immune responses in P. vivax malaria. To this end, we expressed 9 recombinant protein representatives of different PvAMA-1 allelic variants in the yeast Pichia pastoris: Belem, Chesson I, Sal-1, Indonesia XIX, SK0814, TC103, PNG_05_ESP, PNG_62_MU, and PNG_68_MAS. After protein expression and purification, we evaluated the breadth of the immune responses derived from malaria-exposed individuals from the Amazon region. From 611 serum samples of malaria-exposed individuals, 53.68% of them reacted against the PvAMA-1 Belem through ELISA. Positive samples were further tested against recombinant proteins representing the other PvAMA-1 allelic variants. Whereas Sal-1, Chesson I and SK0814 variants were highly recognized by tested serum samples, Indonesia XIX, TC103, PNG_05_ESP, PNG_62_MU, and PNG_68_MAS were only slightly recognized. Moreover, polyclonal sera derived from C57BL/6 mice immunized with the PvAMA-1 Belem protein predominantly recognized Belem, Sal-1, Chesson I, SK0814, and Indonesia XIX through ELISA. Last, ELISA-based competition assays demonstrated that a previous interaction between anti-Belem polyclonal serum and Sal-1, Chesson I, SK0814, or Indonesia XIX proteins could further inhibit antibody binding to the Belem variant. Our human and mouse data suggest the presence of common epitopes or cross-reactivity between Belem, Sal-1, Chesson I, and SK0814 variants. Although the PvAMA-1 Belem variant induces strain-transcendent antibodies, PvAMA-1 variants from Thailand and Papua New Guinea may need to be included in a universal vaccine formulation to achieve protection against P. vivax malaria.

Heterogeneity in response to serological exposure markers of recent Plasmodium vivax infections in contrasting epidemiological contexts

BACKGROUND

Antibody responses as serological markers of Plasmodium vivax infection have been shown to correlate with exposure, but little is known about the other factors that affect antibody responses in naturally infected people from endemic settings. To address this question, we studied IgG responses to novel serological exposure markers (SEMs) of P. vivax in three settings with different transmission intensity.

METHODOLOGY

We validated a panel of 34 SEMs in a Peruvian cohort with up to three years' longitudinal follow-up using a multiplex platform and compared results to data from cohorts in Thailand and Brazil. Linear regression models were used to characterize the association between antibody responses and age, the number of detected blood-stage infections during follow-up, and time since previous infection. Receiver Operating Characteristic (ROC) analysis was used to test the performance of SEMs to identify P. vivax infections in the previous 9 months.

PRINCIPAL FINDINGS

Antibody titers were associated with age, the number of blood-stage infections, and time since previous P. vivax infection in all three study sites. The association between antibody titers and time since previous P. vivax infection was stronger in the low transmission settings of Thailand and Brazil compared to the higher transmission setting in Peru. Of the SEMs tested, antibody responses to RBP2b had the highest performance for classifying recent exposure in all sites, with area under the ROC curve (AUC) = 0.83 in Thailand, AUC = 0.79 in Brazil, and AUC = 0.68 in Peru.

CONCLUSIONS

In low transmission settings, P. vivax SEMs can accurately identify individuals with recent blood-stage infections. In higher transmission settings, the accuracy of this approach diminishes substantially. We recommend using P. vivax SEMs in low transmission settings pursuing malaria elimination, but they are likely to be less effective in high transmission settings focused on malaria control.

Temporal and Microspatial Heterogeneity in Transmission Dynamics of Coendemic Plasmodium vivax and Plasmodium falciparum in Two Rural Cohort Populations in the Peruvian Amazon

BACKGROUND

Malaria is highly heterogeneous: its changing malaria microepidemiology needs to be addressed to support malaria elimination efforts at the regional level.

METHODS

A 3-year, population-based cohort study in 2 settings in the Peruvian Amazon (Lupuna, Cahuide) followed participants by passive and active case detection from January 2013 to December 2015. Incidence and prevalence rates were estimated using microscopy and polymerase chain reaction (PCR).

RESULTS

Lupuna registered 1828 infections (1708 Plasmodium vivax, 120 Plasmodium falciparum; incidence was 80.7 infections/100 person-years (95% confidence interval [CI] , 77.1-84.5). Cahuide detected 1046 infections (1024 P vivax, 20 P falciparum, 2 mixed); incidence was 40.2 infections/100 person-years (95% CI, 37.9-42.7). Recurrent P vivax infections predominated onwards from 2013. According to PCR data, submicroscopic predominated over microscopic infections, especially in periods of low transmission. The integration of parasitological, entomological, and environmental observations evidenced an intense and seasonal transmission resilient to standard control measures in Lupuna and a persistent residual transmission after severe outbreaks were intensively handled in Cahuide.

CONCLUSIONS

In 2 exemplars of complex local malaria transmission, standard control strategies failed to eliminate submicroscopic and hypnozoite reservoirs, enabling persistent transmission.

Early transmission of sensitive strain slows down emergence of drug resistance in Plasmodium vivax

The spread of drug resistance of Plasmodium falciparum and Plasmodium vivax parasites is a challenge towards malaria elimination. P. falciparum has shown an early and severe drug resistance in comparison to P. vivax in various countries. In fact, P. vivax differs in its life cycle and treatment in various factors: development and duration of sexual parasite forms differ, symptoms severity are unequal, relapses present only in P. vivax cases and the Artemisinin-based combination therapy (ACT) is only mandatory in P. falciparum cases. We compared the spread of drug resistance for both species through two compartmental models using ordinary differential equations. The model structure describes how sensitive and resistant parasite strains infect a human population treated with antimalarials. We found that an early transmission,i.e., before treatment and low effectiveness of drug coverage, supports the prevalence of sensitive parasites delaying the emergence of resistant P. vivax. These results imply that earlier attention of both symptomatic cases and reservoirs of P. vivax are essential in controlling transmission but also accelerate the spread of drug resistance.

Dynamics of IgM and IgG responses to the next generation of engineered Duffy binding protein II immunogen: Strain-specific and strain-transcending immune responses over a nine-year period

BACKGROUND

A low proportion of P. vivax-exposed individuals acquire protective strain-transcending neutralizing IgG antibodies that are able to block the interaction between the Duffy binding protein II (DBPII) and its erythrocyte-specific invasion receptor. In a recent study, a novel surface-engineered DBPII-based vaccine termed DEKnull-2, whose antibody response target conserved DBPII epitopes, was able to induce broadly binding-inhibitory IgG antibodies (BIAbs) that inhibit P. vivax reticulocyte invasion. Toward the development of DEKnull-2 as an effective P. vivax blood-stage vaccine, we investigate the relationship between naturally acquired DBPII-specific IgM response and the profile of IgG antibodies/BIAbs activity over time.

METHODOLOGY/PRINCIPAL FINDINGS

A nine-year follow-up study was carried-out among long-term P. vivax-exposed Amazonian individuals and included six cross-sectional surveys at periods of high and low malaria transmission. DBPII immune responses associated with either strain-specific (Sal1, natural DBPII variant circulating in the study area) or conserved epitopes (DEKnull-2) were monitored by conventional serology (ELISA-detected IgM and IgG antibodies), with IgG BIAbs activity evaluated by functional assays (in vitro inhibition of DBPII-erythrocyte binding). The results showed a tendency of IgM antibodies toward Sal1-specific response; the profile of Sal1 over DEKnull-2 was not associated with acute malaria and sustained throughout the observation period. The low malaria incidence in two consecutive years allowed us to demonstrate that variant-specific IgG (but not IgM) antibodies waned over time, which resulted in IgG skewed to the DEKnull-2 response. A persistent DBPII-specific IgM response was not associated with the presence (or absence) of broadly neutralizing IgG antibody response.

CONCLUSIONS/SIGNIFICANCE

The current study demonstrates that long-term exposure to low and unstable levels of P. vivax transmission led to a sustained DBPII-specific IgM response against variant-specific epitopes, while sustained IgG responses are skewed to conserved epitopes. Further studies should investigate on the role of a stable and persistent IgM antibody response in the immune response mediated by DBPII.

Protective Immunity in Mice Immunized With P. vivax MSP119-Based Formulations and Challenged With P. berghei Expressing PvMSP119

The lack of continuous in vitro cultures has been an obstacle delaying pre-clinical testing of Plasmodium vivax vaccine formulations based on known antigens. In this study, we generated a model to test available formulations based on the P. vivax MSP1₁₉ antigen. The Plasmodium berghei strains ANKA and NK65 were modified to express PvMSP1₁₉ instead of the endogenous PbMSP1₁₉. The hybrid parasites were used to challenge C57BL/6 or BALB/c mice immunized with PvMSP1₁₉-based vaccine formulations. The PvMSP1₁₉ was correctly expressed in the P. berghei hybrid mutant lines as confirmed by immunofluorescence using anti-PvMSP1₁₉ monoclonal antibodies and by Western blot. Replacement of the PbMSP1₁₉ by the PvMSP1₁₉ had no impact on asexual growth in vivo. High titers of specific antibodies to PvMSP1₁₉ were not sufficient to control initial parasitemia in the immunized mice, but late parasitemia control and a balanced inflammatory process protected these mice from dying, suggesting that an established immune response to PvMSP1₁₉ in this model can help immunity mounted later during infection.

Differential impact of malaria control interventions on P. falciparum and P. vivax infections in young Papua New Guinean children

INTRODUCTION

As malaria transmission declines, understanding the differential impact of intensified control on Plasmodium falciparum relative to Plasmodium vivax and identifying key drivers of ongoing transmission is essential to guide future interventions.

METHODS

Three longitudinal child cohorts were conducted in Papua New Guinea before (2006/2007), during (2008) and after scale-up of control interventions (2013). In each cohort, children aged 1-5 years were actively monitored for infection and illness. Incidence of malaria episodes, molecular force of blood-stage infections (_molFOB) and population-averaged prevalence of infections were compared across the cohorts to investigate the impact of intensified control in young children and the key risk factors for malaria infection and illness in 2013.

RESULTS

Between 2006 and 2008, P. falciparum infection prevalence, _molFOB, and clinical malaria episodes reduced by 47%, 59% and 69%, respectively, and a further 49%, 29% and 75% from 2008 to 2013 (prevalence 41.6% to 22.1% to 11.2%; _molFOB: 3.4 to 1.4 to 1.0 clones/child/year; clinical episodes incidence rate (IR) 2.6 to 0.8 to IR 0.2 episodes/child/year). P. vivax clinical episodes declined at rates comparable to P. falciparum between 2006, 2008 and 2013 (IR 2.5 to 1.1 to 0.2), while P. vivax _molFOB (2006, 9.8; 2008, 12.1) and prevalence (2006, 59.6%; 2008, 65.0%) remained high in 2008. However, in 2013, P. vivax _molFOB (1.2) and prevalence (19.7%) had also substantially declined. In 2013, 89% of P. falciparum and 93% of P. vivax infections were asymptomatic, 62% and 47%, respectively, were sub-microscopic. Area of residence was the major determinant of malaria infection and illness.

CONCLUSION

Intensified vector control and routine case management had a differential impact on rates of P. falciparum and P. vivax infections but not clinical malaria episodes in young children. This suggests comparable reductions in new mosquito-derived infections but a delayed impact on P. vivax relapsing infections due to a previously acquired reservoir of hypnozoites. This demonstrates the need to strengthen implementation of P. vivax radical cure to maximise impact of control in co-endemic areas. The high heterogeneity of malaria in 2013 highlights the importance of surveillance and targeted interventions to accelerate towards elimination.

Microsatellite analysis reveals connectivity among geographically distant transmission zones of Plasmodium vivax in the Peruvian Amazon: A critical barrier to regional malaria elimination

Despite efforts made over decades by the Peruvian government to eliminate malaria, Plasmodium vivax remains a challenge for public health decision-makers in the country. The uneven distribution of its incidence, plus its complex pattern of dispersion, has made ineffective control measures based on global information that lack the necessary detail to understand transmission fully. In this sense, population genetic tools can complement current surveillance. This study describes the genetic diversity and population structure from September 2012 to March 2015 in three geographically distant settlements, Cahuide (CAH), Lupuna (LUP) and Santa Emilia (STE), located in the Peruvian Amazon. A total 777 P. vivax mono-infections, out of 3264, were genotyped. Among study areas, LUP showed 19.7% of polyclonal infections, and its genetic diversity (H_exp) was 0.544. Temporal analysis showed a significant increment of polyclonal infections and H_exp, and the introduction and persistence of a new parasite population since March 2013. In STE, 40.1% of infections were polyclonal, with H_exp = 0.596. The presence of four genetic clusters without signals of clonal expansion and infections with lower parasite densities compared against the other two areas were also found. At least four parasite populations were present in CAH in 2012, where, after June 2014, malaria cases decreased from 213 to 61, concomitant with a decrease in polyclonal infections (from 0.286 to 0.18), and expectedly variable H_exp. Strong signals of gene flow were present in the study areas and wide geographic distribution of highly diverse parasite populations were found. This study suggests that movement of malaria parasites by human reservoirs connects geographically distant malaria transmission areas in the Peruvian Amazon. The maintenance of high levels of parasite genetic diversity through human mobility is a critical barrier to malaria elimination in this region.

Plasmodium vivax transmission is heterogeneous and discontinuous in the Peruvian Amazon. Such heterogeneity is the result of factors that include, but are not restricted to, the environment, public policies, and characteristics of the parasite, the vector, and human activities. All these factors make P. vivax transmission resilient to interventions. In order to achieve the goals of control and local elimination, P. vivax surveillance must inform how those factors sustain disease transmission in order to focalize and synchronize control strategies. In this study, we implemented molecular surveillance complemented with population genetic tools in the areas of Cahuide, Lupuna, and Santa Emilia located in the Peruvian Amazon. In particular, we characterize the transmission and the parasite genetic variation in these sites from September 2012 to March 2015. The changes in parasite diversity, the wide geographic dispersion of parasite subpopulation and the introduction of a new parasite clone or subpopulation in Lupuna documented in this study suggest that connectivity among the different endemic areas, likely due to human mobility, sustains disease transmission in the region hindering the success of control measures. This information must be considered in the design of current control strategies.

The immunology of Plasmodium vivax malaria

Plasmodium vivax infection, the predominant cause of malaria in Asia and Latin America, affects ~14 million individuals annually, with considerable adverse effects on wellbeing and socioeconomic development. A clinical hallmark of Plasmodium infection, the paroxysm, is driven by pyrogenic cytokines produced during the immune response. Here, we review studies on the role of specific immune cell types, cognate innate immune receptors, and inflammatory cytokines on parasite control and disease symptoms. This review also summarizes studies on recurrent infections in individuals living in endemic regions as well as asymptomatic infections, a serious barrier to eliminating this disease. We propose potential mechanisms behind these repeated and subclinical infections, such as poor induction of immunological memory cells and inefficient T effector cells. We address the role of antibody-mediated resistance to P. vivax infection and discuss current progress in vaccine development. Finally, we review immunoregulatory mechanisms, such as inhibitory receptors, T regulatory cells, and the anti-inflammatory cytokine, IL-10, that antagonizes both innate and acquired immune responses, interfering with the development of protective immunity and parasite clearance. These studies provide new insights for the clinical management of symptomatic as well as asymptomatic individuals and the development of an efficacious vaccine for vivax malaria.

Antibodies to Plasmodium vivax reticulocyte binding protein 2b are associated with protection against P. vivax malaria in populations living in low malaria transmission regions of Brazil and Thailand

Background

The Plasmodium vivax Reticulocyte Binding Protein (PvRBP) family is involved in red blood cell recognition and members of this family are potential targets for antibodies that may block P. vivax invasion. To date, the acquisition of immunity against PvRBPs in low malaria transmission settings and in a broad age group of exposed individuals has not been investigated.

Methodology/Principal findings

Total IgG antibody levels to six members of the PvRBP family (PvRBP1a, PvRBP1b, PvRBP2a, PvRBP2b, a non-binding fragment of PvRBP2c (PvRBP2cNB) and PvRBP2-P2) were measured in samples collected from individuals living in two regions of low P. vivax endemicity in Brazil and Thailand. In both settings, levels of total IgG to PvRBP1a, PvRBP2b, PvRBP2cNB, and PvRBP2P-2 increased significantly with age (rho = 0.17–0.49; P<0.001). IgG responses to PvRBP1a, PvRBP2b and PvRBP2cNB were significantly higher in infected individuals by using Wilcoxon’s signed-rank test (P<0.001). Of the six PvRBPs examined, only antibodies to PvRBP2b were associated with protection against clinical malaria in both settings.

Conclusion/Significance

Our results indicate that PvRBP2b warrants further preclinical development as a blood-stage vaccine candidate against P. vivax. Total IgG responses to PvRBPs were also shown to be promising immunological markers of exposure to P. vivax infection.

Plasmodium vivax preferentially invades young red blood cells called reticulocytes. Successful parasite invasion relies on the interaction between parasite ligands with human red blood cell receptors. The P. vivax reticulocyte binding protein family (PvRBP) plays a role in the invasion process. The role of PvRBP in the natural acquisition of immunity to P. vivax, especially in low transmission settings, is poorly understood. Using samples from longitudinal cohort studies from regions of low P. vivax endemicity in Brazil and Thailand, we showed that antibody responses to PvRBP1a, PvRBP2b, and PvRBP2cNB increased with age and were boosted during infection. In addition, higher antibody levels to PvRBP2b were strongly associated with a lower risk of clinical episodes of P. vivax. Based on these observations, we propose that PvRBP2b warrants further preclinical development as a blood-stage vaccine candidate against P. vivax.

Identification of an Immunogenic Broadly Inhibitory Surface Epitope of the Plasmodium vivax Duffy Binding Protein Ligand Domain

Vivax malaria is the second leading cause of malaria worldwide and the major cause of non-African malaria. Unfortunately, efforts to develop antimalarial vaccines specifically targeting Plasmodium vivax have been largely neglected, and few candidates have progressed into clinical trials. The Duffy binding protein is considered a leading blood-stage vaccine candidate because this ligand’s recognition of the Duffy blood group reticulocyte surface receptor is considered essential for infection. This study identifies a new target epitope on the ligand’s surface that may serve as the target of vaccine-induced binding-inhibitory antibody (BIAb). Understanding the potential targets of vaccine protection will be important for development of an effective vaccine.

The Plasmodium vivax Duffy binding protein region II (DBPII) is a vital ligand for the parasite’s invasion of reticulocytes, thereby making this molecule an attractive vaccine candidate against vivax malaria. However, strain-specific immunity due to DBPII allelic variation in Bc epitopes may complicate vaccine efficacy, suggesting that an effective DBPII vaccine needs to target conserved epitopes that are potential targets of strain-transcending neutralizing immunity. The minimal epitopes reactive with functionally inhibitory anti-DBPII monoclonal antibody (MAb) 3C9 and noninhibitory anti-DBPII MAb 3D10 were mapped using phage display expression libraries, since previous attempts to deduce the 3C9 epitope by cocrystallographic methods failed. Inhibitory MAb 3C9 binds to a conserved conformation-dependent epitope in subdomain 3, while noninhibitory MAb 3D10 binds to a linear epitope in subdomain 1 of DBPII, consistent with previous studies. Immunogenicity studies using synthetic linear peptides of the minimal epitopes determined that the 3C9 epitope, but not the 3D10 epitope, could induce functionally inhibitory anti-DBPII antibodies. Therefore, the highly conserved binding-inhibitory 3C9 epitope offers the potential as a component in a broadly inhibitory, strain-transcending DBP subunit vaccine.

IMPORTANCE Vivax malaria is the second leading cause of malaria worldwide and the major cause of non-African malaria. Unfortunately, efforts to develop antimalarial vaccines specifically targeting Plasmodium vivax have been largely neglected, and few candidates have progressed into clinical trials. The Duffy binding protein is considered a leading blood-stage vaccine candidate because this ligand’s recognition of the Duffy blood group reticulocyte surface receptor is considered essential for infection. This study identifies a new target epitope on the ligand’s surface that may serve as the target of vaccine-induced binding-inhibitory antibody (BIAb). Understanding the potential targets of vaccine protection will be important for development of an effective vaccine.

Analysis of Apical Membrane Antigen (AMA)-1 characteristics using bioinformatics tools in order to vaccine design against Plasmodium vivax

Plasmodium vivax, an intracellular protozoan, causes malaria which is characterized by fever, anemia, respiratory distress, liver and spleen enlargement. In spite of attempts to design an efficient vaccine, there is not a vaccine against P. vivax. Notable advances have recently achieved in the development of malaria vaccines targeting the surface antigens such as Apical Membrane Antigens (AMA)-1. AMA-1 is a micronemal protein synthesized during the erythrocyte-stage of Plasmodium species and plays a significant role in the invasion process of the parasite into host cells. P. vivax AMA-1 (PvAMA-1) can induce strong cellular and humoral responses, indicating that it can be an ideal candidate of vaccine against malaria. Identification and prediction of proteins characteristics increase our knowledge about them and leads to develop vaccine and diagnostic studies. In the present study several valid bioinformatics tools were applied to analyze the various characteristics of AMA-1 such as physical and chemical properties, secondary and tertiary structures, B- cell and T-cell prediction and other important features in order to introduce potential epitopes for designing a high-efficient vaccine. The results demonstrated that this protein had 57 potential PTM sites and only one transmembrane domain on its sequence. Also, multiple hydrophilic regions and classical high hydrophilic domains were predicted. Secondary structure prediction revealed that the proportions of random coil, alpha-helix and extended strand in the AMA-1 sequence were 53.74%, 27.22%, and 19.4%, respectively. Moreover, 5 disulfide bonds were predicted at positions 14-21aa, 162-192aa, 208-220aa, 247-265aa and 354-363aa. The data obtained from B-cell and T-cell epitopes prediction showed that there were several potential epitopes on AMA-1 that can be proper targets for diagnostic and vaccine studies. The current study presented interesting basic and theoretical information regarding PvAMA-1, being important for further studies in order to design a high-efficiency vaccine against malaria.

Molecular epidemiology of residual Plasmodium vivax transmission in a paediatric cohort in Solomon Islands

Background

Following the scale-up of intervention efforts, malaria burden has decreased dramatically in Solomon Islands (SI). Submicroscopic and asymptomatic Plasmodium vivax infections are now the major challenge for malaria elimination in this country. Since children have higher risk of contracting malaria, this study investigated the dynamics of Plasmodium spp. infections among children including the associated risk factors of residual P. vivax burden.

Methods

An observational cohort study was conducted among 860 children aged 0.5–12 years in Ngella (Central Islands Province, SI). Children were monitored by active and passive surveillances for Plasmodium spp. infections and illness. Parasites were detected by quantitative real-time PCR (qPCR) and genotyped. Comprehensive statistical analyses of P. vivax infection prevalence, molecular force of blood stage infection (_molFOB) and infection density were conducted.

Results

Plasmodium vivax infections were common (overall prevalence: 11.9%), whereas Plasmodium falciparum infections were rare (0.3%) but persistent. Although children acquire an average of 1.1 genetically distinct P. vivax blood-stage infections per year, there was significant geographic heterogeneity in the risks of P. vivax infections across Ngella (prevalence: 1.2–47.4%, p < 0.01; _molFOB: 0.05–4.6/year, p < 0.01). Malaria incidence was low (IR: 0.05 episodes/year-at-risk). Age and measures of high exposure were the key risk factors for P. vivax infections and disease. Malaria incidence and infection density decreased with age, indicating significant acquisition of immunity. G6PD deficient children (10.8%) that did not receive primaquine treatment had a significantly higher prevalence (_aOR: 1.77, p = 0.01) and increased risk of acquiring new bloodstage infections (_molFOB _aIRR: 1.51, p = 0.03), underscoring the importance of anti-relapse treatment.

Conclusion

Residual malaria transmission in Ngella exhibits strong heterogeneity and is characterized by a high proportion of submicroscopic and asymptomatic P. vivax infections, alongside sporadic P. falciparum infections. Implementing an appropriate primaquine treatment policy to prevent P. vivax relapses and specific targeting of control interventions to high risk areas will be required to accelerate ongoing control and elimination activities.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2727-9) contains supplementary material, which is available to authorized users.

Mosquito Bite-Induced Controlled Human Malaria Infection with Plasmodium vivax or P. falciparum Generates Immune Responses to Homologous and Heterologous Preerythrocytic and Erythrocytic Antigens

Seroepidemiological studies on the prevalence of antibodies to malaria antigens are primarily conducted on individuals from regions of endemicity. It is therefore difficult to accurately correlate the antibody responses to the timing and number of prior malaria infections. This study was undertaken to assess the evolution of antibodies to the dominant surface antigens of Plasmodium vivax and P. falciparum following controlled human malaria infection (CHMI) in malaria-naive individuals. Serum samples from malaria-naive adults, collected before and after CHMI with either P. vivax (n = 18) or P. falciparum (n = 18), were tested for the presence of antibodies to the circumsporozoite protein (CSP) and the 42-kDa fragment of merozoite surface protein 1 (MSP-1₄₂) of P. vivax and P. falciparum using an enzyme-linked immunosorbent assay (ELISA). Approximately 1 month following CHMI with either P. vivax or P. falciparum, >60% of subjects seroconverted to homologous CSP and MSP-1. More than 50% of the subjects demonstrated reactivity to heterologous CSP and MSP-1₄₂, and a similar proportion of subjects remained seropositive to homologous MSP-1₄₂ >5 months after CHMI. Computational analysis provides insight into the presence of cross-reactive responses. The presence of long-lived and heterologous reactivity and its functional significance, if any, need to be taken into account while evaluating malaria exposure in field settings.

Effectiveness of a Malaria Surveillance Strategy Based on Active Case Detection during High Transmission Season in the Peruvian Amazon

Background: Faced with the resurgence of malaria, malaria surveillance in the Peruvian Amazon incorporated consecutive active case detection (ACD) interventions using light microscopy (LM) as reactive measure in communities with an unusual high number of cases during high transmission season (HTS). We assessed the effectiveness in malaria detection of this local ACD-based strategy. Methods: A cohort study was conducted in June–July 2015 in Mazan, Loreto. Four consecutive ACD interventions at intervals of 10 days were conducted in four riverine communities (Gamitanacocha, Primero de Enero, Libertad and Urco Miraño). In each intervention, all inhabitants were visited at home, and finger-prick blood samples collected for immediate diagnosis by LM and on filter paper for later analysis by quantitative real-time polymerase chain reaction (qPCR). Effectiveness was calculated by dividing the number of malaria infections detected using LM by the number of malaria infections detected by delayed qPCR. Results: Most community inhabitants (88.1%, 822/933) were present in at least one of the four ACD interventions. A total of 451 infections were detected by qPCR in 446 participants (54.3% of total participants); five individuals had two infections. Plasmodium vivax was the predominant species (79.8%), followed by P. falciparum (15.3%) and P. vivax-P. falciparum co-infections (4.9%). Most qPCR-positive infections were asymptomatic (255/448, 56.9%). The ACD-strategy using LM had an effectiveness of 22.8% (detection of 103 of the total qPCR-positive infections). Children aged 5–14 years, and farming as main economic activity were associated with P. vivax infections. Conclusions: Although the ACD-strategy using LM increased the opportunity of detecting and treating malaria infections during HTS, the number of detected infections was considerably lower than the real burden of infections (those detected by qPCR).

Plasmodium genomics: an approach for learning about and ending human malaria

Malaria causes high levels of morbidity and mortality in human beings worldwide. According to the World Health Organization (WHO), about half a million people die of this disease each year. Malaria is caused by six species of parasites belonging to the Plasmodium genus: P. falciparum, P. knowlesi, P. vivax, P. malariae, P. ovale curtisi, and P. ovale wallikeri. Currently, malaria is being kept under control with varying levels of elimination success in different countries. The development of new molecular tools as well as the use of next-generation sequencing (NGS) technologies and novel bioinformatic approaches has improved our knowledge of malarial epidemiology, diagnosis, treatment, vaccine development, and surveillance strategies. In this work, the genetics and genomics of human malarias have been analyzed. Since the first P. falciparum genome was sequenced in 2002, various population-level genetic and genomic surveys, together with transcriptomic and proteomic studies, have shown the importance of molecular approaches in supporting malaria elimination.

Genetic sequence characterization and naturally acquired immune response to Plasmodium vivax Rhoptry Neck Protein 2 (PvRON2)

Background

The genetic diversity of malaria antigens often results in allele variant-specific immunity, imposing a great challenge to vaccine development. Rhoptry Neck Protein 2 (PvRON2) is a blood-stage antigen that plays a key role during the erythrocyte invasion of Plasmodium vivax. This study investigates the genetic diversity of PvRON2 and the naturally acquired immune response to P. vivax isolates.

Results

Here, the genetic diversity of PvRON2_1828–2080 and the naturally acquired humoral immune response against PvRON2_1828–2080 in infected and non-infected individuals from a vivax malaria endemic area in Brazil was reported. The diversity analysis of PvRON2_1828–2080 revealed that the protein is conserved in isolates in Brazil and worldwide. A total of 18 (19%) patients had IgG antibodies to PvRON2_1828–2080. Additionally, the analysis of the antibody response in individuals who were not acutely infected with malaria, but had been infected with malaria in the past indicated that 32 patients (33%) exhibited an IgG immune response against PvRON2.

Conclusions

PvRON2 was conserved among the studied isolates. The presence of naturally acquired antibodies to this protein in the absence of the disease suggests that PvRON2 induces a long-term antibody response. These results indicate that PvRON2 is a potential malaria vaccine candidate.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2543-7) contains supplementary material, which is available to authorized users.

Epidemiology, drug resistance, and pathophysiology of Plasmodium vivax malaria

Malaria, caused by the protozoan parasites of the genus Plasmodium, is a major health problem in many countries of the world. Five parasite species namely, Plasmodium falciparum, P. vivax, P. malariae, P. ovale, and P. knowlesi, cause malaria in humans. Of these, P. falciparum and P. vivax are the most prevalent and account for the majority of the global malaria cases. In most areas of Africa, P. vivax infection is essentially absent because of the inherited lack of Duffy antigen receptor for chemokines on the surface of red blood cells that is involved in the parasite invasion of erythrocytes. Therefore, in Africa, most malaria infections are by P. falciparum and the highest burden of P. vivax infection is in Southeast Asia and South America. Plasmodium falciparum is the most virulent and as such, it is responsible for the majority of malarial mortality, particularly in Africa. Although, P. vivax infection has long been considered to be benign, recent studies have reported life-threatening consequences, including acute respiratory distress syndrome, cerebral malaria, multi-organ failure, dyserythropoiesis and anaemia. Despite exhibiting low parasite biomass in infected people due to parasite's specificity to infect only reticulocytes, P. vivax infection triggers higher inflammatory responses and exacerbated clinical symptoms than P. falciparum, such as fever and chills. Another characteristic feature of P. vivax infection, compared to P. falciparum infection, is persistence of the parasite as dormant liver-stage hypnozoites, causing recurrent episodes of malaria. This review article summarizes the published information on P. vivax epidemiology, drug resistance and pathophysiology.

Asymptomatic Plasmodium vivax parasitaemia in the low-transmission setting: the role for a population-based transmission-blocking vaccine for malaria elimination

Plasmodium vivax remains an important cause of morbidity and mortality across the Americas, Horn of Africa, East and South East Asia. Control of transmission has been hampered by emergence of chloroquine resistance and several intrinsic characteristics of infection including asymptomatic carriage, challenges with diagnosis, difficulty eradicating the carrier state and early gametocyte appearance. Complex human-parasite-vector immunological interactions may facilitate onward infection of mosquitoes. Given these challenges, new therapies are being explored including the development of transmission to mosquito blocking vaccines. Herein, the case supporting the need for transmission-blocking vaccines to augment control of P. vivax parasite transmission and explore factors that are limiting eradication efforts is discussed.

Increasingly inbred and fragmented populations of Plasmodium vivax associated with the eastward decline in malaria transmission across the Southwest Pacific

The human malaria parasite Plasmodium vivax is more resistant to malaria control strategies than Plasmodium falciparum, and maintains high genetic diversity even when transmission is low. To investigate whether declining P. vivax transmission leads to increasing population structure that would facilitate elimination, we genotyped samples from across the Southwest Pacific region, which experiences an eastward decline in malaria transmission, as well as samples from two time points at one site (Tetere, Solomon Islands) during intensified malaria control. Analysis of 887 P. vivax microsatellite haplotypes from hyperendemic Papua New Guinea (PNG, n = 443), meso-hyperendemic Solomon Islands (n = 420), and hypoendemic Vanuatu (n = 24) revealed increasing population structure and multilocus linkage disequilibrium yet a modest decline in diversity as transmission decreases over space and time. In Solomon Islands, which has had sustained control efforts for 20 years, and Vanuatu, which has experienced sustained low transmission for many years, significant population structure was observed at different spatial scales. We conclude that control efforts will eventually impact P. vivax population structure and with sustained pressure, populations may eventually fragment into a limited number of clustered foci that could be targeted for elimination.

Plasmodium vivax is a major human malaria parasite, common in endemic areas outside sub-Saharan Africa, and more difficult to control than other malaria parasite species. The distinct lifecycle biology of P. vivax is thought to contribute to its more stable and efficient transmission allowing the maintenance of high diversity and potentially, gene flow. Independent studies are therefore needed to understand how P. vivax populations respond to changing transmission levels, in order to inform malaria control and elimination efforts. Here we have determined parasite population genetic structure in three countries of the Southwest Pacific, an island chain with a natural west to east decline in transmission intensity (Papua New Guinea > Solomon Islands > Vanuatu). With declining transmission, P. vivax populations experience only a modest decline in diversity but a significant increase in multilocus linkage disequilibrium and population structure, indicating that parasite populations become more inbred and begin to fragment into clustered foci. Analysis of two time points in one study area (Tetere, Solomon Islands) also show similar changes in association with intensifying malaria control. The results indicate that with long term sustained malaria control P. vivax populations will eventually fracture into population clusters that could be targeted for elimination.

A clinical tool to predict Plasmodium vivax recurrence in Malaysia

Background

Recurrence rates of Plasmodium vivax infections differ across various geographic regions. Interestingly, South-East Asia and the Asia-Pacific region are documented to exhibit the most frequent recurrence incidences. Identifying patients at a higher risk for recurrences gives valuable information in strengthening the efforts to control P. vivax infections. The aim of the study was to develop a tool to identify P. vivax- infected patients that are at a higher risk of recurrence in Malaysia.

Methods

Patient data was obtained retrospectively through the Ministry of Health, Malaysia, from 2011 to 2016. Patients with incomplete data were excluded. A total of 2044 clinical P. vivax malaria cases treated with primaquine were included. Data collected were patient, disease, and treatment characteristics. Two-thirds of the cases (n = 1362) were used to develop a clinical risk score, while the remaining third (n = 682) was used for validation.

Results

Using multivariate analysis, age (p = 0.03), gametocyte sexual count (p = 0.04), indigenous transmission (p = 0.04), type of treatment (p = 0.12), and incomplete primaquine treatment (p = 0.14) were found to be predictors of recurrence after controlling for other confounding factors; these predictors were then used in developing the final model. The beta-coefficient values were used to develop a clinical scoring tool to predict possible recurrence. The total scores ranged between 0 and 8. A higher score indicated a higher risk for recurrence (odds ratio [OR]: 1.971; 95% confidence interval [CI]: 1.562–2.487; p ≤ 0.001). The area under the receiver operating characteristic (ROC) curve of the developed (n = 1362) and validated model (n = 682) was of good accuracy (ROC: 0.728, 95% CI: 0.670–0.785, p value < 0.001, and ROC: 0.766, 95% CI: 0.700–0.833, p-value < 0.001, respectively). In both the developed and validated models, area under the ROC curves showed no significant difference in predicting recurrence based on the constructed scoring mechanism (p = 0.399; Z-value: −0.8441; standard error: 0.045).

Conclusions

The developed model to predict recurrence was found to be of good accuracy and could be a useful tool in targeting patients at a higher risk for recurrence for closer monitoring during follow-up, after treatment with primaquine.

High prevalence of very-low Plasmodium falciparum and Plasmodium vivax parasitaemia carriers in the Peruvian Amazon: insights into local and occupational mobility-related transmission

BACKGROUND

The incidence of malaria due both to Plasmodium falciparum and Plasmodium vivax in the Peruvian Amazon has risen in the past 5 years. This study tested the hypothesis that the maintenance and emergence of malaria in hypoendemic regions such as Amazonia is determined by submicroscopic and asymptomatic Plasmodium parasitaemia carriers. The present study aimed to precisely quantify the rate of very-low parasitaemia carriers in two sites of the Peruvian Amazon in relation to transmission patterns of P. vivax and P. falciparum in this area.

METHODS

This study was carried out within the Amazonian-ICEMR longitudinal cohort. Blood samples were collected for light microscopy diagnosis and packed red blood cell (PRBC) samples were analysed by qPCR. Plasma samples were tested for total IgG reactivity against recombinant PvMSP-10 and PfMSP-10 antigens by ELISA. Occupation and age 10 years and greater were considered surrogates of occupation-related mobility. Risk factors for P. falciparum and P. vivax infections detected by PRBC-qPCR were assessed by multilevel logistic regression models.

RESULTS

Among 450 subjects, the prevalence of P. vivax by PRBC-PCR (25.1%) was sixfold higher than that determined by microscopy (3.6%). The prevalence of P. falciparum infection was 4.9% by PRBC-PCR and 0.2% by microscopy. More than 40% of infections had parasitaemia under 5 parasites/μL. Multivariate analysis for infections detected by PRBC-PCR showed that participants with recent settlement in the study area (AOR 2.1; 95% CI 1.03:4.2), age ≥ 30 years (AOR 3.3; 95% CI 1.6:6.9) and seropositivity to P. vivax (AOR 1.8; 95% CI 1.0:3.2) had significantly higher likelihood of P. vivax infection, while the odds of P. falciparum infection was higher for participants between 10 and 29 years (AOR 10.7; 95% CI 1.3:91.1) and with a previous P. falciparum infection (AOR 10.4; 95% CI 1.5:71.1).

CONCLUSIONS

This study confirms the contrasting transmission patterns of P. vivax and P. falciparum in the Peruvian Amazon, with stable local transmission for P. vivax and the source of P. falciparum to the study villages dominated by very low parasitaemia carriers, age 10 years and older, who had travelled away from home for work and brought P. falciparum infection with them.

Proximity to vector breeding site and risk of Plasmodium vivax infection: a prospective cohort study in rural Ethiopia

Background

Despite falling incidence and mortality since the turn of the century, malaria remains an important global health challenge. In the future fight against malaria, greater emphasis will have to be placed on understanding and addressing malaria caused by the Plasmodium vivax parasite. Unfortunately, due to years of neglect and underfunding, there are currently many gaps in knowledge of P. vivax malaria. The aims of the present study were to explore the association between distance to vector breeding site and P. vivax infection in rural Ethiopia, and, secondarily, to test whether this association varies with age.

Methods

A prospective, cohort study of all residents in the Chano Mille Kebele in southern Ethiopia from April 2009 to March 2011 (n = 8121). Weekly household follow up visits included screening for febrile cases (active surveillance). Participants were also asked to contact the local health centre if they experienced subjective fever between visits (passive surveillance). Plasmodium vivax infection was confirmed using microscopy by two independent readers. Information was collected on demographics and household characteristics including GPS-determined distance to vector breeding site. Data was analysed using Cox regression modelling.

Results

Overall the P. vivax infection rate was 12.3/1000 person-years (95% CI 10.5–14.5). Mean household distance to breeding site was 2449 m (range 1646–3717 m). Fully adjusted results showed very strong evidence of an association between proximity to breeding site and P. vivax infection: rate ratio = 3.47 (95% CI 2.15–5.60; P < 0.001) comparing the group closest to the breeding site (distance < 2100 m; n = 1383) to the group furthest away (distance > 2700 m; n = 2460). There was no evidence that age was an effect modifier in the association.

Conclusion

Results showed strong evidence that household proximity to vector breeding site is positively associated with P. vivax infection in rural Ethiopia, and that this association is constant across age groups. The findings might influence how net-distribution and indoor residual spraying campaigns are planned, help guide strategies on water resource development by highlighting potential health effects of man-made dams near human habitats, and add to current educational information given to people living close to breeding sites.

The Biology of Plasmodium vivax

Plasmodium vivax is the second most prevalent cause of malaria worldwide and the leading cause of malaria outside of Africa. Although infections are seldom fatal clinical disease can be debilitating and imposes significant health and economic impacts on affected populations. Estimates of transmission and prevalence intensity can be problematic because many episodes of vivax originate from hypnozoite stages in the liver that have remained dormant from previous infections by an unknown mechanism. Lack of treatment options to clear hypnozoites and the ability to infect mosquitoes before disease symptoms present represent major challenges for control and eradication of vivax malaria. Compounding these challenges is the unique biology of P. vivax and limited progress in development of experimental research tools, thereby hindering development of new drugs and vaccines. Renewed emphasis on vivax malaria research is beginning to make progress in overcoming some of these challenges.

Naturally Acquired Human Antibodies Against Reticulocyte-Binding Domains of Plasmodium vivax Proteins, PvRBP2c and PvRBP1a, Exhibit Binding-Inhibitory Activity

Background

Crucial gaps in our understanding of Plasmodium vivax reticulocyte invasion and protective immunity have hampered development of vivax vaccines. P. vivax exclusively invades reticulocytes that is mediated by the P. vivax reticulocyte-binding proteins (PvRBPs) specifically PvRBP2c and PvRBP1a. Vivax infections in Duffy-null individuals have suggested the evolution of alternate invasion pathways that may be mediated by the PvRBPs. Thus, PvRBPs appear as potential targets for efficacious P. vivax neutralization. However, there are limited data validating their vaccine efficacy. In the absence of vivax invasion assays, binding-inhibitory activity of antibodies has been reported to be associated with protection and a measure of vaccine potential.

Methods

-based analysis was performed of the PvRBP reticulocyte-binding properties and binding-inhibitory activity of specific anti-PvRBP2c/PvRBP1a human antibodies.

Results

PvRBP2c and PvRBP1a displayed a distinct reticulocyte-binding specificity, and their specific reticulocyte-binding domains were mapped within their N-terminal regions. Importantly, naturally acquired antibodies against the reticulocyte-binding domains efficaciously blocked reticulocyte binding of native PvRBPs, suggesting that the human immune system produced functional binding-inhibitory antibodies through exposure to vivax malaria.

Conclusions

Reticulocyte-binding domains of PvRBP2c/PvRBP1a are targets of naturally acquired binding-inhibitory antibodies, substantiating their promise as candidate antigens against which vaccine-inducible immunity could potentially be boosted through natural infections.

Malaria

Malaria is caused in humans by five species of single-celled eukaryotic Plasmodium parasites (mainly Plasmodium falciparum and Plasmodium vivax) that are transmitted by the bite of Anopheles spp. mosquitoes. Malaria remains one of the most serious infectious diseases; it threatens nearly half of the world's population and led to hundreds of thousands of deaths in 2015, predominantly among children in Africa. Malaria is managed through a combination of vector control approaches (such as insecticide spraying and the use of insecticide-treated bed nets) and drugs for both treatment and prevention. The widespread use of artemisinin-based combination therapies has contributed to substantial declines in the number of malaria-related deaths; however, the emergence of drug resistance threatens to reverse this progress. Advances in our understanding of the underlying molecular basis of pathogenesis have fuelled the development of new diagnostics, drugs and insecticides. Several new combination therapies are in clinical development that have efficacy against drug-resistant parasites and the potential to be used in single-dose regimens to improve compliance. This ambitious programme to eliminate malaria also includes new approaches that could yield malaria vaccines or novel vector control strategies. However, despite these achievements, a well-coordinated global effort on multiple fronts is needed if malaria elimination is to be achieved.

Synergistic effect of IL-12 and IL-18 induces TIM3 regulation of γδ T cell function and decreases the risk of clinical malaria in children living in Papua New Guinea

BACKGROUND

γδ T cells are important for both protective immunity and immunopathogenesis during malaria infection. However, the immunological processes determining beneficial or detrimental effects on disease outcome remain elusive. The aim of this study was to examine expression and regulatory effect of the inhibitory receptor T-cell immunoglobulin domain and mucin domain 3 (TIM3) on γδ T cells. While TIM3 expression and function on conventional αβ T cells have been clearly defined, the equivalent characterization on γδ T cells and associations with disease outcomes is limited. This study investigated the functional capacity of TIM3+ γδ T cells and the underlying mechanisms contributing to TIM3 upregulation and established an association with malaria disease outcomes.

METHODS

We analyzed TIM3 expression on γδ T cells in 132 children aged 5-10 years living in malaria endemic areas of Papua New Guinea. TIM3 upregulation and effector functions of TIM3+ γδ T cells were assessed following in vitro stimulation with parasite-infected erythrocytes, phosphoantigen and/or cytokines. Associations between the proportion of TIM3-expressing cells and the molecular force of infection were tested using negative binomial regression and in a Cox proportional hazards model for time to first clinical episode. Multivariable analyses to determine the association of TIM3 and IL-18 levels were conducted using general linear models. Malaria infection mouse models were utilized to experimentally investigate the relationship between repeated exposure and TIM3 upregulation.

RESULTS

This study demonstrates that even in the absence of an active malaria infection, children of malaria endemic areas have an atypical population of TIM3-expressing γδ T cells (mean frequency TIM3+ of total γδ T cells 15.2% ± 12). Crucial factors required for γδ T cell TIM3 upregulation include IL-12/IL-18, and plasma IL-18 was associated with TIM3 expression (P = 0.002). Additionally, we show a relationship between TIM3 expression and infection with distinct parasite clones during repeated exposure. TIM3+ γδ T cells were functionally impaired and were associated with asymptomatic malaria infection (hazard ratio 0.54, P = 0.032).

CONCLUSIONS

Collectively our data demonstrate a novel role for IL-12/IL-18 in shaping the innate immune response and provide fundamental insight into aspects of γδ T cell immunoregulation. Furthermore, we show that TIM3 represents an important γδ T cell regulatory component involved in minimizing malaria symptoms.

Characterizing Antibody Responses to Plasmodium vivax and Plasmodium falciparum Antigens in India Using Genome-Scale Protein Microarrays

Understanding naturally acquired immune responses to Plasmodium in India is key to improving malaria surveillance and diagnostic tools. Here we describe serological profiling of immune responses at three sites in India by probing protein microarrays consisting of 515 Plasmodium vivax and 500 Plasmodium falciparum proteins with 353 plasma samples. A total of 236 malaria-positive (symptomatic and asymptomatic) plasma samples and 117 malaria-negative samples were collected at three field sites in Raurkela, Nadiad, and Chennai. Indian samples showed significant seroreactivity to 265 P. vivax and 373 P. falciparum antigens, but overall seroreactivity to P. vivax antigens was lower compared to P. falciparum antigens. We identified the most immunogenic antigens of both Plasmodium species that were recognized at all three sites in India, as well as P. falciparum antigens that were associated with asymptomatic malaria. This is the first genome-scale analysis of serological responses to the two major species of malaria parasite in India. The range of immune responses characterized in different endemic settings argues for targeted surveillance approaches tailored to the diverse epidemiology of malaria across the world.

Although malaria deaths have fallen by 60% worldwide since 2000, the disease remains a significant public health problem. India has the highest burden of malaria in the South-East Asia Region, where Plasmodium vivax and Plasmodium falciparum are its main causes. While the two major malaria parasite species co-occur in India, their proportion varies across the country. Antibodies in an individual indicate current or past Plasmodium infection, and can be used to identify suitable vaccine candidates, as well as develop novel tools for malaria surveillance. We present the results of a pilot study undertaking the first large-scale characterization of antibody responses to ~1000 Plasmodium antigens at three field sites in India using high-throughput protein microarray technology. Individuals from the eco-epidemiologically diverse sites showed reactivity to 265 P. vivax and 373 P. falciparum antigens, regardless of infection status. Further comparison of individuals with symptomatic and asymptomatic malaria revealed the most immunogenic Plasmodium antigens, as well as antigens that were recognized with greater intensity in individuals that were asymptomatic at the point of sample collection. These results are a valuable addition to existing data from other malaria endemic regions, and will help to expand our understanding of host immunity against the disease.

Characterization of Caveola-Vesicle Complexes (CVCs) Protein, PHIST/CVC-8195 in Plasmodium vivax

Plasmodium vivax produces numerous caveola-vesicle complex (CVC) structures beneath the membrane of infected erythrocytes. Recently, a member helical interspersed subtelomeric (PHIST) superfamily protein, PcyPHIST/CVC-81₉₅, was identified as CVCs-associated protein in Plasmodium cynomolgi and essential for survival of this parasite. Very little information has been documented to date about PHIST/CVC-81₉₅ protein in P. vivax. In this study, the recombinant PvPHIST/CVC-81₉₅ N and C termini were expressed, and immunoreactivity was assessed using confirmed vivax malaria patients sera by protein microarray. The subcellular localization of PvPHIST/CVC-81₉₅ N and C termini in blood stage parasites was also determined. The antigenicity of recombinant PvPHIST/CVC-81₉₅ N and C terminal proteins were analyzed by using serum samples from the Republic of Korea. The results showed that immunoreactivities to these proteins had 61% and 43% sensitivity and 96.9% and 93.8% specificity, respectively. The N terminal of PvPHIST/CVC-81₉₅ which contains transmembrane domain and export motif (PEXEL; RxLxE/Q/D) produced CVCs location throughout the erythrocytic-stage parasites. However, no fluorescence was detected with antibodies against C terminal fragment of PvPHIST/CVC-81₉₅. These results suggest that the PvPHIST/CVC-81₉₅ is localized on the CVCs and may be immunogenic in natural infection of P. vivax.

Global Epidemiology of Plasmodium vivax

Plasmodium vivax is the most widespread human malaria, putting 2.5 billion people at risk of infection. Its unique biological and epidemiological characteristics pose challenges to control strategies that have been principally targeted against Plasmodium falciparum Unlike P. falciparum, P. vivax infections have typically low blood-stage parasitemia with gametocytes emerging before illness manifests, and dormant liver stages causing relapses. These traits affect both its geographic distribution and transmission patterns. Asymptomatic infections, high-risk groups, and resulting case burdens are described in this review. Despite relatively low prevalence measurements and parasitemia levels, along with high proportions of asymptomatic cases, this parasite is not benign. Plasmodium vivax can be associated with severe and even fatal illness. Spreading resistance to chloroquine against the acute attack, and the operational inadequacy of primaquine against the multiple attacks of relapse, exacerbates the risk of poor outcomes among the tens of millions suffering from infection each year. Without strategies accounting for these P. vivax-specific characteristics, progress toward elimination of endemic malaria transmission will be substantially impeded.

Implications of Plasmodium vivax Biology for Control, Elimination, and Research

This paper summarizes our current understanding of the biology of Plasmodium vivax, how it differs from Plasmodium falciparum, and how these differences explain the need for P. vivax-tailored interventions. The article further pinpoints knowledge gaps where investments in research are needed to help identify and develop such specific interventions. The principal obstacles to reduce and eventually eliminate P. vivax reside in 1) its higher vectorial capacity compared with P. falciparum due to its ability to develop at lower temperature and over a shorter sporogonic cycle in the vector, allowing transmission in temperate zones and making it less sensitive to vector control measures that are otherwise effective on P. falciparum; 2) the presence of dormant liver forms (hypnozoites), sustaining multiple relapsing episodes from a single infectious bite that cannot be diagnosed and are not susceptible to any available antimalarial except primaquine, with routine deployment restricted by toxicity; 3) low parasite densities, which are difficult to detect with current diagnostics leading to missed diagnoses and delayed treatments (and protracted transmission), coupled with 4) transmission stages (gametocytes) occurring early in acute infections, before infection is diagnosed.

Asiatic acid influences parasitaemia reduction and ameliorates malaria anaemia in P. berghei infected Sprague-Dawley male rats

BACKGROUND

Current malaria treatment is either "anti-parasitic", "anti-infectivity" or both without addressing the pathophysiological derangement (anti-disease aspect) associated with the disease. Asiatic acid is a natural phytochemical with oxidant, antioxidant and anti-inflammatory properties whose effect on malarial and accompanying pathophysiology are yet to be investigated. Asiatic acid influence in P. berghei-infected Sprague Dawley rats on %parasitaemia and malarial anaemia were investigated.

METHODS

Plasmodium berghei-infected rats (90-120 g) were orally administered with Asiatic acid (5, 10, 20 mg/kg) and 30 mg/kg chloroquine as a positive control. Changes in %parasitaemia and haematological parameters in Asiatic acid administered rats were monitored in a 21 day study and compared to controls.

RESULTS

All animals developed stable parasitaemia (15-20 %) by day 7. Asiatic acid doses suppressed parasitaemia, normalised haematological measurements and influenced biophysical characteristics changes. Most positive changes were associated with intragastric administration of 10 mg/kg Asiatic acid dose. Peak %parasitaemia in Asiatic acid administration occurred at days 12 with a shorter time course compared to day 9 for chloroquine (30 mg/kg) treatment with a longer time course.

CONCLUSIONS

Oral Asiatic acid administration influenced %parasitaemia suppression, ameliorated malarial anaemia and increased biophysical properties on infected animals. Asiatic acid may be a replacement alternative for chloroquine treatment with concomitant amelioration of malaria pathophysiology. Due to different action time courses, Asiatic acid and chloroquine may be possible candidates in combination therapy.

Recent insights into humoral immunity targeting Plasmodium falciparum and Plasmodium vivax malaria

Recent efforts in malaria control have led to marked reductions in malaria incidence. However, new strategies are needed to sustain malaria elimination and eradication and achieve the World Health Organization goal of a malaria-free world. The development of highly effective vaccines would contribute to this goal and would be facilitated by a comprehensive understanding of humoral immune responses targeting Plasmodium falciparum and Plasmodium vivax malaria. New tools are required to facilitate the identification of vaccine candidates and the development of vaccines that induce functional and protective immunity. Here we discuss recent published findings, and unpublished work presented at the 2016 Molecular Approaches to Malaria conference, that highlight advancements in understanding humoral immune responses in the context of vaccine development. Highlights include the increased application of 'omics' and 'Big data' platforms to identify vaccine candidates, and the identification of novel functions of antibody responses that mediate protection. The application of these strategies and a global approach will increase the likelihood of rapid development of highly efficacious vaccines.

Common asymptomatic and submicroscopic malaria infections in Western Thailand revealed in longitudinal molecular and serological studies: a challenge to malaria elimination

Background

Despite largely successful control efforts, malaria remains a significant public health problem in Thailand. Based on microscopy, the northwestern province of Tak, once Thailand’s highest burden area, is now considered a low-transmission region. However, microscopy is insensitive to detect low-level parasitaemia, causing gross underestimation of parasite prevalence in areas where most infections are subpatent. The objective of this study was to assess the current epidemiology of malaria prevalence using molecular and serological detection methods, and to profile the antibody responses against Plasmodium as it relates to age, seasonal changes and clinical manifestations during infection. Three comprehensive cross-sectional surveys were performed in a sentinel village and from febrile hospital patients, and whole blood samples were collected from infants to elderly adults. Genomic DNA isolated from cellular fraction was screened by quantitative-PCR for the presence of Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, Plasmodium ovale and Plasmodium knowlesi. Plasma samples were probed on protein microarray to obtain antibody response profiles from the same individuals.

Results

Within the studied community, 90.2 % of Plasmodium infections were submicroscopic and asymptomatic, including a large number of mixed-species infections. Amongst febrile patients, mixed-species infections comprised 68 % of positive cases, all of which went misdiagnosed and undertreated. All samples tested showed serological reactivity to Plasmodium antigens. There were significant differences in the rates of antibody acquisition against P. falciparum and P. vivax, and age-related differences in species-specific immunodominance of response. Antibodies against Plasmodium increased along the ten-month study period. Febrile patients had stronger antibody responses than asymptomatic carriers.

Conclusions

Despite a great decline in malaria prevalence, transmission is still ongoing at levels undetectable by traditional methods. As current surveillance methods focus on case management, malaria transmission in Thailand will not be interrupted if asymptomatic submicroscopic infections are not detected and treated.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1393-4) contains supplementary material, which is available to authorized users.

An Antibody Screen of a Plasmodium vivax Antigen Library Identifies Novel Merozoite Proteins Associated with Clinical Protection

BACKGROUND

Elimination of Plasmodium vivax malaria would be greatly facilitated by the development of an effective vaccine. A comprehensive and systematic characterization of antibodies to P. vivax antigens in exposed populations is useful in guiding rational vaccine design.

METHODOLOGY/PRINCIPAL FINDINGS

In this study, we investigated antibodies to a large library of P. vivax entire ectodomain merozoite proteins in 2 Asia-Pacific populations, analysing the relationship of antibody levels with markers of current and cumulative malaria exposure, and socioeconomic and clinical indicators. 29 antigenic targets of natural immunity were identified. Of these, 12 highly-immunogenic proteins were strongly associated with age and thus cumulative lifetime exposure in Solomon Islanders (P<0.001-0.027). A subset of 6 proteins, selected on the basis of immunogenicity and expression levels, were used to examine antibody levels in plasma samples from a population of young Papua New Guinean children with well-characterized individual differences in exposure. This analysis identified a strong association between reduced risk of clinical disease and antibody levels to P12, P41, and a novel hypothetical protein that has not previously been studied, PVX_081550 (IRR 0.46-0.74; P<0.001-0.041).

CONCLUSION/SIGNIFICANCE

These data emphasize the benefits of an unbiased screening approach in identifying novel vaccine candidate antigens. Functional studies are now required to establish whether PVX_081550 is a key component of the naturally-acquired protective immune response, a biomarker of immune status, or both.

Maternal-foetal transfer of Plasmodium falciparum and Plasmodium vivax antibodies in a low transmission setting

During pregnancy immunoglobulin G (IgG) antibodies are transferred from mother to neonate across the placenta. Studies in high transmission areas have shown transfer of P. falciparum-specific IgG, but the extent and factors influencing maternal-foetal transfer in low transmission areas co-endemic for both P. falciparum and P. vivax are unknown. Pregnant women were screened weekly for Plasmodium infection. Mother-neonate paired serum samples at delivery were tested for IgG to antigens from P. falciparum, P. vivax and other infectious diseases. Antibodies to malarial and non-malarial antigens were highly correlated between maternal and neonatal samples (median [range] spearman ρ = 0.78 [0.57-0.93]), although Plasmodium spp. antibodies tended to be lower in neonates than mothers. Estimated gestational age at last P. falciparum infection, but not P. vivax infection, was positively associated with antibody levels in the neonate (P. falciparum merozoite, spearman ρ median [range] 0.42 [0.33-0.66], PfVAR2CSA 0.69; P. vivax ρ = 0.19 [0.09-0.3]). Maternal-foetal transfer of anti-malarial IgG to Plasmodium spp. antigens occurs in low transmission settings. P. vivax IgG acquisition is not associated with recent exposure unlike P. falciparum IgG, suggesting a difference in acquisition of antibodies. IgG transfer is greatest in the final weeks of pregnancy which has implications for the timing of future malaria vaccination strategies in pregnant women.

Major Histocompatibility Complex and Malaria: Focus on Plasmodium vivax Infection

The importance of host and parasite genetic factors in malaria resistance or susceptibility has been investigated since the middle of the last century. Nowadays, of all diseases that affect man, malaria still plays one of the highest levels of selective pressure on human genome. Susceptibility to malaria depends on exposure profile, epidemiological characteristics, and several components of the innate and adaptive immune system that influences the quality of the immune response generated during the Plasmodium lifecycle in the vertebrate host. But it is well known that the parasite's enormous capacity of genetic variation in conjunction with the host genetics polymorphism is also associated with a wide spectrum of susceptibility degrees to complicated or severe forms of the disease. In this scenario, variations in genes of the major histocompatibility complex (MHC) associated with host resistance or susceptibility to malaria have been identified and used as markers in host-pathogen interaction studies, mainly those evaluating the impact on the immune response, acquisition of resistance, or increased susceptibility to infection or vulnerability to disease. However, due to the intense selective pressure, number of cases, and mortality rates, the majority of the reported associations reported concerned Plasmodium falciparum malaria. Studies on the MHC polymorphism and its association with Plasmodium vivax, which is the most widespread Plasmodium and the most prevalent species outside the African continent, are less frequent but equally important. Despite punctual contributions, there are accumulated evidences of human genetic control in P. vivax infection and disease. Herein, we review the current knowledge in the field of MHC and derived molecules (HLA Class I, Class II, TNF-α, LTA, BAT1, and CTL4) regarding P. vivax malaria. We discuss particularly the results of P. vivax studies on HLA class I and II polymorphisms in relation to host susceptibility, naturally acquired immune response against specific antigens and the implication of this knowledge to overcome the parasite immune evasion. Finally, the potential impact of such polymorphisms on the development of vaccine candidate antigens against P. vivax will be studied.