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

IgM antibody responses against Plasmodium antigens in neotropical primates in the Brazilian Atlantic Forest

Introduction

Zoonotic transmission is a challenge for the control and elimination of malaria. It has been recorded in the Atlantic Forest, outside the Amazon which is the endemic region in Brazil. However, only very few studies have assessed the antibody response, especially of IgM antibodies, in Neotropical primates (NP). Therefore, in order to contribute to a better understanding of the immune response in different hosts and facilitate the identification of potential reservoirs, in this study, naturally acquired IgM antibody responses against Plasmodium antigens were evaluated, for the first time, in NP from the Atlantic Forest.

Methods

The study was carried out using 154 NP samples from three different areas of the Atlantic Forest. IgM antibodies against peptides of the circumsporozoite protein (CSP) from different Plasmodium species and different erythrocytic stage antigens were detected by ELISA.

Results

Fifty-nine percent of NP had IgM antibodies against at least one CSP peptide and 87% against at least one Plasmodium vivax erythrocytic stage antigen. Levels of antibodies against PvAMA-1 were the highest compared to the other antigens. All families of NP showed IgM antibodies against CSP peptides, and, most strikingly, against erythrocytic stage antigens. Generalized linear models demonstrated that IgM positivity against PvCSP and PvAMA-1 was associated with PCR-detectable blood-stage malaria infection and the host being free-living. Interestingly, animals with IgM against both PvCSP and PvAMA-1 were 4.7 times more likely to be PCR positive than animals that did not have IgM for these two antigens simultaneously.

Discussion

IgM antibodies against different Plasmodium spp. antigens are present in NP from the Atlantic Forest. High seroprevalence and antibody levels against blood-stage antigens were observed, which had a significant association with molecular evidence of infection. IgM antibodies against CSP and AMA-1 may be used as a potential marker for the identification of NP infected with Plasmodium, which are reservoirs of malaria in the Brazilian Atlantic Forest.

Assessing Prevalence and Transmission Rates of Malaria through Simultaneous Profiling of Antibody Responses against Plasmodium and Anopheles Antigens

Reliably assessing exposure to mosquitoes carrying malaria parasites continues to be a challenge due to the lack of reliable, highly sensitive diagnostics with high-throughput potential. Here, we describe an approach that meets these requirements by simultaneously measuring immune responses to both disease vector and pathogen, using an electro-chemiluminescence-based multiplex assay platform. While using the same logistical steps as a classic ELISA, this platform allows for the multiplexing of up to ten antigens in a single well. This simple, reproducible, quantitative readout reports the magnitude, incidence, and prevalence of malaria infections in residents of malaria-endemic areas. By reporting exposure to both insect vectors and pathogen, the approach also provides insights into the efficacy of drugs and/or other countermeasures deployed against insect vectors aimed at reducing or eliminating arthropod-borne diseases. The high throughput of the assay enables the quick and efficient screening of sera from individuals for exposure to Plasmodium even if they are taking drug prophylaxis. We applied this assay to samples collected from controlled malaria infection studies, as well as those collected in field studies in malaria-endemic regions in Uganda and Kenya. The assay was sensitive to vector exposure, malaria infection, and endemicity, demonstrating its potential for use in malaria serosurveillance.

Safety and Tolerability of Mosquito-Bite Induced Controlled Human Infection with P. vivax in Malaria-Naïve Study Participants - Clinical Profile and Utility of Molecular Diagnostic Methods

BACKGROUND

Plasmodium vivax controlled-human-malaria-infection (PvCHMI) is an important tool for the evaluation of drugs, vaccines and pathologies associated with this parasite. However, there is little data on its safety due to the limited number of PvCHMIs performed to-date.

METHODS

We report clinical and laboratory data collected, to include hematological and biochemical profiles and adverse-events, following mosquito-bite induced PvCHMI in malaria-naïve study-participants (ClinicalTrials.gov_NCT01157897). Malaria diagnosis and treatment initiation was based on microscopic analysis of Giemsa-stained slides. Exploratory molecular assays were used to detect parasites using real-time PCR.

RESULTS

Adverse-events (AEs) were mild to moderate and no study-related severe AEs were observed in any of the study participants. Majority of the symptoms were transient, resolving within 48hours. Molecular-diagnostic methods detected parasitemia in 100% of study-participants before malaria diagnosis using microscopy. Of the reported AEs, microscopy detected 67-100%, qPCR 79-100% and qRT-PCR detected 96-100% of the study-participants prior to appearance of symptoms. Almost all the symptoms appeared after the initiation of treatment, likely as a known consequence of drug treatment.

CONCLUSIONS

PvCHMI is safe with majority of the infections being detected prior to the appearance of clinical symptoms, which can be further alleviated by using sensitive molecular methods for clinical diagnosis.

Profiling Humoral Immune Response Against Pre-Erythrocytic and Erythrocytic Antigens of Malaria Parasites Among Neotropical Primates in the Brazilian Atlantic Forest

Human malaria due to zoonotic transmission has been recorded in the Atlantic Forest, an extra-Amazonian area in Brazil, which are a challenge for malaria control. Naturally acquired humoral immune response against pre-erythrocytic and erythrocytic antigens of Neotropical primates (NP) was evaluated here to improve the knowledge about the exposure of those animals to the malaria transmission and support the identification of the potential reservoirs of the disease in the Atlantic Forest. Blood samples of 154 monkeys from three areas of the Atlantic Forest were used to identify IgG antibodies against peptides of the repeat region of the major pre-erythrocytic antigen, the circumsporozoite protein (CSP), of Plasmodium vivax (PvCSP), Plasmodium brasilianum/Plasmodium malariae (Pb/PmCSP), and Plasmodium falciparum (PfCSP) by ELISA. Antibodies against erythrocytic recombinant antigens of P. vivax, Apical membrane antigen 1 (PvAMA-1), Erythrocyte binding protein 2 (PvEBP-2) and domain II of Duffy binding protein (PvDBPII) were also evaluated. Parameters, such as age, sex, PCR positivity, and captivity, potentially associated with humoral immune response were analyzed. Eighty-five percent of NP had antibodies against at least one CSP peptide, and 76% against at least one P. vivax erythrocytic antigen. A high percentage of adults compared to non-adults were seropositive and showed increased antibody levels. Neotropical primates with PCR positive for P. simium had a significantly higher frequency of positivity rate for immune response against PvEBP-2, PvDBPII and also higher antibody levels against PvDBPII, compared to PCR negative NPs for this species. Monkeys with PCR positive for P. brasilianum/P. malariae showed higher frequency of seropositivity and antibody levels against Pb/PmCSP. Levels of antibodies against Pb/PmCSP, PvEBP-2 and PvDBPII were higher in free-living than in captive monkeys from the same area. All Platyrrhine families showed antibodies against CSP peptides, however not all showed IgG against erythrocytic antigens. These findings showed a high prevalence of naturally acquired antibodies against CSP repeats in all studied areas, suggesting an intense exposure to infected-mosquitoes bites of NP from all families. However, mainly monkeys of Atelidae family showed antibodies against P. vivax erythrocytic antigens, suggesting blood infection, which might serve as potential reservoirs of malaria in the Atlantic Forest.

Generation of a Genetically Modified Chimeric Plasmodium falciparum Parasite Expressing Plasmodium vivax Circumsporozoite Protein for Malaria Vaccine Development

Chimeric rodent malaria parasites with the endogenous circumsporozoite protein (csp) gene replaced with csp from the human parasites Plasmodium falciparum (Pf) and P. vivax (Pv) are used in preclinical evaluation of CSP vaccines. Chimeric rodent parasites expressing PfCSP have also been assessed as whole sporozoite (WSP) vaccines. Comparable chimeric P. falciparum parasites expressing CSP of P. vivax could be used both for clinical evaluation of vaccines targeting PvCSP in controlled human P. falciparum infections and in WSP vaccines targeting P. vivax and P. falciparum. We generated chimeric P. falciparum parasites expressing both PfCSP and PvCSP. These Pf-PvCSP parasites produced sporozoite comparable to wild type P. falciparum parasites and expressed PfCSP and PvCSP on the sporozoite surface. Pf-PvCSP sporozoites infected human hepatocytes and induced antibodies to the repeats of both PfCSP and PvCSP after immunization of mice. These results support the use of Pf-PvCSP sporozoites in studies optimizing vaccines targeting PvCSP.

Recombinant Plasmodium vivax circumsporozoite surface protein allelic variants: antibody recognition by individuals from three communities in the Brazilian Amazon

Circumsporozoite protein (CSP) variants of P. vivax, besides having variations in the protein repetitive portion, can differ from each other in aspects such as geographical distribution, intensity of transmission, vectorial competence and immune response. Such aspects must be considered to P. vivax vaccine development. Therefore, we evaluated the immunogenicity of novel recombinant proteins corresponding to each of the three P. vivax allelic variants (VK210, VK247 and P. vivax-like) and of the C-terminal region (shared by all PvCSP variants) in naturally malaria-exposed populations of Brazilian Amazon. Our results demonstrated that PvCSP-VK210 was the major target of humoral immune response in studied population, presenting higher frequency and magnitude of IgG response. The IgG subclass profile showed a prevalence of cytophilic antibodies (IgG1 and IgG3), that seem to have an essential role in protective immune response. Differently of PvCSP allelic variants, antibodies elicited against C-terminal region of protein did not correlate with epidemiological parameters, bringing additional evidence that humoral response against this protein region is not essential to protective immunity. Taken together, these findings increase the knowledge on serological response to distinct PvCSP allelic variants and may contribute to the development of a global and effective P. vivax vaccine.

Liver Enzyme Elevations in Plasmodium falciparum Volunteer Infection Studies: Findings and Recommendations

Malaria volunteer infection studies (VISs) accelerate new drug and vaccine development. In the induced blood-stage malaria (IBSM) model, volunteers are inoculated with erythrocytes infected with Plasmodium falciparum. Observations of elevated liver enzymes in the IBSM model with new chemical entities (NCEs) promoted an analysis of available data. Data were reviewed from eight IBSM studies of seven different NCEs, plus two studies with the registered antimalarial piperaquine conducted between June 2013 and January 2017 at QIMR Berghofer, Brisbane, Australia. Alanine aminotransferase (ALT) was elevated (> 2.5 times the upper limit of normal [×ULN]) in 20/114 (17.5%) participants. Of these, 8.9% (10/114) had moderate increases (> 2.5–5 × ULN), noted in seven studies of six different NCEs ± piperaquine or piperaquine alone, and 8.9% (10/114) had severe elevations (> 5 × ULN), occurring in six studies of six different NCEs ± piperaquine. Aspartate aminotransferase (AST) was elevated (> 2.5 × ULN) in 11.4% (13/114) of participants, across six of the 10 studies. Bilirubin was > 2 × ULN in one participant. Published data from other VIS models, using sporozoite inoculation by systemic administration or mosquito feeding, also showed moderate/severe liver enzyme elevations. In conclusion, liver enzyme elevations in IBSM studies are most likely multifactorial and could be caused by the model conditions, that is, malaria infection/parasite density and/or effective parasite clearance, or by participant-specific risk factors, acetaminophen administration, or direct hepatotoxicity of the test drug. We make recommendations that may mitigate the risk of liver enzyme elevations in future VISs and propose measures to assist their interpretation, should they occur.

The Case for Exploiting Cross-Species Epitopes in Malaria Vaccine Design

The infection dynamics between different species of Plasmodium that infect the same human host can both suppress and exacerbate disease. This could arise from inter-parasite interactions, such as competition, from immune regulation, or both. The occurrence of protective, cross-species (heterologous) immunity is an unlikely event, especially considering that strain-transcending immunity within a species is only partial despite lifelong exposure to that species. Here we review the literature in humans and animal models to identify the contexts where heterologous immunity can arise, and which antigens may be involved. From the perspective of vaccine design, understanding the mechanisms by which exposure to an antigen from one species can elicit a protective response to another species offers an alternative strategy to conventional approaches that focus on immunodominant antigens within a single species. The underlying hypothesis is that certain epitopes are conserved across evolution, in sequence or in structure, and shared in antigens from different species. Vaccines that focus on conserved epitopes may overcome the challenges posed by polymorphic immunodominant antigens; but to uncover these epitopes requires approaches that consider the evolutionary history of protein families across species. The key question for vaccinologists will be whether vaccines that express these epitopes can elicit immune responses that are functional and contribute to protection against Plasmodium parasites.

The Consequences of Mixed-Species Malaria Parasite Co-Infections in Mice and Mosquitoes for Disease Severity, Parasite Fitness, and Transmission Success

The distributions of human malaria parasite species overlap in most malarious regions of the world, and co-infections involving two or more malaria parasite species are common. Little is known about the consequences of interactions between species during co-infection for disease severity and parasite transmission success. Anti-malarial interventions can have disproportionate effects on malaria parasite species and may locally differentially reduce the number of species in circulation. Thus, it is important to have a clearer understanding of how the interactions between species affect disease and transmission dynamics. Controlled competition experiments using human malaria parasites are impossible, and thus we assessed the consequences of mixed-species infections on parasite fitness, disease severity, and transmission success using the rodent malaria parasite species Plasmodium chabaudi, Plasmodium yoelii, and Plasmodium vinckei. We compared the fitness of individual species within single species and co-infections in mice. We also assessed the disease severity of single vs. mixed infections in mice by measuring mortality rates, anemia, and weight loss. Finally, we compared the transmission success of parasites in single or mixed species infections by quantifying oocyst development in Anopheles stephensi mosquitoes. We found that co-infections of P. yoelii with either P. vinckei or P. chabaudi led to a dramatic increase in infection virulence, with 100% mortality observed in mixed species infections, compared to no mortality for P. yoelii and P. vinckei single infections, and 40% mortality for P. chabaudi single infections. The increased mortality in the mixed infections was associated with an inability to clear parasitaemia, with the non-P. yoelii parasite species persisting at higher parasite densities than in single infections. P. yoelii growth was suppressed in all mixed infections compared to single infections. Transmissibility of P. vinckei and P. chabaudi to mosquitoes was also reduced in the presence of P. yoelii in co-infections compared to single infections. The increased virulence of co-infections containing P. yoelii (reticulocyte restricted) and P. chabaudi or P. vinckei (predominantly normocyte restricted) may be due to parasite cell tropism and/or immune modulation of the host. We explain the reduction in transmission success of species in co-infections in terms of inter-species gamete incompatibility.

Fast and fierce versus slow and smooth: Heterogeneity in immune responses to Plasmodium in the controlled human malaria infection model

Controlled human malaria infection (CHMI) is an established model in clinical malaria research. Upon exposure to Plasmodium falciparum parasites, malaria-naive volunteers differ in dynamics and composition of their immune profiles and subsequent capacity to generate protective immunity. CHMI volunteers are either inflammatory responders who have prominent cellular IFN-γ production primarily driven by adaptive T cells, or tempered responders who skew toward antibody-mediated humoral immunity. When exposed to consecutive CHMIs under antimalarial chemoprophylaxis, individuals who can control parasitemia after a single immunization (fast responders) are more likely to be protected against a subsequent challenge infection. Fast responders tend to be inflammatory responders who can rapidly induce long-lived IFN-γ+ T cell responses. Slow responders or even non-responders can also be protected, but via a more diverse range of responses that take a longer time to reach full protective efficacy, in part due to their tempered phenotype. The latter group can be identified at baseline before CHMI by higher expression of inhibitory ligands CTLA-4 and TIM-3 on CD4+ T cells. Delineating heterogeneity in human immune responses to P. falciparum will facilitate rational design and strategy towards effective malaria vaccines.

Antibodies to Cryptic Epitopes in Distant Homologues Underpin a Mechanism of Heterologous Immunity between Plasmodium vivax PvDBP and Plasmodium falciparum VAR2CSA

In this work, we describe a molecular mechanism of heterologous immunity between two distant species of Plasmodium. Our results suggest a mechanism that subverts the classic parasite strategy of presenting highly polymorphic epitopes in surface antigens to evade immunity to that parasite. This alternative immune pathway can be exploited to protect pregnant women from falciparum placental malaria by designing vaccines to cryptic epitopes that elicit broadly inhibitory antibodies against variant parasite strains.

Many pathogens evolve extensive genetic variation in virulence proteins as a strategy to evade host immunity. This poses a significant challenge for the host to develop broadly neutralizing antibodies. In Plasmodium falciparum, we show that a mechanism to circumvent this challenge is to elicit antibodies to cryptic epitopes that are not under immune pressure. We previously discovered that antibodies to the Plasmodium vivax invasion protein, PvDBP, cross-react with P. falciparum VAR2CSA, a distantly related virulence factor that mediates placental malaria. Here, we describe the molecular mechanism underlying this cross-species immunity. We identified an epitope in subdomain 1 (SD1) within the Duffy binding-like (DBL) domain of PvDBP that gives rise to cross-reactive antibodies to VAR2CSA and show that human antibodies affinity purified against a synthetic SD1 peptide block parasite adhesion to chondroitin sulfate A (CSA) in vitro. The epitope in SD1 is subdominant and highly conserved in PvDBP, and in turn, SD1 antibodies target cryptic epitopes in P. falciparum VAR2CSA. The epitopes in VAR2CSA recognized by vivax-derived SD1 antibodies (of human and mouse origin) are distinct from those recognized by VAR2CSA immune serum. We mapped two peptides in the DBL5ε domain of VAR2CSA that are recognized by SD1 antibodies. Both peptides map to regions outside the immunodominant sites, and antibodies to these peptides are not elicited following immunization with VAR2CSA or natural infection with P. falciparum in pregnancy, consistent with the cryptic nature of these target epitopes.