Cross-species analysis of apical asparagine-rich protein of Plasmodium vivax and Plasmodium knowlesi

A novel micronemal protein MP38 is involved in the invasion of merozoites into erythrocytes

The absence of an in vitro cultivation system for Plasmodium vivax hinders the exploration of molecular targets for vaccine development. To address this, functional studies often rely on alternative models, such as P. knowlesi, due to its genetic similarity to P. vivax. This study investigated the role of a novel micronemal protein, PvMP38 (PVX_110945), in both P. vivax and P. knowlesi merozoite invasion of erythrocytes. The full-length ectodomain of PvMP38 was expressed, and polyclonal antibodies were generated to assess its function. PvMP38 was confirmed to localize on micronemal organelle in both P. vivax and P. knowlesi merozoites. In vitro protein-protein interaction assays revealed that PvMP38 binds to Pv12 with high-affinity interaction. A conserved novel complex of Pv12-Pv41-PvMP38 was identified by immunoprecipitation of P. vivax antibodies on P. knowlesi schizont lysates. Linear epitopes of PvMP38 with high and moderate antigenicity were identified in clinical isolates of both species. Invasion inhibition assays demonstrated that a triple antibody combination targeting the PvMP38, Pv12, and Pv41 significantly reduced P. knowlesi merozoite invasion of erythrocytes compared to a single antibody. In addition, CRISPR/Cas9-mediated knockout of P. knowlesi mp38 markedly impaired parasite growth, underscoring its essential role during the asexual stage. These findings identify PvMP38 and its associated complex as promising targets for malaria interventions and highlight the utility of P. knowlesi as a model for investigating P. vivax erythrocyte invasion mechanisms.IMPORTANCEThis manuscript reported an effort in malaria eradication by identifying and functionally characterizing a novel Plasmodium vivax micronemal protein, PvMP38, involved in erythrocyte invasion. A narrow repertoire of an efficacious vaccine targeting P. vivax candidates is being developed due to the lack of continuous in vitro culture. This study addresses a gap in P. vivax research using P. knowlesi as a model for both genome editing and antibody functionality validation. By enhancing the protein-protein interaction screening framework, this study demonstrated that PvMP38 forms a complex with Pv12 and Pv41, opening the approaches to multi-antigen vaccines. The successful application of CRISPR/Cas9 gene editing techniques to disrupt its homolog, the pkmp38 gene, further assesses the protein's significance in the growth and invasion of the parasite. These findings provided valuable insights into the biology of P. vivax and proposed PvMP38 as a promising candidate for malaria intervention strategies.

In silico comparative analysis of genetic diversity and natural selection of Plasmodium knowlesi and Plasmodium vivax etramp11.2 gene

A comprehensive analysis of genetic diversity in field samples is essential for developing an effective strain-transcending malaria vaccine and antigens capable of producing cross-protective immunity can be crucial in combatting malaria. We explored the genetic diversity, natural selection, epitope prediction, and haplotype network analysis of etramp11.2, a potential antigen recently shown to exhibit cross-species reactivity in clinical samples of Plasmodium knowlesi and Plasmodium vivax. We retrieved gene sequences of etramp11.2 from P. knowlesi (n = 42) and P. vivax (n = 112) from public databases and studied the genetic diversity, natural selection, haplotype diversity, and epitope prediction analyses. Both species exhibited low worldwide nucleotide diversity within the cross-reactive domain. Purifying selection was stronger in P. knowlesi compared to P. vivax. Regional analysis in Pvetramp11.2 revealed higher genetic diversity in Africa, followed by North and South America, compared to low nucleotide diversity observed in Asia and Oceania. Haplotype network analysis showed shared clusters and with no geographical clustering of samples. In silico epitope prediction identified six potential epitopes in P. knowlesi and two in P. vivax, including a fully conserved epitope (AERKKRN) in both species. This study provides a preliminary snapshot of ETRAMP11.2 diversity in field samples and its potential for future studies towards a cross-protective vaccine target against both species. Further studies with higher numbers of sequences and functional antibody analyses are needed to validate these findings.

Extracellular vesicles derived from plasmodium-infected red blood cells alleviate cerebral malaria in plasmodium berghei ANKA-infected C57BL/6J mice

RTS,S is the first malaria vaccine recommended for implementation among young children at risk. However, vaccine efficacy is modest and short-lived. To mitigate the risk of cerebral malaria (CM) among children under the age of 5, it is imperative to develop new vaccines. EVs are potential vaccine candidates as they obtain the ability of brain-targeted delivery and transfer plasmodium antigens and immunomodulators during infections. This study extracted EVs from BALB/c mice infected with Plasmodium yoelii 17XNL (P.y17XNL). C57BL/6J mice were intravenously immunized with EVs (EV-I.V. + CM group) or subcutaneously vaccinated with the combination of EVs and CpG ODN-1826 (EV + CPG ODN-S.C. + CM group) on days 0 and 20, followed by infection with Plasmodium berghei ANKA (P.bANKA) on day 20 post-second immunization. We monitored Parasitemia and survival rate. The integrity of the Blood-brain barrier (BBB) was examined using Evans blue staining.The levels of cytokines and adhesion molecules were evaluated using Luminex, RT-qPCR, and WB. Brain pathology was evaluated by hematoxylin and eosin and immunohistochemical staining. The serum levels of IgG, IgG1, and IgG2a were analyzed by enzyme-linked immunosorbent assay. Compared with those in the P.bANKA-infected group, parasitemia increased slowly, death was delayed (day 10 post-infection), and the survival rate reached 75 %-83.3 % in the EV-I.V. + ECM and EV + CPG ODN-S.C. + ECM groups. Meanwhile, compared with the EV + CPG ODN-S.C. + ECM group, although parasitemia was almost the same, the survival rate increased in the EV-I.V. + ECM group.Additionally, EVs immunization markedly downregulated inflammatory responses in the spleen and brain and ameliorated brain pathological changes, including BBB disruption and infected red blood cell (iRBC) sequestration. Furthermore, the EVs immunization group exhibited enhanced antibody responses (upregulation of IgG1 and IgG2a production) compared to the normal control group. EV immunization exerted protective effects, improving the integrity of the BBB, downregulating inflammation response of brain tissue, result in reduces the incidence of CM. The protective effects were determined by immunological pathways and brain targets elicited by EVs. Intravenous immunization exhibited better performance than subcutaneous immunization, which perhaps correlated with EVs, which can naturally cross BBB to play a better role in brain protection.

Characterization of merozoite-specific thrombospondin-related anonymous protein (MTRAP) in Plasmodium vivax and P. knowlesi parasites

Plasmodium vivax, the most widespread human malaria parasite, and P. knowlesi, an emerging Plasmodium that infects humans, are the phylogenetically closest malarial species that infect humans, which may induce cross-species reactivity across most co-endemic areas in Southeast Asia. The thrombospondin-related anonymous protein (TRAP) family is indispensable for motility and host cell invasion in the growth and development of Plasmodium parasites. The merozoite-specific TRAP (MTRAP), expressed in blood-stage merozoites, is supposed to be essential for human erythrocyte invasion. We aimed to characterize MTRAPs in blood-stage P. vivax and P. knowlesi parasites and ascertain their cross-species immunoreactivity. Recombinant P. vivax and P. knowlesi MTRAPs of full-length ectodomains were expressed in a mammalian expression system. The MTRAP-specific immunoglobulin G, obtained from immune animals, was used in an immunofluorescence assay for subcellular localization and invasion inhibitory activity in blood-stage parasites was determined. The cross-species humoral immune responses were analyzed in the sera of patients with P. vivax or P. knowlesi infections. The MTRAPs of P. vivax (PvMTRAP) and P. knowlesi (PkMTRAP) were localized on the rhoptry body of merozoites in blood-stage parasites. Both anti-PvMTRAP and anti-PkMTRAP antibodies inhibited erythrocyte invasion of blood-stage P. knowlesi parasites. The humoral immune response to PvMTRAP showed high immunogenicity, longevity, and cross-species immunoreactivity with P. knowlesi. MTRAPs are promising candidates for development of vaccines and therapeutics against vivax and knowlesi malaria.

Identification of conserved cross-species B-cell linear epitopes in human malaria: a subtractive proteomics and immuno-informatics approach targeting merozoite stage proteins

Human malaria, caused by five Plasmodium species (P. falciparum, P. vivax, P. malariae, P. ovale, and P. knowlesi), remains a significant global health burden. While most interventions target P. falciparum, the species associated with high mortality rates and severe clinical symptoms, non-falciparum species exhibit different transmission dynamics, remain hugely neglected, and pose a significant challenge to malaria elimination efforts. Recent studies have reported the presence of antigens associated with cross-protective immunity, which can potentially disrupt the transmission of various Plasmodium species. With the sequencing of the Plasmodium genome and the development of immunoinformatic tools, in this study, we sought to exploit the evolutionary history of Plasmodium species to identify conserved cross-species B-cell linear epitopes in merozoite proteins. We retrieved Plasmodium proteomes associated with human malaria and applied a subtractive proteomics approach focusing on merozoite stage proteins. Bepipred 2.0 and Epidope were used to predict B-cell linear epitopes using P. falciparum as the reference species. The predictions were further compared against human and non-falciparum databases and their antigenicity, toxicity, and allergenicity assessed. Subsequently, epitope conservation was carried out using locally sequenced P. falciparum isolates from a malaria-endemic region in western Kenya (n=27) and Kenyan isolates from MalariaGEN version 6 (n=131). Finally, physiochemical characteristics and tertiary structure of the B-cell linear epitopes were determined. The analysis revealed eight epitopes that showed high similarity (70-100%) between falciparum and non-falciparum species. These epitopes were highly conserved when assessed across local isolates and those from the MalariaGEN database and showed desirable physiochemical properties. Our results show the presence of conserved cross-species B-cell linear epitopes that could aid in targeting multiple Plasmodium species. Nevertheless, validating their efficacy in-vitro and in-vivo experimentally is essential.

Genetic Diversity and Population Genetic Structure Analysis of Plasmodium knowlesi Thrombospondin-Related Apical Merozoite Protein (TRAMP) in Clinical Samples

The simian malaria parasite Plasmodium knowlesi causes a high number of zoonotic infections in Malaysia. The thrombospondin-related apical merozoite protein (TRAMP) is an essential ligand for binding to the erythrocyte cell surface, whereby it facilitates the invasion. This study is the first attempt to determine the genetic diversity, phylogeography, natural selection and population structure from 97 full-length PkTRAMP gene sequences originating from Malaysia. We found low levels of nucleotide diversity (π~0.0065) for the full-length gene despite samples originating from geographically separated regions (i.e., Peninsular Malaysia and Malaysian Borneo). The rate of synonymous substitutions was significantly higher than that of non-synonymous substitutions, indicating a purifying selection for the full-length gene within the clinical samples. The population genetic analysis revealed that the parasite population is undergoing a significant population expansion. The analysis of the amino acid sequence alignment of 97 PkTRAMP sequences identified 15 haplotypes, of which a major shared haplotype was noted Hap 1 (n = 68, Sarawak; n = 34, Sabah; n = 12, Peninsular Malaysia; n = 22). The phylogenetic analysis using DNA sequences identified two clusters that separated due to geographical distance and three mixed clusters with samples from both Peninsular Malaysia and Malaysian Borneo. Population structure analyses indicated two distinct sub-populations (K = 2). Our findings point to the potential for independent parasite evolution, which could make zoonotic malaria control and elimination even more challenging.

Plasmodium vivax malaria serological exposure markers: Assessing the degree and implications of cross-reactivity with P. knowlesi

Serological markers are a promising tool for surveillance and targeted interventions for Plasmodium vivax malaria. P. vivax is closely related to the zoonotic parasite P. knowlesi, which also infects humans. P. vivax and P. knowlesi are co-endemic across much of South East Asia, making it important to design serological markers that minimize cross-reactivity in this region. To determine the degree of IgG cross-reactivity against a panel of P. vivax serological markers, we assayed samples from human patients with P. knowlesi malaria. IgG antibody reactivity is high against P. vivax proteins with high sequence identity with their P. knowlesi ortholog. IgG reactivity peaks at 7 days post-P. knowlesi infection and is short-lived, with minimal responses 1 year post-infection. We designed a panel of eight P. vivax proteins with low levels of cross-reactivity with P. knowlesi. This panel can accurately classify recent P. vivax infections while reducing misclassification of recent P. knowlesi infections.

Epidemiology of indigenous Plasmodium knowlesi infection in Sarawak, 2011-2019

OBJECTIVES

To characterize the state-wide epidemiology of indigenous knowlesi malaria in Sarawak from 2011-2019.

METHODS

Longitudinal retrospective study based on Sarawak knowlesi malaria surveillance data recorded 2011-2019. Only indigenous cases were included and information extracted for analysis comprised age, sex, occupation, ethnicity, case severity, hospital admission, and parasite density.

RESULTS

Over the 9 years 8473 indigenous knowlesi malaria cases were recorded. Age group 40 to 49 years, males, plantation workers, and Iban communities recorded the highest percentage of cases in each demographic variable. Most of the cases were uncomplicated (85.9%) and 89.5% of the total cases were reported with ≤20,000 parasites/μL of blood. Age group and ethnic group are associated with severity of knowlesi malaria in Sarawak. Multivariable logistic regression indicated that the age group 60+ years had the highest odds of developing severe knowlesi malaria compared to other age groups (AOR 2.48; 95% CI 1.22, 5.02; p=0.012). Bidayuh patients were more likely to develop severe knowlesi malaria than Ibans, the largest ethnic group among knowlesi malaria patients (AOR 1.97; 95% CI 1.31, 2.97; p=0.001).

CONCLUSIONS

Identification of risk groups is important for the implementation of prevention programs and treatments targeting at specific group to combat knowlesi malaria effectively.

Diagnostic performance of a 5-plex malaria immunoassay in regions co-endemic for Plasmodium falciparum, P. vivax, P. knowlesi, P. malariae and P. ovale

Commercial point-of-care tests remain insufficient for accurately detecting and differentiating low-level malaria infections in regions co-endemic with multiple non-falciparum species, including zoonotic Plasmodium knowlesi (Pk). A 5-plex chemiluminescent assay simultaneously measures pan-Plasmodium lactate dehydrogenase (pLDH), P. falciparum (Pf)-LDH, P. vivax (Pv)-LDH, Pf-histidine-rich protein-2 (HRP2), and C-reactive protein. We assessed its diagnostic performance on whole blood (WB) samples from 102 healthy controls and 306 PCR-confirmed clinical cases of Pf, Pv, Pk, P. malariae (Pm) and P. ovale (Po) mono-infections from Southeast-Asia. We confirm its excellent HRP2-based detection of Pf. Cross-reactivity of Pf-LDH with all non-falciparum species tested was observed (specificity 57.3%). Pv-LDH performance was suboptimal for Pv (93.9% sensitivity and 73.9% specificity). Poor specificity was driven by strong Pk cross-reactivity, with Pv-LDH detecting 93.9% of Pk infections. The pan-LDH-to-Pf-LDH ratio was capable of discerning Pv from Pk, and robustly differentiated Pf from Pm or Po infection, useful in regions with hrp2/3 deletions. We tested the platform's performance in plasma for the first time, with WB outperforming plasma for all analytes except Pv-LDH for Pk. The platform is a promising tool for WB malaria diagnosis, although further development is warranted to improve its utility in regions co-endemic for multiple non-falciparum species.

Identification, Mapping, and Genetic Diversity of Novel Conserved Cross-Species Epitopes of RhopH2 in Plasmodium knowlesi With Plasmodium vivax

Malaria is a major public health concern, and any tangible intervention during the pre-elimination phase can result in a significant reduction in infection rates. Recent studies have reported that antigens producing cross-protective immunity can play an important role as vaccines and halt malaria transmission in different endemic regions. In this study, we studied the genetic diversity, natural selection, and discovered novel conserved epitopes of a high molecular weight rhoptry protein 2 (RhopH2) in clinical samples of Plasmodium knowlesi and Plasmodium vivax cross-protective domains, which has been proven to produce cross-protective immunity in both species. We found low levels of nucleotide diversity (P. knowlesi; π ~ 0.0093, SNPs = 49 and P. vivax π ~ 0.0014, SNPs = 23) in P. knowlesi (n = 40) and P. vivax (n = 65) samples in the PkRhopH2 cross-protective domain. Strong purifying selection was observed for both species (P. knowlesi; dS - dN = 2.41, p < 0.009, P. vivax; dS - dN = 1.58, p < 0.050). In silico epitope prediction in P. knowlesi identified 10 potential epitopes, of which 7 epitopes were 100% conserved within clinical samples. Of these epitopes, an epitope with 10 amino acids (QNSKHFKKEK) was found to be fully conserved within all P. knowlesi and P. vivax clinical samples and 80%–90% conservation within simian malaria ortholog species, i.e., P. coatneyi and P. cynomolgi. Phylogenetic analysis of the PkRhopH2 cross-protective domain showed geographical clustering, and three subpopulations of P. knowlesi were identified of which two subpopulations originated from Sarawak, Malaysian Borneo, and one comprised only the laboratory lines from Peninsular Malaysia. This study suggests that RhopH2 could be an excellent target for cross-protective vaccine development with potential for outwitting strain as well as species-specific immunity. However, more detailed studies on genetic diversity using more clinical samples from both species as well as the functional role of antibodies specific to the novel conserved epitope identified in this study can be explored for protection against infection.

Knowlesi malaria: Human risk factors, clinical spectrum, and pathophysiology

Plasmodium knowlesi is endemic across Southeast Asia, and is the commonest cause of zoonotic malaria. The spectrum of clinical disease from P. knowlesi infection ranges from asymptomatic infection, through to severe malaria and death. Over 90% of clinical disease occurs in adults, mostly living in forest edge areas undergoing intensive land use change. With a 24-h asexual life cycle in humans, high parasite counts are possible, but most clinical cases of knowlesi malaria are uncomplicated with low parasitaemia. In co-endemic areas, median parasitaemia in knowlesi malaria is lower than that seen in vivax and falciparum malaria, suggesting a lower fever threshold. Severe malaria occurs in 6-9% of symptomatic adults. Manifestations of severe malaria from P. knowlesi are similar to those seen with falciparum malaria, with the notable absence of coma. Age, parasitaemia, cardiovascular comorbidities and delayed diagnosis are risk factors for severe disease and death, which are only seen in adults. Thrombocytopenia is near-universal in adults, likely related to platelet-red cell binding and clearance. Mechanisms underlying the microvascular sludging seen in fatal disease in non-natural primate hosts and the microvascular accumulation of parasites in fatal human disease are not clear. Marked reductions in deformability of both infected and uninfected red blood cells are associated with disease severity in both humans and other non-natural primate hosts, likely contributing to impaired microvascular perfusion and organ dysfunction. Endothelial activation, endothelial dysfunction, glycocalyx degradation and haemolysis are also associated with, and likely contribute to, severe disease and organ dysfunction, particularly acute kidney injury.

Zoonotic Malaria: Non-Laverania Plasmodium Biology and Invasion Mechanisms

Malaria, which is caused by Plasmodium parasites through Anopheles mosquito transmission, remains one of the most life-threatening diseases affecting hundreds of millions of people worldwide every year. Plasmodium vivax, which accounts for the majority of cases of recurring malaria caused by the Plasmodium (non-Laverania) subgenus, is an ancient and continuing zoonosis originating from monkey hosts probably outside Africa. The emergence of other zoonotic malarias (P. knowlesi, P. cynomolgi, and P. simium) further highlights the seriousness of the disease. The severity of this epidemic disease is dependent on many factors, including the parasite characteristics, host-parasite interactions, and the pathology of the infection. Successful infection depends on the ability of the parasite to invade the host; however, little is known about the parasite invasion biology and mechanisms. The lack of this information adds to the challenges to malaria control and elimination, hence enhancing the potential for continuation of this zoonosis. Here, we review the literature describing the characteristics, distribution, and genome details of the parasites, as well as host specificity, host-parasite interactions, and parasite pathology. This information will provide the basis of a greater understanding of the epidemiology and pathogenesis of malaria to support future development of strategies for the control and prevention of this zoonotic infection.

Using Plasmodium knowlesi as a model for screening Plasmodium vivax blood-stage malaria vaccine targets reveals new candidates

Plasmodium vivax is responsible for the majority of malaria cases outside Africa. Unlike P. falciparum, the P. vivax life-cycle includes a dormant liver stage, the hypnozoite, which can cause infection in the absence of mosquito transmission. An effective vaccine against P. vivax blood stages would limit symptoms and pathology from such recurrent infections, and therefore could play a critical role in the control of this species. Vaccine development in P. vivax, however, lags considerably behind P. falciparum, which has many identified targets with several having transitioned to Phase II testing. By contrast only one P. vivax blood-stage vaccine candidate based on the Duffy Binding Protein (PvDBP), has reached Phase Ia, in large part because the lack of a continuous in vitro culture system for P. vivax limits systematic screening of new candidates. We used the close phylogenetic relationship between P. vivax and P. knowlesi, for which an in vitro culture system in human erythrocytes exists, to test the scalability of systematic reverse vaccinology to identify and prioritise P. vivax blood-stage targets. A panel of P. vivax proteins predicted to function in erythrocyte invasion were expressed as full-length recombinant ectodomains in a mammalian expression system. Eight of these antigens were used to generate polyclonal antibodies, which were screened for their ability to recognize orthologous proteins in P. knowlesi. These antibodies were then tested for inhibition of growth and invasion of both wild type P. knowlesi and chimeric P. knowlesi lines modified using CRISPR/Cas9 to exchange P. knowlesi genes with their P. vivax orthologues. Candidates that induced antibodies that inhibited invasion to a similar level as PvDBP were identified, confirming the utility of P. knowlesi as a model for P. vivax vaccine development and prioritizing antigens for further follow up.

A Chimeric Plasmodium vivax Merozoite Surface Protein Antibody Recognizes and Blocks Erythrocytic P. cynomolgi Berok Merozoites In Vitro

Research on erythrocytic Plasmodium vivax merozoite antigens is critical for identifying potential vaccine candidates in reducing P. vivax disease. However, many P. vivax studies are constrained by its inability to undergo long-term culture in vitro Conserved across all Plasmodium spp., merozoite surface proteins are essential for invasion into erythrocytes and highly expressed on erythrocytic merozoites, thus making it an ideal vaccine candidate. In clinical trials, the P. vivax merozoite surface protein 1 (PvMSP1-19) vaccine candidate alone has shown to have limited immunogenicity in patients; hence, we incorporate the highly conserved and immunogenic C terminus of both P. vivax merozoite surface protein 8 (PvMSP8) and PvMSP1-19 to develop a multicomponent chimeric protein rPvMSP8+1 for immunization of mice. The resulted chimeric rPvMSP8+1 antibody was shown to recognize native protein MSP8 and MSP1-19 of mature P. vivax schizonts. In the immunized mice, an elevated antibody response was observed in the rPvMSP8+1-immunized group compared to that immunized with single-antigen components. In addition, we examined the growth inhibition of these antibodies against Plasmodium cynomolgi (Berok strain) parasites, which is phylogenetically close to P. vivax and sustains long-term culture in vitro Similarly, the chimeric anti-rPvMSP8+1 antibodies recognize P. cynomolgi MSP8 and MSP1-19 on mature schizonts and showed strong inhibition in vitro via growth inhibition assay. This study provides support for a new multiantigen-based paradigm rPvMSP8+1 to explore potential chimeric vaccine candidates against P. vivax malaria using sister species P. cynomolgi.

Plasmodium knowlesi Malaria in Sabah, Malaysia, 2015-2017: Ongoing Increase in Incidence Despite Near-elimination of the Human-only Plasmodium Species

BACKGROUND

Malaysia aims to eliminate malaria by 2020. However, while cases of Plasmodium falciparum and Plasmodium vivax have decreased substantially, the incidence of zoonotic malaria from Plasmodium knowlesi continues to increase, presenting a major challenge to regional malaria control efforts. Here we report incidence of all Plasmodium species in Sabah, including zoonotic P. knowlesi, during 2015-2017.

METHODS

Microscopy-based malaria notification data and polymerase chain reaction (PCR) results were obtained from the Sabah Department of Health and State Public Health Laboratory, respectively, from January 2015 to December 2017. From January 2016 this was complemented by a statewide prospective hospital surveillance study. Databases were matched, and species was determined by PCR, or microscopy if PCR was not available.

RESULTS

A total of 3867 malaria cases were recorded between 2015 and 2017, with PCR performed in 93%. Using PCR results, and microscopy if PCR was unavailable, P. knowlesi accounted for 817 (80%), 677 (88%), and 2030 (98%) malaria cases in 2015, 2016, and 2017, respectively. P. falciparum accounted for 110 (11%), 45 (6%), and 23 (1%) cases and P. vivax accounted for 61 (6%), 17 (2%), and 8 (0.4%) cases, respectively. Of those with P. knowlesi, the median age was 35 (interquartile range: 24-47) years, and 85% were male.

CONCLUSIONS

Malaysia is approaching elimination of the human-only Plasmodium species. However, the ongoing increase in P. knowlesi incidence presents a major challenge to malaria control and warrants increased focus on knowlesi-specific prevention activities. Wider molecular surveillance in surrounding countries is required.

Plasmodium knowlesi infecting humans in Southeast Asia: What's next?

Plasmodium knowlesi, a simian malaria parasite, has been in the limelight since a large focus of human P. knowlesi infection was reported from Sarawak (Malaysian Borneo) in 2004. Although this infection is transmitted across Southeast Asia, the largest number of cases has been reported from Malaysia. The increasing number of knowlesi malaria cases has been attributed to the use of molecular tools for detection, but environmental changes including deforestation likely play a major role by increasing human exposure to vector mosquitoes, which coexist with the macaque host. In addition, with the reduction in human malaria transmission in Southeast Asia, it is possible that human populations are at a greater risk of P. knowlesi infection due to diminishing cross-species immunity. Furthermore, the possibility of increasing exposure of humans to other simian Plasmodium parasites such as Plasmodium cynomolgi and Plasmodium inui should not be ignored. We here review the current status of these parasites in humans, macaques, and mosquitoes to support necessary reorientation of malaria control and elimination in the affected areas.

Spatial and Temporal Analysis of Plasmodium knowlesi Infection in Peninsular Malaysia, 2011 to 2018

The life-threatening zoonotic malaria cases caused by Plasmodium knowlesi in Malaysia has recently been reported to be the highest among all malaria cases; however, previous studies have mainly focused on the transmission of P. knowlesi in Malaysian Borneo (East Malaysia). This study aimed to describe the transmission patterns of P. knowlesi infection in Peninsular Malaysia (West Malaysia). The spatial distribution of P. knowlesi was mapped across Peninsular Malaysia using Geographic Information System techniques. Local indicators of spatial associations were used to evaluate spatial patterns of P. knowlesi incidence. Seasonal autoregressive integrated moving average models were utilized to analyze the monthly incidence of knowlesi malaria in the hotspot region from 2012 to 2017 and to forecast subsequent incidence in 2018. Spatial analysis revealed that hotspots were clustered in the central-northern region of Peninsular Malaysia. Time series analysis revealed the strong seasonality of transmission from January to March. This study provides fundamental information on the spatial distribution and temporal dynamic of P. knowlesi in Peninsular Malaysia from 2011 to 2018. Current control policy should consider different strategies to prevent the transmission of both human and zoonotic malaria, particularly in the hotspot region, to ensure a successful elimination of malaria in the future.

Cross-species reactivity of antibodies against Plasmodium vivax blood-stage antigens to Plasmodium knowlesi

Malaria is caused by multiple different species of protozoan parasites, and interventions in the pre-elimination phase can lead to drastic changes in the proportion of each species causing malaria. In endemic areas, cross-reactivity may play an important role in the protection and blocking transmission. Thus, successful control of one species could lead to an increase in other parasite species. A few studies have reported cross-reactivity producing cross-immunity, but the extent of cross-reactive, particularly between closely related species, is poorly understood. P. vivax and P. knowlesi are particularly closely related species causing malaria infections in SE Asia, and whilst P. vivax cases are in decline, zoonotic P. knowlesi infections are rising in some areas. In this study, the cross-species reactivity and growth inhibition activity of P. vivax blood-stage antigen-specific antibodies against P. knowlesi parasites were investigated. Bioinformatics analysis, immunofluorescence assay, western blotting, protein microarray, and growth inhibition assay were performed to investigate the cross-reactivity. P. vivax blood-stage antigen-specific antibodies recognized the molecules located on the surface or released from apical organelles of P. knowlesi merozoites. Recombinant P. vivax and P. knowlesi proteins were also recognized by P. knowlesi- and P. vivax-infected patient antibodies, respectively. Immunoglobulin G against P. vivax antigens from both immune animals and human malaria patients inhibited the erythrocyte invasion by P. knowlesi. This study demonstrates that there is extensive cross-reactivity between antibodies against P. vivax to P. knowlesi in the blood stage, and these antibodies can potently inhibit in vitro invasion, highlighting the potential cross-protective immunity in endemic areas.

In recent years, malaria initiatives have increasingly shifted focus from achieving malaria control to achieving malaria elimination. However, the interventions used are leading to drastic changes in the proportions of different Plasmodium species causing clinical infection, particularly within Southeast Asia. Little is known about how these different parasite species interact/compete in nature or whether exposure to one species could cause some level of protection against another. We examined cross-reactive antibody responses to key parasite proteins with roles in red blood cell invasion and identified novel cross-species reactivity among the closest of malaria affecting the human population (P. vivax and P. knowlesi). This comprehensive analysis provides evidence that cross-reactive immunity could play an important role in areas where species distributions are perturbed by malaria control measures, and future efforts to identify the specific cross-reactive epitopes involved would be invaluable both to our understanding of malaria immunity and vaccine development.

Updates on malaria incidence and profile in Malaysia from 2013 to 2017

Background

To date, most of the recent publications on malaria in Malaysia were conducted in Sabah, East Malaysia focusing on the emergence of Plasmodium knowlesi. This analysis aims to describe the incidence, mortality and case fatality rate of malaria caused by all Plasmodium species between Peninsular Malaysia and East Malaysia (Sabah and Sarawak) over a 5-year period (2013–2017).

Methods

This is a secondary data review of all diagnosed and reported malaria confirmed cases notified to the Ministry of Health, Malaysia between January 2013 and December 2017.

Results

From 2013 to 2017, a total of 16,500 malaria cases were notified in Malaysia. The cases were mainly contributed from Sabah (7150; 43.3%) and Sarawak (5684; 34.4%). Majority of the patients were male (13,552; 82.1%). The most common age group in Peninsular Malaysia was 20 to 29 years (1286; 35.1%), while Sabah and Sarawak reported highest number of malaria cases in age group of 30 to 39 years (2776; 21.6%). The top two races with malaria in Sabah and Sarawak were Bumiputera Sabah (5613; 43.7%) and Bumiputera Sarawak (4512; 35.1%), whereas other ethnic group (1232; 33.6%) and Malays (1025; 28.0%) were the two most common races in Peninsular Malaysia. Plasmodium knowlesi was the commonest species in Sabah and Sarawak (9902; 77.1%), while there were more Plasmodium vivax cases (1548; 42.2%) in Peninsular Malaysia. The overall average incidence rate, mortality rate and case fatality rates for malaria from 2013 to 2017 in Malaysia were 0.106/1000, 0.030/100,000 and 0.27%, respectively. Sarawak reported the highest average incidence rate of 0.420/1000 population followed by Sabah (0.383/1000). Other states in Peninsular Malaysia reported below the national average incidence rate with less than 0.100/1000.

Conclusions

There were different trends and characteristics of notified malaria cases in Peninsular Malaysia and Sabah and Sarawak. They provide useful information to modify current prevention and control measures so that they are customised to the peculiarities of disease patterns in the two regions in order to successfully achieve the pre-elimination of human-only species in the near future.

Inhibition of parasite invasion by monoclonal antibody against epidermal growth factor-like domain of Plasmodium vivax merozoite surface protein 1 paralog

The Plasmodium vivax merozoite surface protein 1 paralog (PvMSP1P), which has epidermal growth factor (EGF)-like domains, was identified as a novel erythrocyte adhesive molecule. This EGF-like domain (PvMSP1P-19) elicited high level of acquired immune response in patients. Antibodies against PvMSP1P significantly reduced erythrocyte adhesion activity to its unknown receptor. To determine PvMSP1P-19-specific antibody function and B-cell epitopes in vivax patients, five monoclonal antibodies (mAbs) and 18-mer peptides were generated. The mAb functions were determined by erythrocyte-binding inhibition assay and invasion inhibition assay with P. knowlesi. B-cell epitopes of PvMSP1P-19 domains were evaluated by peptide microarray. The pvmsp1p-19 sequences showed limited polymorphism in P. vivax worldwide isolates. The 1BH9-A10 showed erythrocyte binding inhibitory by interaction with the N-terminus of PvMSP1P-19, while this mAb failed to recognize PkMSP1P-19 suggesting the species-specific for P. vivax. Other mAbs showed cross-reactivity with PkMSP1P-19. Among them, the 2AF4-A2 and 2AF4-A6 mAb significantly reduced parasite invasion through C-terminal recognition. The linear B-cell epitope in naturally exposed P. vivax patient was identified at three linear epitopes. In this study, PvMSP1P-19 N-terminal-specific 1BH9-A10 and C-terminal-specific 2AF4 mAbs showed functional activity for epitope recognition suggesting that PvMSP1P may be useful for vaccine development strategy for specific single epitope to prevent P. vivax invasion.

The Plasmodium knowlesi Pk41 surface protein diversity, natural selection, sub population and geographical clustering: a 6-cysteine protein family member

Introduction

The zoonotic malaria parasite Plasmodium knowlesi has currently become the most dominant form of infection in humans in Malaysia and is an emerging infectious disease in most Southeast Asian countries. The P41 is a merozoite surface protein belonging to the 6-cysteine family and is a well-characterized vaccine candidate in P. vivax and P. falciparum; however, no study has been done in the orthologous gene of P. knowlesi. This study investigates the level of polymorphism, haplotypes and natural selection of pk41 genes in clinical isolates from Malaysia.

Method

Thirty-five full-length pk41 sequences from clinical isolates of Malaysia along with four laboratory lines (along with H-strain) were downloaded from public databases. For comparative analysis between species, orthologous P41 genes from P. falciparum, P. vivax, P. coatneyi and P. cynomolgi were also downloaded. Genetic diversity, polymorphism, haplotype and natural selection were determined using DnaSP 5.10 software. Phylogenetic relationships between Pk41 genes were determined using MEGA 5.0 software.

Results

Analysis of 39 full-length pk41 sequences along with the H-strain identified 36 SNPs (20 non-synonymous and 16 synonymous substitutions) resulting in 31 haplotypes. Nucleotide diversity across the full-length gene was low and was similar to its ortholog in P. vivax; pv41. Domain-wise amino acid analysis of the two s48/45 domains indicated low level of polymorphisms for both the domains, and the glutamic acid rich region had extensive size variations. In the central domain, upstream to the glutamate rich region, a unique two to six (K-E)_n repeat region was identified within the clinical isolates. Overall, the pk41 genes were indicative of negative/purifying selection due to functional constraints. Domain-wise analysis of the s48/45 domains also indicated purifying selection. However, analysis of Tajima’s D across the genes identified non-synonymous SNPs in the s48/45 domain II with high positive values indicating possible epitope binding regions. All the 6-cysteine residues within the s48/45 domains were conserved within the clinical isolates indicating functional conservation of these regions. Phylogenetic analysis of full-length pk41 genes indicated geographical clustering and identified three subpopulations of P. knowlesi; one originating in the laboratory lines and two originating from Sarawak, Malaysian Borneo.

Conclusion

This is the first study to report on the polymorphism and natural selection of pk41 genes from clinical isolates of Malaysia. The results reveal that there is low level of polymorphism in both s48/45 domains, indicating that this antigen could be a potential vaccine target. However, genetic and molecular immunology studies involving higher number of samples from various parts of Malaysia would be necessary to validate this antigen’s candidacy as a vaccine target for P. knowlesi.

Within-population genetic diversity and population structure of Plasmodium knowlesi merozoite surface protein 1 gene from geographically distinct regions of Malaysia and Thailand

BACKGROUND

The C-terminal 42 kDa domain of Plasmodium knowlesi merozoite surface protein 1 (PkMSP1) is a potential asexual blood-stage vaccine candidate, however, only a limited number of clinical isolates have been analysed from Malaysia and no inter-country comparative diversity study has been conducted. In the present study, nucleotide diversity, haplotypes and natural selection levels of pkmsp1 in clinical samples from geographically distinct regions of Malaysia and Thailand were investigated. The overall population structure of the parasite from the region was determined.

METHODS

Eleven full-length pkmsp1 sequences obtained from clinical isolates of Malaysia along with the H-strain were downloaded from the database for domain wise characterization of pkmsp1 gene. Additionally, 76 pkmsp-1₄₂ sequences from Thailand and Malaysia were downloaded from the database for intra and inter-population analysis. DnaSP 5.10 and MEGA 5.0 software were used to determine genetic diversity, polymorphism, haplotypes and natural selection. Genealogical relationships were determined using haplotype network tree in NETWORK software v5.0. Population genetic differentiation index (F_ST) of parasites were analysed using Arlequin v3.5.

RESULTS

Sequence analysis of 11 full-length pkmsp1 sequences along with the H-strain identified 477 (8.4%) polymorphic sites, of which 107 were singleton sites. The overall diversity observed in the full-length genes were high in comparison to its ortholog pvmsp1 and the 4 variable domains showed extensive size variations. The nucleotide diversity was low towards the pkmsp1-42 compared to the conserved domains. The 19 kDa domain was less diverse and completely conserved among isolates from Malaysian Borneo. The nucleotide diversity of isolates from Peninsular Malaysia and Thailand were higher than Malaysian Borneo. Network analysis of pkmsp1-42 haplotypes showed geographical clustering of the isolates from Malaysian Borneo and grouping of isolates from Peninsular Malaysia and Thailand. Population differentiation analysis indicated high F_ST values between parasite populations originating from Malaysian Borneo, Peninsular Malaysia and Thailand attributing to geographical distance. Moderate genetic differentiation was observed for parasite populations from Thailand and Peninsular Malaysia. Evidence of population expansion and purifying selection were observed in all conserved domains with strongest selection within the pkmsp1-42 domain.

CONCLUSIONS

This study is the first to report on inter country genetic diversity and population structure of P. knowlesi based on msp1. Strong evidence of negative selection was observed in the 42 kDa domain, indicating functional constrains. Geographical clustering of P. knowlesi and moderate to high genetic differentiation values between populations identified in this study highlights the importance of further evaluation using larger number of clinical samples from Southeast Asian countries.

Molecular and cellular interactions defining the tropism of Plasmodium vivax for reticulocytes

Plasmodium vivax is uniquely restricted to invading reticulocytes, the youngest of red blood cells. Parasite invasion relies on the sequential deployment of multiple parasite invasion ligands. Correct targeting of the host reticulocyte is mediated by two families of invasion ligands: the reticulocyte binding proteins (RBPs) and erythrocyte binding proteins (EBPs). The Duffy receptor has long been established as a key determinant for P. vivax invasion. However, recently, the RBP protein PvRBP2b has been shown to bind to transferrin receptor, which is expressed on reticulocytes but lost on normocytes, implicating the ligand-receptor in the reticulocyte tropism of P. vivax. Furthermore there is increasing evidence for P. vivax growth and sexual development in reticulocyte-enriched tissues such as the bone marrow.

In vitro invasion inhibition assay using antibodies against Plasmodium knowlesi Duffy binding protein alpha and apical membrane antigen protein 1 in human erythrocyte-adapted P. knowlesi A1-H.1 strain

Background

The rapid process of malaria erythrocyte invasion involves ligand–receptor interactions. Inducing antibodies against specific ligands or receptors that abrogate the invasion process is a key challenge for blood stage vaccine development. However, few candidates were reported and remain to be validated for the discovery of new vaccine candidates in Plasmodium knowlesi.

Methods

In order to investigate the efficacy of pre-clinical vaccine candidates in P. knowlesi-infected human cases, this study describes an in vitro invasion inhibition assay, using a P. knowlesi strain adapted to in vitro growth in human erythrocytes, PkA1-H.1. Recombinant proteins of P. knowlesi Duffy binding protein alpha (PkDBPα) and apical membrane antigen 1 (PkAMA1) were produced in Escherichia coli system and rabbit antibodies were generated from immune animals.

Results

PkDBPα and PkAMA1 recombinant proteins were expressed as insoluble and produced as a functional refolded form for this study. Antibodies against PkDBPα and PkAMA1 specifically recognized recombinant proteins and native parasite proteins in schizont-stage parasites on the merozoite organelles. Single and combination of anti-PkDBPα and anti-PkAMA1 antibodies elicited strong growth inhibitory effects on the parasite in concentration-dependent manner. Meanwhile, IgG prevalence of PkDBPα and PkAMA1 were observed in 13.0 and 46.7% in human clinical patients, respectively.

Conclusion

These data provide support for the validation of in vitro growth inhibition assay using antibodies of DBPα and AMA1 in human-adapted P. knowlesi parasite PkA1-H.1 strain.

Electronic supplementary material

The online version of this article (10.1186/s12936-018-2420-4) contains supplementary material, which is available to authorized users.