Immunogenicity of the Plasmodium vivax merozoite surface protein 1 paralog in the induction of naturally acquired antibody and memory B cell responses

The Plasmodium vivax MSP1P-19 is involved in binding of reticulocytes through interactions with the membrane proteins band3 and CD71

The parasite Plasmodium vivax preferentially invades human reticulocytes. Its merozoite surface protein 1 paralog (PvMSP1P), particularly the 19-kDa C-terminal region (PvMSP1P-19), has been shown to bind to reticulocytes, and this binding can be inhibited by antisera obtained by PvMSP1P-19 immunization. The molecular mechanism of interactions between PvMSP1P-19 and reticulocytes during P. vivax invasion, however, remains unclear. In this study, we analyzed the ability of MSP1P-19 to bind to different concentrations of reticulocytes and confirmed its reticulocyte preference. LC-MS analysis was used to identify two potential reticulocyte receptors, band3 and CD71, that interact with MSP1P-19. Both PvMSP1P-19 and its sister taxon Plasmodium cynomolgi MSP1P-19 were found to bind to the extracellular loop (loop 5) of band3, where the interaction of MSP1P-19 with band3 was chymotrypsin sensitive. Antibodies against band3-P5, CD71, and MSP1P-19 reduced the binding activity of PvMSP1P-19 and Plasmodium cynomolgi MSP1P-19 to reticulocytes, while MSP1P-19 proteins inhibited Plasmodium falciparum invasion in vitro in a concentration-dependent manner. To sum up, identification and characterization of the reticulocyte receptor is important for understanding the binding of reticulocytes by MSP1P-19.

Elicitation of T-cell-derived IFN-γ-dependent immunity by highly conserved Plasmodium ovale curtisi Duffy binding protein domain region II (PocDBP-RII)

BACKGROUND

Infections with Plasmodium ovale are widely distributed but rarely investigated, and the resulting burden of disease has been underestimated. Plasmodium ovale curtisi Duffy binding protein domain region II (PocDBP-RII) is an essential ligand for reticulocyte recognition and host cell invasion by P. ovale curtisi. However, the genomic variation, antigenicity and immunogenicity of PocDBP-RII remain major knowledge gaps.

METHODS

A total of 93 P. ovale curtisi samples were collected from migrant workers who returned to China from 17 countries in Africa between 2012 and 2016. The genetic polymorphism, natural selection and copy number variation (CNV) were investigated by sequencing and real-time PCR. The antigenicity and immunogenicity of the recombinant PocDBP-RII (rPocDBP-RII) protein were further examined, and the humoral and cellular responses of immunized mice were assessed using protein microarrays and flow cytometry.

RESULTS

Efficiently expressed and purified rPocDBP-RII (39 kDa) was successfully used as an antigen for immunization in mice. The haplotype diversity (Hd) of PocDBP-RII gene was 0.105, and the nucleotide diversity index (π) was 0.00011. No increased copy number was found among the collected isolates of P. ovale curtisi. Furthermore, rPocDBP-RII induced persistent antigen-specific antibody production with a serum IgG antibody titer of 1: 16,000. IFN-γ-producing T cells, rather than IL-10-producing cells, were activated in response to the stimulation of rPocDBP-RII. Compared to PBS-immunized mice (negative control), there was a higher percentage of CD4+CD44highCD62L- T cells (effector memory T cells) and CD8+CD44highCD62L+ T cells (central memory T cells) in rPocDBP-RII‑immunized mice.

CONCLUSIONS

PocDBP-RII sequences were highly conserved in clinical isolates of P. ovale curtisi. rPocDBP-RII protein could mediate protective blood-stage immunity through IFN-γ-producing CD4+ and CD8+ T cells and memory T cells, in addition to inducing specific antibodies. Our results suggested that rPocDBP-RII protein has potential as a vaccine candidate to provide assessment and guidance for malaria control and elimination activities.

Plasmodium vivax vaccine: What is the best way to go?

Malaria is one of the most devastating human infectious diseases caused by Plasmodium spp. parasites. A search for an effective and safe vaccine is the main challenge for its eradication. Plasmodium vivax is the second most prevalent Plasmodium species and the most geographically distributed parasite and has been neglected for decades. This has a massive gap in knowledge and consequently in the development of vaccines. The most significant difficulties in obtaining a vaccine against P. vivax are the high genetic diversity and the extremely complex life cycle. Due to its complexity, studies have evaluated P. vivax antigens from different stages as potential targets for an effective vaccine. Therefore, the main vaccine candidates are grouped into preerythrocytic stage vaccines, blood-stage vaccines, and transmission-blocking vaccines. This review aims to support future investigations by presenting the main findings of vivax malaria vaccines to date. There are only a few P. vivax vaccines in clinical trials, and thus far, the best protective efficacy was a vaccine formulated with synthetic peptide from a circumsporozoite protein and Montanide ISA-51 as an adjuvant with 54.5% efficacy in a phase IIa study. In addition, the majority of P. vivax antigen candidates are polymorphic, induce strain-specific and heterogeneous immunity and provide only partial protection. Nevertheless, immunization with recombinant proteins and multiantigen vaccines have shown promising results and have emerged as excellent strategies. However, more studies are necessary to assess the ideal vaccine combination and test it in clinical trials. Developing a safe and effective vaccine against vivax malaria is essential for controlling and eliminating the disease. Therefore, it is necessary to determine what is already known to propose and identify new candidates.

Naturally acquired antibody kinetics against Plasmodium vivax antigens in people from a low malaria transmission region in western Thailand

BACKGROUND

Plasmodium vivax (P. vivax) is the dominant Plasmodium spp. causing the disease malaria in low-transmission regions outside of Africa. These regions often feature high proportions of asymptomatic patients with sub-microscopic parasitaemia and relapses. Naturally acquired antibody responses are induced after Plasmodium infection, providing partial protection against high parasitaemia and clinical episodes. However, previous work has failed to address the presence and maintenance of such antibody responses to P. vivax particularly in low-transmission regions.

METHODS

We followed 34 patients in western Thailand after symptomatic P. vivax infections to monitor antibody kinetics over 9 months, during which no recurrent infections occurred. We assessed total IgG, IgG subclass and IgM levels to up to 52 P. vivax proteins every 2-4 weeks using a multiplexed Luminex® assay and identified protein-specific variation in antibody longevity. Mathematical modelling was used to generate the estimated half-life of antibodies, long-, and short-lived antibody-secreting cells.

RESULTS

Generally, an increase in antibody level was observed within 1-week post symptomatic infection, followed by an exponential decay of different rates. We observed mostly IgG1 dominance and IgG3 sub-dominance in this population. IgM responses followed similar kinetic patterns to IgG, with some proteins unexpectedly inducing long-lived IgM responses. We also monitored antibody responses against 27 IgG-immunogenic antigens in 30 asymptomatic individuals from a similar region. Our results demonstrate that most antigens induced robust and long-lived total IgG responses following asymptomatic infections in the absence of (detected) boosting infections.

CONCLUSIONS

Our work provides new insights into the development and maintenance of naturally acquired immunity to P. vivax and will guide the potential use of serology to indicate immune status and/or identify populations at risk.

The presence of circulating antibody secreting cells and long-lived memory B cell responses to reticulocyte binding protein 1a in Plasmodium vivax patients

BACKGROUND

Development of an effective vaccine against blood-stage malaria requires the induction of long-term immune responses. Plasmodium vivax Reticulocyte Binding Protein 1a (PvRBP1a) is a blood-stage parasite antigen which is associated with invasion of red blood cells and induces antibody responses. Thus, PvRBP1a is considered as a target for design of a blood-stage vaccine against vivax malaria.

METHODS

Both cross-sectional and cohort studies were used to explore the development and persistence of long-lived antibody and memory B cell responses to PvRBP1a in individuals who lived in an area of low malaria endemicity. Antibody titers and frequency of memory B cells specific to PvRBP1a were measured during infection and following recovery for up to 12 months.

RESULTS

IgG antibody responses against PvRBP1a were prevalent during acute vivax malaria, predominantly IgG1 subclass responses. High responders to PvRBP1a had persistent antibody responses for at least 12-month post-infection. Further analysis of high responder found a direct relation between antibody titers and frequency of activated and atypical memory B cells. Furthermore, circulating antibody secreting cells and memory B cells specific to PvRBP1a were generated during infection. The PvRBP1a-specific memory B cells were maintained for up to 3-year post-infection, indicating the ability of PvRBP1a to induce long-term humoral immunity.

CONCLUSION

The study revealed an ability of PvRBP1a protein to induce the generation and maintenance of antibody and memory B cell responses. Therefore, PvRBP1a could be considered as a vaccine candidate against the blood-stage of P. vivax.

Acquired antibody responses against merozoite surface protein-119 antigen during Plasmodium falciparum and P.vivax infections in South Indian city of Mangaluru

Merozoite surface protein-1 (MSP-1) of malaria parasites has been extensively studied as a malaria vaccine candidate and the antibody response to this protein is an important indicator of protective immunity to malaria. Mangaluru city and its surrounding areas in southwestern India are endemic to malaria with Plasmodium vivax being the most widespread and prevalent species although P. falciparum also frequently infects. However, no information is available on the level of protective immunity in this population. In this regard, a prospective hospital-based study was performed in malarial patients to assess antibody responses against the 19-kDa C-terminal portion of P. vivax and P. falciparum MSP-1 (MSP-119). Serum samples from 51 healthy endemic controls and 267 infected individuals were collected and anti-MSP-119 antibody levels were analyzed by ELISA. The possible association between the antibody responses and morbidity parameters such as malarial anemia and thrombocytopenia was investigated. Among the 267 infected cases, 144 had P. vivax and 123 had P. falciparum infections. Significant levels of anti-MSP-119 antibody were observed both in P. vivax (123/144; 85.4%) and P. falciparum (108/123; 87.9%) infected individuals. In both type of infections, the major antibody isotypes were IgG1 and IgG3. The IgG levels were found to be increased in patients with severe anemia and thrombocytopenia. The antibody levels were also higher in infected individuals who had several previous infections, although antibodies produced during previous infections were short lived. The predominance of cytophilic anti-MSP-119 IgG1 and IgG3 antibodies suggests the possibility of a dual role of Pv MSP-119 and Pf MSP-119 during malarial immunity and pathogenesis.

Main B-cell epitopes of PvAMA-1 and PvMSP-9 are targeted by naturally acquired antibodies and epitope-specific memory cells in acute and convalescent phases of vivax malaria

Although antibodies are considered critical for malaria protection, little is known about the mechanisms/factors that maintain humoral immunity, especially regarding the induction and maintenance of memory B cells over time. In Brazilian endemic areas, this is the first time that the profile of antibody responses and the occurrence of antigen-specific memory B cells (MBC) against P vivax were investigated during acute malaria and up to six months after parasite clearance. For this, we selected two peptides, PvAMA-1_(S290-K307) and PvMSP-9_(E795-A808) , which represent the apical membrane antigen-1 and merozoite surface protein-9 of P vivax, respectively. Both peptides were previously described as containing linear B-cell epitopes. Our findings were as follows: 1-both peptides were recognized by IgG antibodies at a high frequency (between 24% and 81%) in all study groups; 2-in the absence of infection, the IgG levels remained stable throughout 6 months of follow-up; and 3-PvAMA-1_(S290-K307) and PvMSP-9_(E795-A808) -specific MBCs were detected in all individual groups in the absence of reinfection throughout the follow-up period, suggesting long-lived MBC. However, no positive association was observed between malaria-specific antibody levels and frequency of MBCs over time. Taken together, these results suggest that peptides can be, in the future, an alternative strategy to polypeptidic vaccine formulation.

The persistence of naturally acquired antibodies and memory B cells specific to rhoptry proteins of Plasmodium vivax in patients from areas of low malaria transmission

Background

Rhoptries are the large, paired, secretory organelles located at the apical tip of the malaria merozoite that are considered important for parasite invasion processes. Plasmodium vivax rhoptry proteins have been shown to induce humoral immunity during natural infections. Therefore, these proteins may be potential novel vaccine candidates. However, there is a lack of data on the duration of antibody and memory B cell (MBC) responses. Here, the longitudinal analysis of antibody and MBC responses to the P. vivax rhoptry proteins PvRALP1-Ecto and PvRhopH2 were monitored and analysed in individuals to determine their persistence.

Methods

Thirty-nine samples from P. vivax-infected subjects (age 18–60 years) were recruited to explore the frequency and persistence of antibody and MBC responses against rhoptry proteins (PvRALP1-Ecto and PvRhopH2) using both cross-sectional and longitudinal cohort study designs. Antibody levels were determined by ELISA during clinical malaria, and at 3, 9 and 12 months post-infection. The frequency of MBC sub-sets and presence of rhoptry-specific MBCs in subjects 18 months after treatment were detected by flow cytometry and ELISPOT assay.

Results

The seroprevalence of antibodies against PvRALP1-Ecto and PvRhopH2 proteins was found to be high during acute infection, with IgG1, IgG2 and IgG3 sub-classes predominant. However, these anti-rhoptry responses were short-lived and significantly decreased at 9 months post-infection. To relate the durability of these antibody responses to MBC persistence at post-infection, 18-month post-infection peripheral blood mononuclear cells (PBMCs) samples were taken to detect rhoptry-specific MBCs and frequency of MBC sub-sets, and correlate with antibody responses. These late post-infection samples revealed that rhoptry-specific MBCs were present in about 70% of total subjects. However, the persistence of specific MBCs was not correlated with antibody responses as the majority of malaria subjects who were positive for PvRALP1-Ecto- or PvRhopH2-specific MBCs were seronegative for the rhoptry antigens. The frequencies of classical MBCs were increased after infection, whereas those of activated and atypical MBCs were decreased, indicating that MBC responses could switch from activated or atypical MBCs to classical MBCs after parasite clearance, and were maintained in blood circulating at post-infection.

Conclusion

The study showed that rhoptry antigens induced the development and persistence of MBC responses in P. vivax-infected subjects who lived in a region of low malaria transmission, which were not related to the longevity of antibody responses.

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.

Low Levels of Polymorphisms and Negative Selection in Plasmodum knowlesi Merozoite Surface Protein 8 in Malaysian Isolates

Human infections due to the monkey malaria parasite Plasmodium knowlesi is increasingly being reported from most Southeast Asian countries specifically Malaysia. The parasite causes severe and fatal malaria thus there is a need for urgent measures for its control. In this study, the level of polymorphisms, haplotypes and natural selection of full-length pkmsp8 in 37 clinical samples from Malaysian Borneo along with 6 lab-adapted strains were investigated. Low levels of polymorphism were observed across the full-length gene, the double epidermal growth factor (EGF) domains were mostly conserved, and non-synonymous substitutions were absent. Evidence of strong negative selection pressure in the non-EGF regions were found indicating functional constrains acting at different domains. Phylogenetic haplotype network analysis identified shared haplotypes and indicated geographical clustering of samples originating from Peninsular Malaysia and Malaysian Borneo. This is the first study to genetically characterize the full-length msp8 gene from clinical isolates of P. knowlesi from Malaysia; however, further functional characterization would be useful for future rational vaccine design.

The acquisition of long-lived memory B cell responses to merozoite surface protein-8 in individuals with Plasmodium vivax infection

BACKGROUND

The ability of a malaria antigen to induce effective, long-lasting immune responses is important for the development of a protective malaria vaccine. Plasmodium vivax merozoite surface protein-8 (PvMSP8) has been shown to be immunogenic in natural P. vivax infections and produces both cell-mediated and antibody-mediated immunity. Thus, PvMSP8 has been proposed as a vaccine candidate following fusion with other merozoite antigens in blood stage vaccine design. Here, the long-term responses of antibodies and memory B cells (MBCs) specific to PvMSP8 in individuals were monitored in a longitudinal cohort study.

METHODS

Both cross-sectional surveys and cohort studies were utilized to explore the persistence of antibody and MBC responses to PvMSP8. Antibody titers were detected in individuals with acute disease and those who recovered from an infection for 4 years. The dominant peptide epitope of PvMSP8 recognized by naturally acquired antibodies was examined to observe the durability of the post-infection antibody response. PvMSP8-specific MBCs were also in subjects 4 years post-infection using an enzyme-linked immunospot assay.

RESULTS

The prevalence of antibodies to PvMSP8 was high during and after infection. The antibody levels in individual responders were monitored for up to 12 months post-infection and showed that most patients maintained their seropositive response. Interestingly, the anti-PvMSP8 antibody responses stably persisted in some patients who had recovered from an infection for 4 years. Positive PvMSP8-specific MBCs were also detected at 4 years post-infection. However, analysis in these individuals showed no correlation with the presence or titer of circulating antibody.

CONCLUSION

PvMSP8 had the ability to induce a long-term humoral immune response. The antibodies and MBCs specific for this antigen developed and persisted in subjects who acquired a natural P. vivax infection. Inclusion of the PvMSP8 antigen in blood stage vaccine design should be considered.

Development of B Cell Memory in Malaria

A single exposure to many viral and bacterial pathogens typically induces life-long immunity, however, the development of the protective immunity to Plasmodium parasites is strikingly less efficient and achieves only partial protection, with adults residing in endemic areas often experiencing asymptomatic infections. Although naturally acquired immunity to malaria requires both cell-mediated and humoral immune responses, antibodies govern the control of malarial disease caused by the blood-stage form of the parasites. A large body of epidemiological evidence described that antibodies to Plasmodium antigens are inefficiently generated and rapidly lost without continued parasite exposure, suggesting that malaria is accompanied by defects in the development of immunological B cell memory. This topic has been of focus of recent studies of malaria infection in humans and mice. This review examines the main findings to date on the processes that modulate the acquisition of memory B cell responses to malaria, and highlights the importance of closing outstanding gaps of knowledge in the field for the rational design of next generation therapeutics against malaria.

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.

Genetic diversity and allele frequencies of Plasmodium falciparum msp1 and msp2 in parasite isolates from Bioko Island, Equatorial Guinea

BACKGROUND

Malaria is still a serious public health problem on Bioko Island (Equatorial Guinea), although the number of annual cases has been greatly reduced since 2004 through the Bioko Island Malaria Control Project (BIMCP). A better understanding of malaria parasite population diversity and transmission dynamics is critical for assessing the effectiveness of malaria control measures. The objective of this study is to investigate the genetic diversity of Plasmodium falciparum populations and multiplicity of infection (MOI) on Bioko Island 7 years after BIMCP.

METHODS

A total of 181 patients with uncomplicated P. falciparum malaria diagnosed with microscopy were collected from Bioko Island from January 2011 to December 2014. Parasite DNA was extracted using chelex-100 and species were identified using a real-time PCR followed by high-resolution melting. Plasmodium falciparum msp1 and msp2 allelic families were determined using nested PCR.

RESULTS

Three msp1 alleles (K1, MAD20, and RO33) and two msp2 alleles (FC27 and 3D7) were analysed in all samples. In msp1, the MAD20 allelic family was predominant with 96.69% (175/178) followed respectively by the K1 allelic family with 96.07% (171/178) and R033 allelic family with 70.78% (126/178). In msp2, the FC27 allelic family was the most frequently detected with 97.69% (169/173) compared to 3D7 with 72.25% (125/173). Twenty-six different alleles were observed in msp1 with 9 alleles for K1, 9 alleles for MAD20 and 8 alleles for R033. In msp2, 25 individual alleles were detected with 5 alleles for FC27 and 20 alleles for 3D7. The overall MOI was 5.51 with respectively 3.5 and 2.01 for msp1 and msp2. A significant increase in overall MOI was correlated with the age group of the patients (P = 0.026) or parasite densities (P = 0.04).

CONCLUSIONS

The present data showed high genetic diversity and MOI values among the P. falciparum population in the study, reflecting both the high endemic level and malaria transmission on Bioko Island. These data provide valuable information for surveillance of P. falciparum infection and for assessing the appropriateness of the current malarial control strategies in the endemic area.

Plasmodium vivax Merozoite Surface Protein 1 Paralog as a Mediator of Parasite Adherence to Reticulocytes

Plasmodium vivax parasites preferentially invade reticulocytes in human beings. P. vivax merozoite surface protein 1 (PvMSP1) and PvMSP1 paralog (PvMSP1P) may have important functions in reticulocyte adherence during invasion. These proteins share similar structures, including the presence of two epidermal growth factor (EGF)-like and glycosylphosphatidylinositol (GPI)-anchored domains at the C terminus. However, there have been no reports concerning the functional activity of PvMSP1P in reticulocyte adherence during P. vivax invasion. In this study, the ability of PvMSP1P-19 to bind to reticulocytes and normocytes was analyzed. The reticulocyte binding activity of PvMSP1P-19 was 4.0-fold higher than its normocyte binding activity. The binding of PvMSP1P-19 to reticulocytes and normocytes was inhibited in a dose-dependent manner by antibodies from immunized rabbits and by antibodies from vivax parasite-infected patients. Consistently, antibodies against PvMSP1P inhibited parasite invasion during short-term in vitro cultivation. Similar to the case for PvDBPII binding activity, PvMSP1P-19 binding activity was reduced in chymotrypsin-treated reticulocytes. However, no significant difference between the binding of PvMSP1P-19 to Duffy-positive and Duffy-negative erythrocytes was found. The minimal binding motif of PvMSP1P-19 was characterized using synthetic peptides. The results showed that the residues at amino acid positions 1791 to 1808 may have an important function in mediating merozoite adherence to reticulocytes. The positively charged residues within the EGF-like domain were shown to constitute a key binding motif. This work presents strong evidence supporting the role of PvMSP1P in host target cell selection and invasion of Duffy-independent pathway in P. vivax Moreover, PvMSP1P-19-specific antibodies may confer protection against P. vivax reinvasion.