Characterization of the primary antibody response to Plasmodium falciparum antigens in infants living in a malaria-endemic area

BACKGROUND

The primary antibody (Ab) response to Plasmodium falciparum is a critical step in developing immunity to malaria. Information on the initial Ab responses of babies in malaria-endemic areas is incomplete, in part, because babies receive maternal IgG via transplacental-transfer and usually become infected before maternal IgG wanes. The study aimed to identify the primary IgM and IgG Ab responses to malarial antigens in Cameroonian babies.

METHODS

Infants (n = 70) living in a high malaria transmission area were followed from birth throughout the first year of life (mean 341 ± 42 days, an average of 8.5 time points per infant). Malaria infection was assessed by microscopy and PCR, and IgM and IgG antibodies (Abs) were measured using a multiplex immunoassay to AMA1, EBA-175, MSP1-42, MSP2, MSP3, RESA, LSA1, and CSP.

RESULTS

The half-life of maternal IgG varied among the antigens, ranging from 0.7 to 2.5 months. The first infection of 41% of the babies was sub-microscopic and only 11 to 44% of the babies produced IgM to the above antigens; however, when the first infection was detected by microscopy, 59-82% of the infants made IgM Abs to the antigens. Infants were able to produce IgM even when maternal IgG was present, suggesting maternal Abs did not suppress the baby's initial Ab response. Using longitudinal regression models that incorporated time-varying covariates, infants were found to produce IgG Ab to only AMA-1 when the first infection was sub-microscopic, but they produced IgG Abs to MSP1-42 (3D7, FVO), AMA1 (3D7, FVO) MSP2-FC27, MSP3, RESA, and LSA1, but not MSP 2-3D7, EBA-175, and CSP during their first slide-positive infection. Notably, the primary and secondary IgG responses were short-lived with little evidence of boosting.

CONCLUSIONS

The primary Ab response of babies who had maternal IgG was similar to that reported for primary infections in malaria-naïve adults.

Can growth inhibition assays (GIA) predict blood-stage malaria vaccine efficacy?

An effective vaccine against P. falciparum malaria remains a global health priority. Blood-stage vaccines are an important component of this effort, with some indications of recent progress. However only a fraction of potential blood-stage antigens have been tested, highlighting a critical need for efficient down-selection strategies. Functional in vitro assays such as the growth/invasion inhibition assays (GIA) are widely used, but it is unclear whether GIA activity correlates with protection or predicts vaccine efficacy. While preliminary data in controlled human malaria infection (CHMI) studies indicate a possible association between in vitro and in vivo parasite growth rates, there have been conflicting results of immunoepidemiology studies, where associations with exposure rather than protection have been observed. In addition, GIA-interfering antibodies in vaccinated individuals from endemic regions may limit assay sensitivity in heavily malaria-exposed populations. More work is needed to establish the utility of GIA for blood-stage vaccine development.

Chimeric parasites as tools to study Plasmodium immunology and assess malaria vaccines

The study of pathogen immunity relies upon being able to track antigen specific immune responses and assess their protective capacity. To study immunity to Plasmodium antigens, chimeric rodent or human malaria parasites that express proteins from other Plasmodium species or unrelated species have been developed. Different types of chimeric parasites have been used to address a range of specific questions. Parasites expressing model T cell epitopes have been used to monitor cellular immune responses to the preerythrocytic and blood stages of malaria. Other parasites have been used to assess the functional significance of immune responses targeting particular proteins. Finally, a number of rodent malaria parasites that express vaccine-candidate antigens from P. falciparum and P. vivax have been used in functional assays of vaccine-induced antibody responses. Here, I review the experimental contributions that have been made using these parasites, and discuss the potential of these approaches to continue advancing our understanding of malaria immunology and vaccine research.

Quantifying the importance of MSP1-19 as a target of growth-inhibitory and protective antibodies against Plasmodium falciparum in humans

Background

Antibodies targeting blood stage antigens are important in protection against malaria, but the key targets and mechanisms of immunity are not well understood. Merozoite surface protein 1 (MSP1) is an abundant and essential protein. The C-terminal 19 kDa region (MSP1-19) is regarded as a promising vaccine candidate and may also be an important target of immunity.

Methodology/Findings

Growth inhibitory antibodies against asexual-stage parasites and IgG to recombinant MSP1-19 were measured in plasma samples from a longitudinal cohort of 206 children in Papua New Guinea. Differential inhibition by samples of mutant P. falciparum lines that expressed either the P. falciparum or P. chabaudi form of MSP1-19 were used to quantify MSP1-19 specific growth-inhibitory antibodies. The great majority of children had detectable IgG to MSP1-19, and high levels of IgG were significantly associated with a reduced risk of symptomatic P. falciparum malaria during the 6-month follow-up period. However, there was little evidence of PfMSP1-19 specific growth inhibition by plasma samples from children. Similar results were found when testing non-dialysed or dialysed plasma, or purified antibodies, or when measuring growth inhibition in flow cytometry or microscopy-based assays. Rabbit antisera generated by immunization with recombinant MSP1-19 demonstrated strong MSP1-19 specific growth-inhibitory activity, which appeared to be due to much higher antibody levels than human samples; antibody avidity was similar between rabbit antisera and human plasma.

Conclusions/Significance

These data suggest that MSP1-19 is not a major target of growth inhibitory antibodies and that the protective effects of antibodies to MSP1-19 are not due to growth inhibitory activity, but may instead be mediated by other mechanisms. Alternatively, antibodies to MSP1-19 may act as a marker of protective immunity.

Interferon-γ--central mediator of protective immune responses against the pre-erythrocytic and blood stage of malaria

Immune responses against Plasmodium parasites, the causative organisms of malaria, are traditionally dichotomized into pre-erythrocytic and blood-stage components. Whereas the central role of cellular responses in pre-erythrocytic immunity is well established, protection against blood-stage parasites has generally been ascribed to humoral responses. A number of recent studies, however, have highlighted the existence of cellular immunity against blood-stage parasites, in particular, the prominence of IFN-γ production. Here, we have undertaken to chart the contribution of this prototypical cellular cytokine to immunity against pre-erythrocytic and blood-stage parasites. We summarize the various antiparasitic effector functions that IFN-γ serves to induce, review an array of data about its protective effects, and scrutinize evidence for any deleterious, immunopathological outcome in malaria patients. We discuss the activation and contribution of different cellular sources of IFN-γ production during malaria infection and its regulation in relation to exposure. We conclude that IFN-γ forms a central mediator of protective immune responses against pre-erythrocytic and blood-stage malaria parasites and identify a number of implications for rational malaria vaccine development.

Inhibitory antibodies specific for the 19-kilodalton fragment of merozoite surface protein 1 do not correlate with delayed appearance of infection with Plasmodium falciparum in semi-immune individuals in Vietnam

Inhibitory antibodies specific for the 19-kDa fragment of merozoite surface protein 1 (MSP1(19)) are a significant component of inhibitory responses in individuals immune to malaria. Nevertheless, conflicting results have been obtained in determining whether this antibody specificity correlates with protection in residents of areas where malaria is endemic. In this study, we examined sera collected from a population of semi-immune individuals living in an area of Vietnam with meso-endemicity during a 6-month period. We used two Plasmodium falciparum parasite lines that express either endogenous MSP1(19) or the homologous region from Plasmodium yoelii to measure the MSP1(19)-specific inhibitory activity. We showed that (i) the level of MSP1(19)-specific inhibitory antibodies was not associated with a delay in P. falciparum infection, (ii) MSP1(19)-specific inhibitory antibodies declined significantly during the convalescent period after infection, and (iii) there was no significant correlation between the MSP1(19)-specific inhibitory antibodies and the total antibodies measured by enzyme-linked immunosorbent assay. These results have implications for understanding naturally acquired immunity to malaria and for the development and evaluation of MSP1(19)-based vaccines.

Influence of HLA-DRB1 alleles on antibody responses to PfCP-2.9-immunized and naturally infected individuals

INTRODUCTION

The Plasmodium falciparum chimeric protein, PfCP-2.9, which consists of apical membrane antigen (AMA)-1(III) and merozoite surface protein (MSP)1-19, is a promising asexual-stage malaria vaccine currently being evaluated in clinical trials. This study attempts to investigate the potential association between human leukocyte antigen (HLA)-DRB1 genotype and antibody response against PfCP-2.9 in healthy population and malaria patients.

MATERIALS AND METHODS

We investigated the HLA-DRB1 alleles in 40 participants from phase I trial and 86 malaria patients from southern China by polymerase chain reaction with allele sequence-specific primers. The antibody and cellular response against PfCP-2.9 or its components were measured by enzyme-linked immunosorbent assay and T lymphocyte proliferation assay.

RESULTS

In clinical subjects, the anti-PfCP-2.9 antibody response was likely suppressed by HLA-DR6 alleles, which was consistent with the T lymphocyte proliferation assay. Nevertheless, HLA-DR7 positively correlated with antibody responses in naturally infected individuals while DR8 correlated with weaker antibody responses for all the three recombinant proteins. Moreover, parasitemia was significantly lower in samples with higher antibody levels against PfCP-2.9 or rMSP1-19, but not for rAMA-1(III).

CONCLUSION

These data suggest that antibody responses against PfCP-2.9, AMA-1(III), or MSP1-19 elicited by vaccine formulation or natural infection are controlled by different HLA-II alleles. Moreover, the antibody response to MSP1-19 contributed more to protection immunity than AMA-1(III).