Merozoite Antigens of Plasmodium falciparum Elicit Strain-Transcending Opsonizing Immunity

Abstract

h It is unclear whether naturally acquired immunity to Plasmodium falciparum results from the acquisition of antibodies to multiple, diverse antigens or to fewer, highly conserved antigens. Moreover, the specific antibody functions required for malaria immunity are unknown, and hence informative immunological assays are urgently needed to address these knowledge gaps and guide vaccine development. In this study, we investigated whether merozoite-opsonizing antibodies are associated with protection from malaria in a strain-specific or strain-transcending manner by using a novel field isolate and an immune plasmamatched cohort from Papua New Guinea with our validated assay of merozoite phagocytosis. Highly correlated opsonization responses were observed across the 15 parasite strains tested, as were strong associations with protection (composite phagocytosis score across all strains in children uninfected at baseline: hazard ratio of 0.15, 95% confidence interval of 0.04 to 0.63). Opsonizing antibodies had a strong strain-transcending component, and the opsonization of transgenic parasites deficient for MSP3, MSP6, MSPDBL1, or P. falciparum MSP1-19 (PfMSP1-19) was similar to that of wild-type parasites. We have provided the first evidence that merozoite opsonization is predominantly strain transcending, and the highly consistent associations with protection against diverse parasite strains strongly supports the use of merozoite opsonization as a correlate of immunity for field studies and vaccine trials. These results demonstrate that conserved domains within merozoite antigens targeted by opsonization generate strain-transcending immune responses and represent promising vaccine candidates.

Analysis of antibody responses to selected Plasmodium falciparum merozoite surface antigens in mild and cerebral malaria and associations with clinical outcomes

“…As recently reported, the opsonic phagocytosis assay was shown to correlate with protective immunity [42,57] while the magnitude of the anti-merozoite antibody response correlated with the amount of reactive oxygen species production in the ADRB assay [37]. Very importantly, it has been demonstrated that opsonization capacity in immune sera is not restricted to autologous parasite isolates or by the presence of different MSP gene when using transgenic parasites [58]. These observations explain the efficacy of passive IgG transfer in curing malaria independently of the geographical origin of the IgG pools [8,9].…”

Merozoite Surface Protein 1 from Plasmodium falciparum Is a Major Target of Opsonizing Antibodies in Individuals with Acquired Immunity against Malaria

“…Different immune mechanisms mediated by MSP-1-specific opsonizing antibodies have been described, including (i) the recruitment of monocytes by MSP-1 block 2 antibodies, as measured via the ADCI assay (40), (ii) the fixation of complement factors by antibodies directed against MSP-1 19 and MSP-1 block 2 , as detected via antibody-mediated complementdependent inhibition (23), and (iii) the induction of neutrophil respiratory burst activity, as analyzed via the ADRB assay and reported for antibodies to MSP-1 19 (39) and MSP-1 83 (this work). However, MSP-1 19 antibodies did not seem to play a role in monocyte opsonic phagocytosis, since the OPA response for transgenic P. falciparum merozoites with the MSP-1 19 orthologue from P. chabaudi did not differ from that for wild-type parasites (26). Furthermore, we observed a high level of cross-reactivity between different parasite strains in the ADRB assay, suggesting that antibodies to the highly polymorphic block 2 of MSP-1, which is located within MSP-1 83 , likely play no role in neutrophil respiratory burst activity.…”

“…Thus, cross-reactive regions within MSP-1 appear to be the main targets of opsonizing antibodies that elicit neutrophil respiratory burst activity. Interestingly, opsonizing antibodies that recruit monocytes for merozoite phagocytosis, as measured in the OPA, are effective against 15 different parasite strains, indicating high strain transcendence (26). Thus, opsonizing antibodies that are functional in the ADRB assay or the OPA seem to target mainly cross-reactive regions within merozoite antigens, which would be beneficial for vaccine development.…”

“…***, P Յ 0.001. (18,19,(23)(24)(25)(26)(27). Antibodies that opsonize P. falciparum merozoites and mediate the release of reactive oxygen species from neutrophils can be measured in the ADRB assay.…”

“…Four functional assays have been developed to measure opsonizing antibodies in vitro, i.e., (i) the antibody-dependent cellular inhibition (ADCI) assay (17), (ii) the opsonic phagocytosis assay (OPA) (18,24), (iii) the antibody-dependent complement inhibition (Ab-C) assay (23), and (iv) the antibody-dependent respiratory burst (ADRB) assay (19). Importantly, several studies using these assays show a correlation between opsonizing antibodies and protection against malaria (18,19,(23)(24)(25)(26)(27).…”