Complement-fixing antibodies targeting merozoite invasion of RBCs have recently been shown to be important contributors of protective immunity against P. falciparum malaria. However, it is unknown whether complement fixing antibodies are also induced during P. vivax infection, and whether protein structure or host age impact the induction of functional antibodies. We quantified the induction of complement-fixing antibodies targeting Merozoite Surface Proteins 3α (PvMSP3α) in children and adults with P. vivax malaria.
Plasma samples from 40 individuals (20 children and 20 adults) with P. vivax malaria were collected from a cohort study in Sabah, Malaysia during acute infection and 7 and 28 days following treatment. C1q fixing antibodies and different antibody subclasses targeting different regions of PvMSP3α regions (C-terminal, Block region, N-terminal) were measured by ELISA.
Overall seroprevalence of C1q fixing antibodies was highest against PvMSP3α Block region (77.6%), followed by C-terminal (49.0%) and N-terminal (18.4%). Adults had higher levels of complement fixing antibodies than children. IgG1, IgG3, and IgM were all significantly correlated with C1q fixing antibodies. Between age groups, IgM appeared to be more important in mediating complement fixation in children, compared to IgG3 in adults. Functional antibodies against PvMSP3α increased following acute infection 7 days after treatment, however rapidly waned to baseline at Day 28.
Complement-fixing antibodies are induced during acute P. vivax infection. PvMSP3α is a target of complement fixing antibodies and its immunogenicity was dependent on specific protein region. There were significant differences between induction of IgG1, IgG3, IgM and functional antibodies between children and adults. Complement fixing antibodies are induced during P. vivax infection, and further studies are warranted to understand the role of functional antibodies in protective immunity.