In Papua New Guinea, 1500+ women die every year from childbirth-related causes – 80 times higher than in Australia. And these deaths are, mostly, preventable.
Antibodies against P. falciparum merozoites fix complement to inhibit blood-stage replication in naturally-acquired and vaccine-induced immunity; however, specific targets of these functional antibodies and their importance in protective immunity are unknown. Among malaria-exposed individuals, we show that complement-fixing antibodies to merozoites are more strongly correlated with protective immunity than antibodies that inhibit growth quantified using the current reference assay for merozoite vaccine evaluation. We identify merozoite targets of complement-fixing antibodies and identify antigen-specific complement-fixing antibodies that are strongly associated with protection from malaria in a longitudinal study of children. Using statistical modelling, combining three different antigens targeted by complement-fixing antibodies could increase the potential protective effect to over 95%, and we identify antigens that were common in the most protective combinations. Our findings support antibody-complement interactions against merozoite antigens as important anti-malaria immune mechanisms, and identify specific merozoite antigens for further evaluation as vaccine candidates.
RBC and human serum used for parasite culture and naive controls were provided by the Australian Red Cross Blood bank (Melbourne). We thank all participants and parents/guardians involved in the clinical studies, and the staff of the Papua New Guinea Institute for Medical Research. We thank Janine Stubbs for providing the LALA-mutant MAb. Funding was provided by the National Health and Medical Research Council of Australia (Program grant  and senior research fellowship  to J.G.B., senior research fellowship to I.M., Early Career Fellowship to M.J.B.), the International Centers for Excellence in Malaria Research (AsiaPacific [5U19AI129392]) of the National Institutes of Health (I.M., G.B., M.T.W.), and JSPS KAKENHI(JP26253026) in Japan (T.T.). L.R., M.J.B., L.K., G.F., D.H.O., I.M. and J.G.B. are members of the NHMRC Australian Centre for Research Excellence in Malaria Elimination. The Burnet Institute acknowledges support of the NHMRC Independent Research Institutes Support Scheme and Operational Infrastructure Support of the Victorian State Government.