Functional antibodies against Plasmodium falciparum sporozoites are associated with a longer time to qPCR-detected infection among schoolchildren in Burkina Faso

Background: Individuals living in malaria-endemic regions develop naturally acquired immunity against severe malarial disease, but it is unclear whether immunity that affects the establishment of infections develops following continuous natural exposure. Methods: We cleared schoolchildren in Burkina Faso of possible sub-patent infections and examined them weekly for incident infections by PCR. Plasma samples collected at enrolment were used to quantify antibodies to the pre-eryhrocytic-stage antigens circumsporozoite protein (CSP) and liver stage antigen. Sporozoite gliding inhibition by naturally acquired antibodies was assessed using Plasmodium falciparum NF54 sporozoites; hepatocyte invasion was assessed using the human HC-04 hepatoma cell line and NF54 sporozoites. The associations between these functional pre-erythrocytic immunity phenotypes and time to PCR-detected infection were studied. Results: A total of 51 children were monitored; the median time to first detection of infection by PCR or development of clinical symptoms was 28 days. Anti-CSP antibody titres showed a strong positive association with sporozoite gliding motility inhibition (P<0.0001, Spearman’s ρ=0.76). In vitro hepatocyte invasion was inhibited by naturally acquired antibodies (median invasion inhibition, 19.4% [IQR 15.2-40.9%]), and there was a positive correlation between gliding and invasion inhibition (P=0.02, Spearman’s ρ=0.60). Survival analysis indicated longer time to infection in individuals displaying higher-than-median sporozoite gliding inhibition activity (P=0.01). Conclusions: In summary, functional antibodies against the pre-erythrocytic stages of malaria infection are acquired in children who are repeatedly exposed to Plasmodium parasites. This immune response does not prevent them from becoming infected during a malaria transmission season, but might delay the appearance of blood stage parasitaemia and consequently needs to be considered in the evaluation of malaria vaccines.