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

Abstract

Background: Individuals living in malaria-endemic regions develop immunity against severe malaria, but it is unclear whether immunity against pre-erythrocytic stages that blocks initiation of blood-stage infection after parasite inoculation develops following continuous natural exposure. Methods: We cleared schoolchildren living in an area (health district of Saponé, Burkina Faso) with highly endemic seasonal malaria of possible sub-patent infections and examined them weekly for incident infections by nested PCR. Plasma samples collected at enrolment were used to quantify antibodies to the pre-eryhrocytic-stage antigens circumsporozoite protein (CSP) and Liver stage antigen 1 (LSA-1). In vitro sporozoite gliding inhibition and hepatocyte invasion inhibition by naturally acquired antibodies were assessed using Plasmodium falciparum NF54 sporozoites. Associations between antibody responses, functional pre-erythrocytic immunity phenotypes and time to infection detected by 18S quantitative PCR were studied. Results: A total of 51 children were monitored. Anti-CSP antibody titres showed a 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 inhibition, 19.4% [IQR 15.2-40.9%]), and there were positive correlations between invasion inhibition and gliding inhibition (P=0.005, Spearman’s ρ=0.67) and between invasion inhibition and CSP-specific antibodies (P=0.002, Spearman’s ρ=0.76). Survival analysis indicated longer time to infection in individuals displaying higher-than-median sporozoite gliding inhibition activity (P=0.01), although this association became non-significant after adjustment for blood-stage immunity (P = 0.06). 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. Our approach could not fully separate the effects of pre-erythrocytic-specific and blood-stage-specific antibody-mediated immune responses in vivo; epidemiological studies powered and designed to address this important question should become a research priority.

Plasmodium falciparum–specific IgM B cells dominate in children, expand with malaria, and produce functional IgM

“…Several studies have reported Pf -specific IgM responses in malaria-exposed populations and, in some cases, have shown that these responses associate with protection from clinical malaria ( Boudin et al, 1993 ; Dodoo et al, 2008 ; Richards et al, 2010 ; Adu et al, 2016 ; Oyong et al, 2019 ; Barry et al, 2019 ; Boyle et al, 2019 ). For example, the malaria-resistant Fulani in West Africa have a higher breadth and magnitude of Pf -specific IgM compared with the more susceptible Dogon, whereas Pf -specific IgG did not distinguish between the two groups ( Bolad et al, 2005 ; Arama et al, 2015 ).…”

Higher gametocyte production and mosquito infectivity in chronic compared to incident Plasmodium falciparum infections

“…The rapid infection rates in our study argue against the existence of sterile immune protection 31 , 32 . However, the high proportion of individuals with chronic infections who remained asymptomatic indicates that there was substantial clinical immunity in this age-group 33 .…”