Background: Malaria infection commences when sporozoites are inoculated during a mosquito bite. Sporozoites migrate into the bloodstream and travel to infect hepatocytes, subsequently leading to blood stage infection and clinical disease. Therefore, targeting sporozoites is an attractive strategy for malaria vaccines. The most advanced malaria vaccine, based on the major surface protein of sporozoites, circumsporozoite protein (CSP), only showed low to moderate efficacy in clinical trials, well below the World Health Organization target of ≥75% protective efficacy. Further studies to understand protective immune mechanisms against sporozoites are crucial for developing more effective vaccines.
Method: We first developed opsonic phagocytosis assays to study opsonic phagocytosis by neutrophils and monocytes using P. falciparum sporozoites and fluorescent micro-particles coated with CSP. We also adapted novel assays to further investigate how antibodies interact with specific Fc-receptors to promote opsonic phagocytosis.
Results: Both neutrophils and monocytes from peripheral blood were able to phagocytose sporozoites in an antibody-concentration dependent manner, with neutrophils playing a greater role in phagocytosis. Antibodies to CSP and the central repeat region of CSP effectively promoted opsonic phagocytosis. In a Kenyan cohort of children and adults, antibodies that promoted opsonic phagocytosis were acquired through repeated exposure and were only present at significant levels among adults. We further investigated how antibodies interact with Fc-receptors to promote opsonic phagocytosis. Fcγ-receptor IIa and FcγRIII were crucial for neutrophil opsonic phagocytosis, whereas FcγRI was the main receptor mediating opsonic phagocytosis using the THP1 monocyte model. IgG3 responses appeared to be key determinants of opsonic phagocytosis..
Conclusions: Our data highlight the importance of opsonic antibodies in immunity against sporozoites, and point to important role of neutrophils in clearance of sporozoites from the circulation. Data from this study will facilitate the development of more effective vaccines and help determine mechanisms of immunity of existing sporozoite vaccines.