RTS,S-induced off-target antibody responses in malaria-naïve adults associated with protection against controlled human malaria infection

RTS,S, a WHO-recommended malaria vaccine, targets the circumsporozoite protein (CSP) on the Plasmodium falciparum sporozoite surface. Emerging data suggest off-target (non-CSP) antibody responses are associated with RTS,S-induced protection, but vaccine-derived responses are challenging to differentiate from boosting of naturally acquired immunity in endemic areas. We probed pre-challenge sera from RTS,S-vaccinated malaria-naïve adults who were either protected ( n = 18) or unprotected ( n = 17) against controlled human malaria infection on a custom peptide array with 1747 total variants from 130 non-CSP proteins from across the parasite lifecycle to assess off-target antibody responses with amino acid-level precision. Compared with the unprotected group, the protected group had increased IgG and/or IgA responses to 124 proteins (95%) studied. Using ssearch36, we found that 22 protein regions associated with protection had sequence similarities to CSP epitopes, including regions not found in RTS,S. The finding that off-target antibody responses are induced by RTS,S and are associated with protection in this and other studies suggests that they could be a marker of improved avidity, improved effector functions, or could be mediating improved protection by binding CSP or other proteins. Differentially reactive epitopes without linear sequence homology with CSP could represent conformational epitopes. Further study of these off-target antibody responses could help identify improved strategies for next-generation malaria vaccines.