Nepal, Y.; Francian, A.; Flores-Garcia, Y.; Roberts, B. T.; David, S. A.; Zavala, F.; Chackerian, B.

Vaccines that target the pre-erythrocytic stage of the malaria lifecycle have the potential to provide sterilizing immunity but must elicit sustained, high-titer antibody responses to completely prevent infection. Most pre-erythrocytic vaccines target circumsporozoite protein (CSP), the major surface antigen on Plasmodium falciparum sporozoites. Antibodies targeting distinct epitopes within the central repeat region of CSP have the potential to provide protection from infection, but we have focused on developing vaccines that target a highly vulnerable CSP epitope that is targeted by the potent monoclonal antibody L9. In a previous study, we produced a pre-erythrocytic vaccine displaying a synthetic peptide representing the L9 epitope on Q-beta bacteriophage virus-like particles (VLPs). This vaccine elicited strong anti-CSP antibody responses that protected mice from malaria challenge. Here, we asked whether the structural context of the L9 epitope influences the quality of antibody responses. We compared the immunogenicity and protective efficacy of Q-beta L9 VLPs to recombinant VLPs that display the L9 peptide in a structure that is hypothesized to mimic its native conformation. Recombinant MS2 bacteriophage VLPs displaying various lengths of the L9 epitope were produced and immunogenicity and protective efficacy were evaluated in mice. Our results demonstrate that MS2 L9 VLPs, particularly those displaying longer L9 peptides and in combination with a potent novel adjuvant, elicit strong and durable antibody responses that lower malaria liver burden and prevent infection. We also compared the efficacy of L9-targeted vaccines to the licensed vaccine, RTS,S/AS01E (Mosquirix, GSK). Immunization with Q-beta L9 VLPs, MS2 L9 VLPs, and RTS,S/AS01E provided significant protection from liver-stage infection in a mouse model; immunization with Q-beta L9 VLPs elicited sterilizing immunity in the highest percentage of mice. A combination vaccine consisting of MS2 L9 and Q-beta L9 VLPs, each presenting the L9 epitope in distinct structural forms, provided the strongest protection, reducing liver parasite burden and promoting sterilizing immunity more effectively than the licensed RTS,S/AS01E vaccine.