Disulfide bonds in merozoite surface protein 1 of the malaria parasite impede efficient antigen processing and affect the in vivo antibody response

The 19 kDa C-terminal fragment of the malaria parasite merozoite surface protein 1 (MSP1₁₉) is a leading malaria vaccine candidate. In rodents, high antibody levels to this protein confer protective immunity, and can be generated by immunization with the antigen in adjuvants. In natural human infections, however, MSP1₁₉-specific antibody responses can be short-lived and comparatively low, despite repeated exposure to infection. The tightly folded structure of MSP1₁₉ is stabilized by five or six disulfide bonds. These bonds impede antigen processing and, thereby, may affect the generation of CD4⁺ T cells providing help for B cells. Asparagine endopeptidase could digest unfolded, but not native MSP1₁₉ in vitro. Immunization with unfolded MSP1₁₉ resulted in a faster antibody response, and a combination of unfolded and native MSP1₁₉ increased antibody responses to the native form. Immunization with either form of the antigen activated similar numbers of CD4⁺ T cells, but, unlike the antibody response, CD4⁺ T cells immunized with one form of MSP1₁₉ were able to respond in vitro to the other form of the protein. Although the reduced form of MSP1₁₉ does not induce protective antibodies, our data suggest that inclusion of unfolded protein may improve the efficacy of MSP1₁₉ as a vaccine.