Nature and Specificity of the Required Protective Immune Response That Develops Postchallenge in Mice Vaccinated with the 19-Kilodalton Fragment ofPlasmodium yoeliiMerozoite Surface Protein 1

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Abstract
Immunity induced by the 19-kDa fragment ofPlasmodium yoeliimerozoite surface protein 1 (MSP119) is dependent on high titers of specific antibodies present at the time of challenge and a continuing active immune response postinfection. However, the specificity of the active immune response postinfection has not been defined. In particular, it is not known whether anti-MSP119antibodies that arise following infection alone are sufficient for protection. We developed systems to investigate whether an MSP119-specific antibody response alone both prechallenge and postchallenge is sufficient for protection. We were able to exclude antibodies with other specificities, as well as any contribution of MSP119-specific CD4+T cells acting independent of antibody, and we concluded that an immune response focused solely on MSP119-specific antibodies is sufficient for protection. The data imply that the ability of natural infection to boost an MSP119-specific antibody response should greatly improve vaccine efficacy.