Immune responses to Plasmodium falciparum–merozoite surface protein 1 (MSP1) antigen, II. Induction of parasite‐specific immunoglobulin G in unsensitized human B cells after in vitro T‐cell priming with MSP119

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Abstract
A baculovirus recombinant antigen corresponding to the C‐terminal 19 000 MW fragment of Plasmodium falciparum merozoite surface protein 1 (MSP119), has been used to prime T cells from individuals with no previous exposure to malaria, to provide help for the induction of a parasite specific antibody response in vitro. Although MSP119 alone could induce a small but detectable T‐cell response, which included interleukin‐4 (IL‐4) secretion, this response was significantly increased by the presence of IL‐2. In addition, IL‐4 was shown to synergize with IL‐2 for the induction of antigen‐specific T‐cell responses. If interferon‐γ (IFN‐γ), IL‐12, or neutralizing anti‐IL‐4 antibody was present at the time of priming, the T‐cell responses were abolished. Parasite‐specific immunoglobulin G (IgG) could be detected after secondary restimulation with MSP119, IL‐10 and anti‐CD40 monoclonal antibody in cultures containing MSP119 primed T cells, autologous B cells, IL‐2 and IL‐4. No antibody was secreted in the absence of primed T cells in this B‐cell culture assay. These data show that recombinant MSP119, a leading malaria vaccine candidate, can prime non‐immune human lymphocytes under defined in vitro experimental conditions, which include regulatory cytokines and/or other costimulatory molecules. This is a complementary approach for exploring immunogenic mechanisms of potential vaccine candidates such as P. falciparum antigens in humans.