Proliferative Responses of Mice to a Cloned Plasmodium Falciparum Sporozoite Antigen

Abstract : A peptide fragment of the Plasmodium falciparum circum-sporozoite protein (CSP) containing 30 repeats of the immuno-dominate ASN-ALA-ASN-PRO and two of the VAL ASP variants (R32tet32) is currently being evaluated as a vaccine in man. This R32tet32 peptide, prepared by recombinant DNA technology from a cloned P. falciparum gene fragment, has been examined for its ability to stimulate T-cell proliferation in experimental animals. Groups of mice were injected with either R32tet32 emulsified in Freund's complete adjuvant (CFA), or live, or frozen-thawed P. falciparum sporozoites + CFA. Lymphocytes from such mice were co-cultured with varying doses of R32tet32 or irrelevant antigen. Proliferation was assessed by 3H-thymidine uptake; serum antibody was analyzed by ELISA. A proliferative response was found in mice immunized with R32tet32 + CFA as early as day 7 post-injection, and was persistent through at least day 23. No proliferation in response to R32tet32 was observed in lymphocytes taken from mice injected with live or frozen-thawed sporozoites. All three immunogens induced both IgG antibody to R32tet32. We conclude that exposure to live or frozen-thawed P. falciparum sporozoites + CFA alone is sufficient to generate T- cell helper activity for subsequent antibody production, but that antigen + CFA was necessary to generate significant T-cell proliferative activity. Keywords: Malaria; Vaccines; Immunogens.