Reduced immunogenicity of Plasmodium falciparum gamete surface antigen (Pfs48/45) in mice after disruption of disulphide bonds - evaluating effect of interferon-γ-inducible lysosomal thiol reductase.

Sexual stages of Plasmodium are critical for malaria transmission and stage-specific antigens are important targets for development of malaria transmission-blocking vaccines. Plasmodium falciparum gamete surface antigen (Pfs48/45) is important for male gamete fertility and is being pursued as a candidate vaccine antigen. Vaccine-induced transmission-blocking antibodies recognize reduction-sensitive conformational epitopes in Pfs48/45. Processing and presentation of such disulphide-bond-constrained epitopes is critical for eliciting the desired immune responses. Mice lacking interferon-γ-inducible lysosomal thiol reductase (GILT), an enzyme that mediates reduction of S-S bonds during antigen processing, were employed to investigate immunogenicity of Pfs48/45. It has been well established that the ability to reduce S-S bonds in antigens guides effective T-cell immune responses; however, involvement of GILT in the induction of subsequent B-cell responses has not been explored. We hypothesized that the ability to reduce S-S bonds in Pfs48/45 will impact the generation of T-cell epitopes, and so influence helper T-cell responses required for specific B-cell responses. Non-reduced and reduced and alkylated forms of Pfs48/45 were employed to evaluate immune responses in wild-type and GILT knockout mice and studies revealed important differences in several immune response parameters, including differences in putative T-cell epitope recognition, faster kinetics of waning of Pfs48/45-specific IgG1 antibodies in knockout mice, differential patterns of interferon-γ and interleukin-4 secretions by splenocytes, and possible effects of GILT on induction of long-lived plasma cells and memory B cells responsible for antigen-recall responses. These studies emphasize the importance of antigen structural features that significantly influence the development of effective immune responses.