Evaluation of two homologous Coccidioides posadasii antigens as recombinantly expressed monovalent, divalent, and chimeric vaccine candidates

Evaluation of recombinant proteins of C. posadasii for their protective efficacy in a murine model of coccidioidomycosis has revealed several potential vaccine candidates. One of the most promising is a proline rich-antigen (Ag2/Pra). We have demonstrated that vaccination of C57BL/6 and BALB/c mice with the combination of rAg2/Pra plus rPra2 enhanced protection against a lethal intranasal challenge with Coccidioides, compared to vaccination with either of the single antigens. Enhanced protection was based both on higher percentage of surviving mice (as demonstrated by the BALB/c strain) or significantly better clearance of the fungal burden (C57BL/6). Despite the related primary structures of Ag2/Pra and Pra2, we have identified differences in their helper T cell (Th) and continuous B cell epitope profiles using immune CD4+ T cells and sera from both C57BL/6 and BALB/c mice. The mapping of the Th and continuous B cell epitopes of Ag2/Pra and Pra2 induced by the immunization of C57BL/6 (H-2 b) mice with either rAg2/Pra or rPra2 facilitated the rational design of three divalent chimeric antigens composed of immunoreactive domains selected from both antigens. The three recombinantly expressed and purified divalent chimeric antigens were evaluated for their ability to protect C57BL/6 mice from both sub-lethal and lethal intranasal challenges with Coccidioides (47 and 90 arthroconidia, respectively). The results of these protection experiments provide evidence that the three divalent recombinant chimeric antigens we designed afford a level of protective efficacy that in some cases is statistically no different than the full-length rAg2/Pra plus rPra2 divalent vaccine. However, comparisons of pulmonary fungal burdens (sub-lethal challenge) and survival rates (lethal challenge) indicate that the chimeric antigens in all cases are noticeably (and in some cases statistically) less protective than the combination of rAg2/Pra plus rPra2. These observations suggest that, though we have successfully reconstituted a portion of the protective efficacy that the full-length divalent vaccine (rAg2/Pra plus rPra2) provides C57BL/6 mice, the current immunization protocol and/or design of our divalent chimeric vaccines may not be optimal.