A cryptococcal capsular polysaccharide mimotope prolongs the survival of mice with Cryptococcus neoformans infection

Defined Abs to the Cryptococcus neoformans capsular polysaccharide glucuronoxylomannan (GXM) have been shown to be protective against experimental cryptococcosis. This suggests that if a vaccine could induce similar Abs it might protect against infection. However, the potential use of a GXM-based vaccine has been limited by evidence that GXM is a poor immunogen that can induce nonprotective and deleterious, as well as protective, Abs, and that the nature of GXM oligosaccharide epitopes that can elicit a protective response is unknown. In this study, we investigated whether a peptide surrogate for a GXM epitope could induce an Ab response to GXM in mice. The immunogenicity of peptide-protein conjugates produced by linking a peptide mimetic of GXM, P13, to either BSA, P13-BSA, or tetanus toxoid, P13-tetanus toxoid, was examined in BALB/c and CBA/n mice that received four s.c. injections of the conjugates at 14- to 30-day intervals. All mice immunized with conjugate produced IgM and IgG to P13 and GXM. Challenge of conjugate-immunized mice with C. neoformans revealed longer survival and lower serum GXM levels than control mice. These results indicate that 1) P13 is a GXM mimotope and 2) that it induced a protective response against C. neoformans in mice. P13 is the first reported mimotope of a C. neoformans Ag. Therefore, the P13 conjugates are vaccine candidates for C. neoformans and their efficacy in this study suggests that peptide mimotopes selected by protective Abs deserve further consideration as vaccine candidates for encapsulated pathogens.