Efficacy of immune sera from human immunoglobulin transgenic mice immunized with a peptide mimotope of Cryptococcus neoformans glucuronoxylomannan

The efficacy of antibody mediated immunity against Cryptococcus neoformans has not been established experimentally for human antibodies. Our group has previously shown that immunization with a conjugate consisting of a peptide mimotope of the C. neoformans capsular polysaccharide glucuronoxylomannan (GXM), P13, and diphtheria toxoid (P13-DT) prolonged survival of transgenic mice with human immunoglobulin loci, XenoMouse^® mice, which were challenged with a lethal dose of C. neoformans. In the study reported herein, we determined the efficacy of human antibodies in the sera of immunized XenoMouse^® mice against C. neoformans in passive transfer experiments in naïve BALB/c mice. Survival studies were performed with sera from XenoMouse^® mice expressing human IgG2/kappa (G2/k mice) or IgG4/kappa (G4/k mice) that had been immunized with P13-tetanus toxoid (TT)/Alhydrogel with or without CpG, and G2/k mice that had been immunized with P13-DT/Alhydrogel/CpG or Alhydrogel/CpG, obtained on day 7 (early sera) and days 30 or 35-59 (late sera) after primary immunization. Compared to mice receiving sera from G2/k-PBS-treated mice, the survival of naïve mice was prolonged by both early and late sera from G2/k-P13-DT/Alhydrogel/CpG-immunized mice, but only late sera from G2/k-P13-TT/Alhydrogel/CpG-immunized mice. Late, but not early sera from G2/k-Alhydrogel/CpG-immunized mice also prolonged survival. For all sera, prolongation of survival was associated with GXM-specific serum IgM. Sera from G2/k mice that received P13-TT without CpG, and all groups of G4/k mice had low to undetectable levels of antibody to GXM and were not protective. Our findings suggest that GXM-specific human IgM may be a functional mediator of protection against C. neoformans.