Human antibody responses to vaccination against Cryptococcus neoformans

Abstract

Cryptococcus neoformans is an encapsulated fungal pathogen that causes significant morbidity and mortality worldwide among immunocompromised individuals, especially those with AIDS. Although the role of antibody in natural resistance to C. neoformans is not established, antibodies to the cryptococcal capsular polysaccharide glucuronoxylomannan (GXM) can benefit the host in experimental cryptococcosis. The data presented in this thesis tries to understand the human response to vaccination against C. neoformans and the nature of protective antibodies using an animal model to determine the role that V gene usage plays in protection. The first two chapters describe the response of human immunoglobulin transgenic (XenoMouse RTM) mice to conjugates of a peptide GXM mimotope, P13. P13 conjugates were used to vaccinate mice expressing human IgG2 (G2) or IgG4 (G4). G2 mice vaccinated with P13-diphtheria toxoid (DT) developed an anamnestic IgG response to GXM upon challenge with C. neoformans and manifested longer survival than PBS and DT controls. Passive transfer of sera from P13-DT-vaccinated mice prolonged survival of lethally C. neoformans infected BALB/c mice. G2 mice from the vaccination/challenge experiment receiving the adjuvant CpG manifested a trend towards longer survival after challenge and late sera from these mice prolonged survival compared to PBS controls in passive transfer. The common factor in all of the sera that prolonged survival was the presence of IgM to GXM. The last chapter of this thesis, MAbs that were produced from G2 mice vaccinated with a GXM-DT conjugate are described. Three IgMs to GXM were characterized: G15, which utilizes the VH3-64, and G14 and G19, which utilize VH6. All three MAbs use the same light chain gene, DPK22/A27. G15, a germline antibody, was protective when given to mice in a 100 mug dose, but not at higher or lower doses. This was suggestive of the prozone-like phenomenon described for protective murine IgMs to GXM. G14 and G19 did not protect at any dose. Since the VH3 and VH6 MAbs had different serotype and epitope specificity, certain epitopes may be more likely to elicit human antibodies that are protective. This thesis suggests that the natural repertoire determines the protective response to C. neoformans.