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.

The importance of humoral immunity for resistance to Cryptococcus neoformans is uncertain. A case-controlled study of the human antibody response to C. neoformans comparing the serum antibody profiles of human immunodeficiency virus (HIV)-infected persons who did (HIV+/CM+) or did not (HIV-infected controls) develop cryptococcal meningitis (CM) and HIV-uninfected persons with samples obtained from the Multicenter AIDS Cohort Study was performed. Total immunoglobulin concentrations were determined, and the specificity, isotype, and idiotype expression of antibodies to C. neoformans capsular glucuronoxylomannan were analyzed by ELISA. Compared with the HIV+/CM+ group, the HIV-infected control group had significantly lower levels of total IgM, IgA, and antibodies expressing a certain VH3 determinant. The HIV-infected control group manifested an increase in immunoglobulin levels with a decrease in CD4 lymphocytes. The findings suggest a possible association between reduced expression of certain immunoglobulin subsets and HIV-associated CM.

Antifungal therapy cannot eradicate Cryptococcus neoformans infections in immunosuppressed patient groups. Therefore, adjunctive antibody-based therapy is being considered to enhance host immune responses to C. neoformans. To characterize potentially protective reagents, the idiotypic repertoire of human antibodies to cryptococcal glucuronoxylomannan (GXM) elicited by the investigational conjugate vaccine GXM-tetanus toxoid was examined. The variable genes used by human antibodies to GXM were analyzed with an antigen-based ELISA and mouse monoclonal antibodies (MAbs) that recognize determinants of human VH1, VH3, and VH4 gene segments. Antibodies to GXM were shown to use VH3 gene segments, and antibodies with the greatest binding to GXM also bound to protein A. A VH3-positive human monoclonal IgM prolonged survival of C. neoformans-infected mice. This is the first report that a human antibody is protective against C. neoformans. These results suggest that human MAbs may have promise as therapeutic reagents against cryptococcosis.

Summary Invariant human natural killer T cells (NKT) express a restricted T‐cell receptor (TCR) Vα24Vβ11 repertoire. These cells share both phenotypic and functional similarities between NK and T cells. Given the emerging role of NKT cells as critical cells in bridging the gap between innate and adaptive immunity, we examined their susceptibility to productive human immunodeficiency virus (HIV) infection by T‐tropic, M‐tropic, and primary isolates of HIV. We generated three human NKT cell clones (CA5, CA29, and CA31). Phenotypic characterization of these Vα24+ Vβ11+ clones indicated that they were predominately positive for CD4, CD161, HLA‐DR, CD38, CD45RO, and CD95 expression. The NKT cell clones expressed significantly more surface CCR5 molecules/cell and lower CXCR4 molecules/cell than phytohaemagglutinin‐stimulated peripheral blood mononuclear cells (PBMC). Consistent with the surface expression of CCR5 and CXCR4, the NKT clones were also selectively susceptible to HIV M‐tropic, T‐tropic, and primary isolate infection, as evaluated by both HIV p24 enzyme‐linked immunosorbent assay and intracellular staining of HIV proteins. The amount of p24 production was dependent on the NKT clone studied and the HIV strain used. Clones CA29 and CA31 were also susceptible to HIV IIIB infection. The virions produced by these clones were able to productively infect PHA‐stimulated PBMCs with the same kinetics as for primary infection of CD4+ blast. Collectively, this data demonstrates that NKT cells can be a target for productive HIV infection but with a lag in the time to peak p24 production.