Direct Inhibition of T-Cell Responses by the Cryptococcus Capsular Polysaccharide Glucuronoxylomannan

Abstract

The major virulence factor of the pathogenic fungi Cryptococcus neoformans and C. gattii is the capsule. Glucuronoxylomannan (GXM), the major component of the capsule, is a high-molecular-weight polysaccharide that is shed during cryptococcosis and can persist in patients after successful antifungal therapy. Due to the importance of T cells in the anticryptococcal response, we studied the effect of GXM on the ability of dendritic cells (DCs) to initiate a T-cell response. GXM inhibited the activation of cryptococcal mannoprotein–specific hybridoma T cells and the proliferation of OVA-specific OT-II T cells when murine bone marrow–derived DCs were used as antigen-presenting cells. Inhibition of OT-II T-cell proliferation was observed when either OVA protein or OVA323–339 peptide was used as antigen, indicating GXM did not merely prevent antigen uptake or processing. We found that DCs internalize GXM progressively over time; however, the suppressive effect did not require DCs, as GXM directly inhibited T-cell proliferation induced by anti-CD3 antibody, concanavalin A, or phorbol-12-myristate-13-acetate/ionomycin. Analysis of T-cell viability revealed that the reduced proliferation in the presence of GXM was not the result of increased cell death. GXM isolated from each of the four major cryptococcal serotypes inhibited the proliferation of human peripheral blood mononuclear cells stimulated with tetanus toxoid. Thus, we have defined a new mechanism by which GXM can impart virulence: direct inhibition of T-cell proliferation. In patients with cryptococcosis, this could impair optimal cell-mediated immune responses, thereby contributing to the persistence of cryptococcal infections.

Immune Restoration Diseases Reflect Diverse Immunopathological Mechanisms

“…In contrast to the unmasking form of cryptococcal IRD, relapsing cases are culture negative, supporting the hypothesis that residual cryptococcal antigen is the precipitating factor. Higher opening cerebrospinal fluid (CSF) pressures are observed in cryptococcal IRD patients (67,130,154). These may reflect delayed clearance of residual antigen from the leptomeninges and the precipitating inflammation induced by a Th1 response following ART.…”

Role of the Mannose Receptor in a Murine Model ofCryptococcus neoformansInfection

“…DCs were enriched by positive selection with CD11c ϩ magnetic beads. Mitomycin (50 g/ml)-treated DCs from wild-type mice were plated at 1 ϫ 10 4 per well in 96-well round-bottom plates (19,39). Statistics.…”

“…C57BL/6 mice are relatively susceptible to cryptococcosis (6), and a virulent encapsulated strain of C. neoformans was used for the mortality studies. Moreover, glucuronoxylomannan, the major component of the cryptococcal capsule, directly inhibits T-cell responses (30,39). Thus, while MR-dependent immune responses to MP contribute to host defenses against cryptococcosis, they do not mediate fully protective responses in the mouse model.…”

Dendritic Cell-Based Immunization Ameliorates Pulmonary Infection with Highly Virulent Cryptococcus gattii

“…Previous studies indicated that one of the capsular components of C. gattii may have been involved in immune avoidance or immune suppression and was required for the complete virulence of C. gattii (12,13). Because C. gattii can induce immune avoidance or suppression, an analysis of loss-of-function using gene knockout (KO) mice is not applicable for studying any protective immune responses against C. gattii, and the protective immunity against highly virulent C. gattii has not been well characterized.…”