Optimal T Cell Responses to Cryptococcus neoformans Mannoprotein Are Dependent on Recognition of Conjugated Carbohydrates by Mannose Receptors

Abstract

Cryptococcosis is a leading cause of death among individuals with compromised T cell function. Soluble Cryptococcus neoformans mannoproteins (MP) have emerged as promising vaccine candidates due to their capacity to elicit delayed-type hypersensitivity and Th type 1-like cytokines, both critical to the clearance of this pathogenic yeast. In this study, the mechanisms responsible for the potent immunostimulatory properties of MP were explored. Using Chinese hamster ovary cells expressing human macrophage mannose receptor (MMR), we determined that MP is a MMR ligand. Functionally, competitive blockade of multilectin mannose receptors (MR) on APCs diminished MP-dependent stimulation of primary T cells from immunized mice and the MP-reactive CD4+ T cell hybridoma, P1D6, by 72 and 99%, respectively. Removal of O-linked saccharides from MP by β-elimination inhibited MP-dependent stimulation of P1D6 and primary T cells by 89 and 90%, respectively. In addition, MP-dependent stimulation of P1D6 was abrogated after digestion with proteinase K, suggesting the protein core of MP contributed the antigenic moiety presented by APC. Stimulation of P1D6 by MP also was abolished using APC obtained from invariant chain-deficient mice, demonstrating Ag presentation was MHC class II restricted. Our data suggest that MP is a ligand for the MMR and that T cell stimulation is functionally inhibited either by competitive blockade of MR or by removal of carbohydrate residues critical for recognition. The demonstration that efficient T cell responses to MP require recognition of terminal mannose groups by MMR provides both a molecular basis for the immunogenicity of cryptococcal MP and support for vaccination strategies that target MR.

Tubulation of Class II MHC Compartments Is Microtubule Dependent and Involves Multiple Endolysosomal Membrane Proteins in Primary Dendritic Cells

“…CN is a pathogenic fungus with a complex cell wall structure (45). Although incompletely characterized, the surface of CN engages at least TLR2 and TLR4 (46,47).…”

Antigenic Targeting of the Human Mannose Receptor Induces Tumor Immunity

“…However, the importance of MR in the innate and adaptive immune responses to pathogens is less clear. Some studies support an important role for MR in this capacity (7,8), while other investigations suggest MR is not critical for protection against selected microbial pathogens (9, 10). Adding to the complexity, a number of recent studies suggest that MR cross-linking elicits antiinflammatory responses, and thus may down-regulate innate inflammatory (11)(12)(13).…”

Role of the Mannose Receptor in a Murine Model ofCryptococcus neoformansInfection

“…DCs are the main antigenpresenting cells responsible for the uptake, processing, and presentation of MP to CD4 ϩ T cells (21,22). Blocking receptors for mannose with mannan or deglycosylating MP results in strongly diminished T-cell responses (21,22,32).…”

“…DCs are the main antigenpresenting cells responsible for the uptake, processing, and presentation of MP to CD4 ϩ T cells (21, 22). Blocking receptors for mannose with mannan or deglycosylating MP results in strongly diminished T-cell responses (21,22,32).The MR is a C-type lectin receptor which recognizes terminally mannosylated molecules (24). In addition to MP, a myriad of mannosylated antigens are recognized by the MR, including ManLam from Mycobacterium tuberculosis, the envelope protein gp120 from HIV, the capsular polysaccharide from Streptococcus pneumoniae, and soluble egg antigen from Schistosoma mansoni (21, 37).…”

“…These areas of O-linked and N-linked mannosylation promote the recognition of MP by host receptors for mannose, in particular, the mannose receptor (MR, CD206) and dendritic cell (DC)-specific intercellular adhesion molecule 3 grabbing nonintegrin (CD209) (17,21). DCs are the main antigenpresenting cells responsible for the uptake, processing, and presentation of MP to CD4 ϩ T cells (21,22). Blocking receptors for mannose with mannan or deglycosylating MP results in strongly diminished T-cell responses (21,22,32).…”