Immune modulation during infection by a secreted fungal polysaccharide

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Systemic fungal infections are a major public health concern, annually causing over 1.3 million deaths worldwide and costing healthcare systems billions of dollars. These infections are difficult and expensive to treat due to a limited number of drugs and poor understanding of infectious mechanisms. Cryptococcus neoformans is the major cause of fungal meningitis, annually responsible for approximately 1 million infections and 600,000 deaths worldwide. The majority of these cases are in HIV/AIDS patients. Infections begin in the lungs but in severe cases spread to the brain, causing a difficult-to-treat meningitis infection. C. neoformans produces a large cell surface capsule, the predominant component of which is the polysaccharide glucuronoxylomannan (GXM). GXM is necessary for infection, facilitates C. neoformans's ability to evade phagocytosis and destruction by immune cells, and inhibits immune responses. In addition to cell surface GXM, free secreted GXM (exo-GXM) has been long thought to play a role in infection. Free GXM circulates in serum and cerebrospinal fluid during and after infection. However, its precise role and biogenesis are not understood. We have identified the first mutants in this secretion pathway and will determine how secreted GXM facilitates infection and dissemination of C. neoformans. We find that GXM accumulates in the tissue of infected mice before C. neoformans cells spread from the lungs to those tissues, suggesting that exo-GXM facilitates dissemination. First, we will separate different steps of dissemination and determine how exo-GXM contributes to each. Second, we will determine how exo- GXM inhibits the immune response during infection by testing two hypotheses: 1) exo-GXM blocks immune cell infiltration into tissue and 2) exo-GXM prevents maturation or differentiation of circulating leukocytes. Finally, we will determine the molecular mechanisms of exo-GXM generation and regulation. These experiments will separate exo-GXM function from cell surface GXM function, determine how exo-GXM modulates the mammalian immune system during infection, and help determine whether exo-GXM could be developed as an immunosuppressive drug.