Virulence mechanisms of coccidioides.

Coccidioides is a fungal respiratory pathogen of humans that can cause disease in both immunosuppressed and immunocompetent individuals. We describe here three mechanisms by which the pathogen survives in the hostile host environment: production of a dominant spherule outer wall glycoprotein (SOWgp) that modulates host immune response and results in compromised cell-mediated immunity to coccidioidal infection, depletion of SOWgp presentation on the surface of endospores, which prevents host recognition of the pathogen when the fungal cells are most vulnerable to phagocytic defenses, and induction of elevated production of host arginase I and coccidioidal urease, which contribute to tissue damage at sites of infection. Arginase I competes with inducible nitric oxide synthase (iNOS) in macrophages for the common substrate, L-arginine, and thereby reduces nitric oxide (NO) production and increases the synthesis of host orinithine and urea. Host-derived L-ornithine may promote pathogen growth and proliferation by providing a pool of the monoamine, which could be taken up and used for synthesis of polyamines via metabolic pathways of the parasitic cells. We have shown that high concentrations of Coccidioides- and host-derived urea at infection sites in the presence of urease produced and released by the pathogen, results in secretion of ammonia and contributes to alkalinization of the microenvironment. We propose that ammonia and enzymatically active urease released from spherules during the parasitic cycle of Coccidioides exacerbate the severity of coccidioidal infection by contributing to a compromised immune response to infection and damage of host tissue at foci of infection.