Differentiation of the fungus Histoplasma capsulatum into a pathogen of phagocytes.

Mammalian body temperature triggers differentiation of the fungal pathogen Histoplasma capsulatum into yeast cells. The Drk1 regulatory kinase and an interdependent network of Ryp transcription factors establish the yeast state. Beyond morphology, the differentiation-dependent expression program equips yeasts for invasion and survival within phagosomes. Yeast cells produce α-glucan and the Eng1 endoglucanase which hide yeasts from immune detection. Secretion of yeast phase-specific Sod3 and CatB detoxify phagocyte-derived reactive oxygen molecules. Histoplasma cells adapt to iron and zinc limitation in activated macrophages by production of siderophores and the Zrt2 transporter, respectively. Yeasts also respond to inflammation-associated hypoxia. Histoplasma pathogenicity thus relies on factors controlled by yeast differentiation as well as environment-dependent responses.