Histoplasma capsulatum Depends onDe NovoVitamin Biosynthesis for Intraphagosomal Proliferation

Abstract

ABSTRACTDuring infection of the mammalian host,Histoplasma capsulatumyeasts survive and reside within macrophages of the immune system. Whereas some intracellular pathogens escape into the host cytosol,Histoplasmayeasts remain within the macrophage phagosome. This intracellularHistoplasma-containing compartment imposes nutritional challenges for yeast growth and replication. We identified and annotated vitamin synthesis pathways encoded in theHistoplasmagenome and confirmed by growth in minimal medium thatHistoplasmayeasts can synthesize all essential vitamins with the exception of thiamine. Riboflavin, pantothenate, and biotin auxotrophs ofHistoplasmawere generated to probe whether these vitamins are available to intracellular yeasts. Disruption of theRIB2gene (riboflavin biosynthesis) prevented growth and proliferation of yeasts in macrophages and severely attenuatedHistoplasmavirulence in a murine model of respiratory histoplasmosis. Rib2-deficient yeasts were not cleared from lung tissue but persisted, consistent with functional survival mechanisms but inability to replicatein vivo. In addition, depletion of Pan6 (pantothenate biosynthesis) but not Bio2 function (biotin synthesis) also impairedHistoplasmavirulence. These results indicate that theHistoplasma-containing phagosome is limiting for riboflavin and pantothenate and thatHistoplasmavirulence requiresde novosynthesis of these cofactor precursors. Since mammalian hosts do not rely on vitamin synthesis but instead acquire essential vitamins through diet, vitamin synthesis pathways represent druggable targets for therapeutics.