RNA interference-mediated silencing of the YPS3 gene of Histoplasma capsulatum reveals virulence defects.

The YPS3 gene of Histoplasma capsulatum encodes a protein that is both surface localized in the cell wall of H. capsulatum and released into the culture medium. This protein is produced only during the pathogenic yeast phase of infection and is also expressed differentially in H. capsulatum strains of different virulence levels. In this study, we silenced the YPS3 transcript by using an interfering-RNA strategy and examined the silenced mutants for phenotypic differences in vitro and during infection. The mutants showed no growth defect during in vitro culture in a defined medium at 37 degrees C and appeared to have normal virulence in a RAW 264.7 murine macrophage-like cell line. In a C57BL/6 mouse model of infection, however, the mutants caused significantly decreased fungal burdens, particularly in the peripheral phagocyte-rich tissues of livers and spleens. This defect in organ colonization was evident within 3 days of infection; however, it appeared to be exacerbated at later time points.