A Competitive Infection Model of Hematogenously Disseminated Candidiasis in Mice Redefines the Role of Candida albicans IRS4 in Pathogenesis

Candida albicans IRS4 encodes a protein that regulates phosphatidylinositol-(4,5)-bisphosphate, which was shown to contribute to hematogenously disseminated candidiasis (DC) after several days in the standard mouse model. Our objective was to more accurately define the temporal contributions of IRS4 to pathogenesis. During competition assays in vitro, an irs4-null (Delta irs4) mutant exhibited wild-type fitness. In DC experiments, mice were infected intravenously with the Delta irs4 mutant, strain CAI-12 (1 x 10(5) CFU), or a mixture of the strains (0.5 x 10(5) CFU each). In single-strain infections, quantitative PCR revealed reduced Delta irs4 mutant burdens within kidneys at days 1, 4, and 7 but not 6 h. In competitive infections, the Delta irs4 mutant was outcompeted by CAI-12 in each mouse at >= 6 h (competitive indices, P <= 0.0001). At 4 and 7 days, the Delta irs4 mutant burdens during competitive infections were significantly lower than those during single-strain infections (P = 0.01 and P < 0.001, respectively), suggesting increased susceptibility to inflammatory responses. Phagocytic infiltration of kidneys in response to CAI-12 or competitive infections was significantly greater than that in response to Delta irs4 mutant infection at days 1 and 4 (P < 0.001), and the Delta irs4 mutant was more susceptible to phagocytosis and killing by human polymorphonuclear cells (P = 0.01 and P = 0.006, respectively) and mouse macrophages in vitro (P = 0.04 and P = 0.01, respectively). Therefore, IRS4 contributes to tissue invasion at early stages of DC and mediates resistance to phagocytosis as DC progresses. Microarray analysis revealed remarkably similar gene expression by the Delta irs4 mutant and reference strain CAI-12 within blood, suggesting that IRS4 is not significantly involved in the hematogenous stage of disease. A competitive DC model detects attenuated virulence that is not evident with the standard model.