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 contrib-ute 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 (irs4) mutant exhibited wild-type fitness. In DC experiments, mice were infected intravenously with theirs4mutant, strain CAI-12 (1 105 CFU), or a mixture of the strains (0.5 105 CFU each). In single-strain infections, quantitative PCR revealed reduced irs4mutant burdens within kidneys at days 1, 4, and 7 but not 6 h. In competitive infections, theirs4mutant was outcom-peted by CAI-12 in eachmouse at>6 h (competitive indices, P< 0.0001). At 4 and 7 days, theirs4mutant burdens during com-petitive infections were significantly lower than those during single-strain infections (P 0.01 and P< 0.001, respectively), sug-gesting increased susceptibility to inflammatory responses. Phagocytic infiltration of kidneys in response to CAI-12 or competitive infections was significantly greater than that in response toirs4mutant infection at days 1 and 4 (P< 0.001), and theirs4mutant 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 inva-sion at early stages of DC andmediates resistance to phagocytosis as DC progresses. Microarray analysis revealed remarkably similar gene expression by theirs4mutant and reference strain CAI-12 within blood, suggesting that IRS4 is not significantly involved in the hematogenous stage of disease. A competitive DCmodel detects attenuated virulence that is not evident with th