Candida albicans Cells Lacking AP-2 Have Defective Hyphae and Are Avirulent Despite Increased Host Uptake and Intracellular Proliferation in Macrophages.

Candida albicans is a commensal microbe and opportunistic human pathogen. Candida yeast are recognized and taken up by macrophages via phagocytosis. Macrophage surface receptors bind to specific components of the Candida cell wall. Following phagocytosis, Candida can respond to the host's intracellular environment by switching from a yeast to a hyphal morphology facilitating escape from macrophages and allowing subsequent invasion of host tissues. Various disruptions of Candida's ability to form hyphae have been shown to reduce virulence and fitness in the host. Our previous work concluded that Candida albicans cells lacking AP-2 (apm4Δ/Δ), an endocytic adaptor complex, have increased cell wall chitin and morphologically defective hyphae in vitro. Increased chitin has been correlated with decreased recognition by macrophages, possibly due to masking of cell wall β-glucan, the target for the Dectin-1 immune receptor. Here we test the virulence profiles of apm4Δ/Δ mutant, demonstrating a surprising increase in macrophage phagocytosis that does not occur due to the elevated exposure of β-glucan, highlighting the importance of cell wall components beyond chitin and glucan for macrophage engagement and uptake. Furthermore, the apm4 mutant exhibited parasitism of macrophages, surviving and proliferating within the phagosome, a phenotype that was then replicated with a well-characterized yeast locked mutant, demonstrating the further complexity of C. albicans' ability to evade macrophage responses. Finally, the combined phenotype of reduced hyphal formation but continued proliferation resulted in reduced virulence despite an equivalent burden of infection with wild-type Candida infection, as determined using a zebrafish larval model of candidiasis.