Candida albicans cells lacking AP-2 have defective hyphae and are avirulent despite increased host uptake and intracellular proliferation in macrophages

Abstract

Candida albicans is a commensal microbe and opportunistic human pathogen. The yeast can be recognised and taken up by macrophages via interactions with its cell wall, a complex polysaccharide structure containing several components that are specifically recognised by immune cell receptors. Following uptake Candida can respond in the host environment by switching from a yeast to hyphal morphology which facilitates escape from macrophages and allows subsequent invasion of host tissues. Disruption of Candidas ability to form hyphae results in reduced virulence and fitness for survival in the host environment. Candida albicans cells lacking AP-2, an endocytic adaptor complex, have increased cell wall chitin and morphologically defective hyphae in vitro. Previous studies have correlated increased chitin with decreased recognition by macrophages, possibly due to masking of cell wall beta-glucan which is recognised by dectin receptors. Despite the cell wall changes in the mutant strain there was an unexpected increased uptake of the mutant. Increased chitin did not reduce phagocytosis and additional uptake was not due to compensatory elevated exposure of beta-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-characterised yeast locked mutant. Finally, the combined phenotype of reduced hyphal formation but continued proliferation resulted in reduced virulence despite an equivalent burden of infection to a wild-type Candida infection, as determined using a zebrafish larval model of candidiasis.