You shall not pass! Protective role of autophagic machinery in response to plasma membrane damage triggered by Candida albicans invasion.

Candida albicans (C. albicans) is an opportunistic pathogen causing infections ranging from superficial to life-threatening dissemination, in which C. albicans is able to translocate through the gut barrier into deeper organs. In its filamentous form (hyphae), C. albicans can invade epithelial cells by two mechanisms: epithelial cell-driven endocytosis and C. albicans-driven active penetration of host cell plasma membrane (PM). Autophagic machinery is known to be involved in the epithelial barrier maintenance, especially the intestinal barrier that is continuously challenged by exposure to the gut microbiota or to xenobiotics. The protective role of autophagy during C. albicans infection has been investigated in myeloid cells, however, far less was known regarding its role during infection of epithelial cells. Here, we demonstrated that key proteins of the autophagic machinery and vesicles presenting features of autophagosomes are recruited at C. albicans invasion sites. These events are associated with host PM damage caused by the active penetration of C. albicans. We showed that ATG5 and ATG16L1 proteins contribute to PM repair mediated by lysosomal membrane exocytosis and participate in protection of epithelial cells' integrity against C. albicans-induced cell death. Our findings extend the knowledge on emerging roles of the autophagic machinery in stress-related membrane dynamics.

Membrane protective role of autophagic machinery during infection of epithelial cells by Candida albicans.

Candida albicans (C. albicans) is an opportunistic pathogen causing infections ranging from superficial to life-threatening disseminated infections. In a susceptible host, C. albicans is able to translocate through the gut barrier, promoting its dissemination into deeper organs. C. albicans hyphae can invade human epithelial cells by two well-documented mechanisms: epithelial-driven endocytosis and C. albicans-driven active penetration. One mechanism by which host cells protect themselves against intracellular C. albicans is termed autophagy. The protective role of autophagy during C. albicans infection has been investigated in myeloid cells; however, far less is known regarding the role of this process during the infection of epithelial cells. In the present study, we investigated the role of autophagy-related proteins during the infection of epithelial cells, including intestinal epithelial cells and gut explants, by C. albicans. Using cell imaging, we show that key molecular players of the autophagy machinery (LC3-II, PI3P, ATG16L1, and WIPI2) were recruited at Candida invasion sites. We deepened these observations by electron microscopy analyses that reveal the presence of autophagosomes in the vicinity of invading hyphae. Importantly, these events occur during active penetration of C. albicans into host cells and are associated with plasma membrane damage. In this context, we show that the autophagy-related key proteins ATG5 and ATG16L1 contribute to plasma membrane repair mediated by lysosomal exocytosis and participate in protecting epithelial cells against C. albicans-induced cell death. Our findings provide a novel mechanism by which epithelial cells, forming the first line of defense against C. albicans in the gut, can react to limit C. albicans invasion.

Candida albicans is able to use M cells as a portal of entry across the intestinal barrier in vitro.

Candida albicans is the most frequent yeast responsible for systemic infections in humans. These infections mainly originate from the gastrointestinal tract where C. albicans can invade the gut epithelial barrier to gain access to the bloodstream. Along the gut, pathogens can use Microfold (M) cells as a portal of entry to cross the epithelial barrier. M cells are specialized cells mainly located in the follicule-associated epithelium of Peyer patches. In this study, we used scanning electron and fluorescence microscopy, adhesion and invasion assays and fungal mutants to investigate the interactions of C. albicans with M cells obtained in an established in vitro model whereby enterocyte-like Caco-2 cells co-cultured with the Raji B cell line undergo a phenotypic switch to morphologically and functionally resembling M cells. Our data demonstrate that C. albicans co-localizes with and invades preferentially M cells, providing evidence that the fungus can use M cells as a portal of entry into the intestinal barrier. In addition to active penetration, F-actin dependent endocytosis contributes to internalization of the fungus into M cells through a mechanism involving hypha-associated invasins including Ssa1 and Als3.