Abstract
The fungal cell wall contains glycoproteins that interact with the host immune system. In the prominent pathogenic yeast Candida albicans, Pmr1 acts as a Golgi-resident ion pump that provides cofactors to mannosyltransferases, regulating the synthesis of mannans attached to glycoproteins. To gain insight into a putative conservation of such a crucial process within opportunistic yeasts, we were particularly interested in studying the role of the PMR1 homolog in a low-virulent species that rarely causes candidiasis, Candida guilliermondii. We disrupted C. guilliermondii PMR1 and found that loss of Pmr1 affected cell growth and morphology, biofilm formation, susceptibility to cell wall perturbing agents, mannan levels, and the wall composition and organization. Despite the significant increment in the amount of β1,3-glucan exposed at the wall surface, this positively influenced only the ability of the mutant to stimulate IL-10 production by human monocytes, suggesting that recognition of both mannan and β1,3-glucan, is required to stimulate strong levels of pro-inflammatory cytokines. Accordingly, our results indicate C. guilliermondii sensing by monocytes was critically dependent on the recognition of N-linked mannans and β1,3-glucan, as reported in other Candida species. In addition, chemical remotion of cell wall O-linked mannans was found to positively influence the recognition of C. guilliermondii by human monocytes, suggesting that O-linked mannans mask other cell wall components from immune cells. This observation contrasts with that reported in C. albicans. Finally, mice infected with C. guilliermondii pmr1Δ null mutant cells had significantly lower fungal burdens compared to animals challenged with the parental strain. Accordingly, the null mutant showed inability to kill larvae in the Galleria mellonella infection model. This study thus demonstrates that mannans are relevant for the C. guilliermondii-host interaction, with an atypical role for O-linked mannans.
Highlights
Several members of the Candida genus cause both superficial and systemic candidiasis, the latter resulting in significant morbidity and mortality, especially in immunosuppressed patients (Brown et al, 2012)
To demonstrate that CgPMR1 is the functional ortholog of the CaPMR1, we complemented a C. albicans pmr1 null mutant (Bates et al, 2005) with C. guilliermondii PMR1 under the control of its native promoter
The phenotypical analysis of the mutant strain indicated that C. guilliermondii PMR1 restored the cell wall composition of the C. albicans pmr1 null mutant to levels similar to those found in the wild-type (WT) control cells (Figure 1A)
Summary
Several members of the Candida genus cause both superficial and systemic candidiasis, the latter resulting in significant morbidity and mortality, especially in immunosuppressed patients (Brown et al, 2012). The C. albicans cell wall is composed of structural polysaccharides (chitin, β1,3- and β1,6-glucans) that surround the plasma membrane (inner wall layer) and an outer layer composed of N- and O-linked mannoproteins (Klis et al, 2001) All these components are recognized as pathogen-associated molecular patterns and are capable of stimulating both cytokine production and phagocytosis by innate immune cells (MartínezÁlvarez et al, 2014). A previously generated C. albicans pmr null mutant displayed a strong defect in the cell wall composition and elaboration of both N-linked and O-linked mannans (Bates et al, 2005); and produced a poor cytokine response in human peripheral-blood mononuclear cells (PBMCs) and dendritic cells, and its phagocytosis by macrophages was significantly affected (Netea et al, 2006; Cambi et al, 2008; McKenzie et al, 2010); these findings underscore the relevance of both N- and Olinked mannans for a proper C. albicans-immune cell interaction. We found that C. guilliermondii and C. albicans O-linked mannans are similar in composition, but the former mask other cell wall components from interaction with human PBMCs, preventing a significant increase in cytokine production compared to that generated by the pmr null mutant
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