Candida albicans Cell Wall Glycosylation May Be Indirectly Required for Activation of Epithelial Cell Proinflammatory Responses

Abstract

Oral epithelial cells discriminate between the yeast and hyphal forms of Candida albicans via the mitogenactivated protein kinase (MAPK) signaling pathway. This occurs through phosphorylation of the MAPK phosphatase MKP1 and activation of the c-Fos transcription factor by the hyphal form. Given that fungal cell wall polysaccharides are critical in host recognition and immune activation in myeloid cells, we sought to determine whether ␤-glucan and N-or O-glycosylation was important in activating the MAPK/MKP1/c-Fos hypha-mediated response mechanism and proinflammatory cytokines in oral epithelial cells. Using a series of ␤-glucan and N-and O-mannan mutants, we found that N-mannosylation (via ⌬och1 and ⌬pmr1 mutants) and O-mannosylation (via ⌬pmt1 and ⌬mnt1 ⌬mnt2 mutants), but not phosphomannan (via a ⌬mnn4 mutant) or ␤-1,2 mannosylation (via ⌬bmt1 to ⌬bmt6 mutants), were required for MKP1/c-Fos activation, proinflammatory cytokine production, and cell damage induction. However, the N-and O-mannan mutants showed reduced adhesion or lack of initial hypha formation at 2 h, resulting in little MKP1/c-Fos activation, or restricted hypha formation/pseudohyphal formation at 24 h, resulting in minimal proinflammatory cytokine production and cell damage. Further, the ␣-1,6-mannose backbone of the N-linked outer chain (corresponding to a ⌬mnn9 mutant) may be required for epithelial adhesion, while the ␣-1,2-mannose component of phospholipomannan (corresponding to a ⌬mit1 mutant) may contribute to epithelial cell damage. ␤-Glucan appeared to play no role in adhesion, epithelial activation, or cell damage. In summary, N-and O-mannosylation defects affect the ability of C. albicans to induce proinflammatory cytokines and damage in oral epithelial cells, but this may be due to indirect effects on fungal pathogenicity rather than mannose residues being direct activators of the MAPK/ MKP1/c-Fos hypha-mediated immune response.

Normal Adaptation of Candida albicans to the Murine Gastrointestinal Tract Requires Efg1p-Dependent Regulation of Metabolic and Host Defense Genes

“…This category included genes responsible for N-linked glycosylation, hexosylation, and mannosylation (e.g., MNT2, MNT3, and PMT5), all of which were downregulated in cells colonizing the ileum (see Table S5 in the supplemental material). Because a decreased inflammatory response in response to C. albicans mutants defective in glycosylation was observed (75), decreased or altered patterns of glycosylation may allow colonizing cells to avoid the immune response. In addition, cells with deficiencies in mannosylation are more resistant to phagocytosis by neutrophils (76).…”

Importance of the Candida albicans cell wall during commensalism and infection

“…The mannans of yeast and hyphae differ physically and chemically (e.g. [23 ]) and this may contribute to the differential immune reactivity of these two morphological forms together with morphology associated proteins [47,48 ]. For example, surface-associated pH-regulated antigen 1 protein (Pra1), a predominantly hypha-associated protein, plays a pivotal role in the recognition of C. albicans by human neutrophils and enhances neutrophil antimicrobial responses.…”

Glycosylation of Candida albicans Cell Wall Proteins Is Critical for Induction of Innate Immune Responses and Apoptosis of Epithelial Cells

“…Subsequently, in the same model, we showed that activation of this pro-inflammatory response by C. albicans results in recruitment of polymorphonuclear (PMN) cells and protection against fungal invasion and infection in a toll-like receptor (TLR)4-dependent manner [2]. Others have also confirmed that C. albicans stimulates cytokine production in mucosal monolayer cell lines and primary mucosal cells [10], [11], [12], [13], [14], [15], [16]. However, despite recent progress, the nature of C. albicans surface moieties responsible for epithelial cell immune activation is undefined.…”