Candida albicans cell wall glycosylation may be indirectly required for activation of epithelial cell proinflammatory responses

Oral epithelial cells discriminate between the yeast and hyphal forms of Candida albicans via the mitogen-activated 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 beta-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 beta-glucan and N-and O-mannan mutants, we found that N-mannosylation (via Delta och1 and Delta pmr1 mutants) and O-mannosylation (via Delta pmt1 and Delta mnt1 Delta mnt2 mutants), but not phosphomannan (via a Delta mnn4 mutant) or beta-1,2 mannosylation (via Delta bmt1 to Delta 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 alpha-1,6-mannose backbone of the N-linked outer chain (corresponding to a Delta mnn9 mutant) may be required for epithelial adhesion, while the alpha-1,2-mannose component of phospholipomannan (corresponding to a Delta mit1 mutant) may contribute to epithelial cell damage. beta-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.