Commensal fungi and their cell-wall β-glucans direct differential responses in human intestinal epithelial cells

Intestinal epithelial cells (IECs) are the first to encounter luminal antigens and play an active role in intestinal immune responses. We previously reported that beta-glucans, major fungal cell-wall glycans, induced chemokine secretion by IEC lines in a Dectin-1- and Syk-dependent manner. Here, we show that in contrast to beta-glucans, stimulation of IEC lines with Candida albicans and Saccharomyces cerevisiae did not induce secretion of any of the proinflammatory cytokines IL-8, CCL2, CXCL1, and GM-CSF. Commensal fungi and beta-glucans activated Syk and ERK in IEC lines. However, only beta-glucans activated p38, JNK, and the transcription factors NF-kappa B p65 and c-JUN, which were necessary for cytokine secretion. Furthermore, costimulation of IEC lines with beta-glucans and C. albicans yielded decreased cytokine secretion compared to stimulation with beta-glucans alone. Finally, ex vivo stimulation of human colonic mucosal explants with zymosan and C. albicans, leads to epithelial Syk and ERK phosphorylation, implying recognition of fungi and similar initial signaling pathways as in IEC lines. Lack of cytokine secretion in response to commensal fungi may reflect IECs' response to fungal glycans, other than beta-glucans, that contribute to mucosal tolerance. Skewed epithelial response to commensal fungi may impair homeostasis and contribute to intestinal inflammation.