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Figure S9. Model depicting the proposed role of commensal bacteria in the regulation of TH17 cells in the intestine. Interactions between commensal bacteria and intestinal epithelial cells (1) lead expression of IL-25 (2). Although the factors that induce expression of IL-23 in the absence of commensal bacteria are undefined, IL-25 acts to limit IL-23 expression by macrophages (3) and the subsequent survival and/or expansion of TH17 cells in the intestine (4).