Regulation of Th17 differentiation by epidermal fatty acid-binding protein

Epidermal fatty acid-binding protein, E-FABP, a lipid chaperone, has been shown to regulate the inflammatory function of macrophages and dendritic cells. Herein, we demonstrate that T cell expression of E-FABP promotes Th17 differentiation, while counterregulating development of FoxP3⁺ regulatory T cells (Tregs). In response to immunization with myelin oligodendrocyte glycoprotein peptide (MOG_35-55), E-FABP-deficient mice generated reduced levels of Th17 cells and elevated levels of Tregs, as compared with wild-type mice. Likewise, naive CD4⁺ T cells isolated from E-FABP-deficient mice showed reduced expression of IL-17 and enhanced expression of FoxP3, in vitro, when subjected to Th17 or Treg polarizing conditions, respectively. It has been demonstrated previously that IL-21, induced by IL-6, stimulates the expression of the nuclear receptors retinoic acid-related orphan receptor (ROR)γt and RORα, which in turn induce expression of IL-17. We found that the impaired Th17 differentiation by E-FABP-deficient CD4⁺ T cells was associated with lower levels of IL-21 expression in response to IL-6, as well as reduced expression of RORγt and RORα. However, E-FABP-deficient CD4⁺ T cells expressed significantly higher levels of the nuclear receptor peroxisome proliferator-activating receptor (PPAR)γ than did wild-type CD4 ⁺ T cells, and treatment with the PPARγ antagonist GW9662 restored expression of IL-21, RORγt, RORα, and IL-17 by E-FABP-deficient T cells to wild-type levels. The negative influence of E-FABP deficiency on IL-17 expression was attributed to PPARγ-mediated suppression of IL-6-induced STAT3 activity. Thus, taken together, our data indicate that expression of E-FABP by CD4⁺ T cells contributes to the control of IL-6 stimulation of the IL-21/ROR/IL-17 pathway and to the Th17/Treg counterbalance.