TLR3 drives IRF6-dependent IL-23p19 expression and p19/EBI3 heterodimer formation in keratinocytes

Interferon regulatory factor (IRF) family members impart cell-type specificity to toll-like receptor (TLR) signalling, and we recently identified a role for IRF6 in TLR2 signalling in epithelial cells. TLR3 has a well-characterized role in wound healing in the skin, and here, we examined TLR3-dependent IRF6 functions in human keratinocytes. Primary keratinocytes responded robustly to the TLR3 agonist poly(IC) with upregulation of mRNAs for interferon-β (IFN-β), the interleukin-12 (IL-12) family member IL-23p19 and the chemokines IL-8 and chemokine (C-C motif) ligand 5 (CCL5). Silencing of IRF6 expression enhanced poly(IC)-inducible IFN-β mRNA levels and inhibited poly(IC)-inducible IL-23p19 mRNA expression in primary keratinocytes. Consistent with these data, co-transfection of IRF6 increased poly(IC)-inducible IL-23p19 promoter activity, but inhibited poly(IC)-inducible IFN-β promoter activity in reporter assays. Surprisingly, poly(IC) did not regulate IL-12p40 expression in keratinocytes, suggesting that TLR3-inducible IL-23p19 may have an IL-23-independent function in these cells. The only other IL-12 family member that was strongly poly(IC) inducible was EBI3, which has not been shown to heterodimerize with IL-23p19. Both co-immunoprecipitation and proximity ligation assays revealed that IL-23p19 and EBI3 interact in cells. Co-expression of IL-23p19 and EBI3, as compared with IL-23p19 alone, resulted in increased levels of secreted IL-23p19, implying a functional role for this heterodimer. In summary, we report that IRF6 regulates a subset of TLR3 responses in human keratinocytes, including the production of a novel IL-12 family heterodimer (p19/EBI3). We propose that the TLR3-IRF6-p19/EBI3 axis may regulate keratinocyte and/or immune cell functions in the context of cell damage and wound healing in the skin.