Histone Methylation Directs Myeloid TLR4 Expression and Regulates Wound Healing following Cutaneous Tissue Injury

Myeloid cells are critical for orchestrating regulated inflammation during wound healing. TLRs, particularly TLR4, and its downstream-signaling MyD88 pathway play an important role in regulating myeloid-mediated inflammation. Because an initial inflammatory phase is vital for tissue repair, we investigated the role of TLR4-regulated, myeloid-mediated inflammation in wound healing. In a cutaneous tissue injury murine model, we found that TLR4 expression is dynamic in wound myeloid cells during the course of normal wound healing. We identified that changes in myeloid TLR4 during tissue repair correlated with increased expression of the histone methyltransferase, mixed-lineage leukemia 1 (MLL1), which specifically trimethylates the histone 3 lysine 4 (H3K4me3) position of the TLR4 promoter. Furthermore, we used a myeloid-specific Mll1 knockout (Mll1(f/f)Lyz2(cre+)) to determine MLL1 drives Tlr4 expression during wound healing. To understand the critical role of myeloid-specific TLR4 signaling, we used mice deficient in Tlr4 (Tlr4(-/-)), Myd88 (Myd88(-/-)), and myeloid-specific Tlr4 (Tlr4(f/f)Lyz2(cre+)) to demonstrate delayed wound healing at early time points postinjury. Furthermore, in vivo wound myeloid cells isolated from Tlr4(-/-) and Myd88(-/-) wounds demonstrated decreased inflammatory cytokine production. Importantly, adoptive transfer of monocyte/macrophages from wildtype mice trafficked to wounds with restoration of normal healing and myeloid cell function in Tlr4-deficient mice. These results define a role for myeloid-specific, MyD88-dependent TLR4 signaling in the inflammatory response following cutaneous tissue injury and suggest that MLL1 regulates TLR4 expression in wound myeloid cells.