TLR4, rather than TLR2, regulates wound healing through TGF-β and CCL5 expression

Toll-like receptors (TLRs) have a crucial role in early host defense against invading pathogens. Recent studies suggest that TLRs play important roles in non-infections inflammation and tissue repair and regeneration.

To determine the roles of TLR2 and TLR4 in mouse wound healing using TLR2-deficient (TLR2(-/-)), TLR4-deficient (TLR4(-/-)), and TLR2/TLR4-deficient (TLR2/4(-/-)) mice.

Open wounds made in TLR2(-/-), TLR4(-/-), and TLR2/4(-/-) mice were examined clinically and histologically. Cytokine expression in the wounded skin was also investigated. TGF-β production from macrophages stimulated by hyaluronan, a ligand for TLR2 and TLR4, was evaluated by real-time PCR.

Wound areas in TLR2(-/-), TLR4(-/-), and TLR2/4(-/-) mice were larger than wild-type mice both at days 3 and 7 after wounding, accompanied by decreased numbers of infiltrating macrophages in the dermis and decreased TGF-β and CCL5 mRNA expression in the wounded skin. Immunohistochemistry showed decreased numbers of macrophages expressing TGF-β and reduced CCL5 expression by keratinocytes in the wounded skin from TLR2(-/-), TLR4(-/-), and TLR2/4(-/-) mice compared to wild-type mice. Moreover, TGF-β production from macrophages induced by hyaluronan stimulation in vitro was significantly decreased in the absence of TLRs, especially TLR4. Interestingly, macrophages and wounded skin from TLR2(-/-) mice showed decreased TLR4 mRNA expression compared to wild-type mice, suggesting that the effect of TLR2 deficiency was at least partially dependent on decrease in TLR4. Topical application of TGF-β and CCL5 significantly improved wound healing in TLR-deficient mice.

TLR4, rather than TLR2, regulates wound healing through TGF-β and CCL5 expression.