The Role of Regulatory T Cells and T Cell Cytokines in Skin Wound Repair: Tregs in wound healing

Abstract

Successful wound healing in skin depends on the interplay of a multitude of different cell types, including immune cells, fibroblasts and keratinocytes, which are responding to signals from chemokines, cytokines and growth factors. Activin A is a growth and differentiation factor that is strongly induced at the wound site rapidly after injury and promotes reepithelialization and granulation tissue formation to achieve wound closure and restore tissue integrity. Our laboratory previously showed that overexpression of activin under control of the keratin 14 promoter in the basal layer of the epidermis in mice leads to accelerated wound closure, but at the same time predisposes to an enhanced fibrotic response at later stages of repair. Since modulation of regulatory T cells (Tregs) has been implicated in pro-fibrotic effects associated with tissue repair/remodeling, we investigated if these cells mediate at least in part the pro-healing effects of activin. Therefore, we depleted Tregs in mice that overexpress activin and also in control mice and evaluated the consequences on the wound healing process. Consistent with previous studies, activin accelerated reepithelialization and granulation tissue formation. However, both processes were strongly impaired in the absence of Tregs, independent of the activin transgene. These results demonstrate that Tregs promote wound healing in normal mice and contribute to the healing- promoting effect of activin. To determine the effect of Treg depletion on other immune cells, we established a protocol that allows efficient and reproducible isolation of different cell types from skin and wounds for further flow cytometry analysis. Using this protocol we demonstrated that Treg-depleted wounds show Summary 7 increased infiltration by αβ T cells, both CD4+ and CD4- cells. Since a large fraction of these cells expressed T-bet, but another group expressed GATA-3, these αβ T cells constitute a mixed Th1/Th2-prone population that is responsible for major changes in the cytokine profile of these wounds. While the pro-inflammatory cytokines IL-6 and TNF were detected at elevated levels in wounds compared to non-injured skin in all groups of mice, levels of IL-4 were significantly increased in wounds of Treg-depleted mice. Since overexpression of IL-4 was previously shown to inhibit wound healing, these findings provide a likely explanation for the wound healing defect in Treg depleted mice. We also investigated the importance of cytokine signaling on wound healing in mice deficient for IL-4Rα, IL-22, IFNγ, IL-17A or both IFNγ and IL-17A in comparison to wild-type mice and found that wound closure was improved by a lack in IL-4Rα or IL-22, but strongly impaired in the absence of IL-17A. While the molecular and cellular mechanisms of these alterations remain to be determined, these findings unravel important activities of T cell cytokines in the wound healing response.