Loss of Tumor Necrosis Factor α Potentiates Transforming Growth Factor β-mediated Pathogenic Tissue Response during Wound Healing

Animal cornea is an avascular transparent tissue that is suitable for research on wound healing-related scarring and neovascularization. Here we show that loss of tumor necrosis factor α (TNFα) potentiates the undesirable, pathogenic response of wound healing in an alkali-burned cornea in mice. Excessive invasion of macrophages and subsequent formation of a vascularized scar tissue were much more marked in TNFα-null knockout (KO) mice than in wild-type mice. Such an unfavorable outcome in KO mice was abolished by Smad7 gene introduction, indicating the involvement of transforming growth factor β or activin/Smad signaling. Bone marrow transplantation from wild-type mice normalized healing of the KO mice, suggesting the involvement of bone marrow-derived inflammatory cells in this phenomenon. Co-culture experiments showed that loss of TNFα in macrophages, but not in fibroblasts, augmented the fibroblast activation as determined by detection of α-smooth muscle actin, the hallmark of myofibroblast generation, mRNA expression of collagen Iα2 and connective tissue growth factor, and detection of collagen protein. TNFα in macrophages may be required to suppress undesirable excessive inflammation and scarring, both of which are promoted by transforming growth factor β, and for restoration of tissue architecture in a healing alkali-burned cornea in mice.