Transcription factor T-bet regulates skin sclerosis through its function in innate immunity and via IL-13

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Abstract
Tissue remodeling with fibrosis is a predominant pathophysiological mechanism of many human diseases. Systemic sclerosis is a rare, often lethal, disorder of unknown etiology manifested by dermal fibrosis (scleroderma) and excessive connective tissue deposition in internal organs. Currently, there are no available antifibrotic therapeutics, a reflection of our lack of understanding of this process. Animal models of scleroderma are useful tools to dissect the transcription factors and cytokines that govern fibrosis. A disproportionate increase of type 2 cytokines, like TGF-β and IL-4, more than type 1 cytokines, like IFN-γ, is thought to underlie the pathogenesis of scleroderma. In this study, we show that mice deficient in the transcription factor T-box expressed in T cells (T-bet), a master regulator of type 1 immunity, display increased sensitivity to bleomycin-induced dermal sclerosis. Despite the well-established role of T-bet in adaptive immunity, we also show that RAG2 −/− mice, which lack T and B cells, are vulnerable to bleomycin-induced scleroderma and that RAG2/T-bet double-deficient mice maintain the increased sensitivity to bleomycin observed in T-bet −/− mice. Furthermore, overexpression of T-bet in T cells does not affect the induction of skin sclerosis in this model. Lastly, we show that IL-13 is the profibrotic cytokine regulated by T-bet in this model. Together, we conclude that T-bet serves as a repressor of dermal sclerosis through an IL-13-dependent pathway in innate immune cells. T-bet, and its transcriptional network, represent an attractive target for the treatment of systemic sclerosis and other fibrosing disorders.