Increased Bleomycin-Induced Skin Fibrosis in Mice Lacking the Th1-Specific Transcription Factor T-bet

Abstract

Fibrosis, the pathological hallmark of scleroderma and related conditions, is due to sustained activation of tissue fibroblasts. Accumulating evidence implicates cytokine networks in initiating, and propagating or terminating fibroblast activation, and the specific cytokine phenotype dictates evolution of the fibrotic response toward either resolution or scarring. In particular, cytokines that promote fibroblast proliferation and myofibroblast differentiation and extracellular matrix (ECM) accumulation functionally define a type 2 (Th2) immune response, whereas interferon-γ, which suppresses diverse fibroblast activities, defines a type 1 (Th1) immune response. It remains unclear what role the balance between Th1 and Th2 cytokines plays in the pathogenesis of fibrosis. Here we used bleomycin-induced skin fibrosis as a murine model for human scleroderma in order to study the fibrotic response in mice lacking T-bet, a transcription factor that is essential for initiating Th1 lineage development of CD4+ T lymphocytes. Spleen cells from T-bet null (T-bet^–/–) mice exhibited a typical Th2 cytokine profile ex vivo, with elevated production of interleukin-4 (IL-4), IL-5 and IL-13, and diminished production of interferon-γ. Bleomycin-induced early mast cells and eosinophil accumulation, and eosinophil degranulation, in the lesional tissue were greater in T-bet^–/– mice than in wild-type control mice. At a later time point, T-bet^–/– mice developed significantly more extensive dermal and especially hypodermal fibrosis. Elevated TGF-β expression and intracellular Smad activation were prominent in lesional skin. Infiltrating eosinophils appeared to be an important cellular source of TGF-β. These results demonstrate that in mice lacking T-bet bleomycin induced exaggerated skin fibrosis, suggesting that T-bet has an important physiologic role in regulation of tissue repair by promoting Th1 immune responses that prevent excessive ECM accumulation.

Systemic sclerosis: a prototypic multisystem fibrotic disorder

“…Animal studies provide support for the role of a polarized immune response in the pathogenesis of fibrosis. For example, Th2-polarized cells can induce fibrosis when passively transferred in vivo (41), and mice lacking the Th1-specific transcription factor T-bet show exaggerated skin fibrosis in response to bleomycin (42). Patients with SSc display a relative shift in the Th1-Th2 cytokine balance toward Th2 predominance.…”

CD8+ T cells in systemic sclerosis

“…Animal studies provide support for the role of a polarized immune response in the pathogenesis of fibrosis. Transcriptome analysis in animal models of inflammation has shown that genes involved in wound healing and fibrosis are associated with Th2 polarized responses [30,31], and IL-13 was shown to have an important role in the mouse model of bleomycin-induced fibrosis [32,33].…”

Effector CD8+ T cells in systemic sclerosis patients produce abnormally high levels of interleukin‐13 associated with increased skin fibrosis

“…Type 2 polarized T cells secrete large amounts of IL-4, IL-5, and IL-13 and low levels of type 1 cytokines, such as interferon-␥ (IFN␥) (18). Findings in animal studies provide support for the role of a polarized immune response in the pathogenesis of fibrosis (19,20). Although reports concerning the balance of type 1 and type 2 cytokines in SSc patients are conflicting, most studies suggest that the immunopathologic response is dominated by type 2 cytokines (21-30).…”