Necroinflammatory liver disease in BALB/c background, TGF-β1-deficient mice requires CD4⁺ T cells

The etiology of autoimmune liver disease is poorly understood. BALB/c mice deficient in the immunoregulatory cytokine TGF-β1 spontaneously develop necroinflammatory liver disease, but the immune basis for the development of this pathology has not been demonstrated. Here, we show that BALB/c-TGF-β1^-/- mice exhibit abnormal expansion in hepatic mononuclear cells (MNCs) compared with wild-type littermate control mice, particularly in the T cell and macrophage lineages. To test whether lymphocytes of the adaptive immune system are required for the spontaneous development of necroinflammatory liver disease, BALB/c-TGF- β1^-/- mice were rendered deficient in B and T cells by crossing them with BALB/c-recombinase-activating gene 1^-/- mice. BALB/c-TGF-β1^-/-/recombinase-activating gene 1^-/- double-knockout mice showed extended survival and did not develop necroinflammatory liver disease. The cytolytic activity of BALB/c-TGF-β1^-/- hepatic lymphocytes was assessed using an in vitro CTL assay. CTL activity was much higher in BALB/c-TGF-β1^-/- hepatic MNCs compared with littermate control hepatic MNCs and was particularly pronounced in the CD4⁺ T cell subset. Experimental depletion of CD4⁺ T cells in young BALB/c-TGF-β1^-/- mice prevented the subsequent development of necroinflammatory liver disease, indicating that CD4⁺ T cells are essential for disease pathogenesis in vivo. These data definitively establish an immune-mediated etiology for necroinflammatory liver disease in BALB/c-TGF-β1^-/- mice and demonstrate the importance of CD4⁺ T cells in disease pathogenesis in vivo. Furthermore, TGF-β1 has a critical role in homeostatic regulation of the hepatic immune system, inhibiting the development or expansion of hepatic cytolytic CD4⁺ T cells.