Also, interestingly, the anti-HMGB1 antibody and an antibody against the HMGB1 receptor, which is known as the receptor for advanced glycation end-products (RAGE), possess significant immunosuppressive activity and = 6) (*section. Con A injection. We found that injection of anti-histone H1 antibody could reduce Con A-induced liver damage. Further mechanical analyses revealed that anti-histone H1 antibody altered the intracellular activation LRRC46 antibody of mitogen-activated protein kinase, nuclear factor-B and calcineurin via T-cell receptor signalling, suggesting that anti-histone H1 antibody may protect the liver from Con A-induced injury by inhibiting activation of effector T cells. These findings suggest that anti-histone H1 autoantibody may be a natural immune regulatory factor that protects inflamed livers suffering from autoimmune hepatitis and may lead to T-cell unresponsiveness through the selective regulation of mitogen-activated protein kinase/nuclear factor-B and calcineurin signalling. Keywords: autoantibodies, autoimmunity, liver immunology/disease Introduction In certain class I and class II disparate rat strain combinations such as Dark Agouti (DA) donor and Pieball Virol Glaxo (PVG) recipient, allograft rejection is spontaneously overcome after orthotopic liver transplantation (OLT), resulting in a state of long-lasting and donor-specific tolerance without pharmacological immunosuppression, while skin, heart and renal allografts cause acute rejection.1 In contrast, it has also been shown that recipient Lewis (LEW) rats usually reject a donor DA rat liver within 14 days after OLT.2C6 In our previous reports, we provided evidence that the autoantibody against nuclear histone H1 is a major immunosuppressive factor in the serum during the rejection phase of tolerogenic OLT (DACPVG) but is not induced in rejector OLT (DACLEW).7,8 This phenomenon has also been observed in the case of a clinical drug-free patient who experienced complete cessation of immunosuppressive treatments after living related liver transplantation (LRLT). Anti-histone H1 antibody titre of this drug-free LRLT patient was higher than in healthy volunteers and other LRLT patients who were under immunosuppressive treatment.9,10 Histone H1 normally binds to the linker DNA between nucleosomes, and it is involved in the formation of higher-order chromatin structures.11 Histone H1 possesses various important functions, including a role in transmitting apoptotic signals from the nucleus to the mitochondria, which release apoptogenic factors into the cytoplasm, following DNA double-strand breaks12 and a role in normal dendritic cell (DC) differentiation, based on the evidence that the production and differentiation of DCs in histone H1-deficient mice were reduced significantly.13 Additionally, our later study suggested that the blockade of histone H1 modulates DCs toward a tolerogenic status, decreases the cytotoxicity of rat lymphokine-activated killer and human natural killer cells, and induces CD4+ CD25+ T cells.14 On the other hand, nuclear-binding protein high-mobility group box 1 protein (HMGB1), which was previously thought to function only as a nuclear factor that enhances transcription, was recently discovered to be a crucial cytokine that Hoechst 33342 analog 2 mediates the response Hoechst 33342 analog 2 to infection, injury and inflammation. 15 HMGB1 is passively released from necrotic cells; however, some cell types, including monocytes and macrophages, can secrete HMGB1 actively without dying. 16C18 Exogenous HMGB1 effectively controls DC maturation and function. Also, interestingly, the anti-HMGB1 antibody and an antibody against the HMGB1 receptor, which is known as the receptor for advanced glycation end-products (RAGE), possess significant immunosuppressive activity and = 6) (*section. *Significantly up-regulated as compared with na?ve control serum (= 6) (= 6) (*< 001, respectively). Discussion In this study, we demonstrated that anti-histone H1 antibody possesses not only an immunosuppressive activity7,23,27 but also an anti-inflammatory activity (Fig. 2). Furthermore, anti-histone H1 autoantibody was found to be expressed spontaneously in sera during the recovery stage from Con A-induced liver injury (Fig. 1), suggesting the significance of anti-histone H1 autoantibody for safety against and recovery from autoimmune hepatitis. Autoimmune liver disease is characterized by chronic active hepatitis with serum autoantibody (e.g. anti-nuclear antibody, clean muscle mass antibody, liver-kidney microsome antibody), Hoechst 33342 analog 2 hypergammaglobulinaemia and liver pathology showing necroinflammatory disease and fibrosis.28,29 The measurement of anti-nuclear antibodies is a well-known diagnostic tool for the evaluation of autoimmune disorders. In our earlier study, however, we shown that the.