Autoimmune hepatitis (AIH), like many autoimmune diseases, is most prevalent in young women. The immunological basis of this age and sex susceptibility bias was investigated in a murine model of AIH. Xenoimmunization of 7-week-old female C57BL/6 mice resulted in more severe AIH with higher levels of liver inflammation, serum alanine aminotransferase, specific T-cell cytotoxicity, and autoantibody than younger and older females. Vaccinated males developed minimal liver inflammation and higher percentages of CD4+CD25+FoxP3+ regulatory T cell in peripheral blood mononuclear cells, spleen, and liver than females. Regulatory T cells (Tregs) were virtually absent in liver-lymphocytes infiltrates of females. Castration of C57BL/6 mice, with or without 17β-estradiol supplementation, did not modify susceptibility in males, nor Treg numbers, suggesting minimal contribution of testosterone and estradiol to autoimmune hepatitis (AIH) susceptibility. Xenoimmunized Aire(+/0) mouse displayed similar AIH susceptibility, sex bias, and Tregs numbers as C57BL/6 mice, suggesting that susceptibility in females is not the result of less stringent thymic central tolerance. Autoreactive B cell response against formiminotransferase-cyclodeaminase correlated with disease activity, possibly linking B-cell autoreactivity and AIH pathogenesis. Conclusion: Peripheral tolerance and development of regulatory T cells after self-mimicking antigen exposure, and not sexual hormone nor central tolerance, are the main factors for susceptibility to AIH in females. HEPATOLOGY 2010