AbstractNZB mice spontaneously develop an autoimmune disease characterized by production of anti-RBC, -lymphocyte, and -ssDNA Abs. Evidence suggests that the NZB mouse strain has all of the immunologic defects required to produce lupus nephritis but lacks an MHC locus that allows pathogenic anti-dsDNA Ab production. The capacity to produce diverse autoantibodies in these mice raises the possibility that they possess a generalized defect in self-tolerance. To determine whether this defect is found within the T cell subset, we backcrossed a transgene encoding bovine insulin (BI) onto the NZB background. In nonautoimmune BALB/c mice, the BI transgene induces a profound but incomplete state of T cell tolerance mediated predominantly by clonal anergy. Comparison of tolerance in NZB and BALB/c BI-transgenic mice clearly demonstrated that NZB T cells were at least as tolerant to BI as BALB/c T cells. NZB BI-transgenic mice did not spontaneously produce anti-BI Abs, and following antigenic challenge, BI-specific Ab production was comparably reduced in both BI-transgenic NZB and BALB/c mice. Further, in vitro BI-specific T cell proliferation and cytokine secretion were appropriately decreased for primed lymph node and splenic T cells derived from NZB BI-transgenic relative to their nontransgenic counterparts. These data indicate that a generalized T cell tolerance defect does not underlie the autoimmune disease in NZB mice. Instead, we propose that the T cell-dependent production of pathogenic IgG autoantibodies in these mice arises from abnormal activation of T cells in the setting of normal but incomplete tolerance.