Promiscuous activation and cell cycle entry in T cells from autoimmune animals

Genetic control of T and B lymphocyte activation in nonobese diabetic mice

Type 1 diabetes in nonobese diabetic (NOD) mice is characterized by the infiltration of T and B cells into pancreatic islets. T cells bearing the TCR Vbeta3 chain are disproportionately represented in the earliest stages of islet infiltration (insulitis) despite clonal deletion of most Vbeta3(+) immature thymocytes by the mammary tumor virus-3 (Mtv-3) superantigen (SAg). In this report we showed that a high frequency of NOD Vbeta3(+) T cells that escape deletion are activated in vivo and that this phenotype is linked to the Mtv-3 locus. One potential mechanism of SAg presentation to peripheral T cells is by activated B cells. Consistent with this idea, we found that NOD mice harbor a significantly higher frequency of activated B cells than nondiabetes-prone strains. These activated NOD B cells expressed cell surface molecules consistent with APC function. At the molecular level, the IgH repertoire of activated B cells in NOD mice was equivalent to resting B cells, suggesting a polyclonal response in vivo. Genetic analysis of the activated B cell phenotype showed linkage to Idd1, the NOD MHC haplotype (H-2(g7)). Finally, Vbeta3(+) thymocyte deletion and peripheral T cell activation did not require B cells, suggesting that other APC populations are sufficient to generate both Mtv-3-linked phenotypes. These data provide insight into the genetic regulation of NOD autoreactive lymphocyte activation that may contribute to failure of peripheral tolerance and the pathogenesis of type I diabetes.

The identification of a novel T cell activation state controlled by a diabetogenic gene

The cyclin-dependent kinase inhibitor p27(kip) regulates the cell cycle at the G(1)-S phase restriction point. S phase entry and cell cycle commitment in peripheral T cells requires p27(kip) degradation, normally initiated by the receipt of costimulatory signals such as those provided by B7.1 or IL-2. We have previously reported that T cells from BioBreeding (BB)-diabetes-prone (DP) rats exhibit decreased costimulatory requirements for activation and cell cycle entry. In the present study, we find that peripheral T cell subsets from BB-DP rats demonstrate activation-like characteristics, including significantly reduced levels of p27(kip) as well as increased levels of proliferating cell nuclear Ag (PCNA). Since our previous studies have established that expression of extracellular activation markers are relatively low in unmanipulated peripheral BB-DP T cells; this p27(low) PCNA(high) phenotype represents a novel activation state. Analyses of T cell subsets from congenic rats demonstrate that this phenotype segregates with the lyp diabetogenic locus and that the p27(low) PCNA(high) phenotype is T cell specific. This p27(low) PCNA(high) phenotype is not seen in medullary thymocytes, but appears abruptly in the recent thymic emigrant population, suggesting that the lyp locus does not act directly on cell cycle regulators but rather alters the interaction between T cells and the peripheral environment. These results provide a biochemical basis for costimulation-independent activation and suggest a mechanism whereby a diabetes susceptibility gene contributes to disease development.