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Bcl-xL, a member of the bcl-2 family of cell survival genes, is induced by T cell activation, and induction is prolonged and enhanced by CD28 costimulation. To study the role of bcl-xL induction in T cell survival, and in the events collectively known as costimulation, we used mice transgenic for Bcl-xL expressed on the T cell lineage (Grillot et al., J Exp Med, 183:381). Transgene-negative littermates served as controls. In vitro experiments were performed by stimulating unfractionated lymph node cells with anti-CD3 mAb. Proliferation was measured by labeling cells with the fluorescent dye CFSE and following fluorescence loss by flow cytometry (Wells et al., J Clin Invest, 100:3173), and viability was assessed simultaneously by exclusion of the vital dye TOPRO-3. In the presence of control Ig, both wild-type and Bcl-xL T cells exhibited a similar cell survival rate of 20-50%, with the highest levels of survival occurring in cells which had undergone the most rounds of mitosis. Unexpectedly, when CTLA4Ig was added to the cultures, cell survival was not affected in wild type cells, but was markedly increased to 7O-90% in Bcl-xL T cells, an effect which was particularlv marked in the CD8 subpopulation. These results suggested that Bcl-XL expression might provide a selective survival advantage in the absence of CD28/CTLA4 mediated signals, perhaps blocking Fas mediated cell death. This effect would not be observed in the absence of CTLA4Ig, in which case negative signals through CTLA4 itself could result in bcl-xL "resistant" cell death. To determine if improved survival of Bcl -XL T cells in absence of CD28/CTLA4 signals might affect the ability to immunosuppress these mice with CTLA4IG we examined two immune response: contact hypersensitivity (CHS) and cardiac allograft rejection. While CTLA4Ig blocked CHS by 50-60% in wild type mice (p<O.O1 compared with control Ig), it had no effect in Bcl-xL mice (n=5 mice per group). Similarly, neithr CTLA4Ig nor anti-CD4OL mAb could prolong cardiac allograft survival in Bcl-xL mice (median survival time 28 days), while either agent led to prolonged survival in wild-type mice (median survival > I00 days). Together, these data suggest that induction of cell death by blocking induction of Bcl-xL is an important component of CTLA4Ig-mediated immunosuppression.