The bcl-2 gene encodes an intracellular, membrane-associated protein that protects immature cortical thymocytes from a wide variety of apoptotic stimuli, including glucocorticoids, radiation, and anti-CD3 treatment. Since cortical thymocytes are the primary target cells for thymic positive and negative selection processes, and since these processes are associated with cell death, we evaluated the role of bcl-2 in T cell development in two ways. In the first approach, transgenic mice expressing high levels of Bcl-2 in cortical thymocytes were mated with H-Y T cell receptor (TCR) transgenic mice, the latter being a well-defined system for the study of positive and negative selection of T cells. We found that the bcl-2 transgene had a dramatic effect on positive selection. This was manifested by a greatly increased production of mature thymocytes that were highly skewed towards the CD4-8+ lineage. The change involving CD4-8+ thymocytes occurred not only in bcl-2 transgenic mice, but was also observed in H-Y TCR/bcl-2 doubly transgenic mice, regardless of whether the H-Y TCR was expressed in the selecting (H-2b) or nonselecting (H-2d) environments. Furthermore, a large proportion of CD4-8+ thymocytes produced in H-2b H-Y TCR/bcl-2 doubly transgenic female mice expressed endogenous TCR alpha chains rather than the transgenic TCR alpha chain. These observations are consistent with the model that high expression of Bcl-2 in cortical thymocytes overrides the normal apoptotic pathway. This then allows the selection of CD4-8+ thymocytes expressing TCRs that are otherwise nonselectable. However, the bcl-2 transgene did not protect CD4+8+ thymocytes expressing the male-specific TCR from deletion in male doubly transgenic mice. In the second approach, we determined the level of bcl-2 mRNA expression in populations of thymocytes defined by their CD4/CD8 phenotypes using quantitative reversed transcriptase PCR techniques. Our results indicate that bcl-2 mRNA was expressed at a high level in immature CD4-8- thymocytes and in mature CD4+8- thymocytes. There is a dramatic downregulation of bcl-2 mRNA in CD4+8+ thymocytes, particularly those expressing a low level of TCR. CD4+8+ thymocytes that upregulated their TCR, likely as a result of receiving positive selection signals, also upregulated bcl-2 mRNA. This observation suggests that rescue of immature thymocytes from the programmed cell death pathway by positive selection signals is accompanied by the upregulation of bcl-2 mRNA.

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