A self-reactive T cell population that is not subject to negative selection

In male mice expressing a transgenic alpha beta TCR which recognizes a male antigen (HY), T cells which do not express normal levels of CD8 escape thymic deletion and appear in the periphery. These consist of two distinct populations, one which lacks expression of both CD4 and CD8, and one with low levels of CD8. Neither population has anti-HY reactivity, consistent with the known requirement of this TCR for CD8. We now describe the consequences of expression of both the anti-HY TCR transgene and a constitutive CD8.1 transgene on T cells of male mice. Peripheral T cells in these male 'double transgenic' mice express both the anti-HY TCR and normal levels of CD8, and can proliferate to male antigen in vitro. These cells do not express the endogenous allele of CD8 (CD8.2), suggesting that the increase in CD8 levels due to the CD8.1 transgene leads to the deletion of the CD8.2low population. In contrast, the CD8.1 transgene does not lead to the deletion of the CD8.2- population. This implies that, unlike the majority of alpha beta T cells, TCR+CD4-CD8- cells in TCR transgenic mice are not subject to deletion.