B7-1 Overexpression by Thymic Epithelial Cells Results in Transient and Long-Lasting Effects on Thymocytes and Peripheral T Helper Cells but Does Not Result in Immunodeficiency

The B7-1 (CD80) molecule provides costimulatory function for the activation of T helper lymphocytes upon encounter with antigen. To investigate the role of this molecule in thymocyte maturation, we have generated transgenic (Tg) mice in which CD80 expression is driven by the keratin 14 promoter (K14). This overexpression of CD80 resulted in the loss of detectable cell surface CD28 expression on thymocytes and a significant reduction in both the surface T cell receptor expression and the ratio of CD4+ to CD8+ single-positive thymocytes in Tg animals compared to nontransgenic (non-Tg) controls. While many of these defects were transient, the significant decrease in CD4+ versus CD8+ T cell ratio persisted peripherally. Peripheral T cells from these Tg mice were found to be significantly hyporesponsive to T cell mitogens and in mixed leukocyte reaction, effects that our data indicate are due to reduced IL-2 production by Tg T cells upon activation. Despite these functional defects, immunization with both complex and simple protein antigens produced no differences in the proliferative or humoral responses to these antigens between Tg and non-Tg groups. These data indicate that thymic CD80 signaling results in the deletion of significant numbers of CD4+ T cells but does not culminate in antigen-specific immunodeficiency.