Thymic development in human CD4 transgenic mice: Positive selection occurs after commitment to the CD8 lineage

Developing T cells are positively selected on the basis of their recognition of polymorphic MHCs (major histocompatibility molecules). The CD4 and CD8 co-receptors, together with a compatible TCR-αβ, bind class II and class I MHC, respectively, delivering a signal that allows T cell maturation to proceed along the appropriate co-receptor lineage. During development, thymocytes first co-express CD4 and CD8 and subsequently commit to a single co-receptor phenotype. Debate exists over the stage at which positive selection occurs. Positive selection may precede or accompany co-receptor lineage commitment or occur subsequent to the acquisition of a single co- receptor phenotype. We developed transgenic (TG) mice expressing human CD4 (huCD4) on T cells. To study differentiation of CD8⁺ cells, huCD4 TG mice were bred with mice lacking class I MHC (class I^-). This enabled us to assess maturation of mCD8⁺ mCD4⁺huCD4⁺ thymocytes to mCD8⁺mCD4^-huCD4⁺ cells after interaction between huCD4 and mclass II MHC. In the absence of huCD4, class I^- mice failed to produce mCD8⁺ cells. However, expression of huCD4 allowed development of mCD8⁺mCD4^-huCD4⁺ T cells, suggesting that selection of cells precommitted to the mCD8⁺ mCD4^- lineage occurred. Importantly, CD8⁺ T cells were increased in both the thymus and the periphery of huCD4 TG x class I^- animals, indicating their intrathymic origin. Vβ14⁺ CD8⁺ T cells were also increased, as predicted for positive selection of CD8⁺ cells bearing class II-reactive TCRs. These data provide evidence that positive selection occurs at a late stage of thymocyte differentiation, after commitment to the CD8 lineage.