The majority of postselection CD4⁺ single-positive thymocytes requires the thymus to produce long-lived, functional t cells

We have previously isolated, and characterized in vitro, two subsets of CD4^hi T cell receptor (TCR)^hi single positive (SP) thymocytes: CD8⁻ and CD8¹⁰ In this report, we tlave analyzed phenotypic, functional, and developmental characteristics of these "late" CD4^hi SP thymocyte subsets. The TCK h-phenotype and the elimination of T cells expressing TCK Vß segments reactive with endogenous mouse mammary tumor virus (MMTV) products suggested that both subsets had undergone positive and negative selection. CD8-4^hi thymocytes were functional, as judged by their ability to: (a) induce lethal graft versus host disease (GVHD); (b) survive and expand in peripheral lymphoid organs; and (c) proliferate, rather than undergo apoptosis, in response to in vitro TCK cross-linking. By contrast, CD8¹⁰4^hi cells could not induce GVHD, were unable to expand (and perhaps even survive) in peripheral organs and underwent apoptosis upon TCK cross-linking. However, when reintroduced into the thymus, these cells matured into functional, long-lived CD8-4^hi lymphocytes. These results document an obligatory requirement for the thymic microenvironment in the final maturation of the majority of CD8¹⁰4^hi SP postselection thymocytes, and demonstrate the existence of a previously unrecognized control point in T cell development.