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 CD8(lo). In this report, we have analyzed phenotypic, functional, and developmental characteristics of these 'late' CD4(hi) SP thymocyte subsets. The TCR(hi) phenotype and the elimination of T cells expressing TCR V(β) 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 TCR cross-linking. By contrast, CD8(lo)4(hi) cells could not induce GVHD, were unable to expand (and perhaps even survive) in peripheral organs and underwent apoptosis upon TCR 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 CD4(hi) SP postselection thymocytes, and demonstrate the existence of a previously unrecognized control point in T cell development.