Phenotypic characterization of the post-thymic differentiation of human alloantigen-specific CD8+ cytotoxic T lymphocytes.

Cells comprising the CD8+ T cell population are heterogeneous with regard to their function, maturation, and/or expression of various membrane molecules such as the CD45R Ag. We have previously shown that the functionally distinct subsets of CD4+ lymphocytes identified by anti-CD45R mAb represent cells at different stages of an activation-dependent post-thymic differentiation pathway. In the present studies, we have analyzed the functional capabilities and the lineal relationship of the subsets of CD8+ cells defined by anti-CD45R mAb. Both the CD8+CD45R+ and CD8+CD45R- subpopulations were able to proliferate in response to PHA and anti-CD3 mAb. Similarly, both subsets contained cells with suppressor functions as well as precursor cells which could develop into alloreactive CTL. However, effector CTL derived from each precursor population expressed the CD8+CD45R- phenotype, suggesting that the expression of this Ag might be altered by cell activation or maturation. Studies of purified CD8+CD45R+ cells showed that activation by mitogens or alloantigenic stimuli resulted in a defined sequence of phenotypic changes. First, these cells acquired the expression of the CD29 antigen by a mechanism independent of cell division or the presence of exogenous IL-2. This was followed by the loss of CD45R Ag expression, an event that required exogenous IL-2. When the CD8+CD45R+ precursor cells were stimulated with allogeneic cells, the alloreactive CTL that were generated acquired the CD8+CD45R- phenotype, whereas the CD8+ cells that retained CD45R expression had no CTL activity. The ability to monitor phenotypic changes associated with the post-thymic differentiation of human CD8+ CTL offers a tool for studies of the immune response against viral infections or for assessing allograft rejection.