Large clonal CD8⁺ T cell expansions in elderly humans display a dichotomy of differentiated states that reflect a response to persistent antigen

Healthy humans frequently develop large clonal expansions in the CD8⁺ subset of T lymphocytes. These expansions persist for years and in some individuals constitute the majority of circulating CD8⁺ T cells. Development of these expansions appears to be influenced by increasing age and may be closely related to immune dysfunction in the elderly. The etiology of these clonal expansions, however, is unknown. In this study, elderly individuals with large expansions have been identified with three color flow cytometry using anti-human CD4 and CD8 monoclonal antibodies and a panel of monoclonal antibodies directed against the variable regions of the human T cell receptor. Expansions from 5 elderly individuals were stained for CD28, a surface molecule known to be pivotal in T cell activation. Expression of CD28 is variable in these expansions. In all cases but one, a majority of the clonal cells are negative for CD28, but a small subset maintains expression of this surface marker. Expansions were isolated from peripheral blood on a fluorescence activated cell sorter, and grown in bulk or limiting dilution culture. CD28⁺ cells within the expansion have a high proliferative capacity and can propagate the expansion in bulk culture. CD28⁺ cells however, proliferate poorly and express intracellular perforin, a marker of cytotoxic function. These data demonstrate that a functional heterogeneity exists within clonal CD8⁺ T cell expansions, and they suggest that large clonal expansions in the blood of elderly individuals represent the development of a cytotoxic response to persistent antigens.