Decreased TcR‐CD3+ T cell numbers in healthy aged humans. Evidence that T cell defects are masked by a reciprocal increase of TcR‐CD3−CD2+ natural killer cells

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Abstract
While there is accumulating evidence to indicate the presence of functional abnormalities in T cells from aged healthy humans, their cellular basis remains unclear. By using two‐color immunofluorescence and multiparameter flow cytometry we show that (a) the number of peripheral blood antigen receptor‐positive (TcR‐CD3+) T cells is significantly lower in aged than in young adults; (b) the numbers of E‐rosette‐forming (CD2+) cells are maintained in the elderly due to a reciprocal increase in the frequency of TcR‐CD3− cells, which constitute only a minor lymphocyte subpopulation in young adults, and (c) TcR‐CD3−CD2+5− lymphocytes exhibit the phenotypic features of natural killer (NK) cells. By using functional assays we show the TcR‐CD3−CD2+16+ lymphocytes are indeed NK cells because they are activated by and lyse NK targets. In contrast, they are unresponsive to either phytohemagglutinin or mitogenic CD2 monoclonal antibody stimulation, which in turn activates TcR‐CD3+CD2+16− T cells. We conclude that the increase in TcR‐CD3−CD2+ NK cells masks the T cell reduction in aged humans by normalizing CD2+ cell frequencies. However, NK cells cannot functionally substitute the thymus‐derived lymphocytes they replace.