Age-related changes in the expression of T cell activation antigens following phytohaemagglutinin stimulation

The role of accessory cells (AC) in the temporal expression of several key T-cell-activation-associated antigens has been studied in healthy aged subjects. Compared to responses seen in young adults, phytohaemagglutinin (PHA) induced weak proliferation in peripheral blood mononuclear cells from the aged and lower numbers of T cells expressing CD71, CD25, CD38 or HLA-DR. T cell responses to the monoclonal antibody OKT3, however, were normal. Whereas HLA-DR+ T cell numbers could be increased by raising the AC content (up to 50%) in cultures comprising purified T cells and graded numbers of autologous AC, CD25+ T cell numbers remained largely unaltered. Co-stimulation with PHA + phorbol myristate acetate in the absence of AC restored both proliferation and CD25 expression in the aged. These results indicate that T cells from the healthy aged show selective deficiencies in their capacity to respond to mitogenic stimuli and suggest that impaired PHA responsiveness is due, at least in part, to defective AC-derived signals.