The mechanisms underlying reduced T-cell mitogenic reactivity which accompany human aging were explored in vitro using concanavalin A (Con A) to induce cell proliferation. Peak mitogenic reactivity of both unfractionated mononuclear cells (MNCs) and purified E+ cells from elderly donors was reduced compared to the corresponding cell populations from young individuals. Flow cytometric studies demonstrated that the number of cells entered into the cell cycle at the time of maximum mitogenic stimulation was reduced in the elderly. Peak response to Con A of E+ cells from young donors cocultured with monocytes from old individuals matched that obtained when these cells were cocultured with monocytes from young donors. The response to Con A of cocultures of monocytes from young donors and E+ cells of old donors merely matched the reactivity of old donors' MNCs. These data indicate that changes in intrinsic T-cell properties alone account for the reduced proliferative capacity in the elderly.
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