Interleukin-2 stimulus-response coupling is calcium independent

This study investigated the role of calcium (Ca2+) as a second messenger in the stimulus-response coupling of interleukin 2 (IL 2) binding to its specific receptor that results in lymphocyte proliferation. Although the Ca2+ channel blockers verapamil, nifedipine, and diltiazem induced a dose-dependent inhibition of [3H]-thymidine incorporation by HT-2 cells in response to recombinant human and purified rat IL 2, the stimulation indices of the treated and untreated cells were equivalent. Stimulation of HT-2 cells with recombinant human IL 2 (rIL 2) did not result in an increase in the concentration of intracellular free Ca2+ [( Ca2+]i), using the fluorescent intracellular Ca2+ indicator Fura-2AM. Conversely, partially purified rat IL 2 did cause an increase in [Ca2+]i that was not inhibited by the channel blockers or by chelation of extracellular free Ca2+. Thus, contaminating components in the partially purified rat IL 2, and not the IL 2 itself, resulted in increased [Ca2+]i by mobilization from intracellular stores. These results demonstrate that inhibition of lymphocyte proliferation by verapamil, nifedipine, and diltiazem is not due to an uncoupling of the IL 2 lymphokinetic signal, and stimulation of HT-2 cells using rIL 2 does not increase [Ca2+]i, and thus does not employ Ca2+ as a second messenger.