The changes in [Ca2+]i after mitogen stimulation of individual human peripheral T cells were examined by single cell image analysis to determine the relationship between the Ca2+ signal and functional outcome. Marked heterogeneity in the magnitude of increase in [Ca2+]i, in the lag time of the responses, and in the percentage of T cells that responded to mAb to CD3 and to PHA was observed. However, mitogenic stimuli that induced IL-2 production or DNA synthesis consistently generated increases in [Ca2+]i in individual T cells that were sustained for 1 to 2 h. Soluble mAb to CD3 induced an increase in [Ca2+]i that remained elevated at 60 min and led to IL-2 production and proliferation upon costimulation by phorbol ester. In contrast, cross-linking anti-CD3 with a secondary antibody foreshortened the increase in [Ca2+]i, and IL-2 production and DNA synthesis were inhibited. Immobilized anti-CD3, which can stimulate T cell proliferation and IL-2 production in the absence of phorbol ester, produced a constant sustained elevation in [Ca2+]i that lasted more than 2 h. Similarly, functional responses could be generated by concentrations of PHA that resulted in only a slow increase in [Ca2+]i that continued to rise for 1 to 2 h. Examination of the mitogen-induced sustained increases in [Ca2+]i suggested that an elevation in [Ca2+]i as small as 50 to 100 nM above control mean [Ca2+]i was associated with evidence of T cell activation. Spontaneous oscillatory changes in [Ca2+]i were observed in a small percentage of peripheral T cells although they were noted to occur frequently in Jurkat cells. Mitogenic stimulation did not consistently increase oscillations in peripheral T cells, and neither their frequency nor their magnitude correlated with IL-2 production or DNA synthesis. These observations suggest that oscillatory changes in [Ca2+]i are not a primary determinant of T cell activation. Rather, the data indicate that functional activation of T cells by PHA and anti-CD3 is correlated with the induction of a small, but sustained increase in [Ca2+]i.

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