Previously it has been shown that thymocytes undergo apoptosis, a form of programmed cell death, in response to glucocorticoids. This classic form of apoptosis is prevented by inhibition of protein synthesis. The current paper demonstrates that mature T cells also undergo apoptosis, but that the regulation of apoptosis in spleen T cells differs from that of thymocytes. Mature mouse spleen T cells were shown to die by apoptosis, not necrosis, when cultured without an added stimulus. Assays for apoptosis included internucleosomal DNA cleavage by gel electrophoresis, percent fragmentation of DNA by the diphenylamine method, and percent of cells with hypodiploid DNA by flow cytometry. The percent of apoptotic cells was 2% in fresh spleen T cells, and increased at least until 16 h, when 21% were apoptotic. Dexamethasone caused apoptosis in both thymus and spleen T cells, but only thymocytes showed a requirement for protein synthesis in dexamethasone-induced death. Cycloheximide increased apoptosis in spleen T cells, indicating that apoptosis was controlled by newly synthesized protective proteins. Spontaneous apoptosis was decreased in spleen T cells by protein kinase C activation, and was increased by H7 and staurosporine, which inhibits protein kinases, in contrast with the behavior of thymocytes. The protein kinase A/G inhibitor HA1004 also decreased spleen T cell apoptosis. The contrasting effects of cycloheximide on thymocytes and spleen T cells occurred over the same concentration range, and the same was true for PMA. The dexamethasone dose-response curves were similar, except that a greater proportion of spleen T cells were dexamethasone-resistant. These data support the hypothesis that the apoptosis program in T cells undergoes a transition during their maturation, such that apoptosis in mature T cells is regulated more like that of mature B cells than that of thymocytes.

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