Effect of cyclosporin A on human lymphocyte responses in vitro. V. Analysis of responding T lymphocyte subpopulations in primary MLR with monoclonal antibodies

The populations of T lymphocytes responding in primary MLR in the presence of cyclosporin A (CsA) were analyzed with monoclonal antibodies specific for T cell subpopulations. Initial studies revealed that a small percentage of lymphocytes in CsA-treated primary MLR undergo blastogenesis as detected in a light scattergram on the fluorescence-activated cell sorter (FACS II). In contrast, a much larger percentage of lymphocytes underwent blastogenesis in control MLR without CsA. Virtually no blast cells were detected in autologously stimulated lymphocytes treated with or without CsA. Lymphocyte populations responding in CsA-treated and control MLR harvested on day 6 of culture were windowed by light scatter into low and high light scatter populations (corresponding approximately to small and large lymphocytes, respectively) and analyzed for the presence of specific lymphocyte subpopulations with the OKT4 (specific for helper T cells), OKT8 (specific for cytotoxic/suppressor T lymphocytes), and the OKIa (specific for the backbone of the DR antigen) monoclonal antibodies. Results revealed that in the small population from control and CsA-treated MLR cultures, approximately 27.6 and 21.7% of the cells expressed the OKT8 specificity, whereas 36.1 and 38.6% of cells expressed the OKT4 specificity, respectively. A small percentage of cells in the small population expressed Ia as detected by the OKIa antibody. In the large cell fraction from the CsA-treated cultures, cells expressing the OKT4 specificity were found to be present in proportions approximately equal to those in a similar fraction from control MLR cultures, whereas a slight decrease in cells staining with the OKT8 specificity was observed in the large cell fraction from CsA-treated MLR cultures compared with the control MLR cultures. Similarly, large cells expressing the OKIa specificity were found in greater frequency in control MLR cultures (41.6%) than in CsA-treated MLR cultures (28.4%). Functional activities were assessed in these MLR cultures. Only alloantigen-induced suppressor cell activity was detected in the CsA-treated cultures in the absence of detectable cytotoxic T lymphocyte activity. Moreover, suppressor cell function in both control and CsA-treated MLR cultures could be depleted by treatment with the OKT3, OKT8, and OKIa antibodies plus C, although in some cultures, an OKT4 population appeared to be required for maximal generation or expression of suppressor cell activity. Taken together with the observation that cytotoxic T lymphocytes are not detectable in CsA-treated cultures, these results suggest that only the suppressor lymphocyte subpopulation and a small percentage of T helper cells, which may amplify suppressor cell function, are activated in primary MLR in the presence of CsA.