THE INHIBITORY EFFECT OF CYCLOPHOSPHAMIDE-INDUCED MAC-1(-2 AND IL-4 UTILIZATION IN MLR() NATURAL SUPPRESSOR CELLS ON IL)

Treatment of adult mice with high doses of the immunosuppressive drug cyclophosphamide (CY) induces transient splenic natural suppressor (NS ) cell activity mediated largely by cells bearing the MAC-1(+) cell-su rface marker. Here we show that culture supernatants from mixed lympho cyte reactions (MLR) suppressed by MAC-1(+) NS cells exhibit decreased IL-2 and IL-4 activity in bioassays for these lymphokines. However, i nhibition of MLR was maximal whether the regulatory cells were added a t initiation of culture or 24 hr postinitiation, suggesting that inhib ition of lymphokine synthesis is not likely to be the reason for dimin ished lymphocyte proliferation, since these particular lymphokine gene s are known to be transcribed and expressed during the first 12 hr of culture. Furthermore, flow cytofluorometric analysis demonstrated that the presence of MAC-1(+) NS cells did not alter the percentage of lym phokine producing CD4(+) T cells in MLR. IL-2 receptor (p55) expressio n was also normal in suppressed MLR. The addition of exogenous IL-2 an d/or IL-4 to MLF failed to reverse the inhibitory effect of MAC-1+ NS cells on lymphocyte proliferation, indicating that these regulatory ce lls block the utilization of these lymphokines in MLR. The inhibitory effect of MAC-1(+) NS cells on lymphocyte proliferation in MLR is depe ndent on interferon-gamma, since NS activity was dramatically decrease d in the presence of neutralizing antibodies to interferon-gamma. MAC- 1(+) NS cell-induced suppression of MLR was also diminished in the pre sence of indomethacin, suggesting that prostaglandins play a role in t his NS system.