Suppression of lymphoproliferation by high concentrations of normal human mononuclear leukocytes

Mononuclear leukocytes (MNL) include cells that suppress lymphoproliferation in unstimulated and antigen-stimulated cultures. Suppression is demonstrated by increasing the concentration of cells added to cultures and does not require preactivation of suppressor cells. Suprression of 3H-thymidine incorporation occurred if the high concentrations of MNL were added to cultures before the proliferative responses commenced. This suppressive effect of high cell concentration upon 3H-thymidine incorporation is removed by depleting MNL present in high numbers of cells that adhere to foreign surfaces or by preincubating these cells with cycloheximide, puromycin, or pactamycin. The suppressor cell, which only functions when present in a viable state, is radioresistant, adheres to foreign surfaces, remains active through 5 days in culture, and equates with the presence of a cell that is rich in cytoplasmic esterase. The suppressor cell may be of the monocyte series and did not appear to belong to either the T or the B lymphocyte series. This study provides additional evidence that normal immune reactivity in man is under regulatory control. The suppressor mechanism identified herein with normal human MNL is probably related to a similar type of suppression (but at a much lower cell concentration) that has been described with Hodgkin's disease and solid tumors.