Characterization of lymphocyte inhibition by supernatants of crowded lymphocytoblasts

Crowded lymphocytoblasts produce a factor (or factors) that inhibits the response of peripheral blood lymphocytes to mitogens. This factor is destroyed by pepsin in 1 hr but not by neuraminidase. No activity was lost when the factor was incubated with packed human red blood cells, human lymphocytes, mouse spleen cells, or tissue culture cells. An antiserum was made to the factor which was capable of partially inhibiting its activity. The factor is active at about 25 mug/ml of protein. Maximal inhibition occurs if the factor is present for the duration of the culture period. This factor may play a regulatory role in the immune response.