Immunosuppressive lymphokine derived from natural suppressor cells.

Cloned natural suppressor (NS) cells derived from spleens of total lymphoid irradiated BALB/c mice were incubated with the phorbol ester, PMA, and calcimycin for 4 h. After thorough washing, the induced NS cells were incubated in serum-free medium for 24 h and the supernatants were collected. The supernatants suppressed the MLR between normal adult responder and stimulator spleen cells. There was no Ag specificity or H-2 haplotype restriction of the MLR suppression. The supernatants did not inhibit [3H]thymidine incorporation per se, because they did not suppress mitogen stimulation of spleen cells. Protease digestion of the supernatants removed the suppressive activity, and dialysis studies indicated that the molecular size of the suppressive factor was larger than 50,000 Da and smaller than 100,000 Da. The suppressive activity was stable at 56 degrees C, pH 2, for 1 h. Thus, NS cell clones can be induced to secrete an immunosuppressive lymphokine, NS factor.