Suppression of an in vitro humoral immune response by cultured fetal thymus cells.

We studied the function of fetal thymocytes, derived from thymus progenitor cells of 14-day-old fetuses in an organ culture system, in an in vitro humoral immune response. The unprimed cultured fetal thymocytes suppressed the T-dependent primary immune response to adult syngeneic spleen cells against sheep red blood cells, but failed to suppress the response against the T-independent antigen NIP-POL [4(hydroxy-3-iodo-5-nitrophenyl)acetyl-coupled polymerized flagellin]. The suppression was abolished by anti-Thy-1 serum and complement treatment. Using a differential killing procedure, the suppressor cells were characterized as "low" Thy-1 cells. In addition, they were resistant to hydrocortisone. Suppression could only be achieved when the cultured fetal thymocytes were added during the first 48 h of culture, suggesting that their action occurred during the induction phase of the response. The implications of the high frequency of suppressor cells during fetal life are discussed.