A mutant human T-cell line producing immunosuppressive factor(s).

6T-CEM-20, a subclone of a 6-thioguanine-resistant mutant derived from the human-T-cell line CEM, secreted into the medium, a high titered immunosuppressive factor specific for T cells. The cell-free supernatant was very potent in suppressing, via a noncytotoxic mechanism, mitogen-activated T-cell proliferation, cytotoxic T-cell functions, and pokeweed mitogen (PWM)-stimulated plaque-forming cells (PFC). Lower dilutions of the supernatant (10(-1)-10(-2] abrogated T-cell functions within 24 hr whereas higher dilutions (10(-3)-10(-7] required a culture period of up to 4 days with lymphocytes to arrest T-cell activities. The suppressive activity was most pronounced when the factor was added in the early part of the culture period. The factor was sensitive to heat treatment and both low and high pH (most stable at physiological pH). Preliminary purification with column chromatography indicates that the active moiety was contained in the high-molecular-weight fraction (MW greater than 200,000). Data from coculture experiments suggested that T lymphocytes, which were exposed for 5-12 hr to the active supernatant or the partially purified material, suppressed allogeneic T-cell proliferation in a dose-related manner. This suppressor factor which we called suppressor-activating factor (SAF) might have activated a suppressor population or induced the production of a suppressor factor which in turn mediated the observed suppression. Both the molecular structure and the detailed mechanism of action are under investigation.