A 35 000 dalton T-cell supernatant protein associated with suppression of mitogenic responses

Synthesis of a CEM T-lymphoblast supernatant protein of 35 000 daltons (p35), which is scarcely expressed on CEM membranes, increased 8-fold when cells were grown in crowded cultures. Synthesis of p35 was more dependent upon crowding of cells on the flask bottom (maximum at 2.0 X 10(6) cells/cm2) than upon cell density in the culture medium (maximum at 3.0 X 10(6) cells/ml). Growth medium containing p35 inhibited the response of normal peripheral blood lymphocytes to PHA and to Con A. Supernatant media lacking p35, collected from low-density CEM cultures of from RAJI B lymphoblastoid cells which were grown under low or high-density conditions, did not suppress mitogenic responses of normal lymphocytes. These experiments support the view that the p35 T-cell supernatant protein plays a regulatory role in cell proliferation and possibly in the immune system.