Mode of action of immunosuppressive substances in sera of tumor-bearing mice.

Sera from mice with transplanted 3-methylcholanthrene (MCA)-induced tumors inhibited the proliferative response of normal mouse lymphocytes to mitogens and allogeneic cells. The sera were not toxic to these cells and did not inhibit mitogen responses by an increased binding of mitogen to serum components. The sera could have prevented the initiation of the proliferative response or could have inhibited cells already proliferating. The uptake of 3H-uridine, an event preceding DNA synthesis, was also suppressed. The sera had no effect on proliferation of a normal tissue culture line or an allogeneic tumor cell line induced by MCA. However, sera from tumor-bearing mice slowed growth of the autochthonous tumor cells and allogeneic lymphoma cells but did not completely block their proliferation. Exposure of lymphoid cells to the sera for a period as brief as 1 hour markedly decreased the ability of cells to respond subsequently to mitogens, and washing of the cells did not restore that ability. The inhibitory activity of the sera was only partially removed by absorption with lymphoid cells or cell lines.