Immunosuppression induced in vitro by mastocytoma tumor cells and cell-free extracts.

Suppressed anti-sheep RBC responses occurred when spleen cells from mastocytoma-bearing mice were incubated in vitro and immunized with red blood cells. Marked immunosuppression also occurred when mastocytoma cells were added to spleen cell cultures from normal mice immunized in vitro with the sheep RBC. Suppressed immune responses also occurred when the mastocytoma cells were separated from the normal cells by 0.4-mu nucleopore membranes, but not by dialysis membranes in double chambered culture vessels. Cell-free homogenates prepared from mastocytoma cells, as well as ascitic fluid from tumor-bearing mice, also impaired the responsiveness of normal splenocytes to the SRBC. The suppressive activity of the cell-free homogenates was abolished by heating at 56 degrees C for 30 min. The immunosuppressive activity, however, was retained in supernatant ultracentrifugates of the cell-free homogenate after 100,000 times G for 90 min. Immunosuppression was most evident when mastocytoma extracts were added to cultures of normal splenocytes at the time of in vitro immunization and culture initiation. Suppression was not reversed by washing of the cultured cells and addition of fresh medium. Immunosuppression also occurred when mastocytoma extracts were incubated with spleen cells from allogeneic tumor resistant C57BL/6 mice. Additional physicochemical analyses should provide information as to the nature and specificty of the inhibitory factor(s) and its role as an immunoregulatory substance important in the host-tumor relationship.