Effect of splenectomy upon tumor growth: characterization of splenic tumor-enhancing cells in vivo.

The effect of splenectomy upon neoplastic outgrowth was examined prior to and after implantation of methylcholanthrene-induced C3H/HeJ murine tumors. Splenectomy or sham operation performed 6 and 3 days prior to tumor inoculation significantly facilitated tumor growth compared to nonoperated, control mice. Operative procedures 12 days prior to tumor inoculation had no effect on tumor growth rate, suggesting that facilitated tumor growth was related to surgically induced, transient immunosuppression, rather than to the presence or absence of splenic tissue. On the other hand, 3 days after tumor inoculation, sham operations resulted in significant facilitation, but splenectomy yielded retardation of tumor growth. In local adoptive transfer assays, spleen cells from hosts bearing MCA-F tumors for 3 days nonspecifically facilitated the outgrowth of the antigenically noncrossreactive tumors MCA-F, MCA-D and MCA-C. These findings suggest that the spleen is a reservoir of suppressor cells during early stages of tumor growth, since the suppressor activity was not demonstrate 6 or 12 days after tumor inoculation. The nonspecific splenic suppressor cells were radioresistant (700 rads), capable of phagocytizing carbonyl-iron and adherent to plastic dishes. These findings suggest that perioperative immunodepression resulting in facilitated tumor growth is due to transient nonspecific activation of splenic suppressor macrophages.