Stress induced shedding of a tumor antigen by a rat histiocytic cell line AK-5: a possible mechanism of immune evasion.

AK-5 is a highly immunogenic tumor which stimulates multiple arms of the host immune system. Tumor cells when transplanted subcutaneously grow as solid tumors which are regressed in about 70% of animals, whereas 100% i.p. transplanted animals succumb to the tumor growth. In the present study we demonstrate shedding of an antigen by AK-5 cells when subjected to stress such as low cell number transplantation in vivo, heat shock and serum starvation in vitro. The shed antigen is able to neutralize antitumor antibody in a complement fixation assay. The antigen is also detected in the cell-free ascitic fluid. The size of the shed antigen as recognized by anti-AK-5 antibody is approximately 45 kDa. We have also studied some of the biochemical characteristics of the purified shed antigen. These observations suggest a positive role for the AK-5 antigen in immune recognition which is lost in i.p. transplanted tumor cells due to the shedding of the antigen, thereby escaping the immune invasion.