Detection of soluble tumor-associated antigens in serum of tumor-bearing rats and their immunological role in vivo.

Circulating soluble tumor antigens were detected in the serum of tumor-bearing rats. Sublethally irradiated W/Fu rats inoculated with syngeneic C58(NT)D Gross virus-induced lymphoma served as the source of tumor antigens. Soluble antigens were assessed by specific inhibition of the complement-mediated cytotoxicity of isogenic W/Fu anti-C58(NT)D antibodies against 51Cr tumor target cells. With a s.c. inoculum of 5 X 10(7) tumor cells, circulating tumor antigens were first detected at Day 8, and a maximum concentration was reached by Day 13 to 14, which coincided with the peak of tumor growth and was followed by the sudden death of the animals. Pooled serum from tumor-bearing rats was fractioned on Sephadex G-150 and resulted in one peak that contained all of the antigenic activity. The molecular weight of this fraction was estimated to be 50,000 to 60,000 daltons. Presensitization of normal rats with soluble tumor antigens resulted in a specific acceleration of tumor growth and delay in tumor rejection. Specificity was shown by lack of C58(NT)D tumor enhancement in rats presensitized with serum containing tumor antigens from a syngeneic but antigenically unrelated WR-6 lymphoma. The biological significance of circulating soluble tumor antigen mediating specific immunosuppression against an immunogenic tumor is discussed.