Membrane-associated antigen from the SV40-induced hamster fibrosarcoma, para-7. I. Role in immune complex formation and effector cell blockade.

PARA-7 membrane-associated antigen was prepared by treatment of the tumor cells with pH 9.4 glycine buffer, or by concentration of spent cell culture medium. When admixed with sensitized effector cells, both preparations could specifically block cellular cytotoxicity for PARA-7 target cells. Pretreatment of target cells with antigen did not result in blocking. Incubation of antigen extract with simian virus 40 (SV40) anti-serum caused neutralization of antibody-dependent cellular cytotoxicity (ADCC) with concomitant formation of a factor which blocked at the target cell level but not at the effector cell level. Serum from tumor-bearing hamsters exhibited blocking characteristics comparable to those of the antigen-SV40 teristics comparable to those of the antigen-SV40 antiserum preparation. Washing experiments indicated that PARA-7 antigen was more efficient than PARA-7 antigen-antibody complexes in blocking cell-mediated immunity in vitro. Material extracted from untransformed hamster embryo fibroblasts either by itself or when admixed with SV40 antiserum exhibited no significant blocking activity. These observations support the concept that loss of serum ADCC during progressive tumor growth is due to immune complex formation.