Isolation and Characterization of Canine Venereal Tumor-associated Inhibitory and Blocking Factors

Spontaneous regression of the canine venereal tumor is associated with the production of a serum factor which inhibits in vitro tumor colony-forming units in agar. Logarithmic or persistent tumor growth, on the other hand, is characterized by a serum factor which protects cells against in vitro inhibition (blocking factor). These factors have been characterized by immunochemical methods. Whole regressor and blocking sera were fractionated by Sephadex G-200 filtration and immunoabsorption with rabbit antiserum specific for canine immunoglobulin G_2a. Fractions were characterized by immunoelectrophoresis, radial immunodiffusion, and disc gel electrophoresis. In vitro inhibitory and blocking activity of the whole serum was accounted for by the purified immunoglobulin G_2a. Blocking activity was also found in protein eluted from logarithmically growing tumors. Preparative polyacrylamide electrophoresis revealed five major fractions with blocking activity only in the immunoglobulin G fraction. Tumor eluates and immunoglobulin G isolated from serially removed tumors demonstrated relative inhibitory and blocking activities that correlated with the clinical course of the tumor. Using ultrafiltration and sodium dodecyl sulfate electrophoresis of tumor-associated immunoglobulin G at low pH, it was not possible to identify an antigen complexed to the blocking antibody.