Humoral Components in Tumor-bearing Animals That Inhibit the Appearance of Immunogenicity on Tumor Cells1

actively proliferating. When the cell growth in vivo becomes stationary, sialic acid is added to the galactose on the nonreducing end (17). The antigen molecule was extracted from MM2 cells by hypotonic incubation and was partially purified by sucrose density gradient centrifugation (18). The humoral factor has a specific binding affinity mainly to the saccharide moiety of the antigen molecule. Because of this affinity, the factor can cause specific agglutination of MM2 cells in vitro (19) and, therefore, is called the MM2specific agglutination factor. This agglutination factor is part of the a-globulin fraction of the serum protein from MM2-bearing or MM2-regressor animals and is not cyto toxic to the tumor cells (16). A galactose residue at the nonreducing terminus of the saccharide moiety of the antigen is essential for the binding of the agglutination factor to the antigen (17). Besides the cell line-specific antigen, MM2 cells have surface antigens related to the MTV2 that induced the original tumor in C3H/He mice and also tumor-associated embryonic antigens. Agglutination factors specific for each of them were found in the sera from tumor-bearing and tumor-regressor animals (18). The cell line-specific antigen is by far the most immunogenic of all the substances of these 3 groups (18). Such cell line specific antigens with strong immunogenicity were also found in other ascitic tumor cell lines with near-tetraploid chromosome numbers, such as MM46 and FM3A/R (18). The isolated MM2-specific antigen did not have apprecia ble immunogenicity by itself in syngeneic animals. Specific immunity against transplantation of MM2 cells, however, was induced in C3H/He mice by immunizing the animals with a complex formed by the isolated antigen and the agglutination factor or by MM2 cells•mixedwith serum containing the agglutination factor. The immunized animals produced a y-globulin antibody that had specific binding affinity to the protein moiety of the antigen molecule and that was cytotoxic to the tumor cells in the presence of complement (8). Based on these findings, a biphasic mech anism for the establishment of transplantation immunity was proposed (8). In the 1st phase, which proceeds in tumor-bearing animals, the agglutination factor is formed. The 2nd phase, when a reaction against the protein moiety of the antigen molecule occurs, could be demonstrated using tumor-free animals. This reaction is induced by the antigen when it is combined with the agglutination factor