Tumor-associated surface antigens of a Moloney virus-induced lymphoma, MBL-2, were studied by a 51Cr release, humoral complement-dependent cytotoxicity assay. A syngeneic antiserum obtained from C57BL/6 (B6) animals 40 to 60 days after infection with Moloney sarcoma virus (MSV), and a xenogeneic antiserum prepared by immunizing rabbits with the MBL-2 cells were used in this study. The specificities of the MSV serum and the rabbit anti-MBL-2 serum were examined by the direct cytotoxicity assay with various target cells and by the cytotoxicity inhibition assay with MBL-2 as target cells. B6 leukemia cells, induced by Friend virus (FBL-3), Moloney virus (MBL-2), Rauscher virus (RBL-5), and benzpyrene [EL-4(G+)] could inhibit both sera, and a Gross virus-induced leukemia (E♂G2) was negative. BALB/c leukemias induced by Moloney virus (YC8 and LSTRA) and Rauscher virus (MCDV-12) as well as an A/SN strain leukemia induced by Moloney virus (YAC) also gave positive results whereas a radiation-induced BALB/c leukemia (RL♂1) was negative.

Similar results were obtained with the cytotoxicity inhibition assay when either soluble 3 M KCl extracts or whole cells were tested. Cell surface antigens extracted from MBL-2, RBL-5, and El-4(G+) were positive whereas extracts from E♂G2, normal spleen cells, and P815 (a mastocytoma of DBA/2 mice) were negative.

Despite the identical patterns of the specificity of the two antisera when whole tumor cells or crude extracts were tested, major differences in the specificity of the two antisera were noted when virus reagents were tested. Although the anti-MSV serum was inhibited by intact Moloney or Rauscher virus, the anti-MBL-2 serum was not inhibited by either intact or disrupted viruses. The anti-MSV and heterologous antisera recognized similar but not identical antigens with a distribution broader than the FMR (Friend-Moloney-Rauscher) specificity, but unrelated to Gross cell surface antigens (GCSA). The anti-MBL-2 may be detecting a Moloney virus-associated cell surface antigen that appears not to be a virion antigen.

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