Relationship of Surface Membranes of Lymphoid Cells Transformed by Abelson Murine Leukemia Virus to Tumor Rejection

The Abelson murine leukemia virus (A-MuLV)-transformed lymphoid cell line L1-2 is exceptional since it is rejected following transplantation into syngeneic C57L mice. Furthermore, the serum from recovered mice (Abelson tumor serum) has been used as a source of antibodies to abl antigens present in the M_r120, 000 A-MuLV-encoded gag-abl fusion protein (p120). In addition to p120, this antiserum precipitates from L1-2 cell extracts a minor gag-abl fusion protein with an apparent M_rof 95,000 (p95). Serological and peptide mapping studies indicate that p95 lacks a portion of the abl region that occurs in p120. However, p95 and p120 contain the same substrate sites for tyrosine-specific protein kinase. The presence of abl antigens on the surface membranes of A-MuLV-infected cells was confirmed by rosetting studies using sheep erythrocytes coupled to protein A and by serum adsorption analyses. Because previous workers were unable to label the putative gag-abl cell surface component(s) using chemical probes, we attempted to identify these cell surface molecules by a method that involves direct adsorption of antibodies onto L-[³⁵S]methionine-labeled intact cells followed by cell washing, lysis with detergents, and rapid isolation of antigen:antibody complexes (Krangel, M. S., Orr, H. T., and Strominger, J. L. Cell, 24: 847–858, 1979). However, the gag-abl cell surface components were not recovered in the resulting immune complexes, apparently because they dissociated from their low-avidity antibodies before their isolation could be achieved. Nevertheless, an M_r95,000 protein was identified on the L1-2 cell surface using the Abelson tumor serum as a probe. Extensive serum adsorption analyses demonstrate that this M_r95,000 protein is distinct from the p95 gag-abl fusion protein. Furthermore, the M_r95,000 protein is present on L1-2 cell surfaces but not on more tumorigenic lymphoid or fibroblast cell lines infected with A-MuLV. We propose that immune response directed against the M_r95,000 cell surface protein of L1-2 cells may be involved in the exceptional rejectability of this tumor cell line. These studies provide a logical framework which may be generally useful for the preparation of tumor-specific antisera.