The GIX antigen of murine leukemia virus: an analysis with monoclonal antibodies

The G_IX antigen of murine retrovirus gp70 is a marker for virus replication in various cell types, as well as for T-cell differentiation and leukemogenesis in particular mouse strains. The serological definition of G_IX depends on the cytotoxicity of a particular rat antiserum for thymocytes from strain 129 mice. We have found that two rat monoclonal antibodies, elicited by immunization with AKR ecotropic virus or AKR leukemia cells, co-type closely with G_IX in several assay systems. Like G_IX, the epitopes identified by the monoclonal antibodies are amplified on thymocytes during leukemogenesis in AKR mice. These antibodies are cytotoxic for G_IX⁺ leukemia cells and for fibroblasts infected with G_IX⁺ viral serotypes. Though absorbed by G_IX⁺ thymocytes of various strains, the antibodies are not cytotoxic for these cells. We ascribe the latter result to lower representation of antigenic sites on normal thymocytes, and postulate that more than one epitope may participate in classical G_IX-mediated cytotoxicity. The monoclonal antibodies do not react with viral env products synthesized in the presence of the inhibitor of glycosylation, tunicamycin. Reactivity with the antibodies appears to require a stable configurational change in the env precursor protein coinciding with glycosylation, rather than direct participation of carbohydrate in the antigenic site. Thus subsequent enzymatic removal of carbohydrate chains in vitro does not alter reactivity with the antibodies.