Late in the course of Friend virus (FV)-induced erythroleukemia, leukemic spleen cells express a cell surface retroviral gp70 envelope protein not detected during the early proliferative phase of the disease. Characterization of this gp70 revealed it was unrelated to the input Friend murine leukemia virus (F-MuLV), but antigenically similar to a unique subset of endogenous xenotropic viruses. This gp70 was expressed by murine erythroleukemia cell lines but has not been identified on cell lines of other lineages. A monoclonal antibody (18-6) specifically reactive with this polypeptide was used to examine hematopoietic organs of normal uninoculated mice. This antibody detected a gp70 expressed by a majority of erythroid cells in fetal liver and by a small but significant percentage of normal adult spleen and bone marrow cells. Increased erythropoietic activity induced by treatment of adult mice with phenylhydrazine ( PHZ ) resulted in a seven- to eightfold increase in the frequency of spleen and bone marrow cells expressing this gp70. Peptide map analysis indicated that the 18-6 reactive gp70 expressed by Friend erythroleukemia cells and by cells from normal fetal liver were structurally identical. These results suggested that this unique gp70 was an erythroid-specific differentiation antigen.

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