Two surface antigens expressed on proliferating mouse T lymphocytes defined by rat monoclonal antibodies

A hybrid cell line resulting from the fusion of a Con A-activated normal mouse spleen cell and a transformed mouse T cell (EL-4BU) has been used to prepare and select rat monoclonal antibodies reactive with molecules expressed on the surface of proliferating, as opposed to resting, mouse T cells. In this report, the characterization of two such antigens identified in this way is described. One antigen is a membrane component common to mitogen-activated T and B cells, some bone marrow cells, and various transformed cell lines but is not detectable on either normal thymocytes or the majority of spleen cells by radioimmunoassay or FACS analysis. It has a m.w. of approximately 200,000 daltons under nonreducing conditions and 100,000 daltons under reducing conditions. Antibodies to this antigen precipitate cell-bound transferrin but do not react directly with transferrin itself. It would thus appear that the antigen is the transferrin receptor molecule. The second antigen is not detectable on normal thymocytes, spleen cells, bone marrow cells, or mitogen-stimulated spleen cells but is expressed at high levels on some transformed T cell lines. It, too, appears to be a dimer, with a m.w. of 95,000 daltons under nonreducing conditions, decreasing to 50,000 daltons under reducing conditions. Although the function of the 95,000-dalton antigen is not yet known, its lack of expression on adult T cell populations both before and after activation suggests either a short-lived role at a very early stage of T cell development and/or an association with T cell transformation.