Murine Fcγ receptor proteins: identification of a previously unrecognized molecule with a monoclonal antibody (12-15)

Abstract

Previously we studied differential expression of cell surface molecules between the metastatic murine lymphoma ESb and an adhesion variant ESb-MP. Here we describe the specificity of a monoclonal antibody (12-15) that showed strong binding to the adhesion variant and weak reactivity against ESb cells. The antibody also reacted to lymphoid but not to macrophage-derived cell lines and immunoprecipitated a molecule of approx. 60–69 kDa from ESb-MP cells. N-terminal sequencing of the' antigen revealed identity to the beta protein of mouse Fcγ receptors. Using monoclonal antibodies against Fcγ receptors (2.4G2 and K9.361) in immunofluorescence assays and cDNA probes specific for alpha, betal and beta2 Fc receptor transcripts in Northern blot experiments the differential expression of Fc receptors in ESb and ESb-MP cells was confirmed. Biochemical analysis of endoglycosidase F-treated precipitates revealed that antibody 12-15 reacted to products of all three transcripts with molecular masses for the protein core of 38.5 kDa (betal), 34 kDa (beta2) and 31 kDa alpha). In addition, an unknown protein of 37 kDa (termed beta3) was identified by antibody 12-15 which could also be detected in ESb cells and EL4 cells. Antibodies 2.4G2 and K9.361 did not react to the beta 3 chain but reacted to varying extents to the other Fc proteins in macorphage and lymphoid cells. Comparison by peptide mapping of the novel beta3 chain to betal, beta2 and alpha proteins revealed similar, but also distinct peptides. The tissue-specific reactivity of monoclonal antibody 12-15 is likely to be due to a carbohydrate epitope associated with all Fcγ receptors in lymphoid but not macrophage cell lines.