THE EOSINOPHIL-SPECIFIC CELL-SURFACE ANTIGEN, EOS47, IS A CHICKEN HOMOLOG OF THE ONCOFETAL ANTIGEN MELANOTRANSFERRIN

The EOS47 antigen is a 100-kD cell surface glycoprotein selectively ex pressed by avian retrovirus-transformed eosinophils and their precurso rs. We have purified the EOS47 protein to homogeneity and used peptide sequence information to clone EOS47-encoding cDNAs. The open reading frames from these cDNAs predict a 738 amino acid protein with homology to human melanotransferrin, a membrane-bound, transferrin-like protei n that is expressed at high levels by a subset of melanomas, tumor cel l lines, fetal intestine, and liver, but not by most normal adult tiss ues. The predicted protein sequence of EOS47 displays a 61% sequence i dentity with melanotransferrin and conservation of all 28 cysteine res idues, indicating a similar tertiary structure. The finding that EOS47 lacks several of the iron-coordinating amino acids present in all tra nsferrins suggests that it may be impaired in its ability to bind iron . In nonhematopoietic tissues, EOS47 is expressed at high levels by ep ithelial brush borders of small intestine and kidney and at lower leve ls by cells lining the sinusoids of the liver. Within hematopoietic ti ssues, EOS47 is restricted to a subpopulation of cells (1% to 5%) in b one marrow and early spleen and fluorescence-activated cell sorting of EOS47(+) cells leads to a dramatic (>30-fold) enrichment of peroxidas e(+) eosinophils. In contrast, peripheral blood eosinophils are EOS47( -). suggesting that the antigen is expressed by newly formed eosinophi ls and that expression ceases shortly before these cells emigrate from the bone marrow into the peripheral blood. Our results show that mela notransferrin is a stage-specific marker of eosinophils and should be useful for their isolation and further characterization. (C) 1996 by T he American Society of Hematology.