Characterization of a species of non-specific cross-reacting antigen (NCA) expressed by human monocytic cell lines: structure and expression during cell differentiation

It has been documented that human monocytes/macrophages are reactive with antibodies directed to carcinoembryonic antigen (CEA) and non-specific cross-reacting antigens (NCAs), a group of glycoproteins antigenically cross-reactive with CEA, yet the molecules responsible for this antigenic activity have not been fully clarified. In the present study, among 7 myelomonocytic cell lines tested, 2 monoblastoid lines, U-937 and THP-1, were found to express NCA-50/90, a glycosylphosphatidylinositol-anchored cell-adhesion molecule chiefly expressed on granulocytes. The 2 cell lines showed a reaction pattern with 5 distinct anti-CEA and anti-NCA monoclonal antibodies, similar to that of CHO transfectants expressing recombinant NCA-50/90. Immunoprecipitation and SDS-PAGE analyses identified glycoproteins of about 95 and 55 kDa in U-937 and THP-1 cells, respectively. Deglycosylation of the 2 antigens with N-glycanase gave the same apparent molecular mass of about 45,000, which was also the same as that of the deglycosylated form of the recombinant NCA-50/90. Upon Northern-blot analysis, only one band of approximately 2.5 kb was detected in both cell lines with a cDNA probe for NCA-50/90, which has a broad specificity to the CEA gene family members. cDNA cloning demonstrated that the 2.5-kb clones encode the peptide of NCA-50/90. The expression of NCA-50/90 by U-937 and THP-1 was down-regulated at both the protein and mRNA levels during cell differentiation from monoblastoid to monocyte/macrophage-like cells induced by stimulation with phorbol 12-myristate 13-acetate. Our observations suggest that NCA-50/90 is a differentiation antigen of cells of the monocyte/macrophage lineage as well as of the granulocyte lineage.