Characterization of a species of non‐specific cross‐reacting antigen (NCA) exressed by human monocytic cell lines: Structure and expression during cell differentiation

It has been documented that human monocytes/macro‐phages 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‐I, were found to express NCA‐50/90, a glycosyl‐phosphatidylinositol‐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.