The characterization, molecular cloning, and expression of a novel hematopoietic cell antigen from CD34⁺ human bone marrow cells

The adhesion molecule BEN/SC1/DM-GRASP (BEN) is a marker in the developing chicken nervous system that is also expressed on the surface of embryonic and adult hematopoietic cells such as immature thymocytes, myeloid progenitors, and erythroid progenitors. F84.1 and KG-CAM, two monoclonal antibodies to rat neuronal glycoproteins with similarity to BEN, cross-react with an antigen on rat hematopoietic progenitors, but F84.1 only also recognizes human blood cell progenitors. We have defined the antigen recognized by F84.1 as the hematopoietic cell antigen (HCA). HCA expression was detected on 40% to 70% of CD34⁺ fetal and adult bone marrow cells and mobilized peripheral blood cells. Precursor cell activity for long-term in vitro bone marrow cell culture was confined to the subset of CD34⁺ cells that coexpress HCA. HCA is expressed by the most primitive subsets of CD34⁺ cells, including all rhodamine 123(lo), Thy-1⁺, and CD38(-/lo) CD34⁺ adult bone marrow cells. HCA was also detected on myeloid progenitors but not on early B-cell progenitors. We also describe here the cloning and characterization of cDNAs encoding two variants of the human HCA antigen (huHCA-1 and huHCA-2) and of a cDNA clone encoding rat HCA (raHCA). The deduced amino acid sequences of huHCA and raHCA are homologous to that of chicken BEN. Recombinant proteins produced from either human or rat HCA cDNAs were recognized by F84.1, whereas rat HCA but not human HCA was recognized by antirat KG-CAM. Expression of either form of huHCA in CHO cells conferred homophilic adhesion that could be competed with soluble recombinant huHCA- Fc. The molecular cloning of HCA and the availability of recombinant HCA should permit further evaluation of its role in human and rodent hematopoiesis.