HCA, an immunoglobulin-like adhesion molecule present on the earliest human hematopoietic precursor cells, is also expressed by stromal cells in blood-forming tissues

We have previously shown that the HCA/ALCAM (CD166) glycoprotein, a member of the immunoglobulin family that mediates both hemophilic and heterophilic cell-cell adhesion, via the CD6 ligand, is expressed at the surface of all of the most primitive CD38(-/lo), Thy-1(+), rho123(lo), CD34(+) hematopoie tic cells in human fetal liver and fetal and adult bone marrow. In the pres ent report we show that HCA is also expressed by subsets of stromal cells i n the primary hematopoietic sites that sequentially develop in the human em bryo and fetus, ie, the paraaortic mesoderm, liver, thymus, and bone marrow . Adult bone marrow stromal cells established in vitro, including those der ived from Stro-1(+) progenitors and cells from immortalized cell lines, exp ress HCA. In contrast, no HCA expression could be detected in peripheral ly mphoid tissues, fetal spleen, and lymph nodes. HCA membrane molecules purif ied from marrow stromal cells interact with intact marrow stromal cells, CD 34(+) CD38(-) hematopoietic precursors, and CD3(+) CD6(+) peripheral blood lymphocytes. Finally, low but significant levels of CD6 are here for the fi rst time detected at the surface of CD34(+) rho123(med/lo) progenitors in t he bone marrow and in mobilized blood from healthy individuals. Altogether, these results indicate that the HCA/ALCAM surface molecule is involved in hemophilic or heterophilic (with CD6) adhesive interactions between early h ematopoietic progenitors and associated stromal cells in primary blood-form ing organs. (C) 1999 by The American Society of Hematology.