DIFFERENTIAL EXPRESSION OF ENDOGLIN ON FETAL AND ADULT HEMATOPOIETIC-CELLS IN HUMAN BONE-MARROW

Endoglin, a glycoprotein that is expressed by human endothelial cells, binds TGF-beta 1 and -beta 3 with high affinity. It was originally id entified with the 44G4 mAb that was produced against a human pre-B cel l line. We now report that another anti-pre-B cell mAb, 29-G8, reacts with pro-B and pre-B leukemic cells, but not with mature B and T cells , and recognizes a different epitope of endoglin. The 29-G8 mAb bound specifically to recombinant endoglin and immunoprecipitated a phosphor ylated homodimeric glycoprotein with subunits of M(r) 95,000 from the 697 pre-B cell line. This new Ab removed all of the molecules identifi ed by the prototypic 44G4 anti-endoglin Ab, but the reverse was not tr ue. A subpopulation of 29-G8(+) endoglin molecules on this pre-B cell line was unreactive with the 44G4 mAb, thus suggesting that these anti -endoglin Abs see different epitopes that may discriminate different s pecies of endoglin molecules. Flow cytometric analysis with the 29-G8 mAb revealed two endoglin-positive subpopulations in fetal bone marrow : early B-lineage precursor cells (CD19(+) and CD34(+)), and proerythr oblasts (CD71(+) and glycophorin A(+)). In adult bone marrow, only the proerythroblast subpopulation was observed. Stromal cells derived fro m fetal bone marrow also reacted strongly with the 29-G8 and 44G4 Abs, and these cells responded with enhanced proliferation after stimulati on with either TCF-beta 1 or the anti-endoglin Abs. Thus, endoglin, a specialized component of the TCF-beta receptor system, may play a phys iologic role in the stromal-hemopoietic cell interactions occurring du ring development.