Expression of a divalent cation-dependent erythroblast adhesion receptor by stromal macrophages from murine bone marrow

Stromal macrophages in haemopoietic organs express novel surface receptors that are implicated in trophic interactions with developing blood cells. Macrophages isolated from foetal liver bind erythroblasts (Eb) by a divalent cation-dependent receptor (EbR), whereas stromal macrophages in adult bone marrow and lymphoid organs express a lectin-like receptor, sialoadhesin, which interacts with sialylated structures on sheep erythrocytes and marine haemopoietic cells. In order to learn more about the regulation of these haemagglutinins, we examined binding of Eb by stromal macrophages that had been isolated from adult murine tissues or generated in Dexter-type cultures of bone marrow. Macrophages were purified from bone marrow by collagenase digestion, adherence to a substratum and detachment of clustered haemopoietic cells, and tested for their ability to bind Eb from foetal liver or anaemic adult spleen. Freshly isolated bone marrow macrophages bound Eb mainly by a divalent cation-dependent activity that was not inhibited by neuraminidase treatment of Eb or by specific anti-sialoadhesin monoclonal antibodies, although these macrophages express sialoadhesin and Eb bear a potential ligand for this receptor. Macrophages obtained by digestion from other adult lymphoid tissues also bound Eb by a divalent cation-dependent activity, whereas blood monocytes and lavaged peritoneal macrophages failed to do so. Peritoneal macrophages could be induced to express high levels of sialoadhesin by cultivation in homologous mouse serum, but such macrophages did not acquire sialoadhesin-independent EbR activity. By contrast, high levels of EbR (∼75% rosettes) were detected on bone marrow macrophages derived by cultivation for 3-10 days in medium containing foetal bovine or horse serum and supplemented with 10^-6M dexamethasone, similar to that required to obtain a functionally active stroma in long-term bone marrow culture. EbR activity remained low (<20%) on peritoneal macrophages cultivated under the same conditions. Our studies establish that the EbR is a major cell adhesion receptor on macrophages in adult as well as foetal tissues, that macrophages express distinct, independently regulated haemopoietic cell interaction receptors and that the EbR is a consistent marker of stromal macrophages in situ and in vitro.