Murine fetal liver macrophages bind developing erythroblasts by a divalent cation-dependent hemagglutinin

During mammalian development the fetal liver plays an important role in hematopoiesis. Studies with the macrophage (Mφ)-specific mAb F4/80 have revealed an extensive network of Mφ plasma membranes interspersed between developing erythroid cells in fetal liver. To investigate the interactions between erythroid cells and stromal Mφ, we isolated hematopoietic cell clusters from embryonic day-14 murine fetal liver by collagenase digestion and adherence. Clusters of erythroid cells adhered to glass mainly via Mφ, 94% of which bound 19 ± 11 erythroblasts (Eb) per cell. Bound Eb proliferated vigorously on the surface of fetal liver Mφ, with little evidence of ingestion. The Mφ could be stripped of their associated Eb and the clusters then reconstituted by incubation with Eb in the presence of divalent cations. The interaction required less Ca⁺⁺ than Mg⁺⁺, 100 vs. 250 μM for half-maximal binding, and was mediated by a trypsin-sensitive hemagglutinin on the Mφ surface. After trypsin treatment fetal liver Mφ recovered the ability to bind Eb and this process could be selectively inhibited by cycloheximide. Inhibition tests showed that the Eb receptor differs from known Mφ plasma membrane receptors and fetal liver Mφ did not bind sheep erythrocytes, a ligand for a distinct Mφ hemagglutinin. We propose that fetal liver Mφ interact with developing erythroid cells by a novel nonphagocytic surface hemagglutinin which is specific for a ligand found on Eb and not on mature red cells.