Resident bone marrow macrophages (RBMM) play an important role in clea
rance of nuclei extruded from late-stage erythroblasts (Eb). To invest
igate the nature of macrophage receptors involved in this process, ext
ruded erythroblast nuclei (EEN) were purified by cultivation of erythr
oblasts with erythropoietin, followed by density gradient centrifugati
on. By electron microscopy, the majority of free nuclei had an intact
plasma membrane. EEN bound avidly to RBMM in a divalent cation-indepen
dent manner at both 4 degrees C and 37 degrees C. The use of specific
monoclonal antibodies (MoAbs) and inhibitors showed that this adhesive
interaction was not mediated by previously characterized macrophage r
eceptors involved in recognition of either developing hematopoietic ce
lls or apoptotic cells. The EEN receptor was expressed on resident mac
rophages isolated from murine bone marrow, spleen, lymph node, and per
itoneal cavity, but was completely absent from alveolar macrophages. D
espite high levels of EEN binding to RBMM, few were phagocytosed even
after prolonged culture. Phorbol myristate acetate (PMA) was found to
stimulate phagocytosis, suggesting that this is a regulated process. T
hese findings indicate that EEN are recognized by a novel macrophage r
eceptor and that recognition may be triggered during the membrane remo
deling that accompanies enucleation. (C) 1995 by The American Society
of Hematology.