We describe a novel hemagglutinin which is differentially expressed on murine stromal tissue macrophages. Resident bone marrow macrophages (RBMM), which are physically associated with immature, proliferating hematopoietic cells in vivo, formed striking rosettes with unopsonized sheep erythrocytes (E) in vitro, unlike resident peritoneal macrophages (RPM). Binding of E was macrophage (M phi) specific, not accompanied by ingestion and independent of temperature (0-37 degrees C), divalent cations, and the metabolic inhibitors azide and iodoacetate. Pretreatment of RBMM with trypsin prevented rosette formation, but neuraminidase enhanced it. Conversely, binding was virtually abrogated if E were pretreated with neuraminidase, whereas trypsin pretreatment of the ligand resulted in a slight enhancement. The lectin-like nature of the E receptor (SER), with specificity for sialylated glycoconjugates, was consistent with the inhibition of binding we saw with neuraminyllactose or the ganglioside GD1a (50% inhibition at 5-10 mM and 11 microM, respectively). Expression of SER on freshly isolated RBMM was heterogeneous and exhibited a striking inverse correlation with expression of Ia antigens. During cultivation in 10% FCS, levels of SER on RBMM declined with a half-life of approximately 24 h. Other cell surface changes induced by cultivation included a transient increase in expression of Ia antigen and acquisition of Mac-1. To determine whether SER was expressed on other stromal M phi populations, adherent cells were isolated from various tissues by collagenase digestion or lavage. Binding of E was highest on RBMM and lymph node stromal M phi, at intermediate levels on Kupffer cells and splenic stromal M phi, but was low or undetectable on blood monocytes and thymic, peritoneal, pleural, and bronchoalveolar M phi. SER therefore appeared to be expressed on certain M phi populations embedded in solid tissues but was largely absent from M phi recoverable by lavage. Its absence from monocytes implies that SER is acquired by M phi after entering tissues where it may perform adhesive functions. In bone marrow, SER on RBMM could interact with an appropriate sialylated ligand on murine hematopoietic cells, and could influence their rate of growth and differentiation.

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