An inhibitory rat mAb, SER-4, has been raised to the mouse macrophage (M phi)-restricted hemagglutinin, sheep erythrocyte receptor (SER), which binds unopsonized sheep erythrocytes through recognition of sialylated glycoconjugates. This receptor was originally defined on mouse resident bone marrow M phi where it was implicated in adhesive interactions of these cells with proliferating hematopoietic cells. In the present study using mouse serum-induced thioglycollate-elicited peritoneal M phi (TPM) as a model system for SER expression, mAb SER-4 IgG2a completely blocked rosette formation at 1 microgram/ml. The inhibition was likely to be via steric hindrance rather than through a direct interaction with the putative sialic acid binding site of SER because F(ab')2 and Fab fragments of mAb SER-4 gave a maximum inhibition of 50-60% and 0% respectively, despite binding effectively to the SER-4 antigen (Ag). Immunoprecipitation and Western blotting experiments with cultured M phi or tissue extracts demonstrated that the Ag recognized by SER-4 mAb is a single chain molecule with an apparent Mr by SDS-PAGE of 185 x 10(3) (reduced) or 170 x 10(3) (non-reduced) and is distinct from members of the leukocyte common Ag family. Expression of SER and SER-4 Ag in culture were closely correlated and depended on the presence of mouse serum for optimal induction. Further evidence that the SER-4 Ag is functionally equivalent to SER was provided by immunocytochemistry in which the overall pattern of staining in tissues was consistent with previous rosetting experiments. In the bone marrow, expression of the SER-4 Ag was restricted to the resident bone marrow M phi population with no expression on monocytes. High expression was also observed on stromal M phi within the subcapsular sinus and medullary cords in lymph nodes and on marginal metallophils in the spleen. These results therefore confirm that SER is a novel M phi-restricted receptor whose distribution and properties indicate a role in cellular interactions in hematopoietic and lymphoid tissues.

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