Ligand binding by the pl50,95 antigen of U937 monocytic cells: properties in common with complement receptor type 3 (CR3)

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Abstract
U937 cells (a monocytic cell line) grown in the presence of phorbol myristate acetate were surface labeled with 125I and the iC3b-binding proteins isolated by affinity chromatography on iC3b-Sepharose in the presence of divalent cations. Three polypeptides of 170, 150 and 95 kDa were found to bind specifically to iC3b-Sepharose. The polypeptides of 170 and 95 kDa were identified as the α and the β subunits of CR3 by immunoprecipitation with OKM1 monoclonal antibody. The 150-kDa polypeptide was not immunoprecipitated by antibodies to the α subunit of CR3 or LFA-1. However, the 150-kDa polypeptide, together with the 95-kDa polypeptide, was immunoprecipitated with an anti-β subunit-specific antibody IB4, which immunoprecipitates LFA-1, CR3 and pl50,95. These results indicated that the 150-kDa polypeptide is the a subunit of the pl50,95 antigen. The binding of pl50,95 and CR3 to iC3b- Sepharose is specific as neither binds to C3u-Sepharose. A monoclonal antibody, 3.9, which immunoprecipitated the 150 and a 95-kDa polypeptide from U937 cells was characterized as being directed against the α-subunit of the pl50,95 antigen. Phorbol myristate acetate-stimulated U937 cells form rosettes with EAC3b and EAiC3b but not with EAC3d cells. Monoclonal antibody 3.9 does not inhibit either type of rosetting, but we were unable to exclude a role for pl50,95 in adherence of iC3b-coated particles. Since there is no rosetting with C3d-bearing particles it is unlikely that pl50,95 is a receptor for C3d, a role for pl50,95 which has been suggested by others (Wright, S. D., Licht, M. R. and Silverstein, S. C, Fed. Proc. 1984. 43: 413).

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