Binding and catabolism of aggregated immunoglobulins containing C3b by U937 cells
Fc receptor and C3b receptor (CR1) function on U937 cells, a human monocyte cell line was studied. C3b was incorporated into stable soluble heat aggregates of /sup 125/I-IgM (A-IgM) and /sup 125/I-IgG (A-IgF) by using functionally pure classical pathway components. C3b incorporation was verified by the ability of aggregates to bind to human red cells and by co-sedimentation of /sup 125/I and /sup 131/-I during ultracentrifugation. Cell uptake and degradation of A-IgG C3b was increased up to twofold compared with A-igG not containing C3b molecules. However, A-IgG C3b bound to CR1 after Fc receptors were blocked with nonradiolabeled A-IgG were also not endocytosed and catabolized. Moreover, A-IgM C3b was bound but no degraded by U937 cells. As expected, uptake of A-IgM without C3b was negligible. CR1-mediated binding of A-IgM C3b was specifically inhibited both by a murine monoclonal antibody against the human CR1 that blocks C3b binding and by C3b oligomers generated by trypsin activation of C3, but not by monoclonal antibodies against the iC3b receptor (CR3). The authors conclude that CR1 on U937 cells cause increased binding of A-IgG, and this increased binding leads to increased Fc-mediated endocytosis and catabolism of model immune complexes. However, binding of soluble ligands by CR1 alone, even when binding of soluble ligands by CR1 alone, even when binding is multivalent, does not lead to endocytosis and degradation of soluble ligands bearing C3b.
- Research Organization:
- Univ. of Rochester School of Medicine, NY
- OSTI ID:
- 6845936
- Journal Information:
- J. Immunol.; (United States), Journal Name: J. Immunol.; (United States) Vol. 136:5; ISSN JOIMA
- Country of Publication:
- United States
- Language:
- English
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