Objective. To develop an in vitro model for investigating the mechanism by which autoantibodies in immune complexes (ICs) that are bound to primate erythrocytes via antigen-based heteropolymers (AHPs) are cleared from the circulation and localized to the liver.Methods. IgG anti-double-stranded DNA (antidsDNA) antibodies in ICs with dsDNA were bound to human erythrocytes via complement receptor 1 (CR1) either by opsonization with normal human serum as a complement source or through the use of an AHP, which consists of an anti-CR1 monoclonal antibody (mAb) that is chemically crosslinked with dsDNA. We performed parallel investigations of the mechanism of transfer of both types of erythrocyte-bound ICs to a monocytic cell line (U937). Erythrocytes with CR1-bound ICs were incubated with U937 cells under a variety of conditions, and subsequently, the levels of IgG anti-dsDNA, CR1, AHP, or C3b on both erythrocytes and U937 cells were measured by flow cytometry with appropriate fluorescently labeled probes.Results. In the presence of U937 cells, both the AHP-anti-dsDNA and C3b-opsonized ICs were rapidly removed from the erythrocytes; at 37°C, more than half of the complexes were removed in 2 minutes. Monomeric mouse IgG2a mAb blocked the transfer of both types of complexes by 75%, suggesting that Fc␥ receptor type I (Fc␥RI) is the main phagocyte receptor responsible for the removal of ICs from erythrocytes. Levels of CR1 on the erythrocyte surface were reduced during transfer of the AHP-anti-dsDNA ICs, suggesting that transfer involves a concomitant removal of CR1, presumably by proteolysis.Conclusion. Transfer of AHP-anti-dsDNA ICs from erythrocyte CR1 to model phagocytes occurs by a mechanism that is similar to the natural mechanism of IC clearance, involving recognition by Fc␥RI and removal of erythrocyte CR1 as key steps.
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