Clearance of anti-double-stranded DNA antibodies - The natural immune complex clearance mechanism

Objective. To develop an in vitro model for investigating the mechanism by which autoantibodies in immune complexes (ICs) that are bound to primate er ythrocytes via antigen-based heteropolymers (AHPs) are cleared from the cir culation and localized to the liver, Methods. IgG anti-double-stranded DNA (anti-dsDNA) antibodies in ICs with d sDNA were bound to human erythrocytes via complement receptor 1 (CR1) eithe r 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 investiga tions of the mechanism of transfer of both types of erythrocyte-bound ICs t o a monocytic cell Line (U937), Erythrocytes with CR1-bound ICs were incuba ted with U937 cells under a variety of conditions, and subsequently, the le vels of IgG anti-dsDNA, CR1, AHP, or C3b on both erythrocytes and U937 cell s were measured by flow cytometry with appropriate fluorescently labeled pr obes. Results. In the presence of U937 cells, both the AHP-anti-dsDNA and C3b-ops onized ICs were rapidly removed from the erythrocytes; at 37 degrees C, mor e than half of the complexes were removed in 2 minutes, Monomeric mouse IgG 2a mAb blocked the transfer of both types of complexes by 75%, suggesting t hat Fc gamma receptor type I (Fc gamma RI) is the main phagocyte receptor r esponsible for the removal of ICs from erythrocytes. Levels of CR1 on the e rythrocyte 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 ph agocytes occurs by a mechanism that is similar to the natural mechanism of IC clearance, involving recognition by Fc gamma RI and removal of erythrocy te CR1 as key steps.