Processing of C3b-Opsonized Immune Complexes Bound to Non-Complement Receptor 1 (CR1) Sites on Red Cells: Phagocytosis, Transfer, and Associations with CR1

Abstract

Severe anemia is a lethal complication of Plasmodium falciparum malaria, particularly in children. Recent studies in children with severe P. falciparum anemia have demonstrated elevated levels of E-bound Abs, reduced E-associated complement receptor 1 (CR1) and decay-accelerating factor (DAF), and pronounced splenic enlargement, suggesting a mechanism for E loss involving Abs, complement, and phagocytosis. Motivated by these reports, we have developed an in vitro model in which human E with Abs and complement bound to CR1, DAF, or glycophorin A are incubated with model human macrophages (the THP-1 cell line). Previous work has demonstrated that immune complex (IC) substrates bound to E CR1, either by an Ab or via C3b, are transferred to macrophages with loss of CR1. In this study, we report that IC bound to DAF or glycophorin A by an Ab linkage are also transferred to macrophages. DAF is lost from the E during the transfer of DAF-bound IC, but the transfer of CR1-bound IC does not lead to a significant loss of DAF. Using glycophorin A-bound IC, we observe competition between transfer of IC and phagocytosis of the E: a fraction (≤15%) of the E was phagocytosed, while the remaining E were stripped of IC. We also examined the organization of CR1 and DAF in the presence of E-bound Ab/complement. We find that CR1, but not DAF, colocalizes with IgM mAb-C3b and IC-C3b substrates attached to glycophorin A. We observe that the binding of the IgM mAb-C3b to glycophorin A induces a novel unclustering of CR1.

Hematin Promotes Complement Alternative Pathway-Mediated Deposition of C3 Activation Fragments on Human Erythrocytes: Potential Implications for the Pathogenesis of Anemia in Malaria

“…During childhood malarial anemia, the erythrocytes remaining in the circulation have reduced levels of two key complement proteins: CR1, the immune adherence receptor for C3b, and CD55, a complement control protein that down-regulates deposition of C3b on cells (12,20,22). Moreover, several reports, based on in vitro investigations and clinical observations, have revealed that complement-activation products, in particular C3b and its fragments, can be found on erythrocytes in the bloodstream during many phases of malaria infections (23)(24)(25), and our in vitro studies have demonstrated loss of CD55 upon chelation with anti-CD55 mAbs followed by interaction with model monocyte/macrophages (26). In view of the fact that free hematin can activate complement (27), and because hematin is known to bind to erythrocytes (5,6,21), we have examined the hypothesis that activation of complement, promoted by the binding of hematin to uE in the bloodstream, may play an important role in the pathogenesis of malarial anemia and the destruction of uE.…”

C3b deposition on human erythrocytes induces the formation of a membrane skeleton–linked protein complex

“…In vitro, oligomers of GPA are known to form when IgM cold-reacting antibodies trigger complement activation (33). Other membrane proteins may also be affected when C3b is deposited; for example, Craig et al reported an altered distribution of complement receptor 1 (CR1) in the rbc membrane (34). Together, the SPT and tetherpulling results suggest that the effect of a relatively small number of C3b molecules on GPA and band 3 mobility is amplified by another mechanism.…”

Potential impact of complement regulator deficiencies on hemolytic reactions due to minor ABO-mismatched transfusions

“…12 Complement C3 activation has clinical implications for extravascular hemolysis because it causes increased macrophage-mediated phagocytosis. 28 Hemolysis following ABO-mismatched platelet transfusions is uncommon, with a clinical incidence of ;0.01%. 16,29 However, it is becoming increasingly clear that persistent hemolysis may be clinically relevant in terms of organ failure, thrombosis, and mortality.…”