Microbes as well as immune complexes and other continuously generated inflammatory particles are efficiently removed from the human circulation by red blood cells (RBCs) through a process called immune-adherence clearance. During this process, RBCs use complement receptor 1 (CR1, CD35) to bind circulating complement-opsonized particles and transfer them to resident macrophages in the liver and spleen for removal. We here show that ligation of RBC CR1 by antibody and complement-opsonized particles induces a transient Ca ؉؉ influx that is proportional to the RBC CR1 levels and is inhibited by T1E3 pAb, a specific inhibitor of TRPC1 channels. The CR1-elicited RBC Ca ؉؉ influx is accompanied by an increase in RBC membrane deformability that positively correlates with the number of preexisting CR1 molecules on RBC membranes. Biochemically, ligation of RBC CR1 causes a significant increase in phosphorylation levels of -spectrin that is inhibited by preincubation of RBCs with DMAT, a specific casein kinase II inhibitor. We hypothesize that the CR1-dependent increase in membrane deformability could be relevant for facilitating the transfer of CR1-bound particles from the RBCs to the hepatic and splenic phagocytes.(Blood.
2010;116(26): 6063-6071)
IntroductionIn primates, in contrast to other vertebrates, clearing the intravascular space of complement-opsonized inflammatory particles (eg, microbes and immune complexes) is mediated by circulating red blood cells (RBCs) using complement receptor 1 (CR1, CD35). 1,2 During this process, known as immune-adherence clearance, RBCs immobilize complement-tagged particles and transport them to the liver and spleen where resident macrophages remove the complement-tagged particles and leave the RBCs intact. Immuneadherence clearance acts as a "buffer system," preventing deposition of circulating immune complexes in susceptible organs, such as the kidney, and preventing activation of circulating leukocytes by inflammatory particles. 3,4 We and others have also shown that CR1-mediated immune-adherence promotes more efficient phagocytosis and intracellular killing of complement-opsonized pathogens compared with opsonized pathogens that are free-floating in plasma and not RBC-bound. 5,6 We have previously found that, in circulating human RBCs, CR1 is disperse in RBC plasma membranes, and, after ligation by immune particles, interacts with Fas-associated phosphatase-1 and rearranges into large clusters. 7 Beneath the plasma membrane of RBCs, the spectrin cytoskeleton defines a series of "corrals" that are critical for maintaining RBC shape and deformability and for regulating the range and magnitude of lateral diffusion of most transmembrane proteins. 8 The mechanical attributes of the spectrin meshwork depend critically on the transient phosphorylation of -spectrin, adducin, and protein 4.1R. 9-11 Therefore, we hypothesized that ligation-mediated CR1 clustering is an active process with CR1 directly affecting the phosphorylation status of cytoskeletal proteins and thus the mechanical p...