Introduction to Complement Receptor
Complement receptors (CRs), a number of G protein-coupled receptors (GPCRs), are usually expressed on different immune cells and are responsible for mediating the inflammatory responses in humans. Four types of CRs have been identified, including CR1, CR2, CR3, and CR4, in the past few years of studies. These studies have demonstrated that CRs have been identified in many cell types, such as neutrophils, T cells, B cells, and mast cells. Meanwhile, recent studies have also revealed that these CRs can be considered as a bridge or a linker in target cells and their receptors. Furthermore, CRs can closely bind to several complement fragments to activate the complement cascade. For example, the binding of complement-derived inflammatory mediators and specific CRs has been confirmed in many clinical cases of inflammatory responses. In addition, the structural analysis of various CRs has been conducted by many companies and organizations. These data play an important role in understanding the receptor-ligand interactions of complement family and screening candidate targets for novel drug design.
Fig. 1 General view of complement activation.1
The Structure of Complement Receptors
The complement system consists of many sera or transmembrane proteins that are critical to preventing complement-related damage in humans. In general, complement can be activated by three main pathways, including the classical pathway, lectin pathway, and alternative pathway. CRs are key members of the complement activation family and play a major role in mediating innate recognition of antigen to the Tor B lymphocyte. Of these, CR1 is a key regulator that can activate the complement system of plasma proteins. the structural analysis has shown that CR1 is a membrane-bound, multifunctional glycoprotein with a size of 200-300 kDa. It is usually composed of several domains, such as complement control protein repeats (CCPs). Each domain contains a hydrophobic core and can bind to complement protein C4b and C3b fragments. CR2 belongs to complement activation (RCA) family, contains an SCR extracellular domain, a transmembrane domain, as well as a short intracellular tail. It can enhance the immune responses of B-cells in different complement pathways. CR3 and CR4 are capable of binding of complement-labeled surfaces by macrophages and dendritic cells, which are essential to the formation of humoral immune responses.
The CRs Structure Study in Disease Treatment
Nowadays, CRs have been associated with the pathogenesis of autoimmune disease and have been considered as potential targets to regulate B/T cell immunity in disease therapy. Many attempts have been made to design and develop safe and more effective complement therapeutics solutions for the treatment of different malignancies. Based on the structure-function analyses of CRs, more potent complement inhibitors and therapeutics antibodies derived from CRs can be produced. For instance, the roles of several portions of CR1 primary structure, such as the c-terminal amino acid sequence and the proteolytic cleavage of a peptide bond, have been broadly studied in recent research, thereby offering the possibility of modifying the function of these receptors in disease therapy.
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From Wikipedia: https://commons.wikimedia.org/wiki/File:Complement_pathway.svg
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