See also Regulation of the Complement system .

See also the use of complement receptors for drug targeting (under “pharmacology” and “drug delivery”).

There are three known gene superfamilies of complemen receptors: The short consensus repeat (SCR) modules that code for CR1 and CR2, the beta-2 integrin family members CR3 and CR4, and the immunoglobulin Ig-superfamily member CRIg.

Complment Receptor 1 (CR1) (alsko known as human C3b/C4b receptor or CD35):

Structure: CR1 is a 180-210 kDa glycoprotein consisting of 30 short consensus repeats (SCRs). The structure of the C3b binding site, contained with SCR 15-17 of CR1 (site 2), has been determiend by MRI.

Function: CR1 plays a major role in immune complex clearance. High affinity binding to both C3b and C4b occurs through two distinct sites, each composed of 3 SCRs. The main function of CR1 is to capture ICs on erythrocytes of transport and clearance by the liver. There is a role in phagocytosis for CR1 on neutrophils, but not in tissue macrophages. In addition to its role in clearance of immune complexes, CR1 is a potent inhibitor of both classical and alternative pathway activation through its interaction with the respective convertases. See also Regulation of the Complement system.

Several soluble fragments of CR1 have also been generated via recombinant DNA procedures by eliminating the transmembrane region from the DNA being expressed. The soluble CR1 fragments were functionally active and inhibited in vitro the consequences of complement activation such as neutrophil oxidative burst, complement mediated hemolysis and C3a and C5a production. (WO 94/17822).

Complement receptor type 2 (CR2) (also known as CD21): 

Structure: CR2 contains an extracellular portion having 15 or 16 repeating units known as showrt consensus repeats (SCR domains). The SCR domains have a typical framework of highliy ocnserved residues including 4 cysteines, two prolines, one tryptophane and several other partially conserved glycines and hydrophobic residues. In the ful lenght human CR2 protein sequence, amino acids 1-20 comprsie the elader peptide, amino acids 23-82 comprise SCR1, 91-146 comrpise SCR2, 154-210 comprise SCR3, 215-271 compise s SCR4. The active site (C3d binding site) is located in SCR1-2 (the first two N-terminal SCR domains). These SCR domains are separated by short sequences of variable lenght that serve as spacers. (Gilkeson, WO2007149567). 

Functions: CR2 serves as a receptor for breakdown products of the complement protein C3 such as  C3b, iC3b and C3d cleavage fragments via a binding site located within the first two amino-terminal short consensus repeats (SCRs 1 and 2) of the CR2 protein. Consequently, the SCR1-2 domain of CR2 discriminates between cleaved (i.e., activated) forms of C3 and intact circulating C3. CR2 is present on mature B lymphocytes, CD4 and CD8 positive peripheral T lymphocytes, early thymocytes, epithelial cells and follicular dendritic cells. It plays a role in B cell activation, the generation of immunologic memory, Ig class switching and the regulation of homotypic and heterotypic adhesion. Natural ligands of CR2 include the iC3b and C3d fragments of complement C3. C3d coats foreign antigens and the subsequent cross-liking of CR2 bound C3d to B cell receptors amplifies a signal transduction cascade through the CR2/CD19/CD81 co-activation complex of B cells. 

CR2 is not an inhibitor of ocmplement and it does not bind C3b, unlike the inhibiotrs of complement activation (e.g., DAF, MCP, CR1 and Crry). (Tomlisnon, WO/2004/045520). 

Human CR2 is also the obligate receptor for the Epstein-Barr virus (EBV) through its interactions with the gp350/220 viral membrane protein. CR2 also serves as a receptor for CD23 and has been shown to be essential for normal humoral immunity to T dependent antigens as well as to possibly play an important role in the maintenance of B cell self tolerance and the development of autoimmunity. CR2 has also been shown to mediate the interaction of C3-bound HIV-1 as an immune complex with B cells in a fashion that promotes transfer of virus and infection of CD4T cells.

CR3 and CR4: 

Functions: are transmembrane heterodimers which are involved in adhesion to extracellular matrix and to other cells as well as in recognition of iC3b. They belong to the integrin family and perform functions not only in phagocytosis, but also in leukocyte trafficking and migration, synapse formation and costimulation. Integrin adhesiveness is regulated through a process called inside-out signaling, transforming the integrins form a low to a high affinity binding state. In addition, ligand binding transduces signals from the extracellular domain to the cytoplasm.

CRIg (huSTIgMA, Z39Ig, PRO362): 

Structure: A human CRIg protein was first cloned from a human fetal cDNA library using degenerate primers recognizing conserved Ig domains of human JAM1. Sequencing of several clones revealed an open reading from of 400 amino acids. Blast searches confirmed similarity to Z39Ig, a type 1 transmembrane protein. The novel human protein was originally designated as a “single transmembrane Ig superfamily member macrophage associated” (huSTIgMA, also referred to as PRO362). Subsequently a splice variant of huSTIgMA was cloned, which lacks membrane proximal IgC domain and is 50 amino acids shorter, designated as huSTIgMAshort.

The extracellular IgV domain of CRIg (CRIg-ECD) selectively inhibits the AP pathway by binding to C3b and inhibiting proteolytic activation of C3 and C5. (Bing, “improving therapeutic efficacy of a complement receptor by structure-based affinity maturation” J. biol. chemistry, 284(51), 2009)

Mouse and human CRIg proteins share 83% sequence homology within the IgV domain and they superimpose with a root mean square deviation of 0.37 A for 100C- atoms. Sturctural requirements for C3b binding are also similar in that alanine substitution of residues M86 and D87 or H57 and Q59 showed a greater than 100 fold loss of binding affinity to C3b(Katschke “a novel inhibitor of the alternative pathway of complement reverses inflammation and bone destruction in experimental arthritis” JEM, 204(6) (2007).

Functions: CRIgis a macrophage associated receptor with homology to A33 antigen and JAM1 that is required for the clearance of pathogens form the blood. Next to functioning as a complement receptor for C3 proteins, the extracellular IgV domain of CRIg selectively inhibits the AP by binding to C3b and inhibiting proteolytic activation of C3 and C5. However, CRIg binding affinity for the covnertase subunit C3b is low requiring a relativley high concentraiton of protein to reach near complete complement inhibition. See regulation of the complement system

CRIg is exclusively expressed on tissue resident macrophages and binds to multimers of C3b and iC3b that are covalently attached to particle surfaces. Next to functionaing as an important clearance receptor, CRIg’s extracellular domain inhibits complement activation through the AP, but not the classical pathway. (He “a role of macrophage complement receptor CRIg in immune clearance and inflammation”, Molecular Immunology, 2008, 45(16), pp. 4041-4047.

CRIg variants with enhanced binding affinity:

Ashkenazi (WO 2006/042329 A2) discloses identification of CRIg and its use for the treatment of complement associated diseases. Also disclosed are CRIg variants having at least about 80% amino acid sequence identiy to a native sequence of CRIg. 

Sidhu (12/387794) disclose CRIg variants with combined amino acid substitutions Q64R and M86Y which showed enhanced binding affinity to C3b compared to native wild type sequence. The mutatns were generated by looking at the crystal structure of CRIg in complex with C3b and using phage display to improve binding affinity for CRIg-ECD for C3b. Phage displayd libraries were generated in which codons encoding residues of CRIg-ECD that contact C3b were subjected to limited randomization. Variants with improved binding affinity for C3b were selected and sequenced and recombinatn mutatn CRIg-ECD proteins were expressed and purified. (Bing, “improving therapeutic efficacy of a complement receptor by structure-based affinity maturation” J. biol. chemistry, 284(51), 2009)