Excerpt from Complement Receptor Deficiency

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Background

The complement system, interacting with its regulatory molecules and cellular receptors, plays a central role in the induction and regulation of immunity.

Complement receptors have only been identified in the last 2 decades. They have a wide cellular and tissue distribution, and they play a major role in the mediation of biological responses. Their function in the transporting, processing, and clearing of immune complexes, as well as in neutrophil functions, is pivotal.

Partial or complete deficiencies of the components of the complement system, including its receptors and regulatory proteins, are now described in humans and may be of a genetic or familial origin or acquired.

Research with animals, particularly mice with specifically targeted mutations, has allowed better comprehension of the specific mechanisms involved in deficiency states and the resulting clinical manifestations and/or associated diseases.

The eMedicine Pediatrics article Complement Receptor Deficiency may be helpful.

Pathophysiology

Four distinct complement receptors, CR1, CR2, CR3, and CR4, have been described for the surface-bound complement fraction C3 and its cleavage fragments. Activation of the complement cascade always leads to the enzymatic cleavage of complement fraction C3, which is important in host defense to bacterial infections and phagocytosis.

Complement and complement receptors play a critical role in immune defense by initiating the rapid destruction of invading microorganisms, amplifying the innate and adaptive immune responses, and mediating solubilization and clearance of immune complexes. Defects in the expression of complement or complement receptors may result in loss of tolerance to self-proteins and the development of immune complex–mediated autoimmune diseases such as systemic lupus erythematosus (SLE).¹

Receptors for the anaphylatoxins C3a and C5a have also been identified.

CR1/CD35 and CR2/CD21 are major receptors for activated fragments of C3. They are single-chain molecules present on host cellular membranes and belong to the complement control protein family. They are encoded by separate but linked genes termed regulation of complement activation (RCA) genes, located on the q32 region of chromosome 1. The receptors are widely expressed in humans. They tend to enhance the effects of complement and are highly important in the binding of opsonized immune complexes on B cells.

CR1/CD35 is the receptor for C3b. The CR1 receptor preferentially binds C3b that is covalently attached to immune complexes, and it has a weaker affinity for bound C4b and iC3b. It is a single-chain membrane glycoprotein of approximately 200 kd that has 4 allotypic forms on myeloid cells (eg, erythrocytes, granulocytes, monocytes), lymphoid cells (mostly B cells), follicular dendritic cells, and glomerular podocytes.

The density of CR1 receptors on cell surfaces varies with the cell type and with the activation of the cell for neutrophils and monocytes. Because of their high numbers, red blood cells express an average of 90% of the CR1 receptors despite their having a lower number of antigenic sites per cell. Among healthy individuals, the number of CR1 receptors on the red blood cells varies widely, yet the phenotypic expression, regulated by 2 codominant alleles, is stable.

CR1 has an important role in complement and immune regulation; in phagocytosis and clearance of immune complexes; and in mediating adherence of opsonized bacteria, viruses, and immune complexes. Opsonized immune complexes (coated by C3b and C4b) bind to CR1, mostly on red blood cells, and are cleared through the liver where they can be transferred to CR3-bearing phagocytes and endocytosed. CR1 and CR2 have been shown to influence the immune environment in a B-cell receptor–independent manner.²

CR1 also regulates complement activation by acting as a cofactor for factor I in the cleavage and degradation of bound C3b and C4b to its inactive forms. It also is involved in the generation of ligands for CR2 and CR3, which are believed to be involved in the clearance of immune complexes.

CR2/CD21 interacts with C3b degradation products C3dg and C3d and can act synergistically with the B-cell antigen receptor (BCR) in B-cell activation. CR2/CD21 is a single-chain membrane glycoprotein of approximately 150 kd. C3d-or C3dg-bearing immune complexes can bind to CR2. C3dg and C3d also can bind to the surface of bacteria and fungi, allowing CR2 on B cells and follicular dendritic cells to present the organisms for immune triggering.

CR2 is expressed primarily on lymphoid cells (B and T lymphocytes) and follicular dendritic cells. CR2 plays an important role in the presentation of antigen to specific B and T cells and in the control of B-cell proliferation. Evidence clearly suggests that CR2 is involved in the induction of a primary humoral response. Humoral antibody response is regulated by the direct effect of C3dg on the cell cycle of B cells and by the dependence of the B-cell response to a soluble T-cell–dependent antigen on iC3b and C3dg.

A second component of CR2 binds the Epstein-Barr virus (EBV). EBV requires the CR2 receptor to enter the cell. EBV is an oncogenic herpes virus that is implicated in the pathogenesis of acute mononucleosis, Burkitt lymphoma, and nasopharyngeal carcinoma. It infects and immortalizes B lymphocytes by binding CR2 in vitro.

CR3 (CD11b/18) and CR4 (CD11c/18) both bind to iC3b and promote adhesive interactions of leucocytes with the vascular endothelium. The receptors may act as ligands for adhesion molecules, such as intercellular adhesion molecule 1 (ICAM-1)/CD54 for CR3. They are present on phagocytic cells. Both are members of the beta integrin family. They are heterodimers made of an identical 95-kd beta chains (CD18) and different alpha chains (165 kd for CD11b in CR3 and 150 kd for CD11c in CR4). The beta subunit is shared by another plasma membrane protein lymphocyte function associated antigen (LFA-1 or CD11a/18).

CR3 (CD11b/18) is found on eosinophils, basophils, monocytes, natural killer (NK) cells, most tissue macrophages, and neutrophils. It has a role in cellular adhesion and aggregation, particularly that of neutrophils and monocytes. CR4 (CD11c/18) is similar to CR3 structurally, has a similar affinity for iC3b, and is found on neutrophils.

CR3 and CR4 have an important role in host resistance to infection. iC3b-coated immune complexes have a high affinity for the CR3 receptor on phagocytic cells of the liver and spleen, to where they are transported and degraded.

C3a is a potent anaphylatoxin and proinflammatory mediator generated by proteolytic cleavage of C3 in complement cascade activation. It is involved in the secretion of histamine, smooth muscle contraction, and chemoattraction of eosinophils and mast cells. Findings from recent reports have paradoxically demonstrated an anti-inflammatory function for C3a in vitro. Recently, C3a receptors have been cloned. These receptors are widely distributed on mast cells, neutrophils, basophils, eosinophils, and activated B cells. Deletion of the complement anaphylatoxin C3a receptor attenuates experimental autoimmune encephalomyelitis.³

C5a receptors are expressed on neutrophils, macrophages, basophils, eosinophils, mast cells, and activated T cells, as well as on the epithelia of the proximal tubuli in the kidney, neurons, and glial cells. Activation of C5aR results in degranulation of cells, increased vascular permeability, and edema. In C5aR-deficient mice, the inflammatory response in the skin, lung, and peritoneum is reduced.

CR1 and CR2 deficiency have been found to increase coxsackievirus B3–induced myocarditis, dilated cardiomyopathy, and heart failure by increasing macrophages, interleukin 1-1beta, and immune complex deposition in the heart.¹

Leukocyte adhesion deficiency type 1 (LAD-1) is an autosomal recessive disorder caused by mutations in the ITGB2 (CD18) gene and characterized by recurrent severe infections, impaired pus formation, and defective wound healing. Somatic revertant mosaicism may be seen with LAD-1.⁴ A patient was described with a compound heterozygote bearing 2 different frameshift mutations that abrogate protein expression.

Frequency

International

Partial or complete deficiencies in all components of the complement, as well as its regulatory proteins and receptors, were described in a relatively small number of patients in association with autoimmune or infectious diseases. The deficiencies were either inherited or acquired.

Mortality/Morbidity

Complement plays a major role in the modulation of immune complex formation and its deposition, leading to tissue injury. Deficiencies in complement receptors are associated with a high frequency of immune complex diseases and infections. No predisposition to lymphoreticular malignancies is reported.

Race

No specific racial pattern is noted.

Sex

Receptor deficiencies associated with autoimmune disorders (eg, SLE) are more common in women than in men.

Age

Individuals of all ages may be affected.

• Reduced numbers of cellular receptors are present in preterm and stressed neonates.
• The leucocyte adhesion deficiency syndrome associated with deficiencies of CR3 and CR4 occurs mainly in children.

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