IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 4. Complement

■ The Complement System is one of the core systems of innate Immunity, functioning to distinguish "self" from "non-self". This differentiation is achieved through the presence of regulatory molecules on the Organism's own Cells that suppress complement activation.

■ There are two principal pathways (mechanisms) of complement activation—the classical and alternative pathways. Classical activation is triggered by the binding of immune complexes to C1q, thereby bridging acquired immunity (Antibodies) with innate immunity (complement).

■ A continuous "ticking-over" or spontaneous activation of C3 occurs in Blood Plasma, resulting in the deposition of small numbers of its molecules on the surfaces of both "self" and "non-self" structures. On host cells, regulatory Proteins induce the destruction of bound C3 molecules and inhibit further complement activation. Conversely, on foreign structures lacking regulatory proteins, alternative complement activation is initiated.

■ The presence of an internal thioester bond in C3 and C4 proteins enables them to interact covalently with hydroxyl and amino groups on other molecules. The formation of this bond is a pivotal event in local complement activation at sites of inflammation.

■ Complement activation involves two Amplification mechanisms. The first is known as the "enzyme cascade". The "trigger signal" is the binding of a small number of C1q molecules, which subsequently prompts the sequential Activation of a series of zymogens (proenzymes) that cleave a vastly greater number of C3 molecules.

■ The second amplification mechanism is a positive feedback loop termed the "amplification loop". The Cleavage of a small amount of C3 to form C3b generates the C3 convertase enzyme, which in turn cleaves much more C3. Host cells possess molecules that inhibit this amplification loop by breaking down C3b into inactive products. On foreign structures, the action of the "amplification loop" proceeds unchecked.

■ The effector mechanisms of the complement system are divided into five functional groups: 1) opsonization of microbes for phagocytic uptake; 2) direct destruction of microorganisms via lysis; 3) activation and chemotactic recruitment of leukocytes to inflammatory sites; 4) Processing (specific cleavage) of immune complexes; 5) induction of specific antibodies through, firstly, enhanced localization of Antigens On the surface of B lymphocytes and antigen-presenting cells and, secondly, lowering the threshold for B lymphocyte activation.

■ Pathogenic microbes have evolved mechanisms to evade destruction by the complement system, and in some cases, even exploit it to enhance their pathogenicity.

■ The complement system can contribute to disease Pathogenesis when generalized activation occurs in vivo or when activation targets host Tissues As a result of autoantibody binding.

INTRODUCTION

The term "complement" was originally coined by Ehrlich to describe an "accessory" serum activity without which specific antibodies cannot lyse Bacteria. The discovery of this heat-labile serum activity is generally credited to Bordet (1895), although a similar phenomenon had been described a few years earlier by Nuttall. In 1907, Ferrata, by dialyzing serum against acidified Water, established that complement Proteins can be separated into two fractions: precipitable euglobulins and a water-soluble albumin fraction (pseudoglobulins). Complement activity was manifested only in the presence of both fractions, initially designated as the mid-piece and end-piece, and later as the C'1 and C'2 components. Subsequently, Sachs and Omorokov discovered that cobra venom inactivates another complement component (C'3), while Gordon demonstrated that the next component (C'4) is destroyed by ammonia. The chronological order of discovery of these complement components does not correspond to the sequence in which they enter the activation cascade, which explains the apparent lack of logic in its modern nomenclature.

Complex Nomenclature of the Complement System

The Proteins of the classical activation pathway and the membrane-attack complex are numbered individually and enter the activation sequence in the following order: C1q, C1r, C1s, C4, C2, C3, C5, C6, C7, C8, C9. Many of these are enzyme precursors—proenzymes—that acquire activity only upon cleavage. The designation of an active enzyme is distinguished from its inactive precursor by a superscript bar, for example, C1r. Cleavage products are designated similarly to the parent complement components, but with The addition of lowercase letters—typically "a" for the smaller fragment and "b" for the larger one, such as C3a and C3b. There is one exception to this rule: C2b denotes the smaller fragment and C2a the larger fragment of C2.

Proteins of the alternative activation pathway are termed factors and are designated by single-letter symbols. In text, the word "factor" is usually abbreviated to the initial letter F or omitted entirely, meaning that factor B can be designated by the abbreviation FB or simply B. Regulatory proteins are most frequently denoted by Abbreviations reflecting their functional activity: for example, the protein that accelerates the decay of the classical pathway C3 convertase is designated as DAF (decay-accelerating factor).

Cellular receptors that bind complement components are named According to the abbreviations of their ligands (e.g., C5a receptor) or as marker molecules within the CD system nomenclature. Receptors for the major C3 fragments are numbered separately as complement receptors type 1, 2, 3, and 4 (CR1, CR2, CR3, and CR4). Unfortunately, as a result of this, some receptors in contemporary literature have up to three synonyms, such as C3b receptor = CR1 = CD35.

Complement System Proteins Belong to Diverse Superfamilies

Proteins grouped within a single superfamily—such as IMMUNOGLOBULINS (see Chapter 6)—share many Structural and functional properties. The complement system comprises proteins belonging to several distinct superfamilies.

Classification of complement proteins by superfamily provides a clearer understanding of their structural and functional relationships. This can be illustrated using the superfamily of regulatory complement proteins, also known as regulators of complement activation. These include:

✵ factor H—a plasma protein with an elongated molecular conformation;

✵ C4-binding protein [C4bp]—a heptameric plasma protein featuring a spider-like molecular Structure;

✵ decay-accelerating factor (DAF, CD55)—a Cell membrane protein anchored via a specialized glycophospholipid "tail";

✵ membrane cofactor protein (MCP, CD46)—a transmembrane protein acting as a cofactor for C3b cleavage;

✵ complement receptors type 1 (CR1, CD35) and type 2 (CR2, CD21)—cellular receptors containing transmembrane domains.

The family of regulatory complement proteins is encoded by a group of tightly linked genes located on chromosome 1. Despite clear structural differences, all these proteins share a common domain consisting of approximately 60 amino acid residues, known as the short consensus repeat. This domain may appear multiple times within each protein molecule, forming its structural backbone and likely determining binding Specificity. The synthesis of these proteins is encoded by homologous, tandemly arranged exons.

The six proteins that make up this family also share A number of common Functions in complement activation: factor H, C4bp, DAF, MCP, and CR1 suppress the Formation of the C4b2a and C3bBb complexes, i.e., the C3 convertases of the classical and alternative activation pathways. Some of these proteins share other common functions as well, which are not identical but only partially overlapping. These functions include: suppressing the binding of C2 to C4b and factor B to C3b, inducing the dissociation of C2a from C4b and Bb from C3b, and acting as Cofactors for factor I, the enzyme responsible for the Catabolism of C3b and C4b.

It should be noted that short consensus repeats are also found in other proteins that do not interact with complement proteins; these include the IL-2 receptor, ₂-glycoprotein I, and coagulation factor XIII.

The structure of most complement proteins is "mosaic." The Molecular Basis of the relationship among proteins within different families is becoming clearer thanks to the cloning of their genes. According to current models, evolution involved repeated duplication of exons and their "shuffling" among various genes. Residing simultaneously within different genes, these duplicated DNA segments evolved in parallel and, in many cases, retained similar sequences and functions, although in some instances activity was lost or new activity was acquired.

Many complement proteins represent a "mosaic" of exon products belonging to Genes from different superfamilies. For instance, C1s, an enzyme of the classical pathway, contains Amino Acid Sequence motifs derived from Serine esterase and the low-density lipoprotein receptor, as well as a short consensus repeat found in the regulatory complement protein superfamily. Similarly, C6, C7, C8, and C9—Components of the membrane-attack complex—share common properties with the perforin of cytotoxic T Lymphocytes and eosinophil cationic protein.

Complement activation is a major effector mechanism of inflammation

The complement system is a component of innate immunity and comprises a series of proteins that act sequentially—in a cascade where each enzyme catalyzes The activity of the next. The most important complement component is C3, which is present in blood plasma at concentrations (1–2 mg/mL) comparable to those of certain immunoglobulins.

The two Main Pathways of complement activation reflect its dual role in innate and adaptive immune responses. The classical pathway is associated with adaptive immunity, as the C1q protein interacts with antibodies complexed with an antigen. The alternative pathway of complement activation belongs to innate immune mechanisms, initiating via the non-specific binding of C3b to The surface of a microorganism.

The in vivo activity of individual complement components can be illustrated by disorders caused by deficiencies in these proteins (see ch. 21). Affected patients exhibit an increased susceptibility to recurrent pyogenic bacterial infections, as well as to conditions characterized by elevated autoantibody and immune complex formation. These observations highlight the essential role of complement both in antibacterial defense and in the clearance of immune complexes that could otherwise cause autoimmune and immune-complex diseases (see ch. 25).

Complement activation during inflammation results in:

✵ opsonization of microorganisms and immune complexes;

✵ leukocyte activation;

✵ lysis of target cells (Fig. 4.1).

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Fig. 4.1. 1. Opsonization ("coating") of microorganisms and immune complexes by complement to facilitate their recognition by cells expressing complement receptors. 2. Lysis of target cells. 3. Activation of phagocytes, including macrophages and neutrophils.

Opsonization. This is the enhancement of phagocytosis resulting from the attachment of complement proteins to the surface of targets (microbes, immune complexes, etc.). Equipped with receptors for opsonizing proteins, phagocytic cells bind these targets, which triggers phagocyte activation and the endocytosis or phagocytosis of the targets.

Leukocyte activation. Polymorphonuclear granulocytes and macrophages possess specific receptors for small complement protein fragments generated on target surfaces as a result of the proteolytic cascade. Diffusing into the surrounding microenvironment, these fragments recruit phagocytes (directed cell movement, or chemotaxis) and, upon binding to them, trigger their activation.

Target cell lysis. The proteolytic complement cascade culminates in the insertion of a hydrophobic "probe" into The Lipid Bilayer of the target cell membrane, followed by its osmotic rupture and lysis.

Complement is capable of distinguishing "self" from "non-self." As a component of innate immunity, complement employs mechanisms that allow it to differentiate between self and non-self. The key to this function lies in the immediate binding of C3b to all foreign entities, whether microorganisms or immune complexes; the surfaces of the body's own cells are protected by specialized molecules that very effectively restrict C3b deposition.