Basics of Immunology - Lecture Course by M. V. Skok - Kyiv 2002

Chapter III. Immune Processes at the Organism Level

Appendix 1. The Complement System

The Complement System is an ancient host defense mechanism against Bacteria that emerged long before the adaptive immune system. In higher animals, it complements immune responses and facilitates the destruction of microorganisms, Cells infected with bacteria and Viruses, as well as the host's own transformed cells. The complement system and The Immune System are closely interconnected: immune complexes activate the complement system, while complement components influence the course of immune responses, notably by promoting the clearance of Antigens and antigen-antibody complexes from the bloodstream.

The complement system was discovered in the late 19th century by J. Bordet, who described it as a heat-labile serum component capable of inducing bacterial lysis. It was later revealed that the complement system consists of more than 20 Proteins that interact sequentially in a cascade process, similar to Blood Coagulation and Fibrinolysis, where the product of each reaction acts as a catalyst for the next. Complement proteins belong to globulins, but they are not IMMUNOGLOBULINS; their levels do not increase upon immunization. Complement components are designated by the letter C followed by a number corresponding to the chronological order of their discovery, which does not always reflect the sequence in which they participate in the reaction cascade.

All Components of the complement system can be divided into several functional groups (Table 1).

Table 1. Functional groups of complement proteins.

Component C3 is present in the blood at the highest concentration (up to 1.2 mg/ml). It is with this component that the cascade of reactions leading to the BIOLOGICAL EFFECTS OF complement begins (Fig. 21). Component C3 is cleaved into two fragments: C3a and C3b (note that the letter b denotes the fragment with a higher molecular weight, while the letter a denotes the lower molecular weight fragment. Typically, b components attach to cells or preformed complexes, whereas a components remain in solution). C3b covalently binds to Polysaccharides in The Cell walls of Gram-negative bacteria and to specific receptors on the cells of higher organisms. On The cell membrane, C3b binds component C5, which is cleaved into C5a and C5b. C5b diffuses into the surrounding environment, while C5b remains bound to C3b. C5b initiates the self-assembly of the large membrane attack complex (MAC). It sequentially binds components C6, C7, and C8. The newly formed complex allows the attachment of 12 to 21 molecules of component C9, which pierce the Cell Membrane and form a pore approximately 100 Å in diameter (Fig. 22). Low-molecular-weight compounds can pass freely through such a pore, but proteins and other large biological molecules cannot. As a result, the cell is destroyed via osmotic lysis.

Component C3 contains a characteristic metastable thioester bond in its Structure, which enables The formation of a covalent bond with the cell surface. Under normal conditions, this bond is activated very slowly by Water or plasma factors, yielding a small amount of soluble C3b-like protein. In the presence of magnesium ions, it forms a complex with factor B, designated C3bB. Factor B is then cleaved by factor D into Ba and Bb, forming the C3bBb complex, known as C3 convertase. It exhibits enzymatic activity and can efficiently cleave component C3, thereby triggering the entire MAC formation cascade.

In the absence of infectious agents, this process is tightly regulated: factor B is readily displaced by factor H, and the C3bH complex is attacked by factor I, which inactivates C3b; the latter is subsequently cleaved by Trypsin-like proteases present in Blood Plasma. When Gram-negative bacteria are present in the blood, C3b binds to their membrane lipopolysaccharides, evading binding to factor H, and the newly formed C3 convertase is further stabilized by a specialized protein, properdin. Consequently, as new C3 molecules are cleaved by the convertase, fresh C3b molecules immediately bind to the microbial membrane. Thus, a single catalytic center of C3 convertase can promote the formation and binding of numerous C3b molecules around itself on the membrane, which in turn trigger the MAC cascade, leading to the destruction of the bacterial cell. This pathway of complement activation is evolutionarily the most ancient. It is called the alternative pathway because a more complex pathway involving Antibodies, known as the classical pathway, was discovered first.

The classical pathway of complement activation is initiated by the formation of antigen-antibody immune complexes. Immune complexes containing immunoglobulins of classes M and G are capable of binding complement, with the binding site located within the Fc fragments (specifically the Cμ4 and Cγ2 domains, respectively). These immune complexes bind component C1q. This massive protein consists of six sets of three polypeptide types, featuring a Collagen-like fibrillar region and a globular domain. The fibrillar regions are wound into a triple helix, while the globular domains are grouped for each set of chains, forming a structure resembling a tulip bouquet. In the presence of Calcium Ions, two additional proteins—dimers of C1r and C1s, which are trypsin-like proteases activated upon binding to the immune complex—attach to C1q. This yields component C1, composed of a single C1q molecule and a C1r₂s₂ tetramer (Fig. 23). C1s within the C1 complex cleaves component C4. Like C3, C4 contains an internal thioester bond that allows covalent attachment to the cell membrane. During this process, C4a is released, and C4b binds component C2, forming the C4b2 complex. In the presence of magnesium ions, C1s cleaves C2 into C2a and C2b, and the resulting C4b2b complex acts as a C3 convertase, analogous to C3bBb in The alternative pathway. The formation of C4b2b is regulated by C4-binding protein (C4bp), which prevents C2 binding or degrades the C4b2b convertase.

Thus, There are two main pathways for generating C3 convertase, which triggers the MAC reaction cascade: the classical pathway, mediated by immune complexes and leading to the Formation of the C4b2b convertase, and the alternative pathway, initiated directly by bacteria and leading to the formation of the C3bBb convertase. Recently, it was discovered that certain bacteria and even RNA viruses can bind component C1 and trigger the classical pathway in the absence of immune complexes. Additionally, two Lectins—proteins that specifically bind CARBOHYDRATES of bacterial cell walls—have been found in the serum of all vertebrates: Ra-reactive factor (RaRF) and mannose-binding protein (MBP). These proteins belong to acute-phase proteins, meaning their blood concentrations increase during inflammation. RaRF can dissociate into a C1q-like molecule capable of recognizing polysaccharides and two Polypeptides possessing proteolytic activity. This factor can act directly on C4 and C2 in the absence of C1r or C1s. MBP structurally resembles C1q as well. It binds to a mannose-associated Serine protease in serum, and the resulting complex can effectively activate C1r₂s₂. MBP and RaRF stimulate a third, lectin-dependent pathway of complement activation.

The primary goal of the complement system is to destroy target cell membranes through the formation of the MAC. Furthermore, complement components attached to bacterial cells are recognized by receptors on macrophages, facilitating the phagocytosis of infectious agents. This process is known as opsonization. Complement also helps clear small immune complexes from the body that are poorly recognized by Fc receptors, such as Bacterial toxins and antibody-bound remnants of dead microorganisms. Such complexes bind to CR1 receptors on erythrocytes, which transport them to The Liver and Spleen, where macrophages strip and degrade them without destroying the erythrocyte.

The low-molecular-weight fragments C3a, C4a, and C5a released into the bloodstream upon complement activation mediate the inflammatory process and are referred to as anaphylatoxins. They induce smooth Muscle contraction, increase vascular permeability, and activate mast cells to secrete histamine, serotonin, Proteolytic Enzymes, heparin, chemokines, and inflammatory mediators such as Prostaglandins, thromboxanes, and Leukotrienes. This, in turn, attracts lymphocytes, neutrophils, and phagocytes to the site of inflammation, promoting both phagocytosis and The Development of a specific Immune Response.

Host cells possess specialized regulatory mechanisms to prevent damage to healthy Tissues by complement activity. For example, if the C5b67 complex fails to bind to a target cell, it can be scavenged by low-density Lipoproteins or vitronectin. Such complexes can subsequently bind C8 and C9, but they cannot attach to cell membranes and lack the characteristic transmembrane channel structure. Certain membrane Glycoproteins (protectin, DAF, C8bp, glycophorin A) limit the binding of MAC components to the cell. DAF also accelerates the decay of C4b2b.

Bacteria also have specialized defense mechanisms against complement action:

- some Gram-negative bacteria lack C3b-binding sites;

- others feature long-chain glycoproteins on their surfaces that bind C3b at a distance from the membrane;

- surface enzymes can degrade cell-bound C3b and C4b, and some bacteria rapidly shed them;

- several bacterial species secrete substances that bind complement components in solution;

- some bacteria possess antiphagocytic capsules that prevent phagocytes from reaching bound C3b;

- streptococcal Carboxypeptidase A inactivates the anaphylatoxins C5a and C3a, while other bacteria secrete enzymes that destroy intact complement components.

Summary.

Complement is a system of blood Serum proteins that promote the destruction of infectious agents and infected host cells through both direct lysis and The stimulation of other mechanisms, including the immune response.

Main Functions of the complement system:

1) lysis of target cells via MAC formation;

2) opsonization of target cells to enhance their phagocytosis;

3) clearance of immune complexes from the Circulation;

4) stimulation of the inflammatory response.

Complement activation pathways:

1) classical pathway — triggered by immune complexes; mediated by the C4bC2b convertase;

2) alternative pathway — activated directly by bacterial cells; mediated by the C3bBb convertase;

3) lectin pathway — activated by bacteria; mediated by serum lectins that bind to bacterial membrane polysaccharides.

The effects of complement on cells are mediated by specialized receptors. There are adaptive defense mechanisms in both macro- and microorganisms to protect against complement-mediated destruction.