Classical and alternative pathways of complement activation. The complement system: an overview

Complement system- a complex of complex proteins that are constantly present in the blood. This is a cascade system of proteolytic enzymes, designed for the humoral protection of the body from the action of foreign agents, it is involved in the implementation of the body's immune response. It is an important component of both innate and acquired immunity.

History of the concept

At the end of the 19th century, it was found that blood serum contains a certain “factor” with bactericidal properties. In 1896, a young Belgian scientist Jules Bordet, who worked at the Pasteur Institute in Paris, showed that there are two different substances in the serum, the combined action of which leads to the lysis of bacteria: a thermostable factor and a thermolabile (losing its properties when serum is heated) factor. The thermostable factor, as it turned out, could act only against some microorganisms, while the thermolabile factor had nonspecific antibacterial activity. The thermolabile factor was later named complement. The term "complement" was coined by Paul Ehrlich in the late 1890s. Ehrlich was the author of the humoral theory of immunity and introduced many terms into immunology, which later became generally accepted. According to his theory, cells responsible for immune responses have receptors on their surface that serve to recognize antigens. We now call these receptors "antibodies" (the basis of the variable receptor of lymphocytes is an IgD class antibody attached to the membrane, less often IgM. Antibodies of other classes in the absence of the corresponding antigen are not attached to cells). The receptors bind to a specific antigen, as well as to the heat-labile antibacterial component of the blood serum. Ehrlich called the thermolabile factor "complement" because this component of the blood "serves as a complement" to the cells of the immune system.

Ehrlich believed that there are many complements, each of which binds to its own receptor, just as a receptor binds to a specific antigen. In contrast, Bordet argued that there is only one type of "complement". At the beginning of the 20th century, the dispute was resolved in favor of Bordet; it turned out that complement can be activated with the participation of specific antibodies or independently, in a non-specific way.

General view

Components of the complement system

Complement is a protein system that includes about 20 interacting components: C1 (a complex of three proteins), C2, C3, ..., C9, factor B, factor D and a number of regulatory proteins. All these components are soluble proteins with a mol. weighing from 24,000 to 400,000, circulating in the blood and tissue fluid. Complement proteins are synthesized mainly in the liver and make up approximately 5% of the total globulin fraction of blood plasma. Most are inactive until activated either by an immune response (involving antibodies) or directly by an invading microorganism (see below). One of the possible results of complement activation is the sequential association of the so-called late components (C5, C6, C7, C8 and C9) into a large protein complex that causes cell lysis (lytic, or membrane attack complex). Aggregation of late components occurs as a result of a series of successive proteolytic activation reactions involving early components (C1, C2, C3, C4, factor B and factor D). Most of these early components are proenzymes that are sequentially activated by proteolysis. When any of these proenzymes is specifically cleaved, it becomes the active proteolytic enzyme and cleaves the next proenzyme, and so on. Because many of the activated components bind tightly to membranes, most of these events occur on cell surfaces. The central component of this proteolytic cascade is C3. Its activation by cleavage is the main reaction of the entire complement activation chain. C3 can be activated in two main ways - classical and alternative. In both cases, C3 is cleaved by an enzyme complex called C3 convertase. Two different pathways lead to the formation of different C3 convertases, however, both of them are formed as a result of spontaneous association of two complement components activated earlier in the chain of the proteolytic cascade. C3 convertase cleaves C3 into two fragments, the larger of which (C3b) binds to the target cell membrane next to C3 convertase; as a result, an even larger enzyme complex with an altered specificity is formed - C5-convertase. Then the C5 convertase cleaves C5 and thereby initiates the spontaneous assembly of the lytic complex from the late components - from C5 to C9. Since each activated enzyme cleaves many molecules of the next proenzyme, the activation cascade of early components acts as an enhancer: each molecule activated at the beginning of the entire chain leads to the formation of many lytic complexes.

The main stages of activation of the complement system.

Classical and alternative ways of activation of the complement system.

The complement system works as a biochemical cascade of reactions. Complement is activated by three biochemical pathways: the classical, alternative, and lectin pathways. All three activation pathways produce different variants of C3 convertase (a protein that cleaves C3). classic way(it was discovered first, but evolutionarily new) requires antibodies to activate (specific immune response, adaptive immunity), while alternative and lectin pathways can be activated by antigens without the presence of antibodies (nonspecific immune response, innate immunity). The result of complement activation in all three cases is the same: C3 convertase hydrolyzes C3, creating C3a and C3b and causing a cascade of further hydrolysis of complement system elements and activation events. In the classical pathway, activation of C3 convertase requires the formation of the C4bC2a complex. This complex is formed upon cleavage of C2 and C4 by the C1 complex. The C1 complex, in turn, must bind to class M or G immunoglobulins for activation. C3b binds to the surface of pathogens, which leads to a greater “interest” of phagocytes in C3b-associated cells (opsonization). C5a is an important chemoattractant that helps attract new immune cells to the area of complement activation. Both C3a and C5a have anaphylotoxic activity, directly causing degranulation of mast cells (as a result, release of inflammatory mediators). C5b starts the formation of membrane attack complexes (MACs) consisting of C5b, C6, C7, C8 and polymeric C9. MAC is the cytolytic end product of complement activation. MAC forms a transmembrane channel that causes osmotic lysis of the target cell. Macrophages engulf pathogens labeled by the complement system.

biological functions

Now there are the following functions:

opsonizing function. Immediately following the activation of the complement system, opsonizing components are formed that cover pathogens or immune complexes, attracting phagocytes. The presence of the C3b receptor on the surface of phagocytic cells enhances their attachment to opsonized bacteria and activates the absorption process. This tighter attachment of C3b-bound cells or immune complexes to phagocytic cells has been termed immune attachment phenomenon.
Solubilization (ie dissolution) of immune complexes (C3b molecule). With complement deficiency, immunocomplex pathology (SLE-like conditions) develops. [SLE = systemic lupus erythematosus]
Participation in inflammatory reactions. Activation of the complement system leads to the release of biologically active substances (histamine, serotonin, bradykinin) from tissue basophils (mast cells) and basophilic blood granulocytes, which stimulate the inflammatory response (inflammatory mediators). Biologically active components that are formed during splitting C3 and C5, lead to the release of vasoactive amines, such as histamine, from tissue basophils (mast cells) and blood basophilic granulocytes. In turn, this is accompanied by relaxation of smooth muscles and contraction of capillary endothelial cells, increasing vascular permeability. Fragment C5a and other complement activation products promote chemotaxis, aggregation and degranulation of neutrophils and the formation of oxygen free radicals. Administration of C5a to animals resulted in arterial hypotension, pulmonary vasoconstriction, and increased vascular permeability due to endothelial damage.
Functions of C3a:
- act as a chemotactic factor, causing the migration of neutrophils towards the place of its release;
- induce attachment of neutrophils to the vascular endothelium and to each other;
- activate neutrophils, causing them to develop a respiratory burst and degranulation;
- stimulate the production of leukotrienes by neutrophils.
Cytotoxic or lytic function. In the final stage of activation of the complement system, a membrane attack complex (MAC) is formed from the late complement components, which attacks the membrane of a bacterial or any other cell and destroys it.

Factor C3e, formed by the breakdown of factor C3b, has the ability to cause the migration of neutrophils from the bone marrow, and in this case, be the cause of leukocytosis.

Activation of the complement system

classic way

The classical path is triggered by the activation of the complex C1(it includes one C1q molecule and two C1r and C1s molecules each). The C1 complex binds via C1q to class M and G immunoglobulins associated with antigens. Hexameric C1q is shaped like a bouquet of unopened tulips, the “buds” of which can bind to the α-site of antibodies. A single IgM molecule is sufficient to initiate this pathway, activation by IgG molecules is less efficient and requires more IgG molecules.

С1q binds directly to the surface of the pathogen, this leads to conformational changes in the C1q molecule, and causes the activation of two molecules of C1r serine proteases. They cleave C1s (also a serine protease). The C1 complex then binds to C4 and C2 and then cleaves them to form C2a and C4b. C4b and C2a bind to each other on the surface of the pathogen to form the classical pathway C3 convertase, C4b2a. The appearance of C3 convertase leads to the splitting of C3 into C3a and C3b. C3b forms, together with C2a and C4b, the C5 convertase of the classical pathway. C5 is cleaved into C5a and C5b. C5b remains on the membrane and binds to the C4b2a3b complex. Then C6, C7, C8 and C9 are connected, which polymerizes and a tube appears inside the membrane. Thus, the osmotic balance is disturbed and, as a result of turgor, the bacterium bursts. The classical way is more accurate, since any foreign cell is destroyed in this way.

Alternative path

An alternative pathway is triggered by hydrolysis of C3 directly on the surface of the pathogen. Factors B and D are involved in the alternative pathway. With their help, the formation of the C3bBb enzyme occurs. Protein P stabilizes it and ensures its long-term functioning. Further, PC3bBb activates C3, as a result, C5-convertase is formed and the formation of a membrane attack complex is triggered. Further activation of the terminal complement components occurs in the same way as in the classical pathway of complement activation. In the liquid in the C3bBb complex, B is replaced by the H-factor and, under the influence of a deactivating compound (H), is converted to C3bi. When microbes enter the body, the C3bBb complex begins to accumulate on the membrane, catalyzing the splitting of C3 into C3b and C3a, significantly increasing the concentration of C3b. Another C3b molecule joins the properdin+C3bBb complex. The resulting complex cleaves C5 into C5a and C5b. C5b remains on the membrane. There is a further assembly of MAC with alternate addition of factors C6, C7, C8 and C9. After the connection of C9 with C8, C9 polymerization occurs (up to 18 molecules are crosslinked with each other) and a tube is formed that penetrates the bacterial membrane, water is pumped in and the bacterium bursts.

The alternative pathway differs from the classical one in the following way: activation of the complement system does not require the formation of immune complexes; it occurs without the participation of the first complement components - C1, C2, C4. It also differs in that it works immediately after the appearance of antigens - its activators can be bacterial polysaccharides and lipopolysaccharides (they are mitogens), viral particles, tumor cells.

Lectin (mannose) pathway of activation of the complement system

The lectin pathway is homologous to the classical pathway of activation of the complement system. It uses the mannose-binding lectin (MBL), a protein similar to the classical C1q activation pathway, that binds to mannose residues and other sugars on the membrane, allowing for the recognition of a variety of pathogens. MBL is a serum protein belonging to the group of collectin proteins, which is synthesized mainly in the liver and can activate the complement cascade by directly binding to the surface of the pathogen.

In blood serum, MBL forms a complex with MASP-I and MASP-II (Mannan-binding lectin Associated Serine Protease, MBL-binding serine proteases). MASP-I and MASP-II are very similar to C1r and C1s of the classical activation pathway and may have a common evolutionary ancestor. When several active sites of MBL bind in a certain way to oriented mannose residues on the phospholipid bilayer of the pathogen, MASP-I and MASP-II are activated and cleave the C4 protein into C4a and C4b, and the C2 protein into C2a and C2b. C4b and C2a then combine on the surface of the pathogen to form C3 convertase, and C4a and C2b act as chemoattractants for cells of the immune system.

Regulation of the complement system

The complement system can be very dangerous to host tissues, so its activation must be well regulated. Most of the components are active only as part of the complex, while their active forms can exist for a very short time. If during this time they do not meet with the next component of the complex, then the active forms lose their connection with the complex and become inactive. If the concentration of any of the components is below the threshold (critical), then the work of the complement system will not lead to physiological consequences. The complement system is regulated by special proteins that are found in blood plasma in even higher concentrations than the complement system proteins themselves. The same proteins are present on the membranes of the body's own cells, protecting them from attack by the proteins of the complement system.

Regulatory mechanisms mainly operate at three points.

C1. Inhibitor C1 controls the classical and lectin activation pathways. It acts in two ways: it limits the action of C4 and C2 by binding to C1r and C1s proteases and similarly turns off the lectin pathway by removing MASP enzymes from the MBP complex.
C3 convertase. The lifetime of C3-convertase is reduced by decay accelerating factors. Some of them are found on the surface of their own cells (for example, DAF and CR1). They act on C3 convertases in both the classical and alternative pathways of activation. DAF accelerates the breakdown of the alternative pathway C3 convertase. CR1 (C3b/C4b receptor) is located mainly on the surface of erythrocytes and is responsible for the removal of opsonized immune complexes from blood plasma. Other regulatory proteins are produced by the liver and are dissolved in the blood plasma in an inactive state. Factor I is a serine protease that cleaves C3b and C4b. C4-binding protein (C4BP) cleaves C4 and helps factor I cleave C4b. Factor H binds to glycosaminoglycans that are present on self cells but not on pathogen cells. This protein is a factor I cofactor and also inhibits C3bBb activity.
C9. CD59 and Homologous Limiting Factor inhibit C9 polymerization during the formation of the membrane attack complex, preventing it from forming. Used by HIV and cytomegalovirus to protect against the host's complement system.

The role of the complement system in disease

The complement system plays a large role in many immune-related diseases.

Complement is a complex set of proteins that work together to remove extracellular forms of a pathogen; the system is activated spontaneously by certain pathogens or by the antigen:antibody complex. Activated proteins either directly destroy the pathogen (killer action) or provide better uptake by phagocytes (opsonizing action); or perform the function of chemotactic factors, attracting inflammatory cells to the zone of penetration of the pathogen.

The complex of complement proteins forms cascade systems found in blood plasma. These systems are characterized by the formation of a fast, multiply amplified response to the primary signal due to a cascade process. In this case, the product of one reaction serves as a catalyst for the next, which ultimately leads to the lysis of the cell or microorganism.

There are two main ways (mechanisms) of complement activation - classical and alternative.

The classical complement activation pathway is initiated by the interaction of the C1q complement component with immune complexes (antibodies bound to bacterial cell surface antigens); as a result of the subsequent development of a cascade of reactions, proteins with cytolytic (killer) activity, opsonins, chemoattractants are formed. This mechanism links acquired immunity (antibodies) with innate immunity (complement).

An alternative complement activation pathway is initiated by the interaction of the C3b complement component with the surface of the bacterial cell; activation occurs without the participation of antibodies. This pathway of complement activation is a factor in innate immunity.

In general, the complement system refers to the main systems of innate immunity, the function of which is to distinguish "self" from "non-self". This differentiation in the complement system is carried out due to the presence on the body's own cells of regulatory molecules that suppress complement activation.

Summary. Complement (complement) [lat. complementum- addition]:

1) in immunology - a group of proteins (usually from 9 to 20) that are normally present in the blood serum of vertebrates, which are activated as a result of the body's immune response under the action of both antibodies related to immunoglobulins of the IgG and IgM classes, and bacterial liposaccharides or other compounds; protein complex of blood serum, one of the components of innate immunity. Complement is involved in the regulation of inflammatory processes, activation of phagocytosis and lytic action on cell membranes, and is activated by interaction with the immune complex. The ca system is considered, along with macrophages, as the frontier of the body's immune defense. During complement activation, a cascade of successive reactions of specific limited enzymatic proteolysis occurs, in which the inactive components of complement. become active as a result of cleavage of peptide fragments. The latter have different physiological activity and can be anaphylatoxins (cause smooth muscle contractions, increase vascular permeability, etc.), chemotaxis factors (provide directed cell movement) and leukocytosis, mediators of immune response reactions, participate in the activation of macrophages and lymphocytes, and in the regulation of antibody production. , and also perform some other functions. Fragments of activated complement components also control the biosynthesis and release of interleukins, prostaglandins, and leukotrienes. Complement causes impaired immune responses (may cause autoimmune diseases) and the release of histamine during immediate allergic reactions. The term "complement" was introduced by P. Ehrlich and Yu. Morgenrot in 1900;

2) in genetics - a group of chromosomes produced from a specific nucleus of a gamete or zygote and consisting of one, two or more chromosome sets (H. Darlington, 1932).

History of the concept

General view

Components of the complement system

The main stages of activation of the complement system.

biological functions

Now there are the following functions:

opsonizing function. Immediately following the activation of the complement system, opsonizing components are formed that cover pathogens or immune complexes, attracting phagocytes. The presence of the C3b receptor on the surface of phagocytic cells enhances their attachment to opsonized bacteria and activates the absorption process. This tighter attachment of C3b-bound cells or immune complexes to phagocytic cells has been termed immune attachment phenomenon.
Solubilization (ie dissolution) of immune complexes (C3b molecule). With complement deficiency, immunocomplex pathology (SLE-like conditions) develops. [SLE = systemic lupus erythematosus]
Participation in inflammatory reactions. Activation of the complement system leads to the release of biologically active substances (histamine, serotonin, bradykinin) from tissue basophils (mast cells) and basophilic blood granulocytes, which stimulate the inflammatory response (inflammatory mediators). Biologically active components that are formed during splitting C3 and C5, lead to the release of vasoactive amines, such as histamine, from tissue basophils (mast cells) and blood basophilic granulocytes. In turn, this is accompanied by relaxation of smooth muscles and contraction of capillary endothelial cells, increasing vascular permeability. Fragment C5a and other complement activation products promote chemotaxis, aggregation and degranulation of neutrophils and the formation of oxygen free radicals. Administration of C5a to animals resulted in arterial hypotension, pulmonary vasoconstriction, and increased vascular permeability due to endothelial damage.
Functions of C3a:
- act as a chemotactic factor, causing the migration of neutrophils towards the place of its release;
- induce attachment of neutrophils to the vascular endothelium and to each other;
- activate neutrophils, causing them to develop a respiratory burst and degranulation;
- stimulate the production of leukotrienes by neutrophils.
Cytotoxic or lytic function. In the final stage of activation of the complement system, a membrane attack complex (MAC) is formed from the late complement components, which attacks the membrane of a bacterial or any other cell and destroys it.

Factor C3e, formed by the breakdown of factor C3b, has the ability to cause the migration of neutrophils from the bone marrow, and in this case, be the cause of leukocytosis.

Activation of the complement system

classic way

Alternative path

Lectin (mannose) pathway of activation of the complement system

Regulation of the complement system

Regulatory mechanisms mainly operate at three points.

C1. Inhibitor C1 controls the classical and lectin activation pathways. It acts in two ways: it limits the action of C4 and C2 by binding to C1r and C1s proteases and similarly turns off the lectin pathway by removing MASP enzymes from the MBP complex.
C3 convertase. The lifetime of C3-convertase is reduced by decay accelerating factors. Some of them are found on the surface of their own cells (for example, DAF and CR1). They act on C3 convertases in both the classical and alternative pathways of activation. DAF accelerates the breakdown of the alternative pathway C3 convertase. CR1 (C3b/C4b receptor) is located mainly on the surface of erythrocytes and is responsible for the removal of opsonized immune complexes from blood plasma. Other regulatory proteins are produced by the liver and are dissolved in the blood plasma in an inactive state. Factor I is a serine protease that cleaves C3b and C4b. C4-binding protein (C4BP) cleaves C4 and helps factor I cleave C4b. Factor H binds to glycosaminoglycans that are present on self cells but not on pathogen cells. This protein is a factor I cofactor and also inhibits C3bBb activity.
C9.

Complement is the most important element of the immune system of vertebrates and humans, which plays a key role in the humoral mechanism of the body's defense against pathogens. The term was first introduced by Erlich to refer to a component of blood serum, without which its bactericidal properties disappeared. Subsequently, it was found that this functional factor is a set of proteins and glycoproteins, which, when interacting with each other and with a foreign cell, cause its lysis.

Complement literally translates as "complement". Initially, it was considered just another element that provides the bactericidal properties of live serum. Modern ideas about this factor are much broader. It has been established that complement is a very complex, finely regulated system that interacts with both humoral and cellular factors of the immune response and has a powerful effect on the development of the inflammatory response.

general characteristics

In immunology, the complement system is a group of vertebrate blood serum proteins that exhibit bactericidal properties, which is an innate mechanism of the body's humoral defense against pathogens, capable of acting both independently and in combination with immunoglobulins. In the latter case, complement becomes one of the levers of a specific (or acquired) response, since antibodies by themselves cannot destroy foreign cells, but act indirectly.

The effect of lysis is achieved due to the formation of pores in the membrane of a foreign cell. There may be many such holes. The membrane-perforating complex of the complement system is called MAC. As a result of its action, the surface of the foreign cell becomes perforated, which leads to the release of the cytoplasm to the outside.

Complement accounts for about 10% of all serum proteins. Its components are always present in the blood, without any effect until the moment of activation. All the effects of complement are the result of successive reactions - either splitting its constituent proteins, or leading to the formation of their functional complexes.

Each stage of such a cascade is subject to strict reverse regulation, which, if necessary, can stop the process. Activated complement components exhibit a wide range of immunological properties. In this case, the effects can have both positive and negative effects on the body.

Main Functions and Effects of Complement

The actions of the activated complement system include:

Lysis of foreign cells of bacterial and non-bacterial nature. It is carried out due to the formation of a special complex that is embedded in the membrane and makes a hole in it (perforates).
Activation of the removal of immune complexes.
Opsonization. Attaching to the surfaces of targets, complement components make them attractive to phagocytes and macrophages.
Activation and chemotactic attraction of leukocytes to the focus of inflammation.
formation of anaphylotoxins.
Facilitate the interaction of antigen presenting and B cells with antigens.

Thus, complement has a complex stimulating effect on the entire immune system. However, excessive activity of this mechanism can adversely affect the state of the body. Negative complements include:

Worsening of the course of autoimmune diseases.
Septic processes (subject to mass activation).
Negative effect on tissues in the focus of necrosis.

Defects in the complement system can lead to autoimmune reactions, i.e. damage to healthy tissues of the body by its own immune system. That is why there is such a strict multistage control of the activation of this mechanism.

Complement proteins

Functionally, the proteins of the complement system are divided into components:

Classical way (C1-C4).
Alternative pathway (factors D, B, C3b and properdin).
Membrane attack complex (C5-C9).
Regulatory faction.

The numbers of C-proteins correspond to the sequence of their detection, but do not reflect the sequence of their activation.

Complement regulatory proteins include:

H factor.
C4 binding protein.
Membrane cofactor protein.
Complement receptors of the first and second type.

C3 is a key functional element, since it is after its breakdown that a fragment (C3b) is formed, which attaches to the membrane of the target cell, starting the process of formation of the lytic complex and triggering the so-called amplification loop (positive feedback mechanism).

Activation of the complement system

Complement activation is a cascade reaction in which each enzyme catalyzes the activation of the next. This process can occur both with the participation of the components of acquired immunity (immunoglobulins), and without them.

There are several ways of complement activation, which differ in the sequence of reactions and the set of proteins involved in it. However, all these cascades lead to one result - the formation of a convertase that cleaves the C3 protein into C3a and C3b.

There are three ways to activate the complement system:

Classical.
Alternative.
Lectin.

Among them, only the first one is associated with the acquired immune response system, while the rest have a non-specific nature of action.

In all activation pathways, 2 stages can be distinguished:

Starting (or actually activation) - includes the entire cascade of reactions until the formation of C3 / C5-convertase.
Cytolytic - means the formation of a membrane attack complex (MCF).

The second part of the process is similar in all stages and involves proteins C5, C6, C7, C8, C9. In this case, only C5 undergoes hydrolysis, while the rest simply attach, forming a hydrophobic complex capable of incorporating and perforating the membrane.

The first stage is based on the sequential launch of the enzymatic activity of proteins C1, C2, C3 and C4 by hydrolytic cleavage into large (heavy) and small (light) fragments. The resulting units are denoted by small letters a and b. Some of them carry out the transition to the cytolytic stage, while others play the role of humoral factors of the immune response.

classic way

The classical pathway of complement activation begins with the interaction of the C1 enzyme complex with the antigen-antibody group. C1 is a fraction of 5 molecules:

C1q(1).
C1r(2).
C1s(2).

At the first step of the cascade, C1q binds to immunoglobulin. This causes a conformational rearrangement of the entire C1 complex, which leads to its autocatalytic self-activation and the formation of the active enzyme C1qrs, which cleaves the C4 protein into C4a and C4b. In this case, everything remains attached to the immunoglobulin and, therefore, to the membrane of the pathogen.

After the implementation of the proteolytic effect, the antigen group - C1qrs attaches the C4b fragment to itself. Such a complex becomes suitable for binding to C2, which is immediately cleaved by C1s into C2a and C2b. As a result, the C3 convertase C1qrs4b2a is created, the action of which forms the C5 convertase, which triggers the formation of MAC.

Alternative path

Such activation is otherwise called idle, since the hydrolysis of C3 occurs spontaneously (without the participation of intermediaries), which leads to the periodic, causeless formation of C3 convertase. An alternative path is carried out when the pathogen has not yet formed. The cascade consists of the following reactions:

Blank hydrolysis of C3 to form a C3i fragment.
C3i binds to factor B to form the C3iB complex.
The bound factor B becomes available for cleavage by the D-protein.
The Ba fragment is removed and the C3iBb complex remains, which is the C3 convertase.

The essence of blank activation is that C3-convertase is unstable in the liquid phase and rapidly hydrolyzes. However, upon collision with the membrane of the pathogen, it stabilizes and starts the cytolytic stage with the formation of MAC.

lectin pathway

The lectin pathway is very similar to the classical one. The main difference lies in the first step of activation, which is carried out not through interaction with immunoglobulin, but through the binding of C1q to the terminal mannan groups present on the surface of bacterial cells. Further activation is carried out completely identical to the classical path.

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The complement system, consisting of about 30 proteins, both circulating and expressed on the membrane, is an important effector branch of both innate and antibody-mediated adaptive immune responses. The term "complement" comes from the fact that this temperature-sensitive blood serum material was found to "complement" the ability of antibodies to kill bacteria. Complement is known to play a major role in defense against many infectious microorganisms.

The most important components of its protective function are: 1) the production of opsonins - molecules that increase the ability of macrophages and neutrophils to phagocytosis; 2) the production of anaphylatoxins - peptides that induce local and systemic inflammatory reactions; 3) direct killing of microorganisms.

Other important complement functions are also known, such as enhancing antigen-specific immune responses and maintaining homeostasis (stability within the body) by removing immune complexes and dead or dying cells. We also know that disruption of complement activation can lead to cell and tissue damage in the body.

Complement components are synthesized in the liver, as well as by cells involved in the inflammatory response. The concentration of all complement proteins in the circulating blood is approximately 3 mg/ml. (For comparison: IgG concentration in the blood is about 12 mg/mL) Concentrations of some complement components are high (for example, about 1 mg/mL for C3), while other components (such as factor D and C2) are present in trace amounts. .

Complement activation pathways

The initial stages of complement activation are sequential cascade activation of one after another of its components. At this stage, the activation of one component induces the action of the enzyme, which leads to the activation of the next component in turn. Since one active enzyme molecule is capable of cleaving many substrate molecules, this cascade of reactions amplifies the relatively weak initial signal. These cascade properties of the complement system are similar to those observed in other serum cascades directed towards clot formation and production of kinins, vascular inflammatory mediators.

Upon activation, individual components are split into fragments, denoted by lowercase letters. The smaller of the split fragments is usually denoted by the letter "a", the larger - "b". Historically, however, the larger of the cleaved C2 fragments is usually referred to as C2a and the smaller as C2b. (However, in some texts and articles, fragments of C2 complement components are denoted inversely.) Further cleavage fragments are also denoted in small letters, for example, C3d.

There are three pathways for complement activation: classic, lectin and alternative.

The beginning of each of the pathways of activation is characterized by its own components and processes of recognition, however, at later stages in all three cases, the same components are used. The properties of each activation pathway and the substances that activate them are discussed next.

classic way

The classical activation pathway is so called because it was defined first. The protein components of the classical pathway are designated C1, C2, C9. (The numbers are in the order in which the components were discovered, not in which they are activated.) Antigen-antibody complexes are the main activators of the classical pathway. Thus, the latter is the main effector pathway for activating the humoral adaptive immune response.

Other activators are certain viruses, dead cells and intracellular membranes (eg, mitochondria), immunoglobulin aggregates, and β-amyloid found in plaques in Alzheimer's disease. C-reactive protein is an acute phase protein - a component of the inflammatory response; it attaches to the polysaccharide phosphorylcholine expressed on the surface of many bacteria (eg Streptococcus pneumoniae) and also activates the classical pathway.

The classical pathway is initiated when C1 attaches to an antibody in an antigen-antibody complex, such as an antibody bound to an antigen expressed on the surface of a bacterium (Figure 13.1). Component C1 is a complex of three different proteins: Clq (containing six identical subcomponents) associated with two molecules (each with two) - Clr and Cls. Upon activation of Cl, its globular regions - subcomponents of Clq - bind to a Clq-specific region on the Fc fragments of either one IgM or two closely spaced IgG molecules associated with the antigen (IgG binding is shown in Fig. 13.1).

Thus, IgM and IgG antibodies are effective complement activators. Human immunoglobulins that have the ability to bind to Cl and activate it, in decreasing order of this ability, are: IgM>> IgG3> IgG 1 » IgG2. Immunoglobulins IgG4, IgD, IgA and IgE do not interact with Clq, do not fix or activate it, i.e. do not activate complement via the classical pathway.

After C1 binds to the Cls antigen-antibody complex, it acquires enzymatic activity. This active form is known as Cls-esterase. It splits the next component of the classical path - C4 - into two parts: C4a and C4b. A smaller part - C4a - remains in a dissolved state, and C4b covalently binds to the surface of the bacterium or other activating substance.

The portion of C4b attached to the cell surface then binds C2, which is cleaved by Cls. When C2 is cleaved, a C2b fragment is obtained, which remains in a dissolved state, and C2a. In turn, C2a attaches to C4b on the cell surface to form the C4b2a complex. This complex is called the classical pathway C3 convertase because, as we will see later, this enzyme cleaves the next component, C3.

lectin pathway

The lectin pathway is activated by terminal mannose residues in proteins and polysaccharides located on the surface of the bacterium. These residues are not found on the surface of mammalian cells; therefore, the lectin pathway can be considered as a means of recognizing self and nonself. Because this activation pathway does not require the presence of antibodies, it is part of the innate immune defense system.

On fig. Figure 13.1 shows how bacterial mannose residues bind to the circulating mannose-binding lectin (MBL) complex; similar in structure to the Clq of the classical pathway) and two associated proteases called mannose-associated serine proteases (MASP-1 and -2). This binding activates MAP-1 to subsequently cleave the components of the classical complement pathway, C4 and C2, to form C4b2a, the classical pathway C3 convertase, on the bacterial surface. And MASP-2 has the ability to directly cleave C3. Thus, the lectin pathway after the C3 activation phase is similar to the classical one.

Alternative path

The alternative pathway for complement activation is triggered by almost any foreign substance. The most studied substances include lipopolysaccharides (LPS, also known as gram-negative bacterial cell wall endotoxins), some yeast cell walls, and a protein found in cobra venom (cobra venom factor). Some agents that activate the classical pathway, viruses, immunoglobulin aggregates, and dead cells, also trigger the alternative pathway.

Activation occurs in the absence of specific antibodies. Thus, the alternative complement activation pathway is an effector branch of the innate immune defense system. Some components of the alternative pathway are unique to it (serum factors B and D and properdin, also known as factor P), while others (C3, C3b, C5, C6, C7, C8 and C9) are shared with the classical pathway.

The C3b component appears in the blood in small amounts after spontaneous cleavage of the reactive thiol group in C3. This "pre-existing" C3b is able to bind to the hydroxyl groups of proteins and carbohydrates expressed on cell surfaces (see Figure 13.1). Accumulation of C3b on the cell surface initiates an alternative pathway.

It can occur both on a foreign and on the body's own cell; thus, in terms of the alternate path, it is always running. However, as discussed in more detail below, the body's own cells regulate the course of alternative pathway reactions, while non-self cells do not have such regulatory abilities and cannot prevent the development of subsequent events of the alternative pathway.

Rice. 13.1. Launch of the classical, lectin and alternative pathways. Demonstration of activation of each pathway and formation of C3 convertase

In the next step of the alternative pathway, a whey protein, factor B, binds to C3b on the cell surface to form the C3bB complex. Factor D then cleaves factor B, which is located on the cell surface in the C3bB complex, resulting in a fragment of Ba, which is released into the surrounding fluid, and Bb, which remains associated with C3b. This C3bBb is an alternative pathway C3 convertase that cleaves C3 to C3a and C3b.

Usually C3bBb dissolves quickly, but can be stabilized when combined with properdin (see Fig. 13.1). As a result, properdin-stabilized C3bBb is able to bind and cleave large amounts of C3 in a very short time. Accumulation on the cell surface of these rapidly formed large amounts of C3b leads to an almost "explosive" launch of the alternative pathway. Thus, the binding of properdin to C3bBb creates an alternative pathway amplification loop. The ability of properdin to activate the amplification loop is controlled by the opposite action of regulatory proteins. Therefore, the activation of the alternative path does not occur all the time.

Activation of C3 and C5

C3 cleavage is the main phase for all three activation pathways. On fig. 13.2 shows that C3 convertases in the classical and alternative pathways (C4b2a and C3bBb, respectively) cleave C3 into two fragments. The smaller C3a is a soluble anaphylatoxin protein: it activates cells involved in the inflammatory response. The larger fragment, C3b, continues the activation process of the complement cascade by binding to cell surfaces around the site of activation. As shown below, C3b is also involved in host defense, inflammation, and immune regulation.

Rice. 13.2. Cleavage of component C3 by C3-convertase and component C5 by C5-convertase in the classical and lectin (top) and alternative (bottom) pathways. In all cases, C3 is cleaved into C3b, which is deposited on the cell surface, and C3, which is released into the liquid medium. In the same way, C5 is cleaved into C5b, which is deposited on the cell surface, and C5a, which is released into the liquid medium.

The binding of C3b to C3 convertases, both in the classical and alternative pathways, initiates the binding and cleavage of the next component, C5 (see Fig. 13.2). For this reason, C3 convertases associated with C3b are classified as C5 convertases (C4b2a3b in the classical pathway; C3bBb3b in the alternative). When C5 is cleaved, two fragments are formed. Fragment C5a is released in a soluble form and is an active anaphylatoxin. The C5b fragment binds to the cell surface and forms a nucleus for binding to the terminal complement components.

terminal path

The terminal components of the complement cascade - C5b, C6, C7, C8 and C9 - are common to all activation pathways. They bind to each other and form a membrane attack complex (MAC), which causes cell lysis (Fig. 13.3).

Rice. 13.3 Formation of the membrane attack complex. Complement components of the late phase - C5b-C9 - sequentially connect and form a complex on the cell surface. Numerous C9 components attach to this complex and polymerize to form poly-C9, creating a channel that spans the cell membrane.

The first phase of MAC formation is the attachment of C6 to C5b on the cell surface. C7 then binds to C5b and C6 and penetrates the outer membrane of the cell. Subsequent binding of C8 to C5b67 leads to the formation of a complex that penetrates deeper into the cell membrane. On the cell membrane, C5b-C8 acts as a receptor for C9, a perforin-type molecule that binds to C8.

Additional C9 molecules interact in complex with the C9 molecule, forming polymerized C9 (poly-C9). These poly-C9 form a transmembrane channel that disrupts the osmotic balance in the cell: ions penetrate through it and water enters. The cell swells, the membrane becomes permeable to macromolecules, which then leave the cell. The result is cell lysis.

R. Koiko, D. Sunshine, E. Benjamini