Opsonization- Definition, Mechanism, Opsonins, Examples

Opsonization is a process in which molecules, microbes, or apoptotic cells are chemically modified to have stronger interactions with cell surface receptors on phagocytes and antibodies . This process is important in host defense and makes particles or complexes readily ingestible for uptake by phagocytic cells. Opsonins, such as antibodies and complement components, coat dangerous antigens and mark them for destruction by phagocytes . Without opsonization, the recognition and destruction of invading agents such as bacteria would be inefficient. The complement system plays a major role in opsonization by coating particles such as bacteria with fixed C3 and C4.

Opsonization can be defined as the molecular mechanism whereby molecules, microbes, or apoptotic cells are marked for an immune response or marked dead cells for recycling. An opsonin is any molecule that enhances phagocytosis by marking an antigen for an immune response or marking dead cells for recycling. The purpose of opsonization is to make the antigens palatable to the antibody or the phagocytic cells.

Opsonization can occur via antibodies or the complement system. Antibody-mediated opsonization involves the coating of pathogens with antibodies so that they are recognized and phagocytosed by innate immune cells. Complement-mediated opsonization involves the cleavage of C3 into C3a and C3b, which binds to the surface of the particle and serves as an opsonin. Phagocytic cells have receptors for C3b and Fc portion of IgG antibodies, which mediate binding and ingestion of opsonized particles .

Opsonization is an essential immune process that helps to clear infection and prevent disease. Opsonins have important roles in the immune system like marking of dead and dying cells for clearance by macrophages and neutrophils. Besides, opsonins also aid in activating the complement proteins and destruction of cells by natural killer (NK) cells. Opsonization is one of the ways that the immune system identifies and targets foreign particles for elimination.

Opsonization is the process of coating pathogens with molecules that enhance their recognition and uptake by phagocytic cells. These molecules are called opsonins and they act as bridges between the pathogens and the phagocytes. Opsonins can be either antibodies or complement proteins, which are part of the innate and adaptive immune systems respectively.

Antibody-mediated Opsonization

Antibodies are proteins produced by B cells in response to specific antigens. They have two regions: a variable region that binds to the antigen, and a constant region that interacts with other immune cells or molecules. The constant region of antibodies can bind to Fc receptors on the surface of phagocytes, such as macrophages and neutrophils. This allows the phagocytes to recognize and engulf the antibody-coated pathogens more efficiently. The most effective antibodies for opsonization are IgG and IgM, which have different subclasses and structures that influence their ability to bind to Fc receptors and activate the complement system .

Complement-mediated Opsonization

The complement system is a cascade of proteins that can be activated by different pathways: the classical pathway, which is triggered by antibody-antigen complexes; the alternative pathway, which is initiated by microbial surfaces; and the lectin pathway, which is activated by carbohydrate-binding proteins called lectins . The activation of any of these pathways leads to the cleavage of C3, a central component of the complement system, into C3a and C3b. C3b is a potent opsonin that can bind to microbial surfaces and expose a thioester bond that reacts with hydroxyl or amino groups on the pathogen. C3b can also bind to complement receptors on phagocytes, such as CR1, CR3, and CR4, and facilitate their attachment and ingestion of the opsonized pathogens . C3b can also form complexes with other complement proteins, such as C4b and C5b, that have additional opsonic or lytic functions .

Antibody-mediated opsonization is a process by which a pathogen is marked for phagocytosis by antibodies. Antibodies are part of the adaptive immune system and are produced by plasma cells in response to a specific antigen. Different antigens stimulate different B cells to develop into plasma cells. An antibody’s complex structure enables its specificity to certain antigens. At the end of the light and heavy chains, antibodies have variable regions, also known as antigen-binding sites. These sites allow the antibody to fit like “a lock and key” into the epitopes of specific antigens.

Once the antigen-binding sites are bound to the epitopes on the antigen, the stem region of the antibody binds to the receptor on the phagocytes. This is known as Fc receptor, which recognizes the Fc portion of the antibody molecule. The Fc receptor can bind to different subclasses of IgG antibodies, such as IgG1 and IgG3, but not IgM antibodies. The binding of antibodies to the antigen and the immune cells results in the release of lysis products from the effector cells.

1) Antibodies (A) and pathogens (B) free roam in the blood. 2) The antibodies bind to pathogens, and can do so in different formations such as: opsonization (2a), neutralisation (2b), and agglutination (2c). 3) A phagocyte (C) approaches the pathogen, and Fc region (D) of the antibody binds to one of the Fc receptors (E) on the phagocyte. 4) Phagocytosis occurs as the pathogen is ingested.

The advantage of antibody-mediated opsonization is that it enhances the recognition and clearance of pathogens that are otherwise resistant to phagocytosis, such as encapsulated bacteria. It also activates other immune mechanisms, such as complement system, inflammation, and antibody-dependent cell-mediated cytotoxicity.

Antibody-mediated opsonization is an important defense mechanism against infections and also plays a role in removing apoptotic cells and immune complexes from circulation.

The complement system is composed of over 30 proteins that improve the ability of antibodies and phagocytic cells to fight invading organisms. It initiates phagocytosis by opsonizing antigens. This system is also responsible for enhancing inflammation and cytolysis .

The complement system can be activated by three pathways: the classical pathway, the alternative pathway, and the lectin pathway. All three pathways converge at the cleavage of C3, a central component of the system, into C3a and C3b. C3b is the most important opsonin produced by the complement system, as it can bind to the surface of antigens and serve as a recognition signal for phagocyte receptors .

The classical pathway is initiated by the binding of C1q, a subunit of the C1 complex, to the Fc region of antibodies that are bound to antigens. This activates C1r and C1s, which cleave C4 and C2 into C4a, C4b, C2a, and C2b. C4b and C2a form a complex called C4b2a, which is a C3 convertase that cleaves C3 into C3a and C3b .

The alternative pathway is initiated by the spontaneous hydrolysis of C3 into C3(H2O), which binds to factor B. Factor B is then cleaved by factor D into Ba and Bb. The complex of C3(H2O)Bb is also a C3 convertase that cleaves more C3 into C3a and C3b. Some of the C3b molecules bind to the surface of antigens, where they can bind to factor B and be cleaved by factor D again, forming a stable C3bBb complex that acts as an amplification loop for more C3 cleavage .

The lectin pathway is initiated by the binding of mannose-binding lectin (MBL) or ficolins to carbohydrate structures on the surface of antigens. This activates MBL-associated serine proteases (MASPs), which cleave C4 and C2 into C4a, C4b, C2a, and C2b. The complex of C4b and C2a forms another type of C3 convertase that cleaves C3 into C3a and C3b .

Once a particle is coated with C3b, it must then be recognized by and bound to the surface of a phagocytic cell before it can be ingested. The mechanisms by which these events occur have been partially characterized. All mononuclear phagocytes and polymorphonuclear leukocytes have receptors on their plasma membranes for C3b. These receptors are known as complement receptor 1 (CR1) or CD35. CR1 binds to both C3b and its degradation product iC3b, which are both opsonins for phagocytosis . CR1 also has a role in regulating the complement system by facilitating the cleavage of C3b and C4b by factor I.

Besides CR1, other receptors on phagocytes can also recognize complement opsonins. For example, complement receptor 2 (CR2) or CD21 binds to iC3b and enhances B cell activation. Complement receptor 3 (CR3) or CD11b/CD18 binds to iC3b and enhances phagocytosis by macrophages and neutrophils. Complement receptor 4 (CR4) or CD11c/CD18 also binds to iC3b and enhances phagocytosis by monocytes and dendritic cells.

In summary, complement-mediated opsonization is a process by which antigens are coated with complement proteins such as C3b that facilitate their recognition and phagocytosis by immune cells. This process can be triggered by different pathways that involve different components of the complement system. Complement-mediated opsonization can help clear bacteria, viruses, immune complexes, and tumor cells from the body .

Opsonins are molecules that enhance phagocytosis by marking antigens or other targets for recognition by phagocytic cells. There are many types of opsonins, which can be classified into two main categories: immune and non-immune.

Immune Opsonins

Immune opsonins are produced by the adaptive immune system in response to specific antigens. They include antibodies and some complement proteins.

• Antibodies are proteins produced by B cells that bind to specific antigens on the surface of pathogens or other targets. They have two antigen-binding sites (Fab) and one constant region (Fc) that can interact with Fc receptors on phagocytes. The most effective antibodies for opsonization are IgG subclasses 1 and 3, which have high affinity for Fc receptors and can activate the classical complement pathway.

• Complement proteins are a group of serum proteins that can be activated by three pathways: classical, alternative, and lectin. They form a cascade of reactions that result in the formation of membrane attack complexes (MAC) that lyse target cells, and the generation of opsonins such as C3b, C4b, and C1q. These opsonins bind to complement receptors on phagocytes and facilitate their ingestion of the target.

Non-immune Opsonins

Non-immune opsonins are produced by the innate immune system or other sources and can bind to various targets without prior exposure. They include lectins, collectins, ficolins, pentraxins, and some circulating proteins.

• Lectins are carbohydrate-binding proteins that can recognize specific sugar patterns on the surface of pathogens or other targets. They include mannose-binding lectin (MBL), which initiates the lectin pathway of complement activation and binds to MBL receptors on phagocytes.

• Collectins are a type of lectin that have both carbohydrate-binding domains and collagen-like regions. They include surfactant proteins A and D (SP-A and SP-D), which are secreted by alveolar cells in the lungs and bind to various pathogens and apoptotic cells. They also activate the lectin pathway of complement and interact with collectin receptors on phagocytes.

• Ficolins are another type of lectin that have both carbohydrate-binding domains and fibrinogen-like regions. They include ficolin-1, -2, and -3, which are produced by the liver or leukocytes and bind to N-acetylglucosamine residues on various pathogens and apoptotic cells. They also activate the lectin pathway of complement and interact with ficolin receptors on phagocytes.

• Pentraxins are a family of cyclic multimeric proteins that have a pentameric structure. They include C-reactive protein (CRP) and serum amyloid P component (SAP), which are produced by the liver in response to inflammation or infection. They bind to phosphocholine residues on various pathogens and apoptotic cells and activate the classical pathway of complement. They also interact with Fc receptors and scavenger receptors on phagocytes.

• Circulating proteins are a diverse group of proteins that can act as opsonins for various targets. They include fibronectin, thrombospondin, vitronectin, properdin, coagulation factors, kininogens, histones, and defensins. They bind to different receptors on phagocytes such as integrins, CD36, CD14, CD11b/CD18, CD35, CD55, CD59, TLRs, NLRs, RLRs, etc.

Antibodies are proteins produced by B cells of the adaptive immune system that can bind to specific antigens on the surface of pathogens or other targets. Antibodies can act as opsonins, which are molecules that enhance phagocytosis by marking the targets for recognition by phagocytes.

Antibodies have a Y-shaped structure with two antigen-binding sites (Fab) at the tips of the arms and a constant region (Fc) at the base of the stem. The Fab regions can bind to epitopes on the antigens, while the Fc region can bind to Fc receptors on the surface of phagocytes, such as macrophages, neutrophils, and dendritic cells.

When antibodies coat a target with multiple Fab regions, they form a bridge between the target and the phagocyte, bringing them into close contact and facilitating engulfment. The binding of antibodies to Fc receptors also triggers intracellular signaling pathways that activate the phagocyte and initiate phagocytosis.

Different classes of antibodies have different abilities to opsonize targets. IgG and IgM are the most effective opsonins among the five classes of immunoglobulins (IgA, IgD, IgE, IgG, and IgM). IgG is the most abundant antibody in serum and can bind to four types of Fc receptors (FcγRI, FcγRIIa, FcγRIIb, and FcγRIII) on various phagocytes. IgM is the first antibody produced in response to an infection and can activate the classical pathway of complement system, which generates another opsonin, C3b.

Antibody-mediated opsonization is a specific and efficient mechanism to clear pathogens, dying cells, and diseased cells from the body. It also helps to present antigens to T cells and elicit inflammatory responses. However, some pathogens have evolved strategies to evade antibody-mediated opsonization, such as changing their surface antigens, producing proteases that degrade antibodies, or expressing proteins that block Fc receptors or complement activation.

Complement proteins are part of the innate immune system and are involved in various functions such as inflammation, cytolysis, and opsonization. Opsonization is the process of coating pathogens with molecules that enhance their recognition and phagocytosis by immune cells. Complement proteins that act as opsonins include C3b, C4b, and C1q.

C3b

C3b is a fragment of C3, the central component of the complement system. C3 can be cleaved into C3a and C3b by different pathways: the classical pathway (initiated by antibody-antigen complexes), the lectin pathway (initiated by mannose-binding lectin or ficolins binding to microbial carbohydrates), and the alternative pathway (initiated by spontaneous hydrolysis of C3 or by microbial surfaces). C3b can bind covalently to the surface of pathogens or other particles and serve as an opsonin. C3b can also form a complex with factor B, which can be cleaved by factor D into Ba and Bb. The C3bBb complex is a C3 convertase, which can cleave more C3 into C3a and C3b, amplifying the complement activation and opsonization. C3b can also form a complex with C4b and factor B, which can be cleaved by factor D into Ba and Bb. The C4bC3bBb complex is a C5 convertase, which can cleave C5 into C5a and C5b, initiating the formation of the membrane attack complex (MAC) that can lyse pathogens.

C3b can be recognized by several receptors on phagocytic cells, such as CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18), and CR4 (CD11c/CD18). These receptors can bind to C3b-coated pathogens and mediate their phagocytosis and clearance. CR1 can also act as a cofactor for factor I, which can cleave C3b into inactive fragments iC3b and C3d. This is a regulatory mechanism to prevent excessive complement activation and damage to host cells.

C4b

C4b is a fragment of C4, another component of the complement system. C4 can be cleaved into C4a and C4b by the classical pathway or the lectin pathway. Similar to C3b, C4b can bind covalently to the surface of pathogens or other particles and serve as an opsonin. C4b can also form a complex with C2, which can be cleaved by C1s into C2a and C2b. The C4b2a complex is a C3 convertase, which can cleave C3 into C3a and C3b. The C4b2aC3b complex is a C5 convertase, which can cleave C5 into C5a and C5b.

C4b can be recognized by CR1 on phagocytic cells, which can bind to C4b-coated pathogens and mediate their phagocytosis and clearance. CR1 can also act as a cofactor for factor I, which can cleave C4b into inactive fragments iC4b and C4d.

C1q

C1q is a subunit of the C1 complex, which initiates the classical pathway of complement activation. The classical pathway is triggered by the binding of the globular heads of C1q to antibody-antigen complexes or to some microbial surfaces. This activates the associated serine proteases C1r and C1s, which then cleave C4 and C2 to form the C3 convertase.

C1q can also act as an opsonin by binding directly to some pathogens or apoptotic cells. This binding can be recognized by several receptors on phagocytic cells, such as cC1qR (calreticulin), gC1qR (p33), CD93 (C1qRp), CD91 (LRP), CD11c/CD18 (CR4), CD11c/CD18 (CR4), CD14, CD36, FcγRIIa (CD32), FcγRIIIa (CD16), FcγRIIIb (CD16), FcαRI (CD89), and FcεRI (CD23). These receptors can mediate the phagocytosis and clearance of C1q-coated pathogens or apoptotic cells. C1q can also modulate the expression and function of some of these receptors, as well as cytokine production and cell activation.

Circulating proteins are proteins that are present in the blood plasma and can act as opsonins by binding to specific molecules on the surface of pathogens. These proteins are secreted by the liver or other cells and are part of the innate immune system. They recognize common patterns of molecules on pathogens, such as carbohydrates, lipids, or nucleic acids, and coat them with opsonins to facilitate their recognition and phagocytosis by immune cells. Some of the circulating proteins that function as opsonins are:

• Pentraxins: These are a family of proteins that have a pentameric structure and bind to phosphorylcholine, a component of bacterial cell walls. Pentraxins include C-reactive protein (CRP) and serum amyloid P component (SAP), which are acute-phase proteins that increase in concentration during inflammation. Pentraxins can activate the classical pathway of complement system and bind to Fc receptors on phagocytes .
• Collectins: These are a group of proteins that have a collagen-like domain and a carbohydrate recognition domain. Collectins include mannose-binding lectin (MBL), surfactant proteins A and D (SP-A and SP-D), and ficolin-1, -2, and -3. Collectins bind to mannose, fucose, N-acetylglucosamine, and other sugars on the surface of bacteria, fungi, viruses, and parasites. Collectins can activate the lectin pathway of complement system and bind to C1q receptors on phagocytes .
• Ficolins: These are similar to collectins but have a fibrinogen-like domain instead of a collagen-like domain. Ficolins also bind to sugars on pathogens and can activate the lectin pathway of complement system. Ficolins can also interact with C3b receptors on phagocytes .

Circulating proteins as opsonins play an important role in enhancing the clearance of pathogens from the blood and tissues. They also modulate the inflammatory response and the adaptive immune response by interacting with various cells and molecules of the immune system.

The lectin pathway is one of the three pathways that activate the complement system, a group of proteins that enhance the immune response against pathogens and apoptotic cells. The lectin pathway involves carbohydrate-binding proteins called lectins or collectins that bind to sugars on the surface of microbes or dying cells . The lectins activate serine proteases that trigger a cascade of reactions similar to the classical pathway, but without antibody recognition . The lectin pathway leads to various effector functions such as phagocytosis, cell lysis, inflammation, and adaptive immune response .

The main lectins involved in the lectin pathway are:

• Mannose-binding lectin (MBL), which binds to mannose, glucose, or other sugars with 3- and 4-OH groups placed in the equatorial plane, in terminal positions on carbohydrate or glycoprotein components of microorganisms such as bacteria, fungi and viruses .
• Ficolins, which bind to N-acetylglucosamine residues in complex-type carbohydrates and other ligands on different antigens .
• Collectin-11 (CL-11), which binds to oligomannose structures on bacteria and fungi.

These lectins form oligomers of subunits, which are trimers or higher-order multimers. They associate with mannose-binding lectin-associated serine proteases (MASPs) and MBL-associated proteins (MAps) that are protease zymogens . The MASPs are very similar to C1r and C1s molecules of the classical complement pathway. The MAps are alternative splice products of the MASPs that may regulate their activity.

When the carbohydrate-recognizing heads of the lectins bind to specifically arranged sugars on the surface of a pathogen or a dying cell, MASP-1 and MASP-2 are activated to cleave complement components C4 and C2 into C4a, C4b, C2a, and C2b . MASP-3 may also be involved in this process by activating MASP-1 or MASP-2. C4a and C2b act as potent cytokines, with C4a causing degranulation of mast cells and basophils and C2b acting to increase vascular permeability.

C4b tends to bind to bacterial cell membranes or apoptotic cell surfaces. If it is not then inactivated, it will combine with C2a to form the classical C3 convertase (C4bC2a) on the surface of the target, as opposed to the alternative C3 convertase (C3bBb) involved in the alternative pathway . The C3 convertase cleaves C3 into C3a and C3b. C3a is an anaphylatoxin that induces inflammation. C3b binds to the target surface or opsonizes it for phagocytosis. It can also form a complex with C4bC2a to generate the classical C5 convertase (C4bC2aC3b), which cleaves C5 into C5a and C5b . C5a is another anaphylatoxin that recruits and activates leukocytes. C5b initiates the formation of the membrane attack complex (MAC) that inserts into the target membrane and causes cell lysis .

The lectin pathway is an important part of innate immunity that can recognize and eliminate pathogens and apoptotic cells without prior exposure. It can also link innate and adaptive immunity by activating complement receptors on antigen-presenting cells and B cells . However, dysregulation of the lectin pathway can also contribute to inflammatory diseases such as sepsis, ischemia-reperfusion injury, autoimmune disorders and COVID-19 .

Opsonins are molecules that bind to the surface of pathogens or other targets and enhance their phagocytosis by immune cells. There are many types of opsonins that belong to different branches of the immune system, such as antibodies, complement proteins, and circulating proteins. Here are some examples of common opsonins and their functions:

• IgM antibodies: These are the first type of antibodies produced by B cells in response to an antigen. They have a pentameric structure with 10 antigen-binding sites, which allows them to bind and agglutinate pathogens efficiently. They also activate the classical pathway of complement activation, leading to the deposition of C3b and C4b on the pathogen surface. IgM antibodies are especially effective against encapsulated bacteria and viruses.

• IgG antibodies: These are the most abundant and diverse type of antibodies in the blood. They have a monomeric structure with 2 antigen-binding sites, which allows them to bind and neutralize pathogens and toxins. They also activate the classical pathway of complement activation and bind to Fc receptors on phagocytes, facilitating opsonization and phagocytosis. IgG antibodies are involved in protection against extracellular bacteria, viruses, fungi, and parasites.

• C3b proteins: These are fragments of C3, a central component of the complement system. C3b is generated by the cleavage of C3 by C3-convertases, which can be formed by the classical, alternative, or lectin pathways of complement activation. C3b binds covalently to the surface of pathogens or other targets and acts as an opsonin by binding to complement receptors on phagocytes. C3b also forms part of the C5-convertase, which initiates the formation of the membrane attack complex (MAC) that lyses target cells.

• C4b proteins: These are fragments of C4, another component of the complement system. C4b is generated by the cleavage of C4 by C4-convertases, which can be formed by the classical or lectin pathways of complement activation. C4b binds covalently to the surface of pathogens or other targets and acts as an opsonin by binding to complement receptors on phagocytes. C4b also forms part of the C3-convertase and C5-convertase, which amplify the complement cascade.

• C1q proteins: These are subunits of C1, a complex that initiates the classical pathway of complement activation. C1q binds to IgM or IgG antibodies that are attached to antigens on the pathogen surface. This triggers the activation of C1r and C1s, which cleave C4 and C2, forming the C3-convertase. C1q also acts as an opsonin by binding to receptors on phagocytes and enhancing their phagocytic activity.

• Pentraxins: These are a family of circulating proteins that bind to various ligands on pathogens or apoptotic cells. The most studied pentraxins are serum amyloid P (SAP) and C-reactive protein (CRP), which are acute phase proteins that increase during inflammation. Pentraxins act as opsonins by binding to Fc receptors or complement receptors on phagocytes and promoting their clearance.

• Collectins: These are a family of circulating proteins that have collagen-like domains and carbohydrate recognition domains. The most studied collectins are mannose-binding lectin (MBL) and surfactant proteins A and D (SP-A and SP-D), which are involved in innate immunity in the respiratory tract. Collectins act as opsonins by binding to mannose or other sugars on pathogens or apoptotic cells and enhancing their phagocytosis by macrophages or neutrophils. Collectins also activate the lectin pathway of complement activation by binding to MBL-associated serine proteases (MASPs), which cleave C4 and C2.

• Ficolins: These are a family of circulating proteins that have collagen-like domains and fibrinogen-like domains. The most studied ficolins are ficolin-1 (M-ficolin), ficolin-2 (L-ficolin), and ficolin-3 (H-ficolin), which are involved in innate immunity in the blood. Ficolins act as opsonins by binding to N-acetylglucosamine or other sugars on pathogens or apoptotic cells and enhancing their phagocytosis by macrophages or neutrophils. Ficolins also activate the lectin pathway of complement activation by binding to MASPs, which cleave C4 and C2.

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