Information

13.2A: Opsonization - Biology

13.2A: Opsonization - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Learning Objectives

  • Discuss how antibodies defend the body by way of opsonization. (Include what classes or isotypes of immunoglobulins are involved, the role of the Fab portion of the antibody, the role, if any, of the Fc portion of the antibody, and the role of any complement proteins, if any, involved.)
  • Briefly describe two different ways bacteria may resist opsonization.

Opsonization, or enhanced attachment, refers to the antibody molecules IgG and IgE, the complement proteins C3b and C4b, and other opsonins attaching antigens to phagocytes. This results in a much more efficient phagocytosis.

Opsonization with IgG, IgA, C3b, and C4b

The process starts with antibodies of the isotype IgG, IgA, or IgM being made against a surface antigen of the organism or cell to be phagocytosed. The Fab portions of the antibody react with epitopes of the antigen. The Fc portion of IgG (but not IgM) can then bind to receptors on neutrophils and macrophages thus sticking the antigen to the phagocyte (Figure (PageIndex{1})). The Fc portion of secretory IgA can also bind to macrophages and neutrophils for opsonization.

Figure (PageIndex{1}): Opsonization (Enhanced Attachment). The Fab portion of IgG binds to epitopes of an antigen. The Fc portion can now attach the antigen to Fc receptors on phagocytes for enhanced attachment. This is especially important against encapsulated microbes. C3b and C4b from the complement pathways can also attach antigens to phagocytes.

Flash animation illustrating enhanced attachment by way of IgG.

html5 version of animation for iPad showing enhanced attachment by way of IgG.

The Fc portion of secretory IgA can also bind to macrophages and neutrophils for opsonization.

Alternately, IgG, IgA, and IgM can activate the complement pathways (Figure (PageIndex{2})) and C3b or C4b can stick the antigen to phagocytes (Figure (PageIndex{1})). Like IgG, C3b, and to a lesser extent C4b, can function as opsonins, that is, they can attach antigens to phagocytes.One portion of the C3b binds to proteins and polysaccharides on microbial surfaces; another portion attaches to CR1 receptors on phagocytes, B-lymphocytes, and dendritic cells for enhanced phagocytosis (Figure (PageIndex{3})). (Remember that C3b and C4b are also produced during the alternative complement pathway and the lectin pathway as was discussed in Unit 5.) Activation of the complement pathway also promotes inflammation to bring phagocytes and defense chemicals from the bloodstream to the infection site as discussed later under this topic.

Figure (PageIndex{2}): Activation of C1 during the Classical Complement Pathway. The Fab of 2 molecules of IgG or 1 molecule of IgM bind to epitopes on an antigen. C1, consisting of C1q, C1r, and C1s then binds to the Fc portion of the antibodies. The binding of C1q to the antibody molecules activates the C1r portion of C1 which, in turn, activates C1s. This activation gives C1s enzymatic activity to cleave complement protein C4 into C4a and C4b and complement protein C2 into C2a and C2b.

Flash animation showing the role of C5a in vasodilation, the chemotaxis of phagocytes towards C5a, and their attachment to the opsonin C3b as a result of the complement pathways.

html5 version of animation for iPad showing the role of C5a in vasodilation, the chemotaxis of phagocytes towards C5a, and their attachment to the opsonin C3b as a result of the complement pathways.

Actually, C3b molecule can bind to pretty much any protein or polysaccharide. Human cells, however, produce Factor H that binds to C3b and allows Factor I to inactivate the C3b. On the other hand, substances such as LPS on bacterial cells facilitate the binding of Factor B to C3b and this protects the C3b from inactivation by Factor I. In this way, C3b does not interact with our own cells but is able to interact with microbial cells.

For More Information: Five Classes of Human Antibodies from Unit 6

For More Information: The Complement System from Unit 5

Attachment then promotes destruction of the antigen. Microorganisms are placed in phagosomes (Figure (PageIndex{4})) where they are ultimately digested by lysosomes (Figure (PageIndex{5})). If the antigen is a cell too large to be ingested - such as virus-infected host cells, transplant cells, and cancer cells - the phagocyte empties the contents of its lysosomes directly on the cell for extracellular killing (Figure (PageIndex{6}) and Figure (PageIndex{7})).

Flash animation of opsonization and intracellular destruction.

Flash animation of opsonization and extracellular destruction.

html5 version of animation for iPad showing opsonization and intracellular destruction.

html5 version of animation for iPad showing opsonization and extracellular destruction.

Opsonization is especially important against microorganisms with antiphagocytic structures such as capsules since opsonizing antibodies made against the capsule are able to stick capsules to phagocytes (Figure (PageIndex{8})). In vaccines against pneumococccal pneumonia and Haemophilus influenzae type b, it is capsular polysaccharide that is given as the antigen in order to stimulate the body to make opsonizing antibodies against the encapsulated bacterium.

Flash animation showing phagocytosis of an encapsulated bacterium through opsonization.

html5 version of animation for iPad showing phagocytosis of an encapsulated bacterium through opsonization.

Opsonization with IgE and the promotion of inflammation

The antibody isotype IgE is made against parasitic worms (helminths) and arthropods. The Fab portions of IgE bind to epitopes on the helminth or arthropod while the Fc portion binds to receptors on eosinophils enabling opsonization. In other words, IgE sticks phagocytic eosinophils to helminths and arthropods for the extracellular killing of that organism (Figure (PageIndex{9})).

The Fc portion of IgE also binds to receptors on mast cells and basophils to trigger the release of inflammatory mediators (Figure (PageIndex{10})). The inflammatory response then enables phagocytes and defense chemicals to leave the bloodstream and go to the infected site as will be discussed later under this topic.

Exercise: Think-Pair-Share Questions

Compare and contrast how IgG, IgM, and IgE promote opsonization..

Because of a particular immunodeficiency disorder, a person is unable to produce C3 convertase. Which of the above antibody isotypes could still participate in opsonization? Briefly explain why.

How Bacteria Resist Attachment to Phagocytes

As we learned previously, some bacteria by means of the activities described below are able to resist phagocytic attachment :

  • An outer membrane molecule of Neisseria gonorrhoeae called Protein II and the M-protein of Streptococcus pyogenes allow these bacteria to be more resistant to phagocytic engulfment. The M-protein of S. pyogenes, for example, binds factor H of the complement pathway and this leads to the degradation of the opsonin C3b by factor I and the formation of C3 convertase.
  • Some capsules simply cover the C3b that does bind to the bacterial surface and prevent the C3b receptor on phagocytes from making contact with the C3b (Figure (PageIndex{11})). This is seen with the capsule of Streptococcus pneumoniae.
  • Capsules can also resist unenhanced attachment by preventing the glycoprotein receptors on phagocytes from recognizing the bacterial cell wall components and mannose-containing carbohydrates.
  • S. pneumonia activates the classical complement pathway, but resists C3b opsonization, and complement causes further inflammation in the lungs.
  • Neisseria meningitidis has a capsule composed of sialic acid while Streptococcus pyogenes (group A beta streptococci) has a capsule made of hyaluronic acid. Both of these polysaccharides closely resemble carbohydrates found in human tissue polysaccharides and because they are not recognized as foreign by the lymphocytes that carry out the immune responses, antibodies are not made against these capsules.
  • Some bacteria are able to coat themselves with host proteins such as fibronectin, lactoferrin, or transferrin. This prevents antibody molecules from binding to epitopes on the bacterial surface.
  • Staphylococcus aureus produces protein A while Streptococcus pyogenes produces protein G. Both of these proteins bind to the Fc portion of antibodies, the portion that normally binds to receptors on phagocytes (Figure (PageIndex{12})). In this way the bacteria become coated with antibodies in a way that does not result in opsonization (Figure (PageIndex{13})).
  • Streptococcus pyogenes produces Mac proteins that are able to bind to the receptors on phagocytes to which IgG and C3b normally attach (Figure (PageIndex{14}).and Figure (PageIndex{15})). This blocks opsonization.
  • Pathogenic Yersinia, such as the one that causes plague, contact phagocytes and, by means of a type III secretion system, deliver proteins which depolymerize the actin microfilaments needed for phagocytic engulfment into the phagocytes. Another Yersinia protein degrades C3b and C5a.

Summary

Opsonization, or enhanced attachment, refers to the antibody molecules IgG and IgE, the complement proteins C3b and C4b, and other opsonins attaching antigens to phagocytes. The Fab portions of the antibody IgG react with epitopes of the antigen. The Fc portion of IgG can then bind to neutrophils and macrophages thus sticking the antigen to the phagocyte. The Fc portion of secretory IgA can also bind to macrophages and neutrophils for opsonization. IgG and IgM can activate the classical complement pathway and C3b or C4b can stick the antigen to phagocytes. IgE is made against parasitic worms (helminths) and arthropods. The Fab portions of IgE bind to epitopes on the helminth or arthropod while the Fc portion binds to receptors on eosinophils enabling opsonization.

Questions

Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.

  1. Discuss how antibodies defend the body by way of opsonization. (Include what classes or isotypes of immunoglobulins are involved, the role of the Fab portion of the antibody, the role, if any, of the Fc portion of the antibody, and the role of any complement proteins, if any, involved.) (ans)
  2. We know Streptococcus pneumoniae is encapsulated and capsules resist phagocytosis. Yet the body is eventually able to phagocytose this organism. Describe how. (ans)
  3. Staphylococcus aureus produces an exotoxin called Protein A. Protein A is able to react with the Fc portion of IgG. In terms of humoral immunity, discuss how Protein A may help the Staphylococcus resist phagocytosis. (ans)
  4. The M-protein of Streptococcus pyogenes binds factor H of the complement pathway and allows these bacteria to be more resistant to phagocytic engulfment. Explain how. (ans)
  5. Multiple Choice (ans)

Complement Mediated Opsonization

Different opsonin molecules perform different functions, including antibody mediated opsonization, complement protein mediated opsonization and circulating protein mediated opsonization.

Antibody mediated opsonization

Among these, antibody mediated opsonization is the process by which a pathogen is marked for ingestion and eliminated by a phagocyte.The Fab region of the antibody binds to the antigen, whereas the Fc region of the antibody binds to an Fc receptor on the phagocyte, facilitating phagocytosis. The antigen-antibody complex can also activate complement through the classical complement pathway. Phagocytic cells do not have an Fc receptor for immunoglobulin M (IgM), making IgM ineffective in assisting phagocytosis alone. However, IgM is extremely efficient at activating complement and is, therefore, considered an opsonin. IgG antibodies are also capable of binding immune effector cells via their Fc domain, triggering a release of lysis products from the bound immune effector cell (monocytes, neutrophils, eosinophils, and natural killer cells). This process, called antibody-dependent cellular cytotoxicity / ADCC, can cause inflammation of surrounding tissues and damage to healthy cells.

Complement protein mediated opsonization

The complement system is a part of the innate immune response. C3b, C4b, and C1q are important complement proteins that mediate opsonization. As a part of the alternative complement pathway, the spontaneous activation of a complement cascade converts C3 to C3b, a component that can serve as an opsonin when bound to an antigen's surface. Antibodies can also activate complement via the classical pathway, resulting in deposition of C3b and C4b onto the antigen surface. After C3b has bound to the surface of an antigen, it can be recognized by phagocyte receptors that signal for phagocytosis. Complement receptor 1 is expressed on all phagocytes and recognizes a number of complement opsonins, including C3b and C4b which are both parts of C3-convertase.

Circulating protein mediated opsonization

Pentraxins, collectins, and ficolins are all circulating proteins that mediate opsonization. They are secreted Pattern recognition receptors (PRRs). These molecules coat the microbes as opsonins and enhance neutrophil reactivity against them through a number of mechanisms.


INTRODUCTION

Complement plays an essential role in inflammation and innate immunity against infectious disease. Sepsis caused by Escherichia coli and other gram-negative bacteria is still a serious disease despite the presence of a range of antibiotics [ 1 ]. One main reason for this is the huge and sometimes inappropriate inflammatory response induced by the bacteria themselves or by released bacterial components such as LPS, especially when the bacteria enters the blood stream. This inflammation participates in the tissue damage and multi-organ failure, which are hallmarks of severe sepsis. High levels of secondary inflammatory mediators such as C5a, the terminal complement complex, cytokines, different arachidonic acid metabolites, and acute-phase reactants such as C-reactive protein have been reported in the blood from septic patients [ 2 , 3 4 5 ]. The elevated C5a levels during sepsis impair immune function through stimulation of thymocyte apoptosis [ 6 ] and inhibition of the bactericidal function of neutrophils [ 7 ].

E. coli entering the bloodstream are opsonized rapidly by complement including C1q, mannose-binding lectin (MBL), C4b, and C3b/iC3b [ 8 ], in addition to Igs [ 9 ]. The terminal C5b-9 complex (TCC) induces lysis of certain complement-sensitive bacterial strains. The opsonization of bacteria by complement also facilitates the binding of the bacteria to the adherence complement receptor 1 (CR1) and the integrin and phagocytosis receptor CR3 (CD11b/CD18) on blood leukocytes [ 10 ]. Complement activation in the fluid phase with the release of C5a up-regulates CD11b rapidly and induces oxidative burst [ 11 ]. C5a is also involved in the defect phagocytosis and oxidative burst in granulocytes observed in the cecal ligation/puncture (CLP)-induced sepsis in rats [ 12 ]. Furthermore, C5a mediates a number of other detrimental effects during sepsis including enhanced synthesis of inflammatory mediators and degranulation of granulocytes [ 13 ]. Thus, complement is a double-edged sword and plays a dual role during sepsis.

LPS is a major constituent of the membrane in gram-negative bacteria and may be released from the bacteria when they multiply or lyse [ 14 ]. LPS induces a septic-like condition in vivo including fever, increased body temperature, and synthesis of pro- and anti-inflammatory cytokines by different cell types including macrophages [ 14 ]. LPS binds to soluble or membrane CD14, which functions as a receptor for LPS [ 15 ], after binding of LPS to a LPS-binding protein [ 16 ]. CD14 in monocyte/macrophage cell membranes are attached through a GPI linkage [ 16 ], and the transmembrane signal is mediated through TLR4 [ 17 ] and MyD88 [ 18 ]. The nonreceptor-mediated signaling by LPS is also mediated through CD14 [ 19 ]. LPS and CD14 are therefore important molecules involved in the inflammation induced during gram-negative sepsis. However, blocking of CD14 using mAb in animal models inhibits LPS effects [ 20 ] but has no effect on the mortality during the CLP model of sepsis in mice [ 21 ]. It is interesting that LPS has been reported to activate the transcription factor NF-κB through CR3, indicating a possible link between complement and LPS signaling [ 22 ].

We have shown previously that a specific C5a receptor antagonist (C5aRa) reduced the E. coli-induced expression of the adhesion molecule CD11b dose-dependently [ 11 ]. It is notable that the C5aRa did not inhibit E. coli-induced CD11b up-regulation completely, especially in monocytes, suggesting that other signaling pathways might be involved.

We hypothesized that a combined inhibition of complement and CD14 inhibition would reduce leukocyte activation. Thus, in the present study, the role of complement C3 opsonization, C5a, and LPS/CD14 on E. coli-induced CD11b up-regulation, phagocytosis, and oxidative burst was studied using a novel human whole blood model, where anticoagulation does not interfere with complement activation [ 11 ]. The results showed that the E. coli-induced granulocyte activation was more dependent on complement, and monocyte activation was more dependent on CD14. It is interesting that the activation of both cell types was blocked efficiently by combining complement and CD14 inhibition.