Paper No. : 10 Immunology Module : 23 Complement System Development Team Principal Investigator: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi Co-Principal Investigator: Prof. D.K. Singh Department of Zoology, University of Delhi Paper Coordinator: Prof. Anju Srivastava Department of Zoology, University of Delhi Content Writer: Dr. Sudhir Verma Deen Dayal Upadhyaya College, University of Delhi Content Reviewer: Prof. Sukhmahendra Singh Banaras Hindu University 1 ZOOLOGY Immunology Complement System Description of Module Subject Name ZOOLOGY Paper Name Immunology; Zool 010 Module Name/Title Complement System Module ID 23; Complement System Keywords Complement, Opsonization, Immune Clearance, Cascade, Anaphylatoxin, Classical Pathway, Lectin Pathway, Alternative Pathway, Membrane Attack Complex, Innocent bystander. Contents 1. Learning Outcomes 2. Introduction 3. Historical Perspective 4. Functions 5. Nomenclature 6. Pathways 6.1 Classical Pathway 6.2 Alternative pathway 6.3 Lectin Pathway 6.4 Terminal Pathway: Membrane Attack Complex (MAC) 7. Receptors for Complement Proteins 8. Regulation of Complement System 9. Diseases due to Complement Deficiency 10. Complement Assay Systems 11. Microbial Evasion Strategies from Complement Mediated Damage 12. Summary 2 ZOOLOGY Immunology Complement System 1. Learning Outcomes After studying this module, you shall be able to Know: What complement system is? Why some individuals face recurrent bacterial infections more as compared to others? Learn: How complement system provides immunity to us? Also, how it links innate immunity with adaptive immunity? Identify: The components that constitute and regulate the complement system. Evaluate: The non-specific yet highly organized component of our immunity, the complement cascade system. Analyze: The underlying beauty of our innate immune system in terms of complement. 2. Introduction Complement system was first discovered in 1890 by Jules Bordet as a heat-labile component of serum that „complemented‟ or „augmented‟ its bactericidal properties. It is known to be comprised of around 30 soluble proteins, together which constitute around 10% of total serum proteins. Most of these proteins are synthesized by liver and remain in inactive form in the absence of infection. Evolutionarily, complement system is as ancient as adaptive immune system as even worms and starfishes also have complement system. Complement system is the major component of innate system which is encountered by pathogens that enter our body after breaching physical barriers and antimicrobial defenses. It consists of three different pathways i.e. classical pathway, alternative pathway and lectin pathway. All these pathways ultimately lead to a common terminal pathway, leading to formation of membrane attack complex which subsequently lyses the target cell. Complement system broadly performs three types of functions: host defense against various infections, interface between innate and adaptive immunity and clearance of waste. For many of these biological activities of complement system, complement fragments bind various receptors present on different cell types. Additionally, these receptors regulate the complement activity. Besides these receptors, various other regulatory proteins are there that keep a check on the activity of complement system. The importance of complement system in providing immunity to us lies in the fact that a number of diseases result due to genetic or functional deficiency of different complement proteins. Still there are a number of pathogens which have developed evading strategies from complement mediated damage. The mechanism of evasion strategy and type of microbes possessing these are discussed in later part of this unit. 3. Historical Perspective In 1890s, Jules Bordet at the Pasteur Institute in Paris showed that sheep antiserum to bacterium Vibrio cholera causes lysis of the bacteria. On heating the antiserum this bacteriolytic activity gets lost. But, the same can be restored to the heated serum by adding fresh serum. This fresh serum contained no antibodies directed against the bacterium and was unable to kill the bacterium by itself. Bordet reasoned that this bacteriolytic activity utilizes two different substances: first, the specific antibacterial antibodies, which survive the heating process, and a second, heat-sensitive component 3 ZOOLOGY Immunology Complement System responsible for the lytic activity. Similar experiments were carried out by Paul Ehrlich in Berlin independently and he defined this phenomenon as “the activity of blood serum that completes the action of antibody.” He coined the term complement for the same. Later it was found that complement is interactive composition of large and complex group of proteins. Value Addition Dr. Jules Bordet was honoured with Nobel Prize in physiology and medicine (1919) for his discovery of „complement mediated bacteriolysis‟. Jules Jean Baptiste Vincent Bordet Source: Wellcome Library, London. Public domain, CC BY 4.0, via Wikimedia Commons http://www.nobelprize.org/nobel_prizes/medicine/laureates/1919/bordet-photo.html 4. Functions The functions of complement system are broadly categorized as: Figure 1: Physiological roles of complement system Source: Author 4 ZOOLOGY Immunology Complement System 5. Nomenclature Complement proteins can be named by: i) Numerals e.g. C1, C2, C3…C9, ii) Letter symbols e.g. factor B, factor D, iii) Trivial names e.g. Decay accelerating factor (DAF), homologous restriction factor (HRF) The peptide fragments generated upon cleavage or activation of zymogen complement are denoted by small letters. The smaller fragment resulting from cleavage of a component is designated “a” and the larger fragment is designated as “b” (e.g., C3a, C3b). C2 is an exception to this rule as smaller fragment is called C2b whereas C2a is the larger cleavage fragment. The larger fragments bind to the target near the site of activation, and the smaller fragments diffuse from the site and act as anaphylatoxin. Functional complexes are formed by interaction among complement factors. The complexes having enzymatic activity are designated by a bar over the number or symbol e.g. C4b2a (C3 convertase in classical pathway). 6. Pathways There are three pathways for activation of complement system, which culminate to formation of C5b. These pathways are named as classical pathway, alternative pathway and lectin pathway. The final steps for the formation of membrane attack complex (MAC) are common for all the pathways, termed as terminal pathway. 6.1 Classical Pathway The classical pathway of complement activation starts when the complement C1 recognizes a microbial surface directly or indirectly via antibody bound to pathogen. C1 complement is a complex, made up of a large subunit C1q, which senses the pathogen, and two zymogen subunits C1r and C1s that act as serine proteases. One molecule of C1q and two molecules each of C1r and C1s are stabilized together as C1qr2s2 by Calcium ions. C1q is a hexamer of trimers, each monomer of which contain a globular domain at its amino terminal and collagen like domain on its carboxy terminal. The pathogen recognition activity of C1 lies within the globular domains of C1q. When these globular heads interact with a particular ligand, a conformational change is caused in C1r:C1s, the associated members of C1. It further leads to activation of autocatalytic activity in C1r. The resulting active form of C1r then cleaves C1s to generate active serine protease. C1s acts on two substrates i.e. C4 and C2. C4 is a glycoprotein that consists of three polypeptide chains i.e. α, β and γ. C1s hydrolyzes the C4a i.e. small fragment of C4 from amino terminal of α polypeptide chain and exposes a binding site on remaining C4b component. C2 attaches itself to the exposed binding site on C4b and is cleaved by C1s to C2a and C2b. The smaller fragment C2b (As mentioned earlier, C2 is the exception in nomenclature, C2b being smaller fragment) diffuses away and C2a attaches itself to target surface along with C4b. The resulting C4b2a complex is called as classical C3 convertase enzyme as it activates C3 from its zymogen form to active form. C3 complement consists of two polypeptide chains α and β. C3 convertase hydrolyzes C3a from amino terminal of α chain of C3 and generates C3b. The C3b thus generated, binds to C4b2a complex, giving rise to trimolecular C4b2a3b complex which is termed as C5 convertase. The C5 is cleaved by C5 convertase to C5a that diffuses away and C5b which binds to C6 and starts the membrane attack complex formation (Figure 1). Various proteins involved in classical pathway of complement activation are listed in table 1 below. 5 ZOOLOGY Immunology Complement System Table 1: List of various proteins involved in classical pathway of complement activation Source: Author Figure 2: Overview of the complement activation by classical pathway, leading to formation of C5b, which eventually leads to formation of membrane attack complex. A) C1qr2s2 complex approaches and gets activated (B) upon binding to antigen-antibody complex. (C) C4 is cleaved by activated C1qr2s2 to form C4b which binds with target surface, whereas C4a diffuses away. (D) C2 is cleaved to C2a and C2b. C2a binds with C4b, forming C4b2a i.e. C3 convertase. (E) C3 is cleaved by C3 convertase to form C3a and C3b. C3b binds with C4b2a to form C4b2a3b i.e. C5 convertase. (F) C5 convertase acts upon C5 to form C5b and C5a. C5b initiates formation of membrane attack