Chapter 5 Notes: The Complement System (Kuby Immunology, 8th Edition)

The Complement System – Chapter 5 Notes

• Complement describes a group of serum proteins that circulate in inactive form. When activated, outcomes include:

  • target cell membrane lysis

  • chemotaxis

  • opsonization to enhance phagocytosis

  • inflammation

  • (Fig. 5-1 provides an overview)

• Activation pathways (three) exist and converge on common effector steps:

  • classical pathway

  • lectin pathway

  • alternative pathway

  • All three pathways generate C3b (an important multifunctional complement protein).

  • Classical and lectin pathways use the C3 convertase C4b2a.

  • Alternative pathway uses C3 convertase C3bBb.

  • C5 convertases produce C5a (inflammatory mediator) and C5b (initiates the membrane attack complex, MAC).

  • (Fig. 5-2 depicts pathways and convertases)

• Key convertases and MAC:

  • Classical/lectin C3 convertase: $ext{C4b2a}$

  • Alternative C3 convertase: $ext{C3bBb}$

  • C5 convertases:

  • Classical/lectin: $ext{C4b2a3b}$

  • Alternative: $ext{C3bBb3b}$

  • MAC formation results from deposition of $ext{C5b}$, then C6, C7, C8, and C9 to form a pore in the target membrane.

Classical Pathway Overview

• Requires IgM or IgG binding to a multivalent antigen (soluble immune complexes or membrane-associated).

• Initiation: the C1 complex (C1qr2s2) binds, beginning complement deposition.

• C1 binds Fc on adjacent IgG molecules or on IgM bound to antigen, triggering activation (Fig. 5-3).

• Sequence of events (highlights):

  • C1r and C1s activation leading to cleavage of C4 and C2.

  • C4 is cleaved first; C4b binds near C1 on the membrane.

  • C4b binds C2; C1s cleaves C2, forming the classical C3 convertase: $ext{C4b2a}$.

  • The C3 convertase hydrolyzes many C3 molecules; some C3b fragments combine with the convertase to form the C5 convertase (C4b2a3b).

  • The C5 convertase cleaves C5, initiating MAC assembly.

• Important structural/chemical note:

  • When C4 is cleaved, a thioester bond is exposed in C4b. This bond is reactive and can bind to amino groups on target surfaces; if no target is present, the bond is hydrolyzed and rendered inert (Fig. 5-6).

• Practice question (order of events in classical activation): Place in order from earliest to latest. Enter sequence without spaces or commas.

  • Events: A) C4bC2a is bound to the cell surface as a C3 convertase

  • B) C1r is activated and cleaves C1s

  • C) C3b binds to C3 convertase to form C5 convertase

  • D) C1 complex binds to constant region of antibody

  • E) C1s cleaves C4 and C2

  • Correct order: D → B → E → A → C (i.e., DB EAC with no spaces)

The Lectin Pathway

• Initiated when soluble lectins recognize microbial carbohydrates.

• Lectins (e.g., mannose-binding lectin [MBL]) bind to carbohydrates on microbial surfaces and recruit MASPs (MBL-associated serine proteases).

• MASPs cleave C4 and C2 to form the C3 convertase; subsequent steps are the same as the classical pathway (i.e., proceed to C3 convertase, then C5 convertase, then MAC).

• (Fig. 5-7 summarizes initiators and steps.)

• Initiating and Amplifying Proteins (Table 5-1) – biologically active fragments and functions (classical & lectin pathways):

  • IgM, IgG: Binding to pathogen surface and initiating complement cascade; active in Classical pathway.

  • Mannose-binding lectin (MBL) or ficolins: Bind carbohydrates on microbial surface and initiate complement cascade; active in Lectin pathway.

  • C1: Initiation of classical pathway by binding Ig; C1q with C1r2s2; C1r and C1s are serine proteases.

  • MASP-1 and MASP-2: MASP-2 cleaves C4 and C2 to form C3 convertase; MASP-1 is another serine protease with a function in the lectin pathway.

  • C2 and C4: Cleavage products generate the C3 convertase; Table notes C2a/C2b and C4b/C4c/C4d variants with functional roles in forming or regulating the convertases.

  • C3: Cleavage products C3a (anaphylatoxin) and C3b (opsonin) with roles in forming C5 convertases and in opsonization; iC3b, C3f, C3d, C3dg, C3c are defined with differing receptor interactions (CR3/CR4/CRIg for iC3b; CR2 for C3d/C3dg).

• A note on protein fragments (Table 5-1 commentary):

  • Some authors differentiate C2a as the larger active fragment; others prefer C2b naming for the larger fragment; nomenclature variations exist across texts.

The Alternative Pathway

• Three initiation routes:

  • Alternative tickover pathway

  • Properdin-initiated pathway

  • Protease-initiated pathway (e.g., clotting system crosstalk)

• The Alternative Tickover Pathway (classic description):

  • Small, constitutive cleavage of C3 -> C3(H2O) in fluid phase; in absence of target, C3b is rapidly inactivated.

  • If a surface is available, activated C3b binds to the target membrane.

  • Factor B binds to C3b; Factor D cleaves Factor B to generate Bb.

  • The complex C3bBb on the membrane acts as the C3 convertase.

  • Properdin stabilizes C3bBb, extending its half-life and enabling cleavage of many C3 molecules.

  • Newly formed C3b can bind to C3bBb to form C3bBbC3b, the C5 convertase, which then cleaves C5.

  • The sequence leads to amplification of complement activation on microbial surfaces.

• Figures 5-8 and 5-9 illustrate fluid-phase vs. membrane-bound C3 convertases and the properdin-activated pathway.

• The Alternative Properdin-Activated Pathway:

  • Properdin can directly bind a surface, recruiting C3b and Factor B.

  • Factor D cleaves Factor B to Bb, producing C3bBb (active C3 convertase).

  • Steps thereafter mirror the alternative tickover pathway with identical downstream events.

• The Alternative Protease-Activated Pathway:

  • Initiated by coagulation cascades; thrombin can cleave C3 and C5 in vitro.

  • Platelet activation releases ATP, Ca^{2+}, and serine/threonine kinases that could stabilize C3b in fluid phase.

  • Indication that strong inflammatory reactions could activate complement systems via proteases.

C5 Convertase and the Membrane Attack Complex (MAC)

• All three pathways converge at the formation of the C5 convertase.

• C5 convertase activity cleaves C5 to generate C5a and C5b. C5a is a potent inflammatory mediator; C5b initiates MAC assembly.

• MAC forms a pore in target membranes via deposition of C5b–C9 components, disrupting osmotic integrity and causing cell lysis.

• MAC assembly sequence (simplified): C5b associates with C6, C7, C8, and multiple C9 units to form the pore (Fig. 5-10).

The Proteins of the MAC and Regulation

• MAC components and their roles (Table 5-3):

  • C5: Anaphylatoxin (C5a) and MAC component (C5b)

  • C6–C9: Structural components of MAC; C8 and C9 form the pore; multiple C9 units create the pore lumen.

• Regulation of MAC formation prevents host cell damage and includes inhibitors such as CD59 (Protectin) that blocks MAC insertion and C9 recruitment; soluble S protein (vitronectin) prevents insertion of fluid-phase C5b67.

Complement Receptors and Functions (Opsonization and Immunity Interface)

• Receptors and ligands listed in Tables 5-5 and 5-6; key receptors include:

  • CR1 (CD35): binds C3b, iC3b, C3d, C3dg, C4b, C1q; expressed on erythrocytes, neutrophils, monocytes, macrophages, eosinophils, FDCs, B cells, some T cells; functions: clearance of immune complexes, enhanced phagocytosis, regulation of C3 breakdown.

  • CR2 (CD21): binds C3d, C3dg, iC3b on B cells and FDCs; enhances B-cell activation; B-cell co-receptor; retention of C3d-tagged immune complexes.

  • CR3 (CD11b/CD18, Mac-1): binds iC3b and factor H; mediates adhesion and extravasation; enhances opsonization.

  • CR4 (CD11c/CD18): binds iC3b; supports phagocytosis by monocytes, macrophages, dendritic cells, etc.

  • CRIg (VSIG4): fixed tissue macrophages; mediates iC3b phagocytosis and inhibits alternative pathway.

• Other receptors and ligands:

  • C1qRp (CD93) and SIGN-R1 (CD209): roles in opsonization and T/B cell interactions; C1q and MBL ligands.

  • C3aR and C5aR on granulocytes: stimulate degranulation and inflammatory mediator release (C3a and C5a effects).

• Opsonization concept: complement tagging enhances ingestion and clearance of pathogens and immune complexes by effector cells.

• Complement interfaces with innate and adaptive immunity:

  • Enhances antigen uptake by APCs via binding of MBL, C1q, C3b, C4b to receptors on APCs.

  • Increases B-cell responses through higher avidity of BCR to complement-tagged antigen.

  • Can lyse immature T cells with low sialic acid content; C3a, C5a, and C3b support T-cell growth, differentiation, and survival through their receptors.

Complement in Contraction and Homeostasis

• Complement participates in contraction phase after infection:

  • Aids in disposal of apoptotic cells and bodies.

  • Aids removal/disposal of immune complexes formed during responses.

  • Helps avoid unnecessary inflammation after clearance of infection.

Regulation of Complement Activity

• Complement activity is tightly regulated to prevent host damage:

  • C3 convertases have short half-lives unless stabilized by properdin (Factor P).

  • Self-cells present carbohydrate patterns that favor inactivation of C3b by fluid-phase proteases.

  • Numerous regulatory proteins prevent harm to host cells.

• Key regulatory proteins (Fig. 5-16):

  • C1INH (C1 inhibitor): soluble; promotes dissociation of C1 components; prevents initiation of classical and lectin pathways.

  • DAF (CD55): accelerates decay of C4b2a (classical/lectin) and C3bBb (alternative) convertases; membrane-bound.

  • CR1 (CD35): decays convertases and acts as cofactor for factor I.

  • C4BP: soluble; inhibits classical/lectin convertases; cofactor for factor I.

  • Factor H: soluble; inhibits alternative C3 convertase; cofactor for factor I.

  • Factor I: soluble serine protease; cleaves C3b and C4b with cofactors (MCP/CD46 and CR1).

  • S protein (vitronectin): soluble; binds fluid-phase C5b67 to prevent MAC insertion.

  • CD59 (Protectin): membrane-bound; inhibits MAC, prevents C9 recruitment and pore formation.

  • Carboxypeptidases N, B, R: cleave and inactivate anaphylatoxins C3a and C5a (des-Arg forms).

• Figure 5-16 summarizes multiple regulatory steps (dissociation, decay acceleration, cofactors, MAC inhibition, and anaphylatoxin cleavage).

Regulation Details (Additional Proteins)

• Table 5-6 expands on soluble/membrane-bound regulators and affected pathways, including:

  • Cofactors for factor I (MCP/CD46, CR1): degradation of C3b and C4b on host surfaces.

  • S protein (vitronectin): prevents MAC insertion by binding soluble C5b67.

  • Factor I with cofactors: central regulatory proteolysis of C3b/C4b.

  • CD59 (Protectin): direct MAC inhibition by preventing C9 insertion.

  • Carboxypeptidases N, B, R: des-Arg forms of C3a and C5a, reducing inflammatory signaling.

Complement Deficiencies

• Genetic deficiencies exist for many complement components with variable outcomes:

  • Deficiencies in C1q, C1r, C1s, C4, or C2 often lead to immune complex disorders due to poor clearance.

  • MBL deficiency may increase susceptibility to encapsulated bacterial infections due to reduced opsonization.

  • Animal models exist for many deficiencies for experimental study.

Microbial Evasion of Complement

• Microbes deploy diverse strategies to evade complement:

  • Interfere with the first Ig-mediated steps by depleting antibodies (e.g., staphylococcal protein A) or removing IgG via staphylokinase.

  • Bind/inactivate complement proteins (e.g., S. aureus SCIN binds C3bBb to inactivate it).

  • Protease-mediated destruction of complement components (e.g., elastase/alkaline phosphatase from Pseudomonas degrade C1q, C3, C3b).

  • Microbial mimicry of regulatory proteins (e.g., Streptococcus pyogenes M proteins bind C4BP and factor H to promote decay of C3 convertases on bacterial surfaces).

  • Viral proteins (Variola and Vaccinia) express cofactors for Factor I to degrade C3b and C4b.

  • Disruption of interactions between complement proteins (e.g., C2 receptor trispanning protein from Schistosoma/Trypanosoma disrupts C2a-C4b interaction).

Evolutionary Origins of the Complement System

• Genes encoding complement components belong to five families.

• Evolutionarily, the alternative pathway genes appeared first; terminal components appeared last.

• Complement likely originated to aid phagocytosis before adaptive immunity existed; supports innate immune defense and initiation of humoral responses.

• (Fig. 5-17 depicts evolutionary timeline.)

Complement System Pathways Across Deuterostomes

• Table 5-8 shows distribution of complement pathways among major deuterostome groups:

  • Mammals: classical, lectin, alternative pathways, MAC, antibodies present (all positive).

  • Birds, Reptiles, Amphibians, Teleost fish: similar presence with some variability in antibodies; specific entries noted in the table.

  • Some groups (e.g., Agnathan fish, Tunicates, Echinoderms) show missing or variably conserved components.

Summary Points

• The complement system links innate and adaptive immunity, enhancing defense and shaping immune responses.

• It is tightly regulated to prevent host damage and to promote resolution after infection.

• The system provides insight into evolutionary development of immunity.

Activity: Concept Map (Chapter 5)

• Concept map exercises help relate terms such as:

  • C3 convertase, C5 convertase, PAMP-bound lectins, Properdin, C3(H2O), Proteases, Classical pathway, Lectin pathway, Alternative pathway, Immune complex, MAC.

• Activity steps: think about relationships, draw a concept map, discuss with peers (instructional slides mention an example and solution discussion).

Practice and Quiz Items (Selected Examples)

• Question: Which statement about the complement activation pathways is false?

  • Possible options include: All three form a C3 convertase; All three form a C5 convertase; All three generate C3b; All three generate C4b and C2a; All three lead to MAC formation.

  • (Recall: all three form C3 convertases and C5 convertases; all generate C3b and C4b/C2a are involved in classical/lectin convertases; all converge to MAC formation.)

• Question: Order the following events in classical pathway activation (earliest to latest): D) C1 complex binds to antibody, B) C1r activates C1s, E) C1s cleaves C4 and C2, A) C4bC2a is formed on the surface as C3 convertase, C) C3b binds to the convertase to form C5 convertase.

  • Correct order: D → B → E → A → C (i.e., DBEAC).

Key Formulas and Nomenclature (Quick Reference)

• Classical/lectin C3 convertase: $ext{C4b2a}$

• Alternative C3 convertase: $ext{C3bBb}$

• Classical/lectin C5 convertase: $ext{C4b2a3b}$

• Alternative C5 convertase: $ext{C3bBb3b}$

• MAC composition: $ext{C5b, C6, C7, C8, C9}$

• C1 inhibitor action: inhibits initiation by promoting dissociation of C1r2s2 from C1q.

• Properdin (Factor P) stabilizes C3 convertases on microbial surfaces.

• Des-Arg inactivation of C3a and C5a by carboxypeptidases N/B/R.

Quick Connections to Foundational Concepts

• Innate immunity: immediate, pattern-recognition-driven activation (MBL/ficolins; C1q).

• Adaptive immunity: antigen-antibody complexes drive classical activation; CR2 enhances B-cell responses to complement-tagged antigens.

• Opsonization: covalent C3b/C4b tagging enhances phagocytosis; receptors CR1/CR3/CR4 mediate uptake.

• Inflammation: C3a/C5a recruit and activate leukocytes; regulated to avoid excessive tissue damage.

• Homeostasis: MAC and anaphylatoxins are regulated to prevent host cell lysis and excessive inflammation after infection is cleared.

Practical Takeaways for Exam Preparation

• Be able to identify which convertases form in each pathway and what fragments they generate (C3b, C5a, C5b, MAC).

• Understand the roles of regulatory proteins and how their deficiencies or dysregulations can lead to disease or unchecked inflammation.

• Remember the two main questions type: pathway description/steps and sequence order of events (as shown in classical activation order quiz).

• Recognize microbial evasion strategies and how they counteract each step of the complement cascade.

End of Chapter 5 Notes

If you’d like, I can convert these notes into a condensed study sheet with a one-page quick-reference guide or expand any section with more detailed step-by-step pathways and diagrams.