Exhaustive Study Notes on Chapter 5: The Complement System

Overview of the Complement System

• Definition and Nature of Complement:

  • Complement consists of a group of serum proteins circulating in an inactive precursor state (zymogens).
  • Upon activation, an enzymatic cascade is triggered where each zymogen is sequentially cleaved and activated.
  • Activation leads to several downstream effector responses aimed at eliminating pathogens and maintaining tissue homeostasis.

• Major Biological Outcomes:

  • Target Cell Lysis: Formation of pores in pathogen membranes via the Membrane Attack Complex (MAC).
  • Opsonization: Coating pathogens or immune complexes with complement fragments ($C3b$, $iC3b$) to enhance recognition and ingestion by phagocytic cells.
  • Chemotaxis and Inflammation: Generation of soluble peptides ($C3a$, $C5a$) that attract and activate immune cells.
  • Clearance of Immune Complexes and Apoptotic Cells: Tagging immune complexes and dying cells for clearance in the liver and spleen.

• Functional Categories of Complement Components:

  • Initiators: Pattern-recognition molecules that bind directly to microbial PAMPs, antibodies, or altered host structures (e.g., $C1q$, Mannose-Binding Lectin [MBL], ficolins).
  • Convertase Activators and Enzymatic Mediators: Zymogens that are cleaved to generate active serine proteases, which assemble into enzyme complexes ($C3$ convertase and $C5$ convertase) that amplify the cascade (e.g., $C1r$, $C1s$, $C4b$, $C2a$, Factor B, Factor D).
  • Opsonins: Cleavage fragments that bind covalently to pathogen surfaces and act as tags for phagocytes (e.g., $C3b$, $C4b$).
  • Anaphylatoxins: Soluble inflammatory fragments released during cleavage that induce vascular permeability, smooth muscle contraction, and leukocyte recruitment (e.g., $C3a$, $C5a$).
  • Membrane Attack Complex (MAC) Components: Structural proteins that insert into target cell membranes to assemble a lytic pore (e.g., $C5b$, $C6$, $C7$, $C8$, $C9$).
  • Complement Receptors: Cell-surface receptors that bind specific complement proteins or their fragments to trigger effector functions (e.g., $CR1$, $CR2$, $CR3$, $CR4$, $CRIg$, $C3aR$, $C5aR$).
  • Regulators: Soluble and membrane-bound host proteins that prevent uninhibited complement activation on self-tissues (e.g., $C1INH$, $DAF/CD55$, $MCP/CD46$, Factor I, Factor H, Protectin/CD59).

Pathways of Complement Activation

• Three Distinct Activation Pathways:
  • Complement activation proceeds through three distinct pathways defined by their initiation mechanisms: the Classical Pathway, the Lectin Pathway, and the Alternative Pathway.
  • All three pathways converge at a central enzymatic step: the cleavage of protein $C3$ into $C3a$ and $C3b$ by a $C3\text{ convertase}$.

• Key Convertase Complexes:

  • C3 Convertases:
  • Classical and Lectin Pathways: $C4b2a$
  • Alternative Pathway: $C3bBb$
  • C5 Convertases:
  • Classical and Lectin Pathways: $C4b2a3b$
  • Alternative Pathway: $C3bBbC3b$

• Pathway Convergence and Terminal Stage:

  • $C5\text{ convertases}$ cleave $C5$ into $C5a$ (a potent anaphylatoxin) and $C5b$.
  • $C5b$ initiates the assembly of the Membrane Attack Complex (MAC), driving cell lysis.

The Classical Pathway

• Initiation and Antibody Dependence:

  • Initiated by antibody binding to antigen on pathogen surfaces or within soluble immune complexes.
  • Dependent on antigen-bound $IgM$ or multiple adjacent $IgG$ molecules ($IgG1$, $IgG2$, $IgG3$).
  • Free circulating antibodies do not activate $C1q$ because the Fc regions are not properly configured; binding to antigen induces conformational changes that expose binding sites for $C1q$.

• Structure and Activation of the C1 Complex ($C1qr_2s_2$):

  • The $C1$ complex consists of one $C1q$ molecule non-covalently associated with a tetramer of two $C1r$ and two $C1s$ serine proteases ($C1r_2s_2$).
  • $C1q$ possesses six collagen-like stalks terminating in globular heads that bind the Fc regions of antibodies.
  • Both $C1r$ and $C1s$ are serine proteases.

• Sequential Molecular Cascade:
  1. Binding of $C1q$ to antibody Fc regions induces a conformational change in one $C1r$ molecule, activating its serine protease function.
  2. The activated $C1r$ cleaves and activates the second $C1r$ molecule and subsequently cleaves both $C1s$ molecules.
  3. Active $C1s$ cleaves $C4$ into $C4a$ (released into fluid phase) and $C4b$
  4. $C4b$ exposes an internal thioester bond and attaches covalently to the microbial membrane adjacent to $C1$
  5. $C4b$ binds $C2$, exposing $C2$ to cleavage by $C1s$
  6. $C1s$ cleaves $C2$ into $C2b$ (released; cleavage by plasmin can release C2 kinin, inducing vasodilation) and $C2a$ (remains attached to $C4b$
  7. The membrane-bound complex $C4b2a$ functions as the classical pathway $C3\text{ convertase}$.
  8. $C4b2a$ hydrolyzes many $C3$ molecules into $C3a$ and $C3b$
  9. Some $C3b$ molecules bind directly to $C4b2a$, forming $C4b2a3b$, which functions as the classical pathway $C5\text{ convertase}$.
  10. The $C3b$ component of $C4b2a3b$ binds $C5$, presenting it for cleavage by $C4b2a$ into $C5a$ and $C5b$

• Chemistry and Kinetics of the C4b Thioester Bond:
  • Intact $C4$ contains a hidden thioester bond ($O=C-S$).
  • Proteolytic cleavage by $C1s$ induces a structural shift that exposes the thioester bond.
  • The exposed bond reacts rapidly with primary amino groups ($-NH_2$) or hydroxyl groups ($-OH$) on target surface proteins or carbohydrates, forming a covalent amide or ester linkage.
  • Timing and Hydrolysis: If $C4b$ does not bind a membrane within microseconds, the exposed thioester bond reacts with water ($H_2O$) and undergoes hydrolysis, rendering it permanently inactive in the fluid phase. This timing limitation confines complement deposition strictly to the immediate vicinity of activation, protecting distant host tissues.

The Lectin Pathway

• Initiation Mechanism:

  • The lectin pathway is antibody-independent and relies on host soluble pattern recognition receptors (lectins) binding directly to microbial surface carbohydrates (PAMPs) like mannose, fucose, and N-acetylglucosamine.
  • Lectins involved: Mannose-Binding Lectin (MBL) and Ficolins (e.g., L-ficolin, M-ficolin, H-ficolin).

• MBL and MASP Complex:

  • MBL structurally resembles $C1q$ and circulates in complex with MBL-Associated Serine Proteases (MASP-1 and MASP-2).
  • Unlike $C1q$ (which binds indirectly via antibody), MBL binds directly to carbohydrates on target microbial membranes.
  • MASP-2 is the primary functional serine protease responsible for cleavage; upon MBL binding to carbohydrates, MASP-2 undergoes autoactivation and proceeds to cleave $C4$ and $C2$
  • MASP-1 plays a supportive role in activation.

• Downstream Convergence:

  • Cleavage of $C4$ and $C2$ by MASP-2 generates $C4b$ and $C2a$, assembling the identical $C3\text{ convertase}$ ($C4b2a$) used in the classical pathway.
  • Subsequent enzymatic amplification, $C5\text{ convertase}$ ($C4b2a3b$) assembly, and terminal MAC steps are completely identical to the classical pathway.

The Alternative Pathway

• Unique Features and Activation Modes:

  • The alternative pathway acts independently of antibodies and lectins, serving as a continuous innate surveillance system.
  • Operates via three distinct initiation mechanisms: the Tickover Pathway, the Properdin-Activated Pathway, and the Protease-Activated Pathway.

• Mode 1: The Alternative Tickover Pathway:

  • Fluid-Phase Tickover: Low levels of serum $C3$ undergo spontaneous hydrolysis ("tickover") of its internal thioester bond by water, generating $C3(H_2O)$.
  • Fluid-Phase Convertase Assembly: $C3(H_2O)$ binds Factor B. Factor D then cleaves bound Factor B into $Ba$ (released) and $Bb$, generating the fluid-phase $C3\text{ convertase}$ ($C3(H_2O)Bb$).
  • Fluid-Phase C3 Cleavage: $C3(H_2O)Bb$ cleaves intact serum $C3$ into $C3a$ and $C3b$
  • Membrane Attachment: If generated $C3b$ covalently attaches to a pathogen membrane before hydrolysis occurs, it binds Factor B. Factor D cleaves Factor B into $Ba$ and $Bb$, forming the membrane-bound alternative $C3\text{ convertase}$ ($C3bBb$
  • Stabilization by Properdin: $C3bBb$ is unstable and decays rapidly unless stabilized by Properdin (Factor P).
  • Amplification Loop: Membrane-bound $C3bBb$ rapidly cleaves large quantities of $C3$, depositing extensive $C3b$ on the target surface. This amplification loop supplies $C3b$ to all complement pathways.
  • C5 Convertase Assembly: Newly formed $C3b$ binds to existing $C3bBb$, generating $C3bBbC3b$, the alternative pathway $C5\text{ convertase}$, which cleaves $C5$ into $C5a$ and $C5b$

• Mode 2: The Alternative Properdin-Activated Pathway:
  • Properdin can function as a pattern recognition receptor, binding directly to specific polyanionic structures and PAMPs on target cell surfaces.
  • Bound properdin then recruits pre-existing $C3b$ and Factor B.
  • Factor D cleaves Factor B to $Bb$, forming the stable membrane-bound convertase $C3bPBb$, which cleaves additional $C3$

• Mode 3: The Alternative Protease-Activated Pathway:
  • Cross-talk between the coagulation cascade and complement pathways.
  • Enzymes from the clotting cascade, such as thrombin, can directly cleave $C3$ and $C5$ in vitro during severe tissue injury or systemic inflammation.
  • Platelet activation releases ATP, calcium ions ($Ca^{2+}$), and serine/threonine kinases that stabilize fluid-phase $C3b$, facilitating ongoing complement activation during clotting.

Summary of Initiating and Amplifying Proteins

• Initiation and Convertase Components (Classical, Lectin, Alternative Pathways):

The Membrane Attack Complex (MAC) and Lysis

• Formation Sequence:
  • All three pathways converge at the formation of a $C5\text{ convertase}$ ($C4b2a3b$ or $C3bBbC3b$).
  • $C5\text{ convertase}$ cleaves $C5$ into $C5a$ (released into fluid phase) and $C5b$
  • Stepwise Assembly:
  1. $C5b$ binds non-covalently to the membrane surface and recruits $C6$. $C6$ stabilizes labile $C5b$ (without $C6$, $C5b$ degrades rapidly).
  2. $C7$ binds to $C5b6$, inducing a conformational change that exposes hydrophobic domains that insert into the membrane lipid bilayer ($C5b67$
  3. $C8$ (composed of $C8\text{-}\beta$ and $C8\text{-}\frac{\text{a}}{\text{g}}$ subunits) binds $C5b67$; the $C8\text{-}\beta$ subunit binds $C5b$, while $C8\text{-}\frac{\text{a}}{\text{g}}$ inserts into the membrane, creating a small, unstable pore ($C5b678$
  4. $C5b678$ recruits 10 to 19 molecules of $C9$. $C9$ molecules polymerize into a ring structure, forming a large, hydrophilic pore ($10\text{--}16\text{ nm}$ diameter) spanning the membrane.
  • Lytic Mechanism: The pore permits free diffusion of ions and water across the lipid membrane, destroying cellular osmotic gradients. Water rushes into the cell, leading to swelling and osmotic lysis.

• Proteins of the Membrane Attack Complex:

Complement Receptors and Effector Functions

• Cellular Expression and Receptor Functions:

• CR1 Functions Across Cell Types:

  • Erythrocytes: Bind soluble $C3b$-coated immune complexes in circulation and transport them to the liver and spleen, where resident macrophages ($CRIg^+$ Kupffer cells) remove and destroy the complexes without destroying the red blood cell.
  • Phagocytes: Bind $C3b$- or $C4b$-opsonized pathogens, stimulating internal signaling pathways that drive pseudopod extension and phagocytic ingestion.
  • B Cells: Bind complement-coated antigens, facilitating antigen internalization for processing and presentation on MHC class II molecules to helper T cells.

• CR2 and the B-Cell Co-Receptor Complex:

  • $CR2$ ($CD21$) forms a membrane co-receptor complex on B cells alongside $CD19$ and $CD81$ ($TAPA\text{-}1$).
  • Simultaneous binding of antigen to the B-Cell Receptor (BCR) and antigen-bound $C3d$ to $CR2$ cross-links the co-receptor with the BCR.
  • Cross-linking brings the cytoplasmic tail of $CD19$ into close proximity with BCR-associated kinases, phosphorylating ITAMs and lower the threshold for B-cell activation by up to a thousand-fold.

• Anaphylatoxin Signaling ($C3aR$ and $C5aR$):
  • $C3a$ and $C5a$ bind to seven-transmembrane G-protein-coupled receptors ($C3aR$ and $C5aR$) expressed on myeloid and granulocytic lineages.
  • Granulocyte Activation: Triggers degranulation of mast cells, basophils, and eosinophils, releasing histamine, serotonin, and vasoactive amines.
  • Phagocyte Recruitment: Acts as potent chemoattractants for neutrophils and macrophages, stimulating vascular permeability, diapedesis, cytokine secretion, and the production of reactive oxygen species (oxidative/respiratory burst).

• Opsonization and Immune Complex Clearance:
  • Pathogens tagged with $C3b$, $iC3b$, or $C4b$ are recognized by complement receptors ($CR1$, $CR3$, $CR4$, $CRIg$) on phagocytic membranes, inducing phagocytosis.
  • Soluble immune complexes tagged with $C3b$ are bound by erythrocyte $CR1$, transported through the blood to the liver and spleen, and ingested by tissue macrophages.

• Bridge Between Innate and Adaptive Immunity:

  • Antigen Presentation: Enhances uptake of complement-tagged antigens ($MBL$, $C1q$, $C3b$, $C4b$) by antigen-presenting cells (APCs).
  • B-Cell Activation: Increases the avidity of BCR binding to complement-coated antigens through $CR2$ co-receptor signaling.
  • T-Cell Regulation: Destroys immature T cells with low surface sialic acid; signaling through $C3aR$, $C5aR$, and $C3b$-binding receptors on mature T cells promotes proliferation, differentiation, and cell survival.

• Contraction Phase of Immune Responses:

  • Aids in disposing of apoptotic cells and blebs without triggering inflammatory signaling cascades (e.g., $C1q$ binds phosphatidylserine or apoptotic cell blebs to promote non-inflammatory clearance).
  • Clears remaining immune complexes after antigen elimination, preventing immune-complex accumulation and host tissue damage.

Regulation of Complement Activity

• Passive Regulation Mechanisms:

  • Lability of Components: $C3$ convertases possess inherently short half-lives unless actively stabilized by Properdin.
  • Cell Surface Composition: Eukaryotic self-membranes possess high concentrations of sialic acid and specific carbohydrates that favor binding of fluid-phase regulatory proteins over activating proteins.

• Active Regulatory Mechanisms:

1. Dissociation of C1 Components:
   • C1 Inhibitor (C1INH): A serine protease inhibitor (serpin) that binds covalently to the active sites of $C1r_2s_2$ and MASP-2, causing them to dissociate from $C1q$ or MBL. Inhibits early classical and lectin pathway activation.

2. Decay-Accelerating Activity for C3 Convertases:
   • Regulatory proteins bind assembled $C3\text{ convertases}$ ($C4b2a$ or $C3bBb$) and physically dissociate their catalytic subunits ($C2a$ or $Bb$).
   • Classical Pathway Regulators ($C4b2a$): Decay-Accelerating Factor ($DAF/CD55$), $CR1$ ($CD35$), and $C4\text{-Binding Protein}$ ($C4BP$).
   • Alternative Pathway Regulators ($C3bBb$): $DAF/CD55$, $CR1/CD35$, and Factor H. (Factor H selectively binds negatively charged sialic acid and glycosaminoglycans present on host cell membranes).

3. Factor I Cofactor Activity (Degradation of C3b and C4b):
   • Factor I: A constitutively active soluble serine protease that cleaves membrane-bound $C3b$ and $C4b$ into inactive degradation products ($iC3b$, $C3dg$, $C3c$, $C4c$, $C4d$).
   • Requires membrane-bound or soluble cofactors to function: Membrane Cofactor Protein ($MCP/CD46$), $CR1$ ($CD35$), Factor H, or $C4BP$

4. Inhibition of MAC-Mediated Lysis:
   • Protectin (CD59): A GPI-anchored membrane protein on host cells that binds to assembled $C5b678$ complexes, blocking the recruitment and polymerization of $C9$ and preventing pore assembly.
   • S Protein (Vitronectin): A soluble serum protein that binds fluid-phase $C5b67$ complexes, preventing them from attaching and inserting into host membranes.

5. Cleavage and Inactivation of Anaphylatoxins:
   • Soluble enzymes (Carboxypeptidase N, Carboxypeptidase B, and Carboxypeptidase R) cleave the C-terminal arginine residue from $C3a$ and $C5a$
   • Converts them into des-Arg forms ($C3a\text{ des Arg}$, $C5a\text{ des Arg}$), which possess drastically reduced affinity for $C3aR$ and $C5aR$, dampening inflammatory signaling.

Pathology and Evasion Strategies

• Genetic Complement Deficiencies:

  • Early Classical Pathway Deficiencies ($C1q$, $C1r$, $C1s$, $C4$, $C2$): Impair the processing and clearance of soluble immune complexes and apoptotic cells. Patients present with immune-complex disorders, such as Systemic Lupus Erythematosus (SLE), recurrent bacterial infections, and kidney pathology.
  • MBL Deficiency: Reduces opsonization and phagocytosis of carbohydrate-rich microbes, causing increased frequency and severity of pyogenic bacterial infections, particularly during childhood.
  • Regulatory Protein Deficiencies: Unchecked complement activation leads to tissue injury (e.g., C1INH deficiency causes hereditary angioedema due to overproduction of bradykinin and complement fragments; CD59 deficiency leads to paroxysmal nocturnal hemoglobinuria [PNH] due to complement-mediated lysis of red blood cells).

• Microbial Complement Evasion Mechanisms:

  • Interference with Antibody Binding: Secretion of surface proteins that bind Fc regions of antibodies in an incorrect orientation, preventing $C1q$ recruitment.
  • Degradation of Complement Components: Secretion of microbial proteases that cleavage-inactivate $C3$, $C4$, $C5$, or antibody molecules.
  • Recruitment or Mimicry of Host Regulators: Expressing surface structures that recruit host Factor H or $C4BP$, causing accelerated inactivation of deposited convertases on the microbe surface.
  • Inhibition of MAC Assembly: Expressing surface proteins or thick outer capsules/LPS layers that prevent MAC components from inserting into the lipid membrane.