Immunology - Complement System

Overview of Complement System

• Complement proteins are produced by the liver and continuously present in the blood in an inactive state.

• Mechanisms exist to activate and deactivate complement to prevent unneeded inflammation.

• Complement activation occurs through three main pathways:

  • Classical cascade: Initiated when antibodies tag a microbial surface.

  • Mannospining lectin pathway: Initiated by fungal sugars.

  • Alternative cascade: Initiated by complement alone, occurring on microbial membranes or in the fluid phase.

Complement Protein Functional Classes

• Opsonins: Complement proteins (such as $C3b$ and $C4b$) that bind to membrane surfaces to increase phagocytosis.

• Anaphylatoxins: Inflammatory complement fragments (such as $C3a$ and $C5a$) that trigger mast cell degranulation, histamine release, neutrophil recruitment, and tissue inflammation.

• Membrane Attack Complex (MAC): Structure formed by complement proteins that creates a channel/hole in the membrane, causing microbial cell lysis.

C3 Convertase Assembly and Pathways

• All complement cascades converge at a critical enzymatic step called the $C3$ convertase.

• $C3$ convertase cleaves circulating $C3$ into two parts:

  • $C3a$: Released as an anaphylatoxin to promote inflammation.

  • $C3b$: Functions as a binding component (opsonin) attached to the microbial surface.

• Classical Cascade $C3$ Convertase:

  • Cleavage of $C2$ and $C4$ is initiated by the $C1$ complex.

  • Composed of subunits $C2a + C4b$.

  • Always remains bound to a membrane surface.

• Alternative Cascade $C3$ Convertase:

  • Factor B is cleaved by Factor D to form $Bb$.

  • Composed of subunits $C3b + Bb$.

  • Can occur in the fluid phase; requires stabilization by prohairnuclein (preparatin / propranolone) to remain stable and prevent accidental persistent activation.

C5 Convertase and Membrane Attack Complex (MAC)

• $C5$ Convertase Assembly:

  • Classical pathway composition: $C2a + C4b + C3b$.

  • Alternative pathway composition: $C3b + Bb + C3b$.

  • Cleaves $C5$ into $C5a$ (anaphylatoxin) and $C5b$ (binding component anchored to the membrane).

• Membrane Attack Complex (MAC) Assembly:

  • Anchored $C5b$ sequentially recruits $C6$, $C7$, and $C8$.

  • Multiple $C9$ proteins assemble to construct a transmembrane pore.

  • Insertion of the $C9$ pore causes cell lysis.

Part 2

Complement Anaphylatoxins and Pathways

• Cleavage of C3 and C5 releases anaphylatoxins C3a and C5a, which bind to receptors on basophils and trigger the release of granules containing histamines and prostaglandins to induce intentional inflammation.

• C3a signaling promotes a pro-inflammatory TH1 response, whereas C5a signaling represents a critical activation threshold that recruits and activates diverse immune cell types.

• The mannose-binding lectin (MBL) pathway initiates via MBL protein and MASP complexes, which cleave C4 and C2 to yield C2a and C4b; all subsequent cascade steps are identical to the classical pathway.

• In the classical pathway, initiation occurs when the C1 complex cleaves C4 and C2 to generate C2a and C4b.

Alternative Pathway Mechanics and Thrombin Integration

• The alternative pathway relies on Factor B cleavage by Factor D into Bb, requiring a C3b protein to assemble the C3 convertase amplification loop.

• Properdin serves as the required stabilizing protein; it can bind directly to microbial membranes to recruit C3b and initiate cascade assembly.

• Thrombin from the blood clotting pathway can directly cleave C3 and C5 in vitro, providing dual protection by forming clots to close physical barriers while initiating complement activity against potential pathogens.

• Assembly of C5 convertase requires multiple C3b subunits, meaning C5 cleavage cannot proceed without initial C3 cleavage.

Complement Receptors and Outcomes

• Primary outcomes of complement activation include opsonization (via C3b and C4b oxidants to enhance phagocytosis), inflammatory cell recruitment, and direct pathogen lysis via the membrane attack complex (MAC; composed of C6, C7, C8, and multiple C9 proteins).

• Complement Receptor 1 (CR1): Expressed on red blood cells (to shuttle complement-bound microbes to liver phagocytes for clearance), phagocytes, and B cells.

• Complement Receptor 2 (CR2): Expressed on B cells; binds C3 cleavage products (such as C3b) to provide secondary activation signals that enhance antigen presentation and memory response.

• B cells act as professional antigen-presenting cells by utilizing CR1, CR2, Pattern Recognition Receptors (PRRs), and B cell receptors (BCRs) to increase avidity when capturing complement-tagged microbes.

Antigen Presentation and Immune Contraction

• Antigen presentation pathway: PRRs bind PAMPs $\rightarrow$ processing and loading onto MHC class II $\rightarrow$ presentation to T Cell Receptors (TCR) and CD4 co-receptors on naive T cells.

• Full T cell activation requires co-stimulatory contact between CD28 and CD80/CD86, stabilized by adhesion molecules like LFA-1, driving IL-2 secretion for cell proliferation.

• Complement signaling activates the BCL-2 pathway and IL-2 production for cell survival and proliferation, but sustained activation eventually induces apoptosis to limit hyper-inflammation.

• Immune response contraction involves disposing of apoptotic blebs and immune complexes via C1, C1 receptor, IgM, and MBL; failure of this clearance mechanism results in pathological immune complex deposition.

Regulation of Complement

• C1 inhibitor: Direct serine protease inhibitor that blocks C1 activation.

• Decay Accelerating Factor (DAF): Accelerates the breakdown and inactivation of C3 convertases in both pathways.

• Factor I: Serine protease that degrades C3b and C4b oxidants into inactive fragments.

• Protectin: Inhibits assembly of the C9 core, preventing MAC formation and cell lysis.

• Carboxypeptidase: Cleaves terminal arginine residues from C3a and C5a to eliminate their anaphylatoxin activity.