Complement System—Lectin, Classical, and Alternative Pathways: Activation, Amplification, Regulation, and Effector Functions

Lectin Pathway Activation

  • Initiation: Mannose-binding lectin (MBL) binds to microbial surfaces, recognizing simple carbohydrates such as mannose and N-acetylglucosamine that are abundant on microbes (bacteria, yeast, fungi, viruses). These sugars serve as targets for lectin pathway activation.
  • Microbial surface recognition: Besides mannose and N-acetylglucosamine, acetylated sugars on microbial surfaces can be bound by lectin pathway components, aiding activation.
  • Apoptotic cells and classical pathway linkage: Apoptotic cells release DNA, mitochondrial DNA, and other damage-associated molecules. These products can directly bind C1 and activate the classical pathway, linking cell death processes to complement activation.
  • Net effect on initiation: The lectin pathway is activated by microbial carbohydrates; the classical pathway can be activated by apoptotic cell products binding C1. Both routes feed into the same downstream cascade once C3 is activated.

Classical Pathway Activation by Apoptotic Cells

  • Classical pathway activation mechanism (as discussed): Components released by dying (apoptotic) cells can engage C1, initiating the classical cascade even in the absence of antibodies.
  • Consequence: This provides an alternative route for opsonization and downstream complement effects on dying cells and nearby microbes.

Convergence and Amplification Loop

  • Key idea: Regardless of whether the trigger is classical, lectin, or alternative, once C3 convertase is formed on a surface, amplification loops can drive the cascade forward.
  • Activator surface requirement: Amplification proceeds effectively when there is a surface that supports continued complement activation. If no surface-bound initiator is present, the cascade slows/stops on that surface.
  • Surface specificity and host protection: On self (host) surfaces, regulatory proteins limit activation to protect tissues.
  • Factor H and regulation on self surfaces: Factor H binds C3b on host cells and, with Factor I, facilitates cleavage of C3b to inactive forms, helping to prevent damage to host tissues. If Factor H is missing or dysfunctional, unchecked activation can occur, potentially causing kidney and brain damage due to unregulated complement activity.
  • Properdin (factor P) and stabilizing convertases: Properdin stabilizes the alternative pathway C3 convertase, extending its activity and promoting amplification on suitable surfaces.
  • The shared end-point: All pathways converge at C3 activation and deposition of C3b, forming a central hub for opsonization and further downstream events.

C3 Convertase Formation and Amplification Details

  • Alternative pathway initiation and C3 convertase formation:
    • C3b binds to Factor B on a surface: C3b + B → C3bB
    • Factor D cleaves B on the surface to generate Bb (after D cleavage, B becomes Bb): C3bB → C3bBb
    • Resulting complex C3bBb acts as the C3 convertase (cleaves more C3 into C3a and C3b).
    • Properdin stabilizes C3bBb to prolong activity: C3bBb + P → C3bBbP.
  • C3 cleavage and amplification:
    • C3 convertase cleaves C3 into C3a (an inflammatory mediator) and C3b (opsonin): C3 → C3a + C3b.
    • New C3b molecules can bind additional Factor B and form more C3bBb, amplifying the signal.
  • Transition to C5 convertase:
    • Formation of C5 convertase occurs when C3b binds to the existing C3bBb complex: C3bBb + C3b → C3bBbC3b (the C5 convertase).
    • C5 convertase cleaves C5 into C5a (inflammatory mediator) and C5b.
  • Surface-displayed amplification loop: As more C3b is generated and deposited on the microbe, the cycle continues, increasing opsonization and downstream effects on the surface.

Inflammation Mediators and Receptors

  • C3a and C5a as inflammatory anaphylatoxins:
    • Both C3a and C5a promote inflammation by acting on receptors on leukocytes, leading to recruitment and activation of immune cells.
    • C3a and C5a can stimulate inflammatory responses when receptors on leukocytes bind them.
  • Phagocyte engagement via CR1 (complement receptor 1):
    • Phagocytic cells express CR1, which binds C3b that is coating microbes.
    • This binding promotes phagocytosis and killing of the microbe.
  • Additional inflammatory signaling from C3a and C5a:
    • Leukocytes have receptors for C3a and C5a; engagement enhances inflammatory responses and helps coordinate microbe destruction within phagocytes.
  • Outcomes of inflammation and lysis:
    • Inflammation helps recruit more immune components and facilitates microbial killing.
    • The late steps, particularly MAC formation, lead to lysis of susceptible pathogens.

Late Steps: C5 Convertase and Membrane Attack Complex (MAC)

  • Activation timeline: The late steps start after C5 convertase forms and becomes active.
  • MAC formation:
    • C5b initiates assembly with C6, C7, C8, and C9 to form the membrane attack complex (MAC), which can create pores in target membranes, leading to lysis of the microbe.
  • Inflammatory linkage: The formation of C5a in the late steps also contributes to inflammation, reinforcing the immune response.
  • Shared late-step outcomes across pathways: The late steps are identical across classical and lectin pathways and follow the same sequence to generate MAC and inflammatory mediators.

Regulatory Proteins and Inhibitors

  • Factor H and Factor I regulate the alternative pathway on host cells:
    • Factor H binds surface-bound C3b on host cells.
    • Factor I then acts as a protease to inactivate C3b (via cleavage to iC3b and related fragments), preventing further amplification on self surfaces.
    • If Factor H is defective or absent, regulation fails and there is a higher risk of host tissue damage.
  • Decay-accelerating Factor (DAF, CD55):
    • DAF binds to C3b and C4b and prevents the assembly or stabilizing of C3/C5 convertases, thereby stopping further complement activation on the surface.
  • Membrane Cofactor Protein (MCP, CD46):
    • MCP acts as a cofactor for Factor I, promoting the inactivation of C3b/C4b on host membranes.
  • C1 Inhibitor (C1INH):
    • C1INH prevents activation of the C1 complex in the classical pathway, reducing premature activation.
  • Surface protection and complement control proteins:
    • The host surface possesses regulatory proteins (e.g., Factor H, MCP, DAF) that limit activation and protect host cells from unintended damage.
  • Consequences of regulator mutations:
    • Defects in factor H or other regulators predispose to severe disease due to uncontrolled complement activation on host tissues (e.g., kidney, brain damage referenced in discussion).

Summary of Interconnections and Practical Implications

  • Three pathways converge on C3 activation and deposition of C3b, leading to opsonization and phagocytosis.
  • All pathways eventually generate C5 convertase, producing C5a (inflammation) and C5b (MAC initiation).
  • MAC formation (C5b-9) leads to microbial lysis, while opsonization with C3b enhances phagocytosis via CR1.
  • Inflammatory mediators (C3a and C5a) amplify immune responses and recruit effector cells.
  • Regulation by Factor H, Factor I, DAF, MCP, and C1INH is crucial to prevent host tissue damage; dysregulation can lead to tissue injury (e.g., kidney, brain).
  • Apoptotic cells can engage complement pathways via C1 interaction, linking programmed cell death to immune clearance.

Conceptual Takeaways for Exam Preparation

  • Know the initiation steps for each pathway:
    • Classical: C1q binding to antibody-antigen complexes; C1r/s activates C1s; C4 and C2 form C4b2a (C3 convertase).
    • Lectin: MBL binding to microbial sugars; MASP-1/ MASP-2 activate C4 and C2 to form C4b2a (C3 convertase).
    • Alternative: Tickover activation of C3 to C3b, binding B and D to form C3bBb (C3 convertase) with capstone regulation by properdin.
  • Master regulator concept: All pathways converge on C3 activation; surface protection depends on Factor H, DAF, MCP, and C1INH.
  • Remember the key effector products: C3a, C3b, C5a, C5b, MAC (C5b-9).
  • Receptors and functions to memorize: CR1 on phagocytes; leukocyte receptors for C3a and C5a; MAC-induced lysis.

Key Equations and Notation

  • Classical/lectin pathway C3 convertase:
    • ext{C4b} + ext{C2a}
      ightarrow ext{C4b2a} (C3 convertase of classical/lectin pathways)
  • Alternative pathway C3 convertase and stabilization:
    • ext{C3b} + ext{Bb}
      ightarrow ext{C3bBb} ext{ (C3 convertase)}
    • ext{C3bBb} + ext{P}
      ightarrow ext{C3bBbP} ext{ (stabilized by properdin)}
  • C3 cleavage:
    • ext{C3}
      ightarrow ext{C3a} + ext{C3b}
  • C5 convertase formation:
    • ext{C3bBb} + ext{C3b}
      ightarrow ext{C3bBbC3b} ext{ (C5 convertase)}
  • C5 cleavage:
    • ext{C5}
      ightarrow ext{C5a} + ext{C5b}
  • MAC assembly:
    • ext{C5b} + ext{C6} + ext{C7} + ext{C8} + ext{nC9}
      ightarrow ext{MAC}
  • Opsonization and inflammation:
    • C3b opsonizes; C3a and C5a promote inflammation via leukocyte receptors.

If you want, I can tailor these notes to focus on a specific pathway (classical, lectin, or alternative) or add a quick-reference diagram outline for visual study. I’ve kept the content aligned with the transcript and included explicit mechanisms, regulators, and outcomes for exam prep.