Human Complement System: Activation & Regulation Part 1

Overview of the Complement System

  • A large group of proteins present in blood and tissue that, upon sequential binding and enzymatic activation, promote microbial destruction, cellular phagocytosis, inflammatory responses, and cytolytic attack on pathogens and pathogen-infected cells.
  • Basic properties:
    • Synthesized mainly by cells in the liver, spleen, lung, bone marrow and macrophages.
    • Constitutes about ~10% of serum globulin.
    • Heat sensitive: inactivated by 56°C for 30 minutes.
    • Exists in pro-form (inactive) and must be activated for biological and enzymatic activities.
    • Amounts are relatively stable in blood and not directly affected by antigen stimulation or adaptive immunity, but can be affected by certain diseases (e.g., chronic liver and kidney damage).

Biochemical Features

  • Biochemical features include the following synthesis sites and properties:
    • Key production by liver, spleen, lung, bone marrow, macrophages.
    • Constitutes ~10% of serum globulins.
    • Heat-labile; inactivated at 56°C for 30 min.
    • Circulates as inactive zymogens that become activated in cascades.

Pathways of Complement Activation

  • Three initiating pathways converge on a common terminal pathway:
    • Alternative pathway
    • Lectin pathway (MBL pathway)
    • Classical pathway
  • Key outcomes of activation (common to all pathways):
    • Cleavage of C3 to C3a and C3b.
    • C3b covalently binds to pathogen surfaces, tagging them for destruction (opsonization).
    • Recruitment of inflammatory cells.
    • Direct destruction of pathogens via the membrane attack complex (MAC).
  • Structural/functional recap from the overview figure:
    • Recognizing and binding events lead to activation, then destruction of pathogens.
    • Three stages: Recognition → Activation → Destruction.
Alternative Pathway
  • Initiated on pathogen surfaces without antibody/CRP involvement.
  • Local environment on pathogen surface promotes complement activation.
  • Leads to formation of C3 convertase that cleaves C3 to C3a and C3b.
  • Important convertase components include C3bBb (C3 convertase of the alternative pathway).
Lectin Pathway (MBL Pathway)
  • Triggered by Mannose-Binding Lectin (MBL) binding to mannose residues on pathogen surfaces.
  • Activation involves MASP-1 and MASP-2 (MBL-associated serine proteases).
  • MASP-2 cleaves C4 and C2 to form the classical C3 convertase C4b2a, leading to C3 activation.
Classical Pathway
  • Activated by C-reactive protein or antibodies binding to antigens on pathogen surfaces.
  • Key activators: C-reactive protein (CRP), IgM, and IgG; CRP is an acute-phase protein, not a complement component.
  • Initiation involves C1 components binding to CRP or immune complexes (antigen-antibody)
  • C1 consists of C1q, C1r, and C1s (C1r and C1s are serine proteases).
  • Activation sequence leads to formation of C3 convertase C4b2a, similar to the lectin pathway.

Major Functions and Roles of Complement Components (Selected Examples)

  • Binding to antigen-antibody complexes and pathogen surfaces:
    • C1q (with CRP) and C-reactive protein (CRP) participate in recognizing immune complexes.
    • Mannose-binding lectin (MBL) binds to pathogen surfaces.
  • Activating enzymes:
    • C1r, C1s; MASP-1, MASP-2; Factor D (noted in pathways as catalytic proteases).
  • Membrane-binding proteins and opsonins:
    • C4b, C3b (opsonins) that decorate pathogen surfaces and promote phagocytosis via receptors.
  • Peptide mediators (anaphylatoxins):
    • C3a, C4a, C5a
    • Promote inflammation, recruit leukocytes, increase vascular permeability.
  • Membrane-attack proteins:
    • C5b, C6, C7, C8, C9
  • Complement receptors:
    • CR1 (CD35), CR2 (CD21), CR3 (CD11c/CD18), C1qR, etc.
  • Complement-regulatory proteins:
    • CR1 (CD35), MCP (CD46), CD59, DAF (CD55), C1 esterase inhibitor (C1-INH), etc.

Classical Pathway Factors that Activate the Classical Pathway

  • Key activators: C-reactive protein (CRP), IgM, and IgG binding to pathogen surface.
  • Important points:
    • In innate immunity, CRP is the initiator of the classical pathway.
    • CRP is an acute-phase protein, not a complement component.

C-Reactive Protein (CRP) and Initiation of Classical Pathway

  • CRP binds to the pathogen surface to initiate the classical pathway (first step).
  • Classical pathway components include C1q, C1r, and C1s (C1).
  • IgG and IgM can initiate the classical pathway (adaptive immunity).

Formation of Classical C3 Convertase

  • Classical C3 Convertase is:
    • extC4b2aext{C4b2a}
  • Involves inflammatory mediator CRP and proteases C1r/C1s.
  • C1 binds to CRP or an antigen-antibody complex to begin deposition and activation steps.

Fixation of C4b and C3b on Pathogen Surfaces

  • Process:
    • C1 binds to CRP or antigen:antibody complex on a pathogen surface.
    • Deposition of C4b by C1.
    • Deposition of C3b by the C4b2a (classical C3 convertase) on the pathogen surface.

Formation of Classical C5 Convertase

  • Classical C5 Convertase is formed as: extC4b2a3bext{C4b2a3b}
  • This complex activates C5 to generate C5a (anaphylatoxin) and C5b.
  • Steps illustrated in classical pathway figures include sequential association: C5, C2a, C3b, C4b, etc., culminating in C5a release and C5b initiation of MAC.

Generation of a Pore by MAC on the Pathogen Membrane

  • Terminal complement components assemble to form the MAC:
    • C5b initiates MAC assembly and recruits C6, C7, C8, and multiple C9 molecules.
  • Key idea: The MAC forms a pore in the pathogen membrane, compromising integrity and potentially causing cell death.
  • Sequence (simplified):
    • C5b → binds C6 → binds C7 → binds C8 → recruits multiple C9 to polymerize and form a membrane-spanning channel.

Formation of the Membrane Attack Complex (MAC)

  • MAC components and their approximate serum concentrations (illustrative values):
    • C5b: ∼85 µg/mL
    • C7: ∼60 µg/mL
    • C8: ∼60 µg/mL
    • C9: ∼60 µg/mL
  • Functional roles:
    • On activation, soluble C5b initiates MAC assembly in solution.
    • C6 binds to C5b to stabilize it and form a binding site for C7.
    • C7 binding exposes a hydrophobic region that permits attachment to the cell membrane.
    • C8 binds to the C5b67 complex and exposes a hydrophobic region that inserts into the membrane.
    • Polymerization of C9 onto the C5b678 complex forms the pore that disrupts membrane integrity and can cause cell death.

Pores Formed and Consequences for Target Membranes

  • MAC pores create membrane lesions and can lyse susceptible cells.
  • Pore formation is a key mechanism of direct cytolysis by the complement system.

IgG and Classical Pathway Initiation on Pathogens

  • IgG molecules bound to antigens on bacterial surfaces can initiate the classical pathway when C1q binds to two or more IgG molecules, forming the active C1 complex.
  • IgG bound to soluble multivalent antigen can also initiate the classical pathway.
  • C1q can bind to soluble immune complexes to initiate activation.

IgM Initiation of Classical Pathway on Pathogen Surfaces

  • IgM can initiate the classical pathway when bound to antigens on pathogen surfaces.
  • Key takeaway: IgG and IgM can activate the classical pathway, making it functional in both innate and adaptive immunity.

Complement Regulation and Disease Implications

  • Protective/regulatory proteins prevent unintended activation and host tissue damage:
    • DAF (CD55), MCP (CD46), CR1 (CD35), CD59, C1-INH, etc.
  • Imbalances in complement levels or regulation can be associated with disease:
    • Genetic deficiencies or functional defects in components (e.g., C3, C5, C1q) can lead to increased susceptibility to infections or immune complex diseases.
    • Overactivation or dysregulation can contribute to inflammatory or autoimmune conditions.
  • Pathophysiological context from the lecture materials includes:
    • Chronic liver and kidney diseases may affect complement protein production and stability.
    • Immune complex diseases may involve RBC clearance mechanisms (see below).

Role of Complement Receptor CR1 in Phagocytosis

  • CR1 (CD35) on phagocytes recognizes C3b and C4b on pathogen surfaces or immune complexes.
  • This interaction promotes opsonophagocytosis, enhancing uptake and destruction of opsonized targets by phagocytes.

How Red Blood Cells Remove Immune Complexes from bloodstream

  • Erythrocytes (RBCs) express CR1 (CD35) to bind C3b/C4b-tagged immune complexes.
  • RBCs transport immune complexes to liver and spleen for clearance by phagocytes, aiding detoxification and preventing deposition in tissues.

Anaphylatoxins and Inflammation (C3a, C4a, C5a)

  • Anaphylatoxins contribute to regulation of inflammation by:
    • Chemoattraction and recruitment of inflammatory cells (neutrophils, monocytes).
    • Increasing vascular permeability and promoting inflammation at sites of infection.

Summary of Key Connections and Practical/Philosophical/Clinical Implications

  • The complement system provides a rapid, non-specific defense that bridges innate and adaptive immunity.
  • It functions via proteolytic cascades that amplify responses and orchestrate both direct pathogen destruction and enhanced phagocytosis.
  • Regulation is crucial to prevent host tissue damage; therapeutic modulation of complement components is relevant in autoimmune, inflammatory, and infectious diseases.
  • Understanding the interplay between CRP, IgG/IgM, CR1, and MAC informs diagnostic and therapeutic approaches to infectious and inflammatory diseases.

Notation and Formulas (Quick Reference)

  • Classical C3 Convertase: extC4b2aext{C4b2a}
  • Classical C5 Convertase: extC4b2a3bext{C4b2a3b}
  • Alternative Pathway C3 Convertase: extC3bBbext{C3bBb}
  • MAC composition sequence (simplified):C5b
    ightarrow C6
    ightarrow C7
    ightarrow C8
    ightarrow (C9)_n
  • MAC pore results in disruption of membrane integrity and potential cell lysis.

Key Takeaways for Exam Preparation

  • Know the three activation pathways, their initiators, and the common end-step (C3 activation and MAC formation).
  • Be able to describe the formation and significance of:
    • C3 convertases (C4b2a for classical/lectin; C3bBb for alternative)
    • Classical C5 convertase (C4b2a3b)
    • MAC and the sequential assembly of C5b-678-9 complexes
  • Understand the roles of CR1 in phagocytosis and RBC-mediated immune complex clearance.
  • Recognize the functions of anaphylatoxins and their contribution to inflammation.
  • Recall regulatory proteins and their protective roles to prevent tissue damage.
  • Appreciate how CRP initiates the classical pathway and its distinction as an acute-phase protein rather than a complement component.
  • Be aware of how changes in complement levels relate to disease risk and pathology, including liver/kidney disease impact on complement protein production.