Innate and Adaptive Immunity

• The complement system is a group of serum and cell membrane proteins.
• These proteins interact with each other and other molecules of the innate and adaptive immune systems to create effector functions.
• These functions lead to pathogen recognition and their elimination.
• Complement components are designed by numerals C1-C9, by letter symbols such as Factor D, or by trivial names.
• Activation of the complement system results in multiple outcomes,
  • target cell membrane lysis, chemotaxis, and opsonization to enhance phagocytosis.

Anatomical and Physiological Barriers of the Innate Immune System

• Intact skin
• Lysozyme in tears and saliva
• Cilia lining the lungs
• Low stomach pH

Key Players in Innate Immunity Include

• Eosinophils
• Macrophages
• Mast cells
• Dendritic cells
• Complement proteins
• Mannose-binding lectin
• Antimicrobial peptides
• LPS-binding protein
• C-reactive protein

Key Players in Adaptive Immunity Include

• T cells
• B cells
• Antibodies

Structure of C1 Macromolecule

• Consists of C1q, C1r2S2, and IgG antibody
• C1q exhibits a collagen-like triple helix structure

The Complement System is Activated Via Three Pathways

• Classical pathway is initiated by antigen-antibody immune complexes.
• Lectin pathway is triggered by PAMP recognition by lectins.
• Alternative pathway is initiated by spontaneous hydrolysis.

Classical Pathway Initiation

• IgM or IgG binds to a multivalent antigen.
• This process allows the binding of C1q and starts complement deposition.
• C1 binds Fc (fragment crystallizable region) on adjacent IgG molecules or on IgM bound to an antigen.
• C1q binds antigen-bound antibody, inducing a conformational change in one C1r molecule, activating it.
• The activated C1r then activates the second C1r and the two C1s molecules.
• C1s cleaves C4 and C2.
  • C4 is cleaved first, and C4b binds to the membrane close to C1.
  • C4b binds C2 and exposes it to the action of C1s.
  • C1s cleaves C2, creating the C3 convertase, C4b2a.
• C3 convertase hydrolyzes many C3 molecules; some combine with C3 convertase to form C5 convertase.
• The C3b component of C5 convertase binds C5, permitting C4b2a to cleave C5.

Lectin Pathway

• Initiated when soluble proteins recognize microbial antigens.
• Lectins, like mannose-binding lectin (MBL), bind to microbial surfaces.
• Lectins serve as docking sites for MBL-associated serine proteases (MASPs).
• MASPs cleave C4 and C2 to form the C3 convertase.
• Subsequent steps are the same as the classical pathway.

Alternative Pathway

• Initiated in three ways:
  • The alternative tickover pathway, the alternative properdin-activated pathway, and the alternative protease-activated pathway

Alternative Tickover Pathway

• Small amounts of C3 are always being cleaved.
• Usually quickly inactivated if nothing is around for it to bind to.
• Activated C3b binds to the membrane of a target cell.

Alternative Properdin-Activated Pathway

• Properdin directly binds to a surface and then recruits C3b and Factor B.

Alternative Protease-Activated Pathway

• Clotting cascades stimulate cleavage of complement proteins.
• Thrombin cleaves C3 and C5 in vitro.
• Platelet activation releases ATP, calcium ions, and serine/threonine kinases that could stabilize C3b in fluid phase.

Formation of the Membrane Attack Complex (MAC)

• All three complement pathways converge at the formation of the C5 convertase.
• C5 initiates MAC generation.
• The MAC is the result of deposition of C5b, C6, C7, C8, and C9 on target cell membranes.
• This pore structure disrupts osmotic integrity, causing cell death.

Complement Activity in Host Defense

• Innate defense against infection, interface between innate and adaptive immunity, and contraction phase of the immune response.

Complement Aids in

• Lysis of bacterial and cell membranes via the membrane attack complex (C5b-C9).
• Opsonization, C3b and C4b enhance phagocytosis.
• Induction of inflammation and chemotaxis, C3a and C5a (anaphylatoxins) and their receptors on leukocytes.
• Augmentation of antibody responses, C3b and C4b, and their proteolyzed fragments.
• Enhancement of immunologic memory.
• Enhancement of antigen presentation, MBL, C1q, C3b, C4b, and C5a.
• Clearance of immune complexes and apoptotic cells.
• Induction of regulatory T cells (CD46).

Complement Receptors

• Connect complement-tagged pathogens to effector cells.
• CR1 is found on leukocytes and erythrocytes.
  • On erythrocytes, it helps bring immune complexes to the liver for clearance by phagocytes.
  • On phagocytes, it helps bind to complement-coated bacteria to enhance ingestion and destruction, called opsonization.
  • On B cells, it helps bind to complement-coated antigens, enhancing ingestion for processing and presentation to helper T cells.
• CR2 is found on B cells
  • It binds to C3b on opsonized bacteria/antigens and helps provide secondary signals to B cells through the BCR complex for more efficient activation.
• C3aR/C5aR on granulocytes
  • Stimulates release of proinflammatory cytokines and granule components from basophils, eosinophils, and neutrophils.
• Complement’s enhancement of host defense
  • MAC-induced cell death and the promotion of inflammation and opsonization.
  • Opsonized microbes are easier to ingest/destroy, and opsonized immune complexes are easier to clear.
• It mediates interface between innate and adaptive immunities by
  • Enhancing the antigen uptake of antigen by APCs.
  • Enhancing B-cell response by increasing the avidity of B-cell binding to complement-bound antigen.
  • Lysing immature T cells and facilitating mature T cells' growth, differentiation, and survival.
• It aids in the contraction phase of the immune response, because as lymphocytes are no longer required, complement aids in the disposal of apoptotic cells and bodies.
  • Additionally, it aids in the removal/disposal of immune complexes, avoiding damaging inflammation induction in the absence of antigens.

Regulation of Complement Activity

• Passively regulated by protein stability and cell-surface composition.
• C3 convertase has a short half-life unless stabilized by properdin.
• Self-cells have different carbohydrate structures that are more effectively bound by fluid-phase proteases.
  • These more readily inactivate C3b through hydrolysis, protecting self-cells.
• The C1 inhibitor, C1INH, promotes dissociation of C1 components.
  • It binds in the active site of serine proteases, causing C1r2s2 to dissociate from C1q.
  • This results in no further cleavage of C4 or C2 is possible.
  • Overall, inhibits the initiation of classical and lectin complement pathways.
• Decay accelerating factors promote the decay of C3 convertases.
  • These factors accelerate the decay of C4b2a and C3bBb.
• Factor I degrades C3b and C4b, cleaving membrane-associated C3b and C4b into inactive fragments.
• Protectin (CD59) inhibits the MAC attack.
  • It binds C5b678 complexes deposited on host cells, preventing their insertion into the plasma membrane.
  • Also blocks C9 recruitment, preventing MAC formation.
  • Soluble complement S protein (vitronectin) binds fluid-phase C5b67 to prevent insertion into host cell plasma membranes.

Complement Deficiencies

• Genetic deficiencies have been described for each of the complement components, but outcomes vary.
• Patients with any C1q, C1r, C1s, C4, or C2 deficiency often present with immune complex disorders because of inadequate clearance.
• Those with MBL deficiency can exhibit a greater frequency of infections by encapsulated bacteria because of inefficient opsonization and phagocytosis.

Microbial Complement Evasion Strategies

• Different mechanisms exist and are highly varied.
• Some interfere with the first step of Ig-mediated complement activation.
• Microbial proteins may bind and inactivate complement proteins.
• Microbial proteases destroy complement proteins.
• Some microbes mimic or bind complement regulatory proteins.

Summary of Complement System

• Serves many different purposes, helping to connect innate and adaptive immune responses.
• Tightly controlled but enhances many other responses once activated.
• Understanding the system and its methods of activation and regulation helps to better understand innate immunity and evolution.