The Complement System

The Complement System

Instructor Information

  • Holly Turula, PhD

  • Email: hturula@rvu.edu

Learning Objectives

  1. Compare and contrast activation of the classical, alternative, and lectin complement pathways.

  2. Describe the major functions of complement.

  3. Describe how the complement cascade is regulated.

  4. Describe the immunodeficiencies associated with complement and the major microbial diseases that afflict patients with complement deficiencies.

  5. Explain the relevance of the complement system to clinical medicine.

  6. Describe the CH50 test.

Purpose of the Lecture

  • To learn how complement molecules work together to protect the host from infection.

Reading Assignment

  • Basic Immunology: Functions & Disorders of the Immune System, Abbas & Lichtman

    • Chapter 2: pgs. 39-41, The Complement System

    • Chapter 8: pgs. 164-172, The Complement System

Overview of the Complement System

  • The Complement System includes:

    • Activation

    • Functions

    • Regulation

    • Deficiencies

Complement Protein Production

  • Complement proteins are primarily synthesized in the liver and secreted into the serum.

  • These proteins are referred to as hepatic synthesis plasma proteins and are generally named with a "C" followed by a number (e.g., C3, C9).

  • Many of these proteins exist as inactive zymogens (proenzymes):

    • Upon cleavage, they convert into active components (e.g., C3<br>ightarrowC3a+C3bC3 <br>ightarrow C3a + C3b).

Role of the Complement System

  • The complement system is a crucial component of the innate immune response and operates alongside:

    • B lymphocytes, plasma cells, antibodies

    • Phagocytes, dendritic cells, T lymphocytes, NK cells

    • Epithelial barriers

Activation of the Complement System

  • Complement activation can occur through different pathways, but all converge at C3b.

Pathways of Activation

  • Alternative Pathway:

    1. C3 in serum gets hydrolyzed.

    2. Factor B is recruited.

    3. Factor B is cleaved by Factor D.

    4. A soluble C3 convertase made of C3bBb is produced.

    5. C3 is cleaved and C3b is deposited onto the cell surface.

  • Cell-Associated C3 Convertase:

    1. C3b binds to the pathogen surface.

    2. Factor B is recruited again.

    3. Factor B is cleaved by Factor D.

    4. This produces a cell-associated C3 convertase made of C3bBb.

    5. C3 is cleaved, depositing additional C3b on the pathogen surface.

  • Classical and Lectin Pathways:

    1. Classical Pathway initiates when C1q binds to an antibody (IgM or IgG) or when mannose-binding lectin (MBL) binds to mannose residues.

    2. Bound C1q or MBL cleaves C4 into C4a and C4b and C2 into C2a and C2b.

    3. C4b binds to the pathogen surface.

    4. C2b binds to C4b, generating the C4b2b complex (C3 convertase).

    5. C3 convertase cleaves C3 into C3a and C3b.

Importance of C3 in Complement Activation

  • C3 plays a vital role in the distinction between self and non-self, thereby assisting immune regulation.

C5 Convertase Formation

  • Binding of C3b to the C3 convertase forms the C5 convertase, resulting in the cleavage of C5 into C5a and C5b.

Functions of Complement

The complement system contributes to pathogen elimination through three primary mechanisms:
A. Opsonization and Phagocytosis:

  • C3b binds to microbial surfaces, labeling them for easier identification and digestion by phagocytes.

B. Initiation of Inflammation:

  1. Vascular permeability: Anaphylatoxins (C3a, C4a, C5a) promote leakage and swelling.

  2. Chemotaxis: C3a and C5a recruit immune cells, primarily neutrophils.

  3. Inflammatory activation of immune cells and endothelium.

C. Complement Mediated Cytolysis (via Membrane Attack Complex (MAC)):

  1. Formation of the MAC occurs from reactions between C5b, C6, C7, C8, and C9.

  2. C5 convertase forms, cleaving C5b which binds to the membrane.

  3. C5b, C6, C7, and C8 create a pore in the membrane and recruit C9.

  4. C9 polymerizes to stabilize the pore, leading to osmotic lysis of the pathogen, generally producing pores sized between 70-100Å.

Regulation of the Complement System

  • Regulation is critical to prevent damage to host cells and to avoid uncontrolled complement activation.

  • Two main mechanisms:

    1. Binding that triggers dissociation of complement components.

    2. Proteolytic cleavage of active fragments.

Key Regulatory Factors

  • C1 Inhibitor (C1 INH): Prevents proteolytic activation of C1q and MBL.

  • Decay Accelerating Factor (DAF): Promotes dissociation of C3 convertase.

  • Factor H: Binds to C3b, prevents C5 convertase formation, and acts as a co-factor for Factor I to inactivate C3b.

  • Factor I: Inactivates C3b and C4b, requiring co-factors for activity.

  • Protectin (CD59): Prevents recruitment of C9 to C5b678, thus inhibiting MAC formation and host cell lysis.

Complement Deficiencies

Types of Deficiencies

  • Complement Protein Deficiencies:

    • Early deficiencies (C1-C4)

    • Late deficiencies (C5-C9)

  • Complement Regulatory Protein Deficiencies:

    • C1 INH deficiency, which leads to hereditary angioneurotic edema (HANE).

    • DAF deficiency: Associated with paroxysmal nocturnal hemoglobinuria (PHN).

    • Factor H and I deficiencies lead to unregulated C3b generation.

CH50 Test

  • The CH50 test screens for the activation of the classical complement pathway by measuring the ability of serum to lyse 50% of a standard suspension of erythrocytes coated with antibodies.

  • Result expressed in units as the reciprocal of the dilution that lyses 50% of sensitized RBCs.

Early Complement Deficiencies (C1-C4)

  • C2 Deficiency:

  • Most common; causes failure to clear immune complexes, prevalent in systemic lupus erythematosus (SLE).

  • C3 Deficiency:

  • Results in susceptibility to encapsulated bacteria, leading to severe recurrent infections and life-threatening issues soon after birth.

Late Complement Deficiencies (C5-C9)

  • Generally do not exhibit severe clinical symptoms but have an increased risk of infections by Neisseria gonorrhoeae and N. meningitidis.

Case Study

  • Details of a 21-year-old male who presented with recurrent meningococcal meningitis, evidence points toward potential complement deficiencies (specifically C3 and possibly C5-C9), as suggested by low serum complement component levels and recurrent infections.

Additional Resources

  • Videos on the complement system and pathways available online.

Test Yourself Questions

  1. Which pathway involves components of the adaptive immune response? a. Classical b. Alternative c. Lectin

  2. Which complement molecule aids in opsonization? A. C3a B. C3b C. C5a D. C5b

  3. Which complement component is chemotactic for phagocytes? A. C3 B. C3b C. C5a D. C5b E. C9

  4. In a 2-month-old patient with severe infections and C3 convertase deficiency, which regulatory component would most likely not be involved? a. Factor I b. Factor H c. DAF d. C1-INH e. All of the above

Answers to Test Yourself Questions

  1. Classical – because it involves antibodies.

  2. C3b – essential for opsonization.

  3. C5a – known for its chemotactic properties.

  4. All of the above are involved in C3 regulation.