Complement Cascade

Introduction

The complement cascade is a critical component of the innate immune system, playing a vital role in defending against pathogens. This intricate system involves a series of proteolytic cascades that enhance the ability of antibodies and phagocytic cells to clear microbes and damaged cells, promote inflammation, and attack the pathogen's plasma membrane. The complement system operates through three distinct pathways: the classical, lectin, and alternative pathways. Each pathway has unique triggers and mechanisms but ultimately converges to produce key effector molecules like C3b, C5a, and the membrane attack complex (MAC). Understanding these pathways and their regulation is essential for comprehending the immune response and the development of therapeutic interventions for complement-mediated diseases.

Overview of the Complement System

The complement system is a series of proteolytic cascades that play a critical role in the innate immune system. It enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells, promotes inflammation, and attacks the pathogen's plasma membrane. The main functions of the complement system are:

  • Opsonization: Coating pathogens to enhance phagocytosis.

  • Chemotaxis: Attracting immune cells to the site of infection.

  • Cell lysis: Forming the membrane attack complex (MAC) that punctures pathogen membranes.

  • Clearance of immune complexes and apoptotic cells.

The Classical Pathway

Trigger and Mechanism

The classical pathway is initiated by the binding of the C1 complex (comprising C1q, C1r, and C1s) to antibodies (IgG or IgM) attached to antigens on the pathogen surface. This binding leads to the activation of C1r and C1s, which then cleave C4 and C2 into C4a, C4b, C2a, and C2b.

Formation of C3 and C5 Convertases

C4b and C2b combine to form C4b2b, which acts as a C3 convertase, cleaving C3 into C3a and C3b. C3b then combines with C4b2b to form C4b2b3b, a C5 convertase, which cleaves C5 into C5a and C5b.

Key Molecules

  • C1 Complex: Initiates the classical pathway by binding to immune complexes.

  • C4 and C2: Cleaved into fragments that form C3 and C5 convertases.

  • C3a and C5a: Act as anaphylatoxins, promoting inflammation.

  • C3b: Acts as an opsonin, enhancing phagocytosis.

The Lectin Pathway

Trigger and Mechanism

The lectin pathway is triggered by the binding of mannose-binding lectin (MBL) or other lectins (collectins, ficolins) to carbohydrate patterns on the pathogen surface. This binding activates MBL-associated serine proteases (MASP-1 and MASP-2), which then cleave C4 and C2, similar to the classical pathway.

MASP Complex

The MASP complex acts similarly to the C1 complex, cleaving C4 and C2 to form the C3 convertase C4b2b and subsequently the C5 convertase C4b2b3b.

Key Molecules

  • MBL: Recognizes and binds to carbohydrate patterns on pathogens.

  • MASP-1 and MASP-2: Cleave C4 and C2 to form C3 and C5 convertases.

The Alternative Pathway

Continuous Activation

The alternative pathway is unique because it continuously activates at low levels, a process known as "tick-over." This pathway does not rely on specific pathogen recognition but can be triggered by pathogen surfaces that lack regulatory proteins.

Mechanism

The spontaneous hydrolysis of C3 produces C3b, which binds to factor B. Factor D then cleaves factor B, forming the C3 convertase C3bBb. Properdin stabilizes this convertase, forming C3bBbP, which is more stable and capable of continuous C3 cleavage.

Amplification Loop

The amplification loop involves the generation of more C3b, which feeds back to create additional C3 convertases, enhancing the cascade.

Key Molecules

  • C3: Continuously hydrolyzed to produce C3b.

  • Factor B and D: Involved in the formation of the C3 convertase C3bBb.

  • Properdin: Stabilizes the C3 convertase.

  • C5 Convertase: Formed by adding another C3b to C3bBb, leading to the formation of the MAC.

Complement Inhibitors and Clinical Relevance

Eculizumab and Avacopan

Eculizumab is a monoclonal antibody that inhibits C5, preventing the formation of the MAC. It is used to treat rare conditions such as atypical hemolytic uremic syndrome (aHUS) and paroxysmal nocturnal hemoglobinuria (PNH). Avacopan, an oral C5 inhibitor, has been trialed as a steroid-sparing agent in ANCA-associated vasculitis.

Key Drugs

  • Eculizumab: Blocks C5, preventing the terminal pathway of complement activation.

  • Avacopan: An oral C5 inhibitor used in clinical trials.

Complement Regulation

Regulation Mechanisms

The complement system is tightly regulated to prevent damage to host tissues. Regulatory proteins such as factor H, factor I, and membrane cofactor protein (MCP) deactivate complement components and prevent excessive activation.

Key Regulators

  • Factor H and I: Inactivate C3b and prevent the formation of C3 convertase.

  • MCP (CD46): Cofactor for factor I in the inactivation of C3b.

Complement and Adaptive Immunity

Link to B and T Cells

Complement receptors on B cells (e.g., CR2) enhance the immune response by lowering the activation threshold for B cells. Complement also influences T cell responses, promoting a more robust adaptive immune response.

Key Interactions

  • CR2 on B Cells: Enhances B cell activation and antigen presentation.

  • Influence on T Cells: Promotes adaptive immune responses.

Conclusion

Understanding the complement cascade is crucial for comprehending the innate immune system's role in pathogen defense and its links to adaptive immunity. The classical, lectin, and alternative pathways each have unique triggers and mechanisms but converge to produce key effector molecules like C3b, C5a, and the MAC. Proper regulation is essential to prevent host tissue damage, and targeted therapies like eculizumab highlight the clinical importance of the complement system.