🛡️Immunobiology Unit 8 – Complement System and Inflammation

Key Components of the Complement System

• Consists of over 30 soluble plasma proteins and cell surface receptors that play a crucial role in innate immunity
• Complement proteins are synthesized primarily by the liver and circulate in the blood as inactive precursors (zymogens)
• Three main activation pathways: classical, alternative, and lectin pathways
  • Each pathway is triggered by different stimuli but converges on the formation of C3 convertase
• Central component is C3 (complement component 3), which undergoes cleavage to generate C3a and C3b fragments
  • C3b acts as an opsonin, enhancing phagocytosis of pathogens
• Complement receptors (CR1, CR2, CR3, CR4) on immune cells recognize and bind to complement fragments, facilitating immune responses
• Complement regulatory proteins (e.g., factor H, CD55, CD59) prevent excessive activation and protect host cells from complement-mediated damage
• Terminal pathway leads to the formation of the membrane attack complex (MAC), which creates pores in the target cell membrane, causing cell lysis

Activation Pathways

• Classical pathway is triggered by antigen-antibody complexes (IgG or IgM) binding to C1q
  • C1 complex (C1q, C1r, C1s) initiates a cascade of enzymatic reactions
• Alternative pathway is continuously active at low levels and amplifies complement activation
  • Spontaneous hydrolysis of C3 leads to the formation of C3b, which binds to factor B
  • Factor D cleaves factor B, generating C3 convertase (C3bBb)
• Lectin pathway is activated by the binding of mannose-binding lectin (MBL) or ficolins to carbohydrate patterns on pathogen surfaces
  • MBL-associated serine proteases (MASPs) cleave C4 and C2, forming C3 convertase (C4b2a)
• All three pathways converge on the formation of C3 convertase, which cleaves C3 into C3a and C3b
• C3b binds to C3 convertase, forming C5 convertase, which cleaves C5 into C5a and C5b
  • C5b initiates the assembly of the membrane attack complex (MAC)

Complement Cascade and Its Functions

• Complement cascade involves sequential activation and cleavage of complement proteins, amplifying the immune response
• Three main functions: opsonization, inflammation, and cell lysis
• Opsonization: C3b and iC3b fragments coat pathogen surfaces, enhancing phagocytosis by macrophages and neutrophils
  • Complement receptors (CR1, CR3) on phagocytes recognize and bind to opsonized pathogens
• Inflammation: Anaphylatoxins (C3a, C5a) are released during complement activation and induce local inflammatory responses
  • C5a is a potent chemoattractant for neutrophils and monocytes, promoting their recruitment to the site of infection
  • C3a and C5a increase vascular permeability and cause smooth muscle contraction
• Cell lysis: Formation of the membrane attack complex (MAC) on the target cell membrane leads to osmotic lysis and cell death
  • MAC consists of C5b, C6, C7, C8, and multiple C9 molecules that form a pore in the cell membrane
• Complement also plays a role in the clearance of apoptotic cells and immune complexes, maintaining homeostasis

Inflammatory Response Basics

• Inflammation is a complex biological response to harmful stimuli, such as pathogens, damaged cells, or irritants
• Acute inflammation is a rapid, short-lived response characterized by the cardinal signs: redness, heat, swelling, pain, and loss of function
  • Mediated by the release of inflammatory mediators (e.g., histamine, prostaglandins, cytokines)
• Chronic inflammation is a prolonged response that can last for weeks, months, or even years
  • Associated with various diseases, such as rheumatoid arthritis, atherosclerosis, and cancer
• Inflammatory response involves the coordinated action of immune cells, blood vessels, and molecular mediators
• Vascular changes: Vasodilation and increased permeability of blood vessels allow for the extravasation of fluid, proteins, and immune cells into the affected tissue
• Cellular events: Recruitment of immune cells (neutrophils, monocytes, lymphocytes) to the site of inflammation
  • Neutrophils are the first responders and play a crucial role in phagocytosis and the release of antimicrobial substances
• Resolution of inflammation: Once the harmful stimulus is removed, the inflammatory response subsides, and tissue repair processes begin
  • Macrophages play a key role in the resolution phase by clearing apoptotic cells and debris, and secreting anti-inflammatory cytokines

Cellular Players in Inflammation

• Neutrophils: The first immune cells to arrive at the site of inflammation
  • Attracted by chemokines (e.g., CXCL8/IL-8) and complement anaphylatoxins (C5a)
  • Phagocytose pathogens and release antimicrobial substances (reactive oxygen species, granule enzymes)
  • Neutrophil extracellular traps (NETs) are released to trap and kill microbes
• Monocytes/Macrophages: Recruited to the site of inflammation and differentiate into macrophages
  • Phagocytose pathogens, apoptotic cells, and debris
  • Secrete pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) and chemokines to amplify the inflammatory response
  • Macrophages can polarize into M1 (pro-inflammatory) or M2 (anti-inflammatory) phenotypes depending on the microenvironment
• Dendritic cells: Antigen-presenting cells that link innate and adaptive immunity
  • Capture and process antigens at the site of inflammation
  • Migrate to lymph nodes and present antigens to T cells, initiating adaptive immune responses
• Mast cells: Tissue-resident cells that play a role in allergic reactions and inflammation
  • Release histamine, proteases, and other inflammatory mediators upon activation
  • Contribute to the recruitment of immune cells and the amplification of the inflammatory response
• Lymphocytes (T cells, B cells): Involved in the adaptive immune response and the resolution of inflammation
  • T cells secrete cytokines (IFN-γ, IL-4, IL-17) that modulate the inflammatory response
  • B cells produce antibodies that neutralize pathogens and promote their clearance

Cytokines and Inflammatory Mediators

• Cytokines are small proteins secreted by immune cells that regulate the inflammatory response
• Pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) amplify the inflammatory response
  • TNF-α activates endothelial cells, increases vascular permeability, and promotes the expression of adhesion molecules
  • IL-1β induces fever, stimulates the production of acute-phase proteins, and activates immune cells
  • IL-6 promotes the differentiation of B cells and the production of antibodies, and induces the synthesis of acute-phase proteins
• Anti-inflammatory cytokines (IL-10, TGF-β) downregulate the inflammatory response and promote tissue repair
  • IL-10 inhibits the production of pro-inflammatory cytokines and the activation of macrophages and dendritic cells
  • TGF-β suppresses immune cell activation and promotes the differentiation of regulatory T cells
• Chemokines (CXCL8/IL-8, CCL2/MCP-1) are chemoattractant cytokines that guide the migration of immune cells to the site of inflammation
• Lipid mediators (prostaglandins, leukotrienes) are derived from arachidonic acid and contribute to the inflammatory response
  • Prostaglandins (PGE2) cause vasodilation, increase vascular permeability, and induce pain and fever
  • Leukotrienes (LTB4) are potent chemoattractants for neutrophils and promote their adhesion and activation
• Reactive oxygen species (ROS) and nitric oxide (NO) are produced by immune cells and have antimicrobial and immunomodulatory effects
  • ROS (superoxide, hydrogen peroxide) are generated by the NADPH oxidase complex in phagocytes and contribute to the killing of pathogens
  • NO is synthesized by inducible nitric oxide synthase (iNOS) in macrophages and has vasodilatory and antimicrobial properties

Complement-Inflammation Interplay

• Complement system and inflammatory response are closely interconnected and mutually reinforcing
• Anaphylatoxins (C3a, C5a) generated during complement activation are potent inflammatory mediators
  • C5a is a strong chemoattractant for neutrophils and monocytes, promoting their recruitment to the site of inflammation
  • C3a and C5a increase vascular permeability and induce the release of histamine from mast cells
• Complement activation products (C3b, iC3b) opsonize pathogens and apoptotic cells, facilitating their phagocytosis by macrophages and neutrophils
  • Phagocytosis of complement-opsonized particles triggers the release of pro-inflammatory cytokines (TNF-α, IL-1β) by macrophages
• Inflammatory cytokines (TNF-α, IL-1β, IL-6) can upregulate the expression of complement receptors (CR1, CR3) on immune cells, enhancing complement-mediated phagocytosis
• Complement regulatory proteins (CD55, CD59) are downregulated during inflammation, allowing for increased complement activation and amplification of the inflammatory response
• Complement and inflammation work together to eliminate pathogens, clear damaged cells, and promote tissue repair
  • Dysregulation of either system can lead to chronic inflammation and tissue damage

Clinical Relevance and Disorders

• Complement deficiencies can lead to increased susceptibility to infections and autoimmune diseases
  • Deficiency of early complement components (C1q, C2, C4) is associated with systemic lupus erythematosus (SLE)
  • Deficiency of terminal complement components (C5-C9) predisposes individuals to recurrent Neisseria infections
• Complement overactivation contributes to various inflammatory and autoimmune disorders
  • Paroxysmal nocturnal hemoglobinuria (PNH): Deficiency of GPI-anchored complement regulatory proteins (CD55, CD59) leads to complement-mediated hemolysis
  • Atypical hemolytic uremic syndrome (aHUS): Dysregulation of the alternative pathway due to mutations in complement regulatory proteins (factor H, factor I)
  • Age-related macular degeneration (AMD): Complement activation and inflammation contribute to the development of drusen and retinal damage
• Chronic inflammatory diseases are characterized by persistent inflammation and tissue damage
  • Rheumatoid arthritis: Autoimmune disorder affecting the joints, characterized by synovial inflammation and cartilage and bone destruction
  • Inflammatory bowel disease (Crohn's disease, ulcerative colitis): Chronic inflammation of the gastrointestinal tract, leading to abdominal pain, diarrhea, and ulcerations
  • Psoriasis: Chronic skin disorder characterized by the formation of scaly, erythematous plaques due to hyperproliferation of keratinocytes and infiltration of immune cells
• Anti-inflammatory and immunomodulatory therapies target specific components of the inflammatory response
  • Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) enzymes, reducing the production of prostaglandins
  • Glucocorticoids (e.g., prednisone) have broad anti-inflammatory effects, suppressing the production of cytokines and the activation of immune cells
  • Biologic agents (e.g., TNF-α inhibitors, IL-6 receptor antagonists) target specific cytokines or their receptors to modulate the inflammatory response
• Complement-targeted therapies are being developed for the treatment of complement-mediated disorders
  • Eculizumab: Monoclonal antibody that inhibits C5, approved for the treatment of PNH and aHUS
  • Compstatin: Peptide inhibitor of C3 activation, under investigation for the treatment of age-related macular degeneration and other complement-driven diseases