Immediate Innate Response and Complement Pathways – Study Notes

Innate vs Adaptive immunity (key differences)

  • Innate system:
    • Response is very rapid.
    • Responds to a limited number of targets (recognizes conserved patterns).
    • Response does not get better with time or exposure to pathogens (no memory).
    • Does not generate memory.
  • Adaptive system:
    • Response is slower on first exposure but improves with time/exposure (memory).
    • Can respond to a virtually unlimited variety of targets after learning.
    • Builds specificity and affinity through clonal expansion and maturation.
    • Generates immunological memory that improves response upon re-exposure.
  • Practical exam note: the comparison table for innate vs adaptive is a high-yield source; often the exam question asks you to discuss these differences.

Focus of today's lecture: the immediate innate response

  • Scenario: pathogen breach of epithelial barriers (skin break with pathogen growth in connective tissue OR mucosal epithelium colonization with epithelial damage).
  • Immediate innate response:
    • Preprogrammed, circulating immune proteins ready to act as soon as they encounter a pathogen.
    • Requires no new recognition events by immune cells; these proteins are premade and circulating.
    • Goal: start a cascade to damage/eliminate the pathogen immediately, within the first hours.
  • Key takeaway: this response is fast but relatively non-specific; it works to buy time for the adaptive response to engage.

Macrophages and early pathogen sensing

  • Role of macrophages:
    • Recognize pathogens, phagocytose them, and secrete cytokines to induce inflammation.
    • Inflammation slows pathogen spread, slows growth, recruits more immune cells, and helps activate the adaptive response.
    • Macrophages also act as a garbage disposal system, clearing debris and dead cells, enabling cleaner downstream responses.
  • Recognition mechanisms:
    • Pattern Recognition Receptors (PRRs) detect conserved molecular patterns called Pathogen-Associated Molecular Patterns (PAMPs).
    • Examples include TLRs; tLR4 is highlighted as a receptor for certain bacterial components (e.g., LPS in Gram-negative bacteria).
    • Macrophages do not have a single receptor for each specific pathogen (like an antibody) but recognize broad patterns common to groups of pathogens.
  • All of this happens within minutes to hours after pathogen contact.

The two major ways pathogens harm the host (context for innate defense)

  • Extracellular pathogens (outside cells):
    • Can steal nutrients or secrete toxins directly affecting cells (exotoxins).
    • Exotoxins poison cells from outside the cell; cholera toxin (Vibrio cholerae) is an example.
  • Intracellular pathogens (inside host cells or vesicles):
    • They can release toxins inside the cell or grow inside vesicles (e.g., bacteria inside a phagosome or cytoplasm).
    • Endotoxins, in the sense used here, refer to toxins released within the host cell or within a compromised vesicle, which then poison the cell from the inside.
    • Examples mentioned: Yersinia pestis; Mycobacterium tuberculosis; Mycobacterium leprae (pathogens that can inhabit intracellular niches).
  • Direct cytopathic effect (virus-centric):
    • Virus replication inside a cell leads to cellular dysfunction and death, not merely external toxin action (the virus hijacks host resources).

Antibody-independent and antibody-dependent innate interactions

  • Antibodies and extracellular pathogens:
    • Antibodies are not the first-line actors of the immediate innate response; they will engage later and help tag pathogens for phagocytosis (opsonization) and activate complement via the classical pathway once present.
  • Antibodies are crucial for fine-tuning later responses and for enabling the classical complement pathway when present.

The complement system: overview and core ideas

  • Source of complement proteins:
    • Mostly produced by the liver; hepatocytes are a major source.
    • Other cell types contribute as well.
    • In the liver, these proteins circulate in the bloodstream and are distributed throughout the body.
  • Acute-phase proteins and inflammation:
    • IL-6 released by macrophages during infection signals the liver to produce acute-phase proteins (e.g., mannose-binding lectin, CRP, fibrinogen).
    • Acute-phase proteins are induced by inflammation and help tailor the early response.
  • Mannose-binding lectin (MBL) and MASPs:
    • MBL is a lectin that recognizes mannose residues commonly found on bacterial surfaces; it forms a complex with MASP-1 and MASP-2 serine proteases.
    • This pathway uses lectin binding to activate complement, introducing an alternative route to activation.

The thioester chemistry at the heart of complement activation

  • C3 and C4 have a reactive thioester bond that allows covalent attachment to nearby surfaces once exposed.
  • Activation exposes the thioester, which then reacts with nearby nucleophiles on pathogen surfaces, covalently attaching C3b or C4b to the pathogen.
  • This covalent attachment marks pathogens for destruction (opsonization) and helps amplify the response.
  • Important conceptual image: a thioester is buried inside the protein; upon cleavage, the exposed thioester can bind to the pathogen surface, anchoring the convertases.
  • This chemistry underpins the switch from soluble complements to surface-bound, pathogen-tagging complexes.

Pathways to activation: three routes to the same downstream goal

  • Common endpoint: all pathways lead to formation of surface-bound C3 convertases, deposition of C3b, and generation of C5 convertases that drive MAC formation.

  • Alternative pathway (the primitive, stochastic starter):

    • Starts with C3 spontaneously hydrolyzing in the fluid phase to form C3(H2O) (often denoted as IC3 or iC3 depending on nomenclature).
    • iC3 binds factor B, which is cleaved by factor D into Ba and Bb; Bb forms a soluble C3 convertase with iC3 (iC3Bb).
    • The surface of bacteria promotes a shift where C3b is deposited and binds Bb, generating the surface-bound alternative C3 convertase: C3bBb.
    • This convertase cleaves more C3 into C3a and C3b; the cycle amplifies rapidly on the pathogen surface.
    • Amplification loop: more C3b on the surface leads to more convertases and more C3 activation.
    • Regulation: Factor I (with cofactors such as Factor H and C4BP) trims C3b; DAF (CD55) and MCP (CD46) promote decay of convertases on host cells to protect self.
    • Key regulators and effectors:
    • Factor I (with cofactors) degrades C3b to iC3b and ultimately to inactive fragments.
    • DAF (CD55) and MCP (CD46) degrade C3b on host cells.
    • Protectin (CD59) prevents recruitment of C9 to form the MAC.

  • Lectin pathway (MBL-based, faster response to certain pathogens):

    • Mannose-binding lectin binds to bacterial mannose residues and activates MASP-1 and MASP-2.
    • MASP-2 cleaves C4 into C4a and C4b; C4b attaches to the pathogen surface.
    • C2 is cleaved by MASP-2 into C2a and C2b; C4b2a forms the classical C3 convertase (on the surface): C4b2a.
    • This convertase then acts like the classical pathway to produce C3a, C3b, and downstream C5 convertases, leading to MAC formation.
    • Lectin pathway converges with the alternative pathway after the formation of C3 convertase.

  • Classical pathway (antibody- or CRP-guided):

    • Initiated by C1 complex: C1q binds to antibodies (IgG/IgM) attached to the pathogen, or to C-reactive protein bound to the pathogen surface.
    • C1q binding activates C1r and C1s serine proteases; C1s cleaves C4 and C2 to form the classical C3 convertase: C4b2a.
    • Antibody-independent initiation via CRP and C1 complex is also described.
    • Once C4b2a is formed, it proceeds to cleavage of C3 and generation of the same downstream cascade as the other pathways.

  • Convergence: regardless of initiation, all three pathways generate C3b deposition and then proceed to a common terminal pathway with MAC formation.

The terminal sequence: membrane attack complex (MAC) and pore formation

  • MAC assembly sequence:
    • C5 convertase cleaves C5 into C5a and C5b.
    • C5b initiates assembly by binding C6 and C7; C8 binds next and inserts into the membrane.
    • C9 molecules assemble into a pore that spans the membrane; the MAC disrupts the target cell.
    • Typical MAC pore contains multiple C9 units (the exact number varies; estimates often range around a dozen, e.g., 10–16 C9 subunits; some illustrations show ~13).
  • Consequences:
    • Pore formation compromises membrane integrity and can lead to lysis of the pathogen.
    • The MAC is particularly important in defense against certain encapsulated bacteria; deficiencies in the system increase susceptibility to Neisseria infections.

Anaphylatoxins: C3a and C5a

  • Function:
    • C3a and C5a are potent inflammatory mediators (anaphylatoxins).
    • They promote vascular permeability and vasodilation, aiding immune cell recruitment to the site of infection.
  • Clinical note: systemic (anaphylaxis) would be dangerous, causing widespread vasodilation and dangerous drop in blood pressure; epinephrine can be used to counteract systemic anaphylaxis by constricting vessels and stabilizing blood pressure.

Opsonization and phagocyte engagement

  • Opsonization by C3b:
    • Surface-bound C3b on pathogens enhances recognition by phagocytes via complement receptors.
    • Macrophages express CR1 (CD35) which binds C3b; this allows phagocytosis even if a pathogen lacks its own PRRs for PAMPs.
  • Receptors for cleaved products:
    • CR1 binds C3b on pathogens.
    • CR3 and CR4 recognize iC3b (cleavage product of C3b) and other fragments; these receptors promote phagocytosis.
  • Significance:
    • This system enables efficient clearance of extracellular pathogens by macrophages and other phagocytes.

Regulation to prevent collateral damage: inhibitors and checks

  • Key regulators on host cells to prevent self-damage:
    • DAF (CD55) and MCP (CD46) promote decay of C3 convertases on host cell surfaces.
    • Factor I (with cofactors such as Factor H and C4BP) cleaves C3b to inactivated fragments (iC3b and beyond).
    • Protectin (CD59) prevents assembly of the MAC by blocking recruitment of C9.
  • Defective regulation and disease risk:
    • Deficiency in any of the components or regulators can lead to excessive or misdirected complement activity.
    • Paroxysmal nocturnal hemoglobinuria (PNH) arises when cells lack GPI-anchored proteins like DAF and CD59, making red blood cells susceptible to MAC-mediated lysis;
      episodic hemolysis with hemoglobinuria is characteristic.
  • Protease inhibitors in plasma:
    • Alpha-2-macroglobulin acts as a broad-spectrum protease trap (bait-and-trap) to neutralize serine proteases, preventing excessive proteolysis.
    • Serine protease inhibitors (serpins), including C1 inhibitor, help shut down complement activation and protect host tissues.
  • Balance is essential:
    • Too little complement results in vulnerability to infections.
    • Too much complement can cause tissue damage and autoimmune phenomena; tight temporal control is necessary.
  • NF-kappa B:
    • A central inflammatory transcription factor; when activated, it upregulates acute-phase proteins and then is rapidly shut off to limit inflammation.
    • Its inhibitors keep the response time-limited to prevent chronic inflammation.

Other notes: DEFENSINS and gut immunity

  • Defensins:
    • Small antimicrobial peptides (two major types: alpha and beta defensins).
    • They create pores in bacterial membranes and contribute to innate defense.
  • Paneth cells in the gut:
    • Paneth cells in the gut crypts constantly secrete defensins to defend the gut lumen from pathogens.
    • This represents a specialized tissue defense contributing to overall barrier protection.
  • The breadth of defenses:
    • Many cell types produce various defensins; alpha and beta defensins are different families with similar pore-forming function.

Quick glossary of key players (acronyms and terms you should know)

  • PAMP: Pathogen-Associated Molecular Pattern
  • PRR: Pattern Recognition Receptor
  • TLR4: Toll-like receptor 4 (example receptor recognizing LPS component of Gram-negative bacteria)
  • C3, C4, C5: core complement components; cleavage products include C3a, C3b, C4a, C4b, C5a, C5b
  • C3 convertase: enzymatic complex that cleaves C3 into C3a and C3b
  • C5 convertase: enzymatic complex that cleaves C5 into C5a and C5b
  • MAC: Membrane Attack Complex (C5b-9)
  • CR1 (CD35), CR3 (CD11b/CD18), CR4 (CD11c/CD18): complement receptors on phagocytes
  • DAF (CD55), MCP (CD46): decay-accelerating factors for convertases on host cells
  • Factor I, Factor H, C4BP: regulators that modify and inhibit complement activation
  • C1 inhibitor (C1-INH): a serine protease inhibitor that dampens early classical pathway activation
  • C reactive protein (CRP): an acute-phase protein that can bind to phosphocholine on pathogens and help activate complement via the classical pathway
  • Mannose-binding lectin (MBL) and MASP-1, MASP-2: lectin pathway recognition and proteolytic components
  • Protectin (CD59): MAC inhibitor preventing C9 recruitment
  • Alpha-2-macroglobulin: broad protease inhibitor that traps serine proteases
  • Paroxysmal nocturnal hemoglobinuria (PNH): disease due to lack of GPI-anchored regulators (e.g., DAF, CD59) leading to complement-mediated hemolysis

How the material connects to the bigger picture

  • The innate and complement systems act as a rapid, non-specific first line of defense, buying time for the adaptive immune system to develop targeted responses.
  • The complement system illustrates how body-wide surveillance can be concentrated at infection sites via local amplification (C3b deposition, MAC formation) and inflammation (anaphylatoxins).
  • The interplay between pathogen evasion (e.g., bacterial proteases, surface modifications) and host defense (complement inhibitors, regulatory proteins) highlights the ongoing evolutionary arms race between host and microbe.
  • Clinical relevance includes susceptibility to certain pathogens when components are deficient, and risk of tissue damage if regulation fails (autoimmune-like injury, anaphylaxis).

Quick recap equations and pathways (high-yield anchors)

  • C3 convertase formation and action:
    • ext{Alternative pathway: } C3
      ightarrow C3a + C3b, ext{ via } C3bBb
    • extLectinpathway:C4b2aext(C3convertase)ext{Lectin pathway: } C4b2a ext{ (C3 convertase)}
    • extClassicalpathway:C4b2aext(C3convertase,activatedviaC1q)ext{Classical pathway: } C4b2a ext{ (C3 convertase, activated via C1q)}
  • C5 convertase formation and MAC initiation:
    • ext{C5 convertase (alternative): } C3bBbC3b
      ightarrow C5a + C5b
    • ext{C5 convertase (classical/lectin): } C4b2aC3b
      ightarrow C5a + C5b
    • ext{MAC assembly: } C5b + C6 + C7 + C8 + (C9)_{n}
      ightarrow MAC
  • Anaphylatoxins and vascular effects:
    • C3a, C5a
      ightarrow ext{vasodilation and increased vascular permeability}
  • Opsonization and phagocytosis:
    • C3b ext{ on pathogen surface}
      ightarrow ext{CR1-mediated phagocytosis}
  • Regulation (selected):
    • C3b
      ightarrow ext{iC3b via Factor I + cofactors (e.g., Factor H, C4BP)}
    • C3b ext{ on host cells}
      ightarrow ext{DAF/MCP promote decay of convertases}
    • C9recruitmentblockedbyProtectin(CD59)C9 recruitment blocked by Protectin (CD59)

Note to self (exam strategy)

  • The table contrasting innate vs adaptive is a high-yield source for essays and short-answer questions.
  • Expect questions about how immediate innate responses operate within hours of infection and how they interface with later adaptive responses.
  • Understand the difference between extracellular and intracellular pathogens in terms of the types of immune defenses invoked (e.g., phagocytosis vs MAC-mediated lysis).
  • Remember the clinical correlations (PNH, susceptibility to Neisseria infections) as real-world anchors for the regulation of complement.