Innate Barriers and Complement Pathways – Transcript-Based Study Notes

Three Lines of Defense

• First line: barriers that prevent things from entering (mechanical barriers).
• Second line: innate immunity that is activated; innate cells (e.g., macrophages) signal to other cells to mount a response.
• Third line: adaptive immunity (later chapters will cover this in depth).

Innate Immune Barriers (overview of chapter focus)

• There are multiple innate barriers; the chapter first covers barriers and immediate innate mechanisms, then later chapters cover induced innate responses and adaptive immunity.

Mechanical Barriers

• Skin and skin epithelial cells with tight junctions provide a physical blockade.
• In the gut and elsewhere, flow of fluids helps push pathogens along and expel them.
• In the lungs, ciliated airway epithelial cells move mucus up and out, trapping bacteria.
• Mucus in these regions also contains chemicals targeting bacteria.
• The movement can direct pathogens out of the body (upwards in airway or down the digestive tract).
• Eyes have tear flow that helps expel foreign material; tears keep the eye from drying and help eject irritants.
• Physical barriers are complemented by chemical and immunological mechanisms at these sites.

Chemical Barriers Across Locations

• Skin: fatty acids in sebum create an inhospitable environment for bacteria in hair follicles.
• Antimicrobial peptides (AMPs) are produced at various barrier sites and generally disrupt microbial membranes; some AMPs have other functions but most target microbial membranes.
• The chapter will cover antimicrobial peptides at the end, after discussing the complement system.

Tears, Mucus, and Mechanical Clearance (practical notes)

• Tears in the eyes serve as a mechanical/chemical barrier and aid in expelling particles.
• Mucus traps microbes; ciliated cells then move the mucus (and trapped microbes) toward expulsive routes.

Antimicrobial Peptides (AMPs)

• AMPs disrupt membranes of bacteria (primary mode of action).
• Some AMPs have additional functions, but membrane disruption is the predominant mechanism described here.
• The discussion will return to AMPs at the end of the chapter.

Complement Activation: Overview

• The bulk of the chapter focuses on the complement cascade, a series of proteases in the bloodstream that amplify the immune response.
• There are three pathways to activate the complement system (the outcomes are the same regardless of the pathway):
  • Alternative pathway (spontaneous activation)
  • Lectin pathway (induced by microbial sugars)
  • Classical pathway (antibody-mediated activation)
• The three pathways converge on a shared set of outcomes but are initiated by different triggers.

Key Clarifications

• Proteases are enzymes that cleave other proteins; the complement system uses proteolytic cascades.
• The pathways are named for their initiation: alternative (spontaneous), lectin (sugar-binding), and classical (antibody-mediated).
• The speaker emphasizes that the three pathways ultimately yield similar downstream effects (inflammation, opsonization, and membrane disruption via MAC).
• The order of pathway activation in typical infection: alternative first, lectin second, classical last (antibody-dependent).

The Nine Major Complement Proteins

• There are nine major complement proteins labeled $C<em>1, C</em>2, \cdots, C_9$.
• C1, C2, and C4 are primarily involved in the lectin and classical pathways; they are not used in the alternative pathway.
• The other components (C3, C5, C6, C7, C8, C9) participate in all three pathways.

The Core Outcomes of Complement Activation

• Inflammatory cell recruitment (anaphylatoxins): $C<em>3a$ and $C</em>5a$ promote inflammation and recruit immune cells.
• Opsonization: $C_3b$ coats pathogens, tagging them for phagocytosis.
• Membrane attack: the terminal pathway forms the membrane attack complex (MAC): $C<em>5b + C</em>6 + C<em>7 + C</em>8 + C_9 <br /> ightarrow MAC.$
• The outcomes lead to destruction and clearance of microbes, especially those with membranes.

Alternative Pathway (spontaneous activation)

• Initiation:
  • In the bloodstream, the central component $C<em>3$ spontaneously undergoes a conformational change upon contact with water, producing $C</em>3(H<em>2O)$ (often read as induced $C</em>3$ or $C_3^{*}$).
  • The $C<em>3(H</em>2O)$ form binds factor B, which is then cleaved by factor D to form $C<em>3(H</em>2O)Bb$; this is the fluid-phase C3 convertase.
• First convertase formation:
  • The fluid-phase convertase cleaves $C<em>3$ into $C</em>3a$ and $C_3b$ on encountering a surface.
  • On a microbial surface, $C<em>3b$ binds to surface-bound Bb to form the surface C3 convertase $C</em>3bBb$.
• Surface amplification:
  • The surface convertase $C<em>3bBb$ cleaves more $C</em>3$ to generate additional $C<em>3b$ and $C</em>3a$, amplifying the response.
• Formation of the C5 convertase:
  • When sufficient $C<em>3b$ accumulates, two or more $C3b$ units associate with the surface convertase to form the C5 convertase, commonly denoted as $C</em>3b<em>2Bb$ or $C</em>3bBbC_3b$.
• Downstream cleavage:
  • The C5 convertase cleaves $C<em>5$ into $C</em>5a$ and $C<em>5b$; $C</em>5a$ diffuses away to promote inflammation, while $C_5b$ marks the beginning of the MAC assembly.
• MAC assembly:
  • $C<em>5b$ recruits $C</em>6$, then $C<em>7$, $C</em>8$, and finally $C_9$ to form the membrane attack complex on the pathogen membrane.
• Summary of alternative pathway steps (condensed):
  • $ext{C}<em>3 ightarrow ext{C}</em>3(H_2O)$ (spontaneous)
  • $ext{C}<em>3(H</em>2O) + ext{Bb} <br /> ightarrow ext{C}<em>3(H</em>2O) ext{Bb}$ (with factor D)
  • $ext{C}<em>3 + ext{C}</em>3(H<em>2O) ext{Bb} ightarrow ext{C}</em>3a + ext{C}_3b$ (surface activation)
  • $ext{C}<em>3b + ext{Bb} ightarrow ext{C}</em>3b ext{Bb} <br /> ightarrow ext{C}<em>3b</em>2 ext{Bb} ext{(C5 convertase)}$
  • $ext{C}<em>5 ightarrow ext{C}</em>5a + ext{C}_5b$ (via C5 convertase)
  • $ext{C}<em>5b + ext{C}</em>6 + ext{C}<em>7 + ext{C}</em>8 + ext{C}_9 <br /> ightarrow MAC$
• Key functional outputs:
  • $ext{C}<em>3a, ext{C}</em>5a <br /> ightarrow ext{inflammation (anaphylatoxins)}$
  • $ext{C}_3b <br /> ightarrow ext{opsonization (phagocytosis signal)}$
  • $ext{MAC} <br /> ightarrow ext{membrane disruption and lysis of pathogens}$

Lectin Pathway

• Initiation:
  • Mannose-binding lectin (MBL) binds to specific sugars on microbes.
  • Associated proteases (MASPs) then cleave complement components to propagate activation.
• Core steps:
  • Cleavage of $C<em>4$ and $C</em>2$ by MASPs leads to formation of $C_4b2a$, the C3 convertase for the lectin pathway.
  • The downstream steps mirror the classical pathway after formation of the C3 convertase: cleavage of $C<em>3$ to $C</em>3a + C<em>3b$, formation of C5 convertase (often denoted $C</em>4b2aC<em>3b$), release of $C</em>5a$, and assembly of the MAC via $C<em>5b - C</em>9$.
• Outcome: inflammation, opsonization, and MAC are achieved via the same terminal steps as the other pathways.

Classical Pathway

• Initiation:
  • Activated by antibodies (e.g., IgM or IgG) bound to the surface of microbes.
  • The C1 complex (C1qrs) triggers proteolytic cleavage of C4 and C2.
• Core steps:
  • Cleavage of $C<em>4$ and $C</em>2$ forms the C3 convertase $C_4b2a$.
  • The downstream cascade cleaves $C<em>3$ to $C</em>3a + C<em>3b$, enabling formation of the C5 convertase (e.g., $C</em>4b2aC<em>3b$), followed by $C</em>5a$ release and MAC formation as in the other pathways.
• Outcome: inflammation, opsonization, and MAC formation are shared end results.

Important Notes on Pathways and Nomenclature

• There are three initiation routes, but all converge to the same functional outcomes.
• The alternative pathway uses spontaneous hydrolysis of $C_3$ and is always ready to begin; lectin and classical pathways require microbial recognition or antibody binding, respectively.
• Inhibitions or defects in any step can impair downstream immune responses, increasing susceptibility to infection.

Practical Connections and Study Tips

• Visualize the three pathways as a branching tree that converges to C3 activation, then to C5 activation, and finally to MAC formation.
• Remember the three major outcomes as the triad: inflammation (C3a, C5a), opsonization (C3b), and lysis (MAC via C5b-9).
• Keep straight which components are unique to the classical/lectin pathways (C1, C2, C4) and which are common to all (C3, C5, C6, C7, C8, C9).
• For exam prep, memorize the primary convertases:
  • Alternative pathway: $C<em>3bBb ext{ (surface C3 convertase)}$; C5 convertase: $C</em>3b<em>2Bb ext{ or } C</em>3bBbC_3b$.
  • Classical/lectin pathways: $C<em>4b2a ext{ (C3 convertase)}$; C5 convertase: $C</em>4b2aC_3b$.
• Be comfortable with the terminal steps: once C5 is split into $C<em>5a$ and $C</em>5b$, the MAC forms via $C<em>5b + C</em>6 + C<em>7 + C</em>8 + C_9 <br /> ightarrow MAC$.

Quick Recap (Key Equations and Concepts)

• Mechanical and chemical barriers limit pathogen entry and establishment at barrier sites.
• Antimicrobial peptides disrupt microbial membranes; sebum provides fatty-acid–rich protection on skin.
• The complement system is a protease cascade with three initiation routes that converge on:
  • Inflammation: $C<em>3a, C</em>5a <br /> ightarrow ext{inflammation}</li> <li>Opsonization:$C_3b
    ightarrow ext{opsonization (phagocytosis)}
  • MAC formation: $C<em>5b ightarrow ext{MAC} (C</em>5b-9)</li></ul></li> <li>Alternative pathway convertases and steps (condensed):<ul> <li>$C3 ightarrow C3(H2O) ightarrow C3(H2O)Bb ightarrow C3a + C_3b$</li> <li>Surface:$C3bBb ightarrow C3b2Bb ightarrow C5a + C_5b$</li> <li>MAC assembly:$C5b + C6 + C7 + C8 + C_9
    ightarrow MAC$$
• Lectin and Classical pathways converge to the same C3 convertase and downstream steps, with initiation differing (MBL-MASPs vs antibodies).

Connections to Broader Immunology

• Innate barriers provide the first shield and immediate response, while the complement cascade provides rapid, targeted, and amplified defense with clear effector outcomes.
• Adaptive immunity (to be covered in Chapter 3 and beyond) links to the classical pathway via antibody recognition but the alternative pathway can function independently of antibodies.

Ethical/Practical Implications (Reflective)

• Redundancy in barrier defenses and the complement cascade highlights why partial deficiencies do not always lead to catastrophic failure; however, genetic defects or acquired deficiencies in components can still significantly impair host defense.
• Understanding the timing and triggers of the three pathways helps in designing vaccines and therapeutics that manipulate the complement system to treat infections or inflammatory diseases.