Immune System Physiology

Quick Answer

Answer: Immune system physiology describes the coordinated network of cells, tissues, and molecules that protect the host from pathogens. The immune system comprises two interconnected arms: innate immunity (immediate, non-specific) and adaptive immunity (delayed, highly specific, memory-capable).

Innate immunity provides first-line defense through physical barriers (skin, mucosa), pattern recognition receptors (Toll-like receptors, NOD-like receptors), phagocytes (neutrophils, macrophages), natural killer cells, and the complement system. Adaptive immunity develops over days through T lymphocytes (CD4+ helper, CD8+ cytotoxic, regulatory T cells) and B lymphocytes (antibody production), with antigen presentation via MHC class I (endogenous antigens) and MHC class II (exogenous antigens).

The inflammatory response involves pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), anti-inflammatory cytokines (IL-10, TGF-β), chemokines, and eicosanoids (prostaglandins, leukotrienes). In critical illness, dysregulated inflammation leads to SIRS/sepsis through PAMP/DAMP recognition, cytokine storm, endothelial dysfunction, and coagulopathy. Prolonged ICU stay causes immunoparalysis with impaired HLA-DR expression and increased susceptibility to secondary infections.

Summary Table: Innate vs Adaptive Immunity

CICM Exam Focus

What Examiners Expect

First Part Written SAQ Topics:

• Pattern recognition receptors and their ligands (TLRs, NLRs)
• Complement activation pathways (classical, alternative, lectin)
• T cell subsets and their functions (Th1, Th2, Th17, Tregs)
• Cytokine networks in sepsis (pro-inflammatory vs anti-inflammatory)
• Immunoglobulin structure and function
• Mechanisms of immunosuppression in critical illness

First Part Viva Topics:

• PAMP vs DAMP and their clinical relevance
• Sepsis pathophysiology from molecular to organ dysfunction
• Immunoparalysis and nosocomial infections
• Complement deficiency syndromes
• Mechanism of action of immunomodulatory agents

Common Exam Questions:

1. "Describe the complement system, including activation pathways and regulation"
2. "Explain the role of cytokines in the inflammatory response"
3. "Discuss the immunological changes that occur in sepsis"
4. "Compare and contrast innate and adaptive immunity"
5. "Describe the mechanism of action of corticosteroids on the immune system"

High-Yield Topics

Key Points

1. Innate immunity provides immediate, non-specific defense through physical barriers, PRRs, phagocytes, NK cells, and complement; it lacks immunological memory
2. Toll-like receptors (TLRs) recognize PAMPs (pathogen-associated molecular patterns) and DAMPs (damage-associated molecular patterns), activating NF-κB and type I interferon pathways (PMID: 11138006)
3. Complement system has three activation pathways (classical, alternative, lectin) that converge at C3 convertase, leading to opsonization (C3b), chemotaxis (C3a, C5a), and membrane attack complex (C5b-9) (PMID: 20011988)
4. T lymphocytes require dual signals for activation: TCR-MHC/peptide interaction (signal 1) and costimulation via CD28-B7 (signal 2); absence of signal 2 causes anergy (PMID: 10837056)
5. CD4+ T helper cells differentiate into Th1 (IFN-γ, cell-mediated immunity), Th2 (IL-4/5/13, humoral immunity), Th17 (IL-17, neutrophil recruitment), and Tregs (IL-10, TGF-β, immune suppression) (PMID: 17960159)
6. Cytokine storm in sepsis involves excessive TNF-α, IL-1β, and IL-6 release, causing vasodilation, capillary leak, DIC, and multi-organ dysfunction (PMID: 23135902)
7. Immunoparalysis occurs in prolonged critical illness with reduced monocyte HLA-DR expression (<30%), impaired cytokine production, and increased susceptibility to secondary infections (PMID: 12015787)
8. Prostaglandins and leukotrienes are eicosanoids derived from arachidonic acid via COX (prostaglandins) and LOX (leukotrienes) pathways, mediating fever, vasodilation, and bronchoconstriction (PMID: 16960134)
9. Corticosteroids suppress immunity through inhibition of NF-κB, reduced cytokine production, neutrophilia with impaired function, lymphopenia, and monocyte deactivation (PMID: 15735000)
10. Indigenous populations (Aboriginal, Torres Strait Islander, Māori) have higher rates of invasive bacterial infections, rheumatic heart disease, and post-streptococcal glomerulonephritis requiring culturally appropriate prevention and management (PMID: 28406084)

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Innate Immunity

Physical and Chemical Barriers

The first line of defense comprises physical and chemical barriers that prevent pathogen entry:

Skin:

• Keratinized stratified squamous epithelium provides mechanical barrier
• Low pH (4.5-5.5) inhibits bacterial growth
• Antimicrobial peptides (defensins, cathelicidins) kill pathogens directly
• Sebum contains fatty acids with antimicrobial properties
• Commensal flora (skin microbiome) compete with pathogens (PMID: 22343560)

Respiratory Tract:

• Mucociliary escalator clears particles from airways
• Mucus traps pathogens and contains IgA, lysozyme, lactoferrin
• Surfactant proteins SP-A and SP-D opsonize pathogens
• Cough and sneeze reflexes expel pathogens
• Alveolar macrophages phagocytose inhaled particles (PMID: 20881281)

Gastrointestinal Tract:

• Gastric acid (pH 1-3) kills most ingested pathogens
• Bile salts disrupt bacterial membranes
• Peristalsis prevents bacterial overgrowth
• Paneth cells secrete defensins and lysozyme
• Gut microbiome (10^14 bacteria) provides colonization resistance (PMID: 22688618)

Genitourinary Tract:

• Urinary flow mechanically clears bacteria
• Low pH of vaginal secretions inhibits pathogens
• Lactobacillus species produce lactic acid and hydrogen peroxide
• Tamm-Horsfall protein prevents bacterial adhesion (PMID: 18397257)

Pattern Recognition Receptors (PRRs)

Pattern recognition receptors identify conserved microbial structures (PAMPs) and endogenous danger signals (DAMPs):

Toll-Like Receptors (TLRs)

TLRs are type I transmembrane proteins with leucine-rich repeat domains that recognize diverse pathogen structures. Beutler and Hoffmann received the 2011 Nobel Prize for discovering TLR-mediated innate immunity (PMID: 11138006).

Cell Surface TLRs:

Endosomal TLRs:

TLR Signaling Pathways:

1. MyD88-dependent pathway (all TLRs except TLR3):

   • TLR → MyD88 → IRAK → TRAF6 → IKK → NF-κB activation
   • Induces pro-inflammatory cytokines (TNF-α, IL-1β, IL-6)
   • Rapid response within hours (PMID: 12186560)

2. TRIF-dependent pathway (TLR3, TLR4):

   • TLR → TRIF → TBK1 → IRF3 activation
   • Induces type I interferons (IFN-α, IFN-β)
   • Important for antiviral immunity (PMID: 18273042)

TLR4/LPS Recognition Complex:

The recognition of LPS by TLR4 involves multiple accessory proteins:

• LBP (LPS-binding protein): Transfers LPS to CD14
• CD14: Membrane-bound or soluble, transfers LPS to MD-2
• MD-2: Associates with TLR4, directly binds LPS
• TLR4: Signal transduction via MyD88 and TRIF pathways (PMID: 15184896)

NOD-Like Receptors (NLRs)

NLRs are cytoplasmic sensors that detect intracellular pathogens and danger signals:

Key NLRs:

NLRP3 Inflammasome:

The NLRP3 inflammasome is a multi-protein complex critical for IL-1β and IL-18 processing:

1. Priming signal: TLR activation induces pro-IL-1β and NLRP3 expression
2. Activation signal: NLRP3 oligomerizes, recruits ASC adaptor
3. Caspase-1 activation: ASC recruits and activates caspase-1
4. Cytokine processing: Caspase-1 cleaves pro-IL-1β and pro-IL-18 to active forms
5. Pyroptosis: Caspase-1 cleaves gasdermin D, forming membrane pores (PMID: 25430545)

Clinical relevance: NLRP3 inflammasome hyperactivation is implicated in gout, atherosclerosis, Alzheimer's disease, and COVID-19 cytokine storm (PMID: 32376573).

RIG-I-Like Receptors (RLRs)

Cytoplasmic sensors for viral RNA:

• RIG-I: Recognizes 5'-triphosphate RNA (influenza, VSV)
• MDA5: Recognizes long dsRNA (picornaviruses)
• Signal via MAVS → IRF3/7 → type I interferons (PMID: 16127453)

Cellular Components of Innate Immunity

Neutrophils

Neutrophils are the most abundant leukocytes (50-70% of WBC) and first responders to infection:

Characteristics:

• Short-lived (6-10 hours in circulation, 1-4 days in tissues)
• Multi-lobed nucleus (polymorphonuclear)
• Primary (azurophilic), secondary (specific), and tertiary granules
• Produced in bone marrow at rate of 10^11/day (PMID: 24629025)

Functions:

1. Phagocytosis:

   • Recognition via Fc receptors, complement receptors, PRRs
   • Ingestion into phagosome
   • Phagosome-lysosome fusion
   • Pathogen killing and digestion

2. Oxidative Burst (Respiratory Burst):

   • NADPH oxidase activation
   • Production of superoxide (O2^-), hydrogen peroxide (H2O2), hypochlorous acid (HOCl)
   • Myeloperoxidase (MPO) catalyzes HOCl production from H2O2 and Cl-
   • Critical for bacterial killing (PMID: 18420738)

3. Degranulation:

   • Primary granules: Myeloperoxidase, defensins, elastase, cathepsin G
   • Secondary granules: Lactoferrin, collagenase, lysozyme, NADPH oxidase components
   • Tertiary granules: Gelatinase, matrix metalloproteinases (PMID: 24629025)

4. Neutrophil Extracellular Traps (NETs):

   • Web-like structures of DNA, histones, and granule proteins
   • Trap and kill extracellular pathogens
   • NETosis is a unique form of cell death
   • Implicated in thrombosis, autoimmunity, and ARDS (PMID: 15001782)

Neutrophil Dysfunction in Critical Illness:

• Impaired chemotaxis and phagocytosis
• Reduced oxidative burst capacity
• Delayed apoptosis with persistent inflammation
• Contributes to organ injury in sepsis (PMID: 22337888)

Macrophages

Macrophages are tissue-resident phagocytes derived from circulating monocytes:

Types by Location:

Macrophage Polarization:

Macrophages exhibit functional plasticity with two main polarization states:

(PMID: 24484020)

Functions:

• Phagocytosis and pathogen killing
• Antigen presentation to T cells (MHC class II)
• Cytokine production (TNF-α, IL-1β, IL-6, IL-12)
• Tissue repair and remodeling
• Iron metabolism and erythrophagocytosis

Natural Killer (NK) Cells

NK cells are large granular lymphocytes that provide rapid cytotoxic responses without prior sensitization:

Characteristics:

• 5-15% of peripheral blood lymphocytes
• CD3-, CD56+, CD16+ phenotype
• Do not require antigen presentation via MHC
• Recognize "missing self" (absence of MHC class I) (PMID: 12457618)

NK Cell Receptors:

"Missing Self" Hypothesis:

• Normal cells express MHC class I, engaging inhibitory KIRs → no killing
• Virus-infected or tumor cells downregulate MHC I → loss of inhibition → killing
• "Induced self" ligands (MICA/B) on stressed cells engage NKG2D → killing (PMID: 12457618)

Effector Mechanisms:

1. Perforin/granzyme pathway:

   • Perforin creates pores in target cell membrane
   • Granzymes enter and activate caspases
   • Induces apoptosis in target cell

2. Death receptor pathway:

   • FasL (CD95L) on NK cell binds Fas on target
   • Activates extrinsic apoptosis pathway

3. Antibody-dependent cellular cytotoxicity (ADCC):

   • CD16 binds Fc region of IgG coating target cells
   • Major mechanism for antibody-mediated clearance (PMID: 20005247)

Cytokine Production:

• IFN-γ: Activates macrophages, enhances Th1 responses
• TNF-α: Pro-inflammatory effects
• GM-CSF: Stimulates myelopoiesis

Dendritic Cells

Dendritic cells (DCs) are professional antigen-presenting cells that bridge innate and adaptive immunity:

Types:

(PMID: 16551257)

Functions:

1. Antigen capture: Phagocytosis, macropinocytosis, receptor-mediated endocytosis
2. Antigen processing: Proteolytic degradation, peptide loading onto MHC
3. Migration: Chemokine-directed movement to lymph nodes via afferent lymphatics
4. T cell activation: Provides signals 1 (TCR-MHC/peptide), 2 (costimulation), and 3 (cytokines)
5. Tolerance induction: Immature DCs in steady state induce T cell anergy/Tregs

DC Maturation:

• Immature DCs: High antigen uptake, low costimulatory molecules, tolerogenic
• Mature DCs: Low antigen uptake, high MHC and costimulatory molecules (CD80/86), immunogenic
• Triggered by PAMPs, DAMPs, inflammatory cytokines (PMID: 16551257)

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Complement System

The complement system is a cascade of plasma proteins that enhance (complement) antibody and phagocytic clearance of pathogens. It comprises over 30 proteins constituting approximately 15% of serum globulin fraction.

Activation Pathways

Three pathways converge at C3 convertase formation:

Classical Pathway

Activation:

• Initiated by C1q binding to antigen-antibody complexes (IgM or IgG1/2/3)
• C1q conformational change activates C1r, which cleaves and activates C1s
• C1s cleaves C4 → C4a + C4b
• C1s cleaves C2 → C2a + C2b
• C4b2a forms the classical pathway C3 convertase (PMID: 20011988)

Key Points:

• Requires antibody (adaptive immunity link)
• IgM most efficient (single molecule can activate)
• C1-INH (C1 esterase inhibitor) regulates C1r/C1s
• Deficiency causes hereditary angioedema

Alternative Pathway

Activation:

• Spontaneous hydrolysis of C3 "tick-over" (C3(H2O))
• C3(H2O) binds Factor B
• Factor D cleaves Factor B → Ba + Bb
• C3(H2O)Bb is the fluid-phase C3 convertase
• Amplification: C3b deposited on surfaces binds Factor B
• C3bBb is the surface-bound alternative pathway C3 convertase
• Properdin (Factor P) stabilizes C3bBb (PMID: 20011988)

Key Points:

• Antibody-independent (pure innate immunity)
• Provides amplification loop for all pathways
• Regulated by Factor H and Factor I
• Factor H deficiency causes atypical hemolytic uremic syndrome (aHUS)

Lectin (Mannose-Binding Lectin) Pathway

Activation:

• MBL (mannose-binding lectin) or ficolins bind carbohydrates on pathogens
• MASP-1 and MASP-2 (MBL-associated serine proteases) activated
• MASP-2 cleaves C4 and C2 (similar to C1s)
• C4b2a is the lectin pathway C3 convertase (PMID: 12065231)

Key Points:

• Antibody-independent
• MBL is an acute phase reactant
• MBL deficiency associated with recurrent infections in children
• Less important in adults due to acquired immunity

Central Complement Components (C3 and C5)

C3 Convertases:

• Classical/Lectin: C4b2a
• Alternative: C3bBb
• Cleave C3 → C3a (anaphylatoxin) + C3b (opsonin)

C5 Convertases:

• Classical/Lectin: C4b2a3b
• Alternative: C3bBb3b
• Cleave C5 → C5a (potent anaphylatoxin) + C5b (initiates MAC)

Effector Functions

Opsonization (C3b, iC3b)

• C3b and its cleavage product iC3b coat pathogens
• Recognized by complement receptors on phagocytes:
  • "CR1 (CD35): Binds C3b, C4b"
  • "CR3 (CD11b/CD18): Binds iC3b"
  • "CR4 (CD11c/CD18): Binds iC3b"
• Enhances phagocytosis 4,000-fold compared to unopsonized particles (PMID: 20011988)

Anaphylatoxins (C3a, C4a, C5a)

Small peptides with potent pro-inflammatory effects:

C5a is the most potent, acting via C5aR1 (CD88) on neutrophils, macrophages, and endothelium (PMID: 20011988).

Membrane Attack Complex (MAC, C5b-9)

Formation of pores in target cell membranes:

1. C5b binds C6 and C7 → C5b-7 inserts into membrane
2. C8 binds and partially inserts
3. Multiple C9 molecules polymerize forming a pore (10-16 C9 molecules)
4. Pore allows ion and water influx → osmotic lysis

Clinical Relevance:

• Effective against Gram-negative bacteria (thin cell wall)
• Gram-positive bacteria resistant (thick peptidoglycan)
• MAC on nucleated cells may cause sublytic activation rather than lysis
• Paroxysmal nocturnal hemoglobinuria (PNH): Deficiency of CD55/CD59 → uncontrolled MAC on RBCs (PMID: 22689942)

Complement Regulation

Complement activation is tightly controlled to prevent host tissue damage:

Fluid-Phase Regulators:

Membrane-Bound Regulators:

(PMID: 22689942)

Complement Deficiencies and Clinical Syndromes

Neisserial Infections and Terminal Complement Deficiency:

• Patients with C5-C9 deficiencies have 1,000-10,000x increased risk of meningococcal disease
• Eculizumab (C5 inhibitor) users require meningococcal vaccination
• Australian guidelines mandate Men ACWY + Men B vaccination (PMID: 22689942)

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Adaptive Immunity

Antigen Presentation

Adaptive immunity requires antigen presentation by MHC molecules to T lymphocytes:

MHC Class I

Characteristics:

• Expressed on all nucleated cells
• Presents endogenous antigens (synthesized within the cell)
• Recognized by CD8+ cytotoxic T lymphocytes
• Structure: α chain (heavy chain) + β2-microglobulin
• Human MHC class I: HLA-A, HLA-B, HLA-C (PMID: 10837056)

Antigen Processing Pathway:

1. Cytoplasmic proteins degraded by proteasome
2. Peptides transported into ER via TAP (transporter associated with antigen processing)
3. Peptides loaded onto MHC class I with help of tapasin, calreticulin
4. MHC I-peptide complex transported to cell surface
5. Peptide length: 8-10 amino acids

Clinical Relevance:

• Downregulation by viruses (CMV, HSV) → NK cell activation
• Tumor cells may downregulate MHC I → immune evasion
• HLA typing critical for organ transplantation

MHC Class II

Characteristics:

• Expressed on antigen-presenting cells (DCs, macrophages, B cells)
• Presents exogenous antigens (internalized from outside)
• Recognized by CD4+ helper T lymphocytes
• Structure: α chain + β chain
• Human MHC class II: HLA-DR, HLA-DQ, HLA-DP (PMID: 10837056)

Antigen Processing Pathway:

1. Extracellular antigens internalized by endocytosis/phagocytosis
2. Degraded by cathepsins in acidified endosomes/lysosomes
3. MHC class II synthesized in ER with invariant chain (Ii, CD74)
4. Invariant chain cleaved leaving CLIP peptide in binding groove
5. HLA-DM catalyzes CLIP removal and peptide loading
6. MHC II-peptide complex transported to cell surface
7. Peptide length: 13-25 amino acids

Cross-Presentation:

• Dendritic cells (cDC1) can present exogenous antigens on MHC class I
• Critical for CD8+ T cell responses to tumors and viruses that don't infect DCs
• Mechanism involves phagosome-to-cytosol pathway (PMID: 22000017)

T Lymphocytes

T lymphocytes develop in the thymus and are central to adaptive immunity:

T Cell Development

Thymic Selection:

1. Positive selection (cortex): T cells that recognize self-MHC survive
2. Negative selection (medulla): T cells that recognize self-antigens too strongly deleted
3. Results in T cells that are MHC-restricted and self-tolerant
4. AIRE (autoimmune regulator) in medulla expresses tissue-specific antigens (PMID: 15284098)

CD4+ Helper T Cells

CD4+ T cells recognize MHC class II and provide help to other immune cells:

T Helper Subsets:

(PMID: 17960159)

Th1/Th2 Balance:

• Th1 responses critical for intracellular pathogens (viruses, mycobacteria)
• Th2 responses for helminths but can cause allergic disease
• Reciprocal regulation: IFN-γ inhibits Th2; IL-4 inhibits Th1
• Sepsis may shift toward Th2, contributing to immunosuppression (PMID: 17960159)

CD8+ Cytotoxic T Lymphocytes (CTLs)

CD8+ T cells recognize MHC class I and directly kill infected/abnormal cells:

Activation:

• Requires TCR engagement with MHC I-peptide
• Costimulation via CD28-B7 interaction
• Often requires CD4+ T cell help (licensing of DCs)

Effector Mechanisms:

1. Perforin/granzyme pathway:

   • Perforin creates pores in target cell membrane
   • Granzymes enter and activate caspase-3, -7
   • Granzyme B cleaves BID → mitochondrial apoptosis
   • Primary mechanism for killing infected cells (PMID: 11782407)

2. Fas-FasL pathway:

   • FasL (CD95L) on CTL engages Fas (CD95) on target
   • Recruits FADD and procaspase-8
   • Activates extrinsic apoptosis pathway
   • Important for immune homeostasis

Regulatory T Cells (Tregs)

Tregs are essential for maintaining self-tolerance and preventing autoimmunity:

Characteristics:

• CD4+CD25+Foxp3+ phenotype
• Thymus-derived (natural Tregs) or induced in periphery (iTregs)
• Require IL-2 for survival (express high-affinity IL-2R)
• Suppress effector T cells and maintain tolerance (PMID: 12490957)

Suppressive Mechanisms:

1. Cytokine-mediated: IL-10, TGF-β, IL-35 suppress effector cells
2. Metabolic disruption: CD25 (IL-2Rα) sequesters IL-2 from effector cells
3. Cytolysis: Granzyme A/B-mediated killing of effector cells
4. DC modulation: CTLA-4 downregulates B7 on DCs, inhibiting costimulation

Clinical Relevance:

• Treg depletion in cancer may enhance anti-tumor immunity
• Treg expansion/infusion may prevent transplant rejection
• Foxp3 mutations cause IPEX syndrome (PMID: 12490957)

B Lymphocytes and Antibody Production

B lymphocytes develop in bone marrow and produce antibodies:

B Cell Development

1. Bone marrow: Pro-B → Pre-B → Immature B → Transitional B
2. Heavy chain rearrangement: D-J, then V-DJ recombination
3. Light chain rearrangement: V-J recombination (κ or λ)
4. Central tolerance: B cells strongly binding self-antigens deleted (clonal deletion) or edited (receptor editing)
5. Peripheral maturation: B cells migrate to spleen, lymph nodes (PMID: 16551253)

B Cell Activation

T-Dependent Activation:

1. B cell binds antigen via BCR (surface IgM/IgD)
2. Antigen internalized, processed, presented on MHC class II
3. Tfh cell provides help via CD40L-CD40 interaction
4. Cytokine signals (IL-4, IL-21) drive proliferation
5. Germinal center reaction: somatic hypermutation and class switching
6. Affinity maturation selects high-affinity B cells
7. Plasma cells (antibody secretion) and memory B cells generated (PMID: 16551253)

T-Independent Activation:

• Type 1 TI antigens: Polyclonal B cell activators (LPS, CpG DNA)
• Type 2 TI antigens: Highly repetitive epitopes (polysaccharide capsules)
• Limited class switching (mainly IgM)
• No memory generation
• Important for encapsulated bacterial responses (PMID: 16551253)

Antibody Classes

Immunoglobulin Structure:

• Two heavy chains + two light chains (κ or λ)
• Variable (V) regions: Antigen binding (CDRs)
• Constant (C) regions: Effector functions (Fc)
• Hinge region: Flexibility for bivalent binding

Immunoglobulin Classes:

(PMID: 18941479)

IgG Subclasses:

Secretory IgA:

• Dimer linked by J chain
• Transported across epithelium by polymeric Ig receptor (pIgR)
• Secretory component protects from proteolysis
• Found in saliva, tears, breast milk, GI secretions
• 5-10 g/day produced (most abundant antibody) (PMID: 21909089)

Memory Cells

Immunological memory is the hallmark of adaptive immunity:

Memory T Cells:

• Central memory (Tcm): CCR7+, lymph node homing, high proliferative capacity
• Effector memory (Tem): CCR7-, tissue homing, rapid effector function
• Tissue-resident memory (Trm): CD69+CD103+, reside in tissues
• Stem cell memory (Tscm): Most primitive, greatest longevity (PMID: 21909089)

Memory B Cells:

• Class-switched (IgG, IgA, IgE)
• High-affinity BCR from somatic hypermutation
• Rapid recall response upon re-exposure
• Long-lived plasma cells in bone marrow maintain serum antibody (PMID: 21909089)

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Inflammatory Response

Pro-Inflammatory Cytokines

Tumor Necrosis Factor-α (TNF-α)

Source: Macrophages, monocytes, T cells, NK cells

Production Triggers:

• TLR signaling (especially TLR4/LPS)
• Other cytokines (IL-1, IFN-γ)
• Immune complexes

Biological Effects:

Role in Sepsis:

• Early mediator (peak 1-2 hours after LPS)
• Triggers cytokine cascade (IL-1, IL-6)
• Excessive TNF-α causes shock, DIC, multi-organ failure
• Anti-TNF therapy unsuccessful in sepsis trials (PMID: 9690424)

Interleukin-1 (IL-1)

Forms:

• IL-1α: Membrane-bound and nuclear signaling
• IL-1β: Secreted, requires caspase-1 processing (inflammasome)
• IL-1Ra: Natural receptor antagonist (anakinra = recombinant IL-1Ra)

Biological Effects:

• Fever (hypothalamic PGE2 synthesis)
• Neutrophil recruitment
• Endothelial activation
• Acute phase response
• Synergistic with TNF-α (PMID: 12415258)

Interleukin-6 (IL-6)

Source: Macrophages, T cells, endothelium, fibroblasts

Biological Effects:

• Major inducer of acute phase proteins (CRP, fibrinogen, ferritin)
• Fever
• B cell differentiation to plasma cells
• T cell proliferation and differentiation
• Both pro- and anti-inflammatory properties (PMID: 24907379)

Clinical Significance:

• Elevated IL-6 correlates with sepsis severity and mortality
• Tocilizumab (IL-6R inhibitor) effective in COVID-19 cytokine storm
• CRP is an indirect marker of IL-6 activity (PMID: 24907379)

Anti-Inflammatory Cytokines

Interleukin-10 (IL-10)

Source: Tregs, Th2 cells, macrophages, B cells, DCs

Biological Effects:

• Inhibits macrophage activation
• Suppresses Th1 cytokines (IFN-γ, IL-2, TNF-α)
• Reduces MHC class II expression
• Enhances B cell survival and antibody production
• Critical for preventing immunopathology (PMID: 11390440)

Clinical Significance:

• Elevated IL-10 in sepsis associated with immunoparalysis
• High IL-10/TNF-α ratio predicts poor outcome
• IL-10 deficiency causes inflammatory bowel disease

Transforming Growth Factor-β (TGF-β)

Forms: TGF-β1, TGF-β2, TGF-β3

Biological Effects:

• Suppresses T cell and macrophage activation
• Promotes Treg differentiation (iTregs)
• Wound healing and fibrosis
• Inhibits B cell proliferation
• Context-dependent pro-inflammatory effects (Th17 with IL-6) (PMID: 16551249)

Chemokines

Chemokines are small cytokines that direct leukocyte migration:

Chemokine Families:

(PMID: 17329232)

IL-8 (CXCL8):

• Primary neutrophil chemoattractant
• Produced by macrophages, endothelium, epithelium
• Elevated in ARDS, sepsis, wound fluid
• Potential biomarker for bacterial infection

Eicosanoids

Eicosanoids are lipid mediators derived from arachidonic acid:

Arachidonic Acid Release:

• Phospholipase A2 (PLA2) cleaves membrane phospholipids
• Cytosolic PLA2 (cPLA2) is calcium-activated
• Secretory PLA2 (sPLA2) elevated in sepsis, pancreatitis
• Glucocorticoids inhibit PLA2 (lipocortin/annexin induction) (PMID: 16960134)

Prostaglandins (COX Pathway)

Cyclooxygenase Enzymes:

• COX-1: Constitutive, homeostatic functions (gastric protection, platelet aggregation)
• COX-2: Inducible, inflammatory prostaglandin synthesis

Key Prostaglandins:

(PMID: 16960134)

NSAID Mechanisms:

• Non-selective: Inhibit both COX-1 and COX-2
• COX-2 selective (celecoxib): Spare gastric protection
• Aspirin: Irreversible acetylation of COX (platelet effect for 7-10 days)

Leukotrienes (LOX Pathway)

5-Lipoxygenase (5-LOX) Products:

Leukotriene Receptor Antagonists:

• Montelukast, zafirlukast: CysLT1 receptor antagonists
• Used in asthma maintenance therapy (PMID: 16960134)

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SIRS and Sepsis Pathophysiology

Definitions (Sepsis-3)

The Third International Consensus Definitions for Sepsis (Sepsis-3, 2016):

Sepsis: Life-threatening organ dysfunction caused by a dysregulated host response to infection

• Organ dysfunction: Increase in SOFA score ≥2 points
• qSOFA screening: ≥2 of: RR ≥22, altered mental status, SBP ≤100 mmHg

Septic Shock: Sepsis with circulatory, cellular, and metabolic abnormalities

• Vasopressor requirement to maintain MAP ≥65 mmHg
• Serum lactate >2 mmol/L despite adequate fluid resuscitation
• Hospital mortality >40% (PMID: 26903338)

PAMPs vs DAMPs

PAMPs (Pathogen-Associated Molecular Patterns):

• Conserved microbial structures
• Not present in host cells
• Recognized by PRRs

DAMPs (Damage-Associated Molecular Patterns):

• Endogenous molecules released during cell damage/stress
• Activate same PRRs as PAMPs (sterile inflammation)

(PMID: 23135902)

Cytokine Storm

The cytokine storm is a hyperinflammatory state with excessive cytokine release:

Pathophysiology:

1. Massive PRR activation by PAMPs/DAMPs
2. Overwhelming NF-κB and inflammasome activation
3. Excessive cytokine production (TNF-α, IL-1β, IL-6, IFN-γ)
4. Positive feedback loops amplify inflammation
5. Systemic effects: fever, vasodilation, capillary leak, DIC, organ dysfunction

Clinical Features:

• High-grade fever or hypothermia
• Hemodynamic instability
• ARDS
• Acute kidney injury
• Hepatic dysfunction
• Encephalopathy
• DIC and bleeding (PMID: 23135902)

Associated Conditions:

• Severe sepsis/septic shock
• COVID-19 (cytokine release syndrome)
• Macrophage activation syndrome (MAS)
• Hemophagocytic lymphohistiocytosis (HLH)
• CAR-T cell therapy complications
• Influenza (cytokine storm)

Management Principles:

• Source control (if infectious)
• Supportive care (fluids, vasopressors, organ support)
• Immunomodulation (corticosteroids, tocilizumab, ruxolitinib)
• Specific therapies (anakinra for MAS, etoposide for HLH) (PMID: 32376573)

Endothelial Dysfunction

The endothelium is a central target in sepsis pathophysiology:

Normal Endothelial Functions:

• Barrier function (permeability regulation)
• Anticoagulant surface (thrombomodulin, heparan sulfate, TFPI)
• Vasomotor regulation (NO, prostacyclin)
• Leukocyte trafficking regulation (adhesion molecules)
• Glycocalyx maintenance

Endothelial Activation in Sepsis:

1. Adhesion molecule expression: E-selectin, ICAM-1, VCAM-1 → leukocyte recruitment
2. Procoagulant shift: Tissue factor expression, thrombomodulin loss
3. Barrier dysfunction: Increased permeability → capillary leak
4. Glycocalyx degradation: Heparanase, hyaluronidase release
5. Vasomotor dysfunction: Impaired NO response, vasoplegic state

Biomarkers of Endothelial Injury:

• Soluble thrombomodulin
• Syndecan-1 (glycocalyx degradation)
• Angiopoietin-2 (vascular instability)
• VEGF (permeability) (PMID: 27655888)

Coagulopathy in Sepsis

Sepsis-induced coagulopathy (SIC) and DIC are common complications:

Mechanisms:

1. Tissue factor pathway activation: Monocytes, endothelium express TF → extrinsic pathway
2. Impaired anticoagulant pathways:
   • Antithrombin consumption
   • Protein C/S depletion
   • Thrombomodulin internalization
3. Fibrinolysis inhibition: PAI-1 overexpression, α2-antiplasmin consumption
4. NET formation: Histones, DNA activate coagulation

Clinical Manifestations:

• Microvascular thrombosis → organ dysfunction
• Consumption of factors → bleeding
• Platelet consumption → thrombocytopenia
• DIC: Bleeding + thrombosis simultaneously (PMID: 11897461)

Immunoparalysis

Immunoparalysis is a state of immune suppression following the initial hyperinflammatory phase:

Mechanisms:

1. Lymphocyte apoptosis: Up to 70% reduction in CD4+ T cells
2. Monocyte deactivation: Reduced HLA-DR expression (<30% of normal)
3. Cytokine shift: Decreased TNF-α, increased IL-10
4. Neutrophil dysfunction: Impaired chemotaxis, phagocytosis
5. T cell exhaustion: PD-1 upregulation, anergy
6. Treg expansion: Enhanced immunosuppression (PMID: 12015787)

Clinical Consequences:

• Secondary/nosocomial infections (Pseudomonas, Acinetobacter, Candida)
• Reactivation of latent infections (CMV, HSV, EBV)
• Prolonged ICU stay
• Increased mortality

Biomarkers:

• HLA-DR expression on monocytes (<30% indicates immunoparalysis)
• Ex vivo TNF-α production after LPS stimulation (<200 pg/mL)
• Lymphocyte count and subsets
• IL-10/TNF-α ratio (PMID: 12015787)

Potential Immunostimulatory Therapies:

• GM-CSF (granulocyte-macrophage colony-stimulating factor)
• IFN-γ
• IL-7
• PD-1/PD-L1 inhibitors
• Thymosin α1 (PMID: 23652563)

---

Immunodeficiency in the ICU

Critical Illness-Related Immunosuppression (CIRI)

ICU patients develop acquired immunodeficiency through multiple mechanisms:

Contributing Factors:

(PMID: 27655888)

Nosocomial Infections

ICU-acquired infections are a major cause of morbidity and mortality:

Common ICU-Acquired Infections:

Prevention Strategies:

• Hand hygiene
• Chlorhexidine bathing
• Ventilator care bundles (HOB elevation, oral care, sedation breaks)
• Catheter care bundles (prompt removal, aseptic insertion)
• Antibiotic stewardship (PMID: 30418199)

Reactivation of Latent Infections

Latent viruses reactivate in immunocompromised ICU patients:

Cytomegalovirus (CMV) Reactivation:

• Occurs in 20-40% of CMV-seropositive ICU patients
• Associated with increased mortality, prolonged ventilation, ICU stay
• Mechanism: T cell dysfunction, immunoparalysis
• Diagnosis: CMV PCR (quantitative)
• Treatment threshold: >1,000-10,000 copies/mL (institution-dependent)
• Treatment: Ganciclovir, valganciclovir (PMID: 18195111)

Herpes Simplex Virus (HSV) Reactivation:

• HSV-1 reactivation common in ICU (up to 50% seropositive patients)
• Manifests as oral/labial lesions, tracheobronchitis, pneumonitis
• Treatment: Aciclovir (PMID: 26269610)

Epstein-Barr Virus (EBV) Reactivation:

• Detectable EBV viremia in 40-50% of ICU patients
• Clinical significance uncertain in immunocompetent hosts
• Associated with prolonged ICU stay (PMID: 26269610)

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Immunomodulation in Critical Care

Corticosteroids

Corticosteroids are the most commonly used immunomodulatory agents in ICU:

Mechanisms of Immunosuppression:

1. Transcriptional effects:

   • Inhibit NF-κB → reduced pro-inflammatory cytokines (TNF-α, IL-1, IL-6)
   • Inhibit AP-1 → reduced cytokine/chemokine transcription
   • Induce lipocortin-1 → inhibit PLA2 → reduced eicosanoid synthesis

2. Cellular effects:

   • Neutrophilia (demargination, bone marrow release, delayed apoptosis)
   • Lymphopenia (redistribution, apoptosis)
   • Monocyte deactivation
   • Eosinopenia

3. Vascular effects:

   • Reduced capillary permeability
   • Catecholamine potentiation (PMID: 15735000)

Evidence in Septic Shock:

Current Practice (Surviving Sepsis Campaign 2021):

• Suggest IV corticosteroids (hydrocortisone 200 mg/day) for septic shock requiring escalating vasopressor doses
• Weak recommendation, low quality evidence
• May reduce time to shock resolution (PMID: 34599691)

Corticosteroids in COVID-19 ARDS:

RECOVERY trial (PMID: 32678530) demonstrated:

• Dexamethasone 6mg daily for up to 10 days
• Reduced 28-day mortality in patients requiring oxygen (23.3% vs 26.2%)
• Greatest benefit in mechanically ventilated patients (29.3% vs 41.4%)
• No benefit (possible harm) if not requiring respiratory support

Intravenous Immunoglobulin (IVIG)

IVIG contains pooled IgG from thousands of donors:

Mechanisms of Action:

1. Fc receptor blockade (competitive inhibition)
2. Anti-idiotype antibodies (neutralize autoantibodies)
3. Complement modulation
4. Cytokine and chemokine neutralization
5. Modulation of dendritic cell and T cell function
6. Pathogen-specific antibodies (PMID: 22293433)

Indications in ICU:

(PMID: 22293433)

IVIG in Sepsis:

• Multiple RCTs and meta-analyses show no mortality benefit
• Polyclonal IgG-enriched IVIG (IgGAM) may have benefit (subgroup analyses)
• Not routinely recommended by Surviving Sepsis Campaign

Other Immunomodulatory Agents

GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor):

• Stimulates myeloid cell production and function
• Increases monocyte HLA-DR expression
• May reverse immunoparalysis
• RCTs show improved monocyte function but no mortality benefit (PMID: 24992878)

IFN-γ (Interferon-gamma):

• Activates macrophages
• Enhances antigen presentation
• Used in refractory fungal infections
• May reverse immunoparalysis (PMID: 23652563)

IL-7 (Interleukin-7):

• T cell survival and proliferation factor
• Reverses lymphopenia in sepsis
• Phase 2 trials show safety and T cell recovery (PMID: 29655837)

Anti-PD-1/PD-L1:

• Checkpoint inhibitors reverse T cell exhaustion
• Theoretical benefit in immunoparalysis
• Clinical trials ongoing (PMID: 29655837)

---

Australian/NZ Context

Indigenous Health Considerations

Aboriginal and Torres Strait Islander Populations:

Indigenous Australians experience higher rates of serious infections:

• Invasive pneumococcal disease: 6-10× higher incidence
• Meningococcal disease: 2-3× higher incidence
• Rheumatic heart disease: 8× higher prevalence
• Acute post-streptococcal glomerulonephritis: Endemic in remote communities
• Skin infections: Scabies, impetigo leading to invasive disease
• Tuberculosis: Higher rates in some communities (PMID: 28406084)

Contributing Factors:

• Overcrowded housing (average 4+ people per bedroom in remote communities)
• Poor sanitation and water supply
• Delayed healthcare access (remote/rural locations)
• Comorbidities (diabetes, CKD, rheumatic heart disease)
• Socioeconomic disadvantage

Cultural Considerations in ICU:

• Family and community involvement in decision-making
• Sorry Business (bereavement) may affect treatment decisions
• Traditional healers may be incorporated into care
• Passing on Country preferences (dying at home/community)
• Use of Aboriginal Health Workers and Liaison Officers
• Avoid assumptions; ask about individual cultural needs (PMID: 28406084)

Māori Health (New Zealand):

• Higher rates of rheumatic fever and rheumatic heart disease
• Higher incidence of meningococcal disease
• Increased sepsis-related mortality
• Whānau (extended family) involvement in care
• Tikanga Māori (cultural practices) respected
• Kaumātua (elders) may be consulted for decisions
• Te Tiriti o Waitangi obligations for culturally safe care

Sepsis Epidemiology in Australia/NZ

ANZICS CORE Data:

Australian and New Zealand ICU sepsis data from the ANZICS CORE database:

• Sepsis accounts for approximately 15-20% of ICU admissions
• Hospital mortality: 20-25% (improving over time)
• Average ICU LOS: 5-7 days
• Indigenous patients have higher sepsis incidence and mortality

Australian Sepsis Network:

• National surveillance and quality improvement
• Sepsis pathways and bundles
• Time to antibiotics targets (<1 hour for septic shock)
• Sepsis awareness campaigns (PMID: 30418199)

---

Clinical Applications

Sepsis Management

Surviving Sepsis Campaign 2021 Key Recommendations:

Hour-1 Bundle:

1. Measure lactate (remeasure if >2 mmol/L)
2. Obtain blood cultures before antibiotics
3. Administer broad-spectrum antibiotics
4. Begin fluid resuscitation (30 mL/kg crystalloid for hypotension or lactate ≥4 mmol/L)
5. Start vasopressors if hypotensive during/after fluid resuscitation (target MAP ≥65 mmHg)

Source Control:

• Identify and control source within 6-12 hours
• Drainage, debridement, device removal as appropriate

Immunomodulation:

• Corticosteroids for refractory septic shock
• No IVIG routinely
• Vitamin C, thiamine: Insufficient evidence (PMID: 34599691)

Transplant Immunology

Organ Transplantation Immunology:

Rejection Types:

Immunosuppressive Regimens:

ICU Considerations for Transplant Patients:

• High infection risk (opportunistic pathogens)
• Drug interactions (CYP3A4 inhibitors/inducers)
• Target drug levels for calcineurin inhibitors
• Rejection vs infection differentiation
• Graft-versus-host disease in bone marrow transplant (PMID: 17381597)

Autoimmune Diseases in ICU

Common Autoimmune Emergencies:

(PMID: 22293433)

---

SAQ Practice

SAQ 1: Complement System

Question:

A 22-year-old man presents to the Emergency Department with severe headache, fever, neck stiffness, and petechial rash. Lumbar puncture reveals Gram-negative diplococci. He is the second case of meningococcal disease in his family, with his brother having had meningococcal septicemia at age 18.

(a) Describe the three activation pathways of the complement system and their key initiating factors. (6 marks)

(b) Explain the formation and function of the membrane attack complex (MAC). (4 marks)

(c) What complement deficiency should be suspected in this patient, and why does it predispose to Neisseria infections? (3 marks)

(d) Outline the management approach for this patient, including specific considerations related to complement deficiency. (2 marks)

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Model Answer:

(a) Complement Activation Pathways (6 marks)

Classical Pathway (2 marks):

• Initiated by C1q binding to antigen-antibody complexes (IgM most efficient, IgG1/2/3)
• C1q activation → C1r → C1s → cleaves C4 and C2
• Forms C4b2a (classical C3 convertase)
• Links adaptive immunity to complement system

Alternative Pathway (2 marks):

• Antibody-independent; constitutive low-level activation ("tick-over")
• Spontaneous C3 hydrolysis → C3(H2O) binds Factor B
• Factor D cleaves Factor B → Bb
• C3bBb is the alternative C3 convertase (amplification loop)
• Properdin (Factor P) stabilizes convertase

Lectin Pathway (2 marks):

• Initiated by mannose-binding lectin (MBL) or ficolins binding microbial carbohydrates
• MASP-1 and MASP-2 activated
• MASP-2 cleaves C4 and C2
• Forms C4b2a (same as classical pathway)
• MBL is an acute phase protein

(b) Membrane Attack Complex (4 marks)

Formation (2 marks):

• C5 convertase (C4b2a3b or C3bBb3b) cleaves C5 → C5a + C5b
• C5b binds C6 and C7 sequentially → C5b-7 complex
• C5b-7 inserts into target membrane
• C8 binds and partially penetrates membrane
• Multiple C9 molecules (10-16) polymerize forming a pore

Function (2 marks):

• Transmembrane pore approximately 10 nm diameter
• Allows ion and water influx/efflux
• Osmotic lysis of target cell (especially Gram-negative bacteria)
• Gram-positive bacteria relatively resistant (thick peptidoglycan)
• Sublytic MAC on nucleated cells can cause cellular activation

(c) Complement Deficiency and Neisseria Infections (3 marks)

Suspected Deficiency (1 mark):

• Terminal complement component deficiency (C5, C6, C7, C8, or C9)
• Family history suggests hereditary complement deficiency

Mechanism of Susceptibility (2 marks):

• Neisseria meningitidis has thin lipooligosaccharide outer membrane
• MAC-mediated lysis is the primary complement killing mechanism for Neisseria
• Without functional MAC, bactericidal activity is lost
• Patients have 1,000-10,000× increased risk of meningococcal disease
• May have recurrent infections despite antibody and opsonophagocytic mechanisms

(d) Management (2 marks)

Acute Management (1 mark):

• Immediate IV antibiotics (ceftriaxone 2g)
• Supportive care (fluids, vasopressors if shocked)
• Dexamethasone (before or with first antibiotic dose)

Complement-Specific Considerations (1 mark):

• Complement testing (CH50, AP50, individual components) after recovery
• Meningococcal vaccination (Men ACWY + Men B) if deficiency confirmed
• Vaccination of close contacts
• Consider prophylactic antibiotics for future exposures
• Family screening for complement deficiency

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SAQ 2: Cytokine Storm and Immunoparalysis

Question:

A 68-year-old man with diabetes mellitus is admitted to ICU with septic shock from a perforated sigmoid diverticulum. He is mechanically ventilated, on noradrenaline infusion, and has been in ICU for 14 days following emergency laparotomy. He now develops a new fever and worsening inflammatory markers with Candida albicans isolated from blood cultures.

(a) Describe the key cytokines involved in the initial pro-inflammatory response in sepsis and their biological effects. (4 marks)

(b) Explain the concept of "immunoparalysis" and describe the mechanisms by which it develops following the initial hyperinflammatory phase. (5 marks)

(c) What biomarkers can be used to identify immunoparalysis in this patient? (3 marks)

(d) Discuss potential immunostimulatory therapies that may be considered for critically ill patients with immunoparalysis. (3 marks)

---

Model Answer:

(a) Pro-Inflammatory Cytokines in Sepsis (4 marks)

TNF-α (1 mark):

• Released by macrophages within 1-2 hours of pathogen recognition
• Effects: Endothelial activation (adhesion molecules), fever, neutrophil priming, capillary leak
• Triggers downstream cytokine cascade

IL-1β (1 mark):

• Requires inflammasome activation (NLRP3) and caspase-1 cleavage
• Effects: Fever, neutrophil recruitment, synergistic with TNF-α
• IL-1Ra is natural antagonist

IL-6 (1 mark):

• Major inducer of acute phase response (CRP, fibrinogen, ferritin)
• Produced by macrophages, T cells, endothelium
• Both pro- and anti-inflammatory properties
• Correlates with sepsis severity and mortality

Other Cytokines (1 mark):

• IFN-γ: Macrophage activation, Th1 responses
• IL-8: Neutrophil chemotaxis
• IL-12: NK cell and Th1 activation
• IL-18: Synergizes with IL-12 for IFN-γ production

(b) Immunoparalysis Mechanisms (5 marks)

Definition (1 mark):

• State of immune suppression following initial hyperinflammatory phase
• Characterized by reduced capacity to mount appropriate immune responses
• Typically develops after 3-7 days of critical illness

Mechanisms (4 marks):

1. Lymphocyte Apoptosis (1 mark):

   • Massive apoptosis of CD4+ T cells, B cells, and dendritic cells
   • Up to 70% reduction in circulating lymphocytes
   • Driven by FasL, TNF-α, glucocorticoids

2. Monocyte Deactivation (1 mark):

   • Reduced HLA-DR expression (<30% of normal)
   • Impaired ex vivo TNF-α production to LPS stimulation
   • Shift from M1 (inflammatory) to M2 (anti-inflammatory) phenotype

3. Cytokine Shift (1 mark):

   • Decreased pro-inflammatory cytokines (TNF-α, IL-12)
   • Increased anti-inflammatory cytokines (IL-10, TGF-β)
   • High IL-10/TNF-α ratio

4. T Cell Exhaustion (1 mark):

   • PD-1 (programmed death-1) upregulation on T cells
   • PD-L1 expression on antigen-presenting cells
   • Results in T cell anergy and reduced effector function
   • Increased Treg numbers and suppressive function

(c) Biomarkers of Immunoparalysis (3 marks)

Monocyte HLA-DR (1 mark):

• Flow cytometry measurement
• <8,000 molecules/cell or <30% expression indicates immunoparalysis
• Validated biomarker with prognostic significance

Ex Vivo TNF-α Production (1 mark):

• Whole blood stimulated with LPS
• TNF-α production measured after 4-24 hours
• <200 pg/mL indicates monocyte deactivation

Other Markers (1 mark):

• Absolute lymphocyte count (<500/µL concerning)
• CD4+ T cell count
• IL-10/TNF-α ratio (elevated in immunoparalysis)
• mHLA-DR/CD14 ratio
• PD-1 expression on T cells

(d) Immunostimulatory Therapies (3 marks)

GM-CSF (1 mark):

• Stimulates myeloid cell production and function
• Increases monocyte HLA-DR expression
• RCTs show improved immune markers but unclear mortality benefit
• Dose: 3-8 µg/kg/day subcutaneously

IFN-γ (1 mark):

• Activates macrophages, enhances antigen presentation
• Restores monocyte HLA-DR expression
• Used in refractory fungal infections
• Limited RCT data in sepsis

Emerging Therapies (1 mark):

• IL-7: T cell survival factor, reverses lymphopenia (Phase 2 trials)
• Anti-PD-1/PD-L1: Checkpoint inhibitors to reverse T cell exhaustion
• Thymosin α1: Immune modulator, used in some countries
• Currently no therapy has proven mortality benefit in large RCTs

---

Viva Scenarios

Viva Scenario 1: Sepsis Pathophysiology

Examiner: A 45-year-old woman is admitted to ICU with septic shock secondary to pyelonephritis. She is hypotensive despite fluid resuscitation and has been started on noradrenaline. I'd like to explore the immunological basis of sepsis with you.

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Examiner: Can you explain what happens at the molecular level when bacteria enter the bloodstream?

Candidate: When Gram-negative bacteria enter the bloodstream, the lipopolysaccharide (LPS) component of their outer membrane is recognized by the innate immune system. LPS is transferred from bacteria to the LPS-binding protein (LBP), then to CD14, and finally to the MD-2/TLR4 complex on macrophages and other immune cells.

TLR4 activation triggers two downstream signaling pathways:

1. The MyD88-dependent pathway leads to NF-κB activation and production of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6
2. The TRIF-dependent pathway leads to type I interferon production

This recognition system evolved to rapidly detect and respond to pathogens, but in sepsis, the response becomes dysregulated and harmful.

---

Examiner: What is the difference between PAMPs and DAMPs?

Candidate: PAMPs (Pathogen-Associated Molecular Patterns) are conserved molecular structures found on pathogens but not host cells. Examples include LPS from Gram-negative bacteria, lipoteichoic acid from Gram-positive bacteria, peptidoglycan, flagellin, and viral nucleic acids.

DAMPs (Damage-Associated Molecular Patterns) are endogenous molecules released during cell damage or stress. Examples include HMGB1 (from the nucleus), ATP (from cytoplasm), mitochondrial DNA, uric acid, and histones.

Both PAMPs and DAMPs are recognized by the same pattern recognition receptors and can activate similar inflammatory pathways. This is clinically relevant because sterile injuries like trauma, burns, or ischemia-reperfusion can cause SIRS through DAMP release even without infection.

---

Examiner: Describe the cytokine storm in sepsis.

Candidate: The cytokine storm represents a state of hyperinflammation where there is overwhelming and dysregulated release of pro-inflammatory mediators.

In early sepsis, pathogen recognition triggers massive release of:

• TNF-α (peaks at 1-2 hours): causes endothelial activation, fever, and initiates the cytokine cascade
• IL-1β: requires inflammasome activation, synergizes with TNF-α
• IL-6: induces acute phase response and correlates with severity

This leads to:

• Systemic vasodilation and hypotension
• Increased capillary permeability and edema
• Activation of coagulation cascade (DIC)
• Neutrophil activation and organ infiltration
• Metabolic derangements

The severity depends on pathogen load, host genetics (TLR polymorphisms), and comorbidities. Positive feedback loops amplify inflammation, and the response becomes self-perpetuating.

---

Examiner: The patient has been in ICU for 2 weeks and now develops Candida bloodstream infection. What immunological changes predispose to this?

Candidate: After the initial hyperinflammatory phase, patients develop immunoparalysis, which is a state of acquired immunosuppression.

The key changes include:

1. Lymphocyte depletion: Massive apoptosis of CD4+ T cells, B cells, and dendritic cells—up to 70% reduction

2. Monocyte deactivation: Reduced HLA-DR expression (often <30% of normal), impaired antigen presentation, and reduced TNF-α production in response to LPS

3. Cytokine shift: From pro-inflammatory (TNF-α, IFN-γ) to anti-inflammatory (IL-10, TGF-β)

4. T cell exhaustion: Upregulation of PD-1, causing anergy

5. Neutrophil dysfunction: Impaired chemotaxis and phagocytosis despite elevated numbers

These changes make patients susceptible to:

• Opportunistic infections like Candida
• Reactivation of latent viruses (CMV, HSV, EBV)
• Nosocomial bacterial infections with resistant organisms

---

Examiner: How would you assess for immunoparalysis in this patient?

Candidate: The most validated biomarker is monocyte HLA-DR expression measured by flow cytometry. Values below 8,000 molecules per cell or less than 30% of normal indicate immunoparalysis.

Other assessments include:

• Ex vivo TNF-α production: Whole blood stimulated with LPS; <200 pg/mL indicates monocyte deactivation
• Absolute lymphocyte count: Persistent lymphopenia (<500/µL) is concerning
• CD4+ T cell count: Low counts associated with poor outcomes
• IL-10/TNF-α ratio: Elevated ratio suggests immunosuppressive state

In clinical practice, these tests are not routinely available, so we rely on clinical indicators: prolonged ICU stay, secondary infections, and failure to improve despite source control.

---

Examiner: What are the treatment options for immunoparalysis?

Candidate: Treatment for immunoparalysis remains experimental without proven mortality benefit, but several agents have been studied:

GM-CSF (granulocyte-macrophage colony-stimulating factor):

• Stimulates myeloid cell production and function
• Increases monocyte HLA-DR expression
• RCTs show improved immune markers but unclear clinical benefit
• Dose typically 3-8 µg/kg/day

IFN-γ (interferon-gamma):

• Activates macrophages
• Restores antigen presentation capacity
• Used in refractory fungal infections
• Case reports/series rather than RCT evidence

Emerging therapies:

• IL-7: T cell survival factor, Phase 2 trials show safety and T cell recovery
• Anti-PD-1/PD-L1: May reverse T cell exhaustion
• Thymosin α1: Used in some Asian countries

Currently, supportive care with source control, appropriate antimicrobials, and avoiding additional immunosuppression (steroids if not indicated) remain the mainstays.

---

Viva Scenario 2: Complement System

Examiner: A 19-year-old university student presents with meningococcal septicemia. He requires ICU admission for vasopressor support. His mother mentions his uncle died of meningococcal disease at age 25. I'd like to discuss the complement system with you.

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Examiner: Describe the complement system and its functions.

Candidate: The complement system is a cascade of over 30 plasma proteins that enhance pathogen clearance. It has three main functions:

1. Opsonization: C3b and iC3b coat pathogens and are recognized by complement receptors (CR1, CR3) on phagocytes, enhancing phagocytosis up to 4,000-fold

2. Inflammation: The anaphylatoxins C3a and C5a promote inflammation—C5a is particularly potent as a neutrophil chemoattractant and activator, and increases vascular permeability

3. Direct lysis: The membrane attack complex (C5b-9) forms pores in target cell membranes, causing osmotic lysis—especially effective against Gram-negative bacteria

Additional functions include immune complex clearance, B cell activation, and bridging innate and adaptive immunity.

---

Examiner: Explain the three activation pathways.

Candidate: The three pathways converge at C3 convertase formation:

Classical Pathway:

• Initiated by C1q binding to antibody-antigen complexes
• IgM is most efficient (single molecule sufficient)
• C1 activation leads to cleavage of C4 and C2
• Forms C4b2a (classical C3 convertase)
• Links adaptive immunity to complement

Alternative Pathway:

• Antibody-independent, constitutively active
• "Tick-over" mechanism: spontaneous C3 hydrolysis
• C3(H2O) binds Factor B, cleaved by Factor D
• Forms C3bBb (alternative C3 convertase)
• Provides amplification loop for all pathways
• Regulated by Factor H and Factor I

Lectin Pathway:

• Initiated by MBL or ficolins binding microbial carbohydrates
• MASP-1 and MASP-2 cleave C4 and C2
• Forms C4b2a (same as classical pathway)
• MBL is an acute phase protein

---

Examiner: What is the membrane attack complex and how is it formed?

Candidate: The MAC is a pore-forming complex assembled from terminal complement components.

Formation sequence:

1. C5 convertase cleaves C5 into C5a and C5b
2. C5b sequentially binds C6, then C7
3. C5b-7 complex inserts into the target membrane
4. C8 binds and partially penetrates the membrane
5. Multiple C9 molecules (typically 10-16) polymerize around C8
6. This forms a transmembrane pore approximately 10 nm in diameter

The pore allows uncontrolled ion and water flux, leading to osmotic lysis. This is particularly effective against Gram-negative bacteria with thin outer membranes. Gram-positive bacteria are relatively resistant due to their thick peptidoglycan layer.

On nucleated host cells, sublytic MAC can cause cellular activation rather than lysis.

---

Examiner: Given the family history, what do you suspect?

Candidate: I suspect a terminal complement component deficiency affecting C5, C6, C7, C8, or C9.

The family history with two affected relatives suggests an inherited deficiency. Patients with these deficiencies have 1,000 to 10,000-fold increased risk of Neisseria infections because:

• Neisseria meningitidis has a thin lipooligosaccharide outer membrane
• MAC-mediated lysis is the primary bactericidal mechanism
• Without MAC, opsonization and phagocytosis alone are insufficient

These deficiencies are inherited in autosomal recessive pattern (except properdin deficiency, which is X-linked). Patients may have first infection in adolescence or early adulthood, and recurrent meningococcal disease is common.

---

Examiner: How would you investigate and manage the complement deficiency?

Candidate: Investigation:

1. CH50 (total hemolytic complement): Screens classical pathway; undetectable in terminal component deficiency
2. AH50 (alternative pathway hemolytic activity): Also reduced in terminal deficiencies
3. Individual component assays: C5, C6, C7, C8, C9 levels identify the specific deficiency
4. AP50: Alternative pathway screen

Testing should be performed after recovery from acute infection (acute phase changes affect results).

Management:

Acute:

• Standard sepsis management with appropriate antibiotics
• Ceftriaxone 2g IV is first-line for meningococcal disease

Long-term:

• Vaccination: Meningococcal ACWY and Men B vaccines (may need booster every 3-5 years)
• Education: Recognize early symptoms, seek immediate care
• Prophylactic antibiotics: Some advocate for penicillin prophylaxis
• Medical alert: Bracelet/documentation
• Family screening: Test first-degree relatives

The same approach applies to patients on eculizumab (C5 inhibitor for PNH or aHUS), who are at similar risk for meningococcal disease.

---

Examiner: What other complement deficiencies exist and what are their clinical manifestations?

Candidate: Complement deficiencies have characteristic clinical patterns:

C1q, C2, C4 (early classical pathway):

• SLE-like autoimmune disease
• Impaired clearance of immune complexes and apoptotic cells
• C2 deficiency is most common hereditary complement deficiency

C3 deficiency:

• Severe recurrent bacterial infections (encapsulated organisms)
• Loss of central opsonization
• Very rare, usually fatal in childhood without treatment

Factor H, Factor I:

• Atypical hemolytic uremic syndrome (aHUS)
• C3 glomerulopathy
• Uncontrolled alternative pathway activation

C1-INH (C1 esterase inhibitor):

• Hereditary angioedema
• Uncontrolled contact and complement activation
• Bradykinin-mediated swelling

MBL deficiency:

• Increased childhood infections
• Less significant in adults with developed adaptive immunity
• Common polymorphisms (5-10% population)

Properdin deficiency:

• X-linked
• Meningococcal susceptibility

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References

Landmark Studies and Reviews

1. Akira S, Takeda K, Kaisho T. Toll-like receptors: critical proteins linking innate and acquired immunity. Nat Immunol. 2001;2(8):675-680. PMID: 11477402

2. Beutler B, Poltorak A. The sole gateway to endotoxin response: how LPS was identified as Tlr4, and its role in innate immunity. Drug Metab Dispos. 2001;29(4 Pt 2):474-478. PMID: 11259335

3. Hoffmann JA, Kafatos FC, Janeway CA, Ezekowitz RA. Phylogenetic perspectives in innate immunity. Science. 1999;284(5418):1313-1318. PMID: 10334979

4. Janeway CA Jr, Medzhitov R. Innate immune recognition. Annu Rev Immunol. 2002;20:197-216. PMID: 11861602

5. Takeda K, Kaisho T, Akira S. Toll-like receptors. Annu Rev Immunol. 2003;21:335-376. PMID: 12524386

6. Medzhitov R, Preston-Hurlburt P, Janeway CA Jr. A human homologue of the Drosophila Toll protein signals activation of adaptive immunity. Nature. 1997;388(6640):394-397. PMID: 9237759

7. Poltorak A, He X, Smirnova I, et al. Defective LPS signaling in C3H/HeJ and C57BL/10ScCr mice: mutations in Tlr4 gene. Science. 1998;282(5396):2085-2088. PMID: 9851930

Pattern Recognition Receptors

8. Kawai T, Akira S. The role of pattern-recognition receptors in innate immunity: update on Toll-like receptors. Nat Immunol. 2010;11(5):373-384. PMID: 20404851

9. Martinon F, Mayor A, Tschopp J. The inflammasomes: guardians of the body. Annu Rev Immunol. 2009;27:229-265. PMID: 19302040

10. Lamkanfi M, Dixit VM. Mechanisms and functions of inflammasomes. Cell. 2014;157(5):1013-1022. PMID: 24855941

11. Yoneyama M, Kikuchi M, Natsukawa T, et al. The RNA helicase RIG-I has an essential function in double-stranded RNA-induced innate antiviral responses. Nat Immunol. 2004;5(7):730-737. PMID: 15208624

Complement System

12. Walport MJ. Complement. First of two parts. N Engl J Med. 2001;344(14):1058-1066. PMID: 11287977

13. Walport MJ. Complement. Second of two parts. N Engl J Med. 2001;344(15):1140-1144. PMID: 11297706

14. Ricklin D, Hajishengallis G, Yang K, Lambris JD. Complement: a key system for immune surveillance and homeostasis. Nat Immunol. 2010;11(9):785-797. PMID: 20720586

15. Figueroa JE, Densen P. Infectious diseases associated with complement deficiencies. Clin Microbiol Rev. 1991;4(3):359-395. PMID: 1889047

16. Degn SE, Thiel S. Humoral pattern recognition and the complement system. Scand J Immunol. 2013;78(2):181-193. PMID: 23672610

T Cell Biology

17. Zhu J, Yamane H, Paul WE. Differentiation of effector CD4 T cell populations. Annu Rev Immunol. 2010;28:445-489. PMID: 20192806

18. Mosmann TR, Sad S. The expanding universe of T-cell subsets: Th1, Th2 and more. Immunol Today. 1996;17(3):138-146. PMID: 8820272

19. Sakaguchi S, Sakaguchi N, Asano M, Itoh M, Toda M. Immunologic self-tolerance maintained by activated T cells expressing IL-2 receptor alpha-chains (CD25). J Immunol. 1995;155(3):1151-1164. PMID: 7636184

20. Fontenot JD, Gavin MA, Rudensky AY. Foxp3 programs the development and function of CD4+CD25+ regulatory T cells. Nat Immunol. 2003;4(4):330-336. PMID: 12612578

Cytokines and Inflammation

21. Dinarello CA. Proinflammatory cytokines. Chest. 2000;118(2):503-508. PMID: 10936147

22. Tracey KJ, Cerami A. Tumor necrosis factor: a pleiotropic cytokine and therapeutic target. Annu Rev Med. 1994;45:491-503. PMID: 8198398

23. Heinrich PC, Castell JV, Andus T. Interleukin-6 and the acute phase response. Biochem J. 1990;265(3):621-636. PMID: 1689567

24. Moore KW, de Waal Malefyt R, Coffman RL, O'Garra A. Interleukin-10 and the interleukin-10 receptor. Annu Rev Immunol. 2001;19:683-765. PMID: 11244051

25. Zlotnik A, Yoshie O. Chemokines: a new classification system and their role in immunity. Immunity. 2000;12(2):121-127. PMID: 10714678

Sepsis Pathophysiology

26. Singer M, Deutschman CS, Seymour CW, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA. 2016;315(8):801-810. PMID: 26903338

27. Hotchkiss RS, Monneret G, Payen D. Sepsis-induced immunosuppression: from cellular dysfunctions to immunotherapy. Nat Rev Immunol. 2013;13(12):862-874. PMID: 24232462

28. Angus DC, van der Poll T. Severe sepsis and septic shock. N Engl J Med. 2013;369(9):840-851. PMID: 23984731

29. van der Poll T, van de Veerdonk FL, Scicluna BP, Netea MG. The immunopathology of sepsis and potential therapeutic targets. Nat Rev Immunol. 2017;17(7):407-420. PMID: 28436424

30. Delano MJ, Ward PA. Sepsis-induced immune dysfunction: can immune therapies reduce mortality? J Clin Invest. 2016;126(1):23-31. PMID: 26727230

Immunoparalysis

31. Monneret G, Lepape A, Voirin N, et al. Persisting low monocyte human leukocyte antigen-DR expression predicts mortality in septic shock. Intensive Care Med. 2006;32(8):1175-1183. PMID: 16741700

32. Docke WD, Randow F, Syrbe U, et al. Monocyte deactivation in septic patients: restoration by IFN-gamma treatment. Nat Med. 1997;3(6):678-681. PMID: 9176497

33. Hotchkiss RS, Monneret G, Payen D. Immunosuppression in sepsis: a novel understanding of the disorder and a new therapeutic approach. Lancet Infect Dis. 2013;13(3):260-268. PMID: 23427891

Clinical Trials

34. Sprung CL, Annane D, Keh D, et al. Hydrocortisone therapy for patients with septic shock. N Engl J Med. 2008;358(2):111-124. PMID: 18184957

35. Venkatesh B, Finfer S, Cohen J, et al. Adjunctive glucocorticoid therapy in patients with septic shock. N Engl J Med. 2018;378(9):797-808. PMID: 29347874

36. Annane D, Renault A, Brun-Buisson C, et al. Hydrocortisone plus fludrocortisone for adults with septic shock. N Engl J Med. 2018;378(9):809-818. PMID: 29490185

37. RECOVERY Collaborative Group. Dexamethasone in hospitalized patients with Covid-19. N Engl J Med. 2021;384(8):693-704. PMID: 32678530

38. Evans L, Rhodes A, Alhazzani W, et al. Surviving sepsis campaign: international guidelines for management of sepsis and septic shock 2021. Intensive Care Med. 2021;47(11):1181-1247. PMID: 34599691

Immunomodulation

39. Nimmerjahn F, Ravetch JV. Anti-inflammatory actions of intravenous immunoglobulin. Annu Rev Immunol. 2008;26:513-533. PMID: 18370923

40. Meisel C, Schefold JC, Pschowski R, et al. Granulocyte-macrophage colony-stimulating factor to reverse sepsis-associated immunosuppression: a double-blind, randomized, placebo-controlled multicenter trial. Am J Respir Crit Care Med. 2009;180(7):640-648. PMID: 19590022

41. Francois B, Jeannet R, Daix T, et al. Interleukin-7 restores lymphocytes in septic shock: the IRIS-7 randomized clinical trial. JCI Insight. 2018;3(5):e98960. PMID: 29515037

Australian/NZ Context

42. Carapetis JR, Steer AC, Mulholland EK, Weber M. The global burden of group A streptococcal diseases. Lancet Infect Dis. 2005;5(11):685-694. PMID: 16253886

43. Parnaby MG, Carapetis JR. Rheumatic fever in indigenous Australian children. J Paediatr Child Health. 2010;46(9):527-533. PMID: 20854324

44. Finfer S, Bellomo R, Lipman J, et al. Adult-population incidence of severe sepsis in Australian and New Zealand intensive care units. Intensive Care Med. 2004;30(4):589-596. PMID: 14991093

45. ANZICS. Centre for Outcome and Resource Evaluation 2020 Report. Melbourne: ANZICS; 2021.

Additional Key References

46. Brinkmann V, Reichard U, Goosmann C, et al. Neutrophil extracellular traps kill bacteria. Science. 2004;303(5663):1532-1535. PMID: 15001782

47. Vivier E, Tomasello E, Baratin M, Walzer T, Ugolini S. Functions of natural killer cells. Nat Immunol. 2008;9(5):503-510. PMID: 18425107

48. Steinman RM. Decisions about dendritic cells: past, present, and future. Annu Rev Immunol. 2012;30:1-22. PMID: 22136168

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Related Topics

• Sepsis and Septic Shock
• Acute Respiratory Distress Syndrome
• Coagulation Cascade
• Corticosteroid Pharmacology
• Antimicrobial Pharmacology
• Transplant Immunology
• Nosocomial Infections

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Key Equations and Formulas

Complement Pathway Summary

Classical: Ag-Ab → C1q → C1r/s → C4 + C2 → C4b2a (C3 convertase)
Lectin:   MBL → MASP → C4 + C2 → C4b2a (C3 convertase)
Alternative: C3 tick-over → Factor B + D → C3bBb (C3 convertase)

All pathways → C3 → C3a + C3b → C5 → C5a + C5b → MAC (C5b-9)

Cytokine Timeline in Sepsis

Hours 0-2:   TNF-α peak
Hours 2-4:   IL-1β peak
Hours 4-8:   IL-6 peak
Hours 8-24:  IL-10 peak (anti-inflammatory)
Days 3-7:    Immunoparalysis onset

Immunoparalysis Diagnostic Criteria

Monocyte HLA-DR < 8,000 molecules/cell (or < 30% of normal)
Ex vivo TNF-α production < 200 pg/mL after LPS stimulation
Absolute lymphocyte count < 500/µL
IL-10/TNF-α ratio elevated