2.4 Host Immune Response to Infection and Immunopathology

Host Immune Response to Infection and Immunopathology

The human immune response to microbial invasion is an integrated system comprising innate physical barriers, cellular and humoral effectors, and antigen-specific adaptive immunity. Disruption of these defensive components creates predictable susceptibilities to specific opportunistic pathogens. Mastering immunopathology allows the specialist to correlate host immune deficits with microbiological risks, direct appropriate prophylactic regimens, and manage dysregulated immune responses.

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Innate Immunity and Pattern Recognition Receptors

Innate immunity provides non-specific host protection within minutes of microbial contact. Recognition depends upon germline-encoded Pattern Recognition Receptors (PRRs) that detect invariant molecular motifs unique to microbes, termed Pathogen-Associated Molecular Patterns (PAMPs), or host cellular stress molecules, termed Damage-Associated Molecular Patterns (DAMPs).

Toll-Like Receptors (TLRs)

TLRs are transmembrane glycoproteins situated on cell surfaces or endosomal compartments of dendritic cells, macrophages, and neutrophils:

• TLR4 (with MD-2 and CD14): Recognizes Lipid A of Gram-negative lipopolysaccharide (LPS).
• TLR2 (heterodimerized with TLR1 or TLR6): Recognizes Gram-positive peptidoglycan, lipoteichoic acid, and mycobacterial lipoarabinomannan.
• TLR5: Recognizes bacterial flagellin.
• Endosomal TLRs (TLR3, TLR7, TLR8, TLR9): Sense microbial nucleic acids; TLR3 recognizes viral double-stranded RNA (dsRNA), TLR7/8 recognizes single-stranded RNA (ssRNA), and TLR9 detects unmethylated cytidine-phosphate-guanosine (CpG) DNA motifs characteristic of bacteria and DNA viruses.

Binding of PAMPs to TLRs recruits adaptor proteins (MyD88 or TRIF), activating downstream kinases that phosphorylate I$\kappa$B. This liberates nuclear factor kappa B (NF-$\kappa$B) and activates mitogen-activated protein kinases (MAPK) and interferon regulatory factors (IRFs), driving gene transcription of pro-inflammatory cytokines (TNF-$\alpha$, IL-$1\beta$, IL-6), chemokines (IL-8), and Type I interferons (IFN-$\alpha$, IFN-$\beta$).

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The Complement Cascade

The complement system comprises circulating plasma proteins that function in an enzymatic activation cascade.

Classical Pathway              Lectin Pathway             Alternative Pathway
(Ab-Ag Complexes: IgG/IgM)     (MBL / Ficolins -> Mannose) (Spontaneous C3 Tickover)
         │                                │                          │
         ▼                                ▼                          ▼
      C1qrs                             MASPs                     Factor B & D
         │                                │                          │
         └─────────────────► C4b2a ◄──────┘                          ▼
                              (C3 Convertase)                     C3bBb
                                     │                               │
                                     └──────────────┬────────────────┘
                                                    ▼
                                             Cleaves C3 into:
                                                    │
                                     ┌──────────────┴──────────────┐
                                     ▼                             ▼
                               C3a (Anaphylatoxin)           C3b (Potent Opsonin)
                                                                   │
                                                                   ▼
                                                            C5 Convertase
                                                            (C4b2a3b / C3bBbC3b)
                                                                   │
                                                    ┌──────────────┴──────────────┐
                                                    ▼                             ▼
                                              C5a (Chemotaxin)              C5b
                                                                             │
                                                                             ▼ + C6, C7, C8, poly-C9
                                                                    ┌─────────────────┐
                                                                    │ MAC (C5b-9)     │
                                                                    │ Direct Lysis    │
                                                                    └─────────────────┘

Complement Pathways and Key Mediators

1. Activation Pathways:
   • Classical Pathway: Triggered by antibody-antigen complexes binding C1q (IgM is most potent; IgG1 and IgG3 are active).
   • Lectin Pathway: Mannose-binding lectin (MBL) binds terminal mannose sugars on microbial surfaces, activating MBL-associated serine proteases (MASPs).
   • Alternative Pathway: Continuously active at low levels via spontaneous hydrolysis ("tickover") of C3 to $C3(H_2O)$, which binds Factor B and Factor D to form fluid-phase C3 convertase.
2. Critical Cleavage Products:
   • C3b: Major opsonin that covalently tags microbial surfaces, engaging complement receptor 1 (CR1, CD35) on neutrophils and macrophages to accelerate phagocytosis.
   • C3a and C5a (Anaphylatoxins): Trigger mast cell degranulation, smooth muscle contraction, and marked vascular permeability.
   • C5a (Chemotactic Factor): Potent chemoattractant that directs neutrophils to the focus of infection.
3. Membrane Attack Complex (MAC, C5b-9): C5b recruits C6, C7, C8, and 12–18 molecules of C9 to polymerize a hydrophobic, transmembrane lytic pore (10 nm diameter) across the lipid bilayer of target cells.

Clinical Complement Deficiencies

• Early Classical Pathway (C1, C4, C2) Deficiencies: Impair immune-complex clearance, predisposing to systemic lupus erythematosus (SLE) and recurrent pyogenic infections caused by encapsulated bacteria (S. pneumoniae, H. influenzae).
• C3 Deficiency: Severe, life-threatening susceptibility to recurrent pyogenic infections and encapsulated bacteria, as C3 is the central junction of all three pathways.
• Terminal Pathway (C5, C6, C7, C8, C9) Deficiencies: Specifically impair formation of the Membrane Attack Complex. Gram-positive organisms resist MAC lysis because of their thick peptidoglycan wall, but Gram-negative Neisseria species rely upon direct MAC lytic pores for killing. Terminal deficiencies confer a 1,000- to 10,000-fold increased lifetime risk of invasive meningococcal disease (Neisseria meningitidis) and disseminated gonococcal infection (Neisseria gonorrhoeae).
• Therapeutic C5 Blockade (Eculizumab, Ravulizumab): Humanized monoclonal antibodies that bind C5 and prevent cleavage into C5a and C5b, blocking MAC assembly to treat paroxysmal nocturnal hemoglobinuria (PNH) and atypical hemolytic uremic syndrome (aHUS). Patients receive mandatory meningococcal vaccination (MenACWY conjugate and MenB vaccines) along with daily oral penicillin or macrolide antimicrobial prophylaxis throughout the duration of therapy.

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Neutrophil Kinetics and Phagocytic Killing

Polymorphonuclear neutrophils (PMNs) represent the primary cellular defense against acute bacterial and fungal infections.

Absolute Neutrophil Count (ANC) Stratification

$\text{ANC} (\text{cells/mm}^3) = \text{Total WBC (cells/}\mu\text{L)} \times \left( \frac{\% \text{Segmented Neutrophils} + \% \text{Band Neutrophils}}{100} \right)$

Respiratory Burst and Chronic Granulomatous Disease

Upon phagocytosis, the pathogen is internalized within a phagosome that fuses with primary and secondary lysosomes. Neutrophils then assemble the multi-protein NADPH oxidase enzyme complex (comprising membrane subunits gp91phox, p22phox and cytosolic subunits p47phox, p67phox, p40phox, and Rac2) at the phagolysosomal membrane.

$\text{NADPH} + 2\text{O}_2 \xrightarrow{\text{NADPH Oxidase}} \text{NADP}^+ + \text{H}^+ + 2\text{O}_2^{\bullet -} \quad (\text{Superoxide})$ $2\text{O}_2^{\bullet -} + 2\text{H}^+ \xrightarrow{\text{Superoxide Dismutase}} \text{H}_2\text{O}_2 + \text{O}_2 \quad (\text{Hydrogen Peroxide})$ $\text{H}_2\text{O}_2 + \text{Cl}^- \xrightarrow{\text{Myeloperoxidase (MPO)}} \text{HOCl} + \text{OH}^- \quad (\text{Hypochlorous Acid})$

Chronic Granulomatous Disease (CGD): An inborn error of immunity caused by mutations in NADPH oxidase subunits (X-linked gp91phox deficiency accounts for ~65% of cases). Neutrophils can engulf microbes normally but cannot generate superoxide or hydrogen peroxide.

• Catalase-Negative Pathogens: Microbes such as Streptococcus pneumoniae produce endogenous $\text{H}_2\text{O}_2$ as a metabolic byproduct. Because they lack catalase, this hydrogen peroxide accumulates in the phagolysosome and is weaponized by host myeloperoxidase to kill the bacteria.
• Catalase-Positive Pathogens: Pathogens producing catalase degrade their endogenous hydrogen peroxide ($2\text{H}_2\text{O}_2 \to 2\text{H}_2\text{O} + \text{O}_2$), leaving the neutrophil without oxidative killing capacity. Patients with CGD suffer recurrent life-threatening infections caused by five hallmark catalase-positive organisms:
  1. Staphylococcus aureus
  2. Burkholderia cepacia complex
  3. Serratia marcescens
  4. Nocardia species
  5. Aspergillus species
• Management: Lifelong antibacterial prophylaxis with trimethoprim-sulfamethoxazole, antifungal prophylaxis with itraconazole or voriconazole, and subcutaneous recombinant interferon-gamma (IFN-$\gamma$).

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Adaptive Immunity: Humoral vs. Cell-Mediated

Adaptive immunity provides targeted, clonal pathogen destruction and lasting immunological memory, segregated into humoral (B-cell) and cell-mediated (T-cell) arms.

Comparative Features of Adaptive Immune Deficiencies

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Sepsis Immunopathology: Cytokine Storms to CARS

Sepsis is defined clinically as life-threatening organ dysfunction caused by a dysregulated host response to infection (Sepsis-3). Rather than pure unbridled inflammation, sepsis follows a dynamic, biphasic immunopathological trajectory.

Infection Onset
      │
      ▼
┌────────────────────────────────────────────────────────┐
│ Early Hyper-Inflammatory Phase ("Cytokine Storm")      │
│ - Pro-inflammatory cytokines: TNF-α, IL-1β, IL-6, IL-12 │
│ - Massive endothelial injury, glycocalyx shedding      │
│ - Disseminated intravascular coagulation (DIC)         │
│ - Capillary leak, vasodilation, distributive shock     │
└───────────────────────────┬────────────────────────────┘
                            │
                            ▼
┌────────────────────────────────────────────────────────┐
│ Compensatory Anti-Inflammatory Response Syndrome (CARS)│
│ - Anti-inflammatory cytokines: IL-10, TGF-β, sTNFR     │
│ - Marked apoptosis of CD4+ T-cells, B-cells, dendrites  │
│ - Downregulation of monocytic HLA-DR expression        │
│ - Upregulation of checkpoint inhibitors (PD-1 / PD-L1)  │
│ - Secondary ICU nosocomial infections, CMV reactivation │
└────────────────────────────────────────────────────────┘

1. Early Hyper-Inflammatory Phase: Massive stimulation of PRRs triggers uncontrolled release of early-phase mediators (TNF-$\alpha$ and IL-$1\beta$), followed by systemic amplification via IL-6. These cytokines disrupt the endothelial glycocalyx, activate the tissue factor clotting pathway, impair protein C anticoagulant mechanisms, and cause diffuse microvascular thrombosis, tissue hypoperfusion, and lactic acidosis.
2. Compensatory Anti-inflammatory Response Syndrome (CARS): Occurs concurrently and sequentially to suppress systemic inflammation. Sustained anti-inflammatory signaling driven by IL-10 and transforming growth factor-beta (TGF-$\beta$) suppresses monocyte human leukocyte antigen-DR (HLA-DR) expression (<30% expression indicates severe immunoparalysis). Apoptosis of CD4+ T-cells, B-cells, and dendritic cells leaves patients profoundly immunosuppressed, predisposing ICU patients to secondary nosocomial infections (P. aeruginosa, Acinetobacter, Enterobacterales), opportunistic fungal infections (Candida, Aspergillus), and viral reactivation (CMV, EBV).