Cell-Mediated Immunity: T Cells, NK Cells, and Cytokine-Driven Defense

Context within Chapter 17

  • Current focus: adaptive immunity, specifically the cell-mediated branch
  • Previous lectures concentrated on antibodies (humoral immunity) that operate extracellularly
  • Critical question addressed: How does the body eliminate intracellular threats (e.g., virus-infected cells, intracellular bacteria, transformed/cancerous cells)?
  • Solution: employment of T-cell–driven, cell-mediated immunity

Comparison: Humoral vs Cell-Mediated Immunity

  • Humoral (antibody-mediated)
    • Effectors: antibodies secreted by plasma B cells
    • Targets: free pathogens, toxins, viral particles in body fluids
    • Limitation: cannot enter host cells to deal with intracellular microbes
  • Cell-Mediated
    • Primary effectors: T lymphocytes (CD4⁺, CD8⁺, regulatory) and Natural Killer (NK) cells
    • Targets: infected host cells, fungi, parasites, tissue grafts, malignancies
    • Mechanism: cell-to-cell contact, perforin/granzymes, apoptosis induction, cytokine orchestration

Lymphocyte Lineages & Maturation

  • All immune cells arise from hematopoietic stem cells
  • B cells
    • Mature in bone marrow → differentiate into plasma cells that secrete antibodies
  • T cells
    • Mature in the thymus ("T" for thymus)
    • Undergo positive & negative selection to eliminate self-reactive clones (central tolerance)
    • Exit thymus as naïve T cells → populate secondary lymphoid organs (lymph nodes, spleen, mucosal-associated lymphoid tissue)
    • Unique surface receptor: T-cell receptor (TCR) (not a BCR)

Major T-Cell Classes

  • CD4⁺ T lymphocytes (Helper T cells, Th)
    • Recognize antigen presented on MHC class II molecules of professional antigen-presenting cells (APCs)
    • Function primarily through cytokine secretion → activate & direct other immune cells
  • CD8⁺ T lymphocytes
    • Pre-cursor stage: CTL precursors (CTL-p)
    • Recognize antigen on MHC class I (present on all nucleated cells)
    • Differentiate into Cytotoxic T lymphocytes (CTLs) that directly kill target cells
  • Regulatory T cells (Treg, often CD4⁺CD25⁺FOXP3⁺)
    • Secrete immunosuppressive cytokines
    • Down-regulate immune responses, restore homeostasis, prevent autoimmunity but can hinder anti-tumor immunity

CD4⁺ Helper T-Cell Activation

  • Activation prerequisites:
    1. APC Engulfment & Processing: macrophage/dendritic cell ingests pathogen → proteolytic fragments loaded onto MHC-II
    2. TCR-MHC-II/Peptide engagement: specific Th cell binds via its TCR
    3. Co-stimulatory signals (e.g., CD80/86 on APC with CD28 on Th)
  • Resulting mutual activation (“cross-talk”)
    • APC becomes hyper-activated macrophage (↑ size, membrane ruffling, phagocytosis)
    • Th cell proliferates (clonal expansion) & differentiates into effector subsets

Helper T-Cell Subsets & Their Outputs

  • Th1
    • Key cytokine: Interferon-γ (IFN-γ)
    • Effects: activate macrophages, enhance complement, stimulate opsonizing antibody classes, promote phagocytosis
  • Th2
    • Cytokines: IL-4, IL-5, IL-13
    • Effects: activate B cells → especially class-switch to IgEIgE, recruit & activate eosinophils (anti-parasite)
  • Th17
    • Cytokine: IL-17
    • Effects: potent pro-inflammatory, recruit neutrophils, contribute to barrier immunity; dysregulation linked to autoimmune pathology

Mucosal Sampling & Oral Tolerance

  • M cells (Microfold cells) in gut epithelium ferry luminal antigens to underlying APCs
  • Leads to constant ‘sampling’ of dietary microbes → generation of mucosal immunological memory
  • Illustrates interplay between nutrition, microbiota, and adaptive immunity (not a license to overeat, but conceptually boosts gut defense)

CD8⁺ Cytotoxic T Lymphocytes (CTLs)

  • Often require help from activated Th cells (IL-2, other cytokines) for full activation
  • Target recognition sequence:
    1. Normal nucleated cell presents endogenous peptide on MHC-I
    2. Viral/bacterial/cancer peptides flagged as non-self → recognized by CTL
  • Killing mechanisms
    • Perforin: forms pores in target membrane
    • Granzymes: serine proteases enter via pores, trigger apoptosis (caspase cascade, DNA fragmentation)
    • Engagement of Fas–FasL pathway (intrinsic "kill-switch")
  • Outcome: silent cell death (apoptosis) avoids inflammation versus lytic, inflammatory necrosis if apoptosis fails

Apoptosis vs Necrosis

  • Apoptosis ("polite death")
    • Cell shrinks, DNA laddering, membrane blebbing
    • Bodies phagocytosed without spilling contents → minimal inflammation
  • Necrosis ("violent death")
    • Membrane rupture, release of DAMPs (damage-associated molecular patterns)
    • Triggers inflammation, recruits more immune cells

Regulatory T Cells (Treg)

  • Arrive at inflammatory sites → secrete IL-10, TGF-β, other suppressive cytokines
  • Essential to temper immune response, prevent collateral tissue damage/autoimmunity
  • Clinical relevance: some cancer therapies inadvertently expand Tregs, dampening anti-tumor immunity; ongoing research to modulate this axis

Natural Killer (NK) Cells

  • Innate-like lymphocytes but function in cell-mediated immunity
  • Activation logic: "Missing-self" recognition
    • Healthy cells display MHC-I → NK inhibitory receptors engaged → no kill
    • Virus-infected/tumor cells often down-regulate MHC-I → lack of inhibition → NK cell releases perforin & granzymes
  • Can kill via apoptosis or induce necrosis; also secrete cytokines (IFN-γ) that shape adaptive responses
  • Effective against viruses, tumors, large parasites

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

  • Hybrid pathway merging humoral & cell-mediated arms
  • Steps:
    1. Antibodies bind epitopes on large target (e.g., helminth)
    2. Fc regions engage FcγR or FcεR on macrophages, eosinophils, NK cells
    3. Effector cells release perforin, reactive oxygen species, proteases → extracellular parasite damage
  • Example: eosinophil swarm attacking a fluke worm coated with IgEIgE

Cytokine Network & Therapeutic Applications

  • Cytokines = communication molecules (interleukins, interferons, chemokines)
  • Clinically harnessed: recombinant IL-2 (cancer), IFN-α/β (viral hepatitis, MS), others in clinical trials to skew immunity toward desired pathway
  • Potential for precision immunomodulation (boost CTL, suppress Treg, etc.)

Integrated Flow of Adaptive Immunity (Summary)

  • Humoral arm
    • Naïve B cell + cognate antigen + Th help → clonal expansion
    • Differentiation → plasma cells (antibody factories) & memory B cells
  • Cell-mediated arm
    • APC presents peptide on MHC-II → activates Th
    • Th subsets orchestrate macrophage activation, eosinophil recruitment, B-cell help, CTL activation
    • CTLs (& NK) patrol tissues, kill infected/transformed cells
  • Cross-talk ensures comprehensive defense; absence of one arm (e.g., AIDS depleting CD4⁺ cells) cripples overall immunity

Real-World & Ethical Implications

  • Cancer immunotherapy: balancing CTL activation vs Treg suppression critical for tumor clearance
  • Organ transplantation: cell-mediated immunity recognizes graft MHC differences → risk of rejection; immunosuppressants target T-cell pathways
  • Autoimmunity: breakdown in selection or Treg control leads to self-reactive T cells attacking tissues
  • Vaccinology: next lecture will extend concepts to vaccine strategies that elicit both humoral and cell-mediated memory

Key Takeaways & Self-Check Prompts

  • Identify which MHC class (I vs II) interacts with CD4⁺ vs CD8⁺ T cells
  • Explain how perforin and granzymes collaborate during CTL-mediated killing
  • Contrast Th1 vs Th2 outcomes and relate to pathogen type (intracellular bacteria vs helminths)
  • Describe how NK cells differ from CTLs in activation requirements
  • Predict consequences of Treg depletion or excess in disease contexts

Looking Ahead

  • Next topic: Vaccines – how artificial immunization leverages principles of both humoral and cell-mediated immunity to establish long-term protection.