W4 - T Cells & Hygiene Hypothesis

What are the three distinct sequential signals required from an Antigen-Presenting Cell (APC) for naïve T cell activation, expansion, and differentiation?

• Signal 1 (Activation): Specific binding of the T cell receptor (TCR) to peptide-MHC on the APC.

• Signal 2 (Co-stimulation): Binding of CD28 on the T cell to CD80/CD86 (B7-1/B7-2) on the APC (prevents anergy and promotes survival).

• Signal 3 (Differentiation): Local cytokines secreted by the APC binding to cytokine receptors on the T cell, polarizing it into functional subsets (Th1, Th2, Th17, Treg).

What happens to the peak expanded T cell population after an infection is controlled?

It resolves through two phases: Contraction (~90–95% of effector cells die via apoptosis over 1–2 weeks as antigen clears) and Memory Formation (~5–10% survive to form long-lived memory T cells).

What initiating cytokines and signature transcription/secreted factors drive the Th1 vs. Th2 vs. Th17 lineages?

• Th1: Initiated by IFN-alpha/beta and IL-12; secretes IFN-gamma and TNF-alpha (targets viruses/intracellular bacteria; linked to autoimmunity).

• Th2: Initiated by IL-4; secretes IL-4, IL-5, and IL-13 (targets parasites; linked to asthma/allergies).

• Th17: Initiated by IL-6 + TGF-beta, IL-21, and IL-23; secretes IL-17 (targets extracellular bacteria/fungi; linked to autoimmunity).

What are the major subsets of memory T cells based on their functional spectrum and location?

• T_scm (Stem cell memory): High self-renewal and multipotency.

• T_cm (Central memory): Homs to secondary lymphoid tissues.

• T_rm (Tissue-resident memory): Permanently resides in non-lymphoid peripheral tissues.

• T_em (Effector memory): Recirculates and is equipped for rapid effector function.

What is Central Tolerance (Thymic Education)?

The developmental process in the thymus where immature lymphocytes undergo Positive Selection (surviving if they recognize self-MHC with appropriate affinity) and Negative Selection (clonal deletion/apoptosis if they bind self-antigens with high affinity).

What are the three key peripheral tolerance control mechanisms that act as a fail-safe outside the thymus?

• Regulatory T cells (Tregs): Actively suppress autoreactive cells and excessive inflammation.

• Anergy: Renders self-reactive lymphocytes functionally unresponsiveness if they encounter antigen without co-stimulation.

• Activation-Induced Cell Death (AICD): Triggers apoptosis in persistently activated or self-reactive effector T cells.

What are the four types of hypersensitivity reactions, their primary mediators, and timeframes?

• Type I (Allergy/Anaphylaxis): IgE and Mast Cells; rapid (minutes to hours). Mediated by histamine release.

• Type II (Cytotoxic): Antibody and Complement-mediated cytotoxicity; rapid. (e.g., blood transfusion mismatch).

• Type III (Immune Complex): Antigen-antibody complex deposition in tissues; rapid/intermediate. (e.g., Systemic Lupus Erythematosus).

• Type IV (Delayed-Type): T-cell mediated (CD4+ and CD8+); delayed (takes days to develop). (e.g., Mantoux TB test, contact dermatitis, Type 1 Diabetes).

What are the epidemiological demographics of autoimmune diseases?

They affect 5–12% of Australians, with a strong sex disparity where 75% of affected individuals are women, and prevalence increases with age.

What is the geographical relationship between autoimmune diseases and helminth infections?

There is a clear inverse geographical correlation: regions with high rates of endemic parasitic helminth infections (developing tropical/sub-tropical regions) exhibit low rates of autoimmune pathology, whereas industrialized developed regions (North America, Europe) show the opposite.

What is the core difference between the original Hygiene Hypothesis and the 'Old Friends' Hypothesis?

• Hygiene Hypothesis: Assumes a general lack of early-life infections (colds, flu) prevents overall non-specific immune system training.

• 'Old Friends' Hypothesis: Recognizes that human immune regulation evolved to depend on specific co-evolved symbiotic organisms (helminths, ambient soil mycobacteria, gut microbiota) that actively drive anti-inflammatory pathways and Treg activation via IL-10.

What immunological "seesaw" explains why helminth infections protect against autoimmunity?

Helminths drive anti-inflammatory/regulatory responses via Th2 cytokines (IL-4, IL-5, IL-13) and Tregs (IL-10), which actively counteract and suppress the pro-inflammatory Th1 (driven by IL-12) and Th17 (driven by IL-23) pathways responsible for autoimmune tissue damage.

What is an example of experimental helminth therapy currently researched for inflammatory bowel disease (IBD)?

Administering live parasite eggs (such as Trichuris suis or Necator americanus), which has shown clinical success in driving regulatory T cell (Treg) activity to help patients with Ulcerative Colitis and Crohn’s Disease enter long-term remission.

What are the initiating cytokines, secreted cytokines, target pathogens, and pathological conditions associated with CTLs (CD8+ Cytotoxic T Lymphocytes)?

Initiated by IFN-alpha/beta and IL-12; secretes IFN-gamma, TNF-alpha, Perforin, and Granzymes; targets viruses/intracellular bacteria; pathologically causes cytokine storm.

What initiating molecules and secreted cytokines drive Treg (Regulatory T cell) differentiation, and how is this pathway exploited in disease?

Initiated by TGF-beta + Retinoic Acid (RA); secretes IL-10 and TGF-beta; pathologically exploited in tumour immune evasion.

What are the three key stages of progression in a T cell immune response?

• Initiation: DCs/APCs deliver signal-specific cytokines depending on the infectious agent to drive naïve cell polarisation.

• Amplification: The activated lineage secretes effector cytokines to expand the response and clear the pathogen target.

• Exaggeration: Dysregulated, persistent, or uninhibited activation leads to chronic inflammatory pathologies, autoimmune conditions, hypersensitivities, or tissue damage.

What four key properties of the adaptive immune system are explained by Sir Frank Macfarlane Burnet's Clonal Selection Theory?

1. Diversity: A pre-existing pool of lymphocytes displaying vast antigen receptor specificity generated prior to pathogen exposure.

2. Self-Tolerance: Immature self-reactive lymphocytes binding strongly to self-antigens are deleted in early development.

3. Specificity: A given pathogen selects only the specific lymphocyte clone expressing a matching receptor.

4. Memory: Antigen-driven expansion leaves behind a persistent population of memory cells for faster secondary responses.

What are the cell surface marker phenotypes of T cells as they move from the Bone Marrow through the Thymic Cortex and Medulla?

• Bone Marrow / Fetal Liver: Double-Negative (CD4- CD8- TCR-).

• Thymus Cortex: Double-Positive (CD4+ CD8+ TCR\alpha\beta^low).

• Thymus Medulla (Single-Positive Lineages):

  • Helper T cells (CD4+ CD8- TCR\alpha\beta+)

  • Cytotoxic T cells (CD4- CD8+ TCR\alpha\beta+)

  • gamma\delta T cells (CD4- CD8- TCR\gamma\delta+)

How do Memory Potential and Effector Capacity change along the T cell differentiation spectrum from Naïve to Terminal Effector T cells?

• Memory Potential: Highest in Naïve and Stem Cell Memory (Tscm) subsets, progressively declining toward terminal effector cells.

• Effector Capacity: Lowest in early precursor states (Naïve/T_scm), reaching maximum immediate response in fully differentiated Effector T cells (T_eff).

What are three examples of non-self entities that are normally tolerated by the host immune system?

1. Commensal Microbiome (e.g., E. coli in the gut).

2. Dietary Proteins (e.g., red meat proteins, gluten).

3. Probiotic Microorganisms (e.g., beneficial bacteria in fermented drinks like Yakult).

How is immunodeficiency classified and what are clinical examples of each category?

• Primary (Congenital/Inherited): Caused by genetic defects (e.g., SCID).

• Secondary (Disease/Treatment): Acquired due to environmental factors, infections, or medical treatments (e.g., AIDS/HIV, chemotherapy).

What specific clinical examples correspond to Type 2, Type 3, and Type 4 hypersensitivity reactions?

• Type 2 (Cytotoxic): Blood transfusion mismatch / clinical mismatch reactions.

• Type 3 (Immune Complex): Systemic Lupus Erythematosus (SLE).

• Type 4 (Delayed-Type): Tuberculosis Mantoux test, Contact Dermatitis, and Type 1 Diabetes (beta-cell destruction).

What are the three taxonomic groups of parasitic helminths?

• Nematodes (Roundworms; e.g., Ascaris, hookworms).

• Cestodes (Tapeworms).

• Trematodes (Flukes; e.g., Schistosoma).

What clinical dosing protocol and results were demonstrated using helminth therapy (Trichuris suis) for Inflammatory Bowel Disease (IBD)?

• Protocol: Administering 2,500 parasite eggs every two weeks.

• Ulcerative Colitis Outcome: 50% of patients showed improved pathology.

• Crohn's Disease Outcome: 79% of patients entered remission (maintained for up to 4 years).

Which specific helminth species are investigated in human vs. murine (mouse) models for autoimmune diseases?

• Human Clinical Targets: Necator americanus & Trichuris suis (for IBD); Trichuris trichiura (for Multiple Sclerosis).

• Murine Models: Schistosoma mansoni & Fasciola hepatica (for MS & Rheumatoid Arthritis); Trichinella spiralis & Fasciola hepatica (for Type 1 Diabetes).

Which innate and adaptive cell types are promoted by helminth parasites to skew the immune axis toward tolerance?

• Innate Side: M2-macrophages, Tolerogenic Dendritic Cells (DCs), and Type 2 Innate Lymphoid Cells (ILC2s).

• Adaptive Side: Th2 cells, Regulatory T cells (Treg), and Regulatory B cells (Breg).

What is the primary functional role of CD4+ Helper T cells once activated by professional antigen-presenting cells (pAPCs)?

They differentiate into specialized effector subsets (such as Th1, Th2, Th17, and Treg) that secrete distinct profiles of cytokines to coordinate, help, and regulate other immune cells (like B cells, macrophages, and cytotoxic T cells).

What key cytokines drive the polarization of naive CD4+ T cells into Th1 versus Th2 effector cells, and what are their signature downstream functions?

• Th1 Polarization: Driven by IL-12 and IFN-gamma; secretes IFN-gamma and TNF-alpha to promote cell-mediated immunity, macrophage activation, and intracellular pathogen clearance.

• Th2 Polarization: Driven by IL-4; secretes IL-4, IL-5, and IL-13 to coordinate humoral immunity, antibody class-switching (to IgE), and defense against helminth parasites.

What are Regulatory T cells (Tregs), and what transcription factor and cytokines define their suppressive function?

• Definition: A specialized subpopulation of CD4+ T cells that maintain peripheral self-tolerance and downregulate overactive immune responses.

• Defining Transcription Factor: FoxP3.

• Key Suppressive Cytokines: Secretion of anti-inflammatory cytokines IL-10 and TGF-beta.

What is the core premise of the Hygiene Hypothesis regarding the rising incidence of allergies and autoimmune diseases in modern industrialised societies?

Reduced early-childhood exposure to microbial pathogens, commensal microorganisms, and parasites deprives the developing immune system of necessary immunomodulatory stimuli, leading to a failure in properly calibrating Th1/Th2 regulatory networks and dampening Treg suppression.

How does a lack of early microbial stimulation (the "Old Friends" hypothesis / modified hygiene hypothesis) affect Th1/Th2 balance and allergy susceptibility?

Without early microbial encounters (which typically drive Th1 responses via TLR signaling and IL-12), the immune system's default pathway leans toward a polarized Th2-skewed response, predisposing individuals to IgE-mediated atopic allergies, asthma, and eczema.

What is the role of the Immunological Synapse during antigen presentation between an APC and a T cell?

A structured, organized junction formed at the interface between the T cell and the APC that spatially segregates signaling receptors (central supramolecular activation cluster or cSMAC) from adhesion molecules (pSMAC) to sustain and regulate T-cell activation signals.

How do Induced Regulatory T cells (iTregs) differ functionally from Naturally Occurring Regulatory T cells (nTregs)?

• nTregs: Produced directly in the thymus during T-cell development as a distinct lineage committed to self-tolerance.

• iTregs: Differentiated from mature, naive conventional CD4+ T cells in the peripheral lymphoid tissues under the influence of local microenvironmental cues (such as TGF-beta and retinoic acid).

What is the role of CTLA-4 expression on Tregs in downregulating immune responses?

Tregs constitutively express high levels of CTLA-4, which strips or downregulates costimulatory ligands (CD80/CD86) from the surface of antigen-presenting cells via trans-endocytosis, preventing them from activating conventional effector T cells.

How do Commensal Microbiota interact with the host mucosal immune system to promote immune regulation and prevent allergic hyper-reactivity?

Commensal microbes provide continuous tonic signaling via pattern recognition receptors (e.g., TLRs and NOD-like receptors) and produce short-chain fatty acids (SCFAs like butyrate) that promote the expansion and suppressive function of colonic FoxP3+ regulatory T cells.

What is the concept of "Immune Deviation" in the context of the hygiene hypothesis?

The shift in the balance of the immune response away from a pathological, allergy-prone Th2-dominant state back toward a balanced, robust Th1 and regulatory (Treg) immune profile facilitated by diverse microbial and parasitic exposures early in life.