Cell-Mediated Immunity I – Recognition & Activation (Comprehensive Study Notes)
Page 1
• Source text: Abbas et al., Ch. 1 (pp. 8-10), Ch. 7 (pp. 151-185), Ch. 9 (pp. 217-231)
• Course: Immunology Block 1, Class 13 ─ “Cell-Mediated Immunity I: Recognition & Activation”
• Lecturer: Bidyut Mohanty, PhD (VCOM-Carolinas)
• Time stamp: 08 / 05 / 2025
Page 2 – Objectives
Recall the three hallmark functions of CMI:
– Killing of damaged / infected cells.
– Activation of macrophages.
– Activation of B-lymphocytes.Discriminate among naïve, effector and memory T-cell roles.
Recognize dendritic‐cell (DC) antigen presentation to naïve T cells: relevant molecules (MHC, B7 family, adhesion, cytokines) & anatomic locale (mainly draining lymph nodes / spleen).
Predict fate of a naïve T cell on first Ag encounter (activation → proliferation → differentiation or anergy).
Identify the three signals for T-cell activation:
List steps of T-cell activation & the five structural/signalling components (TCR, CD3, CD4/CD8, adhesion integrins, co-receptors).
Catalogue costimulatory receptors (CD80, CD86 ⇄ CD28; CD40 ⇄ CD40L; CTLA-4 inhibitory role).
Explain adhesion molecule role (LFA-1, VLA-4, ICAM-1, VCAM-1) in stabilising synapse & trafficking.
Enumerate cytokines that drive proliferation/differentiation (IL-2 foremost; also IL-12, IFN-γ, IL-4, IL-5, etc.).
Differentiate ITAM vs. “immunologic synapse.”
Map the four major biochemical pathways: NF-κB, NFAT, Ras/Rac MAP-kinase→AP-1, and PI3-Akt.
State end-results: enzyme activation, adaptor recruitment, production of active transcription factors.
Page 3 – Adaptive Immunity Overview
Humoral (antibody-mediated) vs. Cell-mediated (CMI).
• CMI essential when microbes survive/replicate intracellularly, where antibodies cannot reach.
• Core outcomes:
– Destruction of microbes within phagocytes.
– Killing of infected non-phagocytic host cells.
Page 4 – Defining Features of CMI
• Mediated by T-lymphocytes (no antibody secretion).
• TCR is membrane-bound, structurally related to Ig but distinct.
• Recognise peptide antigens bound to MHC molecules on host cells.
• Two principal T-cell lineages:
– Helper T cells (CD4⁺): secrete cytokines, express membrane molecules to orchestrate immunity.
– Cytotoxic T lymphocytes (CTL, CD8⁺): directly lyse infected host cells.
Page 5 – CD4⁺ vs CD8⁺ Effector Profiles
Naïve → Activated subsets.
• Activated CD4⁺ helpers:
Produce cytokines that recruit leukocytes (inflammation).
Secrete cytokines that induce B-cell IgE switching & activate eosinophils (anti-helminth).
Provide membrane & cytokine help for B-cell antibody production.
• Activated CD8⁺ CTLs:
– Kill virus- or microbe-harbouring target cells.
Page 6 – Cells Participating in CMI
Innate:
• NK T cells – lipid antigen recognition on CD1; cytolysis.
• NK cells – recognise stress/viral ligands; lytic granule release.
• Macrophages – activated by IFN-γ (↑ROS, NO, proteases; ↑MHC II, CD80, CD86).
Adaptive:
• CD4 T helpers (esp. Th1) – secrete IFN-γ → boosts macrophage, NK, CD8 activities.
• CD8 CTLs – specific killing + IFN-γ secretion (positive feedback).
Page 7 – T-Cell-Mediated Responses to Intracellular Pathogens
A. Phagocytes with microbes surviving in phagolysosomes (e.g., Mycobacteria, Listeria, Legionella, fungi, protozoa) activate CD4⁺ Th1.
B. Non-phagocytic infected cells (epithelial, etc.) display antigens via MHC I → CD8⁺ CTL killing.
Dendritic cells prime naïve CD4/CD8 in lymphoid organs → clonal expansion → migration → effector function (granule-mediated lysis; macrophage activation).
Page 8 – NK Cell Regulation
• Killing determined by balance of signals:
– Inhibitory receptors bind self-MHC I (ITIM motifs) → suppress cytolysis.
– Activating receptors bind stress ligands → trigger killing & IFN-γ.
Absence or down-regulation of MHC I (“missing-self”) favours activation.
Page 9 – Macrophage Activation Spectrum
• Classical (M1) activation:
– PRR engagement, FcγR/CR cross-linking, IFN-γ from Th1, CD8, NK.
– Effector outputs: ↑microbicidal ROS/NO, ↑pro-inflam cytokines, ↑Ag presentation.
• Alternative (M2) activation:
– Driven by IL-4, IL-13 (Th2) → tissue repair, fibrosis, anti-inflam.
• De-activation: IL-10, TGF-β, scavenger receptor ligation.
Page 10 – Phases of a T-Cell Response
Antigen recognition.
Activation.
Proliferation (clonal expansion).
Differentiation (effector specialization).
Migration to infection site.
Effector function (kill/control pathogens).
Contraction (apoptosis of most effectors).
Memory maintenance.
Page 11 – Sequence of Events Diagram Highlights
• Naïve CD4⁺ / CD8⁺ in lymphoid organ receive Ag + costimulation.
• IL-2 production/up-regulated converts receptor to high affinity, driving proliferation.
• Effector CD4⁺: activate macrophages, B cells, orchestrate inflammation.
• Effector CD8⁺: CTL cytolysis of infected/tumour cells.
• Memory pools generated.
Page 12 – Definitions of Naïve, Effector, Memory
• Naïve: mature T cells that have not yet encountered cognate Ag.
• Effector: short-lived, immediately functional cells post-activation.
• Memory: long-lived cells poised for rapid, robust secondary response.
Page 13 – Three Major Conceptual Steps in CMI
Antigen recognition/activation.
T-cell proliferation.
T-cell differentiation.
Page 14 – Three-Signal Hypothesis Details
• Signal 1: TCR engagement of MHC-peptide.
• Signal 2: Costimulatory ligand-receptor pairs (e.g., B7/CD28).
• Signal 3: Cytokine binding → directs lineage & magnitude.
Lack of signal 2 ± 3 → anergy, tolerance, or abortive activation.
Page 15 – Reiteration of Three Signals
Same as above (condensed mnemonic: Recognition, Costimulation, Cytokine).
Page 16 – Antigen Presentation Logistics
• DC captures pathogen, migrates to 2° lymphoid organ.
• Antigen processing:
– Endogenous/cytosolic antigens → MHC I → CD8.
– Exogenous/phagolysosomal antigens → MHC II → CD4.
• One MHC molecule can present multiple peptides (promiscuous binding).
• T cells recirculate; rare clones recognising that peptide arrest & engage.
Page 17 – Intracellular vs Extracellular Antigen Routing (Graphic Explanation)
• Exogenous Ag → phagolysosome → loaded on MHC II.
• Endogenous Ag → proteasome → ER → MHC I.
• Vesicular transport brings pMHC to plasma membrane for display.
• DC with pMHC encounters cognate T cell → synapse formation.
Page 18 – Early TCR Signalling Events
• TCR-pMHC binding → conformational shift in CD4/CD8 → Lck activation.
• Lck phosphorylates ITAMs on CD3 ε, γ, δ and ζ-chains (Tyr residues).
• Phospho-ITAMs recruit ZAP-70 → downstream cascade.
Page 19 – Immune Receptor Family Architecture
• Shared motifs: ITAM (activating), ITIM (inhibitory), ITSM.
• TCR complex: αβ heterodimer + CD3γε, δε, ζζ.
Page 20 – Five Structural Components of the Signalling Complex
TCR: ligand recognition.
CD3: transduces signal via ITAMs.
CD4/CD8: MHC co-receptor, source of Lck PTK.
Adhesion integrins (LFA-1, VLA-4) stabilise contacts.
Co-receptors (CD28, CTLA-4) modulate signal strength/direction.
Page 21 – Costimulation Essentials
• Primary pair: CD80 (B7-1) / CD86 (B7-2) on APC ⇄ CD28 on T cell.
• CD40L (T cell) ⇄ CD40 (APC/B cell) amplifies B7 & cytokine expression.
• CTLA-4 competes for B7 with higher affinity, delivering inhibitory ITIM signal – initiates contraction phase & peripheral tolerance.
Page 22 – Mechanistic Cascade of Costimulation (Diagram)
• Un-activated APC (no B7) → Ag recognition alone → T-cell anergy.
• Microbial activation of APC ↑B7 & cytokines.
• CD40L up-regulation on T cell further licenses DC via CD40.
• Result: IL-2 secretion, survival, proliferation, differentiation.
Page 23 – Adhesion Molecules
• Integrins switch from low → high affinity after chemokine signalling.
• Key pairing: on T cell ⇄ on APC.
• VLA-4 (α4β1) ⇄ VCAM-1 critical for effector trafficking to inflamed endothelium.
Page 24 – Membrane Re-organisation
• Lipid rafts cluster signalling molecules into the central SMAC (c-SMAC) surrounded by peripheral SMAC (p-SMAC) of LFA-1/ICAM-1.
• Purpose: efficiency & specificity of signalling.
Page 25 – Immunologic Synapse Functions
• Ensures sustained, directed signalling.
• Focuses secretion of cytokines/lytic granules toward APC/target cell.
• Serves as platform for signal integration and termination cues.
Page 26 – Summary Table (reference figure) of Receptors & Ligands
• Includes addressins, selectins, chemokine receptors guiding homing; not reproduced here but remember high-yield pairs: CCR7-CCL19/21 (LN homing), CXCR3 etc.
Page 27 – Signal 3 (Cytokines)
• Cytokines can be APC-derived (IL-12, IL-6) or paracrine (IFN-γ, IL-4 from nearby T cells).
• Drive expansion (IL-2) & lineage commitment:
– IL-12 + IFN-γ → Th1
– IL-4 → Th2
– IL-6 + TGF-β → Th17
– IL-10 + TGF-β → Treg
Page 28 – LAT Signalosome & Major Pathways
• Sequence: Lck → ZAP-70 → phosphorylated LAT → recruitment of PLC-γ1, GADS, SLP-76, etc.
• Three canonical pathways:
• Add’l survival pathway: PI3K-Akt.
Outcome: transcription of genes for proliferation (IL-2), differentiation (cytokines), migration (chemokine receptors), effector molecules (granzyme, perforin).
Page 29 – Four Signal-Transduction Axes
NF-κB
NFAT
MAPK/AP-1
PI3-Akt (anti-apoptotic, metabolic fitness)
Page 30 – NFAT Cascade Details
• PLC-γ1 cleaves PIP2 → IP3 + DAG.
• IP3 → Ca²⁺ release → calmodulin → calcineurin.
• Calcineurin de-phosphorylates NFAT → nuclear import.
• Cyclosporine & tacrolimus inhibit calcineurin (clinical immunosuppression).
Page 31 – IL-2 Centrality
• TCR/CD28 signals induce IL-2 and expression.
• High-affinity IL-2R (αβγc) forms → autocrine loop drives rapid clonal expansion.
• Therapeutic manipulation: anti-IL-2R antibodies (basiliximab) for transplant.
Page 32 – Cytokine-Driven Differentiation Logic
• APC-secreted cytokines reflect PAMP pattern recognition (e.g., viral dsRNA → IL-12 → Th1 bias).
• Transcription factors:
– T-bet for Th1
– GATA-3 for Th2
– RORγt for Th17
– FoxP3 for Treg
• Establishes stable epigenetic “signature.”
Page 33 – CD4 Helper T-Cell Subsets Snapshot
• Th1: IFN-γ, IL-2 → macrophage activation, IgG opsonisation.
• Th2: IL-4, IL-5, IL-13 → eosinophils, IgE, helminth defense.
• Th17: IL-17, IL-22 → neutrophil recruitment, barrier integrity.
• Tfh: IL-21, CXCR5 expression → B-cell help in germinal centres.
• Treg: IL-10, TGF-β → immune regulation.
Page 34 – Cross-Regulation Principle
• Cytokines are mutually antagonistic:
– IFN-γ inhibits Th2 pathway; IL-4 inhibits Th1.
• Clinical impact: skewed Th2 response in lepromatous leprosy leads to poor intracellular bacilli control.
Page 35 – Integrated Overview of CMI Effector Phase
• Effector CD4/CD8 exit lymphoid organ via S1P gradient, home to inflamed tissue via integrin/chemokine-dependent extravasation.
• CD4 produce cytokines at site → activate resident phagocytes.
• CTL recognise pMHC I on infected cells → release perforin/granzymes, induce apoptosis.
Page 36 – Q&A Slide (No new content)
Page 37 – Required Reading Links
• Abbas & Lichtman chapters – initiation of adaptive responses, TCR complex.
• Accessible through ClinicalKey.
Page 38 – Practice Question 1 (Answer Key Insight)
• Activation of CD8 CTLs is IL-2 dependent (produced mainly by activated CD4 helpers & CTLs themselves). Answer: IL-2.
Page 39 – Practice Question 2 (NK-driven IL-12)
• IL-12 skews toward Th1 → predominant cell-mediated response via Th1. Correct choice: “Primarily a cell-mediated response through Th1 cells.”
Page 40 – References
• Abbas, Lichtman & Pillai, “Cellular & Molecular Immunology.”
• ComBank, journal figures cited on slides.
Page 41 – Supplemental Slides (intro caption)
Page 42 – CMI vs Mycobacteria Case Study
• Mycolic acid/glycolipid antigens presented by CD1 to NK T cells; TL R2 engagement.
• Macrophage infection & phagosomal escape → need Th1 IFN-γ for activation & granuloma formation.
• Primary killers: activated macrophages; back-up cytolysis by NK T, γδ T, CTLs.
Page 43 – CMI & Intracellular Fungi
• Fungi that reside intracellularly (e.g., Histoplasma) demand Th1-driven macrophage activation.
• IFN-γ ↑respiratory burst → fungicidal.
Page 44 – CMI & Parasites
• Many parasites are extracellular yet large; some have intracellular phases (e.g., Plasmodium hepatocyte stage).
• Th2 arm: IL-4→IgE; IL-5→eosinophil degranulation.
• CD8 CTLs may target infected hepatocytes; IFN-γ recruits NK/macrophage help.
Page 45 – NF-κB Pathway Detail
• DAG → PKCθ → phosphorylation & degradation of IκB → NF-κB (p50/p65) nuclear translocation.
• Drives IL-2 transcription, survival gene expression.
Page 46 – Ras/Rac MAPK → AP-1 Pathway
• ZAP-70-dependent adaptors recruit SOS (Ras-GEF) & Vav (Rac-GEF).
• Ras → Raf → MEK → ERK → c-Fos.
• Rac → JNK → c-Jun.
• c-Fos + c-Jun = AP-1 heterodimer → transcription of IL-2, cyclins.
Page 47 – PI3K / Akt Survival Pathway
• CD28 signalling activates PI3K.
• .
• PIP₃ recruits PDK1, Akt → phosphorylation/activation.
• Akt promotes anti-apoptotic proteins (Bcl-xL), metabolism (mTOR), growth.
Page 48 – Lecture Outline Recap
• Introduction → Need for CMI → Key players → NK activation → Macrophage activation → Three-step T-cell response (Activation, Proliferation, Differentiation).
These page-by-page notes capture every major and minor topic, mechanistic detail, examples, numerical or biochemical pathways, clinical correlations, and practice question insights presented throughout the transcript. They are structured to serve as a comprehensive, stand-alone study aid for mastering T-cell recognition and activation in cell-mediated immunity.