DIFFERENTIATION AND FUNCTIONS OF CD8 D CYTOTOXIC T LYMPHOCYTES
Reading File
Finding Sources
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Reading File
Reading File
Reading File
Loading Image
Loading Image
Reading File
Loading Image
I now have all the material needed for a comprehensive answer. Let me compose it.
CD8+ Cytotoxic T Lymphocytes (CTLs): Differentiation and Functions
1. Overview
CD8+ T cells are MHC class I-restricted lymphocytes that differentiate into cytotoxic T lymphocytes (CTLs) - the principal adaptive immune effectors against intracellular pathogens and tumor cells. Naive CD8+ T cells are incapable of killing and must first undergo antigen-driven proliferation and differentiation to acquire cytotoxic machinery.
Induction and Effector Phases of CD8+ T Cell Responses:
Dendritic cells carry microbial antigens to lymph nodes, where naive CD8+ T cells recognize antigen, receive costimulation, proliferate, and differentiate into CTLs. CTLs then enter circulation, migrate to sites of infection, and kill infected cells.
- Cellular and Molecular Immunology, Fig. 11.1
2. Differentiation of Naive CD8+ T Cells Into CTLs
2a. Antigen Presentation: Cross-Presentation
Naive CD8+ T cells require antigen displayed on MHC class I molecules. Since viruses and tumors arise from non-DC cells, a specialized subset of dendritic cells (the classical cDC1 subset) performs cross-presentation: ingesting infected/tumor cells, transferring protein antigens into the cytosol, and processing them through the proteasome-to-ER pathway for MHC class I loading. This is the essential first step for naive CD8+ T cell activation. - Cellular and Molecular Immunology, p. 728
2b. Three Signals for Activation
Like CD4+ T cells, naive CD8+ T cells need three signals:
| Signal | Molecule | Role |
|---|---|---|
| Signal 1 | TCR + peptide-MHC I | Antigen specificity |
| Signal 2 | CD28 + B7 (costimulation) | Prevents anergy |
| Signal 3 | Cytokines (IL-2, IL-12, IFN-γ, IL-27) | Drives proliferation and differentiation |
2c. Role of CD4+ T Helper Cells
In many infections, especially with latent viruses or non-immunogenic tumors, CD8+ T cell activation requires CD4+ T cell help. Two mechanisms operate:
- Licensing of DCs: CD4+ Th cells interact with DCs via CD40L-CD40 signaling, upregulating B7 costimulators so the DC can efficiently prime CD8+ T cells.
- Direct help: IL-2 produced by CD4+ Th cells supports CD8+ T cell proliferation and survival. - Cellular and Molecular Immunology, p. 535
2d. Transcriptional Control - T-bet and Eomes
The master transcription factor T-bet drives CTL differentiation, promoting expression of perforin, granzymes, and cytolytic machinery - similar to its role in Th1 differentiation. T-bet is activated by IL-12, IFN-γ, and TCR signals acting in concert.
After infection is cleared, most CTLs undergo apoptotic contraction. A small subset survives and differentiates into memory CD8+ T cells, preferentially expressing a related transcription factor, Eomes (Eomesodermin), which drives the memory phenotype. Memory cells are characterized by IL-7R expression, enabling long-lived survival and rapid recall responses. Genetic ablation of Eomes profoundly impairs memory generation while having little effect on primary CTL numbers. - Roitt's Essential Immunology, p. 219
3. Effector Functions of CD8+ CTLs
3a. Antigen Recognition and Immune Synapse Formation
CTLs recognize target cells via the TCR + CD8 coreceptor binding peptide-MHC class I. This triggers formation of the immunological synapse - a highly organized junction between the CTL and target cell:
- Signaling patch (central supramolecular activation cluster): contains TCR, CD8, PKC-θ, and LCK
- Secretory region: the site of directed granule exocytosis
- Outer ring: LFA-1 (on CTL) binding ICAM-1 (on target) stabilizes the synapse
The enclosed synaptic gap ensures that lethal molecules are delivered only to the target, not to bystander cells. - Cellular and Molecular Immunology, pp. 734-735
Steps in CTL-Mediated Killing:
The CTL forms an immune synapse with the target cell, exocytoses granules directionally, then detaches - leaving the target to die by apoptosis.
3b. Mechanism 1 - Granule Exocytosis (Perforin/Granzyme Pathway)
This is the principal killing mechanism.
Process:
- TCR activation triggers actin cytoskeleton reorganization
- The microtubule organizing center (MTOC) reorients toward the synapse
- Cytoplasmic granules (modified lysosomes) transport along microtubules to the synapse
- Granule membrane fuses with the CTL plasma membrane at the secretory region
- Granule contents are exocytosed into the synaptic cleft
- Death of the target cell occurs over the following 2-6 hours, even after CTL detachment ("lethal hit")
Key granule proteins:
| Protein | Function |
|---|---|
| Perforin | Homologous to C9 complement; facilitates granzyme entry into target cell cytosol |
| Granzyme A | Serine protease; induces caspase-independent apoptosis |
| Granzyme B | Serine protease; cleaves after Asp residues; directly activates caspases |
| Granzymes H, K | Additional serine proteases in CD8+ CTLs |
| Serglycin | Sulfated proteoglycan; holds granzymes and perforin inactive in granules |
Perforin enables uptake of granzymes into target cell endosomes, from which they escape into the cytosol to activate the caspase cascade and induce apoptosis. - Cellular and Molecular Immunology, pp. 738-740
CTL self-protection: CTLs are shielded from their own granule contents by:
- Cytosolic Spi6 (serine protease inhibitor that antagonizes granzyme B)
- Cathepsin B delivered to the CTL surface on exocytosis, which degrades any stray perforin near the CTL membrane
3c. Mechanism 2 - Fas/FasL Pathway
Activated CTLs express FasL (CD95L) on their surface, which binds Fas (CD95/APO-1) on the target cell. Fas-FasL interaction activates the extrinsic apoptosis pathway via FADD and caspase-8, leading to target cell death. This pathway is particularly important for:
- Killing activated T cells (immune contraction/peripheral tolerance)
- Killing targets that express Fas but may be resistant to granule-mediated killing - Cellular and Molecular Immunology, p. 667 (Fig. 11.6)
3d. Cytokine Production
CD8+ CTLs also secrete cytokines that mediate non-cytotoxic effector functions:
- IFN-γ: activates macrophages for enhanced microbicidal activity (classical activation); assay for frequency of antigen-specific CD8+ T cells
- TNF: contributes to inflammation and direct cytotoxicity
- IL-17: abundant in chronic skin inflammatory diseases (e.g., psoriasis)
CD8+ T cells arrive earlier and in greater numbers than CD4+ T cells in some reactions (e.g., contact sensitivity). - Cellular and Molecular Immunology, p. 745
4. Roles in Host Defense
| Context | Role of CTLs |
|---|---|
| Viral infections | Kill virus-infected cells (viruses live in non-phagosomal compartments inaccessible to antibodies or phagocytes) |
| Intracellular bacteria (M. tuberculosis, Listeria) | Destroy cells harboring cytosol-dwelling bacteria that escape from phagolysosomes |
| Tumor immunity | Kill transformed cells presenting tumor antigens on MHC I |
| DNA elimination | Caspases activated in target cells degrade both host and microbial DNA, preventing spread of infectious DNA |
| Latent viral infections | Keep EBV and other latent viruses in check; CTL defects cause reactivation |
The massive clonal expansion of CD8+ T cells after infection provides a large effector pool. - Cellular and Molecular Immunology, pp. 745-746
5. CTLs as Mediators of Tissue Injury
CTLs can cause harm in certain conditions:
- Hepatitis B and C: The liver damage is caused by CTL/NK killing of infected hepatocytes, not by the virus itself (viruses are non-cytopathic)
- Influenza and other viral infections: Immunopathological injury
- Autoimmune diseases: CD8+ CTLs contribute to tissue destruction (e.g., type 1 diabetes, multiple sclerosis)
- Graft rejection: Rejection of MHC-mismatched allografts
6. CD8+ T Cell Memory
After infection clearance:
- Most CTLs die by apoptosis (contraction phase)
- ~5-10% survive as long-lived memory CD8+ T cells
- Memory cells are functionally quiescent until re-exposed to antigen
- Characterized by IL-7R expression (survival signal) and Eomes-driven transcriptional program
- Tissue-resident memory T cells (T
RM): A subset that stays in non-lymphoid tissues (skin, gut, lung) for extended periods without recirculating, providing frontline protection at sites of prior infection - Cellular and Molecular Immunology, p. 744
7. Clinical Relevance - CTL Defects
Hemophagocytic Lymphohistiocytosis (HLH): Inherited mutations in perforin or granule exocytosis genes (e.g., Munc13-4, syntaxin-11) cause familial HLH - a macrophage activation syndrome. Virus-specific CTLs produce IFN-γ but cannot kill infected cells, leading to:
- Persistent viral antigen → chronic IFN-γ production
- Excessive macrophage activation
- Splenomegaly, hemophagocytosis, multiorgan failure
Treatment: Anti-IFN-γ antibody (emapalumab) is approved for this indication. - Cellular and Molecular Immunology, p. 746
Sources:
- Cellular and Molecular Immunology (Abbas, Lichtman, Pillai), Chapter 11, pp. 727-746
- Roitt's Essential Immunology, Chapter 7, pp. 219-220
- Janeway's Immunobiology 10e, Section 9-13, pp. 396-400
This is a shared conversation. Sign in to Orris to start your own chat.