CD8 1 T Cell Mediated Cytotoxicity

Slide 1 · CD8+ T Cell–Mediated Cytotoxicity

Antigen recognition • granule and death-receptor pathways • tissue injury

Pathology lecture

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Identify the target-recognition rule for CD8+ cytotoxic T lymphocytes (CTLs).

Trace granule-mediated and Fas-dependent apoptosis.

Connect CTL protection with immune-mediated tissue damage.

Apply the mechanism to infection, autoimmunity, transplantation and tumors.

Slide 3 · CTLs: One effector, two possible outcomes

A CD8+ CTL recognizes a target displaying cognate antigen.

The same antigen-specific response may remove a dangerous cell or injure tissue.

Outcome depends on the target and biological context.

Host protection

Host tissue injury

Slide 4 · The defining event is target-cell killing

The effector cell is a differentiated CD8+ T lymphocyte.

Its target is an antigen-expressing host cell.

The lethal event is directed at the recognized target rather than a free microbe.

CTL

Target cell

TCR + CD8

Peptide–MHC I

Schematic

Slide 5 · Why cytotoxicity is classified as type IV

Type IV hypersensitivity is mediated by T cells.

CD8+ CTLs cause injury through direct target-cell death.

Effector

Major action

Typical result

CD8+ CTL

Direct antigen-specific killing

Target-cell apoptosis

CD4+ effector T cell

Cytokine secretion

Leukocyte recruitment / activation

Slide 6 · CTL injury is not the whole of type IV disease

Many T-cell diseases combine cellular killing and inflammation.

A CTL contribution does not exclude CD4+ cytokine responses.

Identify the dominant mechanism, not just the infiltrating cell type.

Direct killing

CTL-mediated target-cell loss

Inflammation

T-cell cytokine response

Overlap

Mechanisms may coexist

Slide 7 · Protective and pathologic cytotoxicity share machinery

Removal of a virus-infected cell may limit the infection.

The same killing process may destroy functional parenchyma.

Type 1 diabetes, hepatitis and graft rejection illustrate different target settings.

Recognize antigen

Trigger target death

Lose target cell

Benefit or injury

Slide 8 · CD8+ T cells are MHC class I–restricted

The T-cell receptor (TCR) recognizes peptide in an MHC class I complex.

CD8 binds the class I molecule as a coreceptor.

Peptide alone is not the physiologic recognition signal for a conventional CTL.

CTL

Target cell

TCR + CD8

Peptide–MHC I

Schematic

Slide 9 · Where MHC class I is expressed

MHC class I is present on nucleated cells and platelets.

Broad expression permits surveillance of infected or transformed host cells.

Erythrocytes are not included among nucleated target cells.

Wide distribution

Nucleated tissues

Immune purpose

Antigen surveillance

Recognition gate

Peptide–MHC class I

Slide 10 · Class I presents cytosolic protein peptides

Cytosolic proteins are broken down by proteasomes.

Peptides enter the endoplasmic reticulum and associate with class I molecules.

Stable peptide–MHC class I complexes travel to the cell surface.

Cytosolic protein

Proteasomal peptides

ER loading on MHC I

Surface presentation

Slide 11 · A viral antigen becomes a cell-surface signal

The viral protein is produced inside an infected host cell.

Its peptides can appear on cell-surface MHC class I.

A CTL recognizing that complex can initiate cytotoxicity.

Intracellular virus

Viral protein peptide

Peptide–MHC I

CTL recognition

Slide 12 · TCR and CD8 play distinct roles

TCR supplies specificity for the presented peptide–MHC complex.

CD8 binds the MHC class I heavy chain.

Together they support recognition and triggering of the cytotoxic response.

CTL

Target cell

TCR + CD8

Peptide–MHC I

Schematic

Slide 13 · Peptide specificity constrains killing

CTL recognition depends on the antigen shown by the target cell.

A nearby cell lacking that recognized complex is not an equivalent target.

Antigen display links the immune response to the cellular source of antigen.

Cognate peptide–MHC I

No cognate complex

Slide 14 · Class I versus class II: A useful contrast

MHC class I displays mainly cytosolic peptides to CD8+ T cells.

MHC class II displays mainly internalized extracellular antigens to CD4+ T cells.

Feature

MHC class I

MHC class II

Peptide source

Mainly cytosolic proteins

Mainly internalized proteins

Recognizing subset

CD8+ T cells

CD4+ T cells

Typical consequence

Cytotoxic killing

Helper / inflammatory response

Slide 15 · Naive CD8+ cells must first become effectors

Naive T cells encounter antigen in secondary lymphoid organs.

Antigen recognition with costimulation initiates activation.

Proliferation and differentiation produce effector CTLs.

Naive CD8+ T cell

Antigen + signals

Clonal expansion

Effector CTL

Slide 16 · Dendritic cells link tissues to lymph nodes

Dendritic cells capture antigens in peripheral tissues.

They carry antigen to draining lymph nodes.

Concentrating antigens helps rare naive antigen-specific T cells encounter them.

Peripheral antigen

Dendritic cell

Draining node

Naive T-cell encounter

Slide 17 · Antigen recognition is signal 1

TCR engagement with peptide–MHC generates antigen-specific input.

Signal 1 selects T cells with appropriate receptor specificity.

Signal 1 is integrated with costimulatory signals during naive-cell activation.

APC displays antigen

Naive TCR recognizes

Signal 1 received

Integrate signal 2

Slide 18 · B7–CD28 supplies costimulation

B7 molecules (CD80/CD86) are expressed by activated APCs.

CD28 is a receptor for B7 on naive T cells.

Signal 1 and signal 2 cooperate to drive activation.

APC peptide–MHC

TCR: Signal 1

B7–CD28: Signal 2

T-cell activation

Slide 19 · IL-2 and costimulation drive expansion

Activated CD8+ cells undergo extensive proliferation.

Costimulators and IL-2 promote clonal expansion.

The resulting effectors increase the capacity to clear antigen-bearing cells.

Activated CD8+

IL-2 + costimulation

Clonal expansion

Larger CTL pool

Slide 20 · Differentiation creates a killing-capable cell

The naive CD8+ T cell is the precursor, not the final killer.

Activation generates effector CTLs.

Effector CTLs eliminate cells harboring microbes in their cytosol.

Naive CD8+ cell

Differentiated CTL

Slide 21 · Effector cells move to peripheral targets

Naive cells circulate through secondary lymphoid organs.

Effector T cells migrate to sites containing microbial antigens.

Recognition and killing occur at the antigen-expressing tissue target.

Lymphoid priming

Effector expansion

Tissue migration

Target recognition

Slide 22 · CTL activity must be distinguished from priming

Dendritic cell interactions generate and expand the response.

Tissue target cells are then recognized by differentiated CTLs.

Phase

Principal location

Core function

Priming

Secondary lymphoid tissue

Activate naive CD8+ cells

Execution

Antigen-bearing tissue

Kill recognized target cells

Slide 23 · The CTL–target interface determines specificity

A CTL contacts a target presenting the cognate peptide–MHC I.

Recognition precedes discharge of cytotoxic mediators.

The targeted cell is then driven toward apoptosis.

CTL

Target cell

TCR + CD8

Peptide–MHC I

Schematic

Slide 24 · Killing is a sequence, not a single molecule

The recognition event identifies the cellular target.

The CTL delivers apoptotic effectors through a cytotoxic pathway.

Caspase activation executes the target-cell death program.

Antigen recognition

Mediator delivery

Caspase activation

Apoptosis

Slide 25 · Why apoptosis is an effective cytotoxic endpoint

Apoptosis dismantles a damaged or dangerous cell.

Cellular contents remain membrane-contained during early fragmentation.

Rapid clearance usually limits leakage and secondary inflammation.

Programmable

Protease-driven dismantling

Contained

Apoptotic bodies

Cleared

Phagocytic disposal

Slide 26 · CTL target cells may be otherwise viable

A target need not be mechanically injured before recognition.

The decisive feature is display of the recognized antigen.

Immune specificity, not spontaneous cell failure, starts the lethal signal.

CTL

Target cell

TCR + CD8

Peptide–MHC I

Schematic

Slide 27 · Perforin and granzymes are the major pathway

CTLs contain lysosome-like cytotoxic granules.

These granules contain preformed perforin, granzymes and other proteins.

Upon recognizing the target, CTLs secrete the cytotoxic complex.

CTL

Target cell

TCR + CD8

Peptide–MHC I

Schematic

Slide 28 · Preformed granules permit a rapid effector response

The mediators are packaged in the differentiated CTL before target contact.

Recognition is followed by release of the granule contents.

The target cell subsequently activates its apoptotic machinery.

Granule storage

Target recognition

Granule secretion

Target apoptosis

Slide 29 · Perforin is a delivery facilitator

The CTL releases a complex containing perforin and granzymes.

The complex enters the target cell through endocytosis.

Perforin facilitates granzyme release into the target-cell cytoplasm.

Secreted complex

Target endocytosis

Perforin action

Cytosolic granzymes

Slide 30 · Granzymes are proteolytic death effectors

Granzymes are proteases.

They cleave and activate caspases within target cells.

Activated caspases drive the execution of apoptosis.

Cytosolic granzymes

Proteolytic cleavage

Caspase activation

Apoptotic death

Slide 31 · Caspase activation is the decisive intracellular switch

Caspases are produced initially as inactive enzymes.

Activation initiates a proteolytic cascade.

Execution-phase enzymes cause cellular fragmentation and death.

Inactive caspases

Initiator activation

Executioner caspases

Cell dismantling

Slide 32 · The target supplies its own apoptotic machinery

The CTL delivers the trigger; The target executes the death program.

Caspases degrade essential proteins and promote DNA fragmentation.

Recognizing this division clarifies cell-mediated cytotoxicity.

CTL: Lethal trigger

Target: Apoptosis machinery

Slide 33 · Granule pathway: Explain the causal chain

A specific peptide–MHC interaction identifies the target.

CTL granules provide perforin and granzymes.

Cytosolic granzymes activate caspases, producing apoptosis.

Peptide–MHC I

Perforin / granzyme

Target caspases

Apoptotic bodies

Slide 34 · Do not equate perforin with target-cell necrosis

The principal downstream effect is apoptosis.

Perforin enables access of granzymes to the target cytosol.

Caspase activation is central to the ensuing target-cell death.

Perforin: Granzyme access

Caspases: Apoptosis

Slide 35 · Fas ligand provides a second CTL route

Activated CTLs may express Fas ligand (FasL).

FasL binds the Fas death receptor on susceptible target cells.

The signal triggers the extrinsic apoptotic pathway.

CTL Fas ligand

Target Fas receptor

Death signaling

Apoptosis

Slide 36 · Fas is a death receptor of the TNF family

Fas is also called CD95.

Engagement by FasL clusters death receptors.

The intracellular death domain recruits apoptotic signaling proteins.

FasL binding

Fas clustering

Death-domain signaling

Caspase cascade

Slide 37 · FADD links Fas to initiator caspases

Engaged Fas recruits the adapter protein FADD.

FADD recruits procaspase-8 (or procaspase-10).

Proximity and cleavage generate active initiator caspase.

Fas engagement

FADD recruitment

Procaspase-8/10

Initiator caspase

Slide 38 · Extrinsic signaling converges on executioners

Initiator caspase-8 activates downstream executioner caspases.

The death program produces proteolysis and fragmentation.

This mechanism is distinct at initiation but shares the apoptotic endpoint.

Fas–FADD

Caspase-8

Executioner caspases

Apoptosis

Slide 39 · Granule and Fas pathways: Same endpoint

Granule route delivers perforin and granzymes to the target.

Fas route uses receptor-mediated initiation of apoptosis.

Property

Granule pathway

Fas pathway

CTL effector

Perforin, granzymes

Fas ligand

Target entry point

Endocytosis / cytosol

Fas death receptor

Convergence

Caspase activation

Caspase activation

Outcome

Apoptosis

Apoptosis

Slide 40 · The granule mechanism is emphasized

Perforin–granzyme release is described as the principal route.

FasL–Fas is presented as a less important CTL killing mechanism.

Importance is context-dependent; Neither route is a form of antibody cytotoxicity.

Major: Granules

Additional: FasL–Fas

Slide 41 · Apoptosis begins with cell shrinkage

The dying cell becomes smaller with denser cytoplasm.

The cytoplasm may become brightly eosinophilic on H&E.

This contrasts with the early swelling typical of necrosis.

Apoptosis: Shrinkage

Necrosis: Swelling

Slide 42 · Chromatin condensation is characteristic

Nuclear chromatin becomes dense and often peripheral.

The nucleus may subsequently fragment.

These changes are structural consequences of the apoptotic program.

Normal nucleus

Chromatin condensation

Nuclear fragmentation

Apoptotic body

Slide 43 · Membrane blebs produce apoptotic bodies

The cell surface develops blebs.

Fragments remain surrounded by plasma membrane.

Apoptotic bodies can contain nuclear or cytoplasmic material.

Cell shrinkage

Membrane blebbing

Bounded fragments

Phagocytic clearance

Slide 44 · Phagocytic clearance limits tissue reaction

Apoptotic bodies expose signals recognized by phagocytes.

Fragments are engulfed before cell contents are widely released.

Individual apoptotic events may evoke little surrounding inflammation.

Apoptotic fragments

Recognition by phagocytes

Engulfment

Limited leakage

Slide 45 · Why a biopsy may underestimate apoptosis

Cell shrinkage and fragmentation occur quickly.

Phagocytes remove the apoptotic remnants efficiently.

A single H&E section may underrepresent the amount of prior cell death.

Rapid changes

Brief morphologic window

Clearance

Short tissue residence

Sampling

Limited snapshot

Slide 46 · Apoptosis versus necrosis in tissue sections

Apoptosis yields cell shrinkage, dense chromatin and discrete bodies.

Necrosis more often includes swelling and plasma-membrane disruption.

Feature

Apoptosis

Necrosis

Cell size

Shrinkage

Swelling

Nuclear change

Condensation / fragmentation

Pyknosis, karyorrhexis, karyolysis

Cell boundary

Contained fragments

Membrane damage

Host response

Often minimal initially

Frequently inflammatory

Slide 47 · CD8+ T cells can also produce IFN-γ

CD8+ cells have activities beyond direct killing.

Interferon-γ (IFN-γ) is a prominent secreted cytokine.

This can contribute to tissue inflammatory reactions.

Direct cytolysis

IFN-γ–linked inflammation

Slide 48 · CTL inflammation can resemble delayed hypersensitivity

CD8+ cytokine responses resembling DTH.

Such reactions are noted after some viral infections.

Some contact-sensitizing exposures may also involve these responses.

CTL secretion

IFN-γ

Pattern

DTH-like inflammation

Settings

Virus / contact sensitizer

Slide 49 · Differentiate primary killing from collateral inflammation

Direct cytotoxicity removes specific antigen-bearing cells.

Cytokine effects recruit or activate other inflammatory participants.

Feature

Cytotoxic effect

Inflammatory effect

Primary mediator

Perforin–granzyme / FasL

IFN-γ and other cytokines

Immediate target

Antigen-presenting cell

Inflammatory network

Main consequence

Cell death

Tissue inflammation

Slide 50 · Viral infection: Protection has a tissue cost

Virally infected cells display peptide on MHC class I.

CTL killing helps eliminate the viral cellular reservoir.

The resulting loss of infected tissue cells can cause disease manifestations.

Virus in host cell

Peptide–MHC I

CTL killing

Clearance ± injury

Slide 51 · Viral hepatitis illustrates immunopathology

Infected hepatocytes can be destroyed by cytotoxic lymphocytes.

For hepatitis A, CTLs and NK cells as mediators of injury.

Hepatocyte damage reflects the host response, not only viral presence.

Infected hepatocyte

CTL / NK recognition

Hepatocyte injury

Hepatitis

Slide 52 · A virus can avoid CTL recognition

Some DNA viruses interfere with MHC class I protein localization.

Impaired peptide display reduces recognition by CD8+ CTLs.

Antigen presentation is therefore a vulnerability in antiviral surveillance.

Viral immune evasion

Reduced class I display

Poor CTL recognition

Infected-cell persistence

Slide 53 · Chronic infection may attenuate CTL potency

T-cell exhaustion in chronic HIV, HBV and HCV infections.

The PD-1 pathway contributes to the exhausted phenotype.

Persistent antigen can coexist with inadequate cellular control.

Chronic infection

Persistent stimulation

T-cell exhaustion

Reduced viral control

Slide 54 · Type 1 diabetes: CTLs attack endocrine cells

Pancreatic β-cell antigens become immune targets.

T-cell inflammation and CTL-mediated β-cell destruction can coexist.

Loss of β-cell mass compromises insulin production.

Islet autoantigen

Autoreactive T cells

β-cell loss

Diabetes

Slide 55 · Insulitis reflects the immune injury setting

Chronic inflammation in pancreatic islets.

The lesion coexists with destruction of insulin-producing β cells.

The tissue context matters beyond counting CD8+ cells.

Site

Pancreatic islet

Response

Inflammatory infiltrate

Result

β-cell destruction

Slide 56 · Transplant rejection: Alloantigens are targets

HLA molecules vary substantially between individuals.

Recipient T cells may recognize donor alloantigens.

Activated CD8+ effectors can damage the graft directly.

Donor HLA disparity

Recipient recognition

CTL differentiation

Graft-cell injury

Slide 57 · Acute cellular rejection has two T-cell arms

CD8+ CTLs directly destroy graft cells.

CD4+ T cells contribute cytokine-mediated inflammation.

Contributor

Mechanism

Tissue consequence

CD8+ CTL

Direct cellular cytotoxicity

Parenchymal cell death

CD4+ T cell

Cytokine-driven inflammation

Inflammatory tissue injury

Slide 58 · Cellular and antibody-mediated rejection differ

Cellular rejection includes CTLs and CD4+ inflammatory responses.

Antibody-mediated rejection targets graft endothelium via antibodies and complement.

Acute rejection arm

Immune effector

Core injury route

Cellular

T lymphocytes

Cytolysis / inflammation

Antibody-mediated

Donor-reactive antibodies

Complement / vascular injury

Slide 59 · Tumor cells can present abnormal peptides

Mutated or abnormal cytosolic proteins generate potential tumor peptides.

MHC class I presents these peptides to antigen-specific CD8+ T cells.

CTLs participate in host immune surveillance of transformed cells.

Abnormal tumor protein

Peptide–MHC I

CTL recognition

Tumor-cell killing

Slide 60 · A tumor microenvironment may weaken CTLs

Immunosuppressive stromal interactions.

TGF-β and related factors may favor Tregs or suppress CD8+ function.

Presence of tumor-specific CTLs does not guarantee effective tumor elimination.

Tumor-associated stroma

Suppressive mediators

Weaker CTL function

Immune evasion

Slide 61 · CD8 positivity alone does not prove cytolysis

CD8 is a T-cell lineage/coreceptor marker, not direct evidence of killing.

Interpretation requires antigen context and evidence of target-cell injury.

A mixed inflammatory lesion may reflect several simultaneous mechanisms.

Phenotype

CD8+ population

Target

Relevant antigen display

Effect

Cell death / injury

Slide 62 · A causal reading of a CTL-rich lesion

Identify which tissue cells display the implicated antigen.

Ask whether apoptotic injury affects those targets.

Separate direct killing from cytokine-mediated secondary damage.

Define target

Establish recognition

Assess apoptosis

Interpret injury

Slide 63 · Why inflammation does not negate apoptosis

The individual apoptotic event may provoke little acute inflammation.

The immune disease itself can include continuing T-cell cytokine secretion.

Tissue-level inflammation and cell-level apoptosis can therefore coexist.

Apoptotic target

Inflamed tissue

Slide 64 · Case: Infected hepatocytes and cell injury

Clinical scenario

A patient has acute viral hepatitis with hepatocyte death.

The infected hepatocyte displays a viral peptide on MHC class I.

Question: What immune effector and pathway can explain the injury?

Reason through the pathway

Identify target

Identify CTL signal

Predict tissue effect

Slide 65 · Case reasoning: The hepatitis paradox

The same CTL response helps limit infected-cell reservoirs.

In an essential parenchymal organ, killing creates clinically important tissue damage.

Protection and immunopathology are two consequences of the same interaction.

Viral control

Hepatocyte loss

Slide 66 · Case: Selective loss of pancreatic β cells

Clinical scenario

An autoimmune islet lesion contains lymphocytic inflammation.

Insulin-producing β cells are progressively lost.

Question: Why can CD8+ cytotoxicity contribute to endocrine failure?

Reason through the pathway

Identify target

Identify CTL signal

Predict tissue effect

Slide 67 · Case: Acute graft dysfunction

Clinical scenario

A transplanted organ develops acute cellular rejection.

The lesion includes recipient T-cell responses to donor HLA.

Question: Distinguish the cytotoxic and cytokine-mediated components.

Reason through the pathway

Identify target

Identify CTL signal

Predict tissue effect

Slide 68 · Case: Impaired tumor immune surveillance

Clinical scenario

A tumor bears abnormal protein-derived epitopes.

Its microenvironment suppresses CD8+ T-cell activity.

Question: Can tumor antigen presentation alone ensure destruction?

Reason through the pathway

Identify target

Identify CTL signal

Predict tissue effect

Slide 69 · From peptide presentation to cellular apoptosis

Cytosolic antigen enters the class I peptide-display pathway.

Activated CTL recognizes cognate peptide–MHC I on the target.

Granules or Fas signaling initiate a caspase-driven apoptotic program.

Peptide–MHC I

Effector CTL

Cytotoxic mechanism

Apoptosis

Slide 70 · One mechanism, four clinicopathologic settings

Viruses: Infected-cell removal may injure parenchyma.

Autoimmunity: Self-tissue cells become antigenic targets.

Context

CTL target

Consequence

Viral disease

Infected host cell

Control ± tissue injury

Type 1 diabetes

Pancreatic β cell

Endocrine cell loss

Allograft rejection

Donor tissue cell

Graft dysfunction

Tumor immunity

Transformed cell

Immune surveillance

Slide 71 · What a pathology trainee should retain

Recognition is antigen-specific and MHC class I–restricted.

Perforin–granzymes dominate; FasL–Fas adds an extrinsic route.

Caspases generate apoptotic morphology and cellular loss.

IFN-γ adds an inflammatory dimension to CD8+ immune responses.

Slide 72 · Selected primary textbook sources

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition.

Immune-system disease: CTL cytotoxicity, antigen presentation and type IV injury.

Cell injury and death: apoptosis and death-receptor pathways.

Clinical examples: viral hepatitis, transplantation and neoplastic immunity.