Natural Killer Cells

Slide 1 · Natural Killer Cells

Recognition, activation, cytotoxicity, and clinicopathologic relevance

Innate surveillance at the interface of cellular stress and host defense

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Recognize

Explain why an NK cell may spare a healthy cell but kill an abnormal one.

Mechanism

Trace inhibitory and activating receptor signals and CD16-mediated ADCC.

Clinical reasoning

Connect cytokine control and NK dysfunction to infection, cancer, and immune disease.

Use receptor balance—not a single marker—as the organizing principle.

Slide 3 · NK cells belong to innate immunity

Innate cellular arm

Acts early after infection or tissue damage

Includes NK cells, phagocytes, and dendritic cells

Adaptive lymphocytes

T and B cells express diverse antigen receptors

Undergo antigen-driven expansion and differentiation

NK cells are lymphocytes classified by their role in innate defense.

Slide 4 · The central task of an NK cell

Recognize and destroy severely stressed or abnormal host cells.

Textbook examples: Virus-infected cells and tumor cells.

An NK cell evaluates properties of the target cell surface.

Effector action can be direct killing or cytokine secretion.

Slide 5 · Three major NK-cell activities

Direct killing

Elimination of infected, stressed, or neoplastically transformed cells.

ADCC

Lysis of IgG-coated targets through CD16 engagement.

Cytokine release

Interferon-γ activates macrophage antimicrobial activity.

Target-cell destruction and macrophage activation are complementary functions.

Slide 6 · NK cells in circulating lymphocytes

Approximately 5%–10% of peripheral blood lymphocytes are NK cells.

This figure describes peripheral blood lymphocyte composition.

Abundance is distinct from the strength of a particular NK response.

The textbook does not specify a diagnostic cutoff for NK-cell deficiency.

Slide 7 · NK cells versus phagocytes

NK cells

Recognize abnormal host cells

Destroy targets; Release IFN-γ

Phagocytes

Ingest microbes and damaged cells

Destroy ingested material and contribute to inflammation

Different mechanisms can cooperate against an intracellular infection.

Slide 8 · NK cells versus dendritic cells

NK-cell emphasis

Effector killing of damaged or infected host cells

Early secretion of macrophage-activating cytokines

Dendritic-cell emphasis

Capture and display protein antigens

Connect innate sensing to T-cell activation

Recognition of danger and presentation of antigen are not interchangeable functions.

Slide 9 · NK cells and cytotoxic T lymphocytes

NK cells

Can kill without prior sensitization

Integrate activating and inhibitory receptor signals

Cytotoxic T lymphocytes

TCR recognizes peptide–MHC complexes

Effector response depends on adaptive T-cell recognition

MHC class I can inhibit NK killing while enabling recognition by many CD8 T cells.

Slide 10 · Three decisions preceding target killing

1. Detect

Does the target display an NK activating ligand?

2. Restrain

Does intact self MHC class I engage an inhibitory receptor?

3. Integrate

Does the net signal favor inhibition or activation?

NK cytotoxicity is governed by an integrated receptor balance.

Slide 11 · Inhibitory and activating signals converge

NK cell

Altered target

MHC I

Stress ligand

Net response integrates both inputs

Activating receptors recognize ligands induced by cellular stress.

Inhibitory receptors recognize self MHC class I molecules.

Opposing receptor inputs regulate whether killing occurs.

Slide 12 · Activating receptors sense cellular stress

Stress-induced surface ligands can stimulate NK-cell activating receptors.

Infection may increase activating-ligand expression.

DNA damage is another example of cellular stress.

The ligand repertoire is diverse; The source does not assign a single universal ligand.

Slide 13 · Triggers associated with NK recognition

Viral infection

Can change the target-cell surface in ways detected by NK cells.

DNA damage

May induce ligands for NK activating receptors.

Transformation

Neoplastic cells may acquire activating ligands and reduce MHC I.

The target is evaluated as an altered self cell.

Slide 14 · Self MHC class I generates a stop signal

NK cell

Healthy cell

MHC I

Low stress

Inhibition → no killing

Inhibitory NK receptors bind self MHC class I on healthy cells.

Engaged inhibition prevents inappropriate killing.

Intact self recognition is a critical safeguard.

Slide 15 · Why normal tissues are protected

Engagement of inhibitory receptors restrains NK-cell cytotoxicity.

Most healthy nucleated cells express MHC class I.

An inhibitory signal can dominate even if activating signals are present.

Protection requires interpreting both sides of the receptor balance.

Slide 16 · Healthy target: Inhibition prevails

NK cell

Healthy cell

MHC I

Low stress

Inhibition → no killing

Self MHC class I is present on the target.

The inhibitory receptor is engaged.

NK activation and target-cell killing are prevented.

Slide 17 · Abnormal target: Activation may prevail

NK cell

Altered target

MHC I ↓

Stress ligand

Activation → killing favored

Stress-induced ligands engage activating receptors.

MHC class I is reduced, weakening inhibitory signaling.

Net activation can result in target-cell killing.

Slide 18 · From viral infection to NK killing

Virus-infected cell

Cellular stress develops

Surface changes

Activating ligands increase; MHC I may decline

NK engagement

Relative inhibition decreases

Effector outcome

Infected cell is destroyed

This is the mechanism illustrated Fig. 6.4.

Slide 19 · From transformation to NK recognition

Cell transformation

Abnormal host cell arises

Altered surface

Activating ligands may increase

Reduced inhibition

MHC class I may be decreased

Net activation

NK-mediated elimination is possible

This as an important mode of tumor-cell surveillance.

Slide 20 · Healthy and stressed targets: Two contrasts

Healthy target

MHC class I retained

Inhibitory receptor engaged

NK cell restrained

Infected or stressed target

MHC class I may be reduced

Activating ligands induced

NK cell can become activated

The decision depends on the combined inputs, not merely whether a virus is present.

Slide 21 · Four receptor-state scenarios

Target surface

Inhibitory engagement

Interpretation

MHC I present; Little stress signal

Engaged

Typical healthy-cell restraint

MHC I present; Some activating ligands

Engaged

Inhibition can still protect

MHC I reduced; Activating ligands present

Weak or absent

NK activation favored

MHC I reduced; Ligand status unknown

Uncertain net balance

Insufficient information for prediction

The last row is deliberately indeterminate: Receptor integration matters.

Slide 22 · Why one signal cannot predict killing

Neither MHC class I status nor an activating ligand should be interpreted in isolation.

An activating ligand may coexist with inhibitory self recognition.

Weak inhibitory input can permit an activating signal to dominate.

The illustrated outcome is a balance of multiple receptor inputs.

Slide 23 · MHC class I has a dual immunologic role

For many CD8 T cells

Displays intracellularly derived peptide antigens

Supports antigen recognition by the TCR

For NK cells

Is recognized by inhibitory receptors

Normally restrains cytotoxicity against self

Loss of MHC I can impair one recognition pathway while removing restraint from another.

Slide 24 · NK recognition does not require priming

NK cells can kill susceptible tumor cells without prior sensitization.

This contrasts with antigen-specific activation of adaptive T cells.

Early activity fits the NK role in innate host defense.

No prior sensitization does not mean indiscriminate target killing.

Slide 25 · NK cells in early antiviral defense

Virus enters cells

An infected host cell develops

Stress recognition

Surface alterations engage NK sensors

NK cytotoxicity

Infected cell is eliminated

Host defense

Viral spread may be limited

This pathway complements, rather than replaces, interferon-mediated antiviral defense.

Slide 26 · NK-cell killing versus type I interferons

NK effector response

Directly eliminates infected cells

Involves target recognition and cytotoxicity

Type I IFN response

Acts on infected and uninfected cells

Induces an antiviral state inhibiting viral replication

Innate antiviral defenses include both cell killing and noncytolytic restriction.

Slide 27 · Intracellular bacteria also engage NK defense

NK cells contribute to early responses to many intracellular bacterial infections.

They are part of the broader innate cell-mediated arm.

IFN-γ from NK cells augments macrophage microbicidal function.

This supports defense against organisms sheltered inside host cells.

Slide 28 · Cytotoxicity is only part of NK function

Target recognition

Stress and self-MHC cues determine activation.

Cell elimination

Activated NK cells destroy selected abnormal cells.

Cytokine coordination

IFN-γ strengthens macrophage killing of ingested microbes.

The effector program acts on both infected targets and other immune cells.

Slide 29 · CD16 links antibodies to NK activity

CD16 is an NK-cell Fc receptor that binds the Fc region of IgG.

IgG recognizes and coats its target through antigen binding.

NK cells contact the antibody-coated target through CD16.

CD16 engagement can initiate antibody-dependent cellular cytotoxicity.

Slide 30 · Antibody-dependent cellular cytotoxicity

IgG coating

Antibody binds target antigen

Fc exposure

IgG Fc regions remain available

CD16 ligation

NK cell contacts bound IgG

Cell lysis

Target is killed without phagocytosis

ADCC uses antibodies as targeting adaptors for innate effector cells.

Slide 31 · Fab specificity and Fc recognition

Antibody Fab region

Binds antigenic structures on the target

Determines antibody target specificity

Antibody Fc region

Is recognized by NK-cell CD16

Connects the coated target to cytotoxic effector action

The NK receptor recognizes the antibody Fc—not the antigen-specific binding site.

Slide 32 · What makes cytotoxicity antibody-dependent?

The target must be coated with IgG that can engage Fc receptors on the effector cell.

Antibody supplies specificity for the target.

CD16 supplies the link to NK-cell killing.

The mechanism is distinct from the stress-ligand recognition schematic.

Slide 33 · ADCC versus opsonophagocytosis

ADCC

NK cells and macrophages can be effectors

Antibody-coated cell is lysed without engulfment

Opsonophagocytosis

Phagocytes bind IgG Fc or complement fragments

Coated material is internalized and destroyed

Both use antibody coating but differ in the immediate cell-destruction mechanism.

Slide 34 · ADCC versus complement-mediated lysis

NK-cell ADCC

Fc receptor-dependent effector cell contact

Target lysis without phagocytosis

Complement membrane attack

Complement cascade assembles membrane attack complex

Membrane integrity is disrupted

Do not label every antibody-associated killing event as ADCC.

Slide 35 · Compare antibody-directed effector pathways

Pathway

Recognition

Immediate mechanism

NK-cell ADCC

CD16 binds IgG Fc

Effector-mediated cell lysis

Phagocytic removal

IgG Fc or deposited C3b

Engulfment and intracellular destruction

Complement attack

Antibody-dependent complement activation

Membrane attack complex injury

Shared antibody recognition need not imply identical mechanisms.

Slide 36 · ADCC in immunopathology: A limitation

ADCC is a biologically established mechanism, but its contribution to common hypersensitivity diseases is uncertain.

The textbook also describes antibody-mediated phagocytosis and complement injury.

Do not automatically attribute a clinical cytopenia to NK-cell ADCC.

Separate a plausible mechanism from a demonstrated disease-specific contribution.

Slide 37 · IFN-γ couples NK cells to macrophages

Intracellular infection

Innate cellular response begins

NK activation

NK cells release IFN-γ

Macrophage response

Macrophage activation increases

Microbe control

Destruction of ingested microbes improves

This collaboration is a central noncytolytic NK contribution.

Slide 38 · IFN-γ is a macrophage-activating cytokine

NK-cell IFN-γ promotes macrophage killing of ingested microbes.

Its target in this context is an immune effector cell.

The major outcome is enhanced destruction of internalized pathogens.

Cytokine secretion can contribute even when the NK cell is not killing the infected cell.

Slide 39 · Type I interferons and IFN-γ are not synonyms

Type I IFNs

Induce antiviral state in infected and nearby cells

Inhibit viral nucleic acids and replication

IFN-γ

Can be secreted by NK cells and T cells

Activates macrophage antimicrobial activity

Both are interferons, but these textbook-described dominant functions differ.

Slide 40 · Cytokines regulating NK responses

IL-2

Stimulates NK-cell proliferation.

IL-15

Stimulates NK-cell proliferation.

IL-12

Enhances NK killing and IFN-γ secretion.

Growth and effector activation are related but distinct regulatory outputs.

Slide 41 · IL-2 supports NK-cell expansion

IL-2 is one of the cytokines stimulating NK-cell proliferation.

Expansion increases the pool of potential innate effector cells.

Robbins groups IL-2 with IL-15 for this effect.

Proliferation should not be confused with immediate target recognition.

Slide 42 · IL-15 supports NK-cell expansion

IL-15 is among the cytokines that stimulate NK-cell proliferation.

IL-15 as a proliferative stimulus.

Its receptor shares a common γ-chain component with other cytokine receptors.

Loss of the common γ-chain can be accompanied by NK-cell deficiency.

Slide 43 · IL-12 enhances NK effector function

IL-12 promotes both killing of target cells and secretion of IFN-γ.

This supports a cytotoxic effector response.

Simultaneous IFN-γ secretion recruits macrophage antimicrobial mechanisms.

These are functional outputs, not surface antigens used for identifying NK cells.

Slide 44 · NK cytokines: Discriminate their effects

Cytokine

Effect emphasized

Interpretation

IL-2

NK-cell proliferation

Expands the cell population

IL-15

NK-cell proliferation

Supports NK availability

IL-12

Killing and IFN-γ secretion

Enhances effector activity

These simplified roles are the ones explicitly described in the source section.

Slide 45 · Coordinating early cellular host defense

Infection-related stress

Infected host cells change

NK receptors

Net activating signal arises

Effector response

Target killing + IFN-γ release

Macrophage cooperation

Greater microbicidal activity

Direct and indirect NK effects can function together.

Slide 46 · IL-15 signaling and immunodeficiency

Some common γ-chain–associated SCID cases also show deficient NK cells.

The common γ-chain is a component of the IL-15 receptor.

Disrupted cytokine signaling can affect more than one lymphocyte population.

A reduced NK population illustrates the clinical importance of cytokine support.

Slide 47 · NK deficiency versus impaired killing

Too few NK cells

A quantitative deficit may follow altered cytokine receptor signaling

Example: Common γ-chain–related immunodeficiency

NK killing machinery fails

Cytotoxic granule formation or deployment is disrupted

Example: Familial forms of HLH

Numerical depletion and functional cytotoxic failure are different biologic problems.

Slide 48 · Defective cytotoxic granules in familial HLH

Several familial HLH mutations impair the formation or deployment of cytotoxic granules by NK cells and CTLs.

The proposed consequence is failure to eliminate some infected cells.

Persistent targets may perpetuate immune-cell stimulation.

This links defective killing to excessive systemic inflammation.

Slide 49 · HLH: Failure to terminate a stimulus

Granule defect

NK/CTL cytotoxicity impaired

Targets persist

Infected cells are not cleared

Ongoing stimulation

Cytotoxic lymphocytes and macrophages remain activated

Systemic effects

Cytokine-rich inflammation and cytopenias

This is a disease mechanism, not merely a numerical deficit in NK cells.

Slide 50 · NK cells can recognize tumor immune escape

Tumor-cell loss of MHC class I may reduce inhibition of NK-cell killing.

Some cancers become poorly recognized by MHC-restricted T-cell pathways.

NK cells can recognize certain MHC I–low targets through receptor balance.

The textbook supports experimental tumor lysis but emphasizes uncertainty in spontaneous human tumor control.

Slide 51 · Experimental tumor killing versus clinical certainty

Established experimental observation

IL-2/IL-15-activated NK cells can lyse human tumor cells

Includes some MHC I–deficient tumor targets

Unresolved clinical inference

Contribution to spontaneous tumor control is not fully established

Immunotherapeutic strategies are under development

Avoid presenting promising laboratory activity as proven clinical efficacy.

Slide 52 · Tumor immunosurveillance: Plausible sequence

Neoplastic stress

Activating surface ligands may rise

MHC I reduction

Inhibitory receptor engagement may fall

NK receptor balance

Activating signal can prevail

Possible elimination

Susceptible tumor cell is killed

A mechanistic account, not proof that all spontaneous tumors are controlled.

Slide 53 · CMV illustrates sophisticated NK evasion

Cytomegalovirus can interfere with both arms of NK-cell receptor signaling.

The virus may express ligands that block NK activating receptors.

MHC class I–like viral proteins may engage NK inhibitory receptors.

Viral MHC downregulation alone does not ensure escape from NK cells.

Slide 54 · CMV evasion: Two receptor-level strategies

Diminish activation

Viral ligands block activating receptors

Positive NK signals are reduced

Strengthen inhibition

Viral MHC I–like proteins engage inhibitory receptors

A negative NK signal is supplied

Both changes can shift the integrated balance away from NK cytotoxicity.

Slide 55 · CD16 and CD56 in lineage assessment

CD16 and CD56 are listed among NK-associated antigens.

CD16 is also expressed by granulocytes.

CD56 is also expressed by a subset of T cells.

No single listed marker should be treated as absolutely NK-specific.

Slide 56 · Markers shared across leukocyte populations

Antigen

Robbins distribution

Diagnostic caution

CD16

NK cells and granulocytes

Not exclusively NK-lineage

CD56

NK cells and a subset of T cells

Not exclusively NK-lineage

CD3

Thymocytes and mature T cells

T-cell lineage context

CD45

All leukocytes

Not lineage-specific

Immunophenotype is interpreted as a pattern, not by an isolated marker.

Slide 57 · Natural killer cells are not NK-T cells

NK cells

Innate effector lymphocytes

Activation regulated by stress/self-MHC signals

NK-T cells

T-cell subset sharing some NK markers

Has a T-cell receptor; Recognizes glycolipids displayed by CD1

Shared surface markers do not make NK and NK-T cells identical.

Slide 58 · Examples of NK-associated neoplasia

NK-cell rarity

NK-origin tumors are uncommon among lymphoid neoplasms.

Large granular lymphocytic leukemia

Can arise from cytotoxic T cells or NK cells.

Extranodal NK-/T-cell lymphoma

Usually NK-derived and strongly associated with EBV.

These are examples from the hematopathology chapter—not a complete classification.

Slide 59 · Large granular lymphocytic leukemia

The Robbins spectrum includes indolent leukemia of cytotoxic T cells or NK cells.

Associated with STAT3 mutations description.

Can coexist with autoimmune phenomena and cytopenias.

The lineage cannot be assumed from the term “large granular” alone.

Slide 60 · Extranodal NK-/T-cell lymphoma

An aggressive neoplasm, usually NK-derived, with a strong EBV association.

The name reflects a related NK/T-cell lymphoma category.

Association with EBV is a clinically relevant etiologic feature.

Definitive subtyping requires disease-specific workup beyond this NK-cell primer.

Slide 61 · Application: A healthy host cell

An NK cell contacts a host cell expressing abundant self MHC class I. The cell also has some activating ligands.

What prevents inappropriate target-cell killing?

The self MHC I–recognizing inhibitory receptor can restrain the NK cell despite activating inputs.

Slide 62 · Application: A virus-infected target

A virus-infected cell shows reduced MHC class I and increased stress-induced activating ligands.

What receptor balance and outcome are expected?

Inhibition is weakened, activation is favored, and susceptible infected cells can be killed.

Slide 63 · Application: An antibody-coated cell

A target cell is coated with IgG. An NK cell engages the exposed Fc region through CD16.

Name the effector mechanism and its distinction from phagocytosis.

Antibody-dependent cellular cytotoxicity: Target lysis without engulfment.

Slide 64 · Application: Cytokine-specific effects

An immune response increases IL-12 signaling to NK cells while other NK growth signals are unchanged.

Which two functional outputs are emphasized?

Increased target-cell killing and enhanced IFN-γ secretion.

Slide 65 · Application: Persistent immune stimulation

An inherited defect impairs cytotoxic-granule deployment by NK cells and CTLs.

Why may severe systemic inflammation develop?

Failure to remove infected targets may perpetuate T-cell and macrophage activation, as described in familial HLH.

Slide 66 · Synthesis: Read the target, then the response

Surface surveillance

Activating ligands indicate stress; Self MHC I provides restraint.

Two killing routes

Direct receptor-balanced cytotoxicity or CD16-mediated ADCC.

System coordination

IFN-γ activates macrophages; IL-2/IL-15/IL-12 regulate NK responses.

NK biology connects innate surveillance to antiviral, antitumor, and antibody-mediated immunity.

Slide 67 · Three distinctions to retain

Do not equate

Low MHC I with automatic NK killing

CD16/56 positivity with definitive NK lineage

Granule failure with NK-cell absence

Instead specify

Net activating versus inhibitory signals

Marker patterns and diagnostic context

Functional versus quantitative immune defects

Precise mechanisms lead to better clinicopathologic interpretation.

Slide 68 · Concluding mechanistic model

NK-cell output is the consequence of receptor balance, antibody recognition, and cytokine context.

Self MHC I restrains; stress-induced ligands can activate.

CD16 couples IgG coating to cytotoxic cell killing.

IFN-γ recruits macrophage function; cytokines modulate growth and activity.