Killer Lymphocytes Lecture Notes

Lecture Outline: Killer Lymphocytes

Functions of Cytotoxic Lymphocytes in Host Defense Effector functions of CTL and NK cells in host defense against infection and tumor

  • n their own.

  • This is crucial in scenarios where:

    • Viruses replicate in cells lacking phagosome/lysosome machinery (e.g., hepatitis viruses in liver cells).

    • Microbes escape phagosomes and reside in the cytosol, rendering vesicular microbicidal mechanisms ineffective.

  • In such intracellular infections, lymphocyte-mediated killing is essential for eradicating the infection reservoir.

Cytotoxic T Lymphocytes (CTLs)

  • CD4+ and CD8+ T cells have distinct yet complementary roles in cell-mediated immunity.

  • CD4+ effector T lymphocytes release cytokines or provide cell-contact-dependent signals, aiding phagocyte-mediated microbe elimination.

  • CD8+ effector cells (CTLs) can kill infected and tumor cells.

    • Eradicate intracellular microbes, especially viruses, even in non-phagocytic cells.

  • Induction Phase:

    • CD8+ T cells recognize antigens presented by dendritic cells in peripheral lymphoid organs.

    • Stimulated to proliferate and differentiate into CTLs and memory cells.

  • Effector Phase:

    • CTLs migrate to infection/tumor sites.

    • Recognize the antigen and kill the cells producing it.

Activation and Function of CD4+ and CD8+ T Cells

Feature

CD8+ (CTL)

CD4+

Source of antigen

Endogenous

Exogenous

MHC requirement

MHC class I

MHC class II

Peptide-binding region size

8–11 residues

10–30 residues

Main effector function

Cytotoxic mediators and cytokines

Cytokines

Co-stimulatory signal

Yes

Yes

Steps in Granule-Mediated Lysis

  • CTL recognizes and binds to the antigen-expressing target cell, leading to activation.

  • CTL releases granule contents into the target cell at the immunologic synapse.

  • Granule contents induce a lethal hit.

  • CTL detaches and can then kill other target cells.

  • CTLs eliminate unwanted cells via two mechanisms:

    • Cytoplasmic granule toxins: perforin (membrane-disrupting protein) and granzymes (serine proteases) induce apoptosis.

    • Engagement of death receptors: FAS (CD95) on target cell binds to FAS ligand (FASL) on the killer cell, triggering caspase-dependent apoptosis.

    • FAS-FASL pathway mainly eliminates self-reactive lymphoid cells.

Major Cytotoxic Proteins

  • Granzymes (A, B, C): serine proteases; granzyme B activates caspases, leading to apoptosis.

  • Perforin: homologous to C9 complement protein; facilitates granzyme entry into the target cell cytosol.

  • Granulysin: alters target cell and microbial membrane permeability, contributing to killing.

  • Activated CTLs express FasL, which binds to Fas on target cells, activating caspases and apoptosis.

  • Studies suggest perforin and granzyme B are the primary mediators of CD8+ CTL killing.

CTLs are Not Damaged During Target Cell Killing

  • CTL is released from its target cell before the target cell dies.

  • Directed granule exocytosis delivers contents into the target cell, away from the CTL.

  • CTL granules contain cathepsin B, which degrades errant perforin molecules near the CTL membrane.

Cytokine Production by CD8+ Effector T Cells

  • CD8+ T cells produce IFN-γ, a macrophage-activating cytokine.

  • IFN-γ secretion measures antigen-specific CD8+ T cell frequency.

  • Both CD4+ Th1 and CD8+ T cells contribute to IFN-γ–induced phagocytic clearance.

  • CD8+ cells can induce inflammatory reactions, like contact sensitivity skin reactions, by producing IFN-γ.

Natural Killer (NK) Cells

  • NK cells kill infected cells, similar to CTLs, without prior sensitization (natural).

  • Derived from bone marrow with lymphoid morphology.

  • Oldest member of the innate lymphocyte (ILC) family and the only cytotoxic ILC.

  • Circulate in blood and lymphoid tissues; constitutdefense against infection and tumorod/spleen.

Functions of NK Cells

  • Provide early defense against viruses and intracellular bacteria.

  • Use germline DNA-encoded receptors to differentiate infected from healthy cells, unlike the clonally distributed receptors of B and T cells.

  • Recognize and eliminate stressed/damaged host cells.

  • Secrete IFN-γ to activate macrophages and modulate adaptive immune responses.

Activation of NK Cells

  • Express an array of receptors (inhibitory, activating, adhesion, cytokine receptors).

  • Function is regulated by the balance between activating and inhibitory receptor signals.

  • NK cells are tolerant to healthy host cells because activating signals are dampened by inhibitory receptors.

  • Tumor cells may lose MHC class I molecule expression, leading to NK cell activation due to loss of inhibitory signal ('missing-self' triggering).

  • 'Stressed' cells upregulate activating ligands for NK cells, overcoming MHC class I molecule inhibition ('stress-induced self' triggering), leading to tumor elimination.

Functions of NK Cells

  • Kill infected cells and produce IFN-γ.

  • Like CTLs, contain granules with proteins (perforin and granzymes) for killing.

  • Important later in viral infections by killing cells that escaped CTLs.

  • NK cell-derived IFN-γ enhances macrophage killing of phagocytosed bacteria.

Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC)

  • NK cells use FcγRIIIA (CD16) to bind to antibody-coated cells.

  • FcγRIII engagement activates NK cells to synthesize/secrete cytokines (IFN-γ) and release granule contents.

  • NK cells bind to antibody-coated cells via Fc receptors and destroy those cells (ADCC).

Learning Points

  • Killer lymphocytes: innate (NK cells) and adaptive (CD8+ T cells).

  • Lytic hit: perforin and granzyme.

  • Differential requirements for CD8+ T and NK cell activation: recognition of self and missing self.

  • Effector functions of CTL and NK cells in host defense against infection and tumor: killing and cytokine production.