Natural Born Killers: NK Cells and CD8+ T Lymphocytes

Question 1

Explain how MHC class I recognition directs cytotoxic T lymphocyte (CTL) activity

Answer 1

1) MHC Class 1 Antigen Processing and Presentation:

• Intracellular proteins are degraded into peptide fragments within the cytoplasm
• Involves the proteasome complex, which generates peptides for transport by the transport associated with antigen processing (TAP) into the ER
• In the ER these peptides are trimmed and loaded onto newly synthesised MHC class 1 molecules, aided by tapasin and calreticulin
• The peptide-MHC class 1 complex is then transported to the cell surface through the Golgi apparatus

2) CTL recognition:

• Cytotoxic T lymphocytes, also known as CD8+ T cells, carry T-cell receptors (TCRs) on their surface that are uniquely structured to recognise specific antigens presented by MHC class I molecules
• Alongside the TCR, the CD8 co-receptors bind to the MHC class 1 molecule (CD3) and assisting in signal transduction that alerts the T cell to the presence of foreign peptide

3) CTL Effector Functions:

• CTLs contain granules that are loaded with enzymes like granzymes and perforin
• Perforin forms pores in the target cell membrane, allowing granzymes to enter the target cell
• Once inside, granzymes trigger apoptosis by activating caspases, a family of proteases that carry out the cell death program
• In addition, CTLs express Fas Ligand (FasL) which can engage with Fas on target cells, providing another pathway to trigger apoptosis

Question 2

Describe and explain how Natural Killer cells help address immune evasion mechanisms employed by viruses

Answer 2

Natural Killer (NK) cells are part of the innate immune system defence against viral infections and certain types of cancer; they recognise and kill infected or transformed cells while sparing normal cells

NK cell inhibitory receptors interact with MHC class 1, under normal circumstances, negative feedback to inhibit the cytotoxic activity of NK cells - “self” recognition

Mechanisms of Action of NK Cells:

1) Direct Killing:

• Recognise and kill infected or cancerous cells without prior sensitisation to specific antigens
• Release of cytotoxic granules containing proteins like perforin and granzymes

2) Cytokine Production:

• Produce cytokines such as IFN-γ
• IFN-γ can inhibit viral replication, increase antigen presentation and processing, and influence the adaptive immune response by promoting Th1 cell and macrophage responses

NK Cells and Immune Evasion by Viruses:

• One of viruses’ immune strategies is the down-regulation of MHC class 1 expression on the surface of infected cells to prevent recognition by CTLs
• This evasion, however, makes these infected cells more susceptible to NK cell-mediated killing
• “missing-self” recognition is when Less MHC class 1 = less inhibitory signals
• Additionally, some viruses induce the expression of stress ligands that are recognised by activating receptors on NK cells

Question 3

Describe and explain how a balance of positive and negative signals determines Natural Killer cell function

Answer 3

Balance of Signals:

• NK cells express both inhibitory and activating receptors on their surface
• The inhibitory receptors recognise self-MHC 1 molecules that are expressed on all healthy cells = inhibitory signal to NK cells, suppressing its cytotoxic activity
• Simultaneously, NK cells have activating receptors that recognise various stress-induced ligands or pathogen-derived structures = positive signal
• Overall balance between these signals determine the activation of cytotoxic function

Positive and Negative Signals:

1) Inhibitory Receptors:

• Killer-cell immunoglobulin-like Receptors (KIRs) and the heterodimeric CD94/NKG2A receptors recognise MHC Class 1 molecules
• By recognising self-MHC class 1 molecules, these inhibitory receptors prevent NK cells from killing normal cells

2) Activating Receptors:

• Natural Cytotoxicity Receptors recognise ligands that are usually absent or only minimally present on the surface of healthy cells but can be upregulated under conditions of cellular stress, infection or malignant transformation

NK Cells and Immune Evasion:

• Viruses and tumours have immune evasion strategies that involve downregulation or alteration of self-MHC class 1 molecule to avoid recognition and destruction by CD8+ CTLs
• In the absence of sufficient MHC class 1 molecules, inhibitory signals to the NK cell are reduced, tipping the balance towards activation
• Viruses and tumors also upregulate the expression of stress-induced ligands, further enhancing the activating signals received by the NK cell

Cytokines and NK Cell Regulation:

• The balance of signals received from activating and inhibitory receptors are also influenced by cytokines
• IL-12, IL-15, IL-18 and type I interferons (ɑ and β)

Question 4

Explain the mechanisms that cytotoxic lymphocytes use to destroy their targets

Answer 4

1) Fas/FasL Death Receptor Pathway:

• Cytotoxic T cells express the Fast Ligand (FasL or CD95L), which can bind to the Fas receptor (CD95) on target cells
• Initiates a cascade of events inside the target cell leading to apoptosis
• Upon FasL binding, Fas receptors trimerises and recruits the adaptor protein FADD (Fas-associated death domain) via their mutual death domains
• This interaction allows FADD to recruit procaspase-8 forming the death-inducing signalling complex (DISC)
• Within the DISC, procaspase-8 is activated to caspase-8 which then initiates the caspase cascade, ultimately leading to apoptosis