Innate Like Recognition Flashcards
What is the function of NK cells?
- NKs are the functional equivalents of cytotoxic T lymphocytes but operate in innate immunity and different form CTLs in their immune recognition
- Kill cells by releasing granzyme and perforin → like CTLs
- However, unlike T cells, killing by NK cells is triggered by germline-encoded receptors that recognize molecules on the surface of malignant cells → these molecules act as positive and negative regulators
- NK cells are classified as part of the innate immune system because of their invariant receptors
- NK cells can be activated in response to interferones or macrophage derived cytokines.
- Killing activity is increased 20-100 fold when NK cells are exposed to IFNa, IFNb or IL-12
- Activated NK cells serve to contain virus infections while the adaptive immune response is generating antigen specific cytotoxic T cells and neutralizing antibodies.
- Patients deficient in these cells susceptible to the early phase of the herpes virus
How do NK cells detect infected vs healthy cells?
- Monitor ‘altered self’ in the form of ‘missing self’ and ‘induced self’
- NK receptors recognizing self-MHC class I are considered the predominant mechanism
- Surveillance monitoring of MHC-I expression provides inhibitor signaling and prevents killing of healthy self cells
- Loss of MHC-I expression (missing self) allows activating NK receptors to promote NK killing
- The outcome of encounter of a target expressing both MHC-I and ligands for activating NK receptors is determined by the net balance of signals received (ie, more inhibitory, or more activating?).
- MHC molecules do not just give a ‘self/non-self/ signal – it gives an indication of how healthy the cell is
- MHC present virus peptides to the NK cell → a number of viruses have targeted this, and decrease MHC expression to try and hide from the immune system.
- Downregulated MHC-1 is also a sign of stressed cells, such as tunour cells.
What are the two types of NK receptor?
- Killer lectin-like receptors (KLRs) → encoded on chromosome 12 in the natural killer gene complex
- Killer immunoglobulin-like receptors (KIRs) → encoded on chromosome 19 in the leukocyte receptor complex
- NK cells express diverse combinations of receptors within a single individual.
- One complicating factor in NK cell activation is that both the activating and inhibitor receptors occur in each family → activating receptors arose from gene duplication and conversion from inhibitor receptors, so their extracellular domains are highly homolgous.
What is the structure of activating NK receptors?
• Activating receptors associate with accessory molecules such as DAP12 which contain
− ITAMs (immunoreceptor tyrosine based activatory motifs) → ligand engagement leads to phosphorylation and signaling leading to NK activation
What is the structure of inhibitory NK receptors?
• Inhibitory receptors have longer cytoplasmic tails which contain:
− ITIMs (immunoreceptor tyrosine based inhibitory motifs) → ligand engagement leads to phosphorylation, which recruits phosphatases that suppress signals that would otherwise lead to activation.
How does the KLR family CD94/NKG2A receptor work?
- Indirectly monitors MHC-1 by interacting with HLA-E
- HLA-E is a non-classical MHC-1 molecule. Has limited polymorphism and presents invariant peptides processed from the leader sequences of HLA-A, B and C
- Peptide is required for its stable assembly and surface expression
- The amount of HLA-E on a cell surface is a measure of the total amount of HLA A, B and C being made, and an indicator of the health status of the cell
- CD94/NKG2A is expressed on most NK cells
- Interaction of this receptor with HLA-E suppresses killing of healthy cells
- → Monitoring in this way works well however, it is insentive to loss of a particular HLA, which can occur in viral infection. It tells you the total HLA, but nothing more specific.
- KIRs however CAN recognize polymorphisms on separate HLAs. Because of this, CD94/NKG2A work together.
Why is NK education needed?
- Although a sizeable minority of NK cells lack recognition of MHC-I, these cells are hyporesponsive rather than hyperfunction
- How are these cells regulated so as to not overtly cause autoimmunity?
- NK education is quantitative, and involves balancing signals for inhibition and activation
- NK cells with a greater number of inhibitory receptors recognizing MHC respond betwee than NK cells with less recognition.
- Individuals with more diverse repertoires of MHC-I have a greater potential for their NK cells to be more responsive.
- In this sense, there are gradations of educated NK cells within indivudals, with some more educated than others.
- NK cells acquire MHC binding inhibitory receptors late in development in the bone marrow.
- CD94/NKG2A expressed first, then the KIR family inhibitor receptors
- Competing bi-directional KIR promoters generates variegated expression of KIRs. So you get cells with different representations of receptors.
- To be effective in detecting changes in MHC-1, NK cells must express at least one inhibitor receptor.
Describe the arming model of NK education
− Positive signals received by an immature NK cell through interactions with MHC-I required to induced functional maturation (arming)
− MHC-I interacts with an inhibitory receptor
− In the arming model, this is proposed to function positively, resulting in NK maturation
− NK cells that fail to interact with MHC-I remain unarmed and unresponseive, and therefore do not attack MHC-I deficient cells.
Describe the disarming model of NK education
− NK cells express a variety of stimulatory and inhibitory receptors
− Only those cell in which inhibitory and stimulatory signals are balanced are allowed to acquire responsiveness
− NK cells that receive unopposed positive signals (eg, in an MHC-I deficient host) → disarmed, rendered unresponsive
- Immature NK cell in the marrow expressed an inhibitor KIR that recognizes self HLA-C on the bone marrow cell → SHP-1 phosphatase activated. No NK cell activation
- Signalling is now present from activating recetors → now a dynamic balance between activating and inhibitory receptors. NK cell is educated
- The educated NK cell leaves the bone marrow and enters the circulation → balance between activating and inhibitor receptors maintained – tickover.
- NK cell encounters a healthy cell and the inhibitor receptor is engaged → inhibitor signals dominate, the NK cell moves on
- NK cell encounters an unhealthy cell and the activating receptors are engaged → activating signals override inhibitor signals, NK cell is activated
What is the role of NKG2D?
Direct sensing of infected cells, independent of MHC.
• NKG2D has a specialized role in activating NK cells:
− NKG2D does not bind HLA-E → its ligands are proteins distantly related to the MHC molecule, but have a completely different function, being produced in response to stress.
− In humans, these ligands are MIC-A and B and the RAET1 protein family.
− These ligands are expressed in response to cellular or metabolic stress, and so are upregulated on infected cells
− NKG2D is expressed on NK cells, y:d T cells and CD8 T cells → recognition of NKG2D ligands by these cells gives potent co-stimulation that enhances their effector functions.
− NKG2D also differs from other activating receptors in the signaling pathway it engages within the cell.
− Other activating receptors associated intracellularly with proteins such as the CD3-zeta chain, the Fc receptor y chain and DAP12 (which all contain ITAMs)
− In contrast, NKG2D binds to DAP10 which does not contain an ITAM, and instead activates the intracellular PI3kinase → enhances the survival and overall activity of the NK cell
What is the role of the FcRIIIA?
Direct sensing of infected cells independent of MHC.
− Strong activating receptor ubiquitously expressed on circulating NK cells
− Low affinity FC receptor, and signals strongly for ADCC
− IgG bound to antigen on the target cell indicates danger and triggers ADCC
Discuss NK cell memory as reviewed by Paust and VonAndrien, 2011.
- Measured with mouse models using two different models of challenge – hapten-induced contact hypersensitivy (CHS) and viral infection.
- Unknown whether NK cells aquire memory via other challenges, or if memory exists in other species.
Evidence of NK mediated acquired immunity:
• SCID mice (lacking B and T cells) acquire sensitization dependent antigen specific memory to contact sensitisers
• Each of these haptens elicits a vigorous CHS response that features the 3 hallmarks of adaptive immunity → the response is learned (requires at least one episode of sensitization), sensitizing antigens are remembered (CHS responses can be elicited as late as 4 months after sensitization) and the ensuing memory is antigen specific (CHS elicited only when identical haptens used for challenge).
• NK cells required and sufficient to elicit CHS responses (antibody mediated depletion of these cells abolishes the response)
• Hapten-specific memory is conferred to naïve mice by adoptive transfer of NK cells from sensitized donors (when the same hapten is used for challenge)
Induction of memory NK cells:
• Ex vivo exposure to activating cytokines IL12 and IL18 elicits a form of memory whereby the primed cells mediated enhanced IFN-y responses after re-stimulation by cytokines
• Thus, NK cells can quire certain memory-like properties even without exposure to a specific antigen, similar to the cytokine driven ‘bystander’ response of CD8 cells.
Maintenance of memory NK cells:
• Once NK cells have been primed in lymph nodes, they migrate to the liver where they persist as long-lived memory cells.
• A potential trafficking molecule is CXCR6, the receptor for CXCL16 (expressed in liver sinusoids)
• This CXCR6-CXCL16 axis may have a key role in the homeostasis and functional regulation of memory NK cells
− Only CXCR6+ NK cells carry transferable memory of haptens and viral antigens
− Blockage of CXCR6 results in loss of NK cell memory
Immune surveillance by memory NK cells:
• Memory NK cells rapidly respond to peripheral hapten or viral challenge for at least 3-4 months after priming.
• The hapten specifc memory population is probably in constant exchance with the blood to patrol peripheral tissues for recall antigen.
Describe y:d T cells
• Resides within the epithalia (eg, skin)
• Divided into two subsets
− one found in lymphoid tissues of all vertebrates. Have highly diversified T cell receptors
− intraepithelial y:d T cells occur variably in different vertebrates, and commonly display receptors of very limited diversity (particularly in the skin, where the y:d T cells are virtually identical)
− On the basis of this limited diversity, it has been suggested that intraepithelial y:d T cells may recognize ligands derived from the epithelium which they reside, but are expressed only when a cell has become infected.
− Candidate ligands are heat shock proteins, MHC-Ib molecules and unorthodox nucleotides
• Do not generally recognize antigen as peptides presented by MHC – instead, seem to recognize antigen directly → thus could recognize and respond rapidly to molecules expressed on many different cell types.
Human Vy9 Vd2 T cells:
• Predominate in blood
• Recognise phosphoantigen presented on cell surfaces.
• these are small phosphorylated metabolites arising from altered tumour metabolism, intracellular infection or other forms of stress
• they are presented by butyrophilin 3A1
Human VyVd1 T cells:
• Recognise glycolipid antigen presented by an MHC-I like molecule called CD1
• These present and bind lipids of self and microbial origin.
Describe B1 cells
- In many ways analogous to intraepithelial y:d T cells → arise early in embryonic development, use a distinct and limited set of gene rearrangements to make their receptors, self-renewing in tissues outside lymphoid organs
- Predominant in peritoneal and plueral cavities
- Seem to make antibody mainly in response to polysaccharide antigen
- Can produce IgM without help from T cells
- First appear within 48 hours, when T cells cannot be involved
- The lack of an antigen-specific interaction with T cells might explain why memory is not generated as a result of the B1 cell response
- Repeated exposure to the same antigen generates similar, or decreased responses with exposure (innate like)
What is interesting about B1 and y:d T cells from an evolutionary perspective?
from an evolutionary perspective, it is interesting that y:d T cells defend body surfaces and B1 cells defend the body cavity. Possible that the two cell types respresent a transitional phase in the evolution of the adaptive response, guarding the two main components of primitive organisms. It is not yet clear whether they are critical to host defense, or are an evolutionary relic.
