The Immunological Synapse Flashcards
What are the 3 signals for T cell priming?
- Signal 1 → Activation signal. not sufficient alone
• Antigen specific activation signal → TCR/MHC:pep/co-receptor (CD4/8) - Signal 2 → Survival signal
• Co-stimulus: co-activation and co-inhibition
• Various potential contributors
• When the innate system interacts with pathogens, it upregulates the expression of co-stimulatory molecules
• eg) B7 / CD28 - Signal 3 → Differentiation signal
• Cytokines
What are examples of co-stimulatory molecules that promote T cell activation?
• CD28 expressed by naïve T cells interacts with CD80/CD86 which are increased on the APC when they have been triggered by a PRR
• On activation, you increase co-stimulation further by increasing ICOS, OX40 and 4-1BB on the T cell and their ligands.
• Co stimulatory signals induce other co-stimulatory molecules, further reinforcing the activation signal → co-stimulatory induction loop.
− eg) When CD28 and CD80 engage, this leads to upregulation of ICOS and CD40L
− These then lead to reinforcement of each others expression
What are the co-stimulatory interactions that inhibit T cell activation?
• Induced during T cell activation to regulate the process
• CTLA-4 is at low levels on the T cell → gets increased as the T cell response proceeds
• CTLA-4 binds CD80 and 86. but more strongly than CD28 → competes
• CTLA-4 is also highly expressed on Tregs
• Another receptor pair are PD-1/PDL → quite high profile over the last few years
− Cancers recruit too much tolerance
− Give patients cancer-targeting T cells as an immunotherapy, but when they reach the tumour, they reach an immunosuppressive environment
− Antibodies against PD1 interefere with the inhibitor interactions so they allow the tumour specific T cells to kill the tumour.
How are DCs activated to prime T cells?
• DCs, macrophages and B cells are main APCs
• Macrophages tend to stay in the tissue
• B cells interact with T cells that are already primed – the T cells give the B cells help
• DCs are the ones required for initial naïve T cell priming
− DCs in the periphery are highly specialized for antigen uptake, but not good at antigen presentation and T cell activation
− However, stress or pathogen encounter causes DC activation:
➢ upregulate MHC and co-stimulatory molecules → now very good at antigen presenting
➢ Decrease expression of CCR1 and 5 (these keep them in the tissue)
➢ Increase expression of CCR7 (this targets them to the lymph node)
− Activated DCs migrate to the LN → now presenting antigen and expressing co-stimulatory molecules and producing cytokines ready for T cell priming
What are the phases of T cell activation in the lymph node?
- T cells enter the LN through HECs and tend to reside in the para-cortex
- Spend 24 hours in the LN → extends to 3-4 days if antigen detected
- DCs enter from the lymph and place themselves around the HEVs in the paracortex
T cell activation by DC in the lymph node:
• In vivo intravital microscopy has allowed us to understand this process
• It was found that rapid, random movement of T cells change during antigen recognition
− T cells in the LNs migrate chaotically at high velocity
− 500-5000 T cells scan each DC every hour
− Antigen recognition is a ‘T cell stop signal’
There is a 3 phase model:
1. First few hours of multiple short contacts of
What are the 4 regions of a T cell immunological synapse?
➢ The iSMAC → devoid of polymerized actin, and the TCR (that is now without signaling molecules) can be internalized
➢ cSMAC → TCR along with co-stimulatory signaling molecules
➢ pSMAC → integrins
➢ dSMAC → CD45 (exclusion zone)
What is the current view of T cell synapse formation? (Kinetic segregation)
The kinetic segregation model of T cell activation:
• As the membranes come together, they push out the larger molecules
• One of these molecules is CD45 phosphatase → if you had T cell molecules trying to signal, the phosphatase would turn this off. Moving them away facilitates signaling.
Model:
• Key molecules on the resting T cell are constantly colliding, providing opportunities for enzymatic reactions.
• On resting T cells, TCRs are randomly phosphorylated by Lck but just as randomly, these are dephosphorylated by CD45 → there is limited net phosphorylation of the TCR in the resting cell
• Cell-cell contact between the APC and the TCR mediated by LFA/ICAM interactions leads to the formation of close-contact zones and the exclusion of membrane tyrosine phosphatases
• CD45 exclusion extends the half-life of phosphorylated species in the close-contact zone
• In the close-contact zone:
➢ A TCR may fail to see the right peptide, and diffuses out of the close contact zone
➢ Another TCR is held passively in the close contact zone by the right MHC-peptide complex, long enough for secondary signaling events to occur (Eg, recruitment of CD4-Lck) which amplifies the signal
➢ A further TCR may be phosphorylated in the close contact zone by free Lck, giving co-receptor independent signaling
• Locally and stochastically, the membranes will separate, allowing CD45 to reset the TCRs and Lcks to their ground states – otherwise the TCR will be internalized
Current view of T cell synapse formation:
- In a resting cell, TCRs seem to exist in nanoclusters of 2-20 TCRs
- Once you get contact with an APC, you start to get microclusters which include co-stimulatory molecules
- Microcluster formation induces actin polymerization, causing membrane spreading and increasing the interaction of the TCR with the MHC:peptide complex on the APC surface
- The membrane will spread to a point of maximal expansion – once you get to this point, the actin starts to contract
- The contraction causes an inward movement of TCR microclusters, with loss of the signaling proteins. As they move in, new microclusters are formed at the periphery → gives a continual flow of microclusters scanning for TCR/peptide:MHC complex interaction
What is the role of TCR enriched microvesicles in the immunological synapse?
Choudhury et al, 2014
Study shows polarized release of TCR enriched microvesicles across the T:B cell synapse to the B cell:
• Central accumulation of TCR, adjacent to secretory domain, both surrounded by an adhesive ring
• TCR located on extracellular microvesicles that bud at the cSMAC centre into the synaptic central cavity
• Retain pMHC binding ability
• Suggested this may play a role in sustaining proliferation of B cells as they separate from the helper T cell
• HIV gag proteins hijacks this process by replacing TCR → budding of virus-like particles across synapses to transport between cells.
Describe the CTL synsape
- cSMAC → TCR and co-stimulatory molecules
- Secretory domain with lytic granules
- pSMAC → integrins – this layer is very tight in a CTL synapse, prevents the lytic granules acting on other cells. They are concentrated at the synapse and only act on the target cell.
- dSMAC → CD45
- In normal T cell, when you get peptide:TCR attachment, the centrosome is located somewhere towards the back
- However, in a CTL, actin rearrangements happen, so that centrosome relocates just below the synapse
- The granules follow these filaments to give targeted release at the point of contact.
- CTLs are ‘serial killers’ – can get multiple kisses of death
- CTL can relocate the centrosome to act on different targets nearby, while still maintaining contacts with the first.
Describe the NK cell synapse
- NK cell either intentionally or accidentally encounters its target cell.
- Integrins LFA-1 and MAC1 (expressed by NK cells) cluster rapidly at the NK-cell synapse following initiation, and mediate firm adhesion.
- initial stage in the commitment to lytic-synapse formation is actin reorganization → formation of F actin networks from pool of monomeric G actin.
- Receptor clustering in the NK-cell lytic synapse is important for the generation of robust signaling
− In the T cell synapse, signaling stems from microclusters of TCR molecules that have moved from the pSMAC to the cSMAC
− Functional microclusters have been identified in NK cells, but these have only been investigated in the SMIC - Polarisation of lytic granules to the synapse beings with movement of granules along the MTOC. Lytic granules aggregate around the MTOC, which then polarizes towards the synapse. Signals required for this include ERK, VAV1 and PYK2
- Actin formation is required for lytic granule polarization, but a discrete region of the actin network needs to be disassembled to create a channel through which the granules gain access to the plasma membrane. Large channels are often observed in the cSMAC of NK-cell lytic synapses.
- After lytic granule release, there is down-modulation of accumulated activating receptors followed by NK cell detatchment from the target cell.
- The cleft formed at the lytic synapse between the NK and target cells creates a protected pocket that is present for 45 minutes after conjugation → this may serve to increase the concentration of lytic granules at the target cell, and protect neighbouring cells.
What diseases effect the NK cell synapse
- LAD → defect in the CD18 component of the leukocyte integrin. NK cells form patients with LAD do not adhere to their target cell, resulting in defective cytotoxicity. Leads to susceptibility to disease and increased leukocytes in the blood because they cant extravasate into tissues.
- Wiskott-Aldrich Syndrome → WASP deficiency leads to haematopoeitic cell specific actin reorganization defect. NK cells have decreased cytoxicity. Patients are susceptible to herpes virus
- Chediak-Higashi syndrome → affect normal formation of lytic granules, leading to ‘giant’ granules. Associated with albinism, caused by aberrant function of melanosomes (equivalent of lytic granules) which pigment the skin.
Describe the B cell synapse
Movement of B cell in LN:
• Naïve B cell encounters antigen in the peripheral lymphoid tissues
• B cells come in through the HEV → go to the B cell region of the cortex
− B cells migrate to the follicle
− After migrating through the FDC network, they travel through the T cell zone before exiting
• Antigen in the lymph comes in through the subcapsular sinus in 3 ways:
− Opsonised antigen
− Small soluble or proteolysed antigen
− Large, cell-borne antigen
• B cells polarize to the immune synapse on antigen stimulation:
− eg) BCR and antigen on FDC:
− MTOC and golgi apparatus in the BCR repositions at the immune synapse
− Lysosomes recruited to the antigen contact site → acidification and release of proteases into the cleft for efficient cleavage and extraction of immobilized antigen from the FDC
− A myosin mediated pulling force triggers invagination and endocytosis of antigen-containing membranes (rips the antigen from FDC – trogocytosis) → contributes to affinity discrimination. Only those with high affinity can do this, the rest will just fall off
− Endosomes fuse with lysosomes → processing and presentation of acquired antigen on MHC-II.
