T Cell Activation and Control

hello everyone I'm John Looney from the University of Rochester I'm a Rheumatologist and a clinical immunologist and I'm delighted to be here today at the Cleveland Clinic to talk to you about T cell activation and control this is part of a series that hopefully just completed chapter 2 on with William Rigby talking about T cell ontogeny the learning objections for this particular section will be to be able to describe the role of T cells in other antigen presenting cells in T cell activation and to identify and define signal one in signal two in terms of T cell activation explain the importance of Co inhibitory pathways for preventing adverse effects from immunological activation to describe immunological exhaustion and how this may prevent control of chronic infections and malignancy and as a sidelight to explain why we actually have cd4 and cd8 t-cells many of the figures from this are from these books from carlin science particularly chapter 9 in the Janeway text so when you get infected with a virus as shown here in this diagram you can see that the virus titer climbs rapidly for the first few days but then plateaus and after about five to six days begins to fall back down what prevents the viral titer from going up and and and allows it to Plateau in those early few days or is the innate immune system where things like interferon alpha and beta are produced to slow down viral growth and replication and NK cells are recruited to the site and K cells or type of lymphocyte that's part of the innate immune system and can be cytotoxic for virally infected cells but what's really required to clear the viral infection is the activation of the adaptive immune system where you have T cells and eliminating those cells that are infected with virus and this is when you have drop in the viral titer and elimination of the virus so we want to talk today about the decision to activate t-cells and looking through the some images for an analogy with this there here's two images which are probably not very representative of the way t-cell activation occurs it's not it's not a random event such as that the flip of the coin depicted on the left and it's not the t-cell you're represented by Hamlet thinking about whether he's going to respond or not it's really more like this walt disney picture you can see here there's a very handsome t-cell being activated by an antigen presenting cell and several things are required you have to have close contact from these two cell types you also have to have specific recognition or signal one and you have to have a positive signal given which would be signal two and we're going to talk about the mechanisms of these these these two signals in the first part of this talk in the second part of the talk we're going to talk about controlling this activation obviously if our t-cell gets too activated you have to have a mechanism for putting on a break and that will be discussed in the second half of this talk here are some actual antigen presenting cells and as you can see they are quite beautiful I've shown in the scanning electron micrographs at the bottom there are dendritic cells there are macrophages and there are b-cells all these cells our professional antigen presenting cells and can activate t-cells the dendritic cells are particularly important and activating naive t-cells whereas both the macrophages and the b-cells are very good at activating memory b-cells once they've initially been activated by the dendritic cells the dendritic cells are part of the innate immune system they are located in these sites where you are going to come in contact with microorganisms such as the skin the respiratory mucosa and the GI mucosa and they're sentinels they're where they're they're an arresting state waiting to be to interact with something that enters the system once they are engaged by a microorganism of some sort that become activated they migrate through the lymph to the lymphatics and in the lymphatics they interact with t-cells and b-cells and cause naive T and B cells to become activated the activated cells and migrate through the lymphatics back to the bloodstream and then can home to the areas of inflammation where the microorganisms were originally entered you enter the system just showing here this is a blow-up of a of a lymph node you can see the dendritic cell coming into the lymph node from the through the lymphatics at the top and it migrates into the para cortical areas shown here in the light blue there it's joined by the T cells and the B cells which migrate into the lymph node via the high endothelial venules that are shown there as a chi HEV in and what happens in the lymph node is that the you have a constant progression of lymphocytes coming through the lymph node and sampling what's going on there and if they interact with a dendritic cell that is activated in a certain way and carry the antigen that they're specific for then they are retained in the lymph node where they become activated and proliferate and then after that activation in proliferation they migrated to the lymphatics and then to the bloodstream so there are two signals that need to come from the dendritic cell signal one and signal to signal one is this specific regulation and signal to the co stimulatory or positive signal this is a picture of the sort of molecular biology that's involved in signal one in signal to signal one is interaction between the T cell antigen receptor shown there in blue on the T cell that's at the bottom of the screen with antigen and MHC expressed on the antigen antigen presenting cells shown in yellow at the top of the screen there are also additional Co receptor molecules such as cd4 cd8 that are involved in this interaction between the T cell antigen receptor and the MHC the second signal involves a cd28 that's expressed on wresting t cells and b 7 1 and b 7 2 that are on the antigen presenting cells v 7 1 in b7 two aren't there and resting antigen presenting cell but as the antigen presenting cells become activated they express these ligands for cd28 and then you can initiate the second signal or co stimulatory signal so in the periphery you have dendritic cells with a lot of receptors that are able to recognize microbes these are the microbes have molecules on them called pathogen associated molecular patterns that tamps there on the right and they can interact with a number of different receptors on antigen presenting cells including receptors that recognize things like bacterial peptides or endotoxin or DNA or RNA so in the periphery the dendritic cells comes into interacts with these pants on the microbes takes up the microbes I get stimulated by the Pam's a migrates through the lymphatics to the lymph nodes and in the lymph nodes now they're quite a different cell they are expressing a number of adhesion molecules such as I cam 1 and I cam - they have b7 - login for cd28 on their surface and they're also secreting cytokines that are correct t-cells and then the t-cells interact with the dendritic cells and become activated this is just one of the first papers that looked at the power of this dual signalling where these are lymphocytes t-cells where that were stimulated either by just anti cd3 alone which activates the binds to it activates the t-cell antigen receptor or they are stimulated by anti cd28 that in that activates the co-stimulatory molecules signal - so you can see with signal one alone or signal two alone you don't get very much but when you have both of those signals you get a tremendous activation of t-cells in this case they're measuring proliferation but you could also look at il-2 production or expression of the il-2 receptor on on the t-cells so what happens if you only get one of these signals if you only get signal 2 which is a co stimulatory signal in other words you're a t-cell that is come into the lymph node and it's interacting with some activated dendritic cells but they don't have the antigen - which are specific nothing much happens you have interacted with the co-stimulatory molecules but since you're not having signal 1 this isn't this isn't really sensed as a positive signal conversely you can also just get signal 1 you can have a non activated dendritic cell carrying antigen particularly self antigen come into the lymph node where it is not expressing b7 1 or b7 2 on its surface so you don't get signal 2 but there there is antigen in the MHC and so you do have the ability of t-cells to recognize and get that signal those t-cells that only get signal 1 without signal 2 may become deleted or energic they certainly don't become activated and so this in this way the dendritic cell is really able to control what's going on with a t-cell inducing both activation and promoting peripheral tolerance so this co-stimulation is really critical in a number of different clinical situations shown in white are the things that we want to happen in other words if you if you have antigen and co-stimulation you get protective immunity and you get tumor immunity and if you have no Co stimulation then you don't get the induction of allergy to harmless environmental antigens you get acceptance of a of a grafted organ and you don't get autoimmunity so obviously you can control a lot of different immunological processes by controlling the co-stimulation if you if you promote co-stimulation then you can enhance protective immunity or tumor immunity if you block Co stimulation you may be able to treat allergic disease graft rejection and autoimmunity this is a little bit of a segue I was going to explain why we have cd4 and cd8 cells and the reason we have cd4 and cd8 cells is because antigen can be presented either by MHC class 2 which would be things like HLA dr or it can be presented by MHC class 1 which would be things like HLA a or B and what what this what you have with cd4 is that it forms a co-receptor with a t-cell antigen receptor in other words both the t-cell antigen receptor and the cd4 bind to the same MHC class 2 molecule the t-cell antigen receptor binds to the specific peptide in the groove of the MHC and the cd4 binds to a in variant portion of the MHC class 2 so you get both the binding affinity of the t-cell antigen receptor and the binding affinity of class of cd4 and also both of those molecules give up can give a positive signals to the cd4 T cell in contrast cd8 is the molecule that binds to MHC class 1 and so when you have a t-cell that is cd8 positive it's much more it has a much easier time interacting with antigen in the context of MHC class 1 there are big differences in how MHC class 1 and MHC class 2 get loaded with antigen MHC class 2 is loaded with antigen from the outside of the cell if you have an external particle that is phagocytose by a macrophage it will enter into a phagocytic vacuole with MHC class 2 and be digested and then the peptides will be actually loaded on to MHC class 2 and that will be routed back to the surface of the cell well then we're then cd4 T cells can recognize that antigen in the context of MHC class 2 similarly on b-cells they can take up antigen via the specific antigen on their surface so anti-tetanus T cells can be cells can take up tetanus antigen by their surface immunoglobulin and then that gets endocytosed and presented again with MHC class 2 that gets routed to the surface of the cell and then the cd4 cells can recognize the antigen in the context of MHC class 2 with things like viruses which are in the cytosol of the cells MHC class 1 is the critical molecule you have a whole sophisticated machinery in in the cytosol of cells that's able to degrade peptides this is the proteasome it degrades peptides into small grade proteins into small peptides which are then transported into the endoplasmic reticulum and there they can be associated with MHC class 1 almost all cells Express MHC class 1 except for neurons and with recites so MHC class 1 presentation is something that all epithelial cells for example are able to do so when you get infected with influenza virus the epithelial cells can process antigen from their cytosol and express it on their surface and then be recognized by cd8 cytotoxic T cells where the infected cells can then be cleared so the immune an immune response is very much like a time where you and this is these are the hopewell rocks up in Nova Scotia that have one of the largest tides in the world on the left you can see low tide where there's a lot of people standing around the bottom of the rocks at high tide the water goes up about 25 feet and would have completely inundated those people that they remain there and then again the tide goes out and and again you can the people come out and you can and you can walk around the rocks so with an immune response you have something very analogous to this you have to build up your immune response to a crescendo you also have to make sure it doesn't have continue to go up that that in fact plateaus and then you have to get it to recede so that you don't continue to have chronic inflammation and have damage to tissue as a result of the adaptive immune response some of the adverse effects of over stimulation are shown here things like cerebral malaria or fulminant hepatitis B or macrophage activation syndrome or the reversal reaction you see with lepromatous leprosy when it gets treated with drugs all of these are examples of over stimulation of the immune system and and this really needs to be controlled so one mechanism the immune system is controlled is by having a co inhibitory receptor and in fact we have many of them that down regulates the immune response of t-cells so here's this diagram is showing ctla4 which is a molecule on activated t-cells that can bind to b7 one and b7 two and then prevent the interaction of those those ligands with cd28 which ordinarily gives you a the second signal for co-stimulation here's cd28 and b7 one shown on the left and the ctla-4 and b7 one shown on the right and what you can see is that the ctla-4 has a distinct advantage it has the capacity to bind multiple b7 ones and form these clusters with a high ability and so it has it will out-compete cd28 and shut off the immune response if you have a resting T cell shown here in the top it already expressed the cd28 and and it also expresses the T cell antigen receptor if you have a resting antigen presenting cell you only get signal 1 through the T cell antigen receptor but if you activate the antigen presenting cell then b7 1 is expressed on the surface and so you can get not only signal one but signal to the T cells become activated they proliferate they have their effector functions and but then you need to one of the other things that happens when the T cells become activated as they begin to express ctla-4 and this now can lead to the inhibition of C cell activation so that the response plateaus and then disappears this figure shows another coast inhibitory molecule called pd-1 which binds to PD one ligand again the PD one is on the T cells at the bottom and the PD one Ligon is at the top and this gives an inhibitory signal that doesn't compete for ligand with the cd28 but in fact interrupts the signaling of cd28 in the cytoplasm of the T cells one of the ways people have figured out how important some of these inhibitory receptors are is by knocking them out in mice when you knock out ctla4 and mice you get a mouse that only lives a few weeks and then it dies because of massive proliferation of lymphoid cells and invade organism and invasion of organs shown in this these photomicrographs as are the pancreas on the left side is a wild-type mouse and you can see a normal pancreas with normal islets on the right hand side is a the ctla4 knockout mice and the pancreas has been completely destroyed by this infiltration of activated t-cells another situation where these inhibitory receptors are important is in malaria in a mouse model of malaria it's been shown that if you give antibodies against ctla-4 and pd1 in other words if you block the KO inhibitory receptors you can induce cerebral malaria so shown on the left is the infiltration of cd8 t-cells which occurs when you block the KO inhibitory molecule ctla-4 or pd-1 this this is what causes the damage is not actually the infection it's the immune response another aspect of KO in addition is that you can develop immunological exhaustion this occurs with when you lose cd8 t-cell function this is seen in a number of different chronic infections and also cancer some of these are shown at the bottom here HIV hepatitis C virus or melanoma are all situations where you can have exhaustion of cd8 T cells so they no longer clear virus or kill the cancerous cells cd8 t-cell exhaustion correlates with having high levels of antigen with having a chronic long duration infection and also correlates with the loss of cd4 helped by blocking inhibitory receptors you can rescue cd8 t-cells from this exhaustion shown here in this diagram are sort of what happens over time and with a chronic infection that's shown in the and the diagonal set of cells were labeled inflammation at the top is what happens when you have a short-term infection an acute infection where you have the effectors generated they produce lots of interferon and they're cytotoxic and they can proliferate and they don't apoptosis and then of course they rapidly disappear once the infection is cleared if the if the infection continues then you have T cells that progressively lose effect or function they become in it unable to lyse other cells they no longer produce il-2 or interferon and they develop they express on their surface a number of inhibitory receptors such as PD one and become much more susceptible to apoptosis so what people are now beginning to look at is whether or not you can give antibodies against PD one and other Co inhibitory receptors and re-establish functions so shown in the middle is sort of the spectrum of cd8 T cells in a chronic infection where you have some that are functioning fairly well and others that are really not functioning at all and Express very high levels of pd-1 if you give an antibody against PD one what you find is you rescue the cells with an intermediate level of expression of the PD one and their surface so in other words cells that are marching down this this this pathway of exhaustion but haven't really hit the bottom can can be rescued with just the anti PD one antibody and and in animal models they're now showing that this can help with chronic infections such as simian immunodeficiency virus but it's hoped in the future is that you can combine anti inhibitory molecules against PD one or against ctla4 with additional stimulators to rescue even a higher number of these cells and treat not only chronic infections such as hepatitis C and HIV but also begin to treat some of the tumors that might be susceptible to immune stimulation so in conclusion co-stimulation is critical for determining whether or not an immune response is induced and the key molecule for Co stimulation is cd28 interacting with b7 1 and b7 2 on the surface of cells if you have inadequate co-stimulation you can have weakened host defenses leading to infection or cancer if you have inappropriate co-stimulation you can have allergies autoimmunity and graft rejection colon abyssion is crucial for putting the brakes on immune responses ctla-4 and pd1 or some of the critical molecules involved in Co inhibition if you have inadequate Co inhibition you can develop autoimmunity or Auto inflammatory diseases if you have inappropriate Co inhibition you can also develop immunological exhaustion in the setting of chronic infection and cancers there are now approved therapies that inhibit co-stimulation one of these ctla-4 IG and this has been approved for both autoimmune diseases and transplantation there are also therapies that inhibit Co in abyssion and these have been approved for malignancies namely anti ctla-4 and they're now being tested in some chronic viral infections given the large number of molecules involved in Co stimulation and Co inhibition it's really very likely that we're going to be seeing a lot of therapies that are targeting these pathways and these will be used not only in autoimmune diseases but also an infection and oncology so the this concludes our talk it's really been a delight to be here at the Cleveland Clinic today thank you very much for your attention

Channel: ClevelandClinicCME

Views: 273,113

Rating: 4.9037237 out of 5

Keywords: T cell, T cell activation, T cell control, T cell biology, T cell antigen recognition, T cell braking, CD4 T cells, CD8 T cells, basic immunology, clinical immunology, immunology, The Bench, Free online CME, Cleveland Clinic CME, John Looney MD

Id: GXVLdbkRkhw

Length: 26min 35sec (1595 seconds)

Published: Tue Feb 12 2013