Session 9: The Immunology of Organ Transplantation
Transcript of Part 1: Introduction to Transplantation: The Immune Response to Allo- and Xenotransplantation
00:00:14.19 Hi, I'm Megan Sykes. 00:00:16.03 I'm a professor at Columbia University, and I'm the Director of the Columbia Center 00:00:21.10 for Translational Immunology. 00:00:23.17 Today's lecture is an introduction to the field of transplantation, particularly transplantation immunology. 00:00:34.16 Transplantation is inseparable from immunology, because organ transplant rejection 00:00:41.02 and graft-versus-host disease are the result of immune responses caused by genetic differences from... 00:00:48.18 between the transplant donor and the recipient. 00:00:53.14 There are a few definitions I'd like you to understand before I go through my lecture, 00:00:58.20 because I will be using some of these words as I go... go forward. 00:01:02.28 Autologous means an organ or tissue from the self. 00:01:07.19 There are some autologous transplants that are done sometimes, 00:01:11.20 for example pancreatic islets in a person whose pancreas needs to be removed due to pancreatitis. 00:01:17.17 The person will get their islets back so they don't get diabetes. 00:01:23.03 Syngeneic is not from the self, but it's from the next best thing, a genetically identical individual 00:01:29.02 -- an identical twin. 00:01:31.20 In mice, we use syngeneic donors all the time because we have inbred mice -- 00:01:39.04 hundreds and thousands of identical twins. 00:01:42.00 In humans, we may have one identical twin in the rare case... in the rare person. 00:01:50.02 Allogeneic is the commonest kind of transplant, that means from another individual, 00:01:54.17 a genetically different individual, of the same species. 00:01:58.13 And that includes anybody who isn't an identical twin: a brother, a sister, a parent. 00:02:04.06 Those are all allogeneic to us. 00:02:06.28 And then finally, xenogeneic. 00:02:08.12 That'll be the topic of my third lecture. 00:02:10.26 A xenogeneic transplant is something that we don't do right now, but we hope to do 00:02:16.10 in the near future. 00:02:17.13 And that involves transplantation from a different species. 00:02:21.21 Okay, well, let's focus for a while on allogeneic organs and tissues. 00:02:27.14 Now, the immune responses that we have to think about when we put an organ 00:02:32.09 from one genetically different individual into another is what we call the host-versus-graft response, 00:02:39.11 the response of the recipient's immune system against the donor, foreign antigens. 00:02:45.25 And T lymphocytes are the main players in this response. 00:02:50.14 Antibodies also happen, and there are specific conditions when antibodies are a concern 00:02:56.00 that I'll talk about later, but many antibody responses are dependent on T cells. 00:03:01.20 And so the T cell can be thought of as a central player in rejection in most instances. 00:03:07.27 Now, in the second part of the slide, I mentioned allogeneic hematopoietic cells. 00:03:15.22 That's what many of you may have heard of as a bone marrow transplant in somebody 00:03:19.22 who has leukemia or lymphoma. 00:03:22.19 That's a commonest reason for doing a hematopoietic cell transplant. 00:03:27.13 It's not always bone marrow. 00:03:28.26 Often, we use mobilized peripheral blood stem cells for a hematopoietic cell transplant. 00:03:35.22 And there, we have to think about immune responses in both directions, because the donor is, 00:03:41.10 again, foreign to the recipient. 00:03:43.16 And again, we have to think about the recipient rejecting the donor. 00:03:47.15 And the main players there are T cells and natural killer cells, and also sometimes antibodies 00:03:53.18 can play a role. 00:03:55.25 But there's another direction we have to think about, because when we do a hematopoietic cell transplant 00:04:01.12 we do things to compromise the recipient's immunity very, very extensively. 00:04:06.10 And then the immune cells that come with the graft can actually attack the recipient. 00:04:12.14 So, it's a donor anti-recipient attack that we call graft-versus-host responses. 00:04:19.03 And T cells and, to some degree, natural killer cells play major roles there. 00:04:25.07 Then, we can think about xenogeneic organs, cells, and tissues. 00:04:30.02 And there, it's mainly the host barrier to the donor that we have to think about, 00:04:35.06 the host-versus-graft response. 00:04:36.09 And it's a very, very strong response involving T cells, natural killer cells, macrophages, 00:04:42.07 and antibodies, even antibodies that aren't T cell dependent. 00:04:45.24 A very strong immune response. 00:04:48.08 Now, there are many ways that we can try to avoid rejection. 00:04:54.02 And I'm speaking now, again, about allogeneic organs, which is the type of organ that 00:04:59.13 we currently transplant. 00:05:00.26 Rarely, we may have an identical twin as a donor. 00:05:04.03 And the very first transplant in humans was done with an identical twin who was able 00:05:09.19 to donate a kidney to his twin brother. 00:05:12.08 Secondly, most... since most people don't have identical twins, we use immunosuppressive drugs. 00:05:19.02 And that really is the standard of care for allowing a graft to be taken without rejection. 00:05:25.14 And finally, I'm going to speak in my second talk about what's sort of 00:05:30.02 the holy grail of transplantation, which is to induce tolerance, immune tolerance. 00:05:34.20 And there have been some recent trials that have actually achieved this in small groups 00:05:39.10 of patients. 00:05:41.05 Now, there's a particular genetic locus that we have to think about when we consider 00:05:47.01 any kind of a transplant. 00:05:48.09 And that's called the HLA, the human leukocyte antigens. 00:05:53.09 Because HLA antigens are extremely polymorphic -- meaning any two individuals who are unrelated 00:06:00.22 are very likely to have completely different HLA genes... and these HLA genes elicit 00:06:08.13 the strongest immune responses by T cells and antibodies. 00:06:13.20 And this slide, here, shows you the importance of HLA matching, that when you have 00:06:23.12 a closely HLA-matched organ -- this is kidney transplantation in a large series -- 00:06:31.01 you can see that the rate of graft loss... now, this is following these grafts over 20 years, that there are... 00:06:39.06 is a constant rate of graft loss over time. 00:06:41.20 Grafts... many grafts eventually get rejected. 00:06:44.21 That's called chronic rejection, which I'll come back to. 00:06:47.24 But these lines are a little bit divergent. 00:06:50.07 The ones that are most closely HLA-matched have the longest half-life, the slowest rate of chronic rejection 00:06:57.17 whereas those that are HLA-mismatched more extensively have 00:07:02.24 poorer outcomes over time. 00:07:04.07 So, HLA is important in this regard. 00:07:08.11 Now, what is the cause of rejection. 00:07:11.00 As I mentioned, T cells are central players in the process. 00:07:15.10 Now, acute rejection is... is something that we avoid. 00:07:20.03 We don't like to lose the organ -- ever -- completely. 00:07:23.24 And that's why we use these immunosuppressive drugs. 00:07:26.12 And if we have an episode of rejection, we can usually control it by 00:07:30.17 adding more immunosuppression temporarily. 00:07:33.16 But as I mentioned, these drugs must be taken chronically. 00:07:38.16 And... otherwise, the graft will almost certainly be rejected. 00:07:43.00 Now, I mentioned chronic rejection as a cause of late graft loss in the slide that 00:07:49.05 I just showed you. 00:07:50.14 Unfortunately, that is an ongoing problem in the field. 00:07:53.25 It hasn't been improved by the improvements that we've made in immunosuppressive therapies 00:08:00.14 in the last few decades. 00:08:02.04 So, that is one reason why tolerance is sort of the holy grail in transplantation, 00:08:07.27 because tolerance would mean the immune system treats the donor as self, and chronic rejection 00:08:13.10 wouldn't happen either. 00:08:15.10 Alright. 00:08:16.10 Now, I've talked about rejection as if it's one thing. 00:08:21.10 But in fact there are several different types of graft rejection. 00:08:25.17 One is caused by pre-existing antibodies. 00:08:30.14 And we call that hyperacute rejection. 00:08:32.15 I'll say more about that. 00:08:34.15 Another kind of antibody-mediated rejection is called acute vascular rejection. 00:08:39.23 And that's when antibodies come... get formed after the transplant, but can cause 00:08:45.00 a fairly rapid rejection once they get formed, whereas hyperacute rejection is caused by antibodies 00:08:51.06 that are there before the transplant. 00:08:53.23 Now, the cellular rejection is one that is mediated, caused, by T cells alone, 00:09:00.24 with no antibody role. 00:09:02.12 But as I mentioned, antibodies also depend on T cells for their formation, 00:09:07.05 so some of this acute vascular rejection is antibody... is T cell-dependent. 00:09:13.06 And then chronic rejection, also, is thought to be dependent on T cells, even though 00:09:18.19 there's fairly good evidence that antibodies also play a role. 00:09:22.24 Okay. 00:09:23.24 Now, the first and second types of rejection -- hyperacute and acute vascular -- 00:09:28.10 we really try very hard to avoid by selecting our donors and recipients, and in some cases doing 00:09:34.15 special preparation of our recipient, if we think they're at high risk of this. 00:09:39.00 So, let's talk a little bit about hyperacute rejection. 00:09:42.09 Hyperacute rejection can occur in a couple of circumstances. 00:09:46.26 One is if we have blood group-mismatched donors, for example if the recipient is blood group B 00:09:52.27 and the donor is blood group A. 00:09:56.01 The recipient has anti-A antibodies in the... in their serum. 00:09:59.28 And that antibody will immediately bind to the blood vessels of the kidney. 00:10:06.08 As soon as the graft is hooked up, it'll bind to the endothelial cells lining the blood vessels. 00:10:13.07 And what that does is it fixes complement, initiates a whole cascade of inflammatory events 00:10:19.22 that ultimately will activate the coagulation cascade and occlude the blood vessels. 00:10:26.08 So, that graft is lost very, very quickly. 00:10:29.19 And the same thing can happen in... even in a blood group-matched situation, but in which 00:10:35.22 the recipient has made antibodies against donor HLA antigens prior to the transplant. 00:10:43.04 And this can happen because of a prior pregnancy, a prior transplant, or due to blood transfusions 00:10:49.22 that the recipient may have had. 00:10:51.18 And it's exactly the same process. 00:10:53.24 The antibodies in the patient circulation bind to the endothelial cells and fix complement 00:11:01.09 and initiate that same coagulation cascade and occlude the blood vessels of the graft. 00:11:07.04 So, as I just said, it can... hyperacute rejection can occur in blood group-mismatched and presensitized 00:11:14.18 recipients. 00:11:16.25 And on the bottom, it says that hyperacute rejection has presented a barrier to xenotransplantation. 00:11:24.27 As I'll discuss in the xenotransplantation lecture, we've found a way of avoiding that. 00:11:31.09 But it was a major limitation to the field of xenotransplantation over the years. 00:11:36.15 Now, just to show you what hyperacute rejection looks like, this is an experimental study 00:11:42.02 where a rat heart was put into a mouse that had lots of natural antibody against that rat. 00:11:51.20 That's the one on the right, here. 00:11:53.07 And you can see that that heart looks black. 00:11:56.04 And that's because that occlusion of the blood vessels has happened in that very short period of time 00:12:01.22 since the graft was put in. 00:12:03.21 This is half an hour after the transplant. 00:12:06.10 Over here, you see a pink looking graft, and that's a much more normal looking graft. 00:12:12.09 And that's because this mouse that got the rat heart didn't have those high levels of 00:12:16.21 preformed antibody against the rat xenograft. 00:12:20.11 It's a xenograft because it's from one species to another. 00:12:25.07 Okay. 00:12:26.07 Now, acute cellular rejection is a major problem. 00:12:33.00 And it's a common cause of what we call rejection episodes. 00:12:37.25 And we think that this whole process, which... in which, again, the T cell is the central player, 00:12:44.05 gets initiated in part because of procedures that involve... are involved in 00:12:51.02 obtaining an organ from a donor, transplanting it to the recipient. 00:12:55.22 And there's a period where there's no blood flow to that graft, called ischemia. 00:13:01.00 And that ischemia can activate all sorts of components of the innate immune system. 00:13:07.13 And the innate immune system is very effective at activating T cells. 00:13:12.16 And it's actually T cells, as I mentioned, that are the real players in this graft rejection. 00:13:19.03 And the strongest rejections are directed against those HLA differences that I mentioned, 00:13:25.10 but also what's called minor histocompatibility antigens, which are basically peptides 00:13:31.03 that are presented by an HLA antigen, but they're peptides of proteins that have some polymorphism, 00:13:37.28 and may differ between the donor and recipient. 00:13:40.21 We have many such polymorphisms. 00:13:43.26 And so, even in the setting of HLA matching, there are many minor histocompatibility antigens 00:13:49.17 that can be presented and cause a T cell-mediated rejection. 00:13:54.15 And this is if you had a really bad T cell-mediated rejection that destroyed the graft, 00:13:59.27 this is what it would look like. 00:14:00.27 It's... it's quite swollen, and it's white. 00:14:03.04 And it's white because of all the leukocytes, the white cells, the lymphocytes, etc, 00:14:08.19 that are infiltrating the graft. 00:14:12.01 As I mentioned, it's a T cell-mediated process. 00:14:15.04 This stain here is an immunostain for... with anti-CD3. 00:14:18.19 And you can see there's a lot of those T cells that express CD3 in the graft. 00:14:25.03 And histologically, you can just see this gross infiltrate of cells with blue nuclei 00:14:30.28 and very little cytoplasm. 00:14:32.11 Those are largely lymphocytes. 00:14:35.02 Okay. 00:14:36.09 Well, what causes this T cell response? 00:14:41.00 There are two types of T cell responses. 00:14:44.10 One is called direct allorecognition, and the other is called indirect allorecognition, 00:14:50.23 in transplantation. 00:14:52.19 And indirect allorecognition is actually like... a lot like any other type of immune response. 00:14:58.04 And I'll tell you why in a minute. 00:15:00.25 Direct allorecognition is quite unique -- it's different from most immune responses -- 00:15:06.19 because it actually is seeing the allogeneic HLA presenting a peptide. 00:15:14.17 And there are many, many such possible allogeneic HLA/peptide complexes on a donor graft. 00:15:23.23 And this elicits a very strong response. 00:15:25.26 And we have both CD4 and CD8 T cells that can recognize this donor. 00:15:32.13 And the essential point of direct allorecognition is that it's directly recognizing antigen 00:15:38.26 presented by a donor cell. 00:15:40.22 So, this is a donor dendritic cell, an antigen-presenting cell that is presenting these donor HLA molecules. 00:15:49.24 And that... that's the direct response. 00:15:52.24 Indirect allorecognition is a little bit different, because it in fact involves a recipient antigen-presenting... 00:16:01.11 presenting cell, here, a recipient dendritic cell. 00:16:04.06 And what happens is that recipient antigen-presenting cell picks up donor antigens, 00:16:09.28 such as these HLA molecules, internalizes them, 00:16:14.27 processes them through the antigen processing pathway, 00:16:18.04 and re-presents them on a recipient MHC molecule, a class II molecule, that is on the surface 00:16:25.13 of the recipient dendritic cell. 00:16:27.12 So, this recipient CD4 T cell, in this slide, is actually recognizing a donor HLA-derived peptide 00:16:36.13 presented by a recipient HLA molecule on a recipient antigen-presenting cell. 00:16:43.02 Alright. 00:16:44.03 Well, HLA differences between donors and recipients activate a very large number of T cells 00:16:51.11 of different specificities. 00:16:53.10 And this results in the response to HLA 00:16:56.23 -- which is the human form of the major histocompatibility complex, the MHC -- 00:17:02.22 being stronger than any ordinary immune response. 00:17:07.08 So, this is an extraordinary response. 00:17:09.17 Now, the reason for this is that T cell receptors are... have evolved to recognize MHC/peptide. 00:17:18.05 And so there's this inherent fit between T cell receptors and MHC/peptide complexes. 00:17:25.01 And T cells get positively selected in the thymus to weakly recognize a self MHC/peptide complex. 00:17:32.15 But what happens, in order to avoid autoimmunity, is that they undergo this other process 00:17:38.02 called negative selection, in the thymus, that weeds out the T cells that strongly recognize 00:17:43.15 self MHC/peptide complexes. 00:17:46.09 But there's no donor cells in that... in your thymus that contribute to that process, 00:17:51.13 so you don't weed out T cells that recognize alloantigens. 00:17:55.16 So, this combination of inherent MHC recognition, and not being weeded out for the donor, 00:18:03.01 results in a lot of T cells that see the donor. 00:18:06.06 And one of the results of this is that we can actually measure responses 00:18:10.26 to an allogeneic HLA-mismatched donor just in cells that we take right out of a person. 00:18:17.03 We don't have to give any immunization to see this. 00:18:20.23 And these two responses, called the mixed lymphocyte reaction, in which we measure 00:18:27.17 either T cell proliferation or cytotoxic T cell activity against the donor cells, 00:18:35.13 are our measures of this very strong response. 00:18:39.15 You don't see that for other types of immune responses. 00:18:42.27 You have to immunize. 00:18:44.05 And in fact, it's been estimated that anywhere from about 1-10% of T cells will recognize 00:18:52.16 a given MHC-mismatched allogeneic donor, which is a huge, huge proportion of this 00:19:00.19 very diverse T cell repertoire that we have. 00:19:03.25 I've been talking mainly about acute rejection up until now. 00:19:07.15 And chronic rejection is another type of rejection that is much more slow than acute rejection. 00:19:15.24 And we've made pretty good inroads into reducing acute rejection and graft loss rates 00:19:21.07 with modern immunosuppression. 00:19:23.10 But we haven't done much to eliminate this slower form of rejection, that we call chronic rejection. 00:19:30.12 And the mechanisms of chronic rejection are not fully understood. 00:19:34.20 They may involve, probably, some of the same processes that I've referred to. 00:19:39.17 But it's thought that indirect recognition plays a major role, mainly because the antigen-presenting cells, 00:19:45.20 the professional APCs that come with the graft, 00:19:49.09 do get replaced over time by the recipient's. 00:19:53.00 So, you have more recipient APCs around to present antigen. 00:19:56.10 And indirect recognition is very effective at inducing antibody responses. 00:20:01.14 And antibodies may play a very important role in many instances of chronic rejection. 00:20:09.09 Now, the reason for that is that the B cells are particularly focused on alloantigens. 00:20:20.23 Because... here's an example of a B cell that has a receptor, a surface immunoglobulin receptor, 00:20:29.03 that recognizes an allogeneic donor HLA class I molecule. 00:20:34.08 And what happens is that B cell, by recognizing that molecule, will selectively pick it up 00:20:41.12 and process it, and present it through its class II molecule to a CD4 cell, shown here. 00:20:50.09 So, that CD4 cell helps that B cell, which has focused its antigen... its peptide antigen that 00:20:57.04 that CD4 cell recognizes, and helps the B cell. 00:21:01.26 So, it's a T cell/B cell interaction. 00:21:04.25 Over here, you have the dendritic cell that initially primed that CD4 cell, but that CD4 00:21:11.02 now can very effectively induce antibody responses by that particular B cell. 00:21:17.00 So, that's the indirect pathway of inducing alloantibody responses. 00:21:22.10 Now, there's a third pathway of allorecognition that's only been recognized in the... 00:21:27.26 in the last few years, and that's called semidirect allorecognition. 00:21:32.12 And what that means is... it... it's still a recipient dendritic cell, a recipient antigen-presenting cell 00:21:41.06 that is presenting antigen to the T cell. 00:21:44.16 But instead of picking up and processing antigen, as we saw for the indirect pathway, over here, 00:21:53.22 what happens is that recipient antigen-presenting cell actually steals entire MHC/peptide complexes 00:22:04.02 away from the donor cell. 00:22:05.19 It picks it up in intact form from the cell membrane, and presents it on its own cell membrane. 00:22:13.15 Some people refer to it as crossdressing. 00:22:15.27 It's the recipient dendritic cell looking like the donor in terms of the HLA/peptide complexes 00:22:23.02 that it presents. 00:22:24.26 And that's important because we know we have donor HLA around as long as we have 00:22:29.20 a donor graft around. 00:22:31.23 And it means that... since the T cells that recognize these donor HLA/peptide complexes 00:22:38.22 are the ones that we originally described as directly alloreactive, it means those cells, 00:22:44.01 which are so abundant, have a constant trigger through this pathway. 00:22:49.19 Okay, how do we prevent all this? 00:22:52.16 I mentioned that lifelong immunosuppressive drugs are the standard of care. 00:22:57.00 That's how we get organs to be accepted for many years in many people. 00:23:02.20 And there are several types of drugs that we use in combination. 00:23:07.27 Steroids are not on this list, but corticosteroids that have very broad effects on 00:23:14.10 the immune system are commonly used as one of three drugs. 00:23:18.25 The second drug is often a cytotoxic drug, such as azathioprine or, more recently, mycophenolate mofetil. 00:23:27.11 And what these [drugs] do is they specifically target proliferating T cells... 00:23:33.02 proliferating cells, like T cells and B cells. 00:23:37.09 And then finally, the third drug is often a calcineurin inhibitor, like cyclosporine 00:23:42.07 or tacrolimus or, instead, rapamycin, 00:23:47.00 all of which are very selective for targeting T cell activation. 00:23:54.08 And rapamycin is a little bit different in its mechanism of action compared to 00:23:59.06 the calcineurin inhibitors. 00:24:02.11 Rapamycin actually inhibits the mammalian target of rapamycin, which is a different pathway, 00:24:08.26 pand may have advantages in sparing a regulatory subset of T cells when it's blocked. 00:24:17.20 Okay. 00:24:18.20 So, that's organ transplantation, rejection, how we prevent rejection. 00:24:23.02 I'm now going to end with just a few slides about hematopoietic cell transplantation, 00:24:28.24 which I mentioned early on. 00:24:30.28 Hematopoietic cell transplantation can involve bone marrow, which used to be the usual type 00:24:37.16 of transplant. 00:24:39.06 But more recently, a lot of people get mobilized peripheral blood instead. 00:24:44.10 And what that involves is treating the donor with a drug, GCSF, or there are some 00:24:50.28 more recent drugs, that actually cause the bone marrow stem cells to leave their home 00:24:55.09 in the bone marrow and go into the circulation. 00:24:58.06 And so now you can just, using a leukapheresis catheter, collect peripheral blood that is 00:25:04.04 very enriched for the bone marrow stem cells that we need for our transplant. 00:25:10.10 Cord blood is another source of hematopoietic cell transplants. 00:25:14.21 And finally, we can actually enrich the hematopoietic stem cells from any of these tissues. 00:25:20.22 CD34 is a marker that is on stem cells, as well as a variety of other cells, 00:25:26.24 and often CD34 selection is used to enrich the stem cells. 00:25:30.25 And the main application of bone marrow... of hematopoietic cell transplantation is 00:25:36.22 in the treatment of hematologic malignancies, like leukemias and lymphomas, 00:25:43.02 but also in treating genetic diseases of blood forming cells, immunodeficiency disease and 00:25:48.15 sickle cell disease, etc. 00:25:50.23 Okay. 00:25:51.23 Well, as I mentioned at the beginning, we have an issue. 00:25:56.00 In addition to the recipient rejecting the donor, we have the issue of graft-versus-host disease 00:26:01.17 when we do hematopoietic cell transplants. 00:26:04.17 And that's in a sense the donor rejecting the recipient, the donor attacking the recipient. 00:26:10.11 And that is also -- just like rejection -- dependent on T cells. 00:26:14.03 They're the key players. 00:26:15.24 And what happens is donor T cells seeing foreign antigens in the heavily compromised recipient 00:26:22.13 get activated, and they attack cells of the... of the epithelium... of the epithelial tissues, 00:26:29.00 skin, anywhere in the intestine... in the intestinal system, and liver are 00:26:34.01 the main targets. 00:26:35.11 And there are acute and chronic forms, just like rejection. 00:26:39.28 And through most of the clinical field of hematopoietic cell transplantation, 00:26:45.20 graft-versus-host disease has been such a big problem when HLA barriers are crossed 00:26:51.07 that it has been necessary to find an HLA-matched donor. 00:26:56.10 And of course that could be a sibling, because we all have two different HLA loci. 00:27:03.03 With any given sibling who's not an identical twin, there's a 1-in-4 chance that there will be 00:27:08.28 a complete HLA match at both loci, at both alleles. 00:27:17.04 Now, that obviously is only 25% of people who are lucky enough to have an HLA-identical sibling, 00:27:23.02 and so this has led to the development of large bone marrow transplant registries, 00:27:28.01 where people have volunteered to be HLA-typed, and then to give their hematopoietic cells 00:27:32.21 to somebody who may need a transplant who they don't know and are unrelated to. 00:27:37.14 But that takes millions and millions of people, because of the extensive polymorphism of HLA. 00:27:43.28 Finding a matched donor is like finding a needle in the haystack, but it's done. 00:27:49.02 Now, recently, the field has advanced so that we are able to perform 00:27:54.11 more extensively mismatched transplants, HLA-mismatched transplants. 00:27:58.16 And this is getting more and more common at multiple centers. 00:28:03.24 Okay. 00:28:04.24 So, what is this whole graft-versus-host disease? 00:28:07.06 How does this happen? 00:28:09.00 So, what happens is we give our allogeneic bone marrow transplant, for example, 00:28:16.13 and that contains mature T cells, okay? 00:28:20.00 Those T cells get infused with the graft, you know, through the vein. 00:28:24.12 And they come in, and they go into the recipient lymphoid structures, like the lymph node 00:28:29.12 and the spleen. 00:28:30.12 And they actually get trapped there, because those structures are so full of recipient 00:28:35.07 antigen-presenting cells. 00:28:36.07 So, over a period of several days, those donor T cells recognizing recipient antigens 00:28:42.07 on these recipient APCs will get activated and expand. 00:28:46.16 And then they will... they will leave the lymph node and go back into the circulation. 00:28:52.11 And if the recipient has been treated with a conditioning that causes inflammation 00:29:00.10 in those epithelial target tissues -- and conditioning being irradiation, chemotherapy, etc -- 00:29:07.19 those tissues themselves are inflamed. 00:29:10.25 And those tissues will provide signals that allow those activated T cells that get in 00:29:17.03 the circulation to enter those epithelial tissues and cause this disease called 00:29:22.19 graft-versus-host disease. 00:29:24.13 Okay. 00:29:25.13 Well, how can we prevent it? 00:29:26.25 The obvious one is to deplete the T cells from the donor graft. 00:29:31.28 And that works. 00:29:33.02 But it has some caveats. 00:29:35.17 First of all, it turns out that those donor T cells help the bone marrow to engraft, 00:29:41.14 to overcome the recipient rejection of the donor. 00:29:43.17 And there is an increase in rejection if we T cell deplete our donors. 00:29:48.09 Secondly, most of these... many of these transplants are done to treat malignant diseases. 00:29:55.00 And it turns out that the T cells in the donor graft are a kind of immunotherapy. 00:30:00.11 They actually kill off residual leukemia or lymphoma cells in the recipient that 00:30:07.20 didn't get killed by the chemo or radiotherapy. 00:30:10.26 And so there are higher relapse rates when we T cell deplete our donor grafts; 00:30:15.11 we take away that immunotherapy. 00:30:18.15 And thirdly, adults have a very hard time reconstituting their immune systems 00:30:24.10 after they've had all this treatment. 00:30:26.04 And if you T cell deplete your donor graft, you are more... the recipient is more prone 00:30:32.05 to serious infections. 00:30:33.23 There's quite a long period of immunodeficiency before the immune system recovers. 00:30:38.09 So, donor T cells also have the risk of anti-infectious immunity. 00:30:45.00 Okay. 00:30:46.00 So, I mentioned that relapse rates are higher when we T cell deplete the donor. 00:30:50.18 And that's because that alloreactivity against the recipient, while it's harmful and 00:30:56.04 causes graft-versus-host disease, it also attacks residual malignant cells in the recipient. 00:31:01.00 And natural killer cells, at least with certain types of malignancies, also have the ability 00:31:06.24 to attack malignant cells. 00:31:09.07 And so they can also be beneficial. 00:31:11.22 And then there are a variety of other strategies people are looking at. 00:31:17.11 Differences... changing immunosuppression, and many, many forms of immunomodulation 00:31:24.25 that are being explored to reduce graft-versus-host disease. 00:31:28.09 And the holy grail is to try and preserve antitumor reactivity in this case. 00:31:35.17 And this can be done in ways that... for example, controlling the trafficking of T cells 00:31:41.13 so you can still use the alloreactivity within the lymphohematopoietic system to give you 00:31:46.23 an anti-leukemia effect. 00:31:49.10 Or many groups are working on specifically targeting tumor antigens with a variety of 00:31:56.07 approaches. 00:31:57.07 So, that's the end of the first presentation. 00:31:59.11 Thank you for your attention.