Session 9: The Immunology of Organ Transplantation
Transcript of Part 3: Xenotransplantation
00:00:14.27 Hi, I'm Megan Sykes. 00:00:16.04 I'm a professor at Columbia University and Director of the Columbia Center 00:00:20.16 for Translational Immunology. 00:00:22.13 I'm going to give you an overview of xenotransplantation in this lecture. 00:00:27.07 So, the field of transplantation is limited by drug treatment-related complications, 00:00:33.20 chronic rejection, and the availability of organs. 00:00:37.15 So, one solution that would overcome all of these problems would be xenotransplantation, 00:00:43.22 meaning transplantation of organs from another species, with induction of tolerance to avoid 00:00:51.11 the drug treatment-related complications and chronic rejection. 00:00:56.15 Currently, many more people need organ transplants than get them. 00:01:02.11 These... these ovals represent the people who need organ transplants, 00:01:08.01 who have end-stage organ failure, 00:01:11.19 versus those... only a fraction of those make it to the waiting list, 00:01:15.15 and only a smaller fraction yet of those actually come to transplant. 00:01:20.00 The unfortunate result of this is that many people actually die waiting for an organ. 00:01:26.19 120,000 people currently are on a waiting list in the United States. 00:01:31.18 And less than a third of these receive transplants, and many die waiting for an organ. 00:01:37.26 So, it would really be nice to have an unlimited supply of organs. 00:01:43.13 And xenotransplantation potentially could fill this need. 00:01:46.28 Most of the field feels that pigs would be the most desirable organ source for human transplantation 00:01:55.23 for a number of reasons, which I'll come back to. 00:01:59.26 However, pigs, and most mammalian species, and also non-mammalian species, in fact, 00:02:07.05 express an antigen that is quite ubiquitous and that causes... has posed a major barrier 00:02:12.19 to xenotransplantation for many years. 00:02:15.21 And that is an epitope called alpha1,3Gal, which is a terminal carbohydrate modification 00:02:23.04 of many glycoproteins and glycolipids, that is present in most species, and it's made 00:02:29.10 by an enzyme called alpha1,3Gal transferase. 00:02:33.01 As it happens, old-world monkeys, and subsequently humans, have a mutation in this enzyme gene 00:02:41.10 and therefore don't make this epitope. 00:02:44.02 And because many species like bacteria and other microbes do have the alpha1,3Gal epitope, 00:02:50.18 we all get exposed to it. 00:02:52.13 And therefore we have antibodies in our circulation that recognize alphaGal. 00:02:57.26 These are called natural antibodies because they're there without any known exposure 00:03:04.02 to a pig or any anything else, but nevertheless they're present in all human sera. 00:03:11.16 And what those can do, if you do a transplant from a pig to an old-world primate, 00:03:18.28 is they can immediately bind to the endothelial cells at the graft, fix complement, and cause 00:03:25.15 hyperacute rejection, or a more delayed form of rejection called delayed xenograft rejection. 00:03:30.15 So, this has been a major obstacle to the field that actually was overcome by 00:03:36.13 a technological advancement, which is the ability to genetically engineer pigs. 00:03:42.11 And in the early 2000s, this gene... this enzyme gene, alpha1,3-galactosyl transferase, 00:03:49.27 was actually knocked out of a pig. 00:03:52.06 And that has really helped to transform the field. 00:03:55.17 Now, these pigs that we use in our studies at Columbia are actually a special line of pigs 00:04:02.07 that have been generated for over 40 years through inbreeding by David Sachs, 00:04:09.13 who was part of our Columbia team, and that are miniature swine. 00:04:13.00 And so they're actually a good size for organ transplantation to human, 00:04:19.26 because they're closer to our size, rather than the thousand pounds that a regular pig can grow to. 00:04:26.00 So, the alpha1,3Gal gene was knocked out of these miniature swine in the early 2000s. 00:04:32.26 And this is a picture of the first such animal, and it was perfectly healthy. 00:04:37.18 And these animals, now, are available for research on a regular basis. 00:04:44.26 Now, other groups also have knocked out alphaGal from more conventional-sized pigs, 00:04:51.27 and this really led to a transformation in the field overall. 00:04:55.15 This sort of shows you the progress of the field in terms of pig organ survival in xenotransplantation 00:05:04.13 to primates. 00:05:05.26 Before 1980, it was minutes. 00:05:08.18 In the 1980s, immunoabsorption procedures were developed to get rid of natural antibodies, 00:05:15.16 and that prolonged graft survivals to hours. 00:05:18.07 In the 1990s, the first transgenic pigs were made that express human complement regulatory proteins, 00:05:24.22 and that, combined with immunosuppress... advances in immunosuppression, permitted xenograft 00:05:31.13 survivals of days to weeks. 00:05:34.04 And then in the 2000s, with the development of these Gal knockout pigs, survivals improved 00:05:39.15 to months. 00:05:40.22 And as I'll show you, in the 2010s, we've come even further than that. 00:05:45.13 Now, there are three approaches, major approaches, to overcome... overcoming xenograft barriers. 00:05:52.11 And these can really be used in combination. 00:05:55.12 One is immunosuppressive therapy. 00:05:57.03 The second is genetic engineering, as I've already mentioned. 00:06:00.24 And the third is tolerance induction. 00:06:02.18 And we think that tolerance is going to be an important component of successful 00:06:08.15 clinical xenotransplantation, because of the very high level of immunity that we have to 00:06:15.10 these highly disparate donors. 00:06:18.15 This is a slide showing a paper from Mohiuddin et al that was published a couple of years ago, 00:06:25.03 that shows how far we've come in getting graft survival from pigs into non-human primates 00:06:32.16 using immunosuppression and genetic engineering. 00:06:36.27 So, this study involved the use of pigs that had been engineered to... they were Gal... 00:06:46.23 alphaGal transferase knockout pigs that had this human complement regulatory protein, 00:06:52.19 CD46, and human thrombomodulin, which inhibits coagulation. 00:06:57.08 And these hearts were transplanted heterotopically, so they're not functioning hearts, 00:07:01.14 but they were put in the abdomen as a sort of accessory heart in these baboons. 00:07:06.11 And what you see is very, very long survival of the animals that got the full immunosuppressive regimen. 00:07:15.10 And unfortunately, the survival was very dependent on high doses of that immunosuppression. 00:07:20.23 As soon as it was reduced, the grafts were rejected. 00:07:24.02 But you can see that the number of days these grafts survived is close to a thousand. 00:07:29.20 So, we now have survival of these heart grafts for several years. 00:07:36.03 Okay. 00:07:37.04 Well, there's other things that can be done to pigs in terms of genetic engineering 00:07:42.20 to help facilitate xenotransplantation. 00:07:46.09 Some people are thinking about removing the major histocompatibility complex antigens, 00:07:52.14 the MHC of the pig, which is referred to as the SLA, so that they can't be seen 00:07:57.18 by the immune system. 00:07:59.03 This is an interesting approach, but it has some limitations. 00:08:02.00 First of all, indirect recognition of processed and presented antigen from the pig could lead 00:08:10.20 to destruction by other mechanisms involving cytokines, etc. 00:08:17.04 Secondly, a lack of MHC will make a cell more prone to be attacked by natural killer cells. 00:08:26.11 This can be overcome with some further genetic engineering, potentially. 00:08:29.10 But thirdly, if a graft doesn't have any SLA molecules, any MHC, it can't present antigen 00:08:37.10 to T cells at all, and so T cells can't protect the graft from infections, 00:08:41.19 so that's a potential problem. 00:08:44.09 So, our approach is not to get rid of the MHC on the... on the donor, but instead 00:08:53.03 to try and re-educate the recipient's immune system, to regard the donor itself, by inducing tolerance. 00:09:00.00 And there's two approaches to tolerance that I'm going to speak about. 00:09:02.28 One is mixed chimerism. 00:09:04.07 And as you'll see, this tolerizes T cells and B cells, and also even natural killer cells. 00:09:10.24 And the second is thymic transplantation, which tolerizes T cells. 00:09:15.22 So, many years ago, we actually tried to develop a non-myeloablative, low toxicity, 00:09:24.15 potentially clinically relevant method of inducing mixed chimerism in the closely related species rat to mice. 00:09:31.05 And this shows you the regimen that involved monoclonal antibodies against T cells 00:09:37.05 and natural killer cells, local radiation to the thymus, and a very low non-myeloablative dose 00:09:43.28 of total body irradiation. 00:09:46.14 And these animals did develop mixed chimerism and were tolerant of their donors. 00:09:51.00 And it showed... this slide shows you that these chimeric animals actually accepted 00:09:57.14 skin grafts from those rat donors without any immunosuppression, 00:10:01.17 whereas they were still competent to reject skin grafts from a third party rat. 00:10:06.11 So, the tolerance was quite specific for that rat bone marrow donor. 00:10:12.22 And interestingly, this chimerism was sort of... it lasted a long time, but only at 00:10:18.17 very, very low levels over time. 00:10:20.22 And yet we could see that the T cell tolerance was mediated by the presence of 00:10:25.13 donor antigen-presenting cell in the recipient thymus that led to central deletion of donor-reactive T cells. 00:10:34.16 Using this model, we were also able to look at how what happens to an innate immune response, 00:10:40.21 particularly natural antibodies. 00:10:44.11 Mice do have natural antibodies against the rat, and that's how we did our first studies. 00:10:48.14 But later, when this alphaGal enzyme was identified... mice, like most species, do have alphaGal, 00:10:55.26 so they don't have anti-Gal natural antibodies, but by knocking out the alphaGal transferase from mice, 00:11:03.04 one could now produce a mouse that resembled a human in having natural antibodies 00:11:07.21 against Gal. 00:11:09.03 And so now we could ask, what happens when we do a rat bone marrow transplant to these mice? 00:11:15.22 The rats do express alphaGal. 00:11:17.24 Will we tolerize the B cells specific for Gal in addition to everything else. 00:11:24.17 And what we found was, indeed, that we could tolerize anti-Gal natural antibody-forming B cells 00:11:29.16 by induction of mixed chimerism. 00:11:33.22 And that we could thereby prevent both T cell-mediated and antibody-mediated rejection. 00:11:39.15 That's illustrated here, where we have several groups of mice, both wild type and 00:11:46.23 alphaGal knockout mice -- so, GalT +/+ and -/- -- that either received the conditioning 00:11:54.00 but no rat bone marrow transplant, 00:11:56.12 or that received conditioning with a rat bone marrow transplant, 00:12:00.05 so that they developed mixed chimerism. 00:12:02.19 And what happens is this conditioning actually really bumps up the level of anti-Gal antibodies 00:12:07.15 in these mice. 00:12:08.15 So, if you just put in a rat heart to one of these mice, it's actually very quickly rejected. 00:12:14.24 Some of them are hyperacutely rejected, others with a more delayed vascular rejection 00:12:20.10 type of pattern. 00:12:21.18 In wild type mice that just get the conditioning, so they don't have the anti-Gal, 00:12:26.17 they still have lots of T cell immunity to the rat, and they reject, by a cellular rejection mechanism, 00:12:32.00 within a week or so. 00:12:33.22 But our mixed chimeras, whether they're wild type or Gal-knockout recipients, 00:12:39.08 uniformly accept those rat heart grafts, showing that both the antibody-mediated 00:12:44.06 and the T cell-mediated rejection processes are avoided by induction of mixed chimerism. 00:12:51.00 This slide illustrates one of those hyperacutely rejected in a conditioned Gal-knockout mouse. 00:12:57.01 You can see that within 30 minutes that graft has turned black. 00:13:00.16 It's completely rejected due to this antibody-mediated mechanism, whereas if you look 00:13:06.14 at the wild type animal here, that doesn't have anti-Gal antibody, the heart is still pink and beating 00:13:13.01 at that 30-minute time point. 00:13:15.25 So, to summarize, mixed chimerism can be induced in Gal transferase-knockout mice 00:13:20.28 using non-myeloablative conditioning and high doses of rat bone marrow. 00:13:25.20 Mixed chimerism in this model leads to rapid tolerization of anti-Gal-secreting B cells. 00:13:31.00 And through a number of studies, we showed that this was true tolerance of the B cells. 00:13:38.14 And in the grafting studies you saw, we were able to prevent hyperacute rejection, 00:13:45.02 delayed xenograft rejection, cellular rejection, and chronic rejection 00:13:49.06 of these primarily vascularized cardiac xenografts. 00:13:54.07 Another innate component of the innate immune system is natural killer cells. 00:13:59.06 And what we observed early on in these rat-to-mouse studies was they actually pose a very strong 00:14:04.21 barrier to engraftment of rat bone marrow, much more so than to allogeneic bone marrow, 00:14:10.27 for example. 00:14:12.25 And so, in this rat-to-mouse mixed chimerism model, we included antibodies to deplete 00:14:18.01 natural killer cells, as well as another sort of innate subset of T cells, the gamma delta T cells, 00:14:24.04 we found also had to be depleted to get mixed chimerism. 00:14:28.11 So, there's more barriers to xenograft... in xenograft... xenogeneic hematopoietic cells 00:14:34.20 than to allogeneic hematopoietic cells from the innate immune system. 00:14:39.23 Okay. 00:14:40.23 Well, what about natural killer cells? 00:14:42.20 So, we deplete them, but they come back after we've induced mixed chimerism with this regimen. 00:14:49.17 We wanted to know whether those NK cells that come back are tolerant to the rat, or whether 00:14:54.09 they still have anti-rat reactivity. 00:14:57.15 Well, this is a complicated slide showing an old-fashioned type of assay for measuring 00:15:03.00 NK cell activity in vitro... in vivo. 00:15:05.11 It's called the IuDR uptake assay. 00:15:08.05 And with this assay, we discovered that induction of mixed chimerism from the rat led to tolerance 00:15:15.19 to the rat, but that it was associated with a global hyporesponsiveness of NK cells. 00:15:21.07 In contrast, conditioning alone did not lead to this effect, so it clearly had to do 00:15:27.11 with the presence of rat chimerism. 00:15:29.27 So, from this, we concluded that mixed xenogeneic chimerism leads to tolerance of T cells, 00:15:36.20 B cells making natural antibodies of all specificities, and natural killer cells. 00:15:41.17 So, in more recent years, we have actually been able to test these ideas and see whether 00:15:47.10 it holds up in the pig/human combination, not by doing human transplants to... 00:15:52.04 pig transplants to humans, but instead by creating human immune systems in immunodeficient mice. 00:15:59.02 And these are what we call humanized mice. 00:16:01.18 And this is a model developed in the laboratory of Yong-Guang Yang several years ago, 00:16:08.24 where he takes immunodeficient mice, gives them a little bit of whole-body irradiation, 00:16:14.11 and then transplants a fetal thymic tissue under the kidney capsule, and gives human 00:16:22.09 bone marrow stem cells intravenously. 00:16:24.24 And he also constructed immunodeficient mice to express porcine cytokine genes that are 00:16:32.15 important for porcine hematopoiesis. 00:16:35.06 And in doing that, could then get pig bone marrow to engraft as well, when given to 00:16:41.07 these mice that get human hematopoietic stem cell grafts. 00:16:45.04 And used this model to ask whether or not tolerance could be achieved by induction of 00:16:52.07 mixed chimerism in the human immune system. 00:16:54.18 And this slide shows you the coexistence of pig and human cells in one immunodeficient mouse, 00:17:01.08 now 25 weeks post-transplant. 00:17:05.00 You can see that there are human cells and pig cells by flow cytometry with specific 00:17:11.03 antibodies. 00:17:12.03 This is a pig antibody, and this is a human CD45 antibody. 00:17:17.02 They're coexisting, lifelong, in these animals. 00:17:21.16 And most importantly, when pig skin grafts were put onto these... these mixed chimeras, 00:17:30.12 it was found that the animals rejected third-party pig skin grafts, but accepted the donor skin grafts. 00:17:40.00 This one animal died at 40 days, but the others showed this pattern of long-term acceptance 00:17:45.23 of the donor skin, and rejection of the third-party skin from the pig. 00:17:53.06 In contrast, animals that are not humanized, they're not reconstituted, they're not able 00:17:57.15 to reject any skin grafts. 00:17:59.21 Whereas those that are reconstituted with the human immune systems but 00:18:04.19 don't get mixed chimerism, they for the most part reject both types of pig skin grafts. 00:18:09.25 So, this shows you that human-specific tolerance to the pig can be induced by induction of 00:18:16.22 this mixed xenogeneic chimerism. 00:18:19.04 So... and with other studies, these results proved that central T cell tolerance of 00:18:26.04 human T cells can be achieved to porcine xenografts through induction of mixed hematopoietic chimerism. 00:18:32.12 More recently in our laboratory, we have asked whether human natural killer cells 00:18:38.00 can also be tolerized to pig by induction of mixed chimerism, 00:18:41.06 as we saw occurred in the rat-to-mouse model. 00:18:45.01 To do this, we've had to take humanized mice and induce mixed chimerism in the way 00:18:50.09 that I just showed you, with pig bone marrow and pig cytokine transgenic recipients. 00:18:56.15 And now do some maneuvers to induce production of human natural killer cells, 00:19:04.24 because they need human IL-15 to be produced. 00:19:06.15 But in doing this, we were able to show that some of these mixed chimeras do indeed 00:19:13.20 show specific tolerance to the pig donor. 00:19:17.06 And that's illustrated here. 00:19:19.15 If you look at the open symbols... 00:19:24.01 I hope you can see them... that both the open and closed symbols represent... the open symbols 00:19:31.06 represent mixed chimeras; the closed symbols represent controls that are not porcine mixed chimeras, 00:19:37.02 but are just humanized; and then we have normal human PBMC with a square 00:19:44.02 and the dashed line. 00:19:45.19 And you can see that the normal human PBMC, and that all these mixed chimeras, 00:19:50.14 have similar killing of this class I-deficient target human cell line, called K562. 00:19:56.05 So, they have function. 00:19:58.15 But if you look at their killing of pig targets, you see that only the non-chimeras 00:20:04.24 kill the human... the pig targets, whereas the mixed chimeras are specifically unresponsive 00:20:11.17 to the pig targets. 00:20:13.22 Now, that's one pattern that we saw. 00:20:16.16 In other animals, we saw this other pattern, called global hyporesponsiveness, 00:20:23.06 where the mixed chimeras, shown with the open symbols, now didn't respond to the common target, 00:20:31.10 K562, and also didn't respond to the pig. 00:20:33.01 So, there were two types of tolerance: one that was desirable, 00:20:37.10 where it was specific for the pig; 00:20:40.06 another where there was a more global dysfunction of the natural killer cells, 00:20:44.24 which is not the ultimate endpoint that we would like. 00:20:49.20 So now we're working on engineering pigs in a way that will make them resistant to 00:20:55.14 human natural killer cells. 00:20:57.26 But to summarize what I've said so far, mixed xenogeneic chimerism leads to tolerance of 00:21:02.22 T cells, B cells, natural killer cells. 00:21:06.28 And so far, these conclusions seem to apply in the human... pig-to-human combination 00:21:11.22 in humanized mice as well as the rat-to-mouse. 00:21:14.13 I haven't shown you human B cell tolerance, but in work that's ongoing, that seems 00:21:19.18 to occur as well. 00:21:21.03 Okay. 00:21:22.03 Well, there's another innate barrier that gets in the way of mixed chimerism, 00:21:27.25 and that is mediated by macrophages. 00:21:31.18 Studies in baboons and also in murine models have shown that mixed chimerism from 00:21:41.13 a highly disparate xenogeneic donor can be extremely short-lived due to destruction by macrophages 00:21:50.21 of those xenogeneic cells as soon as they come in. 00:21:53.05 Now, why are they so quickly eaten by macrophages? 00:21:56.09 Well, it turns out that there is a major molecular interaction that tells macrophages 00:22:04.00 not to eat circulating cells. 00:22:06.13 This is the CD47-SRIPalpha pathway, and this is what prevents our macrophages from normally 00:22:14.04 eating up our erythrocytes. 00:22:16.19 Only when an erythrocyte gets old and loses CD47 expression does it get taken up 00:22:23.06 by a macrophage and destroyed. 00:22:25.28 So, CD47 is a ligand for SIRPalpha, which is an inhibitory receptor expressed on macrophages 00:22:34.20 that gives the macrophage a "don't eat me" signal. 00:22:37.28 As it turns out, porcine CD47 does not transmit that inhibitory signal to human or baboon SIRPalpha, 00:22:48.05 resulting in just unopposed activation of those macrophages, and rapid destruction 00:22:54.12 of the porcine... porcine cells. 00:22:57.02 So, based on this, we hypothesized that another genetic modification of the pig, 00:23:04.26 namely putting in the human CD47 gene into these miniature swine, would make them better 00:23:12.03 hematopoietic cell donors and give us more lasting mixed chimerism. 00:23:15.04 And indeed, that seems to be the case. 00:23:17.16 So, this is a study from David Sachs' lab, showing that peripheral blood stem cells, 00:23:25.11 mobilized stem cells, from the CD47-high transgenic pig led to quite prolonged chimerism, 00:23:33.13 shown in the lower-right quadrant of each of these plots, compared to peripheral blood stem cells 00:23:40.27 from a CD47-low pig, that really didn't express detectable CD... human CD47. 00:23:47.11 So, this CD47 seems to be markedly prolonging the survival of pig hematopoietic cells 00:23:55.27 in the circulat... in the baboon recipient. 00:23:58.21 And this is associated with prolongation of pig skin grafts. 00:24:03.25 So, what you see on this slide is the survival... what these pig skin grafts on these baboons 00:24:11.05 looked like at 14 days, in the control animal that very quickly rejected this skin graft. 00:24:19.26 This animal got pig PBSCs, but without high CD47. 00:24:24.12 And you can see it's rejecting already at 14 days. 00:24:27.17 And histologically there's a lot of rejection there. 00:24:30.22 In contrast, the grafts were much prolonged in the CD47-high pig PBSC recipients. 00:24:40.00 And this graft at 42 days still looks beautiful with, really, very little infiltration of 00:24:46.07 lymphocytes at that point. 00:24:48.12 So, this is a very important aspect of the approach that we're using to induce 00:24:56.05 mixed chimerism and tolerance, now, in non-human primate recipients. 00:25:01.15 The second approach to xenograft tolerance is thymic transplantation. 00:25:06.13 And ultimately, we think this one is going to be useful in conjunction with the hematopoietic chimerism. 00:25:14.11 And this work actually builds on studies that we did back in the 1990s, where we showed that 00:25:20.21 if we took normal immunocompetent mice, removed their thymus, and T cell depleted them 00:25:26.26 temporarily... gave monoclonal antibodies to T cells to the mice, and then put in 00:25:33.21 a pig thymus graft, the pig thymus would replace the mouse thymus that we'd removed, 00:25:39.21 generate a whole new repertoire of T cells. 00:25:41.24 And those T cells were tolerant of the pigs, so that you could put this pig skin graft 00:25:46.04 on to an immunocompetent mouse, and it wasn't rejected. 00:25:50.23 So, we've shown that this same approach works in a human immune system, in human humanized mice. 00:25:58.11 Now, constructing these humanized mice instead of with a human... excuse me... 00:26:03.15 fetal thymus graft, putting in porcine fetal thymus tissue, and then giving the same human stem cells 00:26:11.05 to both groups. 00:26:13.04 And what we find... the graft that we put under the kidney capsule of these mice, 00:26:18.07 the fetal thymus graft, is very, very small: it's one cubic millimeter. 00:26:22.21 And you can see now these are... these are animals that are euthanized after the graft 00:26:27.18 has had a chance to grow. 00:26:29.03 And it was put under the kidney capsule. 00:26:31.26 And this reddish thing is the kidney, and the white thing is the graft that has grown 00:26:37.14 to be just as big as the kidney. 00:26:39.19 And the same occurs if it's a fetal pig thymus tissue, as we see with human thymus tissue. 00:26:50.18 And both of them really look the same in terms of the profile of cells in the thymus graft. 00:26:57.11 So, these are human thymocytes in the human thymus graft showing a very normal CD4/CD8 profile 00:27:02.24 showing regulatory T cells in normal percentages. 00:27:07.05 And this is the same stem cells developing in the pig thymus graft. 00:27:12.01 They're really quite indistinguishable, phenotypically, from what we see in a human thymus graft. 00:27:18.14 And most importantly, once again, putting in this pig thymus, allowing human T cells 00:27:24.04 to develop in a pig thymus, leads to specific tolerance to the pig. 00:27:28.24 So, what we're looking at here is survival of pig skin grafts from the pig... 00:27:35.20 the same genetically similar donor to the pig that gave the bone marrow cells versus 00:27:42.22 a third-party pig that is SLA-mismatched to the donor pig. 00:27:47.06 And in mice with a pig thymus, you can see that the donor pig skin graft is markedly 00:27:54.10 survived... skin grafts are markedly prolonged, whereas the third-party grafts are rejected 00:28:00.15 in most cases. 00:28:02.01 Whereas, when those human T cells developed in a human thymus, they uniformly reject 00:28:09.14 the pig skin grafts. 00:28:10.15 So, we've induced specific skin graft tolerance with this thymus transplant. 00:28:17.16 Applying this in a large animal model, Kaz Yamada and colleagues in our center have developed 00:28:23.28 a regimen for immunosuppression that all... and also have adapted the thymus transplant approach 00:28:30.13 to be primarily vascularized, so it functions more quickly. 00:28:35.20 Rather than just being put as a piece under the kidney capsule... he actually does this 00:28:40.07 in several ways, but one way is to take the donor pig, take out its thymus, 00:28:45.02 cut it into pieces, and put it under the kidney capsule -- multiple pieces of the donor pig -- 00:28:51.12 and then sew that animal back up, and allow that thymus graft to coalesce and grow 00:28:56.22 and become functional. 00:28:59.14 And then, transplanting the thymal kidney en bloc, as a single graft, to a non-human primate. 00:29:05.22 The other approach is to just take out the thymus graft... thymus from the pig and 00:29:10.22 vascularize it primarily in the primate recipient. 00:29:15.06 And the results that we've achieved with this approach are very, very exciting. 00:29:20.10 We... this is an example of an animal that got... a baboon that got 00:29:27.06 a life-supporting pig kidney that has normal kidney function at six months post-transplant, 00:29:35.06 that's sustaining that baboon's life. 00:29:38.06 Normal creatinine. 00:29:39.21 And the kidney looks perfect. 00:29:42.01 There's really no histologic abnormality, and no antibody binding to that kidney. 00:29:49.15 This animal unfortunately had to be euthanized for other reasons. 00:29:54.09 And in vitro studies on this animal, that aren't on this slide, actually showed donor-specific 00:30:00.11 unresponsiveness to that pig. 00:30:03.25 So, we think this is really applicable and very exciting as an approach to tolerance induction. 00:30:10.09 So, to summarize, we can achieve human T cells and NK cell tolerance to porcine xenoantigens 00:30:16.18 via mixed chimerism induction in humanized mice. 00:30:20.24 Mixed xenogeneic chimerism can be enhanced in primates by using human CD47 transgenic pigs 00:30:26.15 as source animals. 00:30:29.08 Porcine thymic transplantation, as a regulatory mechanism to the tolerance, 00:30:34.01 which I didn't go into, 00:30:36.04 but that seems to be very important in suppressing T cells that you can't get rid of prior to the pig transplant. 00:30:43.17 And ultimately, we think a combination of mixed chimerism and thymic transplantation, 00:30:48.16 and some further genetic modifications of the pigs, will make xenotransplantation and 00:30:53.23 tolerance clinically achievable and safe. 00:30:57.16 So, many, many people have contributed to this work. 00:31:02.05 This is a list of some of the key players in the large animal studies. 00:31:06.24 And many people also in the humanized mouse work that I spoke about. 00:31:11.22 So, I'll thank you for your attention and stop there.