Scientist Stories: Jeffrey Bluestone, Mechanisms of Immune Tolerance

Welcome to the Immunological Reviews Podcast. My name is Justyna Stadamlik, and I am delighted to be here with Dr. Jeffrey Bluestone, the A.W. and Mary Margaret Claussen Distinguished Professor and Executive Vice Chancellor and Provost at the University of California, San Francisco. Throughout his career, Dr. Bluestone has made enormous contributions to our understanding of the mechanisms and importance of regulating T cell responses. His work has had far-reaching implications into elucidating the basic immunological mechanisms of tolerance and the development of novel therapeutics. And he is the guest editor of Volume 241 entitled Mechanisms of Tolerance. Dr. Bluestone, during your career, how have you seen the field of immune tolerance develop?

Well, I'm pretty old, so I have seen a lot. If you think back to when I started back in the '70s, we thought of immune tolerance as an active mechanism based on early work by Sir Peter Medawar and Brunet and a number of other people really demonstrating that the thymus in particular caused deletion of all the cells that might respond in the periphery And so we are tolerant because of the central mechanism in the thymus. Over the last 3 to 4 decades, we've learned it's far more complicated, that the thymus isn't as complete as we would hope it to be in deleting all of the self-reactive cells, that the periphery has developed an enormous number of mechanisms that it uses to try to control unwanted immune responses. And the whole field of self/non-self discrimination has really brought in a wealth of different cell types from dendritic cells and macrophages, T cells, B cells, even the innate immune response including NK cells and mast cells that all participate in making these choices. So really what we've learned in the last several decades is that the immune system is a delicate balance. In order to be able to recognize every foreign virus and bacteria in the world, it's created a very robust mechanism for effective immunity. But at the same time, to try to protect against self-reactivity destroying one's pancreas and diabetes or brain and multiple sclerosis, there are just a whole series of interrelated regulatory loops that combine to really shut down the immune response such that it's actually very difficult to initiate an immune response in many settings. And what was once thought to be the the primary responsibility of the immune system to recognize and destroy. Now I think there's a lot to be said for the fact that the immune system is designed to regulate and control, whether it's in pregnancy, whether it's in gut flora and the commensal bacteria that colonize each of us, or whether it's just protecting us from destruction of self-tissue. The immune system is quite highly regulated.

What have been the most seminal discoveries in the field?

Well, I think that there are too many to even consider in one short conversation, but some of the things that have been very close to me in my career that have impacted what I've done have really changed the way I think about immune tolerance. I think the first is the discovery that the periphery plays a very important role. Intolerance. It's funny, we now talk about regulatory cells, regulatory T cells and B cells, but back in the early '80s there was the whole field of suppressor cells which really started a lot of this sense that the immune system is highly controlled. And although a lot of that work was controversial and hasn't panned out, it really opened up, I think, my eyes and many in the field to the notion of this highly regulated network. Of cells that we have. So I'd put among the really important discoveries is the more recent work by Sakaguchi and Shivak and others really identifying cells that really control or regulate the immune response. And when some of the European groups in England and elsewhere have really employed these concepts into transplantation and autoimmunity, it's really helped us to understand how to control an immune response. I think another seminal discovery really was the whole field of co-stimulation, the concept that in order for a T cell to get activated, it needed more than one signal, one through the T cell receptor and then a secondary set of signals that would augment or enhance that activation phenotype of a T cell. And when Kevin Lafferty and and others really identified co-stimulation as a core process in activating the immune system. That was, to me, a very important concept because it meant that when T cells usually interact with self-antigens, they don't respond unless they have an activated dendritic cell or other cell that provides these co-stimulatory signals. Of course, among those, CD28 is the one closest to close to my heart. We have a volume in this book talking about CD28 co-stimulation and its importance. But another volume by Chen Dong, another chapter by Chen Dong also talks about other co-stimulatory molecules like ICOS and others, CD40 ligand, all of which play an important role in manifesting a fully complete T cell response. And there are the cytokines as well, IL-2, IL-4, being a critical cytokine to help promote T cell expansion and co-stimulation in that regard. So this whole concept about co-stimulation to me was really very important. Along with that came the flip side, which I think is going to turn out to be one of the most clinically critical pathways for immune tolerance, and that is breaking it and the importance of these what we call negative co-stimulatory molecules such as CTLA-4 and PD-1, which have been instrumental in now new cancer therapies. And Jim Allison talks about that and how by blocking these negative regulators that turn off immune responses, both the effector T cell and function in the regulatory T cells, that by controlling these negative regulators, you can enhance or promote an immune response against tumors. And that is already in the clinic and hopefully will end up saving people's lives. So in the field of co-stimulation, both the on and the off have been really critical findings. I think another area in co-stimulation— I'm sorry, in immune tolerance which I think is quite seminal is an appreciation for the role of the innate immune response. I think people thought about tolerance purely from the perspective of T cells and B cells. But now we know that inflammation can highly control whether a protein is immunogenic versus tolerogenic. And I think that the work summarized by Larry Turka in his chapter, as well as others in this volume, really highlight the critical importance of the TLR, LPS, and other pathways in which maturation or alteration of dendritic cells and other populations in the immune system really do control whether an antigen is being seen as immunogenic or tolerogenic. And some of the seminal work by Ralph Steinman and Michele Nusenzweig really helped to articulate the different subsets of cells and depending on their state of maturation whether they lead to a protective immune response or not. And that becomes essential in viral infections and in other settings in which the inflammation and innate immune response induced by the pathogen really are what's critical to determine whether you get a productive immune response, which Pam Ohashi speaks to in her chapter in this volume. So I think that's a big area, one that's going to be an important target area for the future, namely the innate system and how that integrates with the T cell and B cell systems in an immune response. Again, there are a lot of areas. I think, and a colleague of mine, Mark Anderson, writes about this in the volume, the whole field now of this transcription factor, AIR, which has really changed the way we think about central and peripheral tolerance. I think everybody saw those as two very separate fields up until the discovery of AIR because people thought the thymus deleted all of the cells that saw self-antigens that were presented in one way or another in the thymus, and then the peripheral systems took care of things once the T cells escaped that would otherwise see self. But now we understand through the discovery of this molecule AIR that it controls the expression of otherwise only peripherally expressed antigens in a small subset of thymocytes, and that controls the deletion of cells that otherwise would only see those proteins in the periphery, like insulin, where it's in the pancreas in the beta cells, but it's also in this small subset of epithelial cells in the thymus. And now more recently, Mark's discovery that this peripheral expression of AIRE can also cause immune modulation in the periphery by air-expressing cells really starts again to link up the central tolerance mechanisms with the peripheral tolerance mechanisms. So that to me is a third absolutely key area. And then I guess the last area that I would focus on as being seminal would be the whole concept of how to exploit basically this whole notion of active suppression, active regulation, and the different subsets of T regulatory and B regulatory cells and how they really can control immunity. And I mentioned early on that that was a conceptual shift from a setting in which people thought clonal deletion and clonal energy were the dominant mechanisms to tolerance to now thinking in everything from pregnancy, which Betz talks about in his chapter, to transplantation, which Wood talks about in her chapter, how these regulatory cells can be really adapted to control tissue-specific responses, to control general immune homeostasis, and to control just the overall immune balance. And the discovery of FOXP3 as the major transcription factor really helped unravel and frankly to validate the notion of a suppressor cell again some 30 years later. And in Rudensky's chapter, he talks about FOXP3 and how it controls the regulatory program. So I'd raise that as being one of the most seminal discoveries over the last 30 years as well.

What do you envision as the next generation of studies?

Yeah, I mean, what's great about immunology is, and all of science, is you never know where the next what the great breakthrough was. It's now only 20 years ago or less that we really could start studying T cell receptors that were cloned. Only 10 years ago that we understood the whole field of TLRs and PAMPs and DAMPs and the role of innate immune response. So it's hard to predict where the future will go. But I'd highlight a couple of things, I think. Number one, I think the future is in human immunology. To a large extent. I think we've learned a lot from our animal models and animal studies. Those are key, but we know the immune system is going to be more complex in humans. We know that epigenetic changes are going to be key, and that really is driving the human genome to express differentially its genes that it has. We know that the biomarkers that we're going to need to be able to identify people who are tolerant versus non-tolerant are going to be dependent on looking in humans who have a mature immune system, that have immune system that's been impacted by viruses and bacteria their whole lives. So we really need to do first-rate human immunology, and I see that a big area in the future. Therapy. I think we are on the verge of having some real tolerogenic therapies that will be applied to humans. In the volume, Jerry Nepom and others talk about their— the work of the Immune Tolerance Network in trying to deliver tolerogenic immunotherapies to patients in allergy, asthma, transplantation, autoimmunity. And there I think we're going to make great strides because we'll be able to use those clinical trials to help us understand the biomarkers, to help us understand what is a tolerant or non-tolerant state. In humans so frankly we can start withdrawing drugs because who would put their kidney or liver at risk right now of rejection if we don't have a good marker to know that when we withdraw the drugs that they won't be targeted by the immune system. So I think we're going to see a lot more clinical trials. I think in those clinical trials we're going to see cell therapy trials where people start putting back in dendritic cells that are tolerogenic. Start putting back in regulatory cells that see alloantigens or see self-antigens, put back in mesenchymal stem cells and other cell types that can regulate, control, or shift the balance. So I see the future being doing clinical trials as a scientific discovery effort as well. And then finally, I think we need to continue to go back inside the cell back to the transcriptome and truly, truly understand the plasticity of the immune system. Because the other thing we've learned in the last few years is that the cells of the immune system are not dogmatic. They're not set in stone for the life of the cell. Regulatory cells can lose some of their stability and turn into effector cells in certain inflammatory settings. Th17 cells can be derived from interfere on gamma-producing cells. We have potentially tolerogenic settings that exist but then get perturbed by the plasticity of the environment that we live in. And so the interface of the environment and genetics and how it controls that plasticity is key. And I think the key to that is really looking inside the nucleus at the genes, how they're transcribed, what controls the program basically of these different cell types. And I suspect that we're going to find that a lot of the plasticity, a lot of the instability of the immune system in a tolerance way is going to be based on a program that's set up in the genome based in part on the genetics but in part on the epigenetics. And the exciting thing is that it already is being applied. I mean, it's only a few years ago that CTLA-4 Ig was approved as a drug and now being used in a number of different autoimmune and now most recently in transplantation. And literally, we're talking about 15 years or maybe 18 years since the time it was first shown that CTLA-4 Ig could induce tolerance, that it's now made it into the clinic and into therapies that are used every day. Anti-CTLA-4 for cancer therapy will happen soon. Anti-PD-1 is making great strides in cancer therapy. But as I say, I think that it's going to continue to grow. I think the concept of cell therapy is going to really take a hold. Maria Roncarolo, in her chapter, talks about the work that's being done in graft-versus-host disease and using regulatory cells as a way to control graft-versus-host disease while allowing graft-versus-leukemia responses to take place. So I think that there's going to be an enormous application of tolerance to the clinic, and we're going to continue to learn the lessons from pregnancy, the lessons from immune-privileged sites as we as we drive that research.

So I know one important facet of your career has been the establishment of the Immune Tolerance Network, and you mentioned it briefly. Can you tell us a little bit more about the goals of that network?

Well, we just celebrated our 10th year, which is very exciting. I've stepped down as director now, and Jerry Nepom directs it. So there's life after the 10-year period. I think the Immune Tolerance Network serves a number of purposes that are important. One is, I think it has created a sense in the community that we're willing to test in patients, in people, tolerogenic therapies in which drugs will actually be removed, and that we can attain a state of tolerance where your immune system is intact while you're not still targeting the tissue that you didn't want to target. So I think it's had a fundamental effect on how the community views therapies going forward. Very specifically, I think it's made some real contributions from early work by David Sachs and his colleagues through an ITN-sponsored trial demonstrating that bone marrow chimerism can lead to long-term survival of kidney allografts without immunosuppression to more recent work in vasculitis where showing that treatment with an anti-B-cell antibody, anti-CD20, can have a long-term effect on Wegener's disease and others in keeping that in check. In the allergy and asthma area, there have also been some great advances and some exciting trials going on looking at, for instance, Getting back to the early MediWar work, if we introduce peanut antigens early on in a baby within 6 months of age, can we induce a state of tolerance to peanut instead of the activation analogies we see later on? So I think we've been able to, through the ITN, challenge some basic concepts, to employ some of the things I talked about in terms of new science, but most importantly to get people to talk to each other, to have the allergists talking to the transplant surgeons, to have the rheumatologist talking to the organ-specific transplanters and really get some real sense of the community and what we're trying to do and couple all of that with a focus on mechanism because at the end of the day, it's not just putting a drug into a patient that's really going to help us come to the solutions. It's going to be studying those patients, understanding what has happened to that immune system when we perturbed it with a given drug, following the natural history of a patient who's prone to type 1 diabetes or prone to allergy to try to figure out what it is about their immune system which is different. And I think when we start to put together those mechanistic studies with the clinical outcomes that happen, that that's where we're going to really learn a lot about what's gonna work and what's not gonna work. And I think the ITN provides a scaffold for being those that the world and anyone in the world can apply and try to get resources to look at.

Dr. Bluestone, thank you very much. It's truly been a pleasure speaking with you today.

Thank you.