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Circulation on the Run


Aug 22, 2022

This week, please join author Kory Lavine and Associate Editor Thomas Eschenhagen as they discuss the article "Donor Macrophages Modulate Rejection After Heart Transplantation."

Dr. Carolyn Lam:            

Welcome to Circulation on the Run, your weekly podcast summary and backstage pass to the journal and its editors. We're your co-hosts. I'm Dr Carolyn Lam, associate editor from the National Heart Center and Duke National University of Singapore.

Dr. Greg Hundley:          

I'm Dr. Greg Hundley, associate editor and director of the Pauley Heart Center at VCU Health in Richmond, Virginia.

Well, Carolyn, this week's feature, we are going to the world of preclinical science and we are going to learn about a very important new finding pertaining to heart transplant rejection, and macrophages may modulate this, but before we get to that feature, how about we grab a cup of coffee and go through some of the other articles in the issue?

Dr. Carolyn Lam:            

I got mine. Would you like to go first, Greg?

Dr. Greg Hundley:          

You bet, Carolyn. Well, my first study comes to us from Dr. Michael Pencino from Duke University. Carolyn, this study was performed to understand the predictive utility of a previously derived polygenic risk score for long-term risk of coronary heart disease and its additive value beyond traditional risk factors and how that might be able to inform prevention strategies. To accomplish this, data from adults aged 20 to 59 free of cardiovascular health disease from the Framingham Offspring Study and the Atherosclerosis Risk in Communities, or ARIC Study, were analyzed. Now, since the polygenic risk score was derived from people of predominantly European ancestry, individuals who self-reported white race were those that were included.

Dr. Carolyn Lam:            

Oh, interesting, so what did they find, Greg?

Dr. Greg Hundley:          

Right, Carolyn. Somewhat surprisingly, they found that, among 9,757 participants, both the traditional risk factor score and the polygenic risk score where significantly associated with incident cardiovascular heart disease in young, early midlife, and late midlife. Now, the delta C index, when the polygenic risk score was added to the traditional risk factor, score was 0.03, 0.02, and 0.002 in the young, the early midlife, and the late-midlife participants, respectively.

Carolyn, despite a statistically significant association between the polygenic risk score and the 30-year risk of cardiovascular heart disease, the C index improved only marginally with the addition of the polygenic risk score to the traditional risk factor model among young adults and did not improve among midlife adults and, thus, Carolyn, the polygenic risk score, an immutable factor, has limited clinical utility for long-term cardiovascular heart disease prediction when added to a traditional risk factor model.

Dr. Carolyn Lam:            

I really like that, Greg, because I think it also tells us that the traditional risk factors, which we can do something about, are still very important. Isn't that great? Well, the next paper is about POTS. Remember what that is? Should I give you a quiz? All right. It's okay. POTS, or Postural Orthostatic Tachycardia Syndrome, is a disorder of orthostatic intolerance that primarily affects females of childbearing age. While the underlying pathophysiology of POTS is not fully understood, it has been suggested that autoimmunity may play a role. Now, the aim of this study was to compare concentrations of autoantibodies to cardiovascular G protein-coupled receptors between 116 POTS patients and 81 healthy controls, and they were from Calgary, Canada, and Malmo, Sweden.

Dr. Greg Hundley:          

Carolyn, really interesting, so what did they find here?

Dr. Carolyn Lam:            

The investigators, led by Dr. Raj from University of Calgary in Canada, found that commercially available autoantibody concentrations to G protein-coupled receptors were not increased or altered in POTS patients relative to healthy controls as assessed using ELISA. Now, while this study suggests that these G protein-coupled receptor autoantibody concentrations alone cannot explain the pathophysiology of POTS, autoantibody activity and signals not picked up by ELISA should still be explored as these results may provide more insights into the pathophysiology of POTS.

Dr. Greg Hundley:          

Very nice, Carolyn. Well, my next study comes to us from the world of pulmonary arterial hypertension. Carolyn, clinical worsening is commonly used as an endpoint in pulmonary arterial hypertension trials. These authors, led by Dr. Steeve Provencher from the Institut Universitaire de Cardiologie Pneumologie de Quebec, aimed to assess the trial-level surrogacy of clinical worsening for mortality in pulmonary artery hypertension trials and whether the various clinical worsening components were similar in terms of frequency of occurrence, treatment-related relative risk reduction and importance to patients.

Dr. Carolyn Lam:            

Okay, so what did they find?

Dr. Greg Hundley:          

Right, Carolyn, so they searched MEDLINE, Embase and the Cochrane Library for trials evaluating the effects of pulmonary arterial hypertension on clinical worsening and, among 35 independent cohorts, so 9,450 patients, the effects of pulmonary arterial hypertension-specific therapies on clinical worsening modestly correlated with mortality. Additionally, study-level clinical worsening was not found to be a surrogate for mortality in pulmonary arterial-hypertension trials. Moreover, components of clinical worsening largely vary in frequency, response to therapy and importance to patients and, thus, are not necessarily interchangeable.

Dr. Carolyn Lam:            

Thank you, Greg. Can I tell you about some other papers in today's issue? There's a Research Letter from Dr. Cosentino on cardiorenal outcomes with ertugliflozin by baseline metformin use, and this is a post hoc analysis of the VERTIS CV trial.

Dr. Greg Hundley:          

Oh, very good, Carolyn. Well, I've got an exchange of letters from Professors Boriani and Steinberg regarding the article “Driving Restrictions and Early Arrhythmias in Patients Receiving a Secondary Prevention Implantable Cardioverter-Defibrillator, the DREAM-ICD-II Study.” There's also an ECG Challenge from Professor Gao entitled “Syncope in a 3-Year-Old Child During the Perioperative Period. What is the diagnosis? What Signs Point Toward Impending Life-threatening Event?”

Then, finally, there's a nice, On My Mind piece from Professor Greenland entitled “Insurance Payers Should Cover Selective Coronary Artery Calcium Testing in Intermediate Risk Primary Prevention Patients.” Well, Carolyn, how about we get on to that feature discussion and dive into the world of rejection after heart transplantation?

Dr. Carolyn Lam:            

Yay. Here we go.

Dr. Greg Hundley:          

Welcome, listeners, to this feature discussion on August 23rd. We have a very interesting article today to discuss with our author and associate editor pertaining to preclinical science and cardiac transplant rejection. Our author today is Dr. Kory Lavine from Washington University in St. Louis and our associate editor today is Dr. Thomas Eschenhagen from Hamburg, Germany. Welcome gentlemen.

Kory, we'll start with you. Can you describe for us some of the background information pertaining to the construct of your study and what was the hypothesis that you wanted to address?

Dr. Kory Lavine:              

Well, thank you for having me. Our study focused on heart transplant rejection, which remains a major clinical challenge that limits both the survival of heart transplant recipients as well as availability of donor hearts. Current clinical practice really focuses on suppressing the immune system in a global way, and that is somewhat effective, but carries important risks that include infection and life-threatening malignancies.

Many studies have appropriately focused on immune cells that infiltrate the transplanted heart that come from the recipient to search for new ways to suppress the immune system safely. What we've understood and learned over the past several years is that the donor heart has its own immune system and its own immune cells, and the majority of those immune cells that come with the donor heart are macrophages that can be broadly divided into two distinct lineages with different functions, tissue-resident macrophages, which lack the cell surface receptors CCR2, and monocyte-derived macrophages with expressed cell surface receptors CCR2. We tested the hypothesis in this study that these macrophages that come with the donor heart remain active for a period of time after transplantation and play important roles in either suppressing or accelerating heart transplant rejection.

Dr. Greg Hundley:          

What was the hypothesis that you wanted to address with your study?

Dr. Kory Lavine:              

Yeah, so our prior work and others' work within this field had suggested that tissue-resident macrophages, CCR2-negative macrophages, are inflammatory, and CCR2-positive macrophages have the opposite functions being inflammatory and play roles in potentiating and initiating inflammation in the heart. In this study, we hypothesized that CCR2-negative macrophages would protect from rejection, while CCR2-positive macrophages may promote heart transplant rejection and could serve as a new therapeutic target to prevent rejection in transplant recipients.

Dr. Greg Hundley:          

Excellent. Kory, can you describe for us the study design that you used to test your hypothesis?

Dr. Kory Lavine:              

Yeah. The study design and approach we used involved a mouse model of heart transplantation where we transplant a donor heart into a recipient mouse that's fully mismatched at all the MHC loci, and this serves as a nice model for both cellular and antibody-mediated rejection. To facilitate tracking these donor macrophages, we used various genetic lineage tracing systems and, to study their phenotypes, we used single-cell RNA sequencing and, to understand their function, we used mouse models that allow us to specifically deplete each of the donor macrophage populations as well as genetic models to manipulate their activation and signaling.

Dr. Greg Hundley:          

The outcome measures were going to be what?

Dr. Kory Lavine:              

Yeah. The outcome measures for transplant rejection in this mouse model are allograph survival, so the survival of the transplanted heart. We're able to directly look at how much rejection is present by histopathology, and then we're able to observe various mechanistic features using detailed phenotyping such as single-cell RNA sequencing and T-cell activation assays.

Dr. Greg Hundley:          

Very nice, Kory. Well, all, our listeners, we're very excited to hear what were your study results?

Dr. Kory Lavine:              

We learned that donor macrophages are dynamic and they survive for a period of time after transplantation or eventually lost due to transplant rejection. When we phenotyped the macrophages that came from the donor heart, we learned that they remained transcriptionally distinct from immune cells that enter the heart that were derived from the recipients, and they had important and distinct functions. If we depleted the tissue-resident macrophages that were CCR2-negative, we observed reduced allograph survival and increased rejection. If we depleted CCR2-positive macrophages that came from the donor heart, we observed improved allograph survival and reduced rejection.

Mechanistically, we learned that CCR2-positive macrophages are activated through a MyD88-dependent pathway and, if we inhibited MyD88 cytokines which controls the expression of pro-inflammatory cytokines and chemokines, we could prolong the survival of the donor heart for a very significant period of time, reduce rejection and prevent the development of T-cells that would attack the donor heart. From a mechanistic aspect, what we uncovered is that this signaling pathway in CCR2-positive macrophages regulated the recruitment of an activation of antigen-presenting cells which played important roles in generating T-cells that would target the transplanted heart.

Dr. Greg Hundley:          

It sounds like a really informative and leap forward in the whole sphere of transplant rejection. Well, listeners, now we're going to turn to our associate editor, Dr. Thomas Eschenhagen.

Thomas, you have many papers come across your desk. What attracted you to this particular paper and then, secondly, how do you put the results of this study really in the context of other research examining heart transplant rejection?

Dr. Thomas Eschenhagen:          

Yeah, thanks for having me. I mean, first, we got attracted by this paper because it's somewhat an out-of-the-box approach. It's not the standard approach to improve the systemic immunosuppression as many studies did and with actually a lot of success over the last 30 years, survivor got much better. There had been a lot of progress in the field of transplantation medicine as we all know, but as Kory said already, we still have 30% rejection, and these immunosuppressions come at a price. Having this study which turns around somehow the argumentation and looks at the donor organ was something which really attracted us. It uses advanced methods and it applies somewhat in a practical way a concept which emerged over the last, I don't know, maybe decade this concept that macrophages are really very different kind of cells. They're all called macrophages, but they're quite different and even maybe in certain respects having opposing effect.

I think many people know about this M1/M2 concept. It's CCR2 receptor positive and negative. It's criticized by some people, but here we see that it really seems to be really important and, of course, then the third argument why we really like the story is that it has a specific, clear translation impact. I mean, looking at the heart, the donor heart, and potentially even treating the donor heart before transplanting it is something which comes immediately out of the story, and that's something which we found super attractive.

Dr. Greg Hundley:          

Really interesting, so really understanding the mechanism and focusing on donor hearts. Well, listeners, let's circle back with Kory.

Kory, given that, what do you think is the next study that really needs to be performed in this sphere of research?

Dr. Kory Lavine:              

I think Thomas said it exactly as we're thinking about it, so the next area that we're really excited to attack and we're hopeful that the field will focus on is ways to build methods and technologies to treat the donor heart between the time of procurement and the time of transplant, when it's being transported and potentially even being perfused for a period of time. We're really interested in finding approaches to identify small molecules and other potential biologic therapies that could be used to prevent the activation of donor CCR2-positive macrophages.

It's a really attractive approach because treating the donor heart ex vivo decreases the risk of adversely affecting other organs that may be transplanted if you're treating the donor, for instance, and it may decrease the risk of immunosuppression and infection by not having to treat the recipient and we're catching the heart in this window where the risks are much lower.

The other area that we're really excited to focus on is trying to identify the exact mediators that are generated from donor CCR2-positive macrophages that mediate the recruitment and activation of antigen-presenting cells because that would represent another potential therapeutic target.

Dr. Greg Hundley:          

Very nice. Thomas, what are your thoughts about what might be the next study to be performed really in this sphere of research?

Dr. Thomas Eschenhagen:

It's obviously something rather a question to Kory than to me, but I agree to what he said. I think it is pretty obvious what are the next steps mechanistically on the one hand, but practically on the other hand. I mean, at this point, we are at the mouse level, so the question is to which extent can this concept be translated into larger animals and then finally in humans? I was wondering, given these newer methods to keep donor hearts alive for long, extended periods, I was wondering which extent you are already collaborating with the respective groups who develop this approach because that obviously would increase the window of opportunity here for drugs. I think it's really an exciting and pretty visible next steps which we see here, and I can just hope that you're going this path and that it will be successful.

Dr. Greg Hundley:          

Kory, any thoughts on those collaborations that Thomas just spoke of?

Dr. Kory Lavine:              

We're definitely establishing collaborations to focus on ex vivo profusion of donor hearts because that's, as Thomas mentioned, is a perfect window to manipulate the immune populations that are within the donor heart. Those studies have to be team science, they have to be collaborative and they have to have a focus on large animals and then moving into clinic. We're definitely forming those collaborations and excited to work as a group.

Dr. Greg Hundley:          

Very nice. Well, listeners, what an exciting paper to discuss here as part of this feature discussion from the world of preclinical science. We want to thank Dr. Kory Lavine from Washington University in St. Louis, Missouri, and also our own associate editor, Dr. Thomas Eschenhagen from Hamburg Germany, for really bringing us this research study highlighting that distinct populations of donor and recipient macrophages coexist within the transplanted heart, and donor CCR2-positive macrophages are key mediators of allograph rejection and deletion of MyD88 signaling in donor macrophages is sufficient to suppress rejection and extend allograph survival.

Well, on behalf of Carolyn and myself, we want to wish you a great week, and we will catch you next week on the run.

Dr. Greg Hundley:          

This program is copyright of the American Heart association, 2022. The opinions expressed by the speakers in this podcast are their own and not necessarily those of the editors or of the American Heart Association. For more, please visit ahajournals.org.