American Journal of Transplantation 2014; 14: 503–504 Wiley Periodicals Inc.
Editorial
C
Copyright 2014 The American Society of Transplantation and the American Society of Transplant Surgeons doi: 10.1111/ajt.12613
Autophagy for Tolerance Induction in Transplantation B. Vokaer1,2,* and A. Le Moine1,2,* 1
Nephrology and Internal Medicine Department, Erasme Hospital, Universite´ Libre de Bruxelles, Anderlecht, Belgium 2 Institute for Medical Immunology, Universite´ Libre de Bruxelles, Gosselies, Belgium Corresponding authors: Benoıˆt Vokaer, [email protected], and Alain Le Moine, [email protected] Received 11 November 2013, revised and accepted for publication 02 December 2013
Autophagy is a homeostatic and ubiquitous process by which cells recycle injured organelles and proteins through intracellular formation of vesicles (autophagosomes), which undergo secondary fusion with lysosomes for degradation. In steady states, this recycling process facilitates cell longevity, particularly in slow-dividing cells such as podocytes where autophagy plays a role in preventing glomeruloscerosis (1). During metabolic stress, autophagosomes provide nutrients through degradation of their cargo. In these two settings, autophagy ensures cell survival and protects against ischemia-reperfusion injury. In other circumstances, such as chemotherapy administration, autophagy is critically involved in the induced programmed cell death that differs from classical apoptosis (2). Thus, autophagy can promote either cellular survival or death depending on the context. Autophagy participates in immune responses at both the innate and adaptive levels. In macrophages, autophagosomes engulf and clear intracellular pathogens after fusion with lysosomes. In T lymphocytes, TCR engagement is a strong inducer of autophagy. On the one hand, by providing nutrients and energy through protein degradation, autophagy is required for optimal T cell activation, cytokine production and proliferation. On the other hand, an excessive burden of autophagic vesicles is deleterious for T lymphocytes and can lead to their death. This situation can occur when the TCR is activated in conditions of nutrient or growth factor starvation, as has been demonstrated by IL-2 deprivation experiments (3). Autophagy also plays a critical role in both central and peripheral tolerance to self-antigens. In thymic epithelial cells, high numbers of autophagosomes fuse with MHC II-
loaded vesicles to allow delivery of self-antigens for positive and negative CD4þ T cell selection. In the periphery, autophagy-related genes stimulate classical phagocytosis in macrophages, a key process for clearance of apoptotic cells that release auto-antigens and trigger autoimmunity. In macrophages, autophagy negatively regulates IL-1b release secondary to NALP3 inflammasome activation by degrading altered mitochondria. In line with this, autophagy might modulate susceptibility to autoimmunity. Genome-wide association studies identified a link between dysfunctional autophagy-related genes (Atg5, Atg16l1) and systemic lupus erythematous, rheumatoid arthritis or Crohn’s disease (2). By using autophagy-deficient Beclinþ/ recipient mice, Verghese et al (4) identified a new role of autophagy in transplant tolerance induction. They have taken advantage of the CD40–CD154 costimulatory blockade coupled with donor-specific transfusion (DST) in a full MHC-mismatched cardiac allograft model. In this stringent combination, tolerance induction requires significant alloreactive T cell deletion. Apoptosis was initially described as the main pathway for activation-induced cell death in the context of costimulatory blockade. Nevertheless, this work demonstrates that normal expression of Beclin-1, a key protein for initiating autophagosome formation, is also required. The lack of cellular death observed in Beclin-1þ/ recipients treated with DST/CD40–CD154 blockade resulted in allograft rejection, enhanced T cell proliferation and IFN-g production. Whereas autophagy was classically correlated with a pro-survival effect, this work illustrates the suitable role of autophagy in inducing cellular death under specific conditions. Of course, it is important to keep in mind that additional roles of Beclin-1 that are independent from autophagy may confound interpretation of the data. How costimulatory blockade and DST induce these ‘‘specific conditions’’ remains to be elucidated. A possible mechanism could rely on autocrine IL-2 deprivation induced by the absence of CD40–CD154 interactions. Indeed, in vitro experiments revealed that IL-2 withdrawal after antiCD3/anti-CD28 TCR stimulation leads to a dramatic increase in autophagy-dependent cellular death (3). At the molecular level, this exacerbated autophagy could result from two successive events. First, TCR-dependent activation of NF-kB stimulates the transcription of the autophagic Beclin-1 gene. Second, the lack of IL-2 receptor stimulation prevents the activation of the mammalian target of rapamycin (mTOR), a potent repressor of autophagy. While regulatory T cells (Treg) from Beclin-1þ/ recipients display
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Figure 1: Dual functions of autophagy. A hypothetical and schematic view of the opposite effects of autophagy either in T cell deletional mechanisms associated with tolerance induction or in T cell activation and allograft rejection is represented. On the upper part, alloreactive TCR engagement in the context of IL-2 starvation, costimulation blockade or the inhibition of the mTOR pathway enhances autophagy leading to T cell death and the shaping of the T cell repertoire allowing graft acceptance. On the bottom, autophagy facilitates T cell activation, survival and effector functions as in allograft rejection.
normal suppressive capacities, the authors demonstrate that effector T cells are less prone to Treg-mediated suppression. Nevertheless, this effect may not be responsible for breaking tolerance since delayed pharmacological blockade of autophagy at day 50 posttransplantation did not restore graft rejection. Since clonal deletion of the alloreactive repertoire requires both apoptosis and autophagy during tolerance induction, this work raises questions about the potential effect of immunosuppressive drugs on these two mechanisms of cellular death. With regard to this specific issue, mTOR inhibitors (rapamycin, everolimus) probably work much more as tolerogenic drugs than calcineurin inhibitors. Indeed, whereas mTOR is a strong inhibitor of autophagy, calcineurin-dependent NFAT activation is clearly implicated in activated T cell apoptosis. This is in agreement with previous experiments that showed that addition of rapamycin to costimulation blockade results in stable allograft tolerance whereas adding cyclosporine A to costimulatory blockade abolishes alloreactive T cell death and long-term allograft acceptance (5,6). Taken together, these observations underline the absolute necessity of incorporating integrated approaches and thoughtful immunosuppression choices into the development of strategies for tolerance induction in transplantation (Figure 1).
Acknowledgment We thank Sandy Field for editing the manuscript.
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Disclosure The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation.
References 1. Hartleben B, Godel M, Meyer-Schwesinger C, et al. Autophagy influences glomerular disease susceptibility and maintains podocyte homeostasis in aging mice. J Clin Invest 2010; 120: 1084–1096. 2. Bhattacharya A, Eissa NT. Autophagy and autoimmunity crosstalks. Front Immunol 2013; 4: 1–7. 3. Li C, Capan E, Zhao Y, et al. Autophagy is induced in CD4þ T cells and important for the growth factor-withdrawal cell death. J Immunol 2006; 177: 5163–5168. 4. Verghese DA, Yadav A, Bizargity P, Murphy B, Heeger PS, Schro¨ppel B. Costimulatory blockade-induced allograft survival requires Beclin1. Am J Transplant 2014; 14: 545–553. 5. Li Y, Li XC, Zheng XX, Wells AD, Turka LA, Strom TB. Blocking both signal 1 and signal 2 of T-cell activation prevents apoptosis of alloreactive T cells and induction of peripheral allograft tolerance. Nat Med 1999; 5: 1298–1302. 6. Taylor PA, Lees CJ, Wilson JM, et al. Combined effects of calcineurin inhibitors or sirolimus with anti-CD40L mAb on alloengraftment under nonmyeloablative conditions. Blood 2002; 100: 3400–3407.
American Journal of Transplantation 2014; 14: 503–504
Editorial
C
Copyright 2014 The American Society of Transplantation and the American Society of Transplant Surgeons doi: 10.1111/ajt.12613
Autophagy for Tolerance Induction in Transplantation B. Vokaer1,2,* and A. Le Moine1,2,* 1
Nephrology and Internal Medicine Department, Erasme Hospital, Universite´ Libre de Bruxelles, Anderlecht, Belgium 2 Institute for Medical Immunology, Universite´ Libre de Bruxelles, Gosselies, Belgium Corresponding authors: Benoıˆt Vokaer, [email protected], and Alain Le Moine, [email protected] Received 11 November 2013, revised and accepted for publication 02 December 2013
Autophagy is a homeostatic and ubiquitous process by which cells recycle injured organelles and proteins through intracellular formation of vesicles (autophagosomes), which undergo secondary fusion with lysosomes for degradation. In steady states, this recycling process facilitates cell longevity, particularly in slow-dividing cells such as podocytes where autophagy plays a role in preventing glomeruloscerosis (1). During metabolic stress, autophagosomes provide nutrients through degradation of their cargo. In these two settings, autophagy ensures cell survival and protects against ischemia-reperfusion injury. In other circumstances, such as chemotherapy administration, autophagy is critically involved in the induced programmed cell death that differs from classical apoptosis (2). Thus, autophagy can promote either cellular survival or death depending on the context. Autophagy participates in immune responses at both the innate and adaptive levels. In macrophages, autophagosomes engulf and clear intracellular pathogens after fusion with lysosomes. In T lymphocytes, TCR engagement is a strong inducer of autophagy. On the one hand, by providing nutrients and energy through protein degradation, autophagy is required for optimal T cell activation, cytokine production and proliferation. On the other hand, an excessive burden of autophagic vesicles is deleterious for T lymphocytes and can lead to their death. This situation can occur when the TCR is activated in conditions of nutrient or growth factor starvation, as has been demonstrated by IL-2 deprivation experiments (3). Autophagy also plays a critical role in both central and peripheral tolerance to self-antigens. In thymic epithelial cells, high numbers of autophagosomes fuse with MHC II-
loaded vesicles to allow delivery of self-antigens for positive and negative CD4þ T cell selection. In the periphery, autophagy-related genes stimulate classical phagocytosis in macrophages, a key process for clearance of apoptotic cells that release auto-antigens and trigger autoimmunity. In macrophages, autophagy negatively regulates IL-1b release secondary to NALP3 inflammasome activation by degrading altered mitochondria. In line with this, autophagy might modulate susceptibility to autoimmunity. Genome-wide association studies identified a link between dysfunctional autophagy-related genes (Atg5, Atg16l1) and systemic lupus erythematous, rheumatoid arthritis or Crohn’s disease (2). By using autophagy-deficient Beclinþ/ recipient mice, Verghese et al (4) identified a new role of autophagy in transplant tolerance induction. They have taken advantage of the CD40–CD154 costimulatory blockade coupled with donor-specific transfusion (DST) in a full MHC-mismatched cardiac allograft model. In this stringent combination, tolerance induction requires significant alloreactive T cell deletion. Apoptosis was initially described as the main pathway for activation-induced cell death in the context of costimulatory blockade. Nevertheless, this work demonstrates that normal expression of Beclin-1, a key protein for initiating autophagosome formation, is also required. The lack of cellular death observed in Beclin-1þ/ recipients treated with DST/CD40–CD154 blockade resulted in allograft rejection, enhanced T cell proliferation and IFN-g production. Whereas autophagy was classically correlated with a pro-survival effect, this work illustrates the suitable role of autophagy in inducing cellular death under specific conditions. Of course, it is important to keep in mind that additional roles of Beclin-1 that are independent from autophagy may confound interpretation of the data. How costimulatory blockade and DST induce these ‘‘specific conditions’’ remains to be elucidated. A possible mechanism could rely on autocrine IL-2 deprivation induced by the absence of CD40–CD154 interactions. Indeed, in vitro experiments revealed that IL-2 withdrawal after antiCD3/anti-CD28 TCR stimulation leads to a dramatic increase in autophagy-dependent cellular death (3). At the molecular level, this exacerbated autophagy could result from two successive events. First, TCR-dependent activation of NF-kB stimulates the transcription of the autophagic Beclin-1 gene. Second, the lack of IL-2 receptor stimulation prevents the activation of the mammalian target of rapamycin (mTOR), a potent repressor of autophagy. While regulatory T cells (Treg) from Beclin-1þ/ recipients display
503
Vokaer and Le Moine
Figure 1: Dual functions of autophagy. A hypothetical and schematic view of the opposite effects of autophagy either in T cell deletional mechanisms associated with tolerance induction or in T cell activation and allograft rejection is represented. On the upper part, alloreactive TCR engagement in the context of IL-2 starvation, costimulation blockade or the inhibition of the mTOR pathway enhances autophagy leading to T cell death and the shaping of the T cell repertoire allowing graft acceptance. On the bottom, autophagy facilitates T cell activation, survival and effector functions as in allograft rejection.
normal suppressive capacities, the authors demonstrate that effector T cells are less prone to Treg-mediated suppression. Nevertheless, this effect may not be responsible for breaking tolerance since delayed pharmacological blockade of autophagy at day 50 posttransplantation did not restore graft rejection. Since clonal deletion of the alloreactive repertoire requires both apoptosis and autophagy during tolerance induction, this work raises questions about the potential effect of immunosuppressive drugs on these two mechanisms of cellular death. With regard to this specific issue, mTOR inhibitors (rapamycin, everolimus) probably work much more as tolerogenic drugs than calcineurin inhibitors. Indeed, whereas mTOR is a strong inhibitor of autophagy, calcineurin-dependent NFAT activation is clearly implicated in activated T cell apoptosis. This is in agreement with previous experiments that showed that addition of rapamycin to costimulation blockade results in stable allograft tolerance whereas adding cyclosporine A to costimulatory blockade abolishes alloreactive T cell death and long-term allograft acceptance (5,6). Taken together, these observations underline the absolute necessity of incorporating integrated approaches and thoughtful immunosuppression choices into the development of strategies for tolerance induction in transplantation (Figure 1).
Acknowledgment We thank Sandy Field for editing the manuscript.
504
Disclosure The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation.
References 1. Hartleben B, Godel M, Meyer-Schwesinger C, et al. Autophagy influences glomerular disease susceptibility and maintains podocyte homeostasis in aging mice. J Clin Invest 2010; 120: 1084–1096. 2. Bhattacharya A, Eissa NT. Autophagy and autoimmunity crosstalks. Front Immunol 2013; 4: 1–7. 3. Li C, Capan E, Zhao Y, et al. Autophagy is induced in CD4þ T cells and important for the growth factor-withdrawal cell death. J Immunol 2006; 177: 5163–5168. 4. Verghese DA, Yadav A, Bizargity P, Murphy B, Heeger PS, Schro¨ppel B. Costimulatory blockade-induced allograft survival requires Beclin1. Am J Transplant 2014; 14: 545–553. 5. Li Y, Li XC, Zheng XX, Wells AD, Turka LA, Strom TB. Blocking both signal 1 and signal 2 of T-cell activation prevents apoptosis of alloreactive T cells and induction of peripheral allograft tolerance. Nat Med 1999; 5: 1298–1302. 6. Taylor PA, Lees CJ, Wilson JM, et al. Combined effects of calcineurin inhibitors or sirolimus with anti-CD40L mAb on alloengraftment under nonmyeloablative conditions. Blood 2002; 100: 3400–3407.
American Journal of Transplantation 2014; 14: 503–504
