Allografts Stimulate Cross‐Reactive Virus‐Specific Memory CD8 T Cells with Private Specificity

Viral infections have been associated with the rejection of transplanted allografts in humans and mice, and the induction of tolerance to allogeneic tissues in mice is abrogated by an ongoing viral infection and inhibited in virus‐immune mice. One proposed mechanism for this ‘heterologous immunity’ is the induction of alloreactive T cell responses that cross‐react with virus‐derived antigens. These cross‐reactive CD8 T cells are generated during acute viral infection and survive into memory, but their ability to partake in the immune response to allografts in vivo is not known. We show here that cross‐reactive, virus‐specific memory CD8 T cells from mice infected with LCMV proliferated in response to allografts. CD8 T cells specific to several LCMV epitopes proliferated in response to alloantigens, with the magnitude and hierarchy of epitope‐specific responses varying with the private specificities of the host memory T cell repertoire, as shown by adoptive transfer studies. Last, we show that purified LCMV‐specific CD8 T cells rejected skin allografts in SCID mice. These findings therefore implicate a potential role for heterologous immunity in virus‐induced allograft rejection.     

Imaging Tolerance Induction in the Classic Medawar Neonatal Mouse Model: Active Roles of Multiple F1‐Donor Cell Types

The immature immune system is uniquely susceptible to tolerance induction and thus an attractive target for immunomodulation strategies for organ transplantation. Newborn mice injected with adult semi‐allogeneic lymphohematopoietic cells accept transplants without immunosuppressive drugs. Early in vivo/in situ events leading to neonatal tolerance remain poorly understood. Here, we show by whole body/organ imaging that injected cells home to lymphoid organs and liver where various F1‐donor cell types selectively alter neonatal immunity. In host thymus, F1‐donor dendritic cells (DC) interact with developing thymocytes and regulatory T cells suggesting a role in negative selection. In spleen and lymph nodes, F1‐donor regulatory T/B cells associate with host alloreactive cells and by themselves prolong cardiac allograft survival. In liver, F1‐donor cells give rise to albumin‐containing hepatocyte‐like cells. The neonatal immune system is lymphopenic, Th‐2 immunodeviated and contains immature DC, suggesting susceptibility to regulation by adult F1‐donor cells. CD8a T cell inactivation greatly enhances chimerism, suggesting that variable emerging neonatal alloreactivity becomes a barrier to tolerance induction. This comprehensive qualitative imaging study systematically shows contribution of multiple in vivo processes leading simultaneously to robust tolerance. These insights into robust tolerance induction have important implications for development of strategies for clinical application.     

Downregulation of cytolytic activity of human effector cells by transgenic expression of human PD‐ligand‐1 on porcine target cells

Cellular rejection is a relevant hurdle for successful pig‐to‐primate xenotransplantion. We have shown previously that the induction of a human anti‐pig T cell response (in vitro activation of CD4⁺ T cells) can be suppressed by the overexpression of human negative costimulatory ligands (e.g. programmed death receptor ligand, PD‐L1) on pig antigen presenting cells. Here, we asked whether PD‐L1 mediated enhancement of negative signaling might also be efficient during the effector phase of human anti‐pig cellular immune responses. The porcine B‐cell line L23 was transfected with human PD‐L1, and clones were selected stably expressing PD‐L1 with low, medium, or high density. Mock‐transfected L23 cells were effectively lysed by human cytotoxic effector cells (IL‐2 activated CD8⁺ T cells and CD56⁺ cells). The lytic potential of the effectors decreased with increasing levels of PD‐L1 and was reduced by about 50% in L23‐PD‐L^high targets. A proportion of activated CD8⁺ effector cells underwent apoptosis when exposed to PD‐L1 expressing L23 cells. These data suggest that the overexpression of PD‐L1 on target cells may (a) trigger negative signals in effector cells that prevent the release of cytolytic molecules and/or (b) induce apoptosis in the attacking effector cells thereby protecting targets from destruction.     

Loss of DRAK2 Signaling Enhances Allogeneic Transplant Survival by Limiting Effector and Memory T Cell Responses

Here, we demonstrate that loss of DRAK2 signaling significantly promotes the acceptance of allogeneic engraftment in two separate transplant models without promoting generalized immunosuppression. Drak2?/? T cells failed to reject allogeneic tumors, and were defective in rejecting Balb/C allogeneic skin grafts on C57BL6/J recipients. A significant fraction of alloreactive Drak2?/? T cells underwent apoptosis following activation, whereas enforced expression of Bcl‐xL in Drak2?/? T cells restored allograft rejection. Formation of allogeneic memory was also greatly hampered in T cells lacking the Drak2 gene. Adoptive transfer of memory T cells from Drak2?/? mice failed to promote the rejection of allogeneic tumors, and such cells led to significantly delayed rejection of skin allografts in the Balb/C‐>C57BL/6J model. Costimulatory blockade by in vivo administration of Cytotoxic T‐Lymphocyte Antigen 4 fusion protein (CTLA4‐Ig) synergized with the DRAK2 deficiency and led to long‐term allogeneic skin graft acceptance. Overall, these results demonstrate that DRAK2 plays an important role in primary and memory T cell responsiveness to allografts. Because previous studies have demonstrated that a loss of DRAK2 does not negatively impact antiviral immunity, the studies here underscore the potential utility of pharmacological blockade of DRAK2 to achieve transplant maintenance without the imposition of generalized immunosuppression.     

The Programmed Death (PD)‐1/PD‐Ligand 1 Pathway Regulates Graft‐Versus‐Host‐Reactive CD8 T Cells After Liver Transplantation

Acute graft‐versus‐host disease (aGVHD) is a life‐threatening complication after solid‐organ transplantation, which is mediated by host‐reactive donor T cells emigrating from the allograft. We report on two liver transplant recipients who developed an almost complete donor chimerism in peripheral blood and bone marrow‐infiltrating T cells during aGVHD. By analyzing these T cells directly ex vivo, we found that they died by apoptosis over time without evidence of rejection by host T cells. The host‐versus‐donor reactivity was selectively impaired, as anti‐third‐party and antiviral T cells were still detectable in the host repertoire. These findings support the acquired donor‐specific allotolerance concept previously established in animal transplantation studies. We also observed that the resolution of aGVHD was not accompanied by an expansion of circulating immunosuppressive CD4/CD25/FoxP3‐positive T cells. In fact, graft‐versus‐host‐reactive T cells were controlled by an alternative negative regulatory pathway, executed by the programmed death (PD)‐1 receptor and its ligand PD‐L1. We found high PD‐1 expression on donor CD4 and CD8 T cells. In addition, blocking PD‐L1 on host‐derived cells significantly enhanced alloreactivity by CD8 T cells in vitro. We suggest the interference with the PD‐1/PD‐L1 pathway as a therapeutic strategy to control graft‐versus‐host‐reactive T cells in allograft recipients.     

Regulatory T cells in immune‐mediated renal disease

Regulatory T cells (Tregs) are CD4+ T cells that can suppress immune responses by effector T cells, B cells and innate immune cells. This review discusses the role that Tregs play in murine models of immune‐mediated renal diseases and acute kidney injury and in human autoimmune kidney disease (such as systemic lupus erythematosus, anti‐glomerular basement membrane disease, anti‐neutrophil cytoplasmic antibody‐associated vasculitis). Current research suggests that Tregs may be reduced in number and/or have impaired regulatory function in these diseases. Tregs possess several mechanisms by which they can limit renal and systemic inflammatory immune responses. Potential therapeutic applications involving Tregs include in vivo induction of Tregs or inducing Tregs from naïve CD4+ T cells or expanding natural Tregs ex vivo, to use as a cellular therapy. At present, the optimal method of generating a phenotypically stable pool of Tregs with long‐lasting suppressive effects is not established, but human studies in renal transplantation are underway exploring the therapeutic potential of Tregs as a cellular therapy, and if successful may have a role as a novel therapy in immune‐mediated renal diseases.     

Regulatory T‐Cell Counter‐Regulation by Innate Immunity Is a Barrier to Transplantation Tolerance

Innate immune signals foster adaptive immunity through activation of antigen‐presenting cells. Recent in vitro evidence suggests that innate signaling may also contribute to immunity by countering the effects of regulatory T cells (T‐regs), counter‐regulation. We present in vivo evidence using a transgenic skin allograft model that the function of T‐regs is lost in the setting of acute skin transplantation but remains intact when grafts were transplanted 1 month prior to allow surgery‐induced inflammation to abate. Our findings identify T‐reg counter‐regulation as a naturally occurring process that accompanies transplantation and an important barrier to T‐reg–mediated tolerance. Our finding further highlights the central role of regulatory cell deactivation in the initiation of the immune response.     

Human Osteoclasts Are Inducible Immunosuppressive Cells in Response to T cell–Derived IFN‐γ and CD40 Ligand In Vitro

Osteoclasts (OCs) are bone resorbing cells whose activity can be regulated by activated T cells and their cytokines. However, the immune function of OCs is largely unknown. In this study, we found that as bystanders, human OCs effectively suppressed T‐cell proliferation induced by allogeneic, microbial antigenic, and T‐cell receptor stimuli in vitro. Mechanism studies revealed that T cell–derived IFN‐γ and CD40 ligand (CD40L) induced the expression of indoleamine 2,3‐dioxygenase (IDO) in OCs, which mediated the immunosuppressive function on T‐cell proliferation through depleting tryptophan. Neutralizing IFN‐γ and blocking CD40L, or silencing or inhibiting IDO in OCs restored T‐cell proliferation in the presence of OCs. Our data reveal a novel function of human OCs as inducible immunosuppressive cells, and a feedback loop between OCs and activated T cells. Thus, this study provides new insight into the mechanism of the immunosuppressive function of OCs, and may be helpful for developing novel therapeutic strategies for human diseases involving both the bone and immune systems. © 2014 American Society for Bone and Mineral Research.     

Costimulatory blockade with mTor inhibition abrogates effector T‐cell responses allowing regulatory T‐cell survival in renal transplantation

The advent of novel immunosuppressive strategies in renal transplantation, with immunomodulatory properties, might facilitate long‐term allograft survival. T‐cell depletion, costimulation‐blockade and mTor inhibition have been shown to favour anti‐donor hyporesponsiveness. Recently, the combination of rATG, belatacept (Bela) and sirolimus (SRL) has been used in kidney transplantation, showing very low incidence of acute rejection and excellent 12‐month graft and patient survival. Herein, we have analysed the 1‐year evolution of memory/effector and regulatory T cells and assessed the donor‐specific T‐cell alloimmune response in a group of these patients and compared with others treated with a calcineurin‐inhibitor(CNI)‐based (rATG/tacrolimus/MMF), and two other Bela‐based regimens (rATG/Bela/MMF and basiliximab/Bela/MMF/steroids). During the first year after transplantation, patients receiving rATG/Bela/SRL had significantly higher percentage of Tregs upon the memory T‐cell compartment and showed a potent anti‐donor suppressive activity. In an in vitro naive and memory/effector T‐cell co‐culture, the combination of costimulation‐blockade and SRL could abrogate both antigen‐specific T‐cell responses as efficiently as using a CNI drug. The combination of T‐cell depletion, costimulation‐blockade and mTor inhibition seems to be able to allow Treg survival and inhibit donor‐specific alloreactive effector immune responses after kidney transplantation in humans.     

Local expression of indoleamine 2,3‐dioxygenase suppresses T‐cell‐mediated rejection of an engineered bilayer skin substitute

Engineered skin substitutes (ESSs) comprising both keratinocytes and fibroblasts can afford many advantages over the use of autologous keratinocyte grafts for the treatment of full‐thickness and partial‐thickness burns. In this study, we investigated the efficacy of a novel ESS containing both genetically altered fibroblasts that express the immunosuppressive factor indoleamine 2,3‐dioxygenase (IDO) and primary keratinocytes from a nonautologous source to confer immune protection of xenogeneic cells cultured in a bilayer ESS. The results show that engraftment of IDO expressing skin substitutes on the back of rats significantly improves healing progression over 7 days compared with both nontreated and non‐IDO‐expressing skin substitutes (p<0.001). Immuno‐staining of CD3 and CD31 suggests that IDO‐expressing skin substitutes significantly suppress T cell infiltration (p<0.001) and improve neovascularization by four‐fold (12.6±1.2 vs. 3.0±1.0 vessel‐like structure/high power field), respectively. In conclusion, we found that IDO expression can improve the efficacy of nonautologous ESS for the purpose of wound healing by mitigating T‐cell infiltration as well as promoting vascularization of the graft.     

Donor‐Specific CD8+Foxp3+ T Cells Protect Skin Allografts and Facilitate Induction of Conventional CD4+Foxp3+ Regulatory T Cells

CD4⁺ regulatory T cells play a critical role in tolerance induction in transplantation. CD8⁺ suppressor T cells have also been shown to control alloimmune responses in preclinical and clinical models. However, the exact nature of the CD8⁺ suppressor T cells, their induction and mechanism of function in allogeneic transplantation remain elusive. In this study, we show that functionally suppressive, alloantigen‐specific CD8⁺Foxp3⁺ T cells can be induced and significantly expanded by stimulating naïve CD8⁺ T cells with donor dendritic cells in the presence of IL‐2, TGF‐β1 and retinoic acid. These CD8⁺Foxp3⁺ T cells express enhanced levels of CTLA‐4, CCR4 and CD103, inhibit the up‐regulation of costimulatory molecules on dendritic cells, and suppress CD4 and CD8 T cell proliferation and cytokine production in a donor‐specific and contact‐dependent manner. Importantly, upon adoptive transfer, the induced CD8⁺Foxp3⁺ T cells protect full MHC‐mismatched skin allografts. In vivo, the CD8⁺Foxp3⁺ T cells preferentially traffic to the graft draining lymph node where they induce conventional CD4⁺Foxp3⁺ T cells and concurrently suppress effector T cell expansion. We conclude that donor‐specific CD8⁺Foxp3⁺ suppressor T cells can be induced and exploited as an effective form of cell therapy for graft protection in transplantation.     

Low‐Dose Rapamycin Treatment Increases the Ability of Human Regulatory T Cells to Inhibit Transplant Arteriosclerosis In Vivo

Regulatory T cells (T_reg) are currently being tested in clinical trials as a potential therapy in cell and solid organ transplantation. The immunosuppressive drug rapamycin has been shown to preferentially promote T_reg expansion. Here, we hypothesized that adjunctive rapamycin therapy might potentiate the ability of ex vivo expanded human T_reg to inhibit vascular allograft rejection in a humanized mouse model of arterial transplantation. We studied the influence of combined treatment with low‐dose rapamycin and subtherapeutic T_reg numbers on the development of transplant arteriosclerosis (TA) in human arterial grafts transplanted into immunodeficient BALB/cRag2^?/?Il2rg^?/? mice reconstituted with allogeneic human peripheral blood mononuclear cell. In addition, we assessed the effects of the treatment on the proliferation and apoptosis of naïve/effector T cells. The combined therapy efficiently suppressed T‐cell proliferation in vivo and in vitro. Neointima formation in the human arterial allografts was potently inhibited compared with each treatment alone. Interestingly, CD4⁺ but not CD8⁺ T lymphocytes were sensitive to T_reg and rapamycin‐induced apoptosis in vitro. Our data support the concept that rapamycin can be used as an adjunctive therapy to improve efficacy of T_reg‐based immunosuppressive protocols in clinical practice. By inhibiting TA, T_reg and rapamycin may prevent chronic transplant dysfunction and improve long‐term allograft survival.     

Low‐Dose IL‐2 for In Vivo Expansion of CD4+ and CD8+ Regulatory T Cells in Nonhuman Primates

IL‐2 is a known potent T cell growth factor that amplifies lymphocyte responses in vivo. This capacity has led to the use of high‐dose IL‐2 to enhance T cell immunity in patients with AIDS or cancer. However, more recent studies have indicated that IL‐2 is also critical for the development and peripheral expansion of regulatory T cells (Tregs). In the current study, low‐dose IL‐2 (1 million IU/m² BSA/day) was administered to expand Tregs in vivo in naïve nonhuman primates. Our study demonstrated that low‐dose IL‐2 therapy significantly expanded peripheral blood CD4⁺ and CD8⁺ Tregs in vivo with limited expansion of non‐Treg cells. These expanded Tregs are mainly CD45RA^? Foxp3 ^high activated Tregs and demonstrated potent immunosuppressive function in vitro. The results of this preclinical study can serve as a basis to develop Treg immunotherapy, which has significant therapeutic potential in organ/cellular transplantation.     

Human TNF-related apoptosis-inducing ligand-expressing dendritic cells from transgenic pigs attenuate human xenogeneic T cell responses

Kemter E, Lieke T, Kessler B, Kurome M, Wuensch A, Summerfield A, Ayares D, Nagashima H, Baars W, Schwinzer R, Wolf E. Human TNF‐related apoptosis‐inducing ligand‐expressing dendritic cells from transgenic pigs attenuate human xenogeneic T cell responses. Xenotransplantation 2012; 19: 40–51. © 2012 John Wiley & Sons A/S. Abstract: Background: Efficient and precise techniques for the genetic modification of pigs facilitate the generation of tailored donor animals for xenotransplantation. Numerous transgenic pig lines exist with the focus on inhibition of the complement system and of humoral immune responses. In addition, immune cell‐based responses need to be controlled to prevent pig‐to‐primate xenograft rejection. Expression of human (hu) TNF‐related apoptosis‐inducing ligand (TRAIL) on porcine cells has the potential to ameliorate human T cell responses. Methods: We generated transgenic pigs expressing human tumor necrosis factor (TNF)‐related apoptosis‐inducing ligand (huTRAIL) under the control of either the mouse H2K^b promoter or a CMV enhancer/chicken β‐actin (CAG) promoter, the latter one (CAG‐huTRAIL) on a GGTA1 knockout/huCD46 transgenic background. The biological activity of huTRAIL was demonstrated by its apoptosis‐inducing effect on Jurkat lymphoma cells. To clarify whether huTRAIL affects also primary immune cells and whether its effects depend on the presence of co‐stimulatory molecules, we exposed human peripheral blood mononuclear cells (PBMC) or isolated T cells to huTRAIL‐expressing porcine fibroblasts or dendritic cells in vitro. Results: H2K^b‐huTRAIL transgenic pigs express huTRAIL mainly in the spleen and secondary lymphoid tissues. The CAG‐huTRAIL construct facilitated huTRAIL expression in multiple organs, the level being at least one order of magnitude higher than in H2K^b‐huTRAIL transgenic pigs. Incubation with huTRAIL‐expressing H2K^b‐huTRAIL transgenic porcine dendritic cells decreased human T cell proliferation significantly without any signs of apoptosis. In spite of the high transgene expression level, CAG‐huTRAIL transgenic fibroblasts did not affect proliferation of human PBMC, independent of their activation state. Conclusions: These results suggest huTRAIL expression on porcine dendritic cells as a possible strategy to attenuate T cell responses against pig‐to‐primate xenografts.     

Prevention of Allograft Rejection by Use of Regulatory T Cells With an MHC‐Specific Chimeric Antigen Receptor

CD4⁺CD25^highFOXP3⁺ regulatory T cells (Tregs) are involved in graft‐specific tolerance after solid organ transplantation. However, adoptive transfer of polyspecific Tregs alone is insufficient to prevent graft rejection even in rodent models, indicating that graft‐specific Tregs are required. We developed a highly specific chimeric antigen receptor that recognizes the HLA molecule A*02 (referred to as A2‐CAR). Transduction into natural regulatory T cells (nTregs) changes the specificity of the nTregs without alteration of their regulatory phenotype and epigenetic stability. Activation of nTregs via the A2‐CAR induced proliferation and enhanced the suppressor function of modified nTregs. Compared with nTregs, A2‐CAR Tregs exhibited superior control of strong allospecific immune responses in vitro and in humanized mouse models. A2‐CAR Tregs completely prevented rejection of allogeneic target cells and tissues in immune reconstituted humanized mice in the absence of any immunosuppression. Therefore, these modified cells have great potential for incorporation into clinical trials of Treg‐supported weaning after allogeneic transplantation.     

Pancreatic islets engineered with a FasL protein induce systemic tolerance at the induction phase that evolves into long‐term graft‐localized immune privilege

We have previously shown that pancreatic islets engineered to transiently display a modified form of FasL protein (SA‐FasL) on their surface survive indefinitely in allogeneic recipients without a need for chronic immunosuppression. Mechanisms that confer long‐term protection to allograft are yet to be elucidated. We herein demonstrated that immune protection evolves in two distinct phases; induction and maintenance. SA‐FasL‐engineered allogeneic islets survived indefinitely and conferred protection to a second set of donor‐matched, but not third‐party, unmanipulated islet grafts simultaneously transplanted under the contralateral kidney capsule. Protection at the induction phase involved a reduction in the frequency of proliferating alloreactive T cells in the graft‐draining lymph nodes, and required phagocytes and TGF‐β. At the maintenance phase, immune protection evolved into graft site‐restricted immune privilege as the destruction of long‐surviving SA‐FasL‐islet grafts by streptozotocin followed by the transplantation of a second set of unmanipulated islet grafts into the same site from the donor, but not third party, resulted in indefinite survival. The induced immune privilege required both CD4⁺CD25⁺Foxp3⁺ Treg cells and persistent presence of donor antigens. Engineering cell and tissue surfaces with SA‐FasL protein provides a practical, efficient, and safe means of localized immunomodulation with important implications for autoimmunity and transplantation.     

Sirolimus Enhances the Magnitude and Quality of Viral‐Specific CD8+ T‐Cell Responses to Vaccinia Virus Vaccination in Rhesus Macaques

Sirolimus is a potent antiproliferative agent used clinically to prevent renal allograft rejection. However, little is known about the effects of maintenance immunosuppressive agents on the immune response to potentially protective vaccines. Here we show that sirolimus paradoxically increases the magnitude and quality of the CD8⁺ T‐cell response to vaccinia vaccination in nonhuman primates, fostering more robust recall responses compared to untreated and tacrolimus‐treated controls. Enhancement of both the central and effector memory compartments of the vaccinia‐specific CD8⁺ T‐cell response was observed. These data elucidate new mechanistic characteristics of sirolimus and suggest immune applications extending beyond its role as an immunosuppressant.     

Immunosuppressive effect of the gut microbiome altered by high‐dose tacrolimus in mice

The alterations induced in gut microbiota by tacrolimus may affect immune function and organ transplantation. Mice were treated with high‐dose tacrolimus for 14 days. The fecal microbiota were analyzed by pyrosequencing the 16S rRNA genes, and the effect on metabolism was predicted using the sequence data. The subgroups of T cells in the serum, gut‐associated lymphoid tissue, and draining lymph nodes were determined by flow cytometry. Tacrolimus treatment significantly altered the relative abundance of Allobaculum, Bacteroides, and Lactobacillus and CD4⁺CD25^hiFoxP3⁺ regulatory T cells in the colonic mucosa and the circulation. These were significantly increased after either tacrolimus treatment or treatment by fecal microbiota transfer from tacrolimus‐treated donors. Further, treatment with low‐dose tacrolimus plus fecal microbiota transfer from high‐dose tacrolimus–altered mice increased skin allograft survival rate in a skin transplantation model. Thus, high‐dose tacrolimus alters the compositions and taxa of the gut microbiota. Administration of these conditioned gut microbiota plus low‐dose tacrolimus resulted in regulation of colonic and systemic immune responses and an increased allograft survival rate. This study demonstrated a new strategy for controlling allograft rejection by combining an immunosuppressive agent with gut microbiome transplantation.     

Radiation and host retinoic acid signaling promote the induction of gut‐homing donor T cells after allogeneic hematopoietic stem cell transplantation

Intestinal graft‐versus‐host disease (GVHD) remains a devastating complication after allogeneic hematopoietic stem cell transplantation (HSCT). Although it has been well established that gut‐tropic donor T cells expressing integrin α4β7 are required to cause intestinal damage, the factors that control the induction of this pathogenic T cell population remain to be identified. Retinoic acid (RA) plays an important role in inducing α4β7 expression on T cells. In this study, we showed that gene expression of retinaldehyde dehydrogenase, the key enzyme involved in RA biosynthesis, is significantly increased in the spleen and mesenteric lymph nodes (MLNs) of irradiated mice. In a C57BL/6‐into‐B6D2F1 allogeneic HSCT model, irradiation significantly increased the induction of α4β7⁺‐donor T cells in mesenteric lymph nodes and spleen. Furthermore, we found that the RA pathway modulates the ability of dendritic cells to imprint gut‐homing specificity on alloreactive T cells. We also showed that host dendritic cell RA signaling influences GVHD risk. Our studies identified radiation and recipient RA signaling as 2 primary factors that dictate the magnitude of gut‐homing donor T cell induction after allogeneic HSCT. Attenuating radiation‐associated inflammation and modulating host RA signaling represent feasible strategies to mitigate intestinal GVHD by reducing gut‐seeking pathogenic donor T cells.     

Short‐term MyD88 inhibition ameliorates cardiac graft rejection and promotes donor‐specific hyporesponsiveness of skin grafts in mice

Recognition of evolutionarily conserved ligands by Toll‐like receptors (TLRs) triggers signaling cascades in innate immune cells to amplify adaptive immune responses. Nearly all TLRs require MyD88 to transduce downstream signaling. MyD88 deficiency has been shown to promote the allograft acceptance in mice. However, direct evidence for therapeutic potential of MyD88 inhibitors remains lacking. Herein, we used a MyD88 inhibitor, namely ST2825, to explore its therapeutic potential and mechanisms in fully allogeneic skin and heart transplant models. Phenotypic maturation of dendritic cells stimulated by TLR ligands was alleviated by ST2825 in parallel with reduced T‐cell proliferation in vitro. A short‐course treatment with ST2825 significantly prolonged cardiac graft survival (mean survival time = 18.5 ± 0.92 days vs. 7.25 ± 0.46 days). ST2825‐treated group had significantly reduced proinflammatory cytokines in allografts compared with control group. ST2825 combined with anti‐CD154 induced long‐term skin allograft acceptance in about one‐third of recipients (>100 days). ‘Skin‐tolerant’ recipients showed attenuated donor‐specific IFN‐γ responses, intact IL‐4 responses, and compromised alloantibody responses. We conclude that MyD88 inhibitor ST2825 attenuates acute cardiac rejection and promotes donor‐specific hyporesponsiveness in stringent skin transplant models. The direct evidence suggests that pharmacological inhibition of MyD88 hold promising potential for transplant rejection.