Augmenter of liver regeneration-mediated mitophagy protects against hepatic ischemia/reperfusion injury

Augmenter of liver regeneration (ALR) is an anti-apoptotic protein found mainly in mitochondria. It protects hepatocytes from ischemia-reperfusion (I/R) injury, but the underlying mechanism is not clear. We found that in rats, delivery of the ALR gene alleviated hepatic I/R injury during orthotopic liver transplantation as evidenced by reduced serum aminotransferase, oxidative stress and apoptosis, and increased expression of autophagy markers. In an in vitro hypoxia/reoxygenation (H/R) model, overexpression of the ALR gene activated autophagy and relieved defective mitophagy via the PINK1/Parkin pathway. Mechanistically, ALR transfection induced the expression of mitofusin 2 (Mfn2) in the H/R model, which led to PINK1 accumulation and mitochondrial translocation of Parkin. Deletion of Mfn2 abolished mitophagy activation induced by ALR transfection, promoted mitochondrial dysfunction, and eventually increased cell apoptosis. Mfn2 administration prevented the inhibition of mitophagy in ALR-knockout (KO) cells, thus attenuated mitochondrial dysfunction and cell apoptosis. In heterozygous ALR-knockout mice treated with a warm I/R injury, marked aggravation of liver injury was associated with mitophagy inhibition and reduction in Mfn2 expression. Taken together, our results confirm that ALR accelerated Parkin translocation and mitophagy via Mfn2, and protected hepatocytes from I/R-induced injury. Our findings provide a novel rationale for the treatment of hepatic I/R injury.

C1q as a potential tolerogenic therapeutic in transplantation

In 1963, Lepow and colleagues resolved C1, the first component of the classical pathway, into three components, which they named C1q, C1r, and C1s. All three of these components were demonstrated to be involved in causing hemolysis in vitro. For over 30 years after that seminal discovery, the primary function attributed to C1q was as part of the C1 complex that initiated the classical pathway of the complement cascade. Then, a series of papers reported that isolated C1q could bind to apoptotic cells and facilitate their clearance by macrophages. Since then, rheumatologists have recognized that C1q is an important pattern recognition receptor (PRR) that diverts autoantigen containing extracellular vesicles from immune recognition. This critical function of C1q as a regulator of immune recognition has been largely overlooked in transplantation. Now that extracellular vesicles released from transplants have been identified as a major agent of immune recognition, it is logical to consider the potential impact of C1q on modulating the delivery of allogeneic extracellular vesicles to antigen presenting cells. This concept has clinical implications in the possible use of C1q or a derivative as a biological therapeutic to down-modulate immune responses to transplants.

Toll-like receptor 3 is an endogenous sensor of cell death and a potential target for induction of long-term cardiac transplant survival

Inflammation posttransplant is directly linked to cell death programs including apoptosis and necrosis. Cell death leads to the release of cellular contents which can promote inflammation. Targeting of these pathways should be an effective strategy to prevent transplant rejection. Toll-like receptor 3 (TLR3) is emerging as a major endogenous sensor of inflammation. In this study, we assessed the role of TLR3 on cell death and transplant rejection. We showed that TLR3 is highly expressed on mouse microvascular endothelial cell (ECs) and the endothelium of cardiac grafts. We demonstrated that TLR3 interacting with dsRNA or self-RNA triggered apoptosis and necroptosis in ECs. Interestingly, TLR3-induced necroptosis led mitochondrial damage. Inhibition of the mitochondrial membrane permeability molecule Cyclophilin D prevented necroptosis in ECs. In vivo, endothelium damage and activities of caspase-3 and mixed lineage kinase domain-like protein were inhibited in TLR3^-/- cardiac grafts compared with C57BL/6 grafts posttransplant (n = 5, p < .001). Importantly, TLR3^-/- cardiac grafts had prolonged survival in allogeneic BALB/c mice (mean survival = 121 ± 67 vs. 31 ± 6 days of C57BL/6 grafts, n = 7, p = .002). In summary, our study suggests that TLR3 is an important cell death inducer in ECs and cardiac grafts and thus a potential therapeutic target in preventing cardiac transplant rejection.

TIGIT regulates apoptosis of risky memory T cell subsets implicated in belatacept-resistant rejection

Belatacept confers increased patient and graft survival in renal transplant recipients relative to calcineurin inhibitors, but is associated with an increased rate of acute rejection. Recent immunophenotypic studies comparing pretransplant T cell phenotypes of patients who reject versus those who remain stable on belatacept identified three potential "risky" memory T cell subsets that potentially underlie belatacept-resistant rejection: CD4⁺ CD28⁺ T_EM , CD8⁺ CD28^null , and CD4⁺ CD57⁺ PD1^- subsets. Here, we compared key phenotypic and functional aspects of these human memory T cell subsets, with the goal of identifying additional potential targets to modulate them. Results demonstrate that TIGIT, an increasingly well-appreciated immune checkpoint receptor, was expressed on all three risky memory T cell subsets in vitro and in vivo in the presence of belatacept. Coculture of human memory CD4⁺ and CD8⁺ T cells with an agonistic anti-TIGIT mAb significantly increased apoptotic cell death of all three risky memory T cell subsets. Mechanistically, TIGIT-mediated apoptosis of risky memory T cells was dependent on FOXP3⁺ Treg, suggesting that agonism of the TIGIT pathway increases FOXP3⁺ Treg suppression of human memory T cell populations. Overall, these data suggest that TIGIT agonism could represent a new therapeutic target to inhibit belatacept-resistant rejection during transplantation.

Circular RNA Foxo3 in cardiac ischemia-reperfusion injury in heart transplantation: A new regulator and target

Ischemia-reperfusion (I/R) injury occurring in heart transplantation (HT) remains as a leading cause of transplant heart graft failure. Circular RNAs (circRNAs) play important roles in gene regulation and diseases. However, the impact of circRNAs on I/R injury during HT remains unknown. This study aims to investigate the role of circular RNA Foxo3 (circFoxo3) in I/R injury in HT. Using an in vivo mouse HT model and an in vitro cardiomyocyte culture model, we demonstrated that circFoxo3 is significantly upregulated in I/R-injured hearts and hypoxia/reoxygenation (H/R)-damaged cardiomyocytes. Knockdown of circFoxo3 using siRNA not only reduces cell apoptosis and death, mitochondrial damage, and expression of apoptosis/death-related genes in vitro, but also protects heart grafts from prolonged cold I/R injury in HT. We also show that circFoxo3 interacts with Foxo3 proteins and inhibits the phosphorylation of Foxo3 and that it indirectly affects the expression of miR-433 and miR-136. In conclusion, circRNA is involved in I/R injury in HT and knockdown of circFoxo3 with siRNA can reduce I/R injury and improve heart graft function through interaction with Foxo3. This study highlights that circRNA is a new type of molecular regulator and a potential target for preventing I/R injury in HT.

Circulating mitochondrial genes detect acute cardiac allograft rejection: Role of the mitochondrial calcium uniporter complex

Acute rejection after heart transplantation increases the risk of chronic dysfunction. Disturbances in mitochondrial function may play a contributory role, however, the relationship between histological signs of rejection in the human transplanted heart and expression levels of circulating mitochondrial genes, such as the mitochondrial Ca²⁺ uniporter (MCU) complex, remains unexplored. We conducted an RNA-sequencing analysis to identify altered mitochondrial genes in serum and to evaluate their diagnostic accuracy for rejection episodes. We included 40 consecutive samples from transplant recipients undergoing routine endomyocardial biopsies. In total, 112 mitochondrial genes were identified in the serum of posttransplant patients, of which 28 were differentially expressed in patients with acute rejection (p < .05). Considering the receiver operating characteristic analysis with an area under the curve (AUC) >0.900 to discriminate patients with moderate or severe degrees of rejection, we found that the MCU system showed a strong capability for detection: MCU (AUC = 0.944, p < .0001), MCU/MCUR1 ratio (AUC = 0.972, p < .0001), MCU/MCUB ratio (AUC = 0.970, p < .0001), and MCU/MICU1 ratio (AUC = 0.970, p < .0001). Mitochondrial alterations are reflected in peripheral blood and are capable of discriminating between patients with allograft rejection and those not experiencing rejection with excellent accuracy. The dysregulation of the MCU complex was found to be the most relevant finding.

Organ Care System Lung resulted in lower apoptosis and iNOS expression in donor lungs

Ischemia-reperfusion (IR) injury after lung transplantation is still today an important complication in up to 25% of patients. The Organ Care System (OCS) Lung, an advanced normothermic ex vivo lung perfusion system, was found to be effective in reducing primary graft dysfunction compared to standard organ care (SOC) but studies on tissue/molecular pathways that could explain these more effective clinical results are lacking. This observational longitudinal study aimed to investigate IR injury in 68 tissue specimens collected before and after reperfusion from 17 OCS and 17 SOC preserved donor lungs. Several tissue analyses including apoptosis evaluation and inducible nitric oxide synthase (iNOS) expression (by immunohistochemistry and real-time reverse transcriptase-polymerase chain reaction) were performed. Lower iNOS expression and apoptotic index were distinctive of OCS preserved tissues at pre- and post-reperfusion times, independently from potential confounding factors. Moreover, OCS recipients had lower acute cellular rejection at the first 6-month follow-up. In conclusion, IR injury, in terms of apoptosis and iNOS expression, was less frequent in OCS- than in SOC-preserved lungs, which could eventually explain a better clinical outcome. Further studies are needed to validate our data and determine the role of iNOS expression as a predictive biomarker of the complex IR injury mechanism.

Xenogeneic transplantation of human WJ-MSCs rescues mice from acute radiation syndrome via Nrf-2-dependent regeneration of damaged tissues

There is an unmet medical need for radiation countermeasures that can be deployed for treatment of exposed individuals during ionizing radiation (IR) accidents or terrorism. Wharton's jelly mesenchymal stem cells (WJ-MSCs) from human umbilical cord have been shown to avoid allorecognition and induce a tissue-regenerating microenvironment, which makes them an attractive candidate for mitigating IR injury. We found that WJ-MSCs protected mice from a lethal dose of IR even when transplanted up to 24 hours after irradiation, and a combination of WJ-MSCs and antibiotic (tetracycline) could further expand the window of protection offered by WJ-MSCs. This combinatorial approach mitigated IR-induced damage to the hematopoietic and gastrointestinal system. WJ-MSCs increased the serum concentration of the cytoprotective cytokines granulocyte colony-stimulating factor (G-CSF) and IL-6 in mice. Knockdown of G-CSF and IL-6 in WJ-MSCs before injection to lethally irradiated mice or transplantation of WJ-MSCs to lethally irradiated Nrf-2 knockout mice significantly nullified the therapeutic protective efficacy. Hence, WJ-MSCs could be a potential cell-based therapy for individuals accidentally exposed to radiation.

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.

Novel cell-permeable p38-MAPK inhibitor efficiently prevents porcine islet apoptosis and improves islet graft function

During islet transplantation, mitogen-activated protein kinase (MAPK) p38 is preferentially activated in response to the isolation of islets and the associated inflammation. Although therapeutic effects of p38 inhibitors are expected, the clinical application of small-molecule inhibitors of p38 is not recommended because of their serious adverse effects on the liver and central nervous system. Here we designed peptides to inhibit p38, which were derived from the sites on p38 that mediate binding to proteins such as MAPK kinases. Peptide 11R-p38I₁₁₀ significantly inhibited the activation of p38. To evaluate the effects of 11R-p38I₁₁₀ , porcine islets were incubated with 10 µmol/L 11R-p38I₁₁₀ or a mutant form designated 11R-mp38I₁₁₀ . After islet transplantation, blood glucose levels reached the normoglycemic range in 58.3% and 0% of diabetic mice treated with 11R-p38I₁₁₀ or 11R-mp38I₁₁₀ , respectively. These data suggest that 11R-p38I₁₁₀ inhibited islet apoptosis and improved islet function. Peptide p38I₁₁₀ is a noncompetitive inhibitor of ATP and targets a unique docking site. Therefore, 11R-p38I₁₁₀ specifically inhibits p38 activation, which may avoid the adverse effects that have discouraged the clinical use of small-molecule inhibitors of p38. Moreover, our methodology to design "peptide inhibitors" could be used to design other inhibitors derived from the binding sites of proteins.

Extracellular vesicles derived from injured vascular tissue promote the formation of tertiary lymphoid structures in vascular allografts

Tertiary lymphoid structures (TLS) accumulate at sites of chronic injury where they function as an ectopic germinal center, fostering local autoimmune responses. Vascular injury leads to the release of endothelial-derived apoptotic exosome-like vesicles (ApoExo) that contribute to rejection in transplanted organs. The purpose of the study was to evaluate the impact of ApoExo on TLS formation in a model of vascular allograft rejection. Mice transplanted with an allogeneic aortic transplant were injected with ApoExo. The formation of TLS was significantly increased by ApoExo injection along with vascular remodeling and increased levels of antinuclear antibodies and anti-perlecan/LG3 autoantibodies. ApoExo also enhanced allograft infiltration by γδT17 cells. Recipients deficient in γδT cells showed reduced TLS formation and lower autoantibodies levels following ApoExo injection. ApoExo are characterized by proteasome activity, which can be blocked by bortezomib. Bortezomib treated ApoExo reduced the recruitment of γδT17 cells to the allograft, lowered TLS formation, and reduced autoantibody production. This study identifies vascular injury-derived extracellular vesicles (ApoExo), as initiators of TLS formation and demonstrates the pivotal role of γδT17 in coordinating TLS formation and autoantibody production. Finally, our results suggest proteasome inhibition with bortezomib as a potential option for controlling TLS formation in rejected allografts.

Mitochondrial permeability regulates cardiac endothelial cell necroptosis and cardiac allograft rejection

Transplantation is invariably associated with programmed cell death including apoptosis and necrosis, resulting in delayed graft function and organ rejection. We have demonstrated the contribution of necroptosis to mouse microvascular endothelial cell (MVEC) death and transplant rejection. Organ injury results in the opening of mitochondrial permeability transition pores (mPTPs), which can trigger apoptotic molecules release that ultimately results in cell death. The effect of mPTPs in the necroptotic pathway remains controversial; importantly, their role in transplant rejection is not clear. In this study, tumor necrosis factor-α triggered MVECs to undergo receptor-interacting protein kinase family (RIPK1/3)-dependent necroptosis. Interestingly, inhibition of mPTP opening could also inhibit necroptotic cell death. Cyclophilin-D (Cyp-D) is a key regulator of the mPTPs. Both inhibition and deficiency of Cyp-D protected MVECs from necroptosis (n = 3, P < .00001). Additionally, inhibition of Cyp-D attenuated RIPK3-downstream mixed-lineage kinase domain-like protein phosphorylation. In vivo, Cyp-D-deficient cardiac grafts showed prolonged survival in allogeneic BALB/c mice posttransplant compared with wild-type grafts (n = 7, P < .0001). Our study results suggest that the mPTPs may be important mechanistic mediators of necroptosis in cardiac grafts. There is therapeutic potential in targeting cell death via inhibition of the mPTP-regulating molecule Cyp-D to prevent cardiac graft rejection.

Receptor tyrosine kinase MerTK suppresses an allogenic type I IFN response to promote transplant tolerance

Recipient infusion of donor apoptotic cells is an emerging strategy for inducing robust transplantation tolerance. Daily clearance of billions of self-apoptotic cells relies on homeostatic engagement of phagocytic receptors, in particular, receptors of the tyrosine kinase family TAM (Tyro3, Axl, and MerTK), to maintain self-tolerance. However, an outstanding question is if allogeneic apoptotic cells trigger the same receptor system for inducing allogeneic tolerance. Here, we employed allogeneic apoptotic splenocytes and discovered that the efferocytic receptor MerTK on recipient phagocytes is a critical mediator for transplantation tolerance induced by this strategy. Our findings indicate that the tolerogenic properties of allogeneic apoptotic splenocytes require MerTK transmission of intracellular signaling to suppress the production of inflammatory cytokine interferon α (IFN-α). We further demonstrate that MerTK is crucial for subsequent expansion of myeloid-derived suppressor cells and for promoting their immunomodulatory function, including maintaining graft-infiltrating CD4⁺ CD25⁺ Foxp3⁺ regulatory T cells. Consequently, recipient MerTK deficiency resulted in failure of tolerance by donor apoptotic cells, and this failure could be effectively rescued by IFN-α receptor blockade. These findings underscore the importance of the efferocytic receptor MerTK in mediating transplantation tolerance by donor apoptotic cells and implicate MerTK agonism as a promising target for promoting transplantation tolerance.

New insights into immune mechanisms of antiperlecan/LG3 antibody production: Importance of T cells and innate B1 cells

Autoantibodies against perlecan/LG3 (anti-LG3) have been associated with increased risks of delayed graft function, acute rejection, and reduced long-term survival. High titers of anti-LG3 antibodies have been found in de novo renal transplants recipients in the absence of allosensitizing or autoimmune conditions. Here, we seek to understand the pathways controlling anti-LG3 production prior to transplantation. Mice immunized with recombinant LG3 produce concomitantly IgM and IgG anti-LG3 antibodies suggesting a memory response. ELISpot confirmed the presence of LG3-specific memory B cells in nonimmunized mice. Purification of B1 and B2 subtypes identified peritoneal B1 cells as the major source of memory B cells reactive to LG3. Although nonimmunized CD4-deficient mice were found to express LG3-specific memory B cells, depletion of CD4⁺ T cells in wild type mice during immunization significantly decreased anti-LG3 production. These results demonstrate that B cell memory to LG3 is T cell independent but that production of anti-LG3 antibodies requires T cell help. Further supporting an important role for T cells in controlling anti-LG3 levels, we found that human renal transplant recipients show a significant decrease in anti-LG3 titers upon the initiation of CNI-based immunosuppression. Collectively, these results identify T cell targeting interventions as a means of reducing anti-LG3 levels in renal transplant patients.

Alpha-1-antitrypsin in cell and organ transplantation

Limited availability of donor organs and risk of ischemia-reperfusion injury (IRI) seriously restrict organ transplantation. Therapeutics that can prevent or reduce IRI could potentially increase the number of transplants by increasing use of borderline organs and decreasing discards. Alpha-1 antitrypsin (AAT) is an acute phase reactant and serine protease inhibitor that limits inflammatory tissue damage. Purified plasma-derived AAT has been well tolerated in more than 30 years of use to prevent emphysema in AAT-deficient individuals. Accumulating evidence suggests that AAT has additional anti-inflammatory and tissue-protective effects including improving mitochondrial membrane stability, inhibiting apoptosis, inhibiting nuclear factor kappa B activation, modulating pro- vs anti-inflammatory cytokine balance, and promoting immunologic tolerance. Cell culture and animal studies have shown that AAT limits tissue injury and promotes cell and tissue survival. AAT can promote tolerance in animal models by downregulating early inflammation and favoring induction and stabilization of regulatory T cells. The diverse intracellular and immune-modulatory effects of AAT and its well-established tolerability in patients suggest that it might be useful in transplantation. Clinical trials, planned and/or in progress, should help determine whether the promise of the animal and cellular studies will be fulfilled by improving outcomes in human organ transplantation.

The Introduction of Human Heme Oxygenase-1 and Soluble Tumor Necrosis Factor-α Receptor Type I With Human IgG1 Fc in Porcine Islets Prolongs Islet Xenograft Survival in Humanized Mice

Apoptosis during engraftment and inflammation induce poor islet xenograft survival. We aimed to determine whether overexpression of human heme oxygenase-1 (HO-1) or soluble tumor necrosis factor-α receptor type I with human IgG1 Fc (sTNF-αR-Fc) in porcine islets could improve islet xenograft survival. Adult porcine islets were transduced with adenovirus containing human HO-1, sTNF-αR-Fc, sTNF-αR-Fc/HO-1 or green fluorescent protein (control). Humanized mice were generated by injecting human cord blood-derived CD34(+) stem cells into NOD-scid-IL-2Rγ(null) mice. Both HO-1 and sTNF-αR-Fc reduced islet apoptosis under in vitro hypoxia or cytokine stimuli and suppressed RANTES induction without compromising insulin secretion. Introduction of either gene into islets prolonged islet xenograft survival in pig-to-humanized mice transplantation. The sTNF-αR-Fc/HO-1 group showed the best glucose tolerance. Target genes were successfully expressed in islet xenografts. Perigraft infiltration of macrophages and T cells was suppressed with decreased expression of RANTES, tumor necrosis factor-α and IL-6 in treatment groups; however, frequency of pig-specific interferon-γ-producing T cells was not decreased, and humoral response was not significant in any group. Early apoptosis of islet cells was suppressed in the treatment groups. In conclusion, overexpression of HO-1 or sTNF-αR-Fc in porcine islets improved islet xenograft survival by suppressing both apoptosis and inflammation. HO-1 or sTNF-αR-Fc transgenic pigs have potential for islet xenotransplantation.

Endothelial Cell Apoptosis Induces TGF-β Signaling-Dependent Host Endothelial-Mesenchymal Transition to Promote Transplant Arteriosclerosis

Endothelial cells (ECs) apoptosis is an initial event in transplant arteriosclerosis (TA), resulting in allograft function loss. To elucidate the precise mechanisms of ECs apoptosis leading to neointimal smooth muscle cells (SMCs) accumulation during TA. We induced apoptosis in cultured ECs by overexpressing p53 through lentivirus-mediated transfection. ECs apoptosis induced the production of transforming growth factor (TGF)-β1 in both apoptotic and neighboring viable cells, leading to increased TGF-β1 in the culture media. Conditioned media from Ltv-p53-transfected ECs further promoted transition of cultured ECs to SM-like cells by activating TGF-β/Smad3, PI3K/Akt/mTOR, and MAPK/ERK signaling in a TGF-β-dependent manner. In transgenic rat aorta transplantation models, inhibition of ECs apoptosis in Bcl-xL(+/+) knock-in rat aortic allografts significantly reduced TGF-β1 production both in allograft endothelia and in blood plasma, which in turn decreased accumulation of SM22α+ cells from transgenic recipient ECs originally marked with EGFP knock-in in neointima and alleviated TA. Systemic treatment with SIS3, AP23573, or PD98059 also prevented recipient ECs-originated SM-like cells accumulation and intima hyperplasia in aortic allografts. These data suggest that allograft EC apoptosis induced recipient endothelial-mesenchymal (smooth muscle) transition via TGF-β signaling, resulting in recipient EC-derived SMC accumulation as a major mechanism of vascular remodeling during TA.

Amyloid formation in human islets is enhanced by heparin and inhibited by heparinase

Islet transplantation is a promising therapy for patients with diabetes, but its long-term success is limited by many factors, including the formation of islet amyloid deposits. Heparin is employed in clinical islet transplantation to reduce clotting but also promotes fibrillization of amyloidogenic proteins. We hypothesized that heparin treatment of islets during pre-transplant culture may enhance amyloid formation leading to beta cell loss and graft dysfunction. Heparin promoted the fibrillization of human islet amyloid polypeptide (IAPP) and enhanced its toxicity to INS-1 beta cells. Heparin increased amyloid deposition in cultured human islets, but surprisingly decreased islet cell apoptosis. Treatment of human islets with heparin prior to transplantation increased the likelihood of graft failure. Removal of islet heparan sulfate glycosaminoglycans, which localize with islet amyloid deposits in type 2 diabetes, by heparinase treatment decreased amyloid deposition and protected against islet cell death. These findings raise the possibility that pretransplant treatment of human islets with heparin could potentiate IAPP aggregation and amyloid formation and may be detrimental to subsequent graft function.

Tempering allorecognition to induce transplant tolerance with chemically modified apoptotic donor cells

The development of organ transplantation as a therapy for end-stage organ failure is among the most significant achievements of 20th century medicine, but chronic rejection remains a barrier to achieving long-term success. Current therapeutic regimens consist of immunosuppressive drugs that are efficient at delaying rejection but are associated with significant risks such as opportunistic infections, toxicity, and malignancy. Thus, the induction of specific immune tolerance to transplant antigens is the coveted aim of researchers. The use of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (ECDI)-treated, autoantigen-coupled syngeneic leukocytes has been developed as a specific immunotherapy in preclinical models of autoimmunity and is currently in a phase II clinical trial for the treatment of multiple sclerosis. In this review, we discuss the use of allogeneic ECDI-treated apoptotic donor leukocytes (allo-ECDI-SP) as a strategy for inducing antigen-specific tolerance in allogeneic transplantation. Allo-ECDI-SP therapy induces long-term systemic immune tolerance to transplant antigens by subverting alloimmune recognition and exploiting apoptotic cell uptake pathways to recapitulate innate mechanisms of peripheral tolerance. Lastly, we discuss potential indications and challenges for transitioning allo-ECDI-SP therapy into clinical practice.

TIM-1 signaling is required for maintenance and induction of regulatory B cells

Apart from their role in humoral immunity, B cells can exhibit IL-10-dependent regulatory activity (Bregs). These regulatory subpopulations have been shown to inhibit inflammation and allograft rejection. However, our understanding of Bregs has been hampered by their rarity, lack of a specific marker, and poor insight into their induction and maintenance. We previously demonstrated that T cell immunoglobulin mucin domain-1 (TIM-1) identifies over 70% of IL-10-producing B cells, irrespective of other markers. We now show that TIM-1 is the primary receptor responsible for Breg induction by apoptotic cells (ACs). However, B cells that express a mutant form of TIM-1 lacking the mucin domain (TIM-1(Δmucin) ) exhibit decreased phosphatidylserine binding and are unable to produce IL-10 in response to ACs or by specific ligation with anti-TIM-1. TIM-1(Δmucin) mice also exhibit accelerated allograft rejection, which appears to be due in part to their defect in both baseline and induced IL-10(+) Bregs, since a single transfer of WT TIM-1(+) B cells can restore long-term graft survival. These data suggest that TIM-1 signaling plays a direct role in Breg maintenance and induction both under physiological conditions (in response to ACs) and in response to therapy through TIM-1 ligation. Moreover, they directly demonstrate that the mucin domain regulates TIM-1 signaling.