Novel Approaches to Immunomodulation for Solid Organ Transplantation.

Despite significant advances in the field of transplantation in the past two decades, current clinically available therapeutic options for immunomodulation remain fairly limited. The advent of calcineurin inhibitor-based immunosuppression has led to significant success in improving short-term graft survival; however, improvements in long-term graft survival have stalled. Solid organ transplantation provides a unique opportunity for immunomodulation of both the donor organ prior to implantation and the recipient post transplantation. Furthermore, therapies beyond targeting the adaptive immune system have the potential to ameliorate ischemic injury to the allograft and halt its aging process, augment its repair, and promote recipient immune tolerance. Other recent advances include expanding the donor pool by reducing organ discard, and bioengineering and genetically modifying organs from other species to generate transplantable organs. Therapies discussed here will likely be most impactful if individualized on the basis of specific donor and recipient considerations.

The T-cell environment: may the regulatory force be with you.

In the study by Sasaki et al. in this issue, the authors studied infusions of ex vivo-expanded regulatory T cells in a highly clinically relevant nonhuman primate kidney transplant model. This commentary will aim to discuss the use of regulatory T cells in the wider context of transplantation, with particular emphasis on the milieu and various engineering potentials to enhance their function, as well as their relationship to other cell populations with regulatory capacity.

Murine kidney transplant outcome is best measured by transdermal glomerular filtration rate.

Mouse kidney transplantation provides a powerful preclinical model for the study of kidney transplant alloimmunity. However, accurate measurement of graft function is difficult because of the inaccuracy of traditional surrogate markers serum creatinine and urea. We report the use of transdermal glomerular filtration rate measurement under the experimental conditions of unilateral nephrectomy and allogeneic kidney transplantation. Our findings demonstrate that transdermal glomerular filtration rate measurement is easy to perform, reproducible, and has more interexperimental consistency than serum creatinine or urea measurements. Most importantly, it significantly reduces the numbers of experimental animals required to detect subtle and yet clinically relevant differences in kidney function as often is the case in experimental murine kidney transplantation models.

Advancing mouse models for transplantation research.

Mouse models have been instrumental in understanding mechanisms of transplant rejection and tolerance, but cross-study reproducibility and translation of experimental findings into effective clinical therapies are issues of concern. The Mouse Models in Transplantation symposium gathered scientists and physician-scientists involved in basic and clinical research in transplantation to discuss the strengths and limitations of mouse transplant models and strategies to enhance their utility. Participants recognized that increased procedure standardization, including the use of prespecified, defined endpoints, and statistical power analyses, would benefit the field. They also discussed the generation of new models that incorporate environmental and genetic variables affecting clinical outcomes as potentially important. If implemented, these strategies are expected to improve the reproducibility of mouse studies and increase their translation to clinical trials and, ideally, new Food and Drug Administration-approved drugs.

Glutamate Chemical Exchange Saturation Transfer (GluCEST) MRI to Evaluate the Rapid Antidepressant Effects of Ketamine in the Hippocampus of Rat Depression Model.

BACKGROUND: Ketamine is a quick acting antidepressant drug, and an accurate detection method is lacking. Ketamine's effects in a rat depression model have not previously been well explored using glutamate chemical exchange saturation transfer (GluCEST). PURPOSE: To investigate the GluCEST changes of chronic unpredictable mild stress (CUMS) rats after receiving either ketamine or saline injection. STUDY TYPE: Randomized animal model trial. ANIMAL MODEL: 12 CUMS and 6 Sprague-Dawley rats. Divided into three groups: ketamine (N = 6), saline (N = 6), and control (N = 6). FIELD STRENGTH/SEQUENCE: 7.0 T/the sequence is GluCEST and 1 H MR spectroscopy (MRS). ASSESSMENT: The CUMS rats were exposed to different stress factors for 8 weeks. The glutamate concentration in the hippocampus was assessed by the GluCEST,1 H MRS, and the high-performance liquid chromatography (HPLC). STATISTICAL TESTS: The t-test, Mann-Whitney U test, and Pearson's correlation. RESULTS: In depression conditions, GluCEST signals were lower in the bilateral hippocampus than in control group. Thirty minutes after ketamine injection, the GluCEST signals in the bilateral hippocampus were higher compared with the saline group (left: 2.99 ± 0.34 [Control] vs. 2.44 ± 0.20 [Saline] vs. 2.85 ± 0.11 [Ketamine]; right: 2.97 ± 0.28 [Control] vs. 2.49 ± 0.25 [Saline] vs. 2.86 ± 0.19 [Ketamine]). In 1 H MRS, significant changes were only observed in the left hippocampus (2.00 ± 0.16 [Control] vs. 1.81 ± 0.09 [Saline] vs. 2.04 ± 0.14 [Ketamine]). Furthermore, HPLC results showed similar trends to those observed in the GluCEST results (left: 2.32 ± 0.22 [Control] vs. 1.96 ± 0.11 [Saline] vs. 2.18 ± 0.11 [Ketamine]; right: 2.35 ± 0.18 [Control] vs. 1.87 ± 0.16 [Saline] vs. 2.09 ± 0.08 [Ketamine]). DATA CONCLUSION: GluCEST can sensitively evaluate the ketamine's antidepressant effects by detecting the fast increase in glutamate concentration. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1.

GluCEST Imaging and Structural Alterations of the Bilateral Hippocampus in First-Episode and Early-Onset Major Depression Disorder.

BACKGROUND: Glutamate dysregulation is one of the key pathogenic mechanisms of major depressive disorder (MDD), and glutamate chemical exchange saturation transfer (GluCEST) has been used for glutamate measurement in some brain diseases but rarely in depression. PURPOSE: To investigate the GluCEST changes in hippocampus in MDD and the relationship between glutamate and hippocampal subregional volumes. STUDY TYPE: Cross-sectional. SUBJECTS: Thirty-two MDD patients (34% males; 22.03 ± 7.21 years) and 47 healthy controls (HCs) (43% males; 22.00 ± 3.28 years). FIELD STRENGTH/SEQUENCE: 3.0 T; magnetization prepared rapid gradient echo (MPRAGE) for three-dimensional T1-weighted images, two-dimensional turbo spin echo GluCEST, and multivoxel chemical shift imaging (CSI) for proton magnetic resonance spectroscopy (1 H MRS). ASSESSMENT: GluCEST data were quantified by magnetization transfer ratio asymmetry (MTRasym ) analysis and assessed by the relative concentration of 1 H MRS-measured glutamate. FreeSurfer was used for hippocampus segmentation. STATISTICAL TESTS: The independent sample t test, Mann-Whitney U test, Spearman's correlation, and partial correlation analysis were used. P < 0.05 was considered statistically significant. RESULTS: In the left hippocampus, GluCEST values were significantly decreased in MDD (2.00 ± 1.08 [MDD] vs. 2.62 ± 1.41 [HCs]) and showed a significantly positive correlation with Glx/Cr (r = 0.37). GluCEST values were significantly positively correlated with the volumes of CA1 (r = 0.40), subiculum (r = 0.40) in the left hippocampus and CA1 (r = 0.51), molecular_layer_HP (r = 0.50), GC-ML-DG (r = 0.42), CA3 (r = 0.44), CA4 (r = 0.44), hippocampus-amygdala-transition-area (r = 0.46), and the whole hippocampus (r = 0.47) in the right hippocampus. Hamilton Depression Rating Scale scores showed significantly negative correlations with the volumes of the left presubiculum (r = -0.40), left parasubiculum (r = -0.47), and right presubiculum (r = -0.41). DATA CONCLUSION: GluCEST can be used to measure glutamate changes and help to understand the mechanism of hippocampal volume loss in MDD. Hippocampal volume changes are associated with disease severity. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1.

Glutamate alterations in the premature infant brain during different gestational ages with glutamate chemical exchange saturation transfer imaging: a pilot study.

OBJECTIVES: To elucidate the change in glutamate levels in preterm infants at different gestational ages by glutamate chemical exchange saturated transfer (GluCEST) magnetic resonance imaging and to compare the difference in glutamate levels among different brain regions between very early preterm infants and middle and late preterm infants. METHODS: Fifty-three preterm infants (59% males; median gestational age = 33.6 weeks) underwent MRI, including conventional MRI and GluCEST. The original data were postprocessed in MATLAB. Correlation analysis was used to determine the relationship between the MTRasym and gestational age. The differences in MTRasym signals among different ROIs were statistically analysed by one-way analysis of variance (ANOVA). The MTRasym difference of the bilateral hemispherical ROI was compared by a paired T test. RESULTS: In all ROIs, glutamate concentration was positively correlated with gestational age. The glutamate concentration in the thalamus was higher than that in the frontal lobe in very early, middle and late preterm infants. A difference in glutamate concentration was not found in the bilateral ROIs. CONCLUSIONS: The concentration of glutamate in the brains of preterm infants of different gestational ages increased with gestational age, which may be one of the factors contributing to the higher incidence of neurodevelopmental dysfunction in very early preterm infants compared to that in middle and late preterm infants. Meanwhile, the glutamate concentrations among different brain regions were also diverse. KEY POINTS: • The glutamate concentration was positively correlated with gestational age in preterm infants of the brain. • Glutamate concentrations were dissimilar in different brain regions of preterm infants. • Glutamate concentration during the process of brain development in premature infants was not found to be asymmetric.

Blocking CCL8-CCR8-Mediated Early Allograft Inflammation Improves Kidney Transplant Function.

BACKGROUND: In kidney transplantation, early allograft inflammation impairs long-term allograft function. However, precise mediators of early kidney allograft inflammation are unclear, making it challenging to design therapeutic interventions. METHODS: We used an allogeneic murine kidney transplant model in which CD45.2 BALB/c kidneys were transplanted to CD45.1 C57BL/6 recipients. RESULTS: Donor kidney resident macrophages within the allograft expanded rapidly in the first 3 days. During this period, they were also induced to express a high level of Ccl8, which, in turn, promoted recipient monocyte graft infiltration, their differentiation to resident macrophages, and subsequent expression of Ccl8. Enhanced graft infiltration of recipient CCR8+ T cells followed, including CD4, CD8, and γδ T cells. Consequently, blocking CCL8-CCR8 or depleting donor kidney resident macrophages significantly inhibits early allograft immune cell infiltration and promotes superior short-term allograft function. CONCLUSIONS: Targeting the CCL8-CCR8 axis is a promising measure to reduce early kidney allograft inflammation.

Impact of infection on transplantation tolerance.

Allograft tolerance is the ultimate goal of organ transplantation. Current strategies for tolerance induction mainly focus on inhibiting alloreactive T cells while promoting regulatory immune cells. Pathogenic infections may have direct impact on both effector and regulatory cell populations, therefore can alter host susceptibility to transplantation tolerance induction as well as impair the quality and stability of tolerance once induced. In this review, we will discuss existing data demonstrating the effect of infections on transplantation tolerance, with particular emphasis on the role of the stage of infection (acute, chronic, or latent) and the stage of tolerance (induction or maintenance) in this infection-tolerance interaction. While the deleterious effect of acute infection on tolerance is mainly driven by proinflammatory cytokines induced shortly after the infection, chronic infection may generate exhausted T cells that could in fact facilitate transplantation tolerance. In addition to pathogenic infections, commensal intestinal microbiota also has numerous significant immunomodulatory effects that can shape the host alloimmunity following transplantation. A comprehensive understanding of these mechanisms is crucial for the development of therapeutic strategies for robustly inducing and stably maintaining transplantation tolerance while preserving host anti-pathogen immunity in clinically relevant scenarios.

Joining Forces in Basic Science: ITS Meeting 2.0.

The second International Transplant Science (ITS) meeting jointly organized by the European Society for Organ Transplantation (ESOT), the American Society of Transplantation (AST), and The Transplantation Society (TTS) took place in May 2022 in one of Europe's most iconic cities: Berlin, Germany. The ITS meeting 2022 was designed to serve as an international platform for scientific discussions on the latest ground-breaking discoveries in the field, while providing an excellent opportunity to present cutting-edge research to the scientific community. We think this is fundamental for the exchange of new ideas and establishment of collaborative work between advanced transplant experts, young professionals and early-stage researchers and students. Scientific sessions tackled hot topics in transplantation such as mechanisms of tolerance, biomarkers, big data and artificial intelligence. Our educational pre-meeting focused on the breakthrough and challenges in single-cell multimodal omics. The program included panel discussions illuminating various topics concerning conflicts and problems related to gender, such as challenges for female scientists. Attendees returned to their institutes with not only profound knowledge of the latest discoveries, technologies, and concepts in basic and translational science, but also inspired and excited after discussions and networking sessions with fellow scientists which have been duly missed during the pandemic.

Acute murine cytomegalovirus disrupts established transplantation tolerance and causes recipient allo-sensitization.

We have previously shown that acute cytomegalovirus (CMV) infection disrupts the induction of transplantation tolerance. However, what impact acute CMV infection would have on the maintenance of established tolerance and on subsequent recipient allo-sensitization is a clinically important unanswered question. Here we used an allogeneic murine islet transplantation tolerance model to examine the impact of acute CMV infection on: (a) disruption of established transplantation tolerance during tolerance maintenance; and (b) the possibility of recipient allo-sensitization by CMV-mediated disruption of stable tolerance. We demonstrated that acute CMV infection abrogated transplantation tolerance during the maintenance stage in 50%-60% recipients. We further demonstrated that acute CMV infection-mediated tolerance disruption led to recipient allo-sensitization by reverting the tolerant state of allo-specific T cells and promoting their differentiation to allo-specific memory cells. Consequently, a second same-donor islet allograft was rejected in an accelerated fashion by these recipients. Our study therefore supports close monitoring for allo-sensitization in previously tolerant transplant recipients in whom tolerance maintenance is disrupted by an episode of acute CMV infection.

Phase 3 trial of human islet-after-kidney transplantation in type 1 diabetes.

Allogeneic islet transplant offers a minimally invasive option for ß cell replacement in the treatment of type 1 diabetes (T1D). The CIT consortium trial of purified human pancreatic islets (PHPI) in patients with T1D after kidney transplant (CIT06), a National Institutes of Health-sponsored phase 3, prospective, open-label, single-arm pivotal trial of PHPI, was conducted in 24 patients with impaired awareness of hypoglycemia while receiving intensive insulin therapy. PHPI were manufactured using standardized processes. PHPI transplantation was effective with 62.5% of patients achieving the primary endpoint of freedom from severe hypoglycemic events and HbA1c  ≤ 6.5% or reduced by ≥ 1 percentage point at 1 year posttransplant. Median HbA1c declined from 8.1% before to 6.0% at 1 year and 6.3% at 2 and 3 years following transplant (P < .001 for all vs baseline), with related improvements in hypoglycemia awareness and glucose variability. The improved metabolic control was associated with better health-related and diabetes-related quality of life. The procedure was safe and kidney allograft function remained stable after 3 years. These results add to evidence establishing allogeneic islet transplant as a safe and effective treatment for patients with T1D and unstable glucose control despite intensive insulin treatment, supporting the indication for PHPI in the post-renal transplant setting.

Glutamate Chemical Exchange Saturation Transfer Imaging and Functional Alterations of Hippocampus in Rat Depression Model: A Pilot Study.

BACKGROUND: Adjusting abnormal glutamate neurotransmission is a crucial mechanism in the treatment of depression. However, few non-invasive techniques could effectively detect changes in glutamate neurotransmitters, and no consensus exists on whether glutamate could affect resting-state function changes in depression. PURPOSE: To study the changes in glutamate chemical exchange saturation transfer (GluCEST) value in the hippocampus of rat model exposed to chronic unpredictable mild stress (CUMS), and to explore the effect of this change on the activity of hippocampal glutamatergic neurons. STUDY TYPE: Prospective animal study. ANIMAL MODEL: Twenty male Sprague-Dawley rats (200-300 g). FIELD STRENGTH/SEQUENCE: 7.0 T scanner. Fat rapid acquisition relaxation enhancement sequence for GluCEST, and echo planner imaging sequence for resting-state functional magnetic resonance imaging (rs_fMRI). ASSESSMENT: Rats were divided into two groups: CUMS group (N = 10) and control group (CTRL, N = 10). The magnetization transfer ratio asymmetry analysis was used to quantify the GluCEST data, and evaluate the rs_fMRI data through the amplitude of low-frequency fluctuation (ALFF) and regional homogeneity (ReHo) analysis. STATISTICAL TESTS: A t-test was used to compare the difference in GluCEST or rs_fMRI between CUMS and CTRL groups. Spearman's correlation was applied to explore the correlation between GluCEST values and abnormal fMRI values in hippocampus. Statistical significance was set at P < 0.05. RESULTS: The GluCEST value in the left hippocampus has changed significantly (3.3 ± 0.3 [CUMS] vs. 3.9 ± 0.4 [CTRL], P < 0.05). In addition, the GluCEST value was significantly positively correlated with the ALFF values (r = 0.5, P < 0. 05, df = 7) and negatively correlated with the ReHo values (r = -0.6, P < 0.05, df = 7). DATA CONCLUSION: GluCEST technique has the feasibility of mapping glutamate changes in rat depression. Glutamate neurotransmitters are important factors affecting the abnormal function of neural activity. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 1.

Dissecting the human kidney allograft transcriptome: single-cell RNA sequencing.

PURPOSE OF REVIEW: Single-cell RNA sequencing (scRNA-seq) has provided opportunities to interrogate kidney allografts at a hitherto unavailable molecular level of resolution. Understanding of this technology is essential to better appreciate the relevant biomedical literature. RECENT FINDINGS: Sequencing is a technique to determine the order of nucleotides in a segment of RNA or DNA. RNA-seq of kidney allograft tissues has revealed novel mechanistic insights but does not provide information on individual cell types and cell states. scRNA-seq enables to study the transcriptome of individual cells and assess the transcriptional differences and similarities within a population of cells. Initial studies on rejecting kidney allograft tissues in humans have identified the transcriptional profile of the active players of the innate and adaptive immune system. Application of scRNA-seq in a preclinical model of kidney transplantation has revealed that allograft-infiltrating myeloid cells follow a trajectory of differentiation from monocytes to proinflammatory macrophages and exhibit distinct interactions with kidney allograft parenchymal cells; myeloid cell expression of Axl played a major role in promoting intragraft myeloid cell and T-cell differentiation. SUMMARY: The current review discusses the technical aspects of scRNA-seq and summarizes the application of this technology to dissect the human kidney allograft transcriptome.

Murine cytomegalovirus dissemination but not reactivation in donor-positive/recipient-negative allogeneic kidney transplantation can be effectively prevented by transplant immune tolerance.

Cytomegalovirus (CMV) reactivation from latently infected donor organs post-transplantation and its dissemination cause significant comorbidities in transplant recipients. Transplant-induced inflammation combined with chronic immunosuppression has been thought to provoke CMV reactivation and dissemination, although sequential events in this process have not been studied. Here, we investigated this process in a high-risk donor CMV-positive to recipient CMV-negative allogeneic murine kidney transplantation model. Recipients were either treated with indefinite immunosuppression or tolerized in a donor-specific manner. Untreated recipients served as controls. Kidney allografts from both immunosuppressed and tolerized recipients showed minimal alloimmunity-mediated graft inflammation and normal function for up to day 60 post-transplantation. However, despite the absence of such inflammation in the immunosuppressed and tolerized groups, CMV reactivation in the donor positive kidney allograft was readily observed. Interestingly, subsequent CMV replication and dissemination to distant organs only occurred in immunosuppressed recipients in which CMV-specific CD8 T cells were functionally impaired; whereas in tolerized recipients, host anti-viral immunity was well-preserved and CMV dissemination was effectively prevented. Thus, our studies uncoupled CMV reactivation from its dissemination, and underscore the potential role of robust transplantation tolerance in preventing CMV diseases following allogeneic kidney transplantation.

Donor apoptotic cell-based therapy for effective inhibition of donor-specific memory T and B cells to promote long-term allograft survival in allosensitized recipients.

Allosensitization constitutes a major barrier in transplantation. Preexisting donor-reactive memory T and B cells and preformed donor-specific antibodies (DSAs) have all been implicated in accelerated allograft rejection in sensitized recipients. Here, we employ a sensitized murine model of islet transplantation to test strategies that promote long-term immunosuppression-free allograft survival. We demonstrate that donor-specific memory T and B cells can be effectively inhibited by peritransplant infusions of donor apoptotic cells in combination with anti-CD40L and rapamycin, and this treatment leads to significant prolongation of islet allograft survival in allosensitized recipients. We further demonstrate that late graft rejection in recipients treated with this regimen is associated with a breakthrough of B cells and their aggressive graft infiltration. Consequently, additional posttransplant B cell depletion effectively prevents late rejection and promotes permanent acceptance of islet allografts. In contrast, persistent low levels of DSAs do not seem to impair graft outcome in these recipients. We propose that B cells contribute to late rejection as antigen-presenting cells for intragraft memory T cell expansion but not to alloantibody production and that a therapeutic strategy combining donor apoptotic cells, anti-CD40L, and rapamycin effectively inhibits proinflammatory B cells and promotes long-term islet allograft survival in such recipients.

Rejection of xenogeneic porcine islets in humanized mice is characterized by graft-infiltrating Th17 cells and activated B cells.

Xenogeneic porcine islet transplantation is a promising potential therapy for type 1 diabetes (T1D). Understanding human immune responses against porcine islets is crucial for the design of optimal immunomodulatory regimens for effective control of xenogeneic rejection of porcine islets in humans. Humanized mice are a valuable tool for studying human immune responses and therefore present an attractive alternative to human subject research. Here, by using a pig-to-humanized mouse model of xenogeneic islet transplantation, we described the human immune response to transplanted porcine islets, a process characterized by dense islet xenograft infiltration of human CD45+ cells comprising activated human B cells, CD4+ CD44+ IL-17+ Th17 cells, and CD68+ macrophages. In addition, we tested an experimental immunomodulatory regimen in promoting long-term islet xenograft survival, a triple therapy consisting of donor splenocytes treated with ethylcarbodiimide (ECDI-SP), and peri-transplant rituximab and rapamycin. We observed that the triple therapy effectively inhibited graft infiltration of T and B cells as well as macrophages, promoted transitional B cells both in the periphery and in the islet xenografts, and provided a superior islet xenograft protection. Our study therefore indicates an advantage of donor ECDI-SP treatment in controlling human immune cells in promoting long-term islet xenograft survival.

Monocytes prime autoreactive T cells after myocardial infarction.

In humans, loss of central tolerance for the cardiac self-antigen α-myosin heavy chain (α-MHC) leads to circulation of cardiac autoreactive T cells and renders the heart susceptible to autoimmune attack after acute myocardial infarction (MI). MI triggers profound tissue damage, releasing danger signals and self-antigen by necrotic cardiomyocytes, which lead to recruitment of inflammatory monocytes. We hypothesized that excessive inflammation by monocytes contributes to the initiation of adaptive immune responses to cardiac self-antigen. Using an experimental model of MI in α-MHC-mCherry reporter mice, which specifically express mCherry in cardiomyocytes, we detected α-MHC antigen in myeloid cells in the heart-draining mediastinal lymph node (MLN) 7 days after MI. To test whether monocytes were required for cardiac self-antigen trafficking to the MLN, we blocked monocyte recruitment with a C-C motif chemokine receptor type 2 (CCR2) antagonist or immune modifying nanoparticles (IMP). Blockade of monocyte recruitment reduced α-MHC antigen detection in the MLN after MI. Intramyocardial injection of the model antigen ovalbumin into OT-II transgenic mice demonstrated the requirement for monocytes in antigen trafficking and T-cell activation in the MLN. Finally, in nonobese diabetic mice, which are prone to postinfarction autoimmunity, blockade of monocyte recruitment reduced α-MHC-specific responses leading to improved tissue repair and ventricular function 28 days after MI. Taken together, these data support a role for monocytes in the onset of pathological cardiac autoimmunity following MI and suggest that therapeutic targeting of monocytes may mitigate postinfarction autoimmunity in humans.NEW & NOTEWORTHY Our study newly identifies a role for inflammatory monocytes in priming an autoimmune T-cell response after myocardial infarction. Select inhibition of monocyte recruitment to the infarct prevents trafficking of cardiac self-antigen and activation of cardiac myosin reactive T cells in the heart-draining lymph node. Therapeutic targeting of inflammatory monocytes may limit autoimmune responses to improve cardiac remodeling and preserve left ventricular function after myocardial infarction.

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.

Immunotherapy of type 1 diabetes: where are we and where should we be going?

Type 1 diabetes (T1D) is a chronic autoimmune disorder characterized by destruction of insulin-producing pancreatic beta cells. Many broad-based immunosuppressive and antigen-specific immunoregulatory therapies have been and are currently being evaluated for their utility in the prevention and treatment of T1D. Looking forward, this review discusses the potential therapeutic use of antigen-specific tolerance strategies, including tolerance induced by "tolerogenic" antigen-presenting cells pulsed with diabetogenic antigens and transfer of induced or expanded regulatory T cells, which have demonstrated efficacy in nonobese diabetic (NOD) mice. Depending on the time of therapeutic intervention in the T1D disease process, antigen-specific immunoregulatory strategies may be employed as monotherapies, or in combination with short-term tolerance-promoting immunoregulatory drugs and/or drugs promoting differentiation of insulin-producing beta cells from endogenous progenitors.