Pathways of allorecognition: implications for transplantation tolerance

Immunopathology of lung transplantation: from infection to rejection and vice versa

Lung transplantation offers a lifesaving option for patients with end-stage lung disease, but it is marred by a high risk of post-transplant infections, particularly involving multidrug-resistant bacteria, Cytomegalovirus, and fungal pathogens. This elevated infection rate, the highest among solid organ transplants, poses a significant challenge for clinicians, particularly within the first year post-transplantation, where infections are the leading cause of mortality. The direct exposure of lung allografts to the external environment exacerbates this vulnerability leading to constant immune stimulation and consequently to an elevated risk of triggering alloimmune responses to the lung allograft. The necessity of prolonged immunosuppression to prevent allograft rejection further complicates patient management by increasing susceptibility to infections and neoplasms, and complicating the differentiation between rejection and infection, which require diametrically opposed management strategies. This review explores the intricate balance between preventing allograft rejection and managing the heightened infection risk in lung transplant recipients.

Allogeneic HLA Humoral Immunogenicity and the Prediction of Donor-Specific HLA Antibody Development

The development of de novo donor-specific HLA antibodies (dnDSAs) following solid organ transplantation is considered a major risk factor for poor long-term allograft outcomes. The prediction of dnDSA development is a boon to transplant recipients, yet the assessment of allo-HLA immunogenicity remains imprecise. Despite the recent technological advances, a comprehensive evaluation of allo-HLA immunogenicity, which includes both B and T cell allorecognition, is still warranted. Recent studies have proposed using mismatched HLA epitopes (antibody and T cell) as a prognostic biomarker for humoral alloimmunity. However, the identification of immunogenic HLA mismatches has not progressed despite significant improvements in the identification of permissible mismatches. Certainly, the prediction of dnDSA development may benefit permissible HLA mismatched organ transplantations, personalized immunosuppression, and clinical trial design. However, characteristics that go beyond the listing of mismatched HLA antibody epitopes and T cell epitopes, such as the generation of HLA T cell epitope repertoires, recipient's HLA class II phenotype, and immunosuppressive regiments, are required for the precise assessment of allo-HLA immunogenicity.

Repeated human leukocyte antigens eplets, importance of typing the partner

INTRODUCTION

Antibody-mediated rejection (AMR) is the most common cause of immune-mediated allograft failure after kidney transplant and impacts allograft survival. Previous sensitization is a major risk factor for development of donor specific antibodies (DSA). AMR can have a wide range of clinical features such as impaired kidney function, proteinuria/hypertension or can be subclinical. HLA molecules have specific regions of antigens binding antibodies called epitopes and eplets are considered essential components responsible for immune recognition. We present a patient with subclinical AMR 1 week post transplantation.

CASE REPORT

A 48-year-old, caucasian woman with end-stage kidney disease (ESKD) secondary to autosomal dominant polycystic kidney disease (ADPKD) on peritoneal dialysis was registered in deceased donor waitlist. She was a hypersensitized patient from 3 prior pregnancies with a calculated panel reactive antibody of 93,48%. She was transplanted through kidney paired exchange donation with no evidence of DSA pre transplantation. Surgery and post-op were unremarkable with excellent and immediate graft function. Per protocol DSA levels on the 5th day was DR1 of 3300 MFI, with an increase in MFI by day 13 with 7820 MFI and a new B41 1979MFI. Allograft kidney biopsy findings were diagnostic of AMR and she was treated with immunoglobulin and plasmapheresis. As early onset AMR post transplantation was observed an anamnestic response was hypothesized from a previous exposure to allo-HLA. We decided to type her husband, her son's father, which was presented with DSA. Mismatch eplet analysis revealed a shared 41 T and 67LQ eplets between the donor and husband, responsible for the reactivity and new HLA class I B41 and HLA class II DR1 DSA, respectively.

DISCUSSION

Shared eplets between the patient husband and donor was responsible for the alloimmune response and early development of DSAs. This case highlights the importance of early monitoring DSA levels in highly sensitized patients after transplant in order to promptly address and lower inflammatory damage. Mismatch eplet analysis can provide a thorough and precise evaluation of immune compatibility providing a useful technique to immune risk stratification, donor selection and post-transplant immunosuppressive therapy and monitoring.

Intestinal Transplant Immunology and Intestinal Graft Rejection: From Basic Mechanisms to Potential Biomarkers

Intestinal transplantation (ITx) remains a lifesaving option for patients suffering from irreversible intestinal failure and complications from total parenteral nutrition. Since its inception, it became obvious that intestinal grafts are highly immunogenic, due to their high lymphoid load, the abundance in epithelial cells and constant exposure to external antigens and microbiota. This combination of factors and several redundant effector pathways makes ITx immunobiology unique. To this complex immunologic situation, which leads to the highest rate of rejection among solid organs (>40%), there is added the lack of reliable non-invasive biomarkers, which would allow for frequent, convenient and reliable rejection surveillance. Numerous assays, of which several were previously used in inflammatory bowel disease, have been tested after ITx, but none have shown sufficient sensibility and/or specificity to be used alone for diagnosing acute rejection. Herein, we review and integrate the mechanistic aspects of graft rejection with the current knowledge of ITx immunobiology and summarize the quest for a noninvasive biomarker of rejection.

Estimation of Sensitization Status in Renal Transplant Recipients by Assessing Indirect Pathway CD4+ T cell Response to Donor Cell-pulsed Dendritic Cell

BACKGROUND

. Generation of donor-specific human leukocyte antigen antibody (DSA) via indirect allorecognition is detrimental to long-term survival of transplant organs. The detection of such immune responses would make it possible to define patients with high risk of sensitization. In this study, we established a novel method for evaluating indirect allorecognition to assess sensitization in kidney transplant recipients.

METHODS

. Recipient CD14+ monocytes were mixed with donor peripheral blood mononuclear cells; cultured in the presence of IL-4, GM-CSF, IL-1β, and TNFα; and used as pulsed dendritic cells (DCs). Cell proliferation and cytokine production were evaluated by carboxyfluorescein diacetate succinimidyl ester-based T cell proliferation assay and Enzyme-Linked ImmunoSpot assay, respectively.

RESULTS

. CD4+ T cell proliferation was strongly observed in following coculture with allogeneic antigen-pulsed DC leading to interferon-γ and IL-21 production. About 1% of CD4+ T cells exhibited Tfh-like phenotype (PD-1highCXCR5+ICOS+CD40L+). Recipient DC pulsed with donor peripheral blood mononuclear cells was cocultured with recipient CD45RA+CD4+ and CD45RA-CD4+ (generally defined as naive and memory in humans, respectively) T cells. Irrespective of preformed or de novo DSA status, CD45RA+CD4+ T cells constantly produced IL-21. In contrast, IL-21-produced CD45RA-CD4+ T cells were significantly higher in preformed DSA-positive patients than those in negative patients (80.8 ± 51.2 versus 14.8 ± 20.4, P < 0.001). In de novo DSA-positive patients, IL-21-produced CD45RA-CD4+ T cells were significantly increased after transplantation compared with before transplantation (9.23 ± 9.08 versus 43.9 ± 29.1, P < 0.001).

CONCLUSIONS

. Assessment of indirect pathway CD4+ T cell response could provide new insights into the underlying mechanism of de novo DSA production, leading to the development of effective strategies against antibody-mediated rejection.

Optimal dose of rituximab in ABO-incompatible kidney transplantation in patients with low anti-A/B antibody titers: A single-center retrospective cohort study

BACKGROUND

The clinical outcomes of ABO-incompatible (ABOi) kidney transplantation have improved with the introduction of desensitization therapy with rituximab. However, rituximab prevents not only antibody-mediated rejection (AMR) but also increases the risk of adverse events, such as infection. For ABOi kidney transplantation in patients with low anti-A/B antibody titers, we previously used a rituximab-free desensitization protocol and then initiated a single dose of 100 mg rituximab in 2016. We retrospectively compared the outcomes of ABOi kidney transplantation in patients with low anti-A/B antibody titers before and after the introduction of rituximab.

METHODS

ABOi kidney transplantations (n = 142) in patients with low anti-A/B antibody titers between 2007 and 2021 were included. Patients were divided into two groups (with and without rituximab) for desensitization. The primary outcomes were the incidence of acute AMR and infection.

RESULTS

Sixty-six patients were desensitized without rituximab (rituximab-free group), and 76 were pretreated with 100 mg rituximab (rituximab group) before transplantation. The incidence of acute AMR was significantly lower in the rituximab group than in the rituximab-free group (.0% [0/76] vs. 7.6% [5/66], respectively; p = .047). Post-transplantation anti-A/B antibody titers were also lower in the rituximab group than in the rituximab-free group. There was no significant difference in the incidence of adverse events, including infections, between the two groups.

CONCLUSION

In ABOi kidney transplantation patients with low anti-A/B antibody titers, the desensitization protocol with a single dose of 100 mg rituximab was effective in preventing acute AMR without increasing the risk of other adverse events.

Antigen Cross-Presentation by Murine Proximal Tubular Epithelial Cells Induces Cytotoxic and Inflammatory CD8+ T Cells

Immune-mediated glomerular diseases are characterized by infiltration of T cells, which accumulate in the periglomerular space and tubulointerstitium in close contact to proximal and distal tubuli. Recent studies described proximal tubular epithelial cells (PTECs) as renal non-professional antigen-presenting cells that stimulate CD4⁺ T-cell activation. Whether PTECs have the potential to induce activation of CD8⁺ T cells is less clear. In this study, we aimed to investigate the capacity of PTECs for antigen cross-presentation thereby modulating CD8⁺ T-cell responses. We showed that PTECs expressed proteins associated with cross-presentation, internalized soluble antigen via mannose receptor-mediated endocytosis, and generated antigenic peptides by proteasomal degradation. PTECs induced an antigen-dependent CD8⁺ T-cell activation in the presence of soluble antigen in vitro. PTEC-activated CD8⁺ T cells expressed granzyme B, and exerted a cytotoxic function by killing target cells. In murine lupus nephritis, CD8⁺ T cells localized in close contact to proximal tubuli. We determined enhanced apoptosis in tubular cells and particularly PTECs up-regulated expression of cleaved caspase-3. Interestingly, induction of apoptosis in the inflamed kidney was reduced in the absence of CD8⁺ T cells. Thus, PTECs have the capacity for antigen cross-presentation thereby inducing cytotoxic CD8⁺ T cells in vitro, which may contribute to the pathology of immune-mediated glomerulonephritis.

Machine learning-based method for tacrolimus dose predictions in Chinese kidney transplant perioperative patients

WHAT IS KNOWN AND OBJECTIVES

Tacrolimus (TAC), a first-line immunosuppressant in solid-organ transplant, has a narrow therapeutic window and large inter-individual variability, which affects its use in clinical practice. Successful predictions using machine learning algorithms have been reported in several fields. However, a comparison of 10 machine learning model-based TAC pharmacogenetic and pharmacokinetic dosing algorithms for kidney transplant perioperative patients of Chinese descent has not been reported. The objective of this study was to screen and establish an appropriate machine learning method to predict the individualized dosages of TAC for perioperative kidney transplant patients.

METHODS

The records of 2551 patients were collected from three transplant centres, 80% of which were randomly selected as a 'derivation cohort' to develop the dose prediction algorithm, while the remaining 20% constituted a 'validation cohort' to validate the final algorithm selected. Important features were screened according to our previously established population pharmacokinetic model of tacrolimus. The performances of the algorithms were evaluated and compared using R-squared and the mean percentage in the remaining 20% of patients.

RESULTS AND DISCUSSION

This study identified several factors influencing TAC dosage, including CYP3A5 rs776746, CYP3A4 rs4646437, haematocrit, Wuzhi capsules, TAC daily dose, age, height, weight, post-operative time, nifedipine and the medication history of the patient. According to our results, among the 10 machine learning models, the extra trees regressor (ETR) algorithm showed the best performance in the training set (R-squared: 1, mean percentage within 20%: 100%) and test set (R-squared: 0.85, mean percentage within 20%: 92.77%) of the derivation cohort. The ETR model successfully predicted the ideal TAC dosage in 97.73% of patients, especially in the intermediate dosage range (>5 mg/day to <8 mg/day), whereby the ideal TAC dosage could be successfully predicted in 99% of the patients.

WHAT IS NEW AND CONCLUSION

The results indicated that the ETR algorithm, which was chosen to establish the dose prediction model, performed better than the other nine machine learning models. This study is the first to establish ETR algorithms to predict TAC dosage. This study will further promote the individualized medication of TAC in kidney transplant patients in the future, which has great significance in ensuring the safety and effectiveness of drug use.

Normothermic ex vivo perfusion of the limb allograft depletes donor leukocytes prior to transplantation

INTRODUCTION

The donor immune compartment plays a central role in graft rejection of the vascularised composite allograft (VCA) by contributing to 'direct presentation'. Using our limb ex vivo normothermic machine perfusion (EVNP) protocol designed for prolonged allograft preservation, this study aimed to assess whether donor leukocytes responsible for allograft rejection are mobilised from the donor compartment.

METHODS

Five genetically different pig forelimbs underwent perfusion via the brachial and radial collateral artery for 6 h after 2 h of cold storage. Oxygenated haemodilute leucocyte-deplete blood was recirculated at normothermia using an extracorporeal perfusion system. Tissue perfusion was evaluated clinically and biochemically via blood perfusate. The temporal kinetics of donor leucocyte extravasation, cytokine secretion and cell-free DNA was characterised in the circulating perfusate.

RESULTS

Flow cytometry revealed increasing populations of viable leukocytes over time, reaching 49 billion leukocytes by 6 h. T (3.0 × 10⁹ cells) and B cells (3.1 × 10⁸ cells) lymphocytes, monocytes (2.7 × 10⁹ cells), granulocytes (8.1 × 10⁹ cells), NK (6.3 × 10⁸) and γδ (8.1 × 10⁸) cells were all identified. Regulatory T cells comprised a minor population (1.6 × 10⁷ cells). There was a cumulative increase in pro-inflammatory cytokines suggesting that the donor limb has the capacity to elicit significant inflammatory responses that could contribute to leucocyte activation and diapedesis.

CONCLUSION

EVNP not only acts as a preservation tool, but could also be utilized to immunodeplete the VCA allograft prior to transplantation. This has clinical implications to mitigate acute rejection and prevent graft dysfunction and supports the future application of machine perfusion in graft preservation and immune modulation.

Review of Gamma-Irradiated Sterile Cornea: Properties, Indications, and New Directions

ABSTRACT

Gamma-irradiated sterile cornea (GISC) is a relatively new graft tissue that follows the 30-year success of irradiation to other tissues (bone and sclera) to lessen antigenic load and infection transmission. It can serve as a graft to cover glaucoma drainage devices, as graft tissue for frank or impending corneal perforations in tectonic keratoplasty, as graft tissue in lamellar keratoplasty, and as carrier tissue for keratoprosthesis. With the limited availability of fresh corneal tissue in less developed countries, GISC may fill a need for corneal tissue worldwide. It can be used in situations where corneal endothelium is not needed and with a greatly reduced risk of rejection and infection because of the effect of irradiation on corneal tissue.

Clinical Significance of Shared T Cell Epitope Analysis in Early De Novo Donor-Specific Anti-HLA Antibody Production After Kidney Transplantation and Comparison With Shared B cell Epitope Analysis

In pre-sensitizing events, immunological memory is mainly created via indirect allorecognition where CD4⁺ T cells recognize foreign peptides in the context of self-HLA class II (pHLA) presented on antigen-presenting cells. This recognition makes it possible for naive CD4⁺ T-helper cells to differentiate into memory cells, resulting in the creation of further antibody memory. These responses contribute to effective secretion of donor-specific anti-HLA antibodies (DSA) after second encounters with the same peptide. Preformed donor-reactive CD4⁺ memory T cells may induce early immune responses after transplantation; however, the tools to evaluate them are limited. This study evaluated shared T cell epitopes (TEs) between the pre-sensitizing and donor HLA using an in silico assay, an alternative to estimate donor-reactive CD4⁺ memory T cells before transplantation. In 578 living donor kidney transplants without preformed DSA, 69 patients had anti-HLA antibodies before transplantation. Of them, 40 had shared TEs and were estimated to have donor-reactive CD4⁺ memory T cells. De novo DSA formation in the early phase was significantly higher in the shared TE-positive group than in the anti-HLA antibody- and shared TE-negative groups (p=0.001 and p=0.02, respectively). In conclusion, evaluation of shared TEs for estimating preformed donor-reactive CD4⁺ memory T cells may help predict the risk of early de novo DSA formation after kidney transplantation.

Pathogen Dose in Animal Models of Hemorrhagic Fever Virus Infections and the Potential Impact on Studies of the Immune Response

Viral hemorrhagic fever viruses come from a wide range of virus families and are a significant cause of morbidity and mortality worldwide each year. Animal models of infection with a number of these viruses have contributed to our knowledge of their pathogenesis and have been crucial for the development of therapeutics and vaccines that have been approved for human use. Most of these models use artificially high doses of virus, ensuring lethality in pre-clinical drug development studies. However, this can have a significant effect on the immune response generated. Here I discuss how the dose of antigen or pathogen is a critical determinant of immune responses and suggest that the current study of viruses in animal models should take this into account when developing and studying animal models of disease. This can have implications for determination of immune correlates of protection against disease as well as informing relevant vaccination and therapeutic strategies.

A post-preservation vascular flush removes significant populations of donor leukocytes prior to lung transplantation

BACKGROUND

Donor leukocytes are intrinsically involved in acute lung allograft rejection, via self-presentation of donor antigens to recipient leukocytes. Therapeutic modalities to remove donor leukocytes are currently unavailable. We evaluated if a vascular flush immediately following preservation can be used for this purpose.

METHODS

A post-preservation flush was performed with STEEN solution in n = 6 porcine lungs following static cold storage. The first 500 ml effluent from the left atrium was collected and an inflammatory profile performed.

RESULTS

A total of 1.17 billion (±2.8 × 10⁸) viable leukocytes were identified within the effluent. T cells were the dominant cell population, representing 82% of the total mobilised leukocytes, of which <0.01% were regulatory T cells. IL-18 was the most abundant cytokine, with a mean concentration of 84,216 pg (±153,552 pg). In addition, there was a mean concentration of 8819 ng (±4415) cell-free mitochondrial DNA.

CONCLUSION

There is an immediate transfer of donor leukocytes, cytokines and damage-associated molecular patterns following reperfusion. Such a pro-inflammatory donor load may enhance alloantigen presentation and drive recipient alloimmune responses. A post-preservation flush may therefore be an effective method for reducing the immune burden of the donor lung prior to transplantation.

Th17/Treg-cell balance in the peripheral blood of pregnant females with a history of recurrent spontaneous abortion receiving progesterone or cyclosporine A

A successful pregnancy requires the maternal immune system to accept a fetus expressing allogeneic paternal antigens and provide competent responses to infections. Accordingly, maternal-fetal immune abnormalities may have an important role in the development of recurrent spontaneous abortion (RSA). Ever since the establishment of the association between immunologic abnormalities and RSA, various types of immune therapy to restore normal immune homeostasis have been increasingly developed. Although previous studies have focused on the maternal-fetal interface, non-invasive examination is of great importance in clinical practice. The present study investigated the balance between type-17 T-helper (Th17) and T-regulatory (Treg) cells in the peripheral blood to improve the current understanding of the pathogenesis of RSA. Imbalances in Th17/Treg cells and associated molecular profiles were observed in patients with RSA. Furthermore, it was determined that the immunosuppressant cyclosporine A reduced the proportion of Th17 cells and promoted Treg-cell dominance by upregulating the expression of co-inhibitory molecules in pregnant females with a history of RSA. Progesterone, the traditional maternal-care drug, also had a certain immunomodulatory role through restoring the levels of several co-inhibitory molecules (including T-cell immunoglobulin mucin family member-3, programmed cell death-1 and cytotoxic T-lymphocyte associated protein-4) in the treatment of RSA. Changes in these immune molecules within the maternal peripheral blood may be indicators for monitoring pregnancy and prediction of RSA.

Design and Optimization of PLGA Particles to Deliver Immunomodulatory Drugs for the Prevention of Skin Allograft Rejection

Background: Recent advancements in therapeutic strategies have attracted considerable attention to control the acute organs and tissues rejection, which is the main cause of mortality in transplant recipients. The long-term usage of immunosuppressive drugs compromises the body immunity against simple infections and decrease the patients' quality of life. Tolerance of allograft in recipients without harming the rest of host immune system is the basic idea to develop the therapeutic approaches after induction of donor-specific transplant. Methods: Controlled and targeted delivery system by using biomimetic micro and nanoparticles as carriers is an effective strategy to deplete the immune cells in response to allograft in an antigen-specific manner. Polylactic-co-glycolic acid (PLGA) is a biocompatible and biodegradable polymer, which has frequently being used as drug delivery vehicle. Results: This review focuses on the biomedical applications of PLGA based biomimetic micro and nano-sized particles in drug delivery systems to prolong the survival of alloskin graft. Conclusion: We will discuss the mediating factors for rejection of alloskin graft, selective depletion of immune cells, controlled release mechanism, physiochemical properties, size-based body distribution of PLGA particles and their effect on overall host immune system.

PIRCHE-II: an algorithm to predict indirectly recognizable HLA epitopes in solid organ transplantation

Human leukocyte antigen (HLA) mismatches between donors and recipients may lead to alloreactivity after solid organ transplantation. Over the last few decades, our knowledge of the complexity of the HLA system has dramatically increased, as numerous new HLA alleles have been identified. As a result, the likelihood of alloreactive responses towards HLA mismatches after solid organ transplantation cannot easily be assessed. Algorithms are promising solutions to estimate the risk for alloreactivity after solid organ transplantation. In this review, we show that the recently developed PIRCHE-II (Predicted Indirectly ReCognizable HLA Epitopes) algorithm can be used to minimize alloreactivity towards HLA mismatches. Together with the use of other algorithms and simulation approaches, the PIRCHE-II algorithm aims for a better estimated alloreactive risk for individual patients and eventually an improved graft survival after solid organ transplantation.

HLA-A promoter, coding, and 3'UTR sequences in a Brazilian cohort, and their evolutionary aspects

HLA-A is the second most polymorphic locus of the human leucocyte antigen (HLA) complex encoding a key molecule for antigen presentation and NK cell modulation. Many studies have evaluated HLA-A variability in worldwide populations, focusing mainly on exons, but the regulatory segments have been poorly characterized. HLA-A variability is particularly high in the segment encoding the peptide-binding groove (exons 2 and 3), which is related to the antigen presentation function and the balancing selection in these segments. Here we evaluate the genetic diversity of the HLA-A gene considering a continuous segment encompassing the extended promoter (1.5 kb upstream of the first translated ATG), all exons and introns, and the entire 3' untranslated region, by using massively parallel sequencing. To achieve this goal, we used a freely available bioinformatics workflow that optimizes read mapping for HLA genes and defines complete sequences using either the phase among variable sites directly observed in sequencing data and probabilistic models. The HLA-A variability detected in a highly admixed population sample from Brazil shows that the HLA-A regulatory segments present few, but divergent sequences. The regulatory segments are in close association with the coding alleles. Both exons and introns are highly variable. Moreover, patterns of molecular diversity suggest that the promoter, in addition to the coding region, might be under the same selective pressure, but a different scenario arises when it comes to exon 4 and the 3'UTR segment.

Immunosuppression following organ transplantation. Part 1: mechanisms and immunosuppressive agents

Solid organ transplantation has revolutionised medical care by providing a definitive cure for a wide spectrum of end-stage medical conditions. This treatment, however, does not come without complications and poses the risks of rejection, life-threatening infection, malignancies and recurrent organ failure, with significant impacts on patient outcomes. One of the major challenges involved in optimising post-transplant outcomes is managing the immune system's response to the transplanted graft and preventing organ rejection. This is mainly accomplished through the use of immunosuppressant agents, which have become a mainstay of treatment for a majority of post-transplant patients. This article, the first of two parts, discusses the concept of immunosuppression and its importance in the care of patients who have received an organ transplant. It focuses on the pathophysiologic mechanisms involved in transplant rejection and discusses the pharmacologic aspects of immunosuppression and its implications for patient care.

Evaluation of current strategies for surveillance and management of donor-specific antibodies: Single-center study

BACKGROUND

Although the presence of donor-specific antibodies (DSA) is known to impact lung allograft, limited data exist regarding DSA management.

METHODS

We did a retrospective study at our center evaluating DSA management in adult lung transplant recipients undergoing lung transplantation between January 1, 2010 and June 30, 2014. Study follow-up was completed through October 2017. All recipients were stratified into 2 groups based on the presence or absence of DSA. Those with DSA were evaluated for the impact of treatment of DSA. The primary outcomes were postlung transplant survival and freedom from bronchiolitis obliterans syndrome (BOS), subset of chronic lung allograft dysfunction (CLAD). Simon-Makuch method was used to estimate overall survival and BOS-free survival to account for DSA as time-dependent covariate. Survival differences between the groups were analyzed using time-dependent Cox proportional hazards model.

RESULTS

Sixty-four percent of 194 total subjects developed post-lung transplant DSA. Overall survival was different with worse survival in the DSA positive group that never cleared DSA (P = .002). BOS-free survival was lower, but did not reach significance in this group. Response to treatment was poor, with only 12 of 47 (25.5%) who received treatment demonstrating clearance of DSA.

CONCLUSIONS

Donor-specific antibodies prevalence is high after lung transplantation. Clearance of DSA correlated with improved outcomes. Current therapeutic strategies against DSA are relatively ineffective. Multicenter collaborative studies will be required to evaluate current treatment strategies and other innovative modalities.

Characterizing the early inflammatory contribution of the donor kidney following reperfusion

Background

Donor kidneys contain a large reservoir of passenger leucocytes that contribute to acute rejection via direct alloantigen presentation and pro-inflammatory cytokine secretion. However, the early contribution of these cells following revascularization has not previously been described. We performed a secondary, high-volume preservation flush following cold storage to characterize the inflammatory contribution of the donor kidney upon reperfusion.

Methods

Porcine kidneys were retrieved using a protocol analogous to current UK clinical practice. Following 2 h of cold static preservation, kidneys underwent a secondary flush with Ringer's solution. The venous effluent was collected and leucocytes phenotyped via flow cytometry. Inflammatory mediators, including cytokines and cell-free DNA, were then assessed to determine the inflammatory contribution of the donor kidney.

Results

Upon reperfusion, a significant population of donor-derived CD45 +  leucocytes mobilized from the renal vasculature via the renal vein [mean 4.738 × 10 8  (SD 1.348 × 10 8 )]. Within this population, T cells were dominant, representing >60% of the leucocyte repertoire. Granulocytes, monocytes and natural killer cells were also identified, but in comparatively lower numbers. Significant concentrations of cytokines and cell-free DNA were also eluted upon reperfusion.

Conclusions

The donor kidney contains a significant immune load that rapidly mobilizes following reperfusion. Performing a secondary preservation flush prior to implantation may reduce this inflammatory burden via diversion of donor leucocytes and inflammatory mediators from entry into the recipient circulation. This may modulate direct presentation and reduce the inflammatory contribution of the donor kidney following transplantation.

Matching donor and recipient based on predicted indirectly recognizable human leucocyte antigen epitopes

The predicted indirectly recognizable human leucocyte antigen (HLA) epitopes (PIRCHE) algorithm is a novel in silico algorithm to determine donor-recipient compatibility. The PIRCHE algorithm determines donor-recipient compatibility by counting the number of mismatched HLA-derived epitopes that are involved in indirect T-cell alloimmune responses; these epitopes are designated as PIRCHE. Over the last few years, the PIRCHE algorithm has been investigated in both hematopoietic stem cell transplantation and solid organ transplantation. This review describes the theory of the algorithm, its application in transplantation, and highlights the future perspectives on the clinical application of the PIRCHE algorithm.

Lymphocytes in Placental Tissues: Immune Regulation and Translational Possibilities for Immunotherapy

Immune modulation at the fetomaternal interface is crucial to ensure that the fetal allograft is not rejected. In the present review, the focus is to describe basic functions of lymphocyte populations and how they may contribute to fetomaternal immune regulation, as well as determining what proportions and effector functions of these cells are reported to be present in placental tissues in humans. Also explored is the possibility that unique cell populations at the fetomaternal interface may be targets for adoptive cell therapy. Increasing the understanding of immune modulation during pregnancy can give valuable insight into other established fields such as allogeneic hematopoietic stem cell transplantation and solid organ transplantation. In these settings, lymphocytes are key components that contribute to inflammation and rejection of either patient or donor tissues following transplantation. In contrast, an allogeneic fetus eludes rejection by the maternal immune system.

Predicted indirectly recognizable HLA epitopes are not associated with clinical outcomes after haploidentical hematopoietic stem cell transplantation

Haploidentical stem cell transplantation (haplo-SCT) provides an alternative method to cure patients with malignant and nonmalignant hematologic diseases who lack a human leukocyte antigen (HLA) matched related or unrelated donor. HLA disparity between donor and patient was the main reason causing lots of clinical immune response. The aim of this study was to investigate whether indirect recognition of mismatched HLA could predict the clinical outcomes in haplo-SCT. The probability of indirect recognition was predicted by the Predicted Indirectly ReCognizable HLA Epitopes (PIRCHE) model. 577 patients with acute leukemia or myelodysplastic syndrome receiving haplo-SCT were enrolled in the study. Patients were divided into 4 quartiles according to PIRCHE-Ⅰ or PIRCHE-Ⅱ. Although the cumulative incidences of chronic graft-versus-host disease (GVHD) were significantly different among the 4 PIRCHE-Ⅰgroups, with 20.4% for group 0-6, 40.5% for group >6-11, 26.1% for group >11-19 and 23.9% for group >19 (P = .007), PIRCHE-Ⅰ was not significantly associated with chronic GVHD in multivariate models (RR, 0.993; 95% CI, 0.858-1.149; P = .926). And no significant associations were observed between PIRCHE-Ⅰ or PIRCHE-Ⅱ and other clinical outcomes. In summary, PIRCHE did not correlate with clinical outcomes and could not predict haplo-SCT outcomes.

HLA Amino Acid Polymorphisms and Kidney Allograft Survival

Background

The association of HLA mismatching with kidney allograft survival has been well established. We examined whether amino acid (AA) mismatches (MMs) at the antigen recognition site of HLA molecules represent independent and incremental risk factors for kidney graft failure (GF) beyond those MMs assessed at the antigenic (2-digit) specificity.

Methods

Data on 240 024 kidney transplants performed between 1987 and 2009 were obtained from the Scientific Registry of Transplant Recipients. We imputed HLA-A, -B, and -DRB1 alleles and corresponding AA polymorphisms from antigenic specificity through the application of statistical and population genetics inferences. GF risk was evaluated using Cox proportional-hazards regression models adjusted for covariates including patient and donor risk factors and HLA antigen MMs.

Results

We show that estimated AA MMs at particular positions in the peptide-binding pockets of HLA-DRB1 molecule account for a significant incremental risk that was independent of the well-known association of HLA antigen MMs with graft survival. A statistically significant linear relationship between the estimated number of AA MMs and risk of GF was observed for HLA-DRB1 in deceased donor and living donor transplants. This relationship was strongest during the first 12 months after transplantation (hazard ratio, 1.30 per 15 DRB1 AA MM; P < 0.0001).

Conclusions

This study shows that independent of the well-known association of HLA antigen (2-digit specificity) MMs with kidney graft survival, estimated AA MMs at peptide-binding sites of the HLA-DRB1 molecule account for an important incremental risk of GF.

In a population of 240 024 kidney transplant recipients using the data of the Scientific Registry of Transplant recipients, the authors demonstrate that, independently of HLA antigen mismatches, estimated amino-acid mismatches at peptide-binding sites of the HLA-DRB1 molecule, accounts for an increased graft failure risk. Supplemental digital content is available in the text.

Transplant trials with Tregs: perils and promises

Modern immunosuppression regimens effectively control acute rejection and decrease graft loss in the first year after transplantation; however, these regimens do not have a durable effect on long-term graft survival owing to a combination of drug toxicities and the emergence of chronic alloimmune responses. Eliminating drugs and their toxicities while maintaining graft acceptance has been the primary aim of cellular therapies. Tregs suppress both autoimmune and alloimmune responses and are particularly effective in protecting allografts in experimental transplant models. Further, Treg-based therapies are selective, do not require harsh conditioning, and do not have a risk of graft-versus-host disease. Trial designs should consider the distinct immunological features of each transplanted organ, Treg preparations, dose, and frequency, and the ability to detect and quantify Treg effects in a given transplant environment. In this Review, we detail the ongoing clinical trials of Treg therapy in liver and kidney transplantation. Integration of Treg biology gleaned from preclinical models and experiences in human organ transplantation should allow for optimization of trial design that will determine the potential efficacy of a given therapy and provide guidelines for further therapeutic development.

Toll-Like Receptor 4 Deficiency Improves Short-term Renal Function but not Long-term Graft Survival in a Fully MHC-Mismatched Murine Model of Renal Allograft Transplantation

BACKGROUND

We have previously demonstrated that absence of myeloid differentiation primary response gene 88 (MyD88) induced donor-specific kidney allograft tolerance. The upstream pathways of MyD88 that mediate this process, however, remain unclear. Toll-like receptor 4 (TLR4) is an innate immune receptor that is dependent upon MyD88 for activity of its dominant signaling pathway. Here, we investigated the role of TLR4 in kidney allograft rejection using a fully major histocompatibility complex-mismatched, life-sustaining, murine model of renal allograft rejection.

METHODS

Donor (BALB/c) and recipient (C57BL/6) mice either both deficient or sufficient for TLR4 underwent heterotopic renal allograft transplantation, with an additional group of mice receiving renal isografts as controls. Survival was assessed up to 100 days posttransplantation. Animals were also sacrificed 14 days posttransplantation for assessment of the acute allograft rejection response.

RESULTS

Both wild-type (WT) and TLR4 allografts showed inferior survival compared to isografts, with no difference in survival between the allograft groups. Serum creatinine was lower in TLR4 allografts at day 14 posttransplantation compared with WT allografts, but this was not sustained by day 100. At day 14 posttransplant, increased CD11c dendritic cell accumulation, expression of IL-2 and indoleamine 2,3-dioxygenase were evident in TLR4 compared with WT allografts, whereas expression of inducible nitric oxide synthase was decreased.

CONCLUSIONS

Acute kidney allograft rejection was modestly attenuated in TLR4 mice; however, long-term allograft survival and function were not affected in our model. Protection against acute rejection may involve increased accumulation of CD11c cells and indoleamine 2,3-dioxygenase expression.

The Biology and Underlying Mechanisms of Cross-Presentation of Exogenous Antigens on MHC-I Molecules

To monitor the health of cells, the immune system tasks antigen-presenting cells with gathering antigens from other cells and bringing them to CD8 T cells in the form of peptides bound to MHC-I molecules. Most cells would be unable to perform this function because they use their MHC-I molecules to exclusively present peptides derived from the cell's own proteins. However, the immune system evolved mechanisms for dendritic cells and some other phagocytes to sample and present antigens from the extracellular milieu on MHC-I through a process called cross-presentation. How this important task is accomplished, its role in health and disease, and its potential for exploitation are the subject of this review.

Expression of a Chimeric Antigen Receptor Specific for Donor HLA Class I Enhances the Potency of Human Regulatory T Cells in Preventing Human Skin Transplant Rejection

Regulatory T cell (Treg) therapy using recipient-derived Tregs expanded ex vivo is currently being investigated clinically by us and others as a means of reducing allograft rejection following organ transplantation. Data from animal models has demonstrated that adoptive transfer of allospecific Tregs offers greater protection from graft rejection compared to polyclonal Tregs. Chimeric antigen receptors (CAR) are clinically translatable synthetic fusion proteins that can redirect the specificity of T cells toward designated antigens. We used CAR technology to redirect human polyclonal Tregs toward donor-MHC class I molecules, which are ubiquitously expressed in allografts. Two novel HLA-A2-specific CARs were engineered: one comprising a CD28-CD3ζ signaling domain (CAR) and one lacking an intracellular signaling domain (ΔCAR). CAR Tregs were specifically activated and significantly more suppressive than polyclonal or ΔCAR Tregs in the presence of HLA-A2, without eliciting cytotoxic activity. Furthermore, CAR and ΔCAR Tregs preferentially transmigrated across HLA-A2-expressing endothelial cell monolayers. In a human skin xenograft transplant model, adoptive transfer of CAR Tregs alleviated the alloimmune-mediated skin injury caused by transferring allogeneic peripheral blood mononuclear cells more effectively than polyclonal Tregs. Our results demonstrated that the use of CAR technology is a clinically applicable refinement of Treg therapy for organ transplantation.

Old game, new players: Linking classical theories to new trends in transplant immunology

The evolutionary emergence of an efficient immune system has a fundamental role in our survival against pathogenic attacks. Nevertheless, this same protective mechanism may also establish a negative consequence in the setting of disorders such as autoimmunity and transplant rejection. In light of the latter, although research has long uncovered main concepts of allogeneic recognition, immune rejection is still the main obstacle to long-term graft survival. Therefore, in order to define effective therapies that prolong graft viability, it is essential that we understand the underlying mediators and mechanisms that participate in transplant rejection. This multifaceted process is characterized by diverse cellular and humoral participants with innate and adaptive functions that can determine the type of rejection or promote graft acceptance. Although a number of mediators of graft recognition have been described in traditional immunology, recent studies indicate that defining rigid roles for certain immune cells and factors may be more complicated than originally conceived. Current research has also targeted specific cells and drugs that regulate immune activation and induce tolerance. This review will give a broad view of the most recent understanding of the allogeneic inflammatory/tolerogenic response and current insights into cellular and drug therapies that modulate immune activation that may prove to be useful in the induction of tolerance in the clinical setting.

Myocardial Gene Expression Profiling to Predict and Identify Cardiac Allograft Acute Cellular Rejection: The GET-Study

Aims

Serial invasive endomyocardial biopsies (EMB) remain the gold standard for acute cellular rejection (ACR) diagnosis. However histological grading has several limitations. We aimed to explore the value of myocardial Gene Expression Profiling (GEP) for diagnosing and identifying predictive biomarkers of ACR.

Methods

A case-control study nested within a retrospective heart transplant patients cohort included 126 patients with median (IQR) age 50 (41–57) years and 111 (88%) males. Among 1157 EMB performed, 467 were eligible (i.e, corresponding to either ISHLT grade 0 or ≥3A), among which 36 were selected for GEP according to the grading: 0 (C_ISHLT, n = 13); rejection ≥3A (R_ISHLT, n = 13); 0 one month before ACR (BR_ISHLT, n = 10).

Results

We found 294 genes differentially expressed between C_ISHLT and R_ISHLT, mainly involved in immune activation, and inflammation. Hierarchical clustering showed a clear segregation of C_ISHLT and R_ISHLT groups and heterogeneity of GEP within R_ISHLT. All EMB presented immune activation, but some R_ISHLT EMB were strongly subject to inflammation, whereas others, closer to C_ISHLT, were characterized by structural modifications with lower inflammation level. We identified 15 probes significantly different between BR_ISHLT and C_ISHLT, including the gene of the muscular protein TTN. This result suggests that structural alterations precede inflammation in ACR. Linear Discriminant Analysis based on these 15 probes was able to identify the histological status of every 36 samples.

Conclusion

Myocardial GEP is a helpful method to accurately diagnose ACR, and predicts rejection one month before its histological occurrence. These results should be considered in cardiac allograft recipients’ care.

What Is Direct Allorecognition?

Direct allorecognition is the process by which donor-derived major histocompatibility complex (MHC)-peptide complexes, typically presented by donor-derived ‘passenger’ dendritic cells, are recognised directly by recipient T cells. In this review, we discuss the two principle theories which have been proposed to explain why individuals possess a high-precursor frequency of T cells with direct allospecificity and how self-restricted T cells recognise allogeneic MHC-peptide complexes. These theories, both of which are supported by functional and structural data, suggest that T cells recognising allogeneic MHC-peptide complexes focus either on the allopeptides bound to the allo-MHC molecules or the allo-MHC molecules themselves. We discuss how direct alloimmune responses may be sustained long term, the consequences of this for graft outcome and highlight novel strategies which are currently being investigated as a potential means of reducing rejection mediated through this pathway.

Ex Vivo Normothermic Perfusion Induces Donor-Derived Leukocyte Mobilization and Removal Prior to Renal Transplantation

Introduction

Ex vivo normothermic perfusion offers an alternative method of organ preservation, allowing donor kidneys to be reanimated and evaluated prior to transplantation. Beyond preservation, it can be used to characterize the immunological contribution of the donor kidney in isolation. Furthermore, it has the potential to be used as an immunomodulatory strategy to manipulate donor kidneys prior to transplantation.

Methods

Explanted porcine kidneys underwent 6 hours of perfusion. Sequential perfusate samples were collected and leukocytes characterized via flow cytometry. An inflammatory profile was generated via cytokine quantification. Cell-free DNA was also determined as markers of cell death.

Results

All kidneys functioned within normal parameters and met the criteria for transplantation at the end of perfusion. Throughout perfusion there were continuous increases in pro-inflammatory cytokines, including large concentrations of interferon-γ, suggesting that perfusion drives a significant inflammatory response. Increasing concentrations in cell-free DNA were also observed, suggesting cell death. During perfusion there was a marked cellular diapedesis of T cells, B cells, natural killer (NK) cells, and monocytes from the kidney into the circuit. Minor populations of granulocytes and macrophages were also detected.

Discussion

We demonstrate that ex vivo normothermic perfusion initiates an inflammatory cytokine storm and release of mitochondrial and genomic DNA. This is likely to be responsible for immune cell activation and mobilization into the circuit prior to transplantation. Interestingly this did not have an impact on renal function. These data therefore suggest that normothermic perfusion can be used to immunodeplete and to saturate the pro-inflammatory capacity of donor kidneys prior to transplantation.

Computational immuno-biology for organ transplantation and regenerative medicine

Organ transplantation and regenerative medicine are adopted platforms that provide replacement tissues and organs from natural or engineered sources. Acceptance, tolerance and rejection depend greatly on the proper control of the immune response against graft antigens, motivating the development of immunological and genetical therapies that prevent organ failure. They rely on a complete, or partial, understanding of the immune system. Ultimately, they are innovative technologies that ensure permanent graft tolerance and indefinite graft survival through the modulation of the immune system. Computational immunology has arisen as a tool towards a mechanistic understanding of the biological and physicochemical processes surrounding an immune response. It comprehends theoretical and computational frameworks that simulate immuno-biological systems. The challenge is centered on the multi-scale character of the immune system that spans from atomistic scales, during peptide-epitope and protein interactions, to macroscopic scales, for lymph transport and organ-organ reactions. In this paper, we discuss, from an engineering perspective, the biological processes that are involved during the immune response of organ transplantation. Previous computational efforts, including their characteristics and visible limitations, are described. Finally, future perspectives and challenges are listed to motivate further developments.

Immune mechanisms of acute and chronic rejection

With the currently available immunosuppression, severe T-cell mediated rejection has become a rare event. With the introduction of modern antibody-detection techniques, such as the L-SAB technology, acute or hyperacute antibody-mediated rejection of the kidney are also seen infrequently. In contrast, chronic antibody-mediated rejection is considered to be a major contributor to graft loss in the late posttransplant phase. Problems in the management of chronic antibody-mediated rejection are effective prevention of the development of alloantibodies against donor HLA and the early identification of patients at risk for this entity. Finally, today there is still noeffective strategy to treat this indolent and slowly progressing form of antibody-mediated rejection. Herein, we review the pathomechanisms of the different forms of rejection and the clinical significance of these entities in human kidney transplantation.

Altered Immunogenicity of Donor Lungs via Removal of Passenger Leukocytes Using Ex Vivo Lung Perfusion

Passenger leukocyte transfer from the donor lung to the recipient is intrinsically involved in acute rejection. Direct presentation of alloantigen expressed on donor leukocytes is recognized by recipient T cells, promoting acute cellular rejection. We utilized ex vivo lung perfusion (EVLP) to study passenger leukocyte migration from donor lungs into the recipient and to evaluate the effects of donor leukocyte depletion prior to transplantation. For this purpose, female pigs received male left lungs either following 3 h of EVLP or retrieved using standard protocols. Recipients were monitored for 24 h and sequential samples were collected. EVLP-reduced donor leukocyte transfer into the recipient and migration to recipient lymph nodes was markedly reduced. Recipient T cell infiltration of the donor lung was significantly diminished via EVLP. Donor leukocyte removal during EVLP reduces direct allorecognition and T cell priming, diminishing recipient T cell infiltration, the hallmark of acute rejection.

Devil Facial Tumor Disease

Devil facial tumor disease (DFTD) is an emergent transmissible cancer exclusive to Tasmanian devils (Sarcophilus harrisii) and threatening the species with extinction in the wild. Research on DFTD began 10 years ago, when nothing was known about the tumor and little about the devils. The depth of knowledge gained since then is impressive, with research having addressed significant aspects of the disease and the devils' responses to it. These include the cause and pathogenesis of DFTD, the immune response of the devils and the immune evasion mechanisms of the tumor, the transmission patterns of DFTD, and the impacts of DFTD on the ecosystem. This review aims to collate this information and put it into the context of conservation strategies designed to mitigate the impacts of DFTD on the devil and the Tasmanian ecosystem.

Clonal CD8+ T Cell Persistence and Variable Gene Usage Bias in a Human Transplanted Hand

Immune prophylaxis and treatment of transplanted tissue rejection act indiscriminately, risking serious infections and malignancies. Although animal data suggest that cellular immune responses causing rejection may be rather narrow and predictable based on genetic background, there are only limited data regarding the clonal breadth of anti-donor responses in humans after allogeneic organ transplantation. We evaluated the graft-infiltrating CD8⁺ T lymphocytes in skin punch biopsies of a transplanted hand over 178 days. Profiling of T cell receptor (TCR) variable gene usage and size distribution of the infiltrating cells revealed marked skewing of the TCR repertoire indicating oligoclonality, but relatively normal distributions in the blood. Although sampling limitation prevented complete assessment of the TCR repertoire, sequencing further identified 11 TCR clonal expansions that persisted through varying degrees of clinical rejection and immunosuppressive therapy. These 11 clones were limited to three TCR beta chain variable (BV) gene families. Overall, these data indicate significant oligoclonality and likely restricted BV gene usage of alloreactive CD8⁺ T lymphocytes, and suggest that changes in rejection status are more due to varying regulation of their activity or number rather than shifts in the clonal populations in the transplanted organ. Given that controlled animal models produce predictable BV usage in T lymphocytes mediating rejection, understanding the determinants of TCR gene usage associated with rejection in humans may have application in specifically targeted immunotherapy.

Cellular and molecular targeting for nanotherapeutics in transplantation tolerance

The induction of donor-specific tolerance to transplanted cells and organs, while preserving immune function as a whole, remains a highly sought after and elusive strategy for overcoming transplant rejection. Tolerance necessitates modulating a diverse array of cell types that recognize and respond to alloantigens, including antigen presenting cells and T lymphocytes. Nanotherapeutic strategies that employ cellular and biomaterial engineering represent an emerging technology geared towards the goal of inducing transplant tolerance. Nanocarriers offer a platform for delivering antigens of interest to specific cell types in order to achieve tolerogenic antigen presentation. Furthermore, the technologies also provide an opportunity for local immunomodulation at the graft site. Nanocarriers delivering a combination of antigens and immunomodulating agents, such as rapamycin, provide a unique technology platform with the potential to enhance outcomes for the induction of transplant tolerance.

Immunogenetic factors in the selection of cord blood units for transplantation: current search strategies and future perspectives

Hematopoietic stem cell transplantation is currently used as a curative treatment for patients with malignant and non-malignant hematologic diseases. Human leukocyte antigen (HLA) matching is a major determinant for hematopoietic stem cell transplantation outcome. For patients lacking a fully HLA-matched donor, umbilical cord blood (UCB) units are alternative sources of hematopoietic stem cells because UCB transplantation allows a less stringent HLA matching. However, selection of the optimal UCB units remains challenging. The current UCB donor selection strategies are based on both cell dose and HLA matching. This Review focuses on the immunogenetic factors that influence UCB donor selection and highlights the future perspectives in UCB donor search.

A Subsequent Human Neural Progenitor Transplant into the Degenerate Retina Does Not Compromise Initial Graft Survival or Therapeutic Efficacy

PURPOSE

Stem and progenitor cell transplantation provides a promising clinical application for treating degenerative retinal diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Our previous studies have shown that a single subretinal injection of human cortical-derived neural progenitor cells (hNPCctx) into cyclosporine-treated Royal College of Surgeons (RCS) rats preserved both photoreceptors and visual function. However, it is still unknown whether nonautologous progenitor cell readministration for sustained vision is efficacious and safe in terms of the initial graft initiating an immune response to a subsequent graft.

METHODS

A cell suspension containing 3×104 hNPCctx into one eye of cyclosporine-treated RCS rats at postnatal day 21 (P21), followed by a second transplantation at P95 into the previously untreated fellow eye.

RESULTS

hNPCctx delayed photoreceptor degeneration and preserved visual function, as measured by electroretinography (ERG), optokinetic response (OKR), and luminance threshold recordings (LTRs). Visual function and photoreceptors of the initially treated eye were still preserved 6 weeks after hNPCctx were injected into the second eye. Antibodies against T-cell markers showed that CD3, CD4, and CD8 T cells were not detected at P90 and P140 in most cases. No detectable level of anti-nestin antibody was found in serum by enzyme-linked immunosorbent assay (ELISA).

CONCLUSIONS

This xenograft study with cyclosporine-treated animals demonstrates that readministration of hNPCctx into the fellow eye did not induce anti-graft immune responses or lower therapeutic efficacy of hNPCctx in preserving vision. Thus, readministration of progenitor cells to sustain long-term efficacy may be an option for long-term therapies of retinal degeneration.

TRANSLATIONAL RELEVANCE

Redosing neural progenitors do not affect the efficacy of the initial grafts in protecting vision or induce unwanted immune responses.

Matching for Human Leukocyte Antigens (HLA) in corneal transplantation - to do or not to do

As many patients with severe corneal disease are not even considered as candidates for a human graft due to their high risk of rejection, it is essential to find ways to reduce the chance of rejection. One of the options is proper matching of the cornea donor and recipient for the Human Leukocyte Antigens (HLA), a subject of much debate. Currently, patients receiving their first corneal allograft are hardly ever matched for HLA and even patients undergoing a regraft usually do not receive an HLA-matched graft. While anterior and posterior lamellar grafts are not immune to rejection, they are usually performed in low risk, non-vascularized cases. These are the cases in which the immune privilege due to the avascular status and active immune inhibition is still intact. Once broken due to infection, sensitization or trauma, rejection will occur. There is enough data to show that when proper DNA-based typing techniques are being used, even low risk perforating corneal transplantations benefit from matching for HLA Class I, and high risk cases from HLA Class I and probably Class II matching. Combining HLA class I and class II matching, or using the HLAMatchmaker could further improve the effect of HLA matching. However, new techniques could be applied to reduce the chance of rejection. Options are the local or systemic use of biologics, or gene therapy, aiming at preventing or suppressing immune responses. The goal of all these approaches should be to prevent a first rejection, as secondary grafts are usually at higher risk of complications including rejections than first grafts.

Interferon-gamma modification of mesenchymal stem cells: implications of autologous and allogeneic mesenchymal stem cell therapy in allotransplantation

Bone marrow-derived mesenchymal stem cells (MSC) have unique immunomodulatory and reparative properties beneficial for allotransplantation cellular therapy. The clinical administration of autologous or allogeneic MSC with immunosuppressive drugs is able to prevent and treat allograft rejection in kidney transplant recipients, thus supporting the immunomodulatory role of MSC. Interferon-gamma (IFN-γ) is known to enhance the immunosuppressive properties of MSC. IFN-γ preactivated MSC (MSC-γ) directly or indirectly modulates T cell responses by enhancing or inducing MSC inhibitory factors. These factors are known to downregulate T cell activation, enhance T cell negative signalling, alter T cells from a proinflammatory to an anti-inflammatory phenotype, interact with antigen-presenting cells and increase or induce regulatory cells. Highly immunosuppressive MSC-γ with increased migratory and reparative capacities may aid tissue repair, prolong allograft survival and induce allotransplant tolerance in experimental models. Nevertheless, there are contradictory in vivo observations related to allogeneic MSC-γ therapy. Many studies report that allogeneic MSC are immunogenic due to their inherent expression of major histocompatibility (MHC) molecules. Enhanced expression of MHC in allogeneic MSC-γ may increase their immunogenicity and this can negatively impact allograft survival. Therefore, strategies to reduce MSC-γ immunogenicity would facilitate "off-the-shelf" MSC therapy to efficiently inhibit alloimmune rejection and promote tissue repair in allotransplantation. In this review, we examine the potential benefits of MSC therapy in the context of allotransplantation. We also discuss the use of autologous and allogeneic MSC and the issues associated with their immunogenicity in vivo, with particular focus on the use of enhanced MSC-γ cellular therapy.

Changes in immunological profile of allogeneic mesenchymal stem cells after differentiation: should we be concerned?

Mesenchymal stem cells (MSCs) are an adult stromal cell population possessing potent differentiation capacity and a potential for use across major histocompatibility complex barriers. Although allogeneic MSCs have potent immunosuppressive properties, evidence also suggests that they elicit a weak allogeneic immune response. However, the effect of induced differentiation on the immunosuppressive ability and immunogenicity of allogeneic MSCs is a potential obstacle when applying MSCs in tissue replacement therapies. These concerns will be explored in this review, with particular emphasis on changes in the cell surface expression of immunogenic markers, changes in the secretion of immunosuppressive molecules and in vivo functional benefits of the cell therapy. We review the literature from a translational point of view, focusing on pre-clinical studies that have utilised and analysed the effects of allogeneic immune responses on the ability of allogeneic MSCs to regenerate damaged tissue in models of bone, heart and cartilage defects.

Immunogenicity and tumorigenicity of pluripotent stem cells and their derivatives: genetic and epigenetic perspectives

One aim of stem cell-based therapy is to utilize pluripotent stem cells (PSCs) as a supplementary source of cells to repair or replace tissues or organs that have ceased to function due to severe tissue damage. However, PSC-based therapy requires extensive research to ascertain if PSC derivatives are functional without the risk of tumorigenicity, and also do not engender severe immune rejection that threatens graft survival and function. Recently, the suitability of induced pluripotent stem cells applied for patient-tailored cell therapy has been questioned since the discovery of several genetic and epigenetic aberrations during the reprogramming process. Hence, it is crucial to understand the effect of these abnormalities on the immunogenicity and survival of PSC grafts. As induced PSC-based therapy represents a hallmark for the potential solution to prevent and arrest immune rejection, this review also summarizes several up-to-date key findings in the field.

Pre-transplant immune state defined by serum markers and alloreactivity predicts acute rejection after living donor kidney transplantation

Acute rejection (AR) remains a major cause for long-term kidney allograft failure. Reliable immunological parameters suitable to define the pre-transplant immune state and hence the individual risk of graft rejection are highly desired to preferably adapt the immunosuppressive regimen in advance. Donor and third party alloreactivities were determined by mixed lymphocyte cultures. Soluble forms of CD25, CD30, and CD44 were detected in patients' serum by ELISA. Various lymphocyte subpopulations were measured using flow cytometry. All patients received triple immunosuppression (tacrolimus/mycophenolate mofetil/steroids) and were grouped according to biopsy results within the first year: rejection-free (RF, n = 13), borderline (BL, n = 5), or acute rejection (AR, n = 7). Patients with AR showed the highest pre-transplant alloreactivities and serum levels (sCD25/sCD30/sCD44) according to the pattern RF < BL < AR. Relying on serum analysis only, multivariate logistic regression (logit link function) yielded a prognostic score for prediction of rejection with 75.0% sensitivity and 69.2% specificity. Patients with rejection showed markedly higher pre-transplant frequencies of CD4(+) /CD8(+) T cells lacking CD28, but lower numbers of CD8(+) CD161(bright) T cells and NK cells than RF individuals. Pre-transplant immune state defined by alloreactivity, serum markers, and particular lymphocyte subsets seems to correlate with occurrence of graft rejection after kidney transplantation. A prognostic score based on pre-transplant serum levels has shown great potential for prediction of rejection episodes and should be further evaluated.

Immunosuppression and allograft rejection following lung transplantation: evidence to date

The enduring success of lung transplantation is built on the use of immunosuppressive drugs to stop the immune system from rejecting the newly transplanted lung allograft. Most patients receive a triple-drug maintenance immunosuppressive regimen consisting of a calcineurin inhibitor, an antiproliferative and corticosteroids. Induction therapy with either an antilymphocyte monoclonal or an interleukin-2 receptor antagonist are prescribed by many centres aiming to achieve rapid inhibition of recently activated and potentially alloreactive T lymphocytes. Despite this generic approach acute rejection episodes remain common, mandating further fine-tuning and augmentation of the immunosuppressive regimen. While there has been a trend away from cyclosporine and azathioprine towards a preference for tacrolimus and mycophenolate mofetil, this has not translated into significant protection from the development of chronic lung allograft dysfunction, the main barrier to the long-term success of lung transplantation. This article reviews the problem of lung allograft rejection and the evidence for immunosuppressive regimens used both in the short- and long-term in patients undergoing lung transplantation.

CXCL9 and CXCL10 accelerate acute transplant rejection mediated by alloreactive memory T cells in a mouse retransplantation model

C-X-C motif chemokine ligand (CXCL) 9 and CXCL10 play key roles in the initiation and development of acute transplant rejection. Previously, higher levels of RANTES expression and secretion were demonstrated in retransplantation or T-cell memory-transfer models. In the present study, the effect of the chemokines, CXCL9 and CXCL10, were investigated in a mouse retransplantation model. BALB/c mice were used as donors, while C57BL/6 mice were used as recipients. In the experimental groups, a heterotopic heart transplantation was performed six weeks following skin grafting. In the control groups, a heterotopic heart transplantation was performed without skin grafting. Untreated mice served as blank controls. The mean graft survival time of the heterotopic heart transplantations was 7.7 days in the experimental group (n=6), as compared with 3.25 days in the control group (n=6; P<0.001). On day three following cardiac transplantation, histological evaluation of the grafts revealed a higher International Society for Heart & Lung Transplantation grade in the experimental group as compared with the control group. In addition, gene expression and serum concentrations of CXCL9, CXCL10, interferon-γ, and interleukin-2 were markedly higher in the experimental group when compared with the control group. Differences between the levels of CXCL9 and CXCL10 in the pre- and post-transplant mice indicated that the chemokines may serve as possible biomarkers to predict acute rejection. The results of the present study demonstrated that CXCL9 and CXCL10 play a critical role in transplantation and retransplantation. High levels of these cytokines during the pre-transplant period may lead to extensive acute rejection. Thus, the observations enhance the understanding of the mechanism underlying the increased expression and secretion of CXCL9 and CXCL10 by alloreactive memory T cells.

Induction of PD-L1 on monocytes: a new mechanism by which IVIg inhibits mixed lymphocyte reactions

Allograft rejection and graft-versus-host disease (GvHD) are frequent complications following solid organ or stem cell transplantation in which T cell activation plays a central role. Despite the development of new immunosuppressive drugs that improve the success rate of transplantation, allograft survival continues to be a challenge. Recently, intravenous immunoglobulin (IVIg) has been proposed as prophylaxis and post-transplant treatment to reduce acute rejection episodes. IVIg is a therapeutic agent that is known to down-modulate T cell functions in patients with autoimmune disorders. To test the hypothesis that this immunomodulatory effect could be beneficial in the context of transplantation, we used mixed lymphocyte reactions (MLR) as an in vitro model of allograft rejection and GvHD. Our results show that IVIg strongly inhibits the MLR as evaluated by IL-2 secretion, a well-known marker of T cell activation. IVIg also modulates the secretion of other pro-(IL-6, IFN-γ) and anti-inflammatory (IL-1RA) cytokines. More importantly, we show that IVIg induces monocytes with a CD80(low) PD-L1(high) phenotype and that blockade of PD-L1 partially abrogates the inhibitory effect of IVIg. We have thus identified a new mechanism by which IVIg inhibits T cell functions in the context of transplantation, supporting the potential usefulness of IVIg in the prevention or treatment of graft rejection and GvHD.

Predicting alloreactivity in transplantation

Human leukocyte Antigen (HLA) mismatching leads to severe complications after solid-organ transplantation and hematopoietic stem-cell transplantation. The alloreactive responses underlying the posttransplantation complications include both direct recognition of allogeneic HLA by HLA-specific alloantibodies and T cells and indirect T-cell recognition. However, the immunogenicity of HLA mismatches is highly variable; some HLA mismatches lead to severe clinical B-cell- and T-cell-mediated alloreactivity, whereas others are well tolerated. Definition of the permissibility of HLA mismatches prior to transplantation allows selection of donor-recipient combinations that will have a reduced chance to develop deleterious host-versus-graft responses after solid-organ transplantation and graft-versus-host responses after hematopoietic stem-cell transplantation. Therefore, several methods have been developed to predict permissible HLA-mismatch combinations. In this review we aim to give a comprehensive overview about the current knowledge regarding HLA-directed alloreactivity and several developed in vitro and in silico tools that aim to predict direct and indirect alloreactivity.