Research opportunities and ethical considerations for heart and lung xenotransplantation research: A report from the National Heart, Lung, and Blood Institute workshop.

Xenotransplantation offers the potential to meet the critical need for heart and lung transplantation presently constrained by the current human donor organ supply. Much was learned over the past decades regarding gene editing to prevent the immune activation and inflammation that cause early organ injury, and strategies for maintenance of immunosuppression to promote longer-term xenograft survival. However, many scientific questions remain regarding further requirements for genetic modification of donor organs, appropriate contexts for xenotransplantation research (including nonhuman primates, recently deceased humans, and living human recipients), and risk of xenozoonotic disease transmission. Related ethical questions include the appropriate selection of clinical trial participants, challenges with obtaining informed consent, animal rights and welfare considerations, and cost. Research involving recently deceased humans has also emerged as a potentially novel way to understand how xeno-organs will impact the human body. Clinical xenotransplantation and research involving decedents also raise ethical questions and will require consensus regarding regulatory oversight and protocol review. These considerations and the related opportunities for xenotransplantation research were discussed in a workshop sponsored by the National Heart, Lung, and Blood Institute, and are summarized in this meeting report.

The landscape of liver transplantation for patients with alcohol-associated liver disease in the United States.

Indications for liver transplants have expanded to include patients with alcohol-associated liver disease (ALD) over the last decade. Concurrently, the liver allocation policy was updated in February 2020 replacing the Donor Service Area with Acuity Circles (ACs). The aim is to compare the transplantation rate, waitlist outcomes, and posttransplant survival of candidates with ALD to non-ALD and assess differences in that effect after the implementation of the AC policy. Scientific Registry for Transplant Recipients data for adult candidates for liver transplant were reviewed from the post-AC era (February 4, 2020-March 1, 2022) and compared with an equivalent length of time before ACs were implemented. The adjusted transplant rates were significantly higher for those with ALD before AC, and this difference increased after AC implementation (transplant rate ratio comparing ALD to non-ALD = 1.20, 1.13, 1.61, and 1.32 for the Model for End-Stage Liver Disease categories 37-40, 33-36, 29-32, and 25-28, respectively, in the post-AC era, p < 0.05 for all). The adjusted likelihood of death/removal from the waitlist was lower for patients with ALD across all lower Model for End-Stage Liver Disease categories (adjusted subdistribution hazard ratio = 0.70, 0.81, 0.84, and 0.70 for the Model for End-Stage Liver Disease categories 25-28, 20-24, 15-19, 6-14, respectively, p < 0.05). Adjusted posttransplant survival was better for those with ALD (adjusted hazard ratio = 0.81, p < 0.05). Waiting list and posttransplant mortality tended to improve more for those with ALD since the implementation of AC but not significantly. ALD is a growing indication for liver transplantation. Although patients with ALD continue to have excellent posttransplant outcomes and lower waitlist mortality, candidates with ALD have higher adjusted transplant rates, and these differences have increased after AC implementation.

Enhancing Kidney Transplantation and the Role of Xenografts: Report of a Scientific Workshop Sponsored by the National Kidney Foundation.

Chronic kidney disease affects an estimated 37 million people in the United States; of these,>800,000 have end-stage renal disease requiring chronic dialysis or a kidney transplant to survive. Despite efforts to increase the donor kidney supply, approximately 100,000 people are registered on the kidney transplant wait-list with no measurable decrease over the past 2 decades. The outcomes of kidney transplantation are significantly better than for chronic dialysis: kidney transplant recipients have lower rates of mortality and cardiovascular events and better quality of life, but wait-list time matters. Time on dialysis waiting for a deceased-donor kidney is a strong independent risk factor for outcomes after a kidney transplant. Deceased-donor recipients with wait-list times on dialysis of<6 months have graft survival rates equivalent to living-donor recipients with waitlist times on dialysis of>2 years. In 2021,>12,000 people had been on the kidney transplant waitlist for ≥5 years. As the gap between the demand for and availability of donor kidneys for allotransplantation continues to widen, alternative strategies are needed to provide a stable, sufficient, and timely supply. A strategy that is gaining momentum toward clinical application is pig-to-human kidney xenotransplantation. This report summarizes the proceedings of a meeting convened on April 11-12, 2022, by the National Kidney Foundation to review and assess the state of pig-to-human kidney xenotransplantation as a potential cure for end-stage renal disease.

Blockade of BAFF Reshapes the Hepatic B Cell Receptor Repertoire and Attenuates Autoantibody Production in Cholestatic Liver Disease.

Defects in biliary transport proteins, MDR3 in humans and Mdr2 in mice, can lead to a spectrum of cholestatic liver disorders. Although B cell disorders and the aberrant Ab production are the leading extrahepatic manifestations of cholestatic liver diseases, the mechanism underlying this phenomenon is incompletely understood. Using mice with deficiency of Mdr2 that progressively develop cholestatic liver disease, we investigated the contributions of BAFF to aberrant IgG autoantibody production and hepatic fibrosis. In Mdr2-/- mice, hepatic B lymphocytes constitutively produced IgG during fibrosis progression, which correlated with elevated serum levels of BAFF, antinuclear Abs (ANA) and immune complexes. The elevated BAFF and ANA titers were also detected in human patients with primary sclerosing cholangitis and hepatobiliary cholangiopathies. Consistent with the higher BAFF levels, liver-specific selection of the focused BCR IgH repertoire was found on hepatic B cells in Mdr2-/- mice. Interestingly, the administration of anti-BAFF mAb in Mdr2-/- mice altered the BCR repertoire on hepatic B lymphocytes and resulted in reduced ANA and immune complex titers. However, anti-BAFF treatment did not attenuate hepatic fibrosis as measured by collagen deposition, hepatic expressions of collagen-1a, α-smooth muscle actin, and mononuclear cell infiltration (CD11b+ Ly-6chi monocytes and CD11b+ Gr1+ neutrophils). Importantly, depletion of B cells by anti-CD20 mAb reduced both hepatic fibrosis and serum levels of ANA and immune complexes. Our findings implicate B cells as the potential therapeutic targets for hepatic fibrosis and targeting BAFF specifically for attenuating the autoantibody production associated with cholestatic liver disease.

Optimization of de novo belatacept-based immunosuppression administered to renal transplant recipients.

Kidney transplant recipients administered belatacept-based maintenance immunosuppression present with a more favorable metabolic profile, reduced incidence of de novo donor-specific antibodies (DSAs), and improved renal function and long-term patient/graft survival relative to individuals receiving calcineurin inhibitor (CNI)-based immunosuppression. However, the rates and severity of acute rejection (AR) are greater with the approved belatacept-based regimen than with CNI-based immunosuppression. Although these early co-stimulation blockade-resistant rejections are typically steroid sensitive, the higher rate of cellular AR has led many transplant centers to adopt immunosuppressive regimens that differ from the approved label. This article summarizes the available data on these alternative de novo belatacept-based maintenance regimens. Steroid-sparing, belatacept-based immunosuppression (following T cell-depleting induction therapy) has been shown to yield AR rates comparable to those seen with CNI-based regimens. Concomitant treatment with belatacept plus a mammalian target of rapamycin inhibitor (mTORi; sirolimus or everolimus) has yielded AR rates ranging from 0 to 4%. Because the optimal induction agent and number of induction doses; blood levels of mTORi; and dose, duration, and use of corticosteroids have yet to be determined, larger prospective clinical trials are needed to establish the optimal alternative belatacept-based regimen for minimizing early cellular AR occurrence.

Anti-C5 Antibody Tesidolumab Reduces Early Antibody-mediated Rejection and Prolongs Survival in Renal Xenotransplantation.

OBJECTIVE: Pig-to-primate renal xenotransplantation is plagued by early antibody-mediated graft loss which precludes clinical application of renal xenotransplantation. We evaluated whether temporary complement inhibition with anti-C5 antibody Tesidolumab could minimize the impact of early antibody-mediated rejection in rhesus monkeys receiving pig kidneys receiving costimulatory blockade-based immunosuppression. METHODS: Double (Gal and Sda) and triple xenoantigen (Gal, Sda, and SLA I) pigs were created using CRISPR/Cas. Kidneys from DKO and TKO pigs were transplanted into rhesus monkeys that had the least reactive crossmatches. Recipients received anti-C5 antibody weekly for 70 days, and T cell depletion, anti-CD154, mycophenolic acid, and steroids as baseline immunosuppression (n = 7). Control recipients did not receive anti-C5 therapy (n = 10). RESULTS: Temporary anti-C5 therapy reduced early graft loss secondary to antibody-mediated rejection and improved graft survival (P < 0.01). Deleting class I MHC (SLA I) in donor pigs did not ameliorate early antibody-mediated rejection (table). Anti-C5 therapy did not allow for the use of tacrolimus instead of anti-CD154 (table), prolonging survival to a maximum of 62 days. CONCLUSION: Inhibition of the C5 complement subunit prolongs renal xenotransplant survival in a pig to non-human primate model.

Brief exposure to hyperglycemia activates dendritic cells in vitro and in vivo.

Dendritic cells are key players in regulating immunity. These cells both activate and inhibit the immune response depending on their cellular environment. Their response to hyperglycemia, a condition common amongst diabetics wherein glucose is abnormally elevated, remains to be elucidated. In this study, the phenotype and immune response of dendritic cells exposed to hyperglycemia were characterized in vitro and in vivo using the streptozotocin-induced diabetes model. Dendritic cells were shown to be sensitive to hyperglycemia both during and after differentiation from bone marrow precursor cells. Dendritic cell behavior under hyperglycemic conditions was found to vary by phenotype, among which, tolerogenic dendritic cells were particularly sensitive. Expression of the costimulatory molecule CD86 was found to reliably increase when dendritic cells were exposed to hyperglycemia. Additionally, hydrogel-based delivery of the anti-inflammatory molecule interleukin-10 was shown to partially inhibit these effects in vivo.

Long-term survival of pig-to-rhesus macaque renal xenografts is dependent on CD4 T cell depletion.

The shortage of available organs remains the greatest barrier to expanding access to transplant. Despite advances in genetic editing and immunosuppression, survival in experimental models of kidney xenotransplant has generally been limited to <100 days. We found that pretransplant selection of recipients with low titers of anti-pig antibodies significantly improved survival in a pig-to-rhesus macaque kidney transplant model (6 days vs median survival time 235 days). Immunosuppression included transient pan-T cell depletion and an anti-CD154-based maintenance regimen. Selective depletion of CD4+ T cells but not CD8+ T cells resulted in long-term survival (median survival time >400 days vs 6 days). These studies suggested that CD4+ T cells may have a more prominent role in xenograft rejection compared with CD8+ T cells. Although animals that received selective depletion of CD8+ T cells showed signs of early cellular rejection (marked CD4+ infiltrates), animals receiving selective CD4+ depletion exhibited normal biopsy results until late, when signs of chronic antibody rejection were present. In vitro study results suggested that rhesus CD4+ T cells required the presence of SLA class II to mount an effective proliferative response. The combination of low pretransplant anti-pig antibody and CD4 depletion resulted in consistent, long-term xenograft survival.

Alterations in Intestinal Microbiota Lead to Production of Interleukin 17 by Intrahepatic γδ T-Cell Receptor-Positive Cells and Pathogenesis of Cholestatic Liver Disease.

BACKGROUND & AIMS: Variants at the ABCB4 or MDR2 locus, which encodes a biliary transport protein, are associated with a spectrum of cholestatic liver diseases. Exacerbation of liver disease has been linked to increased hepatic levels of interleukin (IL) 17, yet the mechanisms of this increase are not understood. We studied mice with disruption of Mdr2 to determine how defects in liver and alteration in the microbiota contribute to production of IL17 by intrahepatic γδ T cells. METHODS: We performed studies with Mdr2-/- and littermate FVB/NJ (control) mice. IL17 was measured in serum samples by an enzyme-linked immunosorbent assay. Mice were injected with neutralizing antibodies against the γδ T-cell receptor (TCR; anti-γδ TCR) or mouse IL17A (anti-IL17A). Livers were collected and bacteria were identified in homogenates by culture procedures; TCRγδ+ cells were isolated by flow cytometry. Fecal samples were collected from mice and analyzed by 16S ribosomal DNA sequencing. Cells were stimulated with antibodies or bacteria, and cytokine production was measured. We obtained tissues from 10 patients undergoing liver transplantation for primary sclerosing cholangitis or chronic hepatitis C virus infection. Tissues were analyzed for cytokine production by γδ TCR+ cells. RESULTS: Mdr2-/- mice had collagen deposition around hepatic bile ducts and periportal-bridging fibrosis with influx of inflammatory cells and increased serum levels of IL17 compared with control mice. Administration of anti-IL17A reduced hepatic fibrosis. Livers from Mdr2-/- mice had increased numbers of IL17A+ γδTCR+ cells-particularly of IL17A+ Vγ6Jγ1 γδ TCR+ cells. Fecal samples from Mdr2-/- mice were enriched in Lactobacillus, and liver tissues were enriched in Lactobacillus gasseri compared with control mice. Mdr2-/- mice also had increased intestinal permeability. The γδ TCR+ cells isolated from Mdr2-/- livers produced IL17 in response to heat-killed L gasseri. Intraperitoneal injection of control mice with L gasseri led to increased serum levels of IL17 and liver infiltration by inflammatory cells; injection of these mice with anti-γδ TCR reduced serum level of IL17. Intravenous injections of Mdr2-/- mice with anti-γδ TCR reduced fibrosis; liver levels of IL17, and inflammatory cells; and serum levels of IL17. γδTCR+ cells isolated from livers of patients with primary sclerosing cholangitis, but not hepatitis C virus infection, produced IL17. CONCLUSIONS: In Mdr2-/- mice, we found development of liver fibrosis and inflammation to require hepatic activation of γδ TCR+ cells and production of IL17 mediated by exposure to L gasseri. This pathway appears to contribute to development of cholestatic liver disease in patients.

What's hot, what's new: Report from the American Transplant Congress 2017.

Significant advances in clinical practice as well as basic and translational science were presented at the American Transplant Congress this year. Topics included innovative clinical trials to recent advances in our basic understanding of the scientific underpinnings of transplant immunology. Key areas of interest included the following: clinical trials utilizing hepatitis C virus-positive (HCV+ ) donors for HCV- recipients, the impact of the new allocation policies, normothermic perfusion, novel treatments for desensitization, attempts at precision medicine, advances in xenotransplantation, the role of mitochondria and exosomes in rejection, nanomedicine, and the impact of the microbiota on transplant outcomes. This review highlights some of the most interesting and noteworthy presentations from the meeting.

Emergency department use among kidney transplant recipients in the United States.

Patients with end-stage renal disease use the emergency department (ED) at a 6-fold higher rate than do other US adults. No national studies have described ED use rates among kidney transplant (KTx) recipients, and the factors associated with higher ED use. We examined a cohort of 132 725 adult KTx recipients in the United States Renal Data System (2005-2013). Data on ED visits, hospitalization, and outpatient nephrology visits were obtained from Medicare claims databases. Nearly half (46.1%) of KTx recipients had at least one ED visit (1.61 ED visits/patient-year [PY]), and 39.7% of ED visits resulted in hospitalization in the first year posttransplantation. ED visit rate was high in the first 30 days (5.26 visits/PY) but declined substantially thereafter (1.81 visits/PY in months 1-3; 1.13 visits/PY in months 3-12 posttransplantation). ED visit rates were higher in the first 30 days versus rates for dialysis patients but less than half the rate thereafter. Female sex, public insurance, medical comorbidities, longer pretransplantation dialysis vintage, and delayed graft function were associated with higher ED use in the first year post-KTx. Policies and strategies addressing potentially preventable ED visits should be promoted to help improve patient care and increase efficient use of ED resources.

Xenoantigen Deletion and Chemical Immunosuppression Can Prolong Renal Xenograft Survival.

OBJECTIVE: Xenotransplantation using pig organs could end the donor organ shortage for transplantation, but humans have xenoreactive antibodies that cause early graft rejection. Genome editing can eliminate xenoantigens in donor pigs to minimize the impact of these xenoantibodies. Here we determine whether an improved cross-match and chemical immunosuppression could result in prolonged kidney xenograft survival in a pig-to-rhesus preclinical model. METHODS: Double xenoantigen (Gal and Sda) knockout (DKO) pigs were created using CRISPR/Cas. Serum from rhesus monkeys (n = 43) was cross-matched with cells from the DKO pigs. Kidneys from the DKO pigs were transplanted into rhesus monkeys (n = 6) that had the least reactive cross-matches. The rhesus recipients were immunosuppressed with anti-CD4 and anti-CD8 T-cell depletion, anti-CD154, mycophenolic acid, and steroids. RESULTS: Rhesus antibody binding to DKO cells is reduced, but all still have positive CDC and flow cross-match. Three grafts were rejected early at 5, 6, and 6 days. Longer survival was achieved in recipients with survival to 35, 100, and 435 days. Each of the 3 early graft losses was secondary to IgM antibody-mediated rejection. The 435-day graft loss occurred secondary to IgG antibody-mediated rejection. CONCLUSIONS: Reducing xenoantigens in donor pigs and chemical immunosuppression can be used to achieve prolonged renal xenograft survival in a preclinical model, suggesting that if a negative cross-match can be obtained for humans then prolonged survival could be achieved.

Transplantation Versus Resection for Hilar Cholangiocarcinoma: An Argument for Shifting Treatment Paradigms for Resectable Disease.

OBJECTIVE: To investigate the influence of type of surgery (transplant vs resection) on overall survival (OS) in patients with hilar cholangiocarcinoma (H-CCA). BACKGROUND: Outcomes after resection for H-CCA are poor, yet transplantation is currently only reserved for well-selected patients with unresectable disease. METHODS: All patients with H-CCA who underwent resection from 2000 to 2015 at 10 institutions were included. Three institutions additionally had active H-CCA transplant protocols with similar selection criteria over similar time periods. RESULTS: Of 304 patients with suspected H-CCA, 234 underwent attempted resection and 70 were enrolled in a transplant protocol. Excluding incomplete/R2 resections (n = 43), patients who were enrolled, but did not undergo transplant (n = 24), and transplants without confirmed H-CCA diagnoses (n = 5), 191 patients underwent curative-intent resection and 41 curative-intent transplant. Compared with resection, transplant patients were younger (52 vs 65 years; P < 0.001), and more frequently had primary sclerosing cholangitis (PSC; 61% vs 2%; P < 0.001) and received chemotherapy and/or radiation (98% vs 57%; P < 0.001). Groups were otherwise similar in demographics and comorbidities. Patients who underwent transplant for confirmed H-CCA diagnosis had improved OS compared with resection (3-year: 72% vs 33%; 5-year: 64% vs 18%; P < 0.001). Among patients who underwent resection for tumors <3 cm with lymph-node negative disease, and excluding PSC patients, transplant was still associated with improved OS (3-year: 54% vs 44%; 5-year: 54% vs 29%; P = 0.03). Transplant remained associated with improved survival on intention-to-treat analysis, even after accounting for tumor size, lymph node status, and PSC (P = 0.049). CONCLUSIONS: Resection for hilar cholangiocarcinoma that meets criteria for transplantation (<3 cm, lymph-node negative disease) is associated with substantially decreased survival compared to transplant for the same criteria with unresectable disease. Prospective trials are needed and justified.

CD4+ Foxp3+ T cells promote aberrant immunoglobulin G production and maintain CD8+ T-cell suppression during chronic liver disease.

Persistent hepatotropic viral infections are a common etiologic agent of chronic liver disease. Unresolved infection can be attributed to nonfunctional intrahepatic CD8+ T-cell responses. In light of dampened CD8+ T-cell responses, liver disease often manifests systemically as immunoglobulin (Ig)-related syndromes due to aberrant B-cell functions. These two opposing yet coexisting phenomena implicate the potential of altered CD4+ T-cell help. Elevated CD4+ forkhead box P3-positive (Foxp3+) T cells were evident in both human liver disease and a mouse model of chemically induced liver injury despite marked activation and spontaneous IgG production by intrahepatic B cells. While this population suppressed CD8+ T-cell responses, aberrant B-cell activities were maintained due to expression of CD40 ligand on a subset of CD4+ Foxp3+ T cells. In vivo blockade of CD40 ligand attenuated B-cell abnormalities in a mouse model of liver injury. A phenotypically similar population of CD4+ Foxp3+, CD40 ligand-positive T cells was found in diseased livers explanted from patients with chronic hepatitis C infection. This population was absent in nondiseased liver tissues and peripheral blood. CONCLUSION: Liver disease elicits alterations in the intrahepatic CD4+ T-cell compartment that suppress T-cell immunity while concomitantly promoting aberrant IgG mediated manifestations. (Hepatology 2017;65:661-677).

Costimulation Blockade in Autoimmunity and Transplantation: The CD28 Pathway.

T cell activation is a complex process that requires multiple cell signaling pathways, including a primary recognition signal and additional costimulatory signals. TCR signaling in the absence of costimulatory signals can lead to an abortive attempt at activation and subsequent anergy. One of the best-characterized costimulatory pathways includes the Ig superfamily members CD28 and CTLA-4 and their ligands CD80 and CD86. The development of the fusion protein CTLA-4-Ig as an experimental and subsequent therapeutic tool is one of the major success stories in modern immunology. Abatacept and belatacept are clinically approved agents for the treatment of rheumatoid arthritis and renal transplantation, respectively. Future interventions may include selective CD28 blockade to block the costimulatory potential of CD28 while exploiting the coinhibitory effects of CTLA-4.

Waitlist Hospital Admissions Predict Resource Utilization and Survival After Renal Transplantation.

OBJECTIVE: To determine whether fitness for transplant can be determined by candidates' hospitalizations although waitlisted. BACKGROUND: Renal transplantation must increasingly serve a population of multiply comorbid patients in an environment defined by organ scarcity and premiums on value-based care. Determining those at excess risk for transplant is critical to these imperatives. METHODS: United States Renal Data Systems patient and claims data for all adult renal transplant recipients between 2000 and 2010 with continuous primary Medicare coverage for 1 year before and after transplantation were examined. Outcomes included readmissions within the first-year post-transplant and 3-year graft and patient survival. Chi-square statistics, Kaplan-Meier methods (log-rank test), and goodness of fit calculations (c-statistics) were performed for models of transplant outcome. RESULTS: Among 37,623 patients, the percentages of patients admitted for 0, 1 to 7, 8 to 14, or 15 or more days in the pretransplant year were 51%, 25%, 11%, and 13%. Overall readmission-free survival at 1 year was 31%. Heavily preadmitted patients were more likely to have a greater length of stay during their transplant admission, and had a greater service needs at discharge. Pretransplant admission strongly predicted more frequent post-transplant admission. Among all factors studied, preadmission was the strongest predictor of post-transplant death, and had a dose-dependent effect on both death and graft loss. CONCLUSIONS: In summary, hospitalization in the year before transplant is an objective, readily ascertainable, and powerful predictor of excess resource utilization and inferior outcome. Incorporation of a rolling assessment of patient hospitalization has potential policy implications for maximizing value in renal transplantation.

Systems biological analyses reveal the hepatitis C virus (HCV)-specific regulation of hematopoietic development.

UNLABELLED: Chronic liver disease is characterized by the liver enrichment of myeloid dendritic cells (DCs). To assess the role of disease on myelopoiesis, we utilized a systems biology approach to study development in liver-resident cells expressing stem cell marker CD34. In patients with endstage liver disease, liver CD34+ cells were comprised of two subsets, designated CD34+CD146+ and CD34+CD146-, and hematopoietic function was restricted to CD34+CD146- cells. Liver CD34 frequencies were reduced during nonalcoholic steatohepatitis (NASH) and chronic hepatitis C virus (HCV) compared to alcohol liver disease (ALD), and this reduction correlated with viral load in the HCV cohort. To better understand the relationship between liver CD34+CD146+ and CD34+CD146- subsets and any effects of disease on CD34 development, we used gene expression profiling and computational modeling to compare each subset during ALD and HCV. For CD34+CD146+ cells, increased expression of endothelial cell genes including von Willebrand factor, VE-cadherin, and eNOS were observed when compared to CD34+CD146- cells, and minimal effects of ALD and HCV diseases on gene expression were observed. Importantly for CD34+CD146- cells, chronic HCV was associated with a distinct "imprint" of programs related to cell cycle, DNA repair, chemotaxis, development, and activation, with an emphasis on myeloid and B lymphocyte lineages. This HCV signature was further translated in side-by-side analyses, where HCV CD34+CD146- cells demonstrated superior hematopoietic growth, colony formation, and diversification compared to ALD and NASH when cultured identically. Disease-associated effects on hematopoiesis were also evident by phenotypic alterations in the expression of CD14, HLA-DR, and CD16 by myeloid progeny cells. CONCLUSION: Etiology drives progenitor fate within diseased tissues. The liver may be a useful source of hematopoietic cells for therapy, or as therapeutic targets.

B cells in clinical transplantation tolerance.

The rarity of tolerance following clinical transplantation has complicated the study of the responsible mechanisms. Several recent studies of kidney transplant recipients have demonstrated an association between increased numbers of B cells with less mature, inhibitory phenotypes and a state of tolerance. While a growing body of evidence from experimental models supports a role for B cells in regulating or suppressing immune responses, a causative role for B cells in clinical transplantation tolerance has yet to be proven. If B cells are conclusively shown to participate in the development of transplantation tolerance, it will be important to define the responsible mechanisms in order to design monitoring assays and immunosuppressive regimens that favor the development of tolerance.

Nonhuman primate models of transplant tolerance: closer to the holy grail.

PURPOSE OF REVIEW: Transplantation tolerance, successful acceptance of an organ without the perils of immunosuppression, has been a central goal of transplant research. Many strategies to achieve this tolerance have been examined over the past three decades, culminating in several human trials of transplant tolerance. This progression from the 'benchtop to the clinic' has depended on the successful implementation of these tolerance strategies in nonhuman primates. This review will examine the described methods of transplant tolerance induction in nonhuman primates. RECENT FINDINGS: Although costimulatory blockade and mixed chimerism have an established record of achieving transplant tolerance in nonhuman primates, some of the most innovative recent techniques of tolerance induction have relied on cellular transfer. This review will fully examine the role of regulatory T-cell transfer and the use of mesenchymal stem/stromal cells to promote tolerance of organ allografts in nonhuman primates. SUMMARY: Use of translational nonhuman primate transplant models is a vital intermediate step to advance new approaches of transplant tolerance induction from the lab to the clinic. This review will explore numerous techniques of tolerance induction that have been piloted in primates, including depletional techniques, induction of mixed hematopoietic chimerism, costimulation blockade, and adoptive transfer of tolerogenic cell populations.

Pre-transplant antibody screening and anti-CD154 costimulation blockade promote long-term xenograft survival in a pig-to-primate kidney transplant model.

Xenotransplantation has the potential to alleviate the organ shortage that prevents many patients with end-stage renal disease from enjoying the benefits of kidney transplantation. Despite significant advances in other models, pig-to-primate kidney xenotransplantation has met limited success. Preformed anti-pig antibodies are an important component of the xenogeneic immune response. To address this, we screened a cohort of 34 rhesus macaques for anti-pig antibody levels. We then selected animals with both low and high titers of anti-pig antibodies to proceed with kidney transplant from galactose-α1,3-galactose knockout/CD55 transgenic pig donors. All animals received T-cell depletion followed by maintenance therapy with costimulation blockade (either anti-CD154 mAb or belatacept), mycophenolate mofetil, and steroid. The animal with the high titer of anti-pig antibody rejected the kidney xenograft within the first week. Low-titer animals treated with anti-CD154 antibody, but not belatacept exhibited prolonged kidney xenograft survival (>133 and >126 vs. 14 and 21 days, respectively). Long-term surviving animals treated with the anti-CD154-based regimen continue to have normal kidney function and preserved renal architecture without evidence of rejection on biopsies sampled at day 100. This description of the longest reported survival of pig-to-non-human primate kidney xenotransplantation, now >125 days, provides promise for further study and potential clinical translation.