Spontaneous Tolerance Research Articles

Higher IL10 Production by Transitional B Cells in Tolerant Kidney Transplant Recipients

Background: Induction of transplantation tolerance would reduce the risk of infection, malignancy and cardiovascular disease caused by immunosupresive drugs in renal transplant recipients. Indices of Tolerance Project, one of the largest studies identifying biomarkers of tolerance, revealed that tolerant kidney transplant recipients, had an expansion of B cells in peripheral blood. In parallel, a B cell subset with regulatory properties has been described in humans as a transitional B cell, characterised by high expression of CD38, CD24 and IL10 production. We set out to test the hypothesis that the expansion of this B cell subset was central in the process of spontaneous tolerance observed in these rare recipients and to test the ability of these cells to present alloantigens. Methods: Peripheral Blood Mononuclear Cells (PBMCs) samples from <tolerant: functionally stable recipients without immunosuppressive drugs, <stable: functionally stable recipients on standard immunosuppression, <chronic rejector: immunosuppressed recipients that presented signs of chronic rejection and <healthy controls were used in the study. B cell subsets from patients were identified by flow cytometry with anti-CD20, anti-CD19, anti-CD27, anti-IgM, anti-IgG, anti-CD24 and anti-CD38. PBMCs were cultured for 3 days alone, with CD40L and CpG in RPMI 10% FCS. After 3 days, activation markers (CD25, CD86 and CD40) were measured by surface staining and IL10, IFN-g and TNF-a were measured by intracellular staining and ELISA. Results: B cells were identified as CD20+CD19+ and memory B cells as CD20+CD19+CD27+. Within the CD27- non-memory B cells we selected the IgD+IgM+ population to study the non-class-switching B cells and then we identified Transitional B cells (regulatory B cells or Bregs) as CD24hiCD38hi. Within the C27- population, transitional B cells were more prevalent in tolerant patients (12,1%), in comparison with stable patients (4,5%), chronic rejectors (2,1%) and healthy controls (7,8%). B cells from all groups of patients increased activation markers and TLR-9 after CD40L and CpG activation, respectively. Tolerant patients displayed a higher percentage of IL10+Bcells in comparison with the rest of the groups in response to CD40L, however the response to CpG was similar in all patient groups. IFN-g-producing Bcells were not identified after any activation and levels of TNF-a were higher after activation with CD40L but not CpG in all of the tested patients. Discussion: CD40L interaction with B cells occurs after encounter with activated T cells. Only in this context B cells from tolerant recipients were producing higher amounts of IL-10. This is a first step towards a firm link between an expansion of transitional B cells and transplantation tolerance. Further studies are under way, to establish alloantigen specificity in this interaction.

Cellular Infiltrates and NFκB Subunit c-Rel Signaling in Kidney Allografts of Patients With Clinical Operational Tolerance

Nuclear factor kappa B (NFκB) plays a potential role in tolerance by orchestrating onset and resolution of inflammation and regulatory T cell differentiation through subunit c-Rel. We characterized cellular infiltrates and expression of NFκB1, c-Rel and its upstream regulators phosphatidylinositol 3-kinase/RAC-alpha serine/threonine kinase, in allograft biopsies from patients with spontaneous clinical operational tolerance (COT). Paraffin-fixed kidney allograft biopsies from 40 patients with COT (n=4), interstitial rejection (IR; n=12), borderline changes (BC; n=12), and long-term allograft function without rejection (NR; n=12) were used in the study. Cellular infiltrates and immunohistochemical expression of key proteins of the NFκB pathway were evaluated in the cortical tubulointerstitium and in cellular infiltrates using digital image analysis software. Results were given as mean±SEM. Biopsies from patients with COT exhibited a comparable amount of cellular infiltrate to IR, BC, and NR (COT, 191±81; IR, 291±62; BC, 178±45; and NR, 210±42 cells/mm) but a significantly higher proportion of forkhead box P3-positive cells (COT, 11%±1.7%; IR, 3.5%±0.70%; BC, 3.4%±0.57%; and NR, 3.7%±0.78% of infiltrating cells; P=0.02). c-Rel expression in cellular infiltrates was significantly elevated in IR, BC, and NR when analyzing the number of positive cells per mm (P=0.02) and positive cells per infiltrating cells (P=0.04). In contrast, tubular PI3K and c-Rel expression were significantly higher in IR and BC but not in NR compared with COT (P=0.03 and P=0.006, respectively). With RAC-alpha serine-threonine kinase, similar tendencies were observed (P=0.2). Allografts from COT patients show significant cellular infiltrates but a distinct expression of proteins involved in the NFκB pathway and a higher proportion of forkhead box P3-positive cells.

Total body irradiation of donors can alter the course of tolerance and induce acute rejection in a spontaneous tolerance rat liver transplantation model

Liver transplantation is an established therapy for end-stage liver diseases. Graft rejection occurs unless the recipient receives immunosuppression after transplantation. This study aimed to explore the mechanism of acute rejection of liver allografts in rats pre-treated with total body irradiation to eliminate passenger lymphocytes and to define the role of CD4(+)CD25(+) regulatory T cells in the induction of immunotolerance in the recipient. Male Lewis rats were used as donors and male DA rats were recipients. Rats were randomly assigned to the following four groups: control group, homogeneity liver transplantation group, idio-immunotolerance group and acute rejection group. After transplantation, the survival time of each group, serum alanine aminotransferase, total bilirubin levels, number of Foxp3(+)CD4(+)CD25(+) regulatory T cells, expression of glucocorticoid-induced tumor necrosis factor receptor on T cell subgroups, histopathology of the hepatic graft and spleen cytotoxic T lymphocyte lytic activity were measured. In the acute rejection group, where donors were preconditioned with total body irradiation before liver transplantation, all recipients died between day 17 and day 21. On day 14, serum alanine aminotransferase increased significantly to (459.2±76.9) U L(-1), total bilirubin increased to (124.1±33.7) μmol L(-1) (P<0.05) and the ratio of Foxp3(+)CD4(+)CD25(+) regulatory T cells decreased significantly to 1.50%±0.50% (P<0.05) compared with the other groups. Analysis of the T cell subpopulations in the acute rejection group varied from the other groups. Histological analysis showed typical changes of acute rejection in the acute rejection group only. Preconditioning of the donors with total body irradiation eliminated passenger lymphocytes of the liver graft, and thus affected the course of tolerance and induced acute rejection after liver transplantation.

Spontaneous Operational Tolerance in Kidney Transplant Recipients: Response

To the Editor: We would like to thank Leventhal et al. (1) for bringing to our attention an inconsistency in Table 1 of our article (2). Due to an error, the time of immunosuppression for operationally tolerant kidney recipients was typed in years whereas it should have been typed in months. Thus, instead of a mean of mean 7 (range 3–12) years, the correct figures are mean 84 (range 36–144) months. The “2–13 years” reported in the text are a misprint. The correct data therefore are those described in Table 1 (we include here a correct version of the table). We apologize for these errors. Leventhal et al. also raised the question of whether kidney recipients requiring no immunosuppression but displaying donor-specific antibodies should ever be considered operationally tolerant. Our group first made the observation that a minority of operationally tolerant kidney recipients exhibits donor-specific antibodies in 2006 (3). In this case-report article, we described two patients with anti-HLA class II antibodies and normal kidney graft function 4 and 10 years after being off on all immunosuppressive drugs. A similar observation was made more recently by Newell et al. (4) in his report on the Immune Tolerance Network cohort of operationally tolerant kidney recipients, in which 1 of 20 recipients also had donor-directed anti-HLA antibodies. In the cohort of operationally tolerant kidney recipients described in this manuscript (2), only 3 of 12 recipients were HLA identical. The remaining nine recipients had one, two, three, three, three, four, four, four and five mismatches. Regarding the three patients with donor-specific antibodies, these were detected in the first case 10 years after transplantation and 9 years after drug withdrawal; in the second case 13 years after transplantation and 8 years after drug withdrawal and in the third case 16 years after transplantation and 2.5 years after withdrawal. To date the 3 recipients still display stable graft function. Unfortunately, we could not establish the relationship between the appearance of anti-HLA antibodies and the discontinuation of immunosuppressive drugs, because these patients were not prospectively followed from the date of transplantation. In any case, we have no data indicating that operationally tolerant kidney recipients bearing donor-specific antibodies have a greater risk of graft deterioration than those who do not. Altogether, the situation appears to be similar to what is observed in “conventional” kidney recipients under maintenance immunosuppression, in whom the pathogenic and clinical meaning of de novo anti-HLA antibodies is far from being clear (5). Until the prognostic value of anti-HLA antibodies is clearly defined in longitudinal studies, we therefore see no reason to exclude this subgroup of patients from studies attempting to characterize operational tolerance in kidney transplantation. We agree with Leventhal et al. that in the absence of kidney biopsies it is not possible to exclude the presence of graft histological abnormalities indicative of low-grade immune reactivity. This is a problem common to all studies that have attempted to characterize operationally tolerant kidney recipients. Nevertheless, we want to stress here that all recipients described in our article still display stable graft function and have done so during a very long period of time in the absence of any immunosuppressive therapy. Concerning our attempt to assess the stability of the circulating B-cell immunophenotype, sequential blood samples were collected from four recipients at the following posttransplant time points: 6 and 11 years after transplantation (4 and 9 years after drug withdrawal); 18 and 22 years after transplantation (5 and 9 years after withdrawal); 23 and 27 years after transplantation (13 and 15 years after withdrawal) and 17 and 18 years after transplantation (5 and 6 years after withdrawal). In this subgroup of patients, only one displayed donor-specific antibodies (detected 10 years after transplantation and 9 years after drug withdrawal). As stated in our article (2), the analysis of these sequentially collected samples revealed that the phenotype of circulating B cells was very stable over time. We are grateful to Leventhal et al. for their supportive comments on our article. The authors of this manuscript have no conflicts of interest to disclose as described by the American Journal of Transplantation.

Spontaneous Operational Tolerance in Kidney Transplant Recipients

Spontaneous Operational Tolerance in Kidney Transplant Recipients

Identification of a novel biomarker gene set with sensitivity and specificity for distinguishing between allograft rejection and tolerance

Here we examined whether the expression of a novel immunoregulatory gene set could be used to predict outcomes in murine models of rapamycin-induced cardiac tolerance, spontaneous hepatic tolerance, and cardiac rejection. The expression of the immunoregulatory gene set was assessed with the GeXP multiplex reverse-transcription polymerase chain reaction (RT-PCR) analysis system, and it was correlated to the pathological and biochemical parameters of the allografts. In rejecting cardiac grafts, the increased expression of an inflammatory set of genes, which included CD45, CD4, CD25, suppressor of cytokine signaling 2, cytotoxic T lymphocyte-associated protein 4 (CTLA4), selectin lymphocyte, interferon-γ (IFN-γ), programmed cell death 1 (Pdcd1), and granzyme B (Gzmb), was seen 8 days after transplantation along with histological evidence of severe allograft rejection. In tolerant cardiac allografts, the expression of fibrinogen-like protein 2 (Fgl2), Pdcd1, killer cell lectin-like receptor G1 (Klrg1), CTLA4, and lymphocyte-activation gene 3 was associated with tolerance. In a model of liver allograft tolerance, the increased expression of lectin galactose-binding soluble 1, Fgl2, CD39, phosphodiesterase 3B, Klrg1, forkhead box P3 (Foxp3), and transforming growth factor β as well as the inflammatory set of genes was observed 8 to 14 days after transplantation (ie, when there was severe inflammatory injury). At a later time when the liver allografts had been fully accepted and were histologically normal, the expression of the inflammatory set of genes returned to the baseline, but the expression of the tolerogenic set of genes was still increased. Genes that were expressed in tolerant cardiac and liver allografts included Fgl2, Klrg1, and Foxp3, whereas genes associated with rejection included CD25, Gzmb, and IFN-γ. Our data indicate that monitoring the graft expression of a novel biomarker gene set with the GeXP multiplex RT-PCR analysis system may allow differentiation between rejection and tolerance.

Abrogation of spontaneous liver tolerance during immune response to Candida albicans: contribution of NKT and hepatic mononuclear cells

Hepatic mononuclear cells (HMC) are a heterogeneous population with innate immune properties involved in the response to several pathogens. Herein, during the primary infection with Candida albicans, we observed dynamic changes in CD3+, NK+ and NKT+ intrahepatic lymphoid subsets and a significant increase in the absolute number of antigen-presenting cells (APC). The liver tolerogenic microenvironment sustained by higher levels of IL-10, transforming growth factor-β and IL-4 was severely modified upon the robust IFN-γ production after the fungal colonization. NKT cells purified from infected animals released significant amounts of IFN-γ and the production of this cytokine was exacerbated after a second contact with the fungus. Interestingly, C. albicans per se was unable to activate tolerogenic NKT cells from naive animals. In vitro experiments performed with HMC cells depleted of the CD11b/c+ population revealed that in the absence of APC, NKT cells are unable to produce IFN-γ in response to C. albicans. Our findings constitute the first evidence that this innate lymphocyte population is involved in the pathogenesis of C. albicans infection.

Oral Immunotherapy for Egg Allergy Clinical and Immunologic Results

Allergy to hen's egg represents an important issue in medical practice: The majority of children with IgE mediated food allergy achieving spontaneous tolerance to egg. However, some children have persistent symptoms beyond the age of 5; for these children we performed oral immunotherapy (OIT) with egg.

Hot-topic debate on tolerance: Immunosuppression withdrawal

1. Liver allografts exhibit intrinsic tolerogenic properties that result in their spontaneous acceptance in many experimental animal models. 2. In clinical transplantation, liver allografts require milder immunosuppression regimens than other organs, are relatively resistant to antibody-mediated rejection, and only very rarely are lost because of immunological insults. 3. A fraction of stable liver transplant recipients can withdraw from all immunosuppression therapy and then maintain normal graft function and not experience rejection. This phenomenon is known as spontaneous operational tolerance (SOT). 4. The intentional discontinuation of immunosuppression in stable liver transplant recipients has led to successful weaning in almost 20% of recipients, but the true prevalence of SOT in unselected recipients is still unknown. 5. The prevalence could be higher in pediatric recipients undergoing transplantation before 1 year of age and in adult recipients with more than 10 years of posttransplant follow-up. 6. Rejection occurring during medically supervised immunosuppression weaning trials tends to be mild and, in the overwhelming majority of cases, can be easily resolved without the administration of high-dose immunosuppression. 7. Tolerant liver recipients exhibit specific transcriptional patterns in peripheral blood and liver tissue that may constitute future diagnostic markers of tolerance. 8. There is still no formal proof that the discontinuation of low-dose immunosuppression in long-term survivors of liver transplantation improves the morbidity and mortality rates associated with immunosuppression therapy.

Donor liver natural killer cells alleviate liver allograft acute rejection in rats

Liver enriched natural killer (NK) cells are of high immune activity. However, the function of donor liver NK cells in allogeneic liver transplantation (LTx) remains unclear. Ten Gy of whole body gamma-irradiation (WBI) from a 60Co source at 0.6 Gy/min was used for depleting donor-derived leukocytes, and transfusion of purified liver NK cells isolated from the same type rat as donor (donor type liver NK cells, dtlNKs) through portal vein was performed immediately after grafting the irradiated liver. Post-transplant survival observation on recipients and histopathological detection of liver grafts were adoptive to evaluate the biological impact of donor liver NK cells on recipients' survival in rat LTx. Transfusion of dtlNKs did not shorten the survival time among the recipients of spontaneous tolerance model (BN to LEW rat) after rat LTx, but prolonged the liver graft survival among the recipients depleted of donor-derived leukocytes in the acute rejection model (LEW to BN rat). Compared to the recipients in the groups which received the graft depleted of donor-derived leukocytes, better survival and less damage in the allografts were also found among the recipients in the two different strain combinations of liver allograft due to transfusion of dtlNKs. Donor liver NK cells alone do not exacerbate liver allograft acute rejection. Conversely, they can alleviate it, and improve the recipients' survival.

The impact of CD4+ CD25+ regulatory T cells on the mouse spontaneous liver transplant tolerance

Objective To examine the contribution of CD4^＋CD25^＋ regulatory T cells to liver transplant tolerance. Methods After injection of anti-CD25 monoclonal antibody （mAb, PC61）, mouse orthotopic liver transplantation was performed and survivals were determined. The paraffin-embedded sections of hepatic allografts were cut and stained with hematoxylin and eosin （HE）. Furthermore, the effect ofCD4^＋CD25^＋regulatory T cells on proliferative response of CD4^＋ T cells and cyto toxicity of CD8^＋ T ceils was examined by depleting these regulatory T cells. Results Depletion of these cells in the recipients but not in the donors before liver transplantation caused rejection. Histological analyses of hepatic allografts with PC61 treatment showed extensive leukocyte infiltration and tissue destruction, whereas those in the control group showed minimal changes. Moreover, elimination of CD4^＋CD25^＋ T cells resulted in the enhancement of both proliferative response of CD4^＋ T cells and eytotoxicity of CD8^＋ T cells against donor-type alloantigen. Conclusions These results suggest that CD4^＋CD25^＋regulatory T cells were important for tolerance induction to hepatic allografts. Key words: CD4^＋CD25^＋regulatory T cells; Liver transplantaton; Immunologieal tolerance

Spontaneous tolerance in kidney transplantation - an instructive, but very rare paradigm*

Recognition and induction of donor-specific immune tolerance have been a major focus of interest in transplantation immunology even before immunosuppressive treatment became available [1]. Several decades of research resulted in reliable protocols for induction of donor-specific tolerance in rodents, but translation of those to nonhuman primates and to patients was extremely challenging [2]. So far only the mixed chimerism approach, which is based on a combined transplantation of bone marrow and kidney from the same living donor, was successful in the clinical setting [3,4]. However, also in these protocols a long and intense conditioning including myelosuppressive treatment limits its routine clinical application. Despite the difficulty in inducing tolerance in humans, anecdotal cases of patients, who have removed themselves from immunosuppression without experiencing graft rejection, have been repeatedly described in the literature. Whether these cases are associated with a state of microchimerism remains a matter of debate [5,6]. Nonetheless, spontaneously tolerant patients represent a unique opportunity to the transplant community: on one hand a better characterization of these patients may provide fundamental knowledge about the immunological regulation of tolerance in humans, on the other hand it might help us to optimize immunosuppressive therapy after transplantation. For these reasons, a large international collaboration was initiated aiming at characterization of this rare group of patients. It was possible to identify a ‘tolerance footprint’ based on gene expression profiles in peripheral blood of spontaneously tolerant kidney and liver recipients [7,8]. However, it is currently unknown how frequently this situation occurs and how such patients should be managed. Obviously, nowadays most spontaneously tolerant patients undergo unnecessary immunosuppressive therapy with potentially deleterious side effects but no benefit, and they could profit from immunosuppression weaning. In this issue of Transplant International, Brouard et al. [9] present their results of the next logical step toward clinical application of tolerance profiling with the intention of identifying patients for an immunosuppression weaning protocol. A particular cohort of long-term stable kidney allograft recipients under cyclosporine-based Transplant International ISSN 0934-0874

Transplantomics and Biomarkers in Organ Transplantation: A Report From the First International Conference

Transplantomics and Biomarkers in Organ Transplantation: A Report From the First International Conference

Induction of Tolerance in Children with Food Allergy

Food allergies are more prevalent during childhood. Relativity few foods are responsible for the vast majority of significant food-induced allergic reactions: milk, egg, peanuts, tree nuts, fish and selfish. The prognosis of these food allergies is not always toward spontaneous tolerance. Predictive factors of not developing spontaneous tolerance were: high levels of specific IgE to food proteins, allergic rhinitis and asthma. To date, the only treatment for food allergies has been to eliminate the offending food from the diet. In recent years, a number of studies have been published regarding desensitization or tolerance induction to food, particularly to cow milk and egg, as an alternative treatment in patients who have not developed spontaneous tolerance. This is a review of the different studies of induction of tolerance with different type of foods and protocols. Oral induction of tolerance appears to be effective in induced short-term desensitization and to be quite safe, but further research is needed using large, high- quality studies that investigate and assesses the long-term efficacy, safety and cost-effectiveness of the induction of oral tolerance. Keywords: Desensitization, food allergy, immunotherapy, induction, tolerance treatment, long-term efficacy, IgE-mediated mechanisms, non-IgE-mediated (cellular) mechanisms, hypersensitivity

The role of co-inhibitory signals in spontaneous tolerance of weakly mismatched transplants

The immune system of female H-2b (C57BL/6) mice is a strong responder against the male minor-H antigen. However rejection or acceptance of such weakly mismatched grafts depends on the type of tissue transplanted. The mechanism responsible for such spontaneous graft acceptance, and its relationship to the natural mechanisms of tolerance of self antigens is unknown. Co-inhibitory molecules negatively regulate immune responses, and are important for self tolerance. We examined whether co-inhibitory molecules play a critical role in “spontaneous” allograft tolerance. Naïve or donor sensitized diabetic female C57BL/6 (B6) wild type (WT), PD-1−/−, and BTLA−/− mice were transplanted with freshly isolated syngeneic male islet grafts. The role of co-inhibitors during priming of anti-donor responses and graft challenge was also assessed using monoclonal antibodies targeting co-inhibitory receptors. Among the co-inhibitor (CTLA-4, PD-1) specific antibodies tested, only anti-PD-1 showed some potential to prevent spontaneous acceptance of male islet grafts. All BTLA−/− and almost all PD-1−/− recipients maintained the ability to spontaneously accept male islet grafts. While spontaneous graft acceptance in naïve recipients was only weakly PD-1 dependent, tolerance induced by the accepted islets was found to be highly PD-1 dependent. Furthermore, spontaneous graft acceptance in pre-sensitized recipients showed an absolute requirement for recipient PD-1 but not BTLA. Thus, the PD-1 pathway, involved in self tolerance, plays a critical role in spontaneous tolerance induced by weakly mismatched grafts in naïve recipients and spontaneous graft acceptance in pre-sensitized recipients.

Prospective highlights of serum glycoproteins in spontaneous tolerance after orthotopic liver transplantation

Background We examined the effects and the mechanisms of serum glycoproteins on spontaneous tolerance after orthotopic liver transplantation (OLT) between DA (RT-1 a) and PVG (RT-1 c) rats. Methods A functional proteome analysis was introduced to investigate differently expressed proteins involved in overcoming major histocompatibility complex (MHC) barriers and lectin blotting was applied to survey the levels of fucosylated proteins. Results Two-dimensional difference gel electrophoresis (2D-DIGE) coupled with mass spectrometry revealed statistically significant changes in the intensity of 19 proteins at 14 and 60 post-OLT days which respectively corresponded to rejection and tolerance. An interaction network analysis of the identified proteins indicated that the interleukin (IL)-6 signaling pathway might be correlated with the immune events in this model. The peak of IL-6 expression occurred during the recovery period might play a role in the remarkable shifts in the immune response towards spontaneous tolerance. Furthermore, increased levels of IL-6-modulated fucosylation of Igh-6 were also observed during the rejection response while fucosylated hemopexin and haptoglobin were obviously upregulated in the tolerogenic period. Conclusions These findings suggest that IL-6 and glycoproteins may play critical roles in this spontaneous tolerogenic DA/PVG OLT model and shed light on prolonging hepatic allograft survival in the future.

Spontaneous pain, pain threshold, and pain tolerance in Parkinson’s disease

The mechanisms underlying pain in Parkinson's disease (PD) are unclear. Although a few studies have reported that PD patients may have low pain threshold and tolerance, none could accurately assess whether there was a correlation between sensory thresholds and demographic/clinical features of PD patients. Thus, tactile threshold, pain threshold, and pain tolerance to electrical stimuli in the hands and feet were assessed in 106 parkinsonian patients (of whom 66 reported chronic pain) and 51 age- and sex-matched healthy subjects. Linear regression models determined relationships between psychophysical parameters and demographic/clinical features. Female gender, severity of disease, medical disease associated with painful symptoms, and dyskinesia were more frequently observed in PD patients experiencing pain, even though dyskinesia did not reach significance. Pain threshold and pain tolerance were significantly lower in PD patients than in control subjects, whereas the tactile threshold yielded comparable values in both groups. Multivariable linear regression analyses yielded significant inverse correlations of pain threshold and pain tolerance with motor symptom severity and Beck depression inventory. Pain threshold and pain tolerance did not differ between PD patients with and without pain. In the former group, there was no relationship between pain threshold and the intensity/type of pain, and number of painful body parts. These findings suggest that pain threshold and pain tolerance tend to decrease as PD progresses, which can predispose to pain development. Female gender, dyskinesia, medical conditions associated with painful symptoms, and postural abnormalities secondary to rigidity/bradikinesia may contribute to the appearance of spontaneous pain in predisposed subjects.

Identification of operationally tolerant liver transplant recipients

KEY POINTS: (1) Liver allografts exhibit intrinsic tolerogenic properties that result in their spontaneous acceptance in many experimental animal models. (2) In clinical transplantation, liver allografts require milder immunosuppression (IS) regimens than other organs, are remarkably resistant to antibody-mediated rejection, and only very rarely are lost because of immunological insults. (3) A fraction of stable liver transplant recipients can withdraw from all IS therapy and then maintain normal graft function and not experience rejection. This phenomenon is known as spontaneous operational tolerance. (4) The intentional discontinuation of IS in stable liver transplant recipients has led to successful weaning in almost 20% of recipients, but the true prevalence of spontaneous operational tolerance in unselected recipients is still unknown. (5) The prevalence could be higher in pediatric recipients undergoing transplantation before 1 year of age and in adult recipients with more than 10 years of posttransplant follow-up. (6) Rejection occurring during medically supervised IS weaning trials tends to be mild and, in the overwhelming majority of cases, can be easily resolved without the administration of high-dose IS. (7) Tolerant liver recipients exhibit specific transcriptional patterns in peripheral blood and in liver tissue that may constitute future diagnostic markers of tolerance. (8) There is still no formal proof that the discontinuation of low-dose IS in long-term surviving liver recipients improves the morbidity and mortality rates associated with IS therapy. Liver Transpl 16:S82-S86, 2010. © 2010 AASLD.

Cardiac allograft acceptance induced by blockade of CD40-CD40L costimulation is dependent on CD4 +CD25 + regulatory T cells

We have demonstrated previously that CD4(+)CD25(+) regulatory T cells (Treg) are important for spontaneous hepatic allograft tolerance. In this study, we examine the role of Treg in cardiac allograft acceptance induced by blockade of the CD40-CD40L pathway. A heterotopic heart transplant model of major histocompatibility complex-mismatched mice was performed. Expression of forkhead/winged helix transcription factor (FoxP3) and/or the number of CD4(+)CD25(+) T cells in allografts and spleens were examined. The effect of Treg from the recipient or the donor on the induction and maintenance of long-term allograft survival was determined. Histologic analyses were also performed. The effects of Treg on CD4(+) and CD8(+) T cells were assessed. The levels of FoxP3 and/or CD4(+)CD25(+) T cells increased in long-surviving allografts and spleens. Depletion of Treg in the recipients but not the donors before transplantation caused rejection. Histologic analyses of allografts with Treg depletion showed extensive leukocyte infiltration and tissue destruction. However, delayed depletion of Treg in long-surviving recipients did not shorten their survival. Treg depletion increased the function of CD4(+) and CD8(+) T cells. Treg in the recipient but not in the donor is essential for long-term survival induced by CD40-CD40L blockade by inhibiting the function of CD4(+) and CD8(+) T cells; however, Treg are not important for maintenance. Both allograft and spleen are critical for induction of successful long-term survival.

Toward a B-cell signature of tolerance?

Operational tolerance is a rare and still unexplained phenomenon in organ transplantation. Pallier et al. analyzed the peripheral B-cell compartment of 12 renal transplant patients with drug-free long-term graft function and found a peculiar blood B-cell phenotype, with an expansion of memory activated B cells and increased expression of inhibitory molecules, suggesting a role of B cells in maintaining graft tolerance.