Immune Reconstitution Following Rabbit Antithymocyte Globulin

Depletional induction therapies are routinely used to prevent acute rejection and improve transplant outcome. The effects of depleting agents on T‐cell subsets and subsequent T‐cell reconstitution are incompletely defined. We used flow cytometry to examine the effects of rabbit antithymocyte globulin (rATG) on the peripheral T‐cell repertoire of pediatric and adult renal transplant recipients. We found that while rATG effectively depleted CD45RA+CD27+ naïve and CD45RO+CD27+ central memory CD4+ T cells, it had little effect on CD45RO+CD27? CD4+ effector memory or CD45RA+CD31?, CD45RO+CD27+ and CD45RO+CD27? CD8+ T cell subsets. When we performed a kinetic analysis of CD31+ recent thymic emigrants and CD45RA+/RO+ T cells, we found evidence for both thymopoiesis and homeostatic proliferation contributing to immune reconstitution. We additionally examined the impact of rATG on peripheral CD4+Foxp3+ T cells. We found that in adults, administration of rATG‐induced peripheral expansion and new thymic emigration of T cells with a Treg phenotype, while CD4+Foxp3+ T cells of thymic origin predominated in children, providing the first evidence that rATG induces Treg in vivo. Collectively our data indicate that rATG alters the balance of regulatory to memory effector T cells posttransplant, providing an explanation for how it positively impacts transplant outcome.     

Costimulatory blockade with mTor inhibition abrogates effector T‐cell responses allowing regulatory T‐cell survival in renal transplantation

The advent of novel immunosuppressive strategies in renal transplantation, with immunomodulatory properties, might facilitate long‐term allograft survival. T‐cell depletion, costimulation‐blockade and mTor inhibition have been shown to favour anti‐donor hyporesponsiveness. Recently, the combination of rATG, belatacept (Bela) and sirolimus (SRL) has been used in kidney transplantation, showing very low incidence of acute rejection and excellent 12‐month graft and patient survival. Herein, we have analysed the 1‐year evolution of memory/effector and regulatory T cells and assessed the donor‐specific T‐cell alloimmune response in a group of these patients and compared with others treated with a calcineurin‐inhibitor(CNI)‐based (rATG/tacrolimus/MMF), and two other Bela‐based regimens (rATG/Bela/MMF and basiliximab/Bela/MMF/steroids). During the first year after transplantation, patients receiving rATG/Bela/SRL had significantly higher percentage of Tregs upon the memory T‐cell compartment and showed a potent anti‐donor suppressive activity. In an in vitro naive and memory/effector T‐cell co‐culture, the combination of costimulation‐blockade and SRL could abrogate both antigen‐specific T‐cell responses as efficiently as using a CNI drug. The combination of T‐cell depletion, costimulation‐blockade and mTor inhibition seems to be able to allow Treg survival and inhibit donor‐specific alloreactive effector immune responses after kidney transplantation in humans.     

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.     

Siplizumab selectively depletes effector memory T cells and promotes a relative expansion of alloreactive regulatory T cells in vitro

Siplizumab, a humanized anti‐CD2 monoclonal antibody, has been used in conditioning regimens for hematopoietic cell transplantation and tolerance induction with combined kidney‐bone marrow transplantation. Siplizumab‐based tolerance induction regimens deplete T cells globally while enriching regulatory T cells (Tregs) early posttransplantation. Siplizumab inhibits allogeneic mixed‐lymphocyte reactions (MLRs) in vitro. We compared the impact of siplizumab on Tregs versus other T cell subsets in HLA‐mismatched allogeneic MLRs using PBMCs. Siplizumab predominantly reduced the percentage of CD4⁺ and CD8⁺ effector memory T cells, which express higher CD2 levels than naïve T cells or resting Tregs. Conversely, siplizumab enriched proliferating CD45RA^? FoxP3^HI cells in MLRs. FoxP3 expression was stable over time in siplizumab‐containing cultures, consistent with enrichment for bona fide Tregs. Consistently, high‐throughput TCRβ CDR3 sequencing of sorted unstimulated and proliferating T cells in MLRs revealed selective expansion of donor‐reactive Tregs along with depletion of donor‐reactive CD4⁺ effector/memory T cells in siplizumab‐containing MLRs. These results indicate that siplizumab may have immunomodulatory functions that may contribute to its success in tolerance‐inducing regimens. Our studies also confirm that naïve in addition to effector/memory T cells contribute to the allogeneic MLR and mandate further investigation of the impact of siplizumab on alloreactive naïve T cells.     

Regulatory T Cells Exhibit Decreased Proliferation but Enhanced Suppression After Pulsing With Sirolimus

Although regulatory T cells (Tregs) suppress allo‐immunity, difficulties in their large‐scale production and in maintaining their suppressive function after expansion have thus far limited their clinical applicability. Here we have used our nonhuman primate model to demonstrate that significant ex vivo Treg expansion with potent suppressive capacity can be achieved and that Treg suppressive capacity can be further enhanced by their exposure to a short pulse of sirolimus. Both unpulsed and sirolimus‐pulsed Tregs (SPTs) are capable of inhibiting proliferation of multiple T cell subpopulations, including CD4⁺ and CD8⁺ T cells, as well as antigen‐experienced CD28⁺CD95⁺ memory and CD28^?CD95⁺ effector subpopulations. We further show that Tregs can be combined in vitro with CTLA4‐Ig (belatacept) to lead to enhanced inhibition of allo‐proliferation. SPTs undergo less proliferation in a mixed lymphocyte reaction (MLR) when compared with unpulsed Tregs, suggesting that Treg‐mediated suppression may be inversely related to their proliferative capacity. SPTs also display increased expression of CD25 and CTLA4, implicating signaling through these molecules in their enhanced function. Our results suggest that the creation of SPTs may provide a novel avenue to enhance Treg‐based suppression of allo‐immunity, in a manner amenable to large‐scale ex vivo expansion and combinatorial therapy with novel, costimulation blockade‐based immunosuppression strategies.     

Regulatory T cell expressed MyD88 is critical for prolongation of allograft survival

MyD88 signaling directly promotes T‐cell survival and is required for optimal T‐cell responses to pathogens. To examine the role of T‐cell‐intrinsic MyD88 signals in transplantation, we studied mice with targeted T‐cell‐specific MyD88 deletion. Contrary to expectations, we found that these mice were relatively resistant to prolongation of graft survival with anti‐CD154 plus rapamycin in a class II‐mismatched system. To specifically examine the role of MyD88 in Tregs, we created a Treg‐specific MyD88‐deficient mouse. Transplant studies in these animals replicated the findings observed with a global T‐cell MyD88 knockout. Surprisingly, given the role of MyD88 in conventional T‐cell survival, we found no defect in the survival of MyD88‐deficient Tregs in vitro or in the transplant recipients and also observed intact cell homing and expression of Treg effector molecules. MyD88‐deficient Tregs also fail to protect allogeneic bone marrow transplant recipients from chronic graft‐versus‐host disease, confirming the observations of defective regulation seen in a solid organ transplant system. Together, our data define MyD88 as having a divergent requirement for cell survival in non‐Tregs and Tregs, and a yet‐to‐be defined survival‐independent requirement for Treg function during the response to alloantigen.     

Risk factors for impaired CD4+ T‐cell reconstitution following rabbit antithymocyte globulin treatment in kidney transplantation

To describe long‐term CD4⁺ T‐cell reconstitution after rabbit antithymocyte globulin (rATG) treatment and identify predictive factors following kidney transplantation. A single‐center retrospective study analyzed lymphocyte subsets in rATG‐treated kidney transplant recipients (1986–2009). 589 patients were analyzed (maximum follow‐up 21 years). A comparator group (n = 298) received an anti‐IL‐2 receptor monoclonal antibody. CD4⁺ T‐cell lymphopenia (<200/mm³) was present in 48.5%, 9.2%, 6.7%,2.0%, and 0% of patients at one, three, five, 10, and 20 years post‐transplant, respectively. CD4⁺ T‐cell count increased during the first 10 years but remained below the pretransplant count even after 20 years. At 1, 3, and 6 months post‐transplant, mean CD4⁺ T‐cell count was significantly lower in patients with CD4⁺ T‐cell lymphopenia at 12 months versus patients without lymphopenia. On multivariate analyses, significant independent predictors for long‐term impaired CD4 T‐cell reconstitution were recipient age, pretransplant CD4⁺ T‐cell count, 12‐month CD4⁺ T‐cell count, and tacrolimus or MMF therapy. Recipient age >40 years was identified as a cutoff point. CD4⁺ T‐cell reconstitution following rATG treatment remains impaired even after 21 years. Most risk factors for long‐term impaired CD4⁺ T‐cell reconstitution may be evaluated pretransplant or are modifiable post‐transplant.     

Regulatory T cells in immune‐mediated renal disease

Regulatory T cells (Tregs) are CD4+ T cells that can suppress immune responses by effector T cells, B cells and innate immune cells. This review discusses the role that Tregs play in murine models of immune‐mediated renal diseases and acute kidney injury and in human autoimmune kidney disease (such as systemic lupus erythematosus, anti‐glomerular basement membrane disease, anti‐neutrophil cytoplasmic antibody‐associated vasculitis). Current research suggests that Tregs may be reduced in number and/or have impaired regulatory function in these diseases. Tregs possess several mechanisms by which they can limit renal and systemic inflammatory immune responses. Potential therapeutic applications involving Tregs include in vivo induction of Tregs or inducing Tregs from naïve CD4+ T cells or expanding natural Tregs ex vivo, to use as a cellular therapy. At present, the optimal method of generating a phenotypically stable pool of Tregs with long‐lasting suppressive effects is not established, but human studies in renal transplantation are underway exploring the therapeutic potential of Tregs as a cellular therapy, and if successful may have a role as a novel therapy in immune‐mediated renal diseases.     

The number of circulating recent thymic emigrants is severely reduced 1 year after a single dose of alemtuzumab in renal transplant recipients

To better understand the kinetics of the delayed reconstitution of peripheral CD4+ T‐cells after depletion with a single administration of alemtuzumab (AL) for renal transplantation, we evaluated in these patients the percentage and absolute number of recent thymic emigrants (RTEs) CD4+ T cells, together with naive and memory subsets, defined by the analysis of CD31, CD45RA and CCR7 expression, and compared with patients treated with a nondepleting protocol based on basiliximab, and with healthy controls. In AL‐treated patients, the number of circulating CD4+ T cells was greatly reduced 1 year after the infusion (P < 0.01), but the proportions of central memory, effector memory and terminally differentiated effector memory subsets among CD4+ cells were significantly increased. On the contrary, the proportion and the absolute number of naïve CD4+ T cells, although progressively increasing with time, were severely reduced. In particular, the absolute number of RTEs had only very slight increase with time (P = 0.049) and was dramatically low 1 year after the therapy (P < 0.01 vs. healthy controls; P < 0.05 vs. basiliximab‐treated transplant recipients). These data suggest that a prolonged defective thymic output after AL therapy in renal transplant recipients is one of the main causes of the persistent CD4+ T‐cell lymphopenia observed in these patients.     

The extent of HLA‐DR expression on HLA‐DR+ Tregs allows the identification of patients with clinically relevant borderline rejection

Regulatory T cells (Tregs) were shown to be involved into the pathogenesis of acute rejection after transplantation. The suppressive activity of the total regulatory T cell pool depends on its percentage of highly suppressive HLA‐DR⁺‐Treg cells. Therefore, both the suppressive activity of the total Treg pool and the extent of HLA‐DR expression of HLA‐DR⁺‐Tregs (MFI HLA‐DR) were estimated in non transplanted volunteers, patients with end‐stage renal failure (ESRF), healthy renal transplant patients with suspicion on rejection, due to sole histological Bord‐R or sole acute renal failure (ARF), and patients with clinically relevant borderline rejection (Bord‐R and ARF). Compared to patients with only Bord‐R or only ARF, the suppressive activity of the total Treg cell pool was exclusively reduced in patients with clinically relevant Bord‐R. In parallel, the HLA‐DR MFI of the DR⁺‐Treg subset was significantly decreased in these patients, due to a significantly lower proportion of DR^high+‐Tregs, which were shown to have the highest suppressive capacity within the total Treg pool. Our findings clearly demonstrate that the determination of the HLA‐DR MFI of the HLA‐DR⁺‐Treg subset allows a highly sensitive, specific and non‐invasive discrimination between patients with clinically relevant Bord‐R (Bord and ARF) and patients with subclinical rejection or other causes of transplant failure.     

Deleterious Effect of CTLA4‐Ig on a Treg‐Dependent Transplant Model

Blockade of the B7:CD28 costimulatory pathway has emerged as a promising therapy to prevent allograft rejection. However, results from the belatacept phase III clinical trial demonstrated a higher rejection rate when compared to cyclosporine, raising concern about potential deleterious effects of this agent. In this study, we investigated the consequences of B7:CD28 blockade by hCTLA4Ig on regulator T cell (Treg) generation in different major histocompatibility complex (MHC) mismatch transplant models. Administration of hCTLA4Ig significantly decreased the amount of Tregs in B6 WT animals and this effect was predominant in thymus‐induced Tregs (Helios⁺). Although hCTLA4Ig prevented rejection in a fully allogeneic mismatch model, it accelerated rejection in a MHC class‐II mismatch model (MST = 26, p < 0.0001), in which long‐term allograft survival is dependent on Tregs. This accelerated rejection was associated with a marked reduction in thymus‐induced Tregs and led to a higher effector/regulatory T‐cell ratio in secondary lymphoid organs and in the allograft. This study confirms the importance of the B7:CD28 pathway in Treg homeostasis in an in vivo transplant model and suggests that hCTLA4Ig therapy may be deleterious in circumstances where engraftment is dependent on Tregs.     

Induction of bona fide regulatory T cells after liver transplantation - the potential influence of polyclonal antithymocyte globulin

T‐cell‐depleting strategies are an integral part of immunosuppressive regimens used in the hematological and solid organ transplant setting. Besides prevention of alloreactivity, treatment with rabbit antithymocyte globulin (rATG) has been related to the induction of immunoregulatory T cells (Treg) in vitro and in vivo. To investigate Treg induced by rATG, we prospectively studied the effect of rATG induction therapy in liver‐transplanted recipients in vivo (n = 28). Treg induction was further evaluated by means of Treg‐specific demethylation region (TSDR) analysis within the FOXP3 locus. Whereas no induction of CD4⁺ CD25^highCD127^? Treg could be observed by phenotypic analysis, we could demonstrate an induction of TSDR⁺ T cells within CD4⁺ T cells exclusively for rATG‐treated patients in the long‐term (day 540) compared with controls (P = NS). Moreover, although in vitro experiments confirm that rATG primarily led to a conversion of CD4⁺ CD25^? into CD4⁺ CD25⁺ T cells displaying immunosuppressive capacities, these cells cannot be classified as bona fide Treg based on their FOXP3 demethylation pattern. Consequently, the generation of Treg after rATG co‐incubation in vitro does not reflect the mechanisms of Treg induction in vivo and therefore the potential clinical relevance of these cells for transplant outcome remains to be determined.     

Kinetic analysis reveals potency of CD4+ CD25bright+ regulatory T-cells in kidney transplant patients

Donor-specific hyporesponsiveness as occurs after allogeneic kidney transplantation may be mediated by repression of effector cells by a specific subset of T-cells: the CD4(+) CD25(bright+) FoxP3(+) regulatory T-cells (Tregs). Here, we examined the suppressive capacity of Tregs isolated from the leukafereses product of 6 kidney transplant recipients, by reconstituting Tregs to responder T-cells at several time-points after initiation of proliferation. We show that Tregs derived from kidney transplant patients potently restrain proliferation to donor-antigens and 3rd party-antigens in classic reconstitution assays (i.e. addition of Tregs at the start of the co-incubation). However, when Tregs were added 5 days after initiation of proliferation, they were still capable of suppressing proliferation to donor-antigens (by 38%) but no longer to 3rd party-antigens. Thus, we conclude that the potency of Tregs to suppress reactivity to specific antigens should be determined by reconstitution to ongoing reactions.     

Rabbit anti‐rat thymocyte immunoglobulin preserves renal function during ischemia/reperfusion injury in rat kidney transplantation

Ischemia/reperfusion (I/R) injury is an important cause of renal graft dysfunction in humans. Increases in cold and warm ischemia times lead to a higher risk of early post‐transplant complications including delayed graft function and acute rejection. Moreover, prolonged cold ischemia is a predictor of long‐term kidney graft loss. The protective effect of rabbit anti‐rat thymocyte immunoglobulin (rATG) was evaluated in a rat model of I/R injury following syngeneic kidney transplantation. Serum creatinine concentration was evaluated at 16 h and 24 h post‐transplant. Animals were sacrificed 24 h post‐transplant for evaluation of histology, infiltrating leukocytes, nitrotyrosine staining, and apoptosis. rATG was effective in preventing renal function impairment, tissue damage and tubular apoptosis associated with I/R only when was given 2 h before transplantation but not at the time of reperfusion. Pretransplant rATG treatment of recipient animals effectively reduced the amount of macrophages, CD4⁺, CD8⁺ T cells and LFA‐1⁺ cells infiltrating renal graft subjected to cold ischemia as well as granzyme‐B expression within ischemic kidney. On the other hand, granulocyte infiltration and oxidative stress were not modified by rATG. If these results will be translated into the clinical setting, pretransplant administration of Thymoglobuline^® could offer the additional advantage over peri‐transplant administration of limiting I/R‐mediated kidney graft damage.     

Clinical Grade Manufacturing of Human Alloantigen‐Reactive Regulatory T Cells for Use in Transplantation

Regulatory T cell (Treg) therapy has the potential to induce transplantation tolerance so that immunosuppression and associated morbidity can be minimized. Alloantigen‐reactive Tregs (arTregs) are more effective at preventing graft rejection than polyclonally expanded Tregs (PolyTregs) in murine models. We have developed a manufacturing process to expand human arTregs in short‐term cultures using good manufacturing practice‐compliant reagents. This process uses CD40L‐activated allogeneic B cells to selectively expand arTregs followed by polyclonal restimulation to increase yield. Tregs expanded 100‐ to 1600‐fold were highly alloantigen reactive and expressed the phenotype of stable Tregs. The alloantigen‐expanded Tregs had a diverse TCR repertoire. They were more potent than PolyTregs in vitro and more effective at controlling allograft injuries in vivo in a humanized mouse model.     

Regulatory T Cells Are Critical to Tolerance Induction in Presensitized Mouse Transplant Recipients Through Targeting Memory T Cells

Memory T cells are a significant barrier to induction of transplant tolerance. However, reliable means to target alloreactive memory T cells have remained elusive. In this study, presensitization of BALB/c mice with C57BL/6 skin grafts generated a large number of OX40⁺CD44^hieffector/memory T cells and resulted in rapid rejection of donor heart allografts. Recognizing that anti‐OX40L monoclonal antibody (mAb) (α‐OX40L) monotherapy prolonged graft survival through inhibition and apoptosis of memory T cells in presensitized recipients, α‐OX40L was added to the combined treatment protocol of LF15–0195 (LF) and anti‐CD45RB (α‐CD45RB) mAb—a protocol that induced heart allograft tolerance in non‐presensitized recipients but failed to induce tolerance in presensitized recipients. Interestingly, this triple therapy restored donor‐specific heart allograft tolerance in our presensitized model that was associated with induction of tolerogenic dendritic cells and CD4⁺CD25⁺Foxp3⁺ T regulatory cells (Tregs). Of note, CD25⁺ T cell depletion in triple therapy recipients prevented establishment of allograft tolerance. In addition, adoptive transfer of donor‐primed effector/memory T cells into tolerant recipients markedly reduced levels of Tregs and broke tolerance. Our findings indicated that targeting memory T cells, by blocking OX40 costimulation in presensitized recipients was very important to expansion of Tregs, which proved critical to development of tolerance.     

Discarded Human Thymus Is a Novel Source of Stable and Long‐Lived Therapeutic Regulatory T Cells

Regulatory T cell (Treg)–based therapy is a promising approach to treat many immune‐mediated disorders such as autoimmune diseases, organ transplant rejection, and graft‐versus‐host disease (GVHD). Challenges to successful clinical implementation of adoptive Treg therapy include difficulties isolating homogeneous cell populations and developing expansion protocols that result in adequate numbers of cells that remain stable, even under inflammatory conditions. We investigated the potential of discarded human thymuses, routinely removed during pediatric cardiac surgery, to be used as a novel source of therapeutic Tregs. Here, we show that large numbers of FOXP3⁺ Tregs can be isolated and expanded from a single thymus. Expanded thymic Tregs had stable FOXP3 expression and long telomeres, and suppressed proliferation and cytokine production of activated allogeneic T cells in vitro. Moreover, expanded thymic Tregs delayed development of xenogeneic GVHD in vivo more effectively than expanded Tregs isolated based on CD25 expression from peripheral blood. Importantly, in contrast to expanded blood Tregs, expanded thymic Tregs remained stable under inflammatory conditions. Our results demonstrate that discarded pediatric thymuses are an excellent source of therapeutic Tregs, having the potential to overcome limitations currently hindering the use of Tregs derived from peripheral or cord blood.     

Clonal Deletion Established via Invariant NKT Cell Activation and Costimulatory Blockade Requires In Vivo Expansion of Regulatory T Cells

Recently, the immune‐regulating potential of invariant natural killer T (iNKT) cells has attracted considerable attention. We previously reported that a combination treatment with a liposomal ligand for iNKT cells and an anti‐CD154 antibody in a sublethally irradiated murine bone marrow transplant (BMT) model resulted in the establishment of mixed hematopoietic chimerism through in vivo expansion of regulatory T cells (Tregs). Herein, we show the lack of alloreactivity of CD8⁺T cells in chimeras and an early expansion of donor‐derived dendritic cells (DCs) in the recipient thymi accompanied by a sequential reduction in the donor‐reactive Vβ‐T cell receptor repertoire, suggesting a contribution of clonal deletion in this model. Since thymic expansion of donor DCs and the reduction in the donor‐reactive T cell repertoire were precluded with Treg depletion, we presumed that Tregs should preform before the establishment of clonal deletion. In contrast, the mice thymectomized before BMT failed to increase the number of Tregs and to establish CD8⁺T cell tolerance, suggesting the presence of mutual dependence between the thymic donor–DCs and Tregs. These results provide new insights into the regulatory mechanisms that actively promote clonal deletion.     

Two‐year follow‐up of a prospective study of circulating regulatory T cells in renal transplant patients

San Segundo D, Fernández‐Fresnedo G, Ruiz JC, Rodrigo E, Benito MJ, Arias M, López‐Hoyos M. Two‐year follow‐up of a prospective study of circulating regulatory T cells in renal transplant patients.
Clin Transplant 2010: 24: 386–393. © 2009 John Wiley & Sons A/S. Abstract: CD4⁺CD25^highFOXP3⁺ regulatory T cells (Tregs) are involved in alloreactivity and may be associated with protection from rejection. Their quantification in peripheral blood could guide clinicians in the management of renal transplant patients. Thus, we prospectively monitored the levels and in vitro suppression of circulating Tregs in 33 renal transplant patients from deceased donors within the first two yr of transplantation. Patients received maintenance immunosuppression with tacrolimus, mofetil mycophenolate and prednisolone. Results showed that peripheral blood Tregs were significantly lower six months after transplantation and recovered to almost basal levels at first post‐transplant year. The number of circulating Tregs increased significantly over basal levels afterwards. The decrease in circulating Tregs at six months may be explained by the high load of tacrolimus, as demonstrated by the inverse correlation between the blood concentration of Tregs and tacrolimus. Likewise, nine patients treated with anti‐CD25 antibodies showed higher numbers of Tregs at six months than those that did not, although differences were not observed later. In conclusion, circulating Tregs decrease in the first six months but recover thereafter up to two yr after kidney transplantation. Such a decrease is favored by high levels of tacrolimus but not by induction protocols with anti‐CD25.