Tracking of TCR‐Transgenic T Cells Reveals That Multiple Mechanisms Maintain Cardiac Transplant Tolerance in Mice

Solid organ transplantation tolerance can be achieved following select transient immunosuppressive regimens that result in long‐lasting restraint of alloimmunity without affecting responses to other antigens. Transplantation tolerance has been observed in animal models following costimulation or coreceptor blockade therapies, and in a subset of patients through induction protocols that include donor bone marrow transplantation, or following withdrawal of immunosuppression. Previous data from our lab and others have shown that proinflammatory interventions that successfully prevent the induction of transplantation tolerance in mice often fail to break tolerance once it has been stably established. This suggests that established tolerance acquires resilience to proinflammatory insults, and prompted us to investigate the mechanisms that maintain a stable state of robust tolerance. Our results demonstrate that only a triple intervention of depleting CD25⁺ regulatory T cells (Tregs), blocking programmed death ligand‐1 (PD‐L1) signals, and transferring low numbers of alloreactive T cells was sufficient to break established tolerance. We infer from these observations that Tregs and PD‐1/PD‐L1 signals cooperate to preserve a low alloreactive T cell frequency to maintain tolerance. Thus, therapeutic protocols designed to induce multiple parallel mechanisms of peripheral tolerance may be necessary to achieve robust transplantation tolerance capable of maintaining one allograft for life in the clinic.     

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.     

Disruption of Transplant Tolerance by an “Incognito” Form of CD8 T Cell–Dependent Memory

Several approaches successfully achieve allograft tolerance in preclinical models but are challenging to translate into clinical practice. Many clinically relevant factors can attenuate allograft tolerance induction, including intrinsic genetic resistance, peritransplant infection, inflammation, and preexisting antidonor immunity. The prevailing view for immune memory as a tolerance barrier is that the host harbors memory cells that spontaneously cross‐react to donor MHC antigens. Such preexisting “heterologous” memory cells have direct reactivity to donor cells and resist most tolerance regimens. In this study, we developed a model system to determine if an alternative form of immune memory could also block tolerance. We posited that host memory T cells could potentially respond to donor‐derived non‐MHC antigens, such as latent viral antigens or autoantigens, to which the host is immune. Results show that immunity to a model nonself antigen, ovalbumin (OVA), can dramatically disrupt tolerance despite undetectable initial reactivity to donor MHC antigens. Importantly, this blockade of tolerance was CD8⁺ T cell–dependent and required linked antigen presentation of alloantigens with the test OVA antigen. As such, this pathway represents an unapparent, or “incognito,” form of immunity that is sufficient to prevent tolerance and that can be an unforeseen additional immune barrier to clinical transplant tolerance.     

The Changed Balance of Regulatory and Naive T Cells Promotes Tolerance after TLI and Anti‐T‐Cell Antibody Conditioning

The goal of the study was to determine how the changed balance of host naïve and regulatory T cells observed after conditioning with total lymphoid irradiation (TLI) and antithymocyte serum (ATS) promotes tolerance to combined organ and bone marrow transplants. Although previous studies showed that tolerance was dependent on host natural killer T (NKT) cells, this study shows that there is an additional dependence on host CD4⁺CD25⁺ Treg cells. Depletion of the latter cells before conditioning resulted in rapid rejection of bone marrow and organ allografts. The balance of T‐cell subsets changed after TLI and ATS with TLI favoring mainly NKT cells and ATS favoring mainly Treg cells. Combined modalities reduced the conventional naïve CD4⁺ T cells 2800‐fold. The host type Treg cells that persisted in the stable chimeras had the capacity to suppress alloreactivity to both donor and third party cells in the mixed leukocyte reaction. In conclusion, tolerance induction after conditioning in this model depends upon the ability of naturally occurring regulatory NKT and Treg cells to suppress the residual alloreactive T cells that are capable of rejecting grafts.     

The Programmed Death (PD)‐1/PD‐Ligand 1 Pathway Regulates Graft‐Versus‐Host‐Reactive CD8 T Cells After Liver Transplantation

Acute graft‐versus‐host disease (aGVHD) is a life‐threatening complication after solid‐organ transplantation, which is mediated by host‐reactive donor T cells emigrating from the allograft. We report on two liver transplant recipients who developed an almost complete donor chimerism in peripheral blood and bone marrow‐infiltrating T cells during aGVHD. By analyzing these T cells directly ex vivo, we found that they died by apoptosis over time without evidence of rejection by host T cells. The host‐versus‐donor reactivity was selectively impaired, as anti‐third‐party and antiviral T cells were still detectable in the host repertoire. These findings support the acquired donor‐specific allotolerance concept previously established in animal transplantation studies. We also observed that the resolution of aGVHD was not accompanied by an expansion of circulating immunosuppressive CD4/CD25/FoxP3‐positive T cells. In fact, graft‐versus‐host‐reactive T cells were controlled by an alternative negative regulatory pathway, executed by the programmed death (PD)‐1 receptor and its ligand PD‐L1. We found high PD‐1 expression on donor CD4 and CD8 T cells. In addition, blocking PD‐L1 on host‐derived cells significantly enhanced alloreactivity by CD8 T cells in vitro. We suggest the interference with the PD‐1/PD‐L1 pathway as a therapeutic strategy to control graft‐versus‐host‐reactive T cells in allograft recipients.     

Amnion‐Derived Multipotent Progenitor Cells Support Allograft Tolerance Induction

Donor‐specific immunological tolerance using high doses of bone marrow cells (BMCs) has been demonstrated in mixed chimerism‐based tolerance induction protocols; however, the development of graft versus host disease remains a risk. Here, we demonstrate that the co‐infusion of limited numbers of donor unfractionated BMCs with human amnion‐derived multipotent progenitor cells (AMPs) 7 days post–allograft transplantation facilitates macrochimerism induction and graft tolerance in a mouse skin transplantation model. AMPs + BMCs co‐infusion with minimal conditioning led to stable, mixed, multilineage lymphoid and myeloid macrochimerism, deletion of donor‐reactive T cells, expansion of CD4⁺CD25⁺Foxp3⁺ regulatory T cells (T_regs) and long‐term allograft survival (>300 days). Based on these findings, we speculate that AMPs maybe a pro‐tolerogenic cellular therapeutic that could have clinical efficacy for both solid organ and hematopoietic stem cell transplant applications.     

Prolongation of allograft survival by passenger donor regulatory T cells

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT‐regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor‐strain lymphocytes was first assessed by identifying circulating donor‐derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT‐regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT‐regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor‐strain nT‐regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT‐regs. In summary, following transplantation, passenger donor‐strain nT‐regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor‐derived nT‐regs may hold potential as a cellular therapy to improve transplant outcomes.     

Maintaining T cell tolerance of alloantigens: Lessons from animal studies

Achieving host immune tolerance of allogeneic transplants represents the ultimate challenge in clinical transplantation. It has become clear that different cells and mechanisms participate in acquisition versus maintenance of allograft tolerance. Indeed, manipulations which prevent tolerance induction often fail to abrogate tolerance once it has been established. Hence, elucidation of the immunological mechanisms underlying maintenance of T cell tolerance to alloantigens is essential for the development of novel interventions that preserve a robust and long lasting state of allograft tolerance that relies on T cell deletion in addition to intra‐graft suppression of inflammatory immune responses. In this review, we discuss some essential elements of the mechanisms involved in the maintenance of naturally occurring or experimentally induced allograft tolerance, including the newly described role of antigen cross‐dressing mediated by extracellular vesicles.     

Flt3L‐mobilized dendritic cells bearing H2‐Kbm1 apoptotic cells do not induce cross‐tolerance to CD8+ T cells across a class I MHC mismatched barrier

Tolerization of allogeneic CD8⁺ T cells is still a pending issue in the field of transplantation research to achieve long‐term survival. To test whether dendritic cells (DC) bearing allogeneic major histocompatibility complex (MHC) class I mismatched apoptotic cells could induce cross‐tolerance to alloreactive CD8⁺ T cells, the following experimental strategy was devised. Rag2/γ_c KO B6 mice were treated with Fms‐like tyrosine kinase 3 ligand (Flt3L)‐transduced B16 melanoma cells to drive a rapid expansion and mobilization of DC in vivo. Of all DC populations expanded, splenic CD11c⁺CD103⁺CD8α⁺ DC were selectively involved in the process of antigen clearance of X‐ray irradiated apoptotic thymocytes in vivo. Considering that CD11c⁺CD103⁺CD8α⁺ DC selectively take up apoptotic cells and that they are highly specialized in cross‐presenting antigen to CD8⁺ T cells, we investigated whether B6 mice adoptively transferred with Flt3L‐derived DC loaded with donor‐derived apoptotic thymocytes could induce tolerance to bm1 skin allografts. Our findings on host anti‐donor alloresponse, as revealed by skin allograft survival and cytotoxic T lymphocyte assays, indicated that the administration of syngeneic DC presenting K^bm1 donor‐derived allopeptides through the indirect pathway of antigen presentation was not sufficient to induce cross‐tolerance to alloreactive CD8⁺ T cells responding to bm1 alloantigens in a murine model of skin allograft transplantation across an MHC class I mismatched barrier.     

Adoptive Transfer of Renal Allograft Tolerance in a Large Animal Model

Our recent studies in an inbred swine model demonstrated that both peripheral and intra‐graft regulatory cells were required for the adoptive transfer of tolerance to a second, naïve donor‐matched kidney. Here, we have asked whether both peripheral and intra‐graft regulatory elements are required for adoptive transfer of tolerance when only a long‐term tolerant (LTT) kidney is transplanted. Nine highly‐inbred swine underwent a tolerance‐inducing regimen to prepare LTT kidney grafts which were then transplanted to histocompatible recipients, with or without the peripheral cell populations required for adoptive transfer of tolerance to a naïve kidney. In contrast to our previous studies, tolerance of the LTT kidney transplants alone was achieved without transfer of additional peripheral cells and without strategies to increase the number/potency of regulatory T cells in the donor. This tolerance was systemic, since most subsequent, donor‐matched challenge kidney grafts were accepted. These results confirm the presence of a potent tolerance‐inducing and/or tolerance‐maintaining cell population within LTT renal allografts. They suggest further that additional peripheral tolerance mechanisms, required for adoptive transfer of tolerance to a naïve donor‐matched kidney, depend on peripheral cells that, if not transferred with the LTT kidney, require time to develop in the adoptive host.     

Plasmacytoid Dendritic Cells: No Longer an Enigma and Now Key to Transplant Tolerance?

Plasmacytoid (p) dendritic cells (DC) are a specialized subset of DC whose primary role was initially defined by the production of type I interferons in response to viral infection. They are now known to also possess a repertoire of functions capable of determining T cell fate and activation. Under homeostatic conditions, non‐lymphoid tissue‐resident pDC play a critical role in the regulation of mucosal immunity, as well as the development of central and peripheral tolerance. Although these cells display a number of characteristics that differ from conventional DC, particularly altered costimulatory molecule expression and poor allostimulatory capacity when interacting with T cells, this phenotype favors the generation of alloantigen‐specific regulatory CD4⁺ or CD8⁺ T cells critical to the development of graft tolerance. In this minireview, we discuss pDC ontogeny, functional biology and the emerging data that demonstrate the importance of pDC in the induction of tolerance, as well as recent studies that define mechanisms underlying pDC‐mediated tolerance to both solid organ and haematopoietic stem cell transplants. We also highlight their use in clinical settings and the potential of pDC both as targets and cellular therapeutic agents to improve the outcome of organ transplantation.     

Direct versus Indirect Allorecognition: Visualization of Dendritic Cell Distribution and Interactions During Rejection and Tolerization

Interactions of donor and recipient dendritic cells (DCs) with CD4+ T cells determine the alloantigenic response in organ transplantation, where recipient T cells respond either directly to donor MHC, or indirectly to processed donor MHC allopeptides in the context of recipient MHC molecules. The present study evaluates donor and recipient alloantigen-presenting DC trafficking and their interactions with CD4+ T cells in the lymph nodes (LNs) and the spleen under tolerogenic treatment with anti-CD2 plus anti-CD3 mAb compared with untreated rejecting conditions. CX3CR1(GFP) BALB/c (I-A(d)) donor hearts were transplanted into C57BL/6 (I-A(b)) mice and quantification of donor DC direct (GFP+ or I-A(d+)) and recipient DC indirect (YAe+) trafficking and interactions with host CD4+ T cells was performed by fluorescent microscopy. Our data indicate that although both direct and indirect interactions between CD4+ T cells and donor and recipient DCs occur shortly after engraftment, only indirect presentation persists in the LN, but not the spleen, of tolerized recipients. These data suggest that distinct anatomic lymphoid compartments play a critical role in peripheral tolerance induction and maintenance, and persistent indirect presentation to CD4+ T cells within the LNs is an important process during tolerization.     

Combined Anti‐CD154/CTLA4Ig Costimulation Blockade‐Based Therapy Induces Donor‐Specific Tolerance to Vascularized Osteomyocutaneous Allografts

Tolerance induction by means of costimulation blockade has been successfully applied in solid organ transplantation; however, its efficacy in vascularized composite allotransplantation, containing a vascularized bone marrow component and thus a constant source of donor‐derived stem cells, remains poorly explored. In this study, osteomyocutaneous allografts (alloOMCs) from Balb/c (H2^d) mice were transplanted into C57BL/6 (H2^b) recipients. Immunosuppression consisted of 1 mg anti‐CD154 on day 0, 0.5 mg CTLA4Ig on day 2 and rapamycin (RPM; 3 mg/kg per day from days 0–7, then every other day for 3 weeks). Long‐term allograft survival, donor‐specific tolerance and donor–recipient cell trafficking were evaluated. Treatment with costimulation blockade plus RPM resulted in long‐term graft survival (>120 days) of alloOMC in 12 of 15 recipients compared with untreated controls (median survival time [MST] ≈10.2 ± 0.8 days), RPM alone (MST ≈33 ± 5.5 days) and costimulation blockade alone (MST ≈45.8 ± 7.1 days). Donor‐specific hyporesponsiveness in recipients with viable grafts was demonstrated in vitro. Evidence of donor‐specific tolerance was further assessed in vivo by secondary donor‐specific skin graft survival and third‐party graft rejection. A significant increase of Foxp3⁺ regulatory T cells was evident in tolerant animals. Donor cells populated peripheral blood, thymus, and both donor and recipient bone marrow. Consequently, combined anti‐CD154/CTLA4Ig costimulation blockade‐based therapy induces donor‐specific tolerance in a stringent murine alloOMC transplant model.     

Tolerogenic Dendritic Cells: All Present and Correct?

Although well‐recognized for their sentinel role and, when activated, their immunostimulatory function, bone marrow‐derived dendritic cells (DC) possess inherent tolerogenic (tol) ability. Under quiescent conditions, these cells maintain central and peripheral self tolerance. When appropriately conditioned, in vitro or in vivo, they inhibit innate and adaptive immunity to foreign antigens, including memory T‐cell responses. This suppressive function is mediated by various mechanisms, including the expansion and induction of antigen‐specific regulatory T cells. Extensive experience in rodent models and recent work in nonhuman primates, indicate the potential of pharmacologically‐modified, tol DC (tolDC) to regulate alloimmunity in vivo and to promote lasting, alloantigen‐specific T‐cell unresponsiveness and transplant survival. While there are many questions yet to be addressed concerning the functional biology of tolDC in humans, these cells offer considerable potential as natural, safe and antigen‐specific regulators for long‐term control of the outcome of organ and hematopoietic cell transplantation. This minireview surveys recent findings that enhance understanding of the functional biology and therapeutic application of tolDC, with special reference to transplantation.     

Treg‐Therapy Allows Mixed Chimerism and Transplantation Tolerance Without Cytoreductive Conditioning

Establishment of mixed chimerism through transplantation of allogeneic donor bone marrow (BM) into sufficiently conditioned recipients is an effective experimental approach for the induction of transplantation tolerance. Clinical translation, however, is impeded by the lack of feasible protocols devoid of cytoreductive conditioning (i.e. irradiation and cytotoxic drugs/mAbs). The therapeutic application of regulatory T cells (Tregs) prolongs allograft survival in experimental models, but appears insufficient to induce robust tolerance on its own. We thus investigated whether mixed chimerism and tolerance could be realized without the need for cytoreductive treatment by combining Treg therapy with BM transplantation (BMT). Polyclonal recipient Tregs were cotransplanted with a moderate dose of fully mismatched allogeneic donor BM into recipients conditioned solely with short‐course costimulation blockade and rapamycin. This combination treatment led to long‐term multilineage chimerism and donor‐specific skin graft tolerance. Chimeras also developed humoral and in vitro tolerance. Both deletional and nondeletional mechanisms contributed to maintenance of tolerance. All tested populations of polyclonal Tregs (FoxP3‐transduced Tregs, natural Tregs and TGF‐β induced Tregs) were effective in this setting. Thus, Treg therapy achieves mixed chimerism and tolerance without cytoreductive recipient treatment, thereby eliminating a major toxic element impeding clinical translation of this approach.     

The Knife's Edge of Tolerance: Inducing Stable Multilineage Mixed Chimerism but With a Significant Risk of CMV Reactivation and Disease in Rhesus Macaques

Although stable mixed‐hematopoietic chimerism induces robust immune tolerance to solid organ allografts in mice, the translation of this strategy to large animal models and to patients has been challenging. We have previously shown that in MHC‐matched nonhuman primates (NHPs), a busulfan plus combined belatacept and anti‐CD154‐based regimen could induce long‐lived myeloid chimerism, but without T cell chimerism. In that setting, donor chimerism was eventually rejected, and tolerance to skin allografts was not achieved. Here, we describe an adaptation of this strategy, with the addition of low‐dose total body irradiation to our conditioning regimen. This strategy has successfully induced multilineage hematopoietic chimerism in MHC‐matched transplants that was stable for as long as 24 months posttransplant, the entire length of analysis. High‐level T cell chimerism was achieved and associated with significant donor‐specific prolongation of skin graft acceptance. However, we also observed significant infectious toxicities, prominently including cytomegalovirus (CMV) reactivation and end‐organ disease in the setting of functional defects in anti‐CMV T cell immunity. These results underscore the significant benefits that multilineage chimerism‐induction approaches may represent to transplant patients as well as the inherent risks, and they emphasize the precision with which a clinically successful regimen will need to be formulated and then validated in NHP models.     

Increased apoptosis of immunoreactive host cells and augmented donor leukocyte chimerism, not sustained inhibition of B7 molecule expression are associated with prolonged cardiac allograft survival in mice preconditioned with immature donor dendritic cells plus anti-CD40L mAb.

BACKGROUND: We previously reported the association among donor leukocyte chimerism, apoptosis of presumedly IL-2-deficient graft-infiltrating host cells, and the spontaneous donor-specific tolerance induced by liver but not heart allografts in mice. Survival of the rejection-prone heart allografts in the same strain combination is modestly prolonged by the pretransplant infusion of immature, costimulatory molecule-(CM) deficient donor dendritic cells (DC), an effect that is markedly potentiated by concomitant CM blockade with anti-CD40L (CD154) monoclonal antibody (mAb). We investigated whether the long survival of the heart allografts in the pretreated mice was associated with donor leukocyte chimerism and apoptosis of graft-infiltrating cells, if these end points were similar to those in the spontaneously tolerant liver transplant model, and whether the pretreatment effect was dependent on sustained inhibition of CM expression of the infused immature donor DC. In addition, apoptosis was assessed in the host spleen and lymph nodes, a critical determination not reported in previous studies of either spontaneous or "treatment-aided" organ tolerance models. METHODS: Seven days before transplantation of hearts from B10 (H-2b) donors, 2x10(6) donor-derived immature DC were infused i.v. into C3H (H-2k) recipient mice with or without a concomitant i.p. injection of anti-CD40L mAb. Donor cells were detected posttransplantation by immunohistochemical staining for major histocompatibility complex class II (I-Ab) in the cells of recipient lymphoid tissue. CM expression was determined by two-color labeling. Host responses to donor alloantigen were quantified by mixed leukocyte reaction, and cytotoxic T lymphocyte (CTL) assays. Apoptotic death in graft-infiltrating cells and in areas of T-dependent lymphoid tissue was visualized by terminal deoxynucleotidyltransferase-catalyzed dUTP-digoxigenin nick-end labeling and quantitative spectrofluorometry. Interleukin-2 production and localization were estimated by immunohistochemistry. RESULTS: Compared with control heart transplantation or heart transplantation after only DC administration, concomitant pretreatment with immature donor DC and anti-CD40L mAb caused sustained elevation of donor (I-Ab+) cells (microchimerism) in the spleen including T cell areas. More than 80% of the I-Ab+ cells in combined treatment animals also were CD86+, reflecting failure of the mAb to inhibit CD40/ CD80/CD86 up-regulation on immature DC in vitro after their interaction with host T cells. Donor-specific CTL activity in graft-infiltrating cells and spleen cell populations of these animals was present on day 8, but decreased strikingly to normal control levels by day 14. The decrease was associated with enhanced apoptosis of graft-infiltrating cells and of cells in the spleen where interleukin-2 production was inhibited. The highest levels of splenic microchimerism were found in mice with long surviving grafts (>100 days). In contrast, CTL activity was persistently elevated in control heart graft recipients with comparatively low levels of apoptotic activity and high levels of interleukin-2. CONCLUSION: The donor-specific acceptance of rejection-prone heart allografts by recipients pretreated with immature donor DC and anti-CD40L mAb is not dependent on sustained inhibition of donor DC CM (CD86) expression. Instead, the pretreatment facilitates a tolerogenic cascade similar to that in spontaneously tolerant liver recipients that involves: (1) chimerism-driven immune activation, succeeded by deletion of host immune responder cells by apoptosis in the spleen and allograft that is linked to interleukin-2 deficiency in both locations and (2) persistence of comparatively large numbers of donor-derived leukocytes. These tolerogenic mechanisms are thought to be generic, explaining the tolerance induced by allografts spontaneously, or with the aid of various kinds of immunosuppression.     

Donor‐targeted serotherapy as a rescue therapy for steroid‐resistant acute GVHD after HLA‐mismatched kidney transplantation

Acute graft‐versus‐host disease (GVHD) is a rare but frequently lethal complication after solid organ transplantation. GVHD occurs in unduly immunocompromised hosts but requires the escalation of immunosuppression, which does not discriminate between host and donor cells. In contrast, donor‐targeted therapy would ideally mitigate graft‐versus‐host reactivity while sparing recipient immune functions. We report two children with end‐stage renal disease and severe primary immune deficiency (Schimke syndrome) who developed severe steroid‐resistant acute GVHD along with full and sustained donor T cell chimerism after isolated kidney transplantation. Facing a therapeutic dead end, we used a novel strategy based on the adoptive transfer of anti‐HLA donor‐specific antibodies (DSAs) through the transfusion of highly selected plasma. After approval by the appropriate regulatory authority, an urgent nationwide search was launched among more than 3800 registered blood donors with known anti‐HLA sensitization. Adoptively transferred DSAs bound to and selectively depleted circulating donor T cells. The administration of DSA‐rich plasma was well tolerated and notably did not induce antibody‐mediated rejection of the renal allografts. Acute GVHD symptoms promptly resolved in one child. This report provides a proof of concept for a highly targeted novel therapeutic approach for solid organ transplantation–associated GVHD.     

Induction of Allograft Tolerance by Monoclonal CD3 Antibodies: A Matter of Timing

Despite remarkable progress in organ transplantation through the development of a wealth of immunosuppressive drugs highly effective at controlling acute rejection, two major problems still remain, the loss of transplants due to chronic rejection and the growing number of sensitized recipients due to previous transplants, transfusions or pregnancies. Induction of immune tolerance appears to be the only way to curb this complex situation. Here we describe that a therapy, already successfully used to restore immune tolerance to self‐antigens in overt autoimmunity, is effective at promoting transplant tolerance. We demonstrate that a short low‐dose course with CD3 antibodies started after transplantation, at the time of effector T cell priming to alloantigens, induces permanent acceptance of fully mismatched islet allografts. Mechanistic studies revealed that antigen‐specific regulatory and effector T cells are differentially affected by the treatment. CD3 antibody treatment preferentially induces apoptosis of activated alloreactive T cells which is mandatory for tolerance induction. In contrast, regulatory T cells are relatively spared from CD3 antibody‐induced depletion and can transfer antigen‐specific tolerance thus arguing for their prominent role in sustaining long‐term graft survival.