Peripheral deletion after bone marrow transplantation with costimulatory blockade has features of both activation-induced cell death and passive cell death

Role of thymus in operational tolerance induction in limb allograft transplant model

BACKGROUND

In this study, we evaluated the role of host thymus in tolerance induction in composite tissue allografts (CTA) across major histocompatibility complex (MHC) barrier during a 7-day alphabeta- T-cell receptor (TCR)/ cyclosporine A (CsA) protocol.

MATERIALS AND METHODS

A total of 62 limb allograft transplants were studied. Euthymic (group A) and thymectomized (group B) Lewis recipients (LEW, RT1(1)) received vascularized hind-limb allografts from hybrid Lewis x Brown-Norway (F1), (LBN, RT1(1+n)) donors. Mixed lymphocyte reaction (MLR) and skin grafting assessed donor-specific tolerance in vitro and in vivo, respectively. Flow cytometry determined the efficacy of immunosuppressive protocols and the presence of donor-specific chimerism. Immunocytochemistry revealed the presence of donor-specific cells in the lymphoid organs of recipients.

RESULTS

Isograft transplants survived indefinitely. For thymectomized rats, the median survival time (MST) of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR extended MST to 16 days, and CsA therapy extended it to 30 days. Using the alphabeta-TCR/CsA protocol, the MST of allografts was 51 days. For euthymic rats, the MST of limb allograft in non-treated recipients was 7 days; monotherapy with alphabeta-TCR or CsA extended MST to 13 or 22 days, respectively. Treatment with alphabeta-TCR/CsA resulted in indefinite allografts survival (MST=370 days). MLR and skin grafting confirmed donor-specific tolerance in euthymic recipients. Flow cytometry showed stable chimerism in the euthymic rats and transient chimerism in thymectomized limb recipients. Immunoperoxidase staining revealed the persistence of donor-derived cells in the lymphoid tissues of euthymic recipients.

CONCLUSION

We found that the presence of thymus was imperative for the induction of donor-specific tolerance in rat hind-limb composite tissue allografts using a alphabeta-TCR/CsA protocol.

Anti-mouse CD154 antibody treatment facilitates generation of mixed xenogeneic rat hematopoietic chimerism, prevents wasting disease and prolongs xenograft survival in mice

BACKGROUND

The induction of xenogeneic hematopoietic chimerism is an attractive approach for overcoming the host response to xenografts, but establishing xenogeneic chimerism requires severe myeloablative conditioning of the recipient. The goal of this study was to determine if co-stimulation blockade would facilitate chimerism and xenograft tolerance in irradiation-conditioned concordant recipients.

METHODS

Wistar Furth rat bone marrow (BM) cells were injected into irradiation-conditioned C57BL/6 mice with or without co-administration of anti-mouse CD154 monoclonal antibody (mAb). Chimerism was quantified by flow cytometry, and mice were transplanted with WF rat skin and islet xenografts.

RESULTS

Blockade of CD40-CD154 interaction facilitated establishment of xenogeneic chimerism in mice conditioned with 600 cGy irradiation. Anti-CD154 mAb was not required for establishment of chimerism in mice treated with 700 cGy. However, mice irradiated with 700 cGy but not treated with anti-CD154 mAb developed a "graft-versus-host disease (GVHD)-like" wasting syndrome and died, irrespective of their development of chimerism. Xenogeneic chimeras established with irradiation and anti-CD154 mAb treatment exhibited prolonged skin and, in many cases, permanent islet xenograft survival. Chimerism was unstable and eventually lost in most recipients. Skin xenografts were rejected even in mice that remained chimeric, whereas most islet xenografts survived to the end of the observation period.

CONCLUSIONS

Blockade of host CD40-CD154 interaction facilitates the establishment of xenogeneic chimerism and prevents wasting disease and death. Chimerism permits prolonged xenograft survival, but chimerism generated in this way is unstable over time. Skin xenografts are eventually rejected, whereas most islet xenografts survive long term and perhaps permanently.

Molecular signature of mice T lymphocytes following tolerance induction by allogeneic BMT and CD40-CD40L costimulation blockade

Tolerance induction by mixed chimerism and costimulation blockade is a promising approach to avoid immunosuppression, but the molecular basis of tolerant T lymphocytes remains elusive. We investigated the genome-wide gene expression profile of murine T lymphocytes after tolerance induction by allogeneic bone marrow transplantation (BMT) and costimulatory blockade using the anti-CD40L antibody MR1. Molecular functions, biological processes, cellular locations, and coregulation of identified genes were determined. A total of 113 unique genes exhibited a significant differential expression between the lymphocytes of MR1-treated Tolerance (TOL) and untreated recipients Control (CTRL). The majority of genes upregulated in the TOL group are involved in several signal transduction cascades such as members of the MAPKKK cascade (IL6, Tob2, Stk39, and Dusp24). Other genes involved in lymphocyte differentiation and highly expressed in the TOL group are lymphotactin, the estrogen receptors (ERs) and the suppressor of cytokine signaling 7. Common transcription factors such as ER 1 alpha, GATA-binding protein 1, insulin promoter factor 1, and paired-related homeobox 2 could be identified in the promoter regions of upregulated genes in the TOL group. These data suggest that T lymphoctes of tolerant mice exhibit a distinct molecular expression profile, which needs to be evaluated in other experimental tolerance models to determine whether it is a universal signature of tolerance.

Low dose busulfan facilitates chimerism and tolerance in a murine model

T cell depletion, sirolimus and "mega" dose donor specific bone marrow (DSBM) infusion promotes stable multilineage chimerism and indefinite survival of skin allografts in completely mismatched mice. The purpose of this study is to determine whether the addition of low dose busulfan can reduce the amount of DSBM required while preserving efficacy. C57BL/6 recipients of BALB/c skin allografts were treated with alphaCD4 and alphaCD8 monoclonal antibodies, DSBM, sirolimus and various doses of busulfan. The kinetics and phenotype of chimerism and the presence of clonal deletion of alloreactive T-cells were defined using flow cytometry. In vitro reactivity was determined using mixed lymphocyte culture. Second skin grafts confirmed the presence of tolerance. All doses of busulfan resulted in engraftment when combined with this regimen using a reduced dose of donor marrow. The level, kinetics and character of chimerism observed were dose related. Chimerism was associated with indefinite allograft acceptance (>200 days). Tolerance was documented both in vitro/in vivo and was associated with clonal deletion. Addition of a single low dose of busulfan to an established tolerance protocol reduced the required DSBM dose by over 80% while still promoting comparable levels of donor chimerism and donor-specific tolerance.

The role of non-deletional tolerance mechanisms in a murine model of mixed chimerism with costimulation blockade

Peripheral and central clonal deletion are important tolerance mechanisms in models using bone marrow transplantation (BMT) with costimulation blockade (CB). However, since tolerance can be found before peripheral deletion is complete and since elimination of recipient CD4(+) cells at the time of BMT prevents tolerance induction, we investigated the potential roles of regulation and anergy in such a murine model. We found that transient elimination of CD25(+) cells or neutralization of IL2 immediately after BMT and CB prevented the induction of skin graft tolerance. Cotransfer into SCID mice of CD4(+) cells taken from chimeras early after BMT, together with naive recipient-type CD4(+) cells significantly prolonged donor skin graft survival. In contrast, cotransfer of CD4(+) cells harvested from chimeras late after BMT did not prolong donor skin graft survival. Besides, depletion of CD25(+) cells in established chimeras several months post-BMT did not break tolerance. In vivo administration of recombinant IL2 inhibited chimerism and tolerance neither early nor late post-BMT, arguing against a decisive role for classical anergy. Thus, CD4 cell-mediated regulation contributes significantly to tolerance induction early after BMT, but appears to have no critical role in the maintenance of tolerance.

Everolimus and basiliximab permit suppression by human CD4+CD25+ cells in vitro

Immunosuppressive drugs are essential for the prevention of acute transplant rejection but some may not promote long-term tolerance. Tolerance is dependent on the presence and regulatory function of CD4(+)CD25(+) T cells in a number of animal models. The direct effects of immunosuppressive drugs on CD4(+)CD25(+) cells, particularly those that interfere with IL-2 signaling are uncertain. We studied the effects of the rapamycin derivative everolimus and the anti-CD25 monoclonal antibody basiliximab on the regulatory capacity of human CD4(+)CD25(+) cells in vitro. Both drugs permitted the suppression of proliferation and IFN-gamma secretion by CD4(+)CD25(-) cells responding to allogeneic and other polyclonal stimuli; CTLA-4 expression was abolished on CD4(+)CD25(+) cells without compromising their suppressive ability. Everolimus reduced IFN-gamma secretion by CD4(+)CD25(-) cells before the anti-proliferative effect: this is a novel finding. Exogenous IL-2 and IL-15 could prevent the suppression of proliferation by CD4(+)CD25(+) cells and the drugs could not restore suppression. By contrast, suppression of IFN-gamma secretion was only slightly impeded with the exogenous cytokines. Finally, CD4(+)CD25(+) cells were more resistant than CD4(+)CD25(-) cells to the pro-apoptotic action of the drugs. Together these data suggest that CD4(+)CD25(+) cells may still exert their effects in transplant patients taking immunosuppression that interferes with IL-2 signaling.

Tolerance in mixed chimerism - a role for regulatory cells?

The establishment of mixed hematopoietic chimerism induces life-long donor-specific organ graft tolerance while obviating the need for chronic immunosuppression. Recent advances have dramatically reduced the conditioning toxicity required to achieve mixed chimerism. We argue that the achievement of high levels of donor chimerism ensures life-long deletion of donor-reactive T cells, precluding and obviating the need for regulatory mechanisms in the maintenance of tolerance. However, in situations where high levels of donor chimerism cannot be established or sustained, control of immune responsiveness can be achieved through additional mechanisms, including regulatory T cells.

Transplantation of hematopoietic stem cells for induction of unresponsiveness to organ allografts

Although it has been recognized since the early days of Owen and Medawar that engraftment of donor stem cells, induced in utero spontaneously or intentionally neonatally, results in life-long unresponsiveness to donor alloantigens. However, successful induction of transplantation tolerance in adult life still represents an unsolved problem. Engraftment of donor stem cells using conventional modalities involves intensive myeloablative or lymphoablative immunosuppression, which is associated with toxicity and mortality and such methods are not suitable for organ allograft recipients. In this chapter, we present an innovative approach for induction of donor-specific unresponsiveness to bone marrow and organ allografts without myeloablative conditioning. Our methods is based on cyclophosphamide-induced, alloantigen-primed lymphocyte depletion. Cyclophosphamide is administered 1 day following infusion of donor hematopoietic cells, thus eliminating predominantly host T lymphocytes reacting against donor cell challenge, and resulting in relative unresponsiveness to donor alloantigens. Subsequently, life-long tolerance to fully mismatched donor skin allografts can be accomplished by a second infusion of stem cells from the same donor, with donor T cells displacing residual alloreactive host cells that may have escaped deletion. Taken together, we believe that induction of true permanent and specific tolerance to organ allografts using donor hematopoietic cells could become a clinical reality in the foreseeable future.

Tolerance induction in clinical transplantation

Introduction of modern immunosuppressive agents has led to great success of allotransplantation in humans, and survival rates for all solid organs have been dramatically improved. However, a constant proportion of organs is lost every year due to chronic allograft rejection and immunosuppressive drug toxicity. This has led to a situation where, despite the of donor organ shortage, about one third of the patients on the kidney transplant waiting list are listed for a retransplant. The induction of donor-specific tolerance has the potential of at least partially resolving this problem, since it might prevent chronic rejection and drug toxicity at the same time. For a variety of protocols, successful tolerance induction has been demonstrated in rodent models. However, translation of such protocols to large animal models and on clinical trials has turned out to be very difficult. This review briefly describes mechanisms and barriers to transplantation tolerance, and then focuses on pre-clinical and clinical studies in non-human primates and humans. We have divided the strategies into two groups, based on the principle mechanisms of tolerance induction: the first group are protocols not using hematopoietic stem cell transplantation (HCT) as part of there regimen. They rely mainly on intensive T cell depletion (either by total body irradiation, total lymphoid irradiation or treatment with T cell-depleting agents such as anti-thymocyte globulin, anti-CD52 antibody or CD3 immunotoxin), which have been combined with costimulatory blockade, signaling blockade or donor antigen infusion. The second group are HCT-based protocols combining HCT with T cell-depleting agents and cytoreductive treatment. So far, only two protocols (one with total lymphoid irradiation and anti-thymocyte globulin, but no HCT; one with HCT, cyclophosphamide, anti-thymocyte globulin and thymic irradiation) have been translated into successful human studies. We summarize and discuss the results of these trials and suggest goals for further studies for the development tolerance protocols applicable for a broad population of allograft recipients.

CD154 blockade for induction of mixed chimerism and prolonged renal allograft survival in nonhuman primates

Costimulatory blockade with anti-CD154 monoclonal antibody (aCD154) prolongs allograft survival in nonhuman primates, but has not reliably induced tolerance when used alone. In the current studies, we evaluated the effect of adding CD154 blockade to a chimerism inducing nonmyeloablative regimen in primates. We observed a significant improvement of donor bone marrow (DBM) engraftment, which has been associated with a lower incidence of acute rejection and long-term survival of renal allografts without the need for previously required splenectomy. Among the long-term survivors, four never showed evidence of rejection, with the longest survival exceeding 1700 days following discontinuation of immunosuppression. Nevertheless, late chronic rejection was observed in three of eight recipients, indicating the necessity of further modifications of the regimen. Control recipients receiving no DBM or donor splenocytes in place of DBM rejected their allografts. Thus, DBM engraftment with, at least, transient mixed chimerism appears essential for induction of allograft tolerance using this conditioning regimen. Modification of the original mixed chimerism approach, by the addition of costimulatory blockade, has been shown to enhance mixed chimerism and induce renal allograft tolerance with less morbidity in nonhuman primates.

Immunobiology of acute graft-versus-host disease

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). The immunobiology of acute GVHD is complex and can be conceptualized to be a three-step process. In step 1, the conditioning regimen (irradiation and/or chemotherapy) leads to the damage and activation of host tissues and induces the secretion of inflammatory cytokines TNF-alpha and IL-1. As a consequence expression of MHC antigens and adhesion molecules is increased, thus enhancing the recognition of host alloantigens by donor T cells. Donor T-cell activation in step 2 is characterized by donor T-cell interaction with host APCs and subsequent proliferation, differentiation, and secretion of cytokines. Cytokines such as IL-2 and IFN-gamma enhance T-cell expansion, induce cytotoxic T cells (CTL) and natural killer (NK) cell responses, and prime additional mononuclear phagocytes to produce TNF-alpha and IL-1. These inflammatory cytokines in turn stimulate production of inflammatory chemokines, thus recruiting effector cells into target organs. In step 3, effector functions of mononuclear phagocytes are triggered via a secondary signal provided by lipopolysaccharide (LPS) that leaks through the intestinal mucosa damaged during step 1. This mechanism may result in the amplification of local tissue injury and further promotion of an inflammatory response, which, together with the CTL and NK components, leads to target tissue destruction in the transplant host.

Production of donor T cells is critical for induction of donor-specific tolerance and maintenance of chimerism

Nonmyeloablative conditioning has significantly reduced the morbidity associated with bone marrow transplantation. The donor hemopoietic cell lineage(s) responsible for the induction and maintenance of tolerance in nonmyeloablatively conditioned recipients is not defined. In the present studies we evaluated which hemopoietic stem cell-derived components are critical to the induction of tolerance in a total body irradiation-based model. Recipient B10 mice were pretreated with mAbs and transplanted with allogeneic B10.BR bone marrow after conditioning with 100-300 cGy total body irradiation. The proportion of recipients engrafting increased in a dose-dependent fashion. All chimeric recipients exhibited multilineage donor cell production. However, induction of tolerance correlated strictly with early production of donor T cells. The chimeras without donor T cells rejected donor skin grafts and demonstrated strong antidonor reactivity in vitro, while possessing high levels of donor chimerism. These animals lost chimerism within 8 mo. Differentiation into T cells was aborted at a prethymic stage in recipients that did not produce donor T cells. Moreover, donor Ag-driven clonal deletion of recipient T cells occurred only in chimeras with donor T cells. These results demonstrate that donor T cell production is critical in the induction of transplantation tolerance and the maintenance of durable chimerism. In addition, donor T cell production directly correlates with the deletion of potentially alloreactive cells.

Donor T cells lacking Fas ligand and perforin retain the capacity to induce severe GvHD in minor histocompatibility antigen mismatched bone-marrow transplantation recipients

Graft-versus-host disease (GvHD) is a frequent impediment to therapeutically successful allogeneic bone-marrow transplantation (BMT). This investigation further examines the roles of two potential donor cytotoxic effector mechanisms previously implicated in tissue pathogenesis. Cytotoxically double deficient (B6-cdd) T cells (lacking functional fas ligand and perforin) and wild-type (B6-wt) donor T-cell transplantation in a minor antigen-mismatched BMT model (C57BL/6 --> C3H.SW) resulted in similar mortality and weight loss. Histopathologic findings revealed mononuclear infiltrates and cellular atrophy in GvHD target tissues (liver, stomach) in recipients of B6-wt and B6-cdd donor T cells. Both recipients also exhibited GvH-associated lymphohematopoietic compartment (LHC) alterations as evidenced by inverted CD4:CD8 ratios and B-cell hypoplasia. Notably, transplants using recombinant inbred mHAg disparate recipients demonstrated that B6-cdd T cells induced lethal GvHD in CXBE but not CXBG recipients: the same pattern induced by B6-wt T cells. This observation is consistent with previous findings that cytotoxic T lymphocyte (CTL) responses against CXBG and CXBE antigens did not correlate with GvH responses in these strains. In contrast with the typical pattern of donor T-cell expansion and contraction, T cells lacking perforin and FasL function exhibited extensive expansion postBMT. In summary, these findings support the notion that donor anti-host cytotoxicity by way of the two major pathways is not a prerequisite for induction of GvHD. In addition, the results suggest that this model will be useful to investigate the regulation of allogeneic donor T-cell expansion after major histocompatibility complex-matched allogeneic BMT.

Stem cell–mediated tolerance inducing strategies in organ transplantation

The scope of possible tools to modulate the recipients immune response towards tolerance induction basically includes deletional and non deletional mechanisms, which are currently targeted by various strategies including monoclonal antibodies, cytokine deviation, chimerism induction and the support of regulating T-cells. Here we summarize the main findings in the field derived from experimental animal studies and currently performed clinical studies. This review focuses to give a clinically relevant overview over relevant tolerance inducing concepts, taking into consideration risk profiles and clinical efficacy associated with specific immunosuppressive regiments currently applied in the clinical setting of transplantation.

Regulation of expression of Bcl-2 protein family member Bim by T cell receptor triggering

Bim, a proapoptotic BH3-only member of the Bcl-2 protein family, is required for central and peripheral deletion of T lymphocytes. Mechanisms regulating Bim activity in T cells remain poorly understood. We show that expression of Bim is up-regulated in human T cells after polyclonal or specific T cell receptor triggering. Induction of Bim was affected by the agonistic potency of MHC:peptide ligands. Peptides that failed to induce Bim expression, failed to induce apoptosis in specific T cells, whereas partially agonistic ligands, which trigger death receptor-independent activation-induced cell death (AICD), induced Bim, but were inefficient in up-regulating Bcl-X(L). Activation of protein kinase C and calcineurin appeared to be necessary and sufficient for Bim up-regulation after T cell receptor ligation. Immunosuppressive drugs known to prevent T cell deletion in vivo, such as cyclosporin A or FK506, blocked Bim up-regulation and rescued T cells from death receptor-independent AICD, whereas rapamycin, which allows the development of stable immunological tolerance, did not exhibit these activities. These results define a new mode of Bim regulation, strongly implicate Bim as a mediator of AICD, and suggest that Bim up-regulation can be targeted to influence the outcome of specific immune responses.

Mechanisms of early peripheral CD4 T-cell tolerance induction by anti-CD154 monoclonal antibody and allogeneic bone marrow transplantation: evidence for anergy and deletion but not regulatory cells

Anti-CD154 (CD40L) monoclonal antibody (mAb) plus bone marrow transplantation (BMT) in mice receiving CD8 cell-depleting mAb leads to long-term mixed hematopoietic chimerism and systemic donor-specific tolerance through peripheral and central deletional mechanisms. However, CD4(+) T-cell tolerance is demonstrable in vitro and in vivo rapidly following BMT, before deletion of donor-reactive CD4 cells is complete, suggesting the involvement of other mechanisms. We examined these mechanisms in more detail. Spot enzyme-linked immunosorbent (ELISPOT) analysis revealed specific tolerization (within 4 to 15 days) of both T helper 1 (Th1) and Th2 cytokine responses to the donor, with no evidence for cytokine deviation. Tolerant lymphocytes did not significantly down-regulate rejection by naive donor-reactive T cells in adoptive transfer experiments. No evidence for linked suppression was obtained when skin expressing donor alloantigens in association with third-party alloantigens was grafted. T-cell receptor (TCR) transgenic mixing studies revealed that specific peripheral deletion of alloreactive CD4 T cells occurs over the first 4 weeks following BMT with anti-CD154. In contrast to models involving anti-CD154 without BMT, BMT with anti-CD154 leads to the rapid induction of anergy, followed by deletion of pre-existing donor-reactive peripheral CD4(+) T cells; the rapid deletion of these cells obviates the need for a regulatory cell population to suppress CD4 cell-mediated alloreactivity.

Pathophysiology of acute graft-versus-host disease

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). The pathophysiology of acute GVHD is complex and can be conceptualized to be a three-step process based on murine studies. In step 1, the conditioning regimen leads to the damage and activation of host tissues and induces the secretion of inflammatory cytokines. As a consequence, the expression of MHC antigens and adhesion molecules is increased enhancing the recognition of host alloantigens by donor T cells. Donor T-cell activation in step 2 is characterized by donor T cell interaction with host APCs and subsequent proliferation, differentiation and secretion of cytokines. Cytokines such as IL-2 and IFN-gamma enhance T-cell expansion, induce cytotoxic T cells (CTL) and natural killer (NK) cell responses and prime additional mononuclear phagocytes to produce TNF-alpha and IL-1. These inflammatory cytokines in turn stimulate production of inflammatory chemokines, thus recruiting effector cells into target organs. In step 3, effector functions of mononuclear phagocytes are triggered via a secondary signal provided by lipopolysaccharide (LPS) that leaks through the intestinal mucosa damaged during step 1. This mechanism may result in the amplification of local tissue injury and further promotion of an inflammatory response, which, together with the CTL and NK components, leads to target tissue destruction in the transplant host. The following review discusses the three-step process of the pathophysiology of experimental acute GVHD.

Allogeneic parenchymal and hematopoietic tissues differ in their ability to induce deletion of donor-reactive T cells

The establishment of immune tolerance to self antigen expressed exclusively in the periphery is a crucial yet incompletely understood feature of the immune system. A dominant concept of peripheral tolerance has been that exposure of T cells to signal one, the TCR-MHC interaction, in the absence of signal two, or costimulation, is a major mechanism of peripheral tolerance. This model suggests that any cell type that expresses MHC-peptide complexes, be they of self or foreign origin, should have the capacity to tolerize antigen-specific T cells when critical costimulatory interactions are interrupted. However, a spectrum of responses, from permanent engraftment to rapid rejection, has been observed in various transplantation models utilizing costimulatory blockade. Therefore we undertook a series experiments to directly assess the tolerogenic potential of donor hematopoietic and parenchymal cells. We find that allogeneic tissues differ profoundly in their ability to promote peripheral tolerance concurrent with combined blockade of B7-CD28 and CD40-CD40L pathways. Non-vascularized and vascularized parenchymal grafts as well as donor-specific transfusions promote varying degrees of donor-specific hyporesponsiveness, but fail to induce donor-reactive T-cell deletion; whereas establishment of stable hematopoietic chimerism promotes specific tolerance mediated by deletion of donor-reactive cells in the periphery.

Long-term survival of intestinal allografts induced by costimulation blockade, busulfan and donor bone marrow infusion

Tolerance-inducing strategies that infuse donor bone marrow cells in conjunction with costimulation blockade have not been applied to intestinal transplantation. Intestines from BALB/c mice were transplanted into C57BL/6 recipients treated with anti-CD40L mAb, CTLA4-Ig, donor bone marrow, and busulfan. The majority of mice transplanted after completion of this regimen developed hematopoietic macrochimerism, although the degree of chimerism varied widely between recipients, and experienced long-term allograft survival. T cells from these mice demonstrated donor-specific hyporesponsiveness in vitro. However, T cells from chimeric mice proliferated to donor alloantigen in vivo. Furthermore, chimeric mice bearing intestinal allografts were capable of rejecting subsequently placed donor-strain skin grafts. These data suggest that although long-term allograft survival occurs in the absence of acute or chronic rejection, recipient mice are not completely unresponsive to donor alloantigens. When intestinal transplantation was performed at the time of initial bone marrow infusion (initiation of the chimerism protocol), most recipients failed to develop chimerism and promptly rejected the intestinal allograft. Although this is the most effective protocol that we have tested using this stringent model of transplantation, our observations suggest that modifications will be necessary before it can be reliably applied to the transplantation of highly immunogeneic organs like the intestine.

Hematopoietic chimerism and central tolerance created by peripheral-tolerance induction without myeloablative conditioning

Allogeneic hematopoietic chimerism leading to central tolerance has significant therapeutic potential. Realization of that potential has been impeded by the need for myeloablative conditioning of the host and development of graft-versus-host disease (GVHD). To surmount these impediments, we have adapted a costimulation blockade-based protocol developed for solid organ transplantation for use in stem cell transplantation. The protocol combines donor-specific transfusion (DST) with anti-CD154 mAb. When applied to stem cell transplantation, administration of DST, anti-CD154 mAb, and allogeneic bone marrow leads to hematopoietic chimerism and central tolerance with no myeloablation and no GVHD. Tolerance in this system results from deletion of both peripheral host alloreactive CD8+ T cells and nascent intrathymic alloreactive CD8+ T cells. In the absence of large numbers of host alloreactive CD8+ T cells, the transfusion that precedes transplantation need not be of donor origin, suggesting that both allospecific and non-allospecific mechanisms regulate engraftment. Agents that interfere with peripheral transplantation tolerance impair establishment of chimerism. We conclude that robust allogeneic hematopoietic chimerism and central tolerance can be established in the absence of host myeloablative conditioning using a peripheral transplantation tolerance protocol.

Induction of tolerance for islet transplantation for type 1 diabetes

Type 1 diabetes is an autoimmune disorder characterized by selective destruction of pancreatic b cells and absolute insulin deficiency. Even when treated well, control is imperfect and complications inevitable. Advances in immunosuppressive drugs and preparation of donor islets have recently made curative islet transplantation a reality for type 1 diabetes. Unfortunately, short-term side effects and long-term health risks of lifelong systemic immunosuppression compromise the otherwise extraordinary benefits that accrue from a successful graft. Our current goal is to obviate the need for immunosuppression and achieve islet graft tolerance. New protocols based on costimulation blockade have brought us close to that goal, inducing states of both peripheral and central transplantation tolerance. These have overcome both allograft rejection and recurrent autoimmunity, but potentially detrimental effects of environmental agents on tolerance are not yet fully understood. Studies of the underlying mechanisms have provided new insights into the nature of both tolerance and autoimmunity.

Mechanisms of tolerance induction through the transplantation of donor hematopoietic stem cells: central versus peripheral tolerance

The transplantation of donor hematopoietic stem cells has been used successfully in numerous experimental settings to induce donor-specific tolerance. After appropriate host conditioning, hematopoietic stem-cell transplantation leads to a lasting state of donor macrochimerism that is associated with a robust form of tolerance. One of the key factors in the success of this approach is its reliance on intrathymic clonal deletion to ensure lifelong tolerization of newly developing T cells. Evidence for ongoing central deletion comes from studies following superantigen-reactive T cells and from experiments using mice transgenic for an alloreactive T-cell receptor. In protocols inducing tolerance through macrochimerism, the preexisting mature T-cell repertoire is controlled by either globally destroying all T cells before the hematopoietic cell transplantation or, in more recent models, by tolerizing it through co-stimulation blockade. The peripheral mechanisms induced by hematopoietic stem-cell transplantation and co-stimulation blockade include both extrathymic clonal deletion and the nondeletional mechanisms anergy, suppression, or both. In addition to these immunologic hurdles, a physiologic engraftment barrier has to be surmounted for the successful induction of mixed chimerism. This can be achieved by cytoreductive host treatment or by the infusion of high numbers of donor hematopoietic cells. A detailed delineation of the mechanisms responsible for tolerance induction after hematopoietic stem-cell transplantation is expected to help in the translation of these experimental protocols to clinical organ transplantation.

The influence of immunosuppressive drugs on tolerance induction through bone marrow transplantation with costimulation blockade

We recently developed a murine protocol for the induction of allogeneic mixed chimerism and tolerance employing nonmyeloablative total body irradiation (TBI), standard-dose bone marrow transplantation (BMT), and costimulation blockade (cobl) with an anti-CD154 monoclonal antibody (mAb) plus CTLA4Ig. We now evaluated whether a short course (1 month) of immunosuppressive drugs, which would be ethically required in the clinical setting of organ transplantation to prevent graft loss in case tolerance is not achieved, interferes with tolerance induced with this regimen. Our results show that calcineurin inhibitors (cyclosporin A [CyA] or tacrolimus [FK]) inhibit development of long-term chimerism and abrogate tolerance induction in this model. Rapamycin (rapa), methylprednisolone (MP), FTY720, and mycophenolate mofetil (MMF), in contrast, have no negative effect on chimerism or tolerance development. Peripheral deletion of donor-reactive T cells, which usually occurs in the weeks following BMT in this model, is blocked by CyA and FK, but not by the other drugs tested. Furthermore, we found that the additional use of compatible immunosuppressive drugs (rapa plus MMF plus MP) allows the dose of TBI to be reduced, so that mixed chimerism and donor skin-graft acceptance can be achieved with 1 Gy using clinically feasible cell numbers. Thus, this protocol of BMT with costimulation blockade can be safely combined with a clinically tested immunosuppressive regimen to permit success with a lower dose of irradiation. These results should facilitate clinical application of this tolerance strategy.

The effect on the proliferation and apoptosis of alloreactive T cells of cell dose in a murine MHC-mismatched hematopoietic cell transplantation model

Activation-induced cell death (AICD) of lymphocytes is an apoptotic pathway involved in the control of T-cell homeostasis. The magnitude of graft-vs.-host disease (GVHD) following allogeneic hematopoietic cell transplantation may be attenuated by the enhancement of AICD. The aim of the present study was to clarify the effect of T cell dose upon the fate (proliferation or apoptosis) of individual activated T cells in a murine GVHD model. To this end, we investigated the kinetics of the proliferation and apoptosis of donor T cells in recipient spleens in the early stage of a fully major histocompatibility complex (MHC)-mismatched murine transplantation model from C57BL/6 (H-2(b)) to lethally-irradiated (8.5 Gy) BALB/c (H-2(d)) mice. To track the behavior of alloreactive lymphocytes in vivo, we used the fluorescent cytoplasmic dye carboxyfluorescein diacetate succinimidyl ester in combination with flow cytometry. Engraftment of donor T cells to recipient spleens was almost completed within 24 h after transfer. After that, at higher doses of transferred cells, the donor T cells actively divided for up to 72 h resulting in a 30-fold increase in cell number at the maximum cell dose (2.0 x 10(7)). As the transferred cell dose decreased, the proliferation of T cells tended to be suppressed. At cell doses of 0.5 x 10(7) or less, the proliferation of T cells was profoundly suppressed, ultimately resulting in little proliferation of donor T cells observed from 24 to 72 h at the minimum cell dose (0.1 x 10(7)). The frequency of Annexin-V-positive cells was found to increase gradually as the transferred cell dose decreased. Thus, an increase in apoptotic events appeared to play an important role in the suppression of the proliferation of T cells at lower splenocyte doses. Further analyses revealed that Fas ligand (FasL)-positive T cells were observed exclusively among T cells that divided at least 5 times, and that all of them were positive for Annexin-V, indicating that they were in the process of apoptosis. Together with our finding that the frequency of apoptosis increased with the progression of cell division, these findings strongly suggest that AICD occurred through the Fas/FasL system and that AICD increased as the dose of donor T cells participating in the allogeneic response decreased. When relatively small numbers of T cells are confronted with an excess of antigen, they disappear. This process is called clonal exhaustion-deletion. Our results support the idea that AICD is involved in the process of clonal exhaustion-deletion. Relevant to the clinical aspects of hematopoietic cell transplantation, our findings indicate that AICD may be associated with tolerance induction in T-cell-depleted transplantation from HLA-mismatched donors, in which T cells contaminating marrow grafts do not need to be completely removed for achieving tolerance between donors and recipients. Furthermore, our results indicate that a small change in the quantitative balance between antigens and T cells responding to them leads to a large difference in the fate of T cells activated in response to MHC-incompatible antigens. Thus, the size of the T cell dose is one of the important considerations in tolerance induction, GVHD and rejection.

Stem cells engineered to express self-antigen to treat autoimmunity

Autoimmune diseases are incurable. We propose a combination curative strategy using haematopoietic stem cells and gene therapy. The strategy should establish molecular chimerism with culprit antigen expressed by donor antigen-presenting cells in the thymus, resulting in immunological tolerance. Our strategy is based on the intrathymic tolerance and disease resistance established by transgenic expression of antigen in the thymus under control of an MHC class II promoter. We propose that transplantation with genetically engineered stem cells provides an advance over current autologous bone marrow transplantation, in that it should remove the significant risk of disease relapse.

The complementary roles of deletion and regulation in transplantation tolerance

Neonatal tolerance of alloantigens was described in mice nearly half a century ago, but unfortunately, the translation of these early findings into the clinical arena proved to be much more challenging than was first anticipated. However, the past decade has seen considerable progress in our understanding of the mechanisms that contribute to transplantation tolerance in experimental models. This review outlines our current understanding of the mechanisms of allograft tolerance, emphasizing the complementary roles of deletion and regulation of alloreactive T cells.

Mechanisms of transplant tolerance induction using costimulatory blockade

The potential use of costimulation-blocking reagents to induce transplantation tolerance has recently created considerable excitement. Recent evidence has begun to delineate the mechanisms by which these powerful effects occur. It has become increasingly clear, firstly, that T cell costimulation is mediated by a delicate network of signaling pathways and, secondly, that interference with these systems can lead to numerous different tolerance mechanisms, including immune regulation, anergy and deletion.

Th1 cytokines, programmed cell death, and alloreactive T cell clone size in transplant tolerance

The Th1 cytokines IL-2 and IFN-gamma, which inhibit T cell proliferation and promote activation induced cell death, may be required to diminish alloreactive T cell numbers and to foster tolerance across full allogeneic barriers. However, we hypothesized that these cytokines might be dispensable when the alloreactive T cell clone size is relatively small, as is seen in recipients of minor-mismatched grafts. We show that alloreactive T cell clone size of C57BL/6 mice against multiple minor-mismatched 129X1/sv mice was approximately 4-9-fold smaller than that against MHC-mismatched BALB/c mice. In the MHC-mismatched combination, CD28-B7 blockade by CTLA4Ig induced long-term graft survival in wild-type recipients, but this treatment was ineffective in IFNgamma(-/-) or IL-2(-/-) recipients. In contrast, in the minor-mismatched combination, CTLA4Ig induced long-term allograft survival in wild-type, IFNgamma(-/-), and IL-2(-/-) recipients. Bcl-x(L) transgenic animals, which are defective in "passive" T cell death, are likewise sensitive to the effects of CTLA4Ig only in the setting of the minor-mismatch grafts. Therefore, the alloreactive T cell clone size is an important determinant affecting the need for Th1 cytokines and T cell death in tolerance induction. These data have implications for the design of tolerance strategies in transplant recipients with varying degrees of MHC mismatching.

Th1 cytokines, programmed cell death, and alloreactive T cell clone size in transplant tolerance

The Th1 cytokines IL-2 and IFN-gamma, which inhibit T cell proliferation and promote activation induced cell death, may be required to diminish alloreactive T cell numbers and to foster tolerance across full allogeneic barriers. However, we hypothesized that these cytokines might be dispensable when the alloreactive T cell clone size is relatively small, as is seen in recipients of minor-mismatched grafts. We show that alloreactive T cell clone size of C57BL/6 mice against multiple minor-mismatched 129X1/sv mice was approximately 4-9-fold smaller than that against MHC-mismatched BALB/c mice. In the MHC-mismatched combination, CD28-B7 blockade by CTLA4Ig induced long-term graft survival in wild-type recipients, but this treatment was ineffective in IFNgamma(-/-) or IL-2(-/-) recipients. In contrast, in the minor-mismatched combination, CTLA4Ig induced long-term allograft survival in wild-type, IFNgamma(-/-), and IL-2(-/-) recipients. Bcl-x(L) transgenic animals, which are defective in "passive" T cell death, are likewise sensitive to the effects of CTLA4Ig only in the setting of the minor-mismatch grafts. Therefore, the alloreactive T cell clone size is an important determinant affecting the need for Th1 cytokines and T cell death in tolerance induction. These data have implications for the design of tolerance strategies in transplant recipients with varying degrees of MHC mismatching.

Role for thymic and splenic regulatory CD4+ T cells induced by donor dendritic cells in allograft tolerance by LF15-0195 treatment

A 20-day treatment with LF15-0195, a deoxyspergualine analogue, induced allograft tolerance in a fully MHC-mismatched heart allograft model in the rat. Long-term allografts displayed minimal cell infiltration with no signs of chronic rejection. CD4+ spleen T cells from tolerant LF15-0195-treated recipients were able to suppress in vitro proliferation of allogeneic CD4+ T cells and to transfer tolerance to second syngeneic recipients, demonstrating dominant suppression by regulatory cells. A significant increase in the percentage of CD4+CD25+ T cells was observed in the thymus and spleen from tolerant LF15-0195-treated recipient. In vitro direct stimulation with donor APCs demonstrated that CD4+ regulatory T cells proliferated weakly and expressed low levels of IFN-gamma, IL-10, and IL-2. CD4+CD25+ cell depletion increased IL-2 production by CD4+CD25- thymic cells, but not splenic cells. Moreover, tolerance was transferable with splenic and thymic CD4+CD25+ cells, but also in 50% of cases with splenic CD4+CD25- cells, demonstrating that CD25 can be a marker for regulatory cells in the thymus, but not in the periphery. In addition, we presented evidences that donor APCs were required to induce tolerance and to expand regulatory CD4+ T cells. This study demonstrates that LF15-0195 treatment induces donor APCs to expand powerful regulatory CD4+CD25+/- T cells present in both the central and peripheral compartments.

Tolerance through bone marrow transplantation with costimulation blockade

The routine induction of tolerance in organ transplant recipients remains an unattained goal. The creation of a state of mixed chimerism through allogeneic bone marrow transplantation leads to robust donor-specific tolerance in several experimental models and this approach has several features making it attractive for clinical development. One of its major drawbacks, however, has been the toxicity of the required host conditioning. The use of costimulation blocking reagents (anti-CD 154 monoclonal antibodies and the fusion protein CTLA4Ig) has led to much less toxic models of mixed chimerism in which global T cell depletion of the host is no longer necessary and which has even allowed the elimination of all cytoreductive treatment when combined with the injection of very high doses of bone marrow cells. In this overview we will briefly discuss general features of tolerance induction through bone marrow transplantation, will then describe recent models using costimulation blockade to induce mixed chimerism and will review the mechanisms of tolerance found with these regimens. Finally we will attempt to identify issues related to the clinical introduction of bone marrow transplantation with costimulation blockade which remain unresolved.

Biology and significance of T-cell apoptosis in the liver

The liver has emerged as an organ with distinct immunological properties. In this review, we summarize evidence that shows that the liver can remove apoptotic, or non-apoptotic but activated, CD8+ T cells from the circulation and induce apoptosis in these activated T cells by either active or passive mechanisms. Hepatitis viruses, particularly hepatitis C virus, often establish persistent infection. We review evidence that suggests that these viruses exploit intrahepatic tolerance mechanisms to protect themselves from immune attack.

The CD154-CD40 costimulatory pathway in transplantation

The CD154-CD40 pathway is one of the critical costimulatory pathways that are required for full activation of T cells during alloimmune responses. Blockade of this pathway with anti-CD154 antibodies has been reported to prolong allograft survival in experimental transplantation models and to induce tolerance in some instances. However, anti-CD154 monotherapy cannot induce tolerance in "stringent" models such as skin and islet transplantation and is not sufficient to prevent chronic graft vasculopathy in vascularized organ transplantation. Therefore, combined therapies of anti-CD154 antibodies plus donor-specific transfusion, bone marrow infusion, or B7 blockade by CTLA4-Ig have been tried, and synergistic effects for tolerance induction have been reported. Furthermore, the efficacy of CD154 blockade in primate models has been confirmed for islet and kidney transplantation. The mechanisms of CD154 blockade in vivo include CTLA4-dependent anergy or regulation, T-cell apoptosis, and induction of regulatory cells. Finally, anti-CD154 antibody therapy has been reported to result in unexpected thromboembolic complications in both primates and humans, although the etiology of these conditions remains unclear. In addition, not all antibodies cause this side effect. Clinical trials with humanized anti-CD154 monoclonal antibodies are underway in severe autoimmune diseases, but its development in transplantation is unclear at this time.

Thymic re-entry of mature activated T cells and increased negative selection in vascularized allograft recipients

Transplantation tolerance is a dynamic state that involves several homeostatic mechanisms intrinsic to the host. One of these mechanisms is activation-induced T cell death (AICD). However, it is unclear where AICD takes place during alloreactive responses. Since activated T cells can re-enter the thymus, we hypothesized that mature T cells activated by an allograft could be deleted upon re-entry into the thymus. To test this hypothesis, we used wild-type or 2C TCR transgenic mice receiving syngeneic or allogeneic heterotopic, vascularized heart grafts. First, we demonstrated that ex vivo CFSE-labelled T cells re-entered the thymus when transferred into allograft recipients but not when transferred into isograft recipients. Next, we compared the changes in cell subset numbers and incidence of apoptosis in the thymi and spleens of allograft or isograft recipients. Seven days after transplantation, at a time in which all the allografts were undergoing rejection, cells expressing donor-MHC class II molecules had migrated to the thymus and to the spleen. In the thymus of allograft recipients, overall cellularity was significantly reduced by 40% and associated with an increase in the number of double negative (CD4-CD8-) thymocytes and a decrease in double positive (CD4+CD8+) thymocytes, consistent with increased negative selection of thymocytes. Additionally, thymi of allograft recipients showed an increase in the number of recently activated, mature T cells (TCRhi, CD25+, CD44+) and a significant increase in the number of apoptotic cells, especially in the thymic medulla, that involved mature T cells as indicated by the TCRhi, CD44+, CD4 or CD8 single positive phenotype. Spleens of allograft recipients were increased in size and cellularity but did not show any of the changes in cell subsets seen in the thymi. Our data show that after allografting there is an increase in apoptotic cell death that is associated with negative selection of developing thymocytes as well as of alloreactive mature T cells that have re-entered the thymus upon activation in the periphery. This may occur upon migration of graft-derived antigen-presenting cells to the thymus.

Understanding the alloresponse: new approaches to graft-versus-host disease prevention

Graft-versus-host disease (GVHD) has been the primary limitation to the wider application of allogeneic bone marrow transplantation (BMT). GVHD occurs when donor T cells react to host antigens on antigen-presenting cells (APCs) and attack host tissues, with sequential activation of donor T cells and monocytes/macrophages. The net effects of dysregulated cytokine production in this complex system are the severe inflammatory manifestations that we recognize as clinical acute GVHD. Long-term outcomes are also adversely affected by chronic GVHD, which has distinctive clinical and pathologic manifestations that mimic autoimmune disease, although its exact pathogenesis remains ambiguous. The ultimate goal for preventing GVHD is the induction of specific tolerance to host antigens, thereby maintaining favorable aspects of donor immunity. Tolerance may be achieved by costimulatory blockade, deletion of activated cells, suppression by regulatory T cells, and immune deviation. This report will focus on these mechanisms as they relate to the pathophysiology of acute GVHD.

Mechanisms involved in the establishment of tolerance through costimulatory blockade and BMT: lack of requirement for CD40L-mediated signaling for tolerance or deletion of donor-reactive CD4+ cells

We have previously shown that high levels of multiline-age mixed hematopoietic chimerism and systemic T-cell tolerance can be achieved in mice without myeloablation through the use of anti-CD40L and costimulatory blockade alone (plus CTLA4Ig) or with recipient CD8 depletion and allogeneic bone marrow transplantation. Chimeric mice permanently accept donor skin grafts (> 100 days), and rapidly reject third-party grafts. The mechanisms by which costimulatory blockade facilitates the engraftment of allogeneic hematopoietic cells have not been defined. To further understand the in vivo mechanisms by which the administration of anti-CD40L mAb facilitates the engraftment of donor bone marrow and rapidly tolerizes CD4+ T cells, we analyzed the establishment of chimerism and tolerance in CD40L -/- mice. We demonstrate here that anti-CD40L mAb treatment is required only to prevent CD40L/CD40 interactions, and that no signal to the T cell through CD40L is necessary for the induction of CD4+ tolerance. Peripheral deletion of donor-reactive CD4+ T cells occurs rapidly in CD40L -/- mice receiving bone marrow transplantation (BMT), indicating that this deletion in the presence of anti-CD40L is not due to targeting of activated CD4+ cells by the antibody. Complete CD4+ cell tolerance is observed by both skin graft acceptance and in vitro assays before deletion is complete, indicating that additional mechanisms play a role in inducing CD4+ T-cell tolerance as the result of BMT in the presence of CD40/CD40L blockade.

Therapeutic approaches for transplantation

Developments in marrow and organ transplantation are mutually interactive. There have been several recent advances in stem cell transplantation: to ensure engraftment using larger doses of stem cells; to substantially reduce the incidence of graft-versus-host disease and marrow rejection using monoclonal antibodies; and to reduce toxicity of the preparative regimen through use of so-called nonmyeloablative regimens (mini-transplants). These advances may pave the way for generation of mixed hemopoietic chimerism as an aid to achieving tolerance to organ transplants. The use of short courses of T-cell-depleting antibodies, such as CD3 immunotoxin in primates and CAMPATH-1H in humans, has demonstrated that long-term graft survival may be possible without substantive long-term immunosuppressive treatment of the recipient. The demonstration in rodents that nondepleting antibodies to T cells can give rise to powerful regulatory mechanisms that maintain tolerance to grafts has initiated a major research effort in understanding how these regulatory T cells work, with the prospect of new therapeutic modalities to mimic or enhance their function.