Requirement for T-cell apoptosis in the induction of peripheral transplantation tolerance

Successful conversion from conventional immunosuppression to anti-CD154 monoclonal antibody costimulatory molecule blockade in rhesus renal allograft recipients

BACKGROUND

Several conventional forms of immunosuppression have been shown to antagonize the efficacy of anti-CD154 monoclonal antibody- (mAb) based costimulatory molecule blockade immunotherapy. Our objective was to determine if allograft recipients treated with a conventional immunosuppressive regimen could be sequentially converted to anti-CD154 mAb monotherapy without compromising graft survival.

METHODS

Outbred juvenile rhesus monkeys underwent renal allotransplantation from MHC-disparate donors. After a 60-day course of triple therapy immunosuppression with steroids, cyclosporine, and mycophenolate mofetil, monkeys were treated with: (1) cessation of all immunosuppression (control); (2) seven monthly doses of 20 mg/kg hu5C8 (maintenance), or; (3) 20 mg/kg hu5C8 on posttransplant days 60, 61, 64, 71, 79, and 88 followed by five monthly doses (induction+maintenance). Graft rejection was defined by elevation in serum creatinine>1.5 mg/dl combined with histologic evidence of rejection.

RESULTS

Graft survival for the three groups were as follows: group 1 (control): 70, 75, >279 days; group 2 (maintenance): 83, 349, >293 days, and; group 3 (induction+maintenance): 355, >377, >314 days. Acute rejection developing in two of four monkeys after treatment with conventional immunosuppression was successfully reversed with intensive hu5C8 monotherapy.

CONCLUSIONS

Renal allograft recipients can be successfully converted to CD154 blockade monotherapy after 60 days of conventional immunosuppression. An induction phase of anti-CD154 mAb appears to be necessary for optimal conversion. Therefore, although concurrent administration of conventional immunosuppressive agents including steroids and calcineurin inhibitors has been shown to inhibit the efficacy of CD154 blockade, sequential conversion from these agents to CD154 blockade appears to be effective.

Regulatory T cells induced by 1 alpha,25-dihydroxyvitamin D3 and mycophenolate mofetil treatment mediate transplantation tolerance

1alpha,25-dihydroxyvitamin D3, the active form of vitamin D3, and mycophenolate mofetil, a selective inhibitor of T and B cell proliferation, modulate APC function and induce dendritic cells (DCs) with a tolerogenic phenotype. Here we show that a short treatment with these agents induces tolerance to fully mismatched mouse islet allografts that is stable to challenge with donor-type spleen cells and allows acceptance of donor-type vascularized heart grafts. Peritransplant macrophages and DCs from tolerant mice express down-regulated CD40, CD80, and CD86 costimulatory molecules. In addition, DCs from the graft area of tolerant mice secrete, upon stimulation with CD4+ cells, 10-fold lower levels of IL-12 compared with DCs from acutely rejecting mice, and induce a CD4+ T cell response characterized by selective abrogation of IFN-gamma production. CD4+ but not CD8+ or class II+ cells from tolerant mice, transferred into naive syngeneic recipients, prevent rejection of donor-type islet grafts. Graft acceptance is associated with impaired development of IFN-gamma-producing type 1 CD4+ and CD8+ cells and an increased percentage of CD4+CD25+ regulatory cells expressing CD152 in the spleen and in the transplant-draining lymph node. Transfer of CD4+CD25+ cells from tolerant but not naive mice protects 100% of the syngeneic recipients from islet allograft rejection. These results demonstrate that a short treatment with immunosuppressive agents, such as 1alpha,25-dihydroxyvitamin D3/mycophenolate mofetil, induces tolerance to islet allografts associated with an increased frequency of CD4+CD25+ regulatory cells that can adoptively transfer transplantation tolerance.

Complexities of CD28/B7: CTLA-4 costimulatory pathways in autoimmunity and transplantation

Recent advances in the understanding of T cell activation have led to new therapeutic approaches in the treatment of immunological disorders. One attractive target of intervention has been the blockade of T cell costimulatory pathways, which result in more selective effects on only those T cells that have encountered specific antigen. In fact, in some instances, costimulatory pathway antagonists can induce antigen-specific tolerance that prevents the progression of autoimmune diseases and organ graft rejection. In this review, we summarize the current understanding of these complex costimulatory pathways including the individual roles of the CD28, CTLA-4, B7-1 (CD80), and B7-2 (CD86) molecules. We present evidence that suggests that multiple mechanisms contribute to CD28/B7-mediated T cell costimulation in disease settings that include expansion of activated pathogenic T cells, differentiation of Th1/Th2 cells, and the migration of T cells into target tissues. Additionally, the negative regulatory role of CTLA-4 in autoimmune diseases and graft rejection supports a dynamic but complex process of immune regulation that is prominent in the control of self-reactivity. This is most apparent in regulation of the CD4(+)CD25(+)CTLA-4(+) immunoregulatory T cells that control multiple autoimmune diseases. The implications of these complexities and the potential for use of these therapies in clinical immune intervention are discussed.

Transgenic expression of CD95 ligand on thyroid follicular cells confers immune privilege upon thyroid allografts

Constitutive Fas ligand (FasL) expression by specialized cells in the body participates in the immune privilege status of tissues containing these cells. This property has been used to prevent rejection of allogeneic grafts. Nevertheless, the mechanism responsible for such protection has not been fully elucidated. Unfortunately, grafting of FasL transgenic (TG) tissues has been unsuccessful. We have generated TG mice expressing FasL (soluble + membrane bound) on thyroid follicular cells (TFC), and used them to show that ectopic FasL expression prevents thyroid allograft rejection. FasL expression on TFC led to markedly decreased anti-allogeneic, cytotoxic, and helper T lymphocyte activities. The alloantibody response in TG thyroid recipients was either completely inhibited or switched toward a T2-Ab response. Surprisingly, the beneficial effect of FasL on TG thyroid grafts was abolished by host CD4(+) T cell depletion. Host CD8(+) T cell depletion improved nontransgenic (NTG), but not TG graft survival. Altogether, our results suggest that FasL-induced tolerance is concomitant with a move away from a T1 type response, and a CD4 T cell-mediated regulation of the allocytotoxic T cell response. These results were dependent upon the level of FasL expression on TFC, in that low expression of FasL led to a less marked effect compared with the effect observed with high expression of FasL. These results provide some insight into the role of FasL in regulating destructive alloimmune responses in the case of whole organ grafting, and they have important implications for the development of FasL-based immunotherapy in organ transplantation.

Requirements for the promotion of allogeneic engraftment by anti-CD154 (anti-CD40L) monoclonal antibody under nonmyeloablative conditions

The promotion of alloengraftment in the absence of global immune suppression and multiorgan toxicity is a major goal of transplantation. It is demonstrated that the infusion of a single modest bone marrow dosage in 200 cGy-irradiated recipients treated with anti-CD154 (anti-CD40L) monoclonal antibody (mAb) resulted in chimerism levels of 48%. Reducing irradiation to 100 or 50 cGy permitted 24% and 10% chimerism, respectively. In contrast, pan-T-cell depletion resulted in only transient engraftment in 200 cGy-irradiated recipients. Host CD4(+) cells were essential for alloengraftment as depletion of CD4(+) cells abrogated engraftment in anti-CD154-treated recipients. Strikingly, the depletion of CD8(+) cells did not further enhance engraftment in anti-CD154 mAb-treated recipients in a model in which rejection is mediated by both CD4(+) and CD8(+) T cells. However, anti-CD154 mAb did facilitate engraftment in a model in which only CD8(+) T cells mediate rejection. Furthermore, CD154 deletional mice irradiated with 200 cGy irradiation were not tolerant of grafts, suggesting that engraftment promotion by anti-CD154 mAb may not simply be the result of CD154:CD40 blockade. Together, these data suggest that a CD4(+) regulatory T cell may be induced by anti-CD154 mAb. In contrast to anti-CD154 mAb, anti-B7 mAb did not promote donor engraftment. Additionally, the administration of either anti-CD28 mAb or anti-CD152 (anti-CTLA-4) mAb or the use of CD28 deletional recipients abrogated engraftment in anti-CD154 mAb-treated mice, suggesting that balanced CD28/CD152:B7 interactions are required for the engraftment-promoting capacity of anti-CD154 mAb. These data have important ramifications for the design of clinical nonmyeloablative regimens based on anti-CD154 mAb administration.

A short course of methylprednisolone immunosuppression inhibits both rejection and spontaneous acceptance of rat liver allografts

BACKGROUND

The effects of immunosuppressive drugs on transplant tolerance have not been extensively studied, although their effect on rejection is well established.

METHODS

We examined the effects of a short course of treatment with the immunosuppressive drug methylprednisolone (MP) on the survival of PVG liver allografts in Dark Agouti (DA) recipients that accepted the livers and in Lewis recipients that rejected the livers. Infiltration of liver allografts was examined by immunohistochemical staining of liver sections, and apoptosis was measured by terminal deoxynucleotide transferase-mediated dUTP nick end labeling.

RESULTS

A 5-day course of MP (days 0 to 4) led to rejection of four of six livers (mean survival time [MST] 99 days) in DA recipients compared with long-term survival (MST >100 days) in untreated animals. Delayed administration of MP (days 3 to 7) exacerbated rejection in DA recipients, and all eight animals rejected the graft (MST 68.5 days). Treatment of Lewis recipients with MP did not significantly prolong survival when administered from days 0 to 4 (MST 13 days), although delay of administration improved the outcome. Treatment from days 3 to 7 resulted in an MST of 21 days, whereas treatment from days 7 to 11 resulted in an MST of 41.5 days. MP treatment from day 3 to day 7 reduced T cells and interleukin 2 receptor-expressing cells but increased the numbers of apoptotic cells infiltrating both DA and Lewis strain allografts.

CONCLUSIONS

These results show that immunosuppression with MP inhibits both spontaneous tolerance and rejection of liver allografts in a rat model and question the efficacy of administering MP to all liver allograft recipients from the time of transplantation.

Rapamycin induces apoptosis in monocyte- and CD34-derived dendritic cells but not in monocytes and macrophages

Rapamycin (Rapa), a recently introduced immunosuppressive drug, seems to be effective in preventing acute allograft rejection. Although its antiproliferative effect on T lymphocytes has been investigated extensively, its effect on the initiators of the immune response, the dendritic cells (DCs), is not known. Therefore, the effect of Rapa on monocyte- (mo-DCs) and CD34(+)-derived DCs in vitro but also on other myeloid cell types, including monocytes and macrophages, was examined. The present study shows that Rapa does not affect phenotypic differentiation and CD40L-induced maturation of mo-DCs. However, Rapa dramatically reduced cell recovery (40%-50%). Relatively low concentrations of Rapa (10(-9) M) induced apoptosis in both mo-DCs and CD34(+)-derived DCs, as visualized by phosphatidylserine exposure, nuclear condensation and fragmentation, and DNA degradation. In contrast, Rapa did not affect freshly isolated monocytes, macrophages, or myeloid cell lines. The sensitivity to Rapa-induced apoptosis was acquired from day 2 onward of mo-DC differentiation. Rapa exerts its apoptotic effect via a reversible binding to the cytosolic receptor protein FKBP-12, as demonstrated in competition experiments with FK506, which is structurally related to Rapa. Partial inhibition of Rapa-induced apoptosis was obtained by addition of ZVAD-fmk, which implies caspase-dependent and caspase-independent processes. The fact that Rapa exerts a specific effect on DCs but not on monocytes and macrophages might contribute to the unique actions of Rapa in the prevention of allograft rejection and other immune responses.

Advances in transplantation tolerance

Immunosuppressive drugs developed in the past two decades have improved the short-term survival of organ allografts, but tolerance has not been achieved and almost all transplant recipients continue to require drugs throughout life. Graft rejection arises from the cognate interaction of T cells with antigen-presenting cells, the recognition of alloantigen through the T-cell receptor, and the delivery of accessory stimulation signals. Once activated by the specific antigen, replicating T cells die if they are re-exposed to the same antigen. Since depletion of antigen-activated T cells is one critical mechanism of transplantation tolerance, drugs such as ciclosporin that interfere with activation-induced T-cell death could inhibit tolerance, whereas drugs such as mycophenolate mofetil, that induce the death of activated T cells, could facilitate tolerance. Other tolerance mechanisms depend on inactivation rather than elimination of allograft reactive T cells. When antigen recognition occurs without costimulation through the CD28 and CD154 accessory receptors, or in absence of cell division, T cells become unresponsive. Thus, inhibitors of CD28 and CD154, and inhibition of T-cell division by rapamycin promotes transplantation tolerance.

Western and Chinese antirheumatic drug-induced T cell apoptotic DNA damage uses different caspase cascades and is independent of Fas/Fas ligand interaction

Spontaneous or therapeutic induction of T cell apoptosis plays a critical role in establishing transplantation tolerance and maintaining remission of autoimmune diseases. We investigated the mechanisms of apoptosis induced by Chinese and Western antirheumatic drugs (ARDs) in human T cells. We found that hydroxychloroquine, Tripterygium wilfordii hook F, and tetrandrine (Tet), but not methotrexate, at therapeutic concentrations can cause T cell death. In addition, Tet selectively killed T cells, especially activated T cells. Although ARD-induced cytotoxicity was mediated through apoptotic mechanisms, Fas/Fas ligand interaction was not required. We further demonstrated that the processes of phosphatidylserine externalization and DNA damage along the ARD-induced T cell apoptotic pathway could operate independently, and that selective inhibition of DNA damage by caspase inhibitors did not prevent T cells from undergoing cell death. Moreover, we found that Tet- and Tripterygium wilfordii hook F-induced T cell DNA damage required caspase-3 activity, and hydroxychloroquine-induced T cell DNA damage was mediated through a caspase-3- and caspase-8-independent, but Z-Asp-Glu-Val-Asp-fluomethyl ketone-sensitive, signaling pathway. Finally, the observation that ARD-induced activation of caspase-3 in both Fas-sensitive and Fas-resistant Jurkat T cells indicates that Fas/Fas ligand interaction plays no role in ARD-induced T cell apoptosis. Our observations provide new information about the complex apoptotic mechanisms of ARDs, and have implications for combining Western and Chinese ARDs that have different immunomodulatory mechanisms in the therapy of autoimmune diseases and transplantation rejection.

mTOR inhibitors: an overview

Inhibitors of the mammalian target of rapamycin are a new class of immunosuppressants. In contrast to other macrolides, such as tacrolimus and cyclosporine A, they do not inhibit calcineurin and thus signal I of T-cell activation. By inhibiting signal III, the mechanism of action and side effects of sirolimus (rapamycin) and its derivative RAD are distinct from other immunosuppressants. Reports of synergism with cyclosporine A and tacrolimus in preclinical and clinical studies, avoidance of nephrotoxicity, and possible treatment or prevention of chronic allograft rejection are leading to high expectations for this new class of immunosuppressants. Furthermore, studies evaluating tolerance induction are being conducted. This review summarizes preclinical and clinical results published to date and exploits the future value of sirolimus and RAD for clinical transplantation.

TGF-beta released by apoptotic T cells contributes to an immunosuppressive milieu

T cell apoptosis is critical to development and homeostasis of the immune system. The most salient feature of apoptosis is the lack of an attendant inflammatory response or tissue damage. Here, we present evidence that apoptotic T cells release TGF-beta, thereby contributing to an immunosuppressive milieu. Apoptotic T cells released not only latent but also bio-active TGF-beta. Nonetheless, TGF-beta transcription was not upregulated, suggesting release of existing rather than synthesis of new TGF-beta. Localized within the intracellular membrane-bound compartment, which includes mitochondria, TGF-beta was redistributed into the cytosol following loss of mitochondrial membrane potential. TGF-beta secreted from apoptotic T cells inhibited proinflammatory cytokine production by activated macrophages to foster immune suppression. These findings broaden the potential mechanisms whereby induction of immune tolerance or deficiency occurs through T cell deletion.

Long-term induction of immune tolerance after blockade of CD40-CD40L interaction in a mouse model of hemophilia A

A factor VIII-deficient knockout mouse was used as a model for severe hemophilia A to characterize the immune response to recombinant human factor VIII (fVIII) and to study new approaches for induction of immune tolerance to fVIII. Mice initially received periodic injections of fVIII in doses similar to those used for the treatment of human hemophilia A. To induce immune tolerance, a hamster monoclonal antibody specific for murine CD40 ligand (CD40L or CD154) was injected with fVIII. Control mice received fVIII alone or fVIII and hamster immunoglobulin G. After treatment, humoral and cellular immune responses were evaluated. Ninety-five percent of anti-CD40L-treated mice had lower titers of anti-fVIII antibody (less than 1 microg/mL) compared with fVIII-injected control mice (mean, 18 microg/mL). To determine whether anti-CD40L treatment induces long-term immune tolerance, mice were rechallenged 3 times with fVIII alone. At 150 days after treatment, 12 of 22 anti-CD40L-treated mice remained tolerant to fVIII (anti-fVIII antibody titers less than 1 microg/mL). However, tolerant mice immunized with tetanus toxoid (TT) developed high anti-TT antibody, demonstrating that tolerance is fVIII specific. T cells from tolerant mice showed impaired proliferative responses after stimulation with fVIII in vitro and lack of production of the cytokines interleukin-2 (IL-2), IL-4, interferon gamma, and IL-10. These results demonstrate that long-term immune tolerance to fVIII was effectively induced after early blockade of CD40-CD40L interaction. In addition, the lack of tolerance in this model was associated with the expression of a Th2 phenotype.

Advances in transplant immunobiology

The present review briefly addresses the most recent knowledge acquired in the field of transplant immunology. A particular emphasis is placed on articles published during the past 12-18 months that have focused on allorecognition, dendritic cells and tolerance.

Posttransplant administration of donor leukocytes induces long-term acceptance of kidney or liver transplants by an activation-associated immune mechanism

Donor leukocytes play a dual role in rejection and acceptance of transplanted organs. They provide the major stimulus for rejection, and their removal from the transplanted organ prolongs its survival. Paradoxically, administration of donor leukocytes also prolongs allograft survival provided that they are administered 1 wk or more before transplantation. Here we show that administration of donor leukocytes immediately after transplantation induced long-term acceptance of completely MHC-mismatched rat kidney or liver transplants. The majority of long-term recipients of kidney transplants were tolerant of donor-strain skin grafts. Acceptance was associated with early activation of recipient T cells in the spleen, demonstrated by a rapid increase in IL-2 and IFN-gamma at that site followed by an early diffuse infiltrate of activated T cells and apoptosis within the tolerant grafts. In contrast, IL-2 and IFN-gamma mRNA were not increased in the spleens of rejecting animals, and the diffuse infiltrate of activated T cells appeared later but resulted in rapid graft destruction. These results define a mechanism of allograft acceptance induced by donor leukocytes that is associated with activation-induced cell death of recipient T cells. They demonstrate for the first time that posttransplant administration of donor leukocytes leads to organ allograft tolerance across a complete MHC class I plus class II barrier, a finding with direct clinical application.

Anergy induction by dimeric TCR ligands

T cells that recognize particular self Ags are thought to be important in the pathogenesis of autoimmune diseases. In multiple sclerosis, susceptibility is associated with HLA-DR2, which can present myelin-derived peptides to CD4(+) T cells. To generate molecules that target such T cells based on the specificity of their TCR, we expressed a soluble dimeric DR2-IgG fusion protein with a bound peptide from myelin basic protein (MBP). Soluble, dimeric DR2/MBP peptide complexes activated MBP-specific T cells in the absence of signals from costimulatory or adhesion molecules. This initial signaling through the TCR rendered the T cells unresponsive (anergic) to subsequent activation by peptide-pulsed APCs. Fluorescent labeling demonstrated that anergic T cells were initially viable, but became susceptible to late apoptosis due to insufficient production of cytokines. Dimerization of the TCR with bivalent MHC class II/peptide complexes therefore allows the induction of anergy in human CD4(+) T cells with a defined MHC/peptide specificity.

Immune activation is required for the induction of liver allograft tolerance: implications for immunosuppressive therapy

Liver transplants in many animal models are unusual because often they are not rejected even when transplanted across complete major histocompatibility complex barriers without immunosuppression. Their paradoxical behavior is even more obvious when the immune mechanism of acceptance is examined. Instead of acceptance resulting from a lack of immune response to the graft, the opposite occurs, and there is an unusual extensive increase in immune activation in acceptance compared with rejection. This abnormal extensive immune activation is driven by donor leukocytes transferred with the liver and results in death of the recipient cells that would normally reject the transplant. Some forms of immunosuppression inhibit this activation-associated liver transplant tolerance. The significance of these findings and possible means to design future treatment protocols for clinical transplantation that optimize management of liver transplant recipients are discussed.

Requirement for natural killer T (NKT) cells in the induction of allograft tolerance

In this study, we investigated the role of Valpha14 natural killer T (NKT) cells in transplant immunity. The ability to reject allografts was not significantly different between wild-type (WT) and Valpha14 NKT cell-deficient mice. However, in models in which tolerance was induced against cardiac allografts by blockade of lymphocyte function-associated antigen-1/intercellular adhesion molecule-1 or CD28/B7 interactions, long-term acceptance of the grafts was observed only in WT but not Valpha14 NKT cell-deficient mice. Adoptive transfer with Valpha14 NKT cells restored long-term acceptance of allografts in Valpha14 NKT cell-deficient mice. The critical role of Valpha14 NKT cells to mediate immunosuppression was also observed in vitro in mixed lymphocyte cultures in which lymphocyte function-associated antigen-1/intercellular adhesion molecule-1 or CD28/B7 interactions were blocked. Experiments using IL-4- or IFN-gamma-deficient mice suggested a critical contribution of IFN-gamma to the Valpha14 NKT cell-mediated allograft acceptance in vivo. These results indicate a critical contribution of Valpha14 NKT cells to the induction of allograft tolerance and provide a useful model to investigate the regulatory role of Valpha14 NKT cells in various immune responses.

FasL is important in costimulation blockade-resistant skin graft rejection

BACKGROUND

Simultaneous blockade of the CD40 and CD28 costimulatory pathways is effective in prolonging allograft survival in murine and primate models. Recent data suggest that intact apoptotic pathways are crucial for the induction of hyporesponsiveness by costimulation blockade. We have studied the impact of fas/fasL signaling, an important T cell apoptotic pathway, on the effects of costimulation blockade. Methods. Wild type, lpr (fas deficient), and gld (fasL deficient), mice were used as donors and recipients in the murine skin graft model. Allograft survival was compared in untreated and costimulation blockade (500 microg anti-CD40L and 500 microg CTLA4-Ig, days 0, 2, 4, 6) treated recipients. In some recipients, CD4+ T cells were depleted using rat anti-murine CD4 (100 microg day -3, -2, -1, and weekly).

RESULTS

gld mice treated with costimulation blockade enjoy a significantly greater increase in skin allograft survival than do wild-type mice. This effect is not replicated using lpr donors or recipients. Experiments in which CD4+ cells were depleted demonstrate that fasL is not necessary for CD8-mediated allograft rejection, and that depletion of CD4+ cells eliminates some of the survival advantage induced by costimulation blockade.

CONCLUSIONS

FasL is not required for the establishment of costimulation blockade induced hyporesponsiveness, but rather appears to be required for normal costimulation blockade resistant rejection. Fas expression is not critical for costimulation blockade resistant rejection, suggesting that fasL may be interacting with other receptors. Further, it appears that CD4+ cells are important in the maintenance of allograft protection induced by costimulation blockade in this model.

Prevention of lethal acute graft-versus-host disease in mice by oral administration of T helper 1 inhibitor, TAK-603

Acute graft-versus-host diseases (GVHD) is a major cause of morbidity and mortality in patients undergoing allogeneic bone marrow transplantation (BMT). T helper 1 (Th1)-type cytokines such as interferon-gamma or tumor necrosis factor-alpha have been implicated in the pathogenesis of acute GVHD. TAK-603 is a new quinoline derivative, which is now in clinical trials for use as a disease-modifying antirheumatic drug. In preclinical studies, it inhibited delayed-type hypersensitivity, but not Arthus-type reaction, in mice, and selectively suppressed Th1 cytokine production. Thus, the present study was designed to investigate whether the Th1 inhibitor (TAK-603) ameliorates lethal acute GVHD in a mouse model. Administration of TAK-603 into BALB/c mice given 10 Gy total body irradiation followed by transplantation of bone marrow and spleen cells from C57BL/6 mice markedly reduced the mortality in association with minimal signs of GVHD pathology in the liver, intestine, and skin. TAK-603 reduced not only the production of Th1-type cytokines, but also the proportion of Th1 cells in CD4(+) helper T cells in this GVHD mouse model. These results suggest that TAK-603 could be a potent therapeutic agent for acute lethal GVHD.

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

Two major pathways of death of previously activated T cells have been described: activation-induced cell death can be triggered by restimulating activated T cells with high concentrations of Ag, is Fas-dependent, is not influenced by proteins of the Bcl family, and is blocked by cyclosporin A; in contrast, passive cell death is induced by the withdrawal of growth factors and activation stimuli, is Fas-independent, and is blocked by Bcl family proteins. We examined the role of these two forms of cell death in the peripheral deletion of donor-reactive host T cells after allogeneic bone marrow transplantation and costimulatory blockade with anti-CD154 plus CTLA4Ig in two murine models. The substantial decline in donor-reactive CD4 cells seen in wild-type recipients 1 wk after bone marrow transplantation with costimulatory blockade was largely inhibited in Fas-deficient recipients and in Bcl-x(L)-transgenic recipients. We observed these effects both in a model involving low-dose total body irradiation and a conventional dose of bone marrow, and in a radiation-free regimen using high-dose bone marrow transplantation. Furthermore, cyclosporin A did not completely block the deletion of donor-reactive CD4(+) T cells in recipients of bone marrow transplantation with costimulatory blockade. Thus, the deletion of donor-reactive T cells occurring early after bone marrow transplantation with costimulatory blockade has features of both activation-induced cell death and passive cell death. Furthermore, these in vivo data demonstrate for the first time the significance of in vitro results indicating that proteins of the Bcl family can prevent Fas-mediated apoptosis under certain circumstances.

Apoptosis of superantigen-activated T cells induced by mycophenolate mofetil treatment

BACKGROUND

Mycophenolate mofetil (MMF), an ester prodrug of mycophenolic acid (MPA), is a potent immunosuppressive agent used in clinical organ transplantation. MPA preferentially inhibits the type II isoform of inosine monophosphate dehydrogenase, depletes GTP, suppresses transfer of mannose and fucose to glycoproteins, and prevents lymphocyte proliferation in vivo. Whether MMF can also delete activated T cells in vivo by triggering an apoptotic signal was addressed in this study. To this end we analyzed the activity of MMF in mice injected with the bacterial superantigen staphylococcal enterotoxin B (SEB). Superantigens bind to MHC class II molecules without requirement for processing, and activate subsets of CD4+ and CD8+ T cells whose T cell receptor beta chains express Vbeta family-specific homologous sequences. This model that shares several features with direct allorecognition has the unique advantage of allowing a precise monitoring of activated T cells.

METHODS

BALB/c mice treated with MMF (100 mg/kg/ day) or vehicle were injected with SEB. Serum cytokines, CD4+ and CD8+ Vbeta8+ cells were monitored in blood and lymphoid tissues, and apoptosis was determined by externalization of membrane phosphatidyl serine, double strand DNA breaks, and expression of B220 antigen by Vbeta8+ cells.

RESULTS

MMF treatment decreased tumor necrosis factor alpha, interferon gamma, and interleukin-10 secretion induced by SEB. It did not modify other early activation events (blast transformation, CD69 and CD25 expression) but completely inhibited SEB-induced expansion of Vbeta8+ cells by inducing apoptosis of SEB-reactive T cells. A similar effect was observed in CD95-ligand-deficient mice. Repeated SEB injections associated with MMF resulted in a marked decrease of CD8+ Vbeta8+ T cells. SEB-induced increase of Vbeta8+ thymocytes was not prevented by MMF treatment.

CONCLUSION

Results obtained in this in vivo model suggest that MMF treatment may induce deletion of activated peripheral T cells and decrease early cytokine responses.

Quantifying the frequency of alloreactive T cells in vivo: new answers to an old question

Alloreactive T cell precursor frequency was measured in vivo using fluorescent dye labeling in combination with novel models based on lymphocyte activation and recovery. CFSE-labeled C57BL/6 (H-2(b)) spleen and lymph node cells were adoptively transferred to C57BL/6xDBA F(1) (H-2(b/d)) recipients, a parent-->F(1) MHC mismatch in which only donor cells respond. Recipients were sacrificed at serial time points to assess engraftment efficiency, and the extent of donor cell activation and proliferation. These data were used to calculate alloreactive T cell frequencies that varied 30-fold (0.71 +/- 0.31% to 21.05 +/- 3.62%), depending upon whether it was assumed that all donor cells injected became established and were capable of responding, or that only those present at later time points (24-72 h) were available to respond. By measuring the number of cells established in the recipient 24 h after transfer, before proliferation, we calculated an in vivo alloreactive frequency of approximately 7%. Using CD69 expression at 48 h to quantify activation, we found that 40-50% of the alloactivated CD4(+) donor T cells do not divide. Studies of cotransferred congenic and allogeneic cells demonstrated that bystander proliferation does not occur. We conclude that accurate calculations of alloreactive precursor frequency must account for both proliferation and cell engraftment. When this is done, a high percentage of alloreactive T cells exists across an MHC mismatch, but not all alloreactive cells proliferate in vivo. Bystander proliferation is negligible, revealing exquisite specificity to the alloresponse. These data provide a novel approach to quantify alloreactive T cell responses during specific immunomodulatory strategies in vivo.

Interleukin-2 and interferon-gamma double knockout mice reject heterotopic cardiac allografts

BACKGROUND

Several studies suggest that MHC-mismatched allografts reject with a Th1 or Th2 immune response, but these models all have low-level expression of the Th1 cytokines interleukin (IL)-2 and interferon-gamma (IFN-gamma).

METHODS

We interbred mice with single targeted gene disruptions for IL-2 and IFN-gamma to establish IL-2 + IFN-gamma double knockout (DKO) mice. Heterotopic cardiac allografts from DBA/2j (H2d) donors were transplanted WT, IL-2 knockout (KO), IFN-gamma KO, and DKO recipients (C57BL/6x129; H2b). Cytokine transcripts from allografts and DKO splenocytes were analyzed by reverse transcription polymerase chain reaction.

RESULTS

DKO mice had a cytokine profile and IgG1/ IgG2a isotype ratio characteristic of Th2 deviation. DKO recipients rejected heterotopic cardiac allografts faster than IL-2 KO mice, but significantly slower than WT and IFN-gamma KO mice (P<0.01). Analysis of the rejecting DKO recipients showed intragraft Th2 cytokine expression.

CONCLUSION

The combined absence of IL-2 and IFN-gamma in the setting of Th2 deviation does not prevent allograft rejection.

Cutting edge: anti-CD154 therapeutic antibodies induce infectious transplantation tolerance

Nondepleting anti-CD154 (CD40 ligand) mAbs have proven effective in inducing transplantation tolerance in rodents and primates. In the induction phase, anti-CD154 Ab therapy is known to enhance apoptosis of Ag reactive T cells. However, this may not be the sole explanation for tolerance, as we show in this study that tolerance is maintained through a dominant regulatory mechanism which, like tolerance induced with CD4 Abs, manifests as infectious tolerance. Therefore, tolerance induced with anti-CD154 Abs involves not only the deletion of potentially aggressive T cells, but also a contagious spread of tolerance to new cohorts of graft-reactive T cells as they arise.

Donor and recipient leukocytes in organ allografts of recipients with variable donor-specific tolerance: with particular reference to chronic rejection

We have attributed organ engraftment to clonal exhaustion-deletion of host-versus-graft and graft-versus-host reactions that are reciprocally induced and governed by migratory donor and recipient leukocytes. The so-called donor passenger leukocytes that migrate from the allograft into the recipients have been thoroughly studied (chimerism), but not the donor leukocytes that remain in, or return to, the transplanted organ. Therefore, using flow cytometry we determined the percentage and lineages of donor leukocytes in cell suspensions prepared from Lewis (LEW) cardiac allografts to 100 days posttransplantation. The LEW hearts were transplanted to naïve untreated Brown Norway (BN) recipients (group 2), to naïve BN recipients treated with a 28-day or continuous course of tacrolimus (TAC) (groups 3 and 4), and to drug-free BN recipients pretolerized by earlier bone marrow cell (BMC) or orthotopic LEW liver transplantation (groups 5 and 6). The findings in the heart cell suspensions were correlated with the results from parallel histopathologic-immunocytochemical studies and other studies of the grafts and of host tissues. Although the LEW heart allografts were rejected in 9.6 days by the unmodified recipients of group 2, all beat for 100 days in the recipients of groups 3 through 6. Nevertheless, all of the long-surviving cardiac allografts (but not the isografts in group 1) were the targets of an immune reaction at 5 days, reflected by dramatic increases in the ratio of leukocytes to nonleukocyte nucleated cells from normal values of 1:5-1:6 to 1:1-5:1 and by manifold other evidence of a major inflammatory event. The acute changes returned to baseline by 100 days in the chronic rejection (CR) free hearts of groups 4 and 6, but not in the CR-afflicted hearts of short-course TAC group 3 or the less-severely damaged hearts of the BMC-prime group 5. The freedom from CR in groups 4 and 6 was associated with a large donor contribution to the intracardiac leukocyte population at 5 days (28.6% and 22% in the respective groups) and at 100 days (30.5% in group 4 and 8.4% in group 6) compared with 2% and 1.2% at 100 days in the CR-blighted allografts of the partially tolerant animals of groups 3 and 5. Whether large or small, the donor leukocyte fraction always included a subset of class II leukocytes that had histopathologic features of dendritic cells. These class II(+) cells were of mixed myeloid (CD11b/c(+)) and lymphoid lineages; their migration was markedly inhibited by TAC and accelerated by donor-specific priming and TAC discontinuance. Although a large donor leukocyte population and a normal leukocyte/nonleukocyte cell ratio were associated with freedom from CR, these findings and the lineage profile of the intracardiac leukocytes were not associated with tolerance in the animals of groups 3 and 4 under active TAC treatment. The findings in this study, singly and in their entirety, are compatible with our previously proposed leukocyte migration-localization paradigm of organ allograft acceptance and tolerance.

Suppression of primary T-cell responses and induction of alloantigen-specific hyporesponsiveness in vitro by the Janus kinase inhibitor tyrphostin AG490

BACKGROUND

Tyrphostin AG490 has recently been shown to block interleukin (IL)-2 receptor gamma-chain-associated Janus kinase 3. Here, we analyzed the effect of AG490 on T-cell alloresponses in vitro.

METHODS

For the evaluation of T-cell activation, DNA synthesis, surface marker expression, cytokine secretion, intracellular calcium mobilization, early protein tyrosine phosphorylation, and apoptosis were measured.

RESULTS

AG490 effectively inhibited T-cell proliferation in human mixed lymphocyte culture (MLC) even when added 4 days after culture initiation. Inhibition of IL-2-dependent proliferation in T-cell blasts and the incapability of IL-2 or IL-15 to restore proliferation in AG490-treated MLC suggests interference with cytokine receptor signaling. T-cell receptor-triggered early protein tyrosine phosphorylation, calcium mobilization, up-regulation of CD69, and initial CD25 expression were not affected. Interestingly, AG490 substantially inhibited production of IL-2 and interferon-gamma in T cells stimulated with alloantigen or via CD3 and CD28. In CD28-independent activation models (e.g., stimulation with phorbol myristate acetate plus ionomycin), however, cytokine secretion was not inhibited. Pretreatment of primary MLC with AG490 resulted in substantial down-regulation of secondary responses to cells from the original donor as opposed to third-party cells or phytohemagglutinin. Unresponsiveness was induced also in T cells stimulated with CD3 monoclonal antibody. Induction of apoptosis in polyclonally activated T cells and the incapability of IL-2 to reverse specific hyporesponsiveness, suggest programmed cell death as an important mechanism underlying antigen-specific down-regulation of alloresponses.

CONCLUSIONS

We demonstrate that AG490 blocks different manifestations of T-cell activation. This and its ability to induce alloantigen-specific hyporesponsiveness point to a potential use for interfering with alloreactivities in vivo.

Dendritic cells and prospects for transplantation tolerance

The past year has witnessed the resolution of some long-standing enigmas surrounding the immunobiology of dendritic cells, illuminating their opposing roles in peripheral tolerance and allograft rejection. Nevertheless these advances have posed many new questions, the answers to which may subtly influence our approach to the treatment of rejection while bringing ever closer the prospect of donor-specific transplanation tolerance.

The role of T cell apoptosis in transplantation tolerance

Rejection of fully MHC-mismatched allografts entails the direct recognition of donor MHC molecules (direct antigen presentation) and the activation of an unusually large mass of alloreactive T cells. There is compelling evidence that apoptotic cell death of activated T cells is a critical initial step in the induction of peripheral allograft tolerance with regimens that are not inherently lymphoablative and that therapies that block T cell activation and T cell apoptosis also block the acquisition of tolerance. Thus, T cell apoptosis may play an important role in reducing the size of cytopathic T cell clones and this process may also promote the development and expansion of immune regulatory cells that are essential in the maintenance of allograft tolerance.

Differential effects of cyclosporine A, methylprednisolone, mycophenolate, and rapamycin on CD154 induction and requirement for NFkappaB: implications for tolerance induction

BACKGROUND

Recent experimental data indicate that the targeting of the costimulatory molecule CD40-ligand (CD154) may well offer an opportunity for tolerance induction in transplant recipients and patients with autoimmune diseases, although the optimal therapeutic strategy for clinical application of CD154 monoclonal antibody (mAb) is unclear.

METHODS

We undertook vascularized heterotopic cardiac allograft transplantation in completely MHC-mismatched mice, treated recipients with CD154 mAb plus various immunosuppressive agents, and performed flow cytometric analysis of CD154 expression by T cells activated in vitro in the presence of corresponding immunosuppressive agents. We also tested the extent to which CD154 induction was NFkappaB-dependent by using NFkappaB/p50-deficient mice as allograft recipients and as source of cells for in vitro studies of CD154 induction, and through use of proteasome inhibitors to block IkappaBalpha degradation and NFKB activation in wild-type mice.

RESULTS

Concomitant use of cyclosporin A or methylprednisolone, but not rapamycin or mycophenolate, inhibited CD154 mAb-induced allograft survival. The differential effects of these agents on CD154 mAb-induced tolerance correlated with their capacity to inhibit activation-induced CD154 expression on CD4+ T cells. Full expression of CD154 expression was found to require NF-kappaB activation, and CD154 mAb was ineffective in NF-kappaB/p50 deficient allograft recipients or control mice in which NF-kappaB activation was blocked by proteasome inhibition.

CONCLUSIONS

Strategies to use CD154 mAb clinically must take into account the effects of immunosuppressive agents on CD154 induction, which seems to be at least partially NF-kappaB dependent. Our data suggest that ligation of surface-expressed CD154 provides an important signal that modulates T cell activation and thereby contributes to the effects of CD154 mAb, in addition to previously recognized actions involving blockade of CD40/CD154-dependent cell activation and activation-induced cell death.

CD40-CD40 ligand-independent activation of CD8+ T cells can trigger allograft rejection

In experimental transplantation, blockade of CD40-CD40 ligand (CD40L) interactions has proved effective at permitting long-term graft survival and has recently been approved for clinical evaluation. We show that CD4+ T cell-mediated rejection is prevented by anti-CD40L mAb therapy but that CD8+ T cells remain fully functional. Furthermore, blocking CD40L interactions has no effect on CD8+ T cell activation, proliferation, differentiation, homing to the target allograft, or cytokine production. We conclude that CD40L is not an important costimulatory molecule for CD8+ T cell activation and that following transplantation donor APC can activate recipient CD8+ T cells directly without first being primed by CD4+ T cells.

Immunosuppressive drugs: the first 50 years and a glance forward

During the past 50 years, many immunosuppressive drugs have been described. Often their mechanisms of action were established long after their discovery. Eventually these mechanisms were found to fall into five groups: (i) regulators of gene expression; (ii) alkylating agents; (iii) inhibitors of de novo purine synthesis; (iv) inhibitors of de novo pyrimidine synthesis; and (v) inhibitors of kinases and phosphatases. Glucocorticoids exert immunosuppressive and anti-inflammatory activity mainly by inhibiting the expression of genes for interleukin-2 and other mediators. Cyclophosphamide metabolites alkylate DNA bases and preferentially suppress immune responses mediated by B-lymphocytes. Methotrexate and its polyglutamate derivatives suppress inflammatory responses through release of adenosine; they suppress immune responses by inducing the apoptosis of activated T-lymphocytes and inhibiting the synthesis of both purines and pyrimidines. Azathioprine metabolites inhibit several enzymes of purine synthesis. Mycophenolic acid and mizoribine inhibit inosine monophosphate dehydrogenase, thereby depleting guanosine nucleotides. Mycophenolic acid induces apoptosis of activated T-lymphocytes. A leflunomide metabolite and Brequinar inhibit dihydroorotate dehydrogenase, thereby suppressing pyrimidine nucleotide synthesis. Cyclosporine and FK-506 (Tacrolimus) inhibit the phosphatase activity of calcineurin, thereby suppressing the production of IL-2 and other cytokines. In addition, these compounds have recently been found to block the JNK and p38 signaling pathways triggered by antigen recognition in T-cells. In contrast, rapamycin inhibits kinases required for cell cycling and responses to IL-2. Rapamycin also induces apoptosis of activated T-lymphocytes. Immunosuppressive and anti-inflammatory compounds in development include inhibitors of p38 kinase and of the type IV isoform of cyclic AMP phosphodiesterase which is expressed in lymphocytes and monocytes.A promising future application of immunosuppressive drugs is their use in a regime to induce tolerance to allografts. The role of leukocytes in grafts, and the induction of apoptosis of clones of responding T-lymphocytes, is discussed.