Costimulation blockade, busulfan, and bone marrow promote titratable macrochimerism, induce transplantation tolerance, and correct genetic hemoglobinopathies with minimal myelosuppression

Local irradiation enhances congenic donor pluripotent hematopoietic stem cell engraftment similarly in irradiated and nonirradiated sites

Long-term multilineage chimerism is achieved in CD45 congenic mice receiving high bone marrow doses with or without mediastinal irradiation (MI). Increased donor chimerism results in MI-treated compared with nonirradiated animals, suggesting that MI makes "space" for engraftment of donor pluripotent hematopoietic stem cells (PHSCs). We have now examined whether space is systemic or whether increased engraftment of donor marrow in locally irradiated mice is confined to the irradiated bones. While increased donor chimerism was observed in irradiated bones compared with nonirradiated bones of MI-treated animals 4 weeks following bone marrow transplantation (BMT), these differences were minimal by 40 weeks. MI-treated chimeras contained more adoptively transferable donor PHSCs in the marrow of both irradiated and distant bones compared with non-MI-treated chimeras. Similar proportions of donor PHSCs were present in irradiated and nonirradiated bones of locally irradiated mice at both 4 and 40 weeks. Irradiated bones contained more donor short-term repopulating cells than distant bones at 4 weeks, but not 40 weeks, after BMT. Our study suggests that local proliferation of donor PHSCs in mice receiving local irradiation rapidly leads to a systemic increase in donor PHSC engraftment.

Anti-CD20 treatment depletes B-cells in blood and lymphatic tissue of cynomolgus monkeys

INTRODUCTION

Macaque species offer a valuable model for translational allo-transplantation and tolerance studies. Cardiac allograft vasculopathy in Macaca fascicularis is associated with elaboration of anti-donor antibodies. Since T-independent pathways of B cell activation have been described, and anti-B cell strategies have proven to be a fruitful tolerogenic adjunct in rodent and xenogenic models, here we investigate whether an anti-CD20 antibody (rituximab) would be useful to deplete B-cells in a pre-clinical allo-transplantation setting in macaques.

METHODS

Three cynomolgus macaques which had previously rejected a cardiac allograft and one with concurrent subacute vascular rejection were treated weekly with rituximab 20 mg/kg i.v. for 4 and 2 weeks, respectively. B-cell levels (CD19+ cells) were measured by flow cytometry in peripheral blood, spleen, lymph node and bone marrow cells at various intervals after initiation of treatment. B-cells and plasma cells were also analyzed by immunohistochemistry at necropsy in spleen, lymph node, tonsil and thymus tissue sections. Anti-donor antibody titers were measured by flow cytometry.

RESULTS

B-cells expressing CD19 were not detectable in the peripheral blood in any animal within 24 h after initial treatment, or over the ensuing month. At necropsy, the germinal centers in spleen and lymph node were completely depleted of CD20+ B-cells in 2 animals, leaving a hypocellular trabecular pattern around preserved plasma cell follicles. Substantial but incomplete depletion of B-cells was demonstrated in the other 2 animals, in each instance immunohistochemical findings in spleen and lymph node exhibiting higher sensitivity for residual B-cells compared to FACS. Anti-donor antibody titers exhibited kinetics similar to untreated animals over this short follow-up.

COMMENT

Treatment with anti-CD20 very efficiently depletes peripheral and tissue B-cells but not plasma cells in this macaque species. Biopsy of lymph node is necessary and may be sufficient to assess B-cell clearance in secondary lymphoid organs in this model.

Reprogramming immune responses: enabling cellular therapies and regenerative medicine

Recent advances in cellular therapies have led to the emergence of a multidisciplinary scientific approach to developing therapeutics for a wide variety of diseases and genetic disorders. Although most cell-based therapies currently consist of heterogeneous cell populations, it is anticipated that the standard of care will eventually be well-characterized stem cell lines that can be modified to meet the individual needs of the patient. Many challenges have to be overcome, however, before such "designer cells" can become a clinical reality. One of the major hurdles will be to prevent immune rejection of the therapeutic cells. A patient's immune system may react to genetically modified or allogeneic cells as foreign, leading to their destruction. We propose that specific reprogramming of the immune system to accept cellular therapies can be accomplished by establishing hematopoietic chimerism. Successful engraftment of hematopoietic stem cells (HSCs), which have the same origin as those cells intended for therapeutic use, should lead to a re-education of the immune system so that the donor cells are recognized as self and will not be rejected. Developing safe, nontoxic protocols for reprogramming the immune system is critical to the success of this approach. Two major requirements exist for achieving stable HSC engraftment: (A) depletion or displacement of host stem cells, and (B) adequate immune suppression. Available data indicate that an agent such as busulfan is effective in depleting stem cells and that immune suppression can be accomplished with monoclonal antibodies that specifically target immune-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.

Tolerance induction by costimulator blockade in 100 cGy treated hosts with varying degrees of genetic disparity

Long-term multilineage allochimerism can be obtained in H2-mismatched B6.SJL to BALB/c transplants with host irradiation of 100 cGy, donor spleen cell pre-exposure and costimulator blockade with anti-CD40 ligand (CD40L) antibody. We evaluated this allochimerism approach in murine marrow transplants with different degrees of major histocompatibility complexe (MHC) mismatching; these include: (1) H2-mismatched transplant H2Kk to H2Kb, (2) full haplo-identical transplant H2Kbd to H2Kbk, (3) a partial haplo-identical transplant H2Kd to H2Kbd and (4) an MHC class II mismatch. Levels of chimerism increased up to 12 weeks and then stayed relatively stable up to 1 year after transplant. At 18 weeks post-transplant, the H2-mismatched, haplo-identical, partial haplo-identical and class II-mismatch transplants evidenced 17.9+/-4.4, 40.7+/-0.9, 25.1+/-4.19 and 33.7+/-3.5% donor chimerism, respectively. Dropping the anti-CD40 antibody treatment and spleen cells or changing the schedule of antibody to one injection, in haplo-identical or full-mismatched transplants resulted in no donor-derived chimerism. On the other hand, these still resulted in minor chimerism in class II-mismatched transplants. Lineage analysis of peripheral blood at 6 and 12 months post-transplant demonstrated a significant shift toward increased chimeric lymphocytes and decreased chimeric granulocytes in the full H2 as compared with haplo-identical or class II transplants. Transplantation with anti-CD40L antibody eliminated both graft-versus-leukemia and graft-versus-host disease (GVHD) and delayed lymphocyte infusion did not rescue animals from fatal leukemia. In conclusion, under the conditions of our tolerization regimen, a haplo transplant gives higher engraftment levels than a full H2 mismatch, and despite lower engraftment levels, a class II-mismatched transplant can be successfully accomplished with only 100 cGy and no CD40L blockade.

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.

Heterologous immunity provides a potent barrier to transplantation tolerance

Many strategies have been proposed to induce tolerance to transplanted tissue in rodents; however, few if any have shown equal efficacy when tested in nonhuman primate transplant models. We hypothesized that a critical distinction between specific pathogen-free mice and nonhuman primates or human patients is their acquired immune history. Here, we show that a heterologous immune response--specifically, virally induced alloreactive memory--is a potent barrier to tolerance induction. A critical threshold of memory T cells is needed to promote rejection, and CD8(+) "central" memory T cells are primarily responsible. Finally, treatment with deoxyspergualin, an inhibitor of NF-kappa B translocation, together with costimulation blockade, synergistically impairs memory T cell activation and promotes antigen-specific tolerance of memory. These data offer a potential explanation for the difficulty encountered when inducing tolerance in nonhuman primates and human patients and provide insight into the signaling pathways essential for memory T cell activation and function.

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.

Conventional immunosuppression is compatible with costimulation blockade-based, mixed chimerism tolerance induction

T-cell costimulatory blockade has emerged as an effective strategy to prevent allograft rejection in experimental models. We and others have reported that the beneficial effects of costimulation blockade can be negated when combined with certain immunosuppressants. The current study evaluates the compatibility of various immunosuppressive agents in a costimulation blockade-based, mixed chimerism tolerance protocol. The addition of conventional agents, including calcineurin inhibitors, did not interfere with tolerance induction. All mice developed multilineage macrochimerism and accepted donor allografts. Analysis of specific T-cell receptor utilization demonstrated selective deletion of donor-reactive T cells. Challenge with donor and third-party allografts confirmed donor-specific tolerance. Clinical introduction of costimulation blockade-based strategies will likely incorporate currently approved immunosuppressive agents. While it has been reported that certain conventional agents are detrimental to costimulation blockade-based strategies, our results suggest that these agents could safely be combined in clinical trials when used as part of a nonmyelosuppressive, mixed chimerism-based tolerance strategy.

Towards a myeloablative regimen with clinical potential: I. Treosulfan conditioning and bone marrow transplantation allow induction of donor-specific tolerance for skin grafts across full MHC barriers

To investigate whether we could create a radiation-free conditioning regimen to induce permanent mixed and multilineage chimerism and donor-specific tolerance, we treated recipient mice with anti-T-cell antibodies, varying and fractionated doses of Treosulfan and fully MHC disparate bone marrow cells. Treosulfan is mainly used in the treatment of ovarian cancer. It is a structural analog of busulfan, but it does not induce severe hepatotoxicity or veno-occlusive disease at or above the maximum tolerated dose, lacks significant nonhematological toxicity and has limited organ toxicity. We report here the successful induction of permanent mixed multilineage chimerism and donor-specific tolerance as was proven by skin transplantation and IFN-gamma ELISPOT. In conclusion, because of its lower nonhematological toxicity, compared with other myeloablative regimens (eg irradiation or busulfan admin- istration), Treosulfan could be a better candidate for conditioning to induce donor-specific allograft tolerance.

Tolerance: of mice and men

Little is known about the effect of an individual's immune history on his or her response to an allogeneic tissue transplant. An important study now reveals that individuals harboring virally-induced memory T cells that are cross reactive with donor alloantigen are resistant to conventional strategies designed to induce transplant tolerance.

Nonmyeloablative conditioning is sufficient to allow engraftment of EGFP-expressing bone marrow and subsequent acceptance of EGFP-transgenic skin grafts in mice

Immunologic reactions against gene therapy products may prove to be a frequent problem in clinical gene therapy protocols. Enhanced green fluorescence protein (EGFP) is commonly used as a marker in gene transfer protocols, and immune responses against EGFP-expressing cells have been documented. The present study was designed to investigate the effect of a pharmacologic, nonmyeloablative, conditioning regimen on the development of EGFP+ donor/recipient mixed bone marrow chimerism and ensuing tolerance to EGFP-expressing transplants. To this end, C57BL/6J (B6) mice were treated with soluble formulations of either busulfan (Busulfex) or the closely related compound treosulfan, followed by transplantation of bone marrow cells from EGFP-transgenic (B6-EGFP.Tg) donor mice. Such conditioning regimens resulted in long-term persistence of donor EGFP+ cells among various hematopoietic lineages from blood, bone marrow, and thymus. Stable hematopoietic chimeras transplanted at 10 to 17 weeks after bone marrow transplantation (BMT) with B6-EGFP.Tg skin grafts all accepted their transplants, whereas non-EGFP chimeric B6 control animals were able to mount rejection of the EGFP+ B6 skin grafts. Control third-party grafts from major histocompatibility complex (MHC)-mismatched mice were rejected within 20 days, indicating that acceptance of EGFP-expressing skin grafts was the result of specific immune tolerance induction by the transplantation of EGFP-transgenic bone marrow. Long-term tolerance to EGFP in chimeric recipients was confirmed by the absence of anti-EGFP-reactive T cells and antibodies. These results broaden the therapeutic potential for using hematopoietic molecular chimerism in nonmyeloablated recipients as a means of preventing rejection of genetically modified cells.

Heterologous immunity provides a potent barrier to transplantation tolerance

Many strategies have been proposed to induce tolerance to transplanted tissue in rodents; however, few if any have shown equal efficacy when tested in nonhuman primate transplant models. We hypothesized that a critical distinction between specific pathogen-free mice and nonhuman primates or human patients is their acquired immune history. Here, we show that a heterologous immune response--specifically, virally induced alloreactive memory--is a potent barrier to tolerance induction. A critical threshold of memory T cells is needed to promote rejection, and CD8(+) "central" memory T cells are primarily responsible. Finally, treatment with deoxyspergualin, an inhibitor of NF-kappa B translocation, together with costimulation blockade, synergistically impairs memory T cell activation and promotes antigen-specific tolerance of memory. These data offer a potential explanation for the difficulty encountered when inducing tolerance in nonhuman primates and human patients and provide insight into the signaling pathways essential for memory T cell activation and function.

Role of the CTLA4 pathway in hyporesponsiveness induced by intratracheal delivery of alloantigen

BACKGROUND

The authors previously reported that intratracheal delivery (ITD) of donor alloantigen induced donor-specific hyporesponsiveness to C57BL/10 cardiac allografts in CBA recipients and that blockade of the B7 pathways abrogated that hyporesponsiveness. In this study, the authors used a CD28-deficient model to evaluate which signal, either through CD28 or cytotoxic T-lymphocyte-associated antigen (CTLA4), is involved in the induction of hyporesponsiveness.

METHODS

Seven days before transplantation of hearts from C3H/HeJ (H2k) mice into C57BL/6 (H2b) or CD28-deficient (C57BL/6 background) mice, the transplant recipients were given ITD of donor splenocytes (1 x 10(7)), alone or in combination with human CTLA4-immunoglobulin (Ig) (200 microg).

RESULTS

ITD of C3H splenocytes induced donor-specific hyporesponsiveness to C3H cardiac grafts in C57BL/6 recipients (graft median survival time [MST], 40 days). Administration of CTLA4-Ig concurrently with ITD abrogated the prolonged allograft survival (MST, 12 days). Interestingly, ITD of C3H splenocytes induced prolonged survival of C3H allografts in CD28-deficient recipients (MST, 55 days). Furthermore, administration of CTLA4-Ig combined with ITD of C3H splenocytes abrogated the prolonged survival of C3H allografts in CD28-deficient recipients (MST, 7 days), whereas recipients given isotype-control antibody in combination with ITD of splenocytes had prolonged survival of C3H allografts (MST, 58 days).

CONCLUSIONS

Taken together, the authors' findings indicate that a signal through CTLA4, rather than through CD28, plays an important role in the induction of hyporesponsiveness by ITD of alloantigen in this model.

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.

Blockade of CD40-mediated signaling is sufficient for inducing islet but not skin transplantation tolerance

Treatment of mice with a single donor-specific transfusion (DST) plus a brief course of anti-CD154 mAb to block CD40-mediated signaling uniformly induces donor-specific transplantation tolerance. Survival of islet allografts in treated mice is permanent, but skin grafts eventually fail unless recipients are thymectomized. The nature of the cellular mechanisms involved and the basis for the difference in survival of islet vs skin allografts are not known. In this study, we used CD40 knockout mice to investigate the role of CD40-mediated signaling in each component of the tolerance induction protocol: the DST, the graft, and the host. When CD40-mediated signaling was eliminated in only the DST or the graft, islet allografts were rapidly rejected. However, when CD40 signaling was eliminated in the host, approximately 40% of the islet allografts survived. When CD40 signaling was eliminated in the DST, the graft, and the host, islet grafts survived long term (>84 days), whereas skin allografts were rapidly rejected ( approximately 13 days). We conclude that transplantation tolerance induction in mice treated with DST and anti-CD154 mAb requires blockade of CD40-mediated signaling in the DST, the graft, and the host. Blockade of CD40-mediated signaling is necessary and sufficient for inducing islet allograft tolerance and is necessary but not sufficient for long-term skin allograft survival. We speculate that a requirement for regulatory CD4(+) T cells in skin allograft recipients could account for this differential response to tolerance induction.

Primary and secondary immunocompetence in mixed allogeneic chimeras

Targeted disruption of T cell costimulatory pathways, particularly CD28 and CD40, has allowed for the development of minimally myeloablative strategies for the induction of mixed allogeneic chimerism and donor-specific tolerance across full MHC barriers. In this study we analyze in depth the ability of mixed allogeneic chimeras in two strain combinations to mount effective host-restricted and donor-restricted antiviral CD4 and CD8 responses, as well as the impact of development of mixed chimerism on the maintenance of pre-existing memory populations. While antiviral CD8 responses in mixed chimeras following acute viral infection with lymphocytic choriomeningitis virus Armstrong or vaccinia virus are largely host-restricted, donor-restricted CD8 responses as well as host- and donor-restricted CD4 responses are also readily detected, and virus is promptly cleared. We further demonstrate that selection of donor-restricted T cells in mixed chimeras is principally mediated by bone marrow-derived cells in the thymus. Conversely, we find that mixed chimeras exhibit a deficit in their ability to deal with a chronic lymphocytic choriomeningitis virus clone 13 infection. Encouragingly, pre-existing memory populations are largely unaffected by the development of high level mixed chimerism and maintain the ability to control viral rechallenge. Our results suggest that while pre-existing T cell memory and primary immunocompetence to acute infection are preserved in mixed allogeneic chimeras, MHC class I and/or class II tissue matching may be required to fully preserve immunocompetence in dealing with chronic viral infections.

Human CD34(+) blood cells induce T-cell unresponsiveness to specific alloantigens only under costimulatory blockade

OBJECTIVES

The immunogenic role of human CD34(+) cells in allogeneic hematopoietic stem cell transplantation is controversial. In this study we tested the role of CD40 and CTLA4 ligands on CD34(+) cell costimulation of HLA-mismatched lymphocytes.

MATERIALS AND METHODS

An anti-CD40L monoclonal antibody (hu5C8) and/or CTLA4-Ig molecule were used in primary mixed lymphocyte culture (MLC) with irradiated CD34(+) blood cells and allogeneic responders. Then, secondary MLC, cytotoxic activity, and effector cytokine expression and production were measured.

RESULTS

Each reagent was able to reduce anti-CD34(+) cell alloreactivity, but only the combination of the anti-CD40L monoclonal antibody and CTLA4-Ig induced greater than 90% inhibition of T-cell response in primary MLC and prevented generation of cytotoxic T cells when priming with purified CD34(+) cells. Importantly, responder cells activated by allogeneic CD34(+) cells in the presence of anti-CD40L monoclonal antibody and CTLA4-Ig entered a state of antigen-specific unresponsiveness while responding to third party antigen, tetanus toxoid, or phytohemagglutinin, and showed suppression of interferon-gamma and increase of interleukin-10 expression and release. Interestingly, addition of interleukin-2 in secondary MLC did not reverse T-cell anergy.

CONCLUSIONS

The results demonstrate that human CD34(+) blood progenitors stimulate T-cell responses potently and can induce T-cell unresponsiveness only when both B7:CD28 and CD40:CD40L pathways are blocked, with an increase of interleukin-10-producing cells. Therefore, our data allow design of in vivo studies aimed at achieving T-cell tolerance across HLA barriers by using purified CD34(+) cells and costimulatory blockade.

Cutting edge: persistent viral infection prevents tolerance induction and escapes immune control following CD28/CD40 blockade-based regimen

A continuing concern with CD28 and/or CD40 blockade-based strategies to induce tolerance and mixed chimerism is their potential to disrupt protective immunity to preexisting infections. In this report, we find that preexisting persistent infection with lymphocytic choriomeningitis virus (LCMV) clone 13 prevents the induction of tolerance, mixed chimerism, and donor-reactive T cell deletion. Mice continue to be refractory to tolerance induction even after viremia has been resolved and virus is present only at very low levels in peripheral tissues. Conversely, we find that the full tolerance regimen, or costimulation blockade alone, specifically inhibits already ongoing antiviral immune responses, leading to an inability to control viremia. These findings suggest that ongoing T cell responses continue to depend on costimulatory interactions in the setting of a chronic infection and provide insight into potential risks following costimulation blockade posed by chronic or latent viral infections such as hepatitis C, EBV, and CMV.

Immunotherapy as a means to induce transplantation tolerance

There have been several recent advances in the use of immunotherapy to induce transplantation tolerance. These include newer and safer protocols to create hematopoietic chimerism, the development of more-powerful T cell depleting antibodies, the identification of additional costimlulatory pathways as molecular targets and the identification of a role for suppressor cells in transplant tolerance.

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.

Prevention of chronic rejection in murine cardiac allografts: a comparison of chimerism- and nonchimerism-inducing costimulation blockade-based tolerance induction regimens

We have previously described a nonirradiation-based regimen combining costimulation blockade, busulfan, and donor bone marrow cells that promotes stable, high level chimerism, deletion of donor-reactive T cells, and indefinite survival of skin allografts in mice. The purpose of the current study is to determine the efficacy of this tolerance regimen in preventing acute and chronic rejection in a vascularized heart graft model and to compare this regimen with other putative tolerance protocols. Mice receiving costimulation blockade (CTLA4-Ig and anti-CD40 ligand) alone or in combination with donor cells enjoyed markedly prolonged heart graft survival and initially preserved histological structure. However, tolerance was not achieved, as evidenced by the eventual onset of chronic rejection characterized by obliterative vasculopathy and the rejection of secondary skin grafts. In contrast, following treatment with costimulation blockade, busulfan, and bone marrow, heart grafts survived indefinitely without detectable signs of chronic rejection or structural damage, even 100 days after placement of a secondary donor skin graft. We detected multilineage chimerism in peripheral blood, spleen, lymph nodes, and thymus, and peripheral deletion of donor-reactive cells was complete by day 90. These findings indicate that only the CD40/CD28 blockade chimerism induction regimen prevents both acute and chronic rejection of vascularized organ transplants. Further testing of these strategies in a preclinical large animal model is warranted.

Promotion of skin graft tolerance across MHC barriers by mobilization of dendritic cells in donor hemopoietic cell infusions

Flt3 ligand (FL) dramatically increases the number of immunostimulatory dendritic cells (DC) and their precursors in bone marrow (BM) and secondary lymphoid tissues. Herein we tested the ability of FL-mobilized donor hemopoietic cells to promote induction of skin graft tolerance across full MHC barriers. C57BL/10 (B10; H2(b), IE(-)) mice were given 10(8) spleen cells (SC) from normal or FL-treated, H-2-mismatched B10.D2 (H2(d), IE(+)) donors i.v. on day 0, 200 mg/kg i.p. cyclophosphamide on day 2, and 10(7) T cell-depleted BM cells from B10.D2 mice on day 3. B10.D2 skin grafting was performed on day 14. Indefinite allograft survival (100 days) was induced in recipients of FL-SC, but not in mice given normal SC. Tolerance was associated with blood macrochimerism and was confirmed by second-set skin grafting with donor skin 100 days after the first graft. In tolerant mice, peripheral donor-reactive T cells expressing TCR Vbeta11 were deleted selectively. Immunocompetence of tolerant FL-SC-treated mice was proven by rapid rejection of third-party skin grafts. To our knowledge this is the first report that mobilization of DC in donor cell infusions can be used to induce skin graft tolerance across MHC barriers, accompanied by specific deletion of donor-reactive T cells.

Viral abrogation of stem cell transplantation tolerance causes graft rejection and host death by different mechanisms

Tolerance-based stem cell transplantation using sublethal conditioning is being considered for the treatment of human disease, but safety and efficacy remain to be established. We have shown that mouse bone marrow recipients treated with sublethal irradiation plus transient blockade of the CD40-CD154 costimulatory pathway develop permanent hematopoietic chimerism across allogeneic barriers. We now report that infection with lymphocytic choriomeningitis virus at the time of transplantation prevented engraftment of allogeneic, but not syngeneic, bone marrow in similarly treated mice. Infected allograft recipients also failed to clear the virus and died. Postmortem study revealed hypoplastic bone marrow and spleens. The cause of death was virus-induced IFN-alphabeta. The rejection of allogeneic bone marrow was mediated by a radioresistant CD8(+)TCR-alphabeta(+)NK1.1(-) T cell population. We conclude that a noncytopathic viral infection at the time of transplantation can prevent engraftment of allogeneic bone marrow and result in the death of sublethally irradiated mice treated with costimulation blockade. Clinical application of stem cell transplantation protocols based on costimulation blockade and tolerance induction may require patient isolation to facilitate the procedure and to protect recipients.

Rapamycin and T cell costimulatory blockade as post-transplant treatment promote fully MHC-mismatched allogeneic bone marrow engraftment under irradiation-free conditioning therapy

Hematopoietic macrochimerism, established by bone marrow transplantation, can be used as an approach for treating autoimmune disease and inducing transplant tolerance. In this study, we investigated whether a stable, high level of fully MHC-mismatched hematopoietic macrochimerism can be induced by using irradiation-free protocols, and whether rapamycin and T cell costimulatory blockades (anti-CD40L monoclonal antibody (mAb) and CTLA4Ig) as post-transplant treatment promote bone marrow engraftment. Donor-specific blood transfusion (DST), anti-lymphocyte serum (ALS), busulfan, and cyclophosphamide were given pretransplantation. Balb/c (H-2(d)) bone marrow cells, at a dose of 4 x 10(7), were infused into each C57BL/6 mouse (H-2(b)). Rapamycin, anti-CD40L mAb, and CTLA4Ig were then administered, either alone or in combination. Without ALS or busulfan and cyclophosphamide, macrochimerism can only rarely be induced. Donor-specific transfusion (DST) enhances induction of hematopoietic macrochimerism. Rapamycin, anti-CD40L mAb and CTLA4Ig, alone or in combination, induce a stable and high level of hematopoietic macrochimerism. In the chimeric mice, donor-derived cells were detected in all lymphohematopoietic tissues and donor-specific tolerance was induced in vitro. We conclude that a stable and high level of fully MHC-mismatched hematopoietic macrochimerism can be induced in mice after transplanting a single modest dose of bone marrow cells without irradiation. Rapamycin and T cell costimulatory blockade as post-transplant treatment promote bone marrow engraftment.

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.

A cure for murine sickle cell disease through stable mixed chimerism and tolerance induction after nonmyeloablative conditioning and major histocompatibility complex-mismatched bone marrow transplantation

The morbidity and mortality associated with sickle cell disease (SCD) is caused by hemolytic anemia, vaso-occlusion, and progressive multiorgan damage. Bone marrow transplantation (BMT) is currently the only curative therapy; however, toxic myeloablative preconditioning and barriers to allotransplantation limit this therapy to children with major SCD complications and HLA-matched donors. In trials of myeloablative BMT designed to yield total marrow replacement with donor stem cells, a subset of patients developed mixed chimerism. Importantly, these patients showed resolution of SCD complications. This implies that less toxic preparative regimens, purposefully yielding mixed chimerism after transplantation, may be sufficient to cure SCD without the risks of myeloablation. To rigorously test this hypothesis, we used a murine model for SCD to investigate whether nonmyeloablative preconditioning coupled with tolerance induction could intentionally create mixed chimerism and a clinical cure. We applied a well-tolerated, nonirradiation-based, allogeneic transplantation protocol using nonmyeloablative preconditioning (low-dose busulfan) and costimulation blockade (CTLA4-Ig and anti-CD40L) to produce mixed chimerism and transplantation tolerance to fully major histocompatibility complex-mismatched donor marrow. Chimeric mice were phenotypically cured of SCD and had normal RBC morphology and hematologic indices (hemoglobin, hematocrit, reticulocyte, and white blood cell counts) without evidence of graft versus host disease. Importantly, they also showed normalization of characteristic spleen and kidney pathology. These experiments demonstrate the ability to produce a phenotypic cure for murine SCD using a nonmyeloablative protocol with fully histocompatibility complex-mismatched donors. They suggest a future treatment strategy for human SCD patients that reduces the toxicity of conventional BMT and expands the use of allotransplantation to non-HLA-matched donors.

The role of TNF receptor and TNF superfamily molecules in organ transplantation

The rapid increase in the number of molecules demonstrated to regulate immune responses has provided new opportunities for manipulation of the recipient immune response to transplanted organs. Molecules belonging to the TNF receptors and TNF superfamily are increasingly recognized as playing a major role in the regulation of immune responses to tumor, viral, and autoantigens. The mechanisms by which these molecules regulate immune responses are diverse. TNF receptor-related molecules have been shown to control the development of secondary lymphoid organs, affect the activation and survival of T cells and antigen presenting cells, and alter cytokine and chemokine production. An increasing amount of data suggest that some TNFR superfamily members are particularly important for the function of CD8+ T cells. Based on our current understanding of these molecules it seems highly likely that strategies that target selected TNFR/TNF superfamily molecules will be useful for controlling or preventing the rejection of transplanted organs and tissues.

Characterization of virus-mediated inhibition of mixed chimerism and allospecific tolerance

Simultaneous blockade of the CD28 and CD40 T cell costimulatory pathways has been shown to effectively promote skin allograft survival in mice. Furthermore, blockade of one or both of these pathways has played a central role in the development of strategies to induce mixed hematopoietic chimerism and allospecific tolerance. It has recently been observed that the beneficial effects of CD40 blockade and donor splenocytes in prolonging skin graft survival can be abrogated by some viral infections, including lymphocytic choriomeningitis virus (LCMV). In this study, we show that LCMV infection prevents prolonged allograft survival following CD28/CD40 combined blockade. We further show that LCMV prevents the induction of allospecific tolerance and mixed hematopoietic chimerism, while delay of infection for 3-4 wk posttransplant has no effect on tolerance induction. Because of reports of anti-H-2(d) activity following LCMV infection, we assayed the ability of LCMV-specific T cells to respond to alloantigen at a single cell level. Although we confirm that LCMV infection induces the generation of alloreactive cells, we also demonstrate that LCMV-specific T cells do not divide in response to alloantigen. The alloresponse suppressed by costimulation blockade is restored by LCMV infection and correlates with increased dendritic cell maturation. We hypothesize that the costimulation blockade-resistant rejection mediated by LCMV could be partly attributable to the up-regulation of alternative costimulatory pathways subsequent to LCMV-induced dendritic cell maturation.