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

PPARγ/ETV2 axis regulates endothelial-to-mesenchymal transition in pulmonary hypertension

Endothelial-to-mesenchymal transition (EndoMT) plays an important role in pulmonary hypertension (PH) but the molecular mechanisms regulating EndoMT remain to be defined. We demonstrate that the axis of the transcription factors PPARγ (Peroxisome Proliferator-Activated Receptor gamma) and ETV2 (ETS variant 2) play important roles in the pathogenesis of PH. Decreased levels of the expression of PPARγ and ETV2 along with reduced endothelial and increased EndoMT markers are consistently observed in lungs and pulmonary artery endothelial cells (PAECs) of idiopathic pulmonary arterial hypertension patients, in hypoxia-exposed mouse lungs, human PAECs, and in induced-EndoMT cells. Etv2 +/- mice spontaneously developed PH and right ventricular hypertrophy (RVH), associated with increased EndoMT markers and decreased EC markers. Interestingly, chronic hypoxia exacerbated right ventricular systolic pressure and RVH in Etv2 +/- mice. PPARγ transcriptionally activates the ETV2 promoter. Consistently, while mice overexpressing endothelial PPARγ increases the expression of ETV2 and endothelial markers with reduced EndoMT markers, endothelial PPARγ KO mice show decreased ETV2 expression and enhanced EndoMT markers. Inducible overexpression of ETV2 under induced-EndoMT cell model reduces number of cells with mesenchymal morphology and decreases expression of mesenchymal markers with increased EC makers, compared to control. Therefore, our study suggests that PPARγ-ETV2 signaling regulates PH pathogenesis through EndoMT.

Donor derived hematopoietic stem cell niche transplantation facilitates mixed chimerism mediated donor specific tolerance

Compelling experimental evidence confirms that the robustness and longevity of mixed chimerism (MC) relies on the persistence and availability of donor-derived hematopoietic stem cell (HSC) niches in recipients. Based on our prior work in rodent vascularized composite allotransplantation (VCA) models, we hypothesize that the vascularized bone components in VCA bearing donor HSC niches, thus may provide a unique biologic opportunity to facilitate stable MC and transplant tolerance. In this study, by utilizing a series of rodent VCA models we demonstrated that donor HSC niches in the vascularized bone facilitate persistent multilineage hematopoietic chimerism in transplant recipients and promote donor-specific tolerance without harsh myeloablation. In addition, the transplanted donor HSC niches in VCA facilitated the donor HSC niches seeding to the recipient bone marrow compartment and contributed to the maintenance and homeostasis of stable MC. Moreover, this study provided evidences that chimeric thymus plays a role in MC-mediated transplant tolerance through a mechanism of thymic central deletion. Mechanistic insights from our study could lead to the use of vascularized donor bone with pre-engrafted HSC niches as a safe, complementary strategy to induce robust and stable MC-mediated tolerance in VCA or solid organ transplantation recipients.

PPARγ increases HUWE1 to attenuate NF-κB/p65 and sickle cell disease with pulmonary hypertension

Sickle cell disease (SCD)-associated pulmonary hypertension (PH) causes significant morbidity and mortality. Here, we defined the role of endothelial specific peroxisome proliferator-activated receptor γ (PPARγ) function and novel PPARγ/HUWE1/miR-98 signaling pathways in the pathogenesis of SCD-PH. PH and right ventricular hypertrophy (RVH) were increased in chimeric Townes humanized sickle cell (SS) mice with endothelial-targeted PPARγ knockout (SSePPARγKO) compared with chimeric littermate control (SSLitCon). Lung levels of PPARγ, HUWE1, and miR-98 were reduced in SSePPARγKO mice compared with SSLitCon mice, whereas SSePPARγKO lungs were characterized by increased levels of p65, ET-1, and VCAM1. Collectively, these findings indicate that loss of endothelial PPARγ is sufficient to increase ET-1 and VCAM1 that contribute to endothelial dysfunction and SCD-PH pathogenesis. Levels of HUWE1 and miR-98 were decreased, and p65 levels were increased in the lungs of SS mice in vivo and in hemin-treated human pulmonary artery endothelial cells (HPAECs) in vitro. Although silencing of p65 does not regulate HUWE1 levels, the loss of HUWE1 increased p65 levels in HPAECs. Overexpression of PPARγ attenuated hemin-induced reductions of HUWE1 and miR-98 and increases in p65 and endothelial dysfunction. Similarly, PPARγ activation attenuated baseline PH and RVH and increased HUWE1 and miR-98 in SS lungs. In vitro, hemin treatment reduced PPARγ, HUWE1, and miR-98 levels and increased p65 expression, HPAEC monocyte adhesion, and proliferation. These derangements were attenuated by pharmacological PPARγ activation. Targeting these signaling pathways can favorably modulate a spectrum of pathobiological responses in SCD-PH pathogenesis, highlighting novel therapeutic targets in SCD pulmonary vascular dysfunction and PH.

Vascularized composite allotransplantation combined with costimulation blockade induces mixed chimerism and reveals intrinsic tolerogenic potential

Vascularized composite allotransplantation (VCA) has become a valid therapeutic option to restore form and function after devastating tissue loss. However, the need for high-dose multidrug immunosuppression to maintain allograft survival is still hampering more widespread application of VCA. In this study, we investigated the immunoregulatory potential of costimulation blockade (CoB; CTLA4-Ig and anti-CD154 mAb) combined with nonmyeoablative total body irradiation (TBI) to promote allograft survival of VCA in a fully MHC-mismatched mouse model of orthotopic hind limb transplantation. Compared with untreated controls (median survival time [MST] 8 days) and CTLA4-Ig treatment alone (MST 17 days), CoB treatment increased graft survival (MST 82 days), and the addition of nonmyeloablative TBI led to indefinite graft survival (MST > 210 days). Our analysis suggests that VCA-derived BM induced mixed chimerism in animals treated with CoB and TBI + CoB, promoting gradual deletion of alloreactive T cells as the underlying mechanism of long-term allograft survival. Acceptance of donor-matched secondary skin grafts, decreased ex vivo T cell responsiveness, and increased graft-infiltrating Tregs further indicated donor-specific tolerance induced by TBI + CoB. In summary, our data suggest that vascularized BM-containing VCAs are immunologically favorable grafts promoting chimerism induction and long-term allograft survival in the context of CoB.

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

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

Advances in targeting co-inhibitory and co-stimulatory pathways in transplantation settings: the Yin to the Yang of cancer immunotherapy

In the past decade, the power of harnessing T-cell co-signaling pathways has become increasingly understood to have significant clinical importance. In cancer immunotherapy, the field has concentrated on two related modalities: First, targeting cancer antigens through highly activated chimeric antigen T cells (CAR-Ts) and second, re-animating endogenous quiescent T cells through checkpoint blockade. In each of these strategies, the therapeutic goal is to re-ignite T-cell immunity, in order to eradicate tumors. In transplantation, there is also great interest in targeting T-cell co-signaling, but with the opposite goal: in this field, we seek the Yin to cancer immunotherapy's Yang, and focus on manipulating T-cell co-signaling to induce tolerance rather than activation. In this review, we discuss the major T-cell signaling pathways that are being investigated for tolerance induction, detailing preclinical studies and the path to the clinic for many of these molecules. These include blockade of co-stimulation pathways and agonism of coinhibitory pathways, in order to achieve the delicate state of balance that is transplant tolerance: a state which guarantees lifelong transplant acceptance without ongoing immunosuppression, and with preservation of protective immune responses. In the context of the clinical translation of immune tolerance strategies, we discuss the significant challenge that is embodied by the fact that targeted pathway modulators may have opposing effects on tolerance based on their impact on effector vs regulatory T-cell biology. Achieving this delicate balance holds the key to the major challenge of transplantation: lifelong control of alloreactivity while maintaining an otherwise intact immune system.

Mechanisms of Tolerance Induction by Hematopoietic Chimerism: The Immune Perspective

Hematopoietic chimerism is one of the effective approaches to induce tolerance to donor‐derived tissue and organ grafts without administration of life‐long immunosuppressive therapy. Although experimental efforts to develop such regimens have been ongoing for decades, substantial cumulative toxicity of combined hematopoietic and tissue transplants precludes wide clinical implementation. Tolerance is an active immunological process that includes both peripheral and central mechanisms of mutual education of coresident donor and host immune systems. The major stages include sequential suppression of early alloreactivity, establishment of hematopoietic chimerism and suppressor cells that sustain the state of tolerance, with significant mechanistic and temporal overlap along the tolerization process. Efforts to devise less toxic transplant strategies by reduction of preparatory conditioning focus on modulation rather than deletion of residual host immunity and early reinstitution of regulatory subsets at the central and peripheral levels. Stem Cells Translational Medicine2017;6:700–712

Bone marrow chimerism as a strategy to produce tolerance in solid organ allotransplantation

PURPOSE OF REVIEW

Clinical transplant tolerance has been most successfully achieved combining hematopoietic chimerism with kidney transplantation. This review outlines this strategy in animal models and human transplantation, and possible clinical challenges.

RECENT FINDINGS

Kidney transplant tolerance has been achieved through chimerism in several centers beginning with Massachusetts General Hospital's success with mixed chimerism in human leukocyte antigen (HLA)-mismatched patients and the Stanford group with HLA-matched patients, and the more recent success of the Northwestern protocol achieving full chimerism. This has challenged the original view that stable mixed chimerism is necessary for organ graft tolerance. However, among the HLA-mismatched kidney transplant-tolerant patients, loss of mixed chimerism does not lead to renal-graft rejection, and the development of host Foxp3+ regulatory T cells has been observed. Recent animal models suggest that graft tolerance through bone marrow chimerism occurs through both clonal deletion and regulatory immune cells. Further, Tregs have been shown to improve chimerism in animal models.

SUMMARY

Animal studies continue to suggest ways to improve our current clinical strategies. Advances in chimerism protocols suggest that tolerance may be clinically achievable with relative safety for HLA-mismatched kidney transplants.

Hematopoietic stem cells and solid organ transplantation

Solid organ transplantation provides lifesaving therapy for patients with end stage organ disease. In order for the transplanted organ to survive, the recipient must take a lifelong cocktail of immunosuppressive medications that increase the risk for infections, malignancies and drug toxicities. Data from many animal studies have shown that recipients can be made tolerant of their transplanted organ by infusing stem cells, particularly hematopoietic stem cells, prior to the transplant. The animal data have been translated into humans and now several clinical trials have demonstrated that infusion of hematopoietic stem cells, along with specialized conditioning regimens, can permit solid organ allograft survival without immunosuppressive medications. This important therapeutic advance has been made possible by understanding the immunologic mechanisms by which stem cells modify the host immune system, although it must be cautioned that the conditioning regimens are often severe and associated with significant morbidity. This review discusses the role of hematopoietic stem cells in solid organ transplantation, provides an understanding of how these stem cells modify the host immune system and describes how newer information about adaptive and innate immunity might lead to improvements in the use of hematopoietic stem cells to induce tolerance to transplanted organs.

Alloreactive Regulatory T Cells Allow the Generation of Mixed Chimerism and Transplant Tolerance

The induction of donor-specific transplant tolerance is one of the main goals of modern immunology. Establishment of a mixed chimerism state in the transplant recipient has proven to be a suitable strategy for the induction of long-term allograft tolerance; however, current experimental recipient preconditioning protocols have many side effects, and are not feasible for use in future therapies. In order to improve the current mixed chimerism induction protocols, we developed a non-myeloablative bone-marrow transplant (NM-BMT) protocol using retinoic acid (RA)-induced alloantigen-specific Tregs, clinically available immunosuppressive drugs, and lower doses of irradiation. We demonstrate that RA-induced alloantigen-specific Tregs in addition to a NM-BMT protocol generates stable mixed chimerism and induces tolerance to allogeneic secondary skin allografts in mice. Therefore, the establishment of mixed chimerism through the use of donor-specific Tregs rather than non-specific immunosuppression could have a potential use in organ transplantation.

Methyl-Guanine-Methyl-Transferase Transgenic Bone Marrow Transplantation Allows N,N-bis(2-chloroethyl)-Nitrosourea Driven Donor Mixed-Chimerism Without Graft-Versus-Host Disease, and With Donor-Specific Allograft Tolerance

BACKGROUND

Transplant tolerance has been achieved by mixed chimerism in animal models and in a limited number of kidney transplant patients. However, these mixed-chimerism strategies were limited either by loss of long-term mixed chimerism or risk of graft-versus-host disease (GVHD). Selective bone marrow (BM) engraftment using marrow protective strategies are currently reaching clinical use. In this study, we tested the utility of methyl-guanine-methyl-transferase (MGMT)-transgenic-C57BL/6 BM into a major histocompatibility complex mismatched-BALB/c model followed by N,N-bis(2-chloroethyl)-nitrosourea (BCNU) treatment to enhance donor-cell engraftment and then evaluated transplant tolerance induction.

METHODS

A single-dose of anti-CD8 antibody and busulfan was administered into BALB/c-host-mice at day 1. The BALB/c-mice also received costimulatory blockade through multiple-doses of anti-CD40L antibody. 10 × 10(6) BM-cells from MGMT-transgenic-mice were transplanted into host BALB/c mice at day 0. The BCNU was administered at 4 time points after BM transplantation (BMT). Heterotopic donor C57BL/6 cardiac allografts were performed at day 243 after BMT. Skin transplantation with third-party CBA, host BALB/c and donor C57BL/6 grafts was performed at day 358 after BMT.

RESULTS

The BALB/c-mice showed long-term stable and high-level donor-cell engraftment with MGMT transgenic C57BL/6 BMT after BCNU treatment, demonstrating full reconstitution and donor cardiac-allograft tolerance and no GVHD with expanded donor and host Foxp3 T regulatory cells. Further, skin grafts from donor, host, and third party showed good immune function with rejection of third-party grafts from all mice and benefit from enhanced chimerism after BCNU with less cell infiltrate and no chronic rejection in the donor skin grafts of BCNU treated mice compared no BCNU treated mice.

CONCLUSIONS

High-level mixed chimerism without GVHD can be achieved using MGMT transgenic BM in a mixed-chimerism model receiving BCNU across a major histocompatibility complex mismatch. Enhanced mixed chimerism leads to long-term donor-specific allograft tolerance.

Establishment of transplantation tolerance via minimal conditioning in aged recipients

Mixed hematopoietic chimerism is a powerful means of generating donor-specific tolerance, allowing long-term graft acceptance without lifelong dependence on immunosuppressive drugs. To avoid the need for whole body irradiation and associated side effects, we utilized a radiation-free minimal conditioning regime to induce long-term tolerance across major histocompatibility barriers. We found that low-dose busulfan, in combination with host T cell depletion and short-term sirolimus-based immunosuppression, facilitated efficient donor engraftment. Tolerance was achieved when mice were transplanted with whole or T cell-depleted bone marrow, or purified progenitor cells. Tolerance induction was associated with an expansion in regulatory T cells and was not abrogated in the absence of a thymus, suggesting a dominant or compensatory peripheral mode of tolerance. Importantly, we were able to generate durable chimerism and tolerance to donor skin grafts in both young and aged mice, despite age-related thymic atrophy and immune senescence. Clinically, this is especially relevant as the majority of transplant recipients are older patients whose immune recovery might be dangerously slow and would benefit from radiation-free minimal conditioning regimes that allow efficient donor engraftment without fully ablating the recipient immune system.

γδ T cells restrain extrathymic development of Foxp3+-inducible regulatory T cells via IFN-γ

Inducible Treg (iTreg) cells generated from Ag-stimulated naïve CD4(+) T cells in the periphery play an important role in regulating immune responses. TGF-β is a key cytokine that promotes this conversion process; however, how this process is regulated in vivo remains unclear. Here, we report that γδ T cells play a crucial role in controlling iTreg generation and suppressor function. Ag-induced iTreg generation was significantly enhanced in C57BL/6 mice in the absence of γδ T cells. Inhibition of iTreg conversion was mediated by IFN-γ produced by activated γδ T cells but not by activated CD4(+) T cells. BM chimera experiments further confirmed γδ-derived IFN-γ-dependent mechanism in regulating iTreg generation in vivo. Lastly, human peripheral blood γδ T cells also interfere with iTreg conversion via IFN-γ. Our results suggest a novel function of γδ T cells in limiting the generation of iTreg cells, potentially balancing immunity and tolerance.

Mechanistic and therapeutic role of regulatory T cells in tolerance through mixed chimerism

PURPOSE OF REVIEW

Although substantial advances in transplantation medicine have improved short-term graft survival, long-term outcome after organ transplantation is unsatisfactory. The induction of donor-specific tolerance as a potential solution remains an unmet need. Mixed chimerism established through transplantation of donor bone marrow is an appealing tolerance strategy, but widespread clinical application is prevented by the toxicity of recipient conditioning, which is required for achieving bone marrow engraftment. Clonal deletion - both central and peripheral - has long been recognized as a cardinal mechanism in experimental mixed chimerism models.

RECENT FINDINGS

Several recent studies have delineated the importance of nondeletional, regulatory mechanisms for the induction of tolerance through mixed chimerism. Moreover, the therapeutic application of recipient regulatory T cells (Tregs) has been combined with the transplantation of donor bone marrow. Such a 'Treg-chimerism' protocol leads to engraftment of conventional doses of fully allogeneic bone marrow and to donor-specific tolerance without the need for any cytotoxic conditioning.

SUMMARY

Regulatory mechanisms play a major role in mixed chimerism protocols. Treg therapy is exceptionally effective in achieving bone marrow engraftment without cytotoxic recipient treatment, thereby eliminating a major toxic factor preventing widespread application of the mixed chimerism strategy.

T-regulatory cell treatment prevents chronic rejection of heart allografts in a murine mixed chimerism model

Background

The mixed chimerism approach induces donor-specific tolerance in both pre-clinical models and clinical pilot trials. However, chronic rejection of heart allografts and acute rejection of skin allografts were observed in some chimeric animals despite persistent hematopoietic chimerism and tolerance toward donor antigens in vitro. We tested whether additional cell therapy with regulatory T cells (Tregs) is able to induce full immunologic tolerance and prevent chronic rejection.

Methods

We recently developed a murine “Treg bone marrow (BM) transplantation (BMT) protocol” that is devoid of cytoreductive recipient pre-treatment. The protocol consists of a moderate dose of fully mismatched allogeneic donor BM under costimulation blockade, together with polyclonal recipient Tregs and rapamycin. Control groups received BMT under non-myeloablative irradiation and costimulation blockade without Treg therapy. Multilineage chimerism was followed by flow cytometry, and tolerance was assessed by donor-specific skin and heart allografts.

Results

Durable multilineage chimerism and long-term donor skin and heart allograft survival were successfully achieved with both protocols. Notably, histologic examination of heart allografts at the end of follow-up revealed that chronic rejection is prevented only in chimeras induced with the Treg protocol.

Conclusions

In a mouse model of mixed chimerism, additional Treg treatment at the time of BMT prevents chronic rejection of heart allografts. As the Treg-chimerism protocol also obviates the need for cytoreductive recipient treatment it improves both efficacy and safety over previous non-myeloablative mixed chimerism regimens. These results may significantly impact the development of protocols for tolerance induction in cardiac transplantation.

Signal one and two blockade are both critical for non-myeloablative murine HSCT across a major histocompatibility complex barrier

Non-myeloablative allogeneic haematopoietic stem cell transplantation (HSCT) is rarely achievable clinically, except where donor cells have selective advantages. Murine non-myeloablative conditioning regimens have limited clinical success, partly through use of clinically unachievable cell doses or strain combinations permitting allograft acceptance using immunosuppression alone. We found that reducing busulfan conditioning in murine syngeneic HSCT, increases bone marrow (BM):blood SDF-1 ratio and total donor cells homing to BM, but reduces the proportion of donor cells engrafting. Despite this, syngeneic engraftment is achievable with non-myeloablative busulfan (25 mg/kg) and higher cell doses induce increased chimerism. Therefore we investigated regimens promoting initial donor cell engraftment in the major histocompatibility complex barrier mismatched CBA to C57BL/6 allo-transplant model. This requires full myeloablation and immunosuppression with non-depleting anti-CD4/CD8 blocking antibodies to achieve engraftment of low cell doses, and rejects with reduced intensity conditioning (≤75 mg/kg busulfan). We compared increased antibody treatment, G-CSF, niche disruption and high cell dose, using reduced intensity busulfan and CD4/8 blockade in this model. Most treatments increased initial donor engraftment, but only addition of co-stimulatory blockade permitted long-term engraftment with reduced intensity or non-myeloablative conditioning, suggesting that signal 1 and 2 T-cell blockade is more important than early BM niche engraftment for transplant success.

Mixed chimerism through donor bone marrow transplantation: a tolerogenic cell therapy for application in organ transplantation

PURPOSE OF REVIEW

Organ transplantation is the state-of-the-art treatment for end-stage organ failure; however, long-term graft survival is still unsatisfactory. Despite improved immunosuppressive drug therapy, patients are faced with substantial side effects and the risk of chronic rejection with subsequent graft loss. The transplantation of donor bone marrow for the induction of mixed chimerism has been recognized to induce donor-specific tolerance a long time ago, but safety concerns regarding toxicities of current bone marrow transplantation (BMT) protocols impede widespread application.

RECENT FINDINGS

Recent studies in nonhuman primates and kidney transplant patients have demonstrated successful induction of allograft tolerance even though--in contrast to murine models--only transient chimerism was achieved. Progress toward the development of nontoxic murine BMT protocols revealed that Treg therapy is a potent therapeutic adjunct eliminating the need for cytotoxic recipient conditioning. Furthermore, new insight into the mechanisms underlying tolerization of CD4 and CD8 T cells in mixed chimeras has been gained and has identified possible difficulties impeding clinical translation.

SUMMARY

This review will address the recent advances in murine models as well as findings from the first clinical trials for the induction of tolerance through mixed chimerism. Both the potential for more widespread clinical application and the remaining hurdles and challenges of this tolerance approach will be discussed.

Mixed chimerism and split tolerance: mechanisms and clinical correlations

Establishing hematopoietic mixed chimerism can lead to donor-specific tolerance to transplanted organs and may eliminate the need for long-term immunosuppressive therapy, while also preventing chronic rejection. In this review, we discuss central and peripheral mechanisms of chimerism induced tolerance. However, even in the long-lasting presence of a donor organ or donor hematopoietic cells, some allogeneic tissues from the same donor can be rejected; a phenomenon known as split tolerance. With the current goal of creating mixed chimeras using clinically feasible amounts of donor bone marrow and with minimal conditioning, split tolerance may become more prevalent and its mechanisms need to be explored. Some predisposing factors that may increase the likelihood of split tolerance are immunogenicity of the graft, certain donor-recipient combinations, prior sensitization, location and type of graft and minimal conditioning chimerism induction protocols. Additionally, split tolerance may occur due to a differential susceptibility of various types of tissues to rejection. The mechanisms involved in a tissue's differential susceptibility to rejection include the presence of polymorphic tissue-specific antigens and variable sensitivity to indirect pathway effector mechanisms. Finally, we review the clinical attempts at allograft tolerance through the induction of chimerism; studies that are revealing the complex relationship between chimerism and tolerance. This relationship often displays split tolerance, and further research into its mechanisms is warranted.

Anti-LFA-1 or rapamycin overcome costimulation blockade-resistant rejection in sensitized bone marrow recipients

While costimulation blockade-based mixed chimerism protocols work well for inducing tolerance in rodents, translation to preclinical large animal/nonhuman primate models has been less successful. One recognized cause for these difficulties is the high frequency of alloreactive memory T cells (Tmem) found in the (pre)clinical setting as opposed to laboratory mice. In the present study, we therefore developed a murine bone marrow transplantation (BMT) model employing recipients harboring polyclonal donor-reactive Tmem without concomitant humoral sensitization. This model was then used to identify strategies to overcome this additional immune barrier. We found that B6 recipients that were enriched with 3 × 10(7) T cells isolated from B6 mice that had been previously grafted with Balb/c skin, rejected Balb/c BM despite costimulation blockade with anti-CD40L and CTLA4Ig (while recipients not enriched developed chimerism). Adjunctive short-term treatment of sensitized BMT recipients with rapamycin or anti-LFA-1 mAb was demonstrated to be effective in controlling Tmem in this model, leading to long-term mixed chimerism and donor-specific tolerance. Thus, rapamycin and anti-LFA-1 mAb are effective in overcoming the potent barrier that donor-reactive Tmem pose to the induction of mixed chimerism and tolerance despite costimulation blockade.

The road to purified hematopoietic stem cell transplants is paved with antibodies

Hematopoietic progenitor cell replacement therapy remains a surprisingly unrefined process. In general, unmanipulated bone marrow or mobilized peripheral blood (MPB) grafts which carry potentially harmful passenger cells are administered after treating recipients with high-dose chemotherapy and/or radiotherapy to eradicate malignant disease, eliminate immunologic barriers to allogeneic cell engraftment, and to 'make space' for rare donor stem cells within the stem cell niche. The sequalae of such treatments are substantial, including direct organ toxicity and nonspecific inflammation that contribute to the development of graft-versus-host disease (GVHD) and poor immune reconstitution. Passenger tumor cells that contaminate autologous hematopoietic grafts may contribute to relapse post-transplant. Use of antibodies to rid grafts of unwanted cell populations, and to eliminate or minimize the need for nonspecifically cytotoxic therapies used to condition transplant recipients, will dramatically improve the safety profile of allogeneic and gene-modified autologous hematopoietic stem cell therapies.

Evidence for kidney rejection after combined bone marrow and renal transplantation despite ongoing whole-blood chimerism in rhesus macaques

Although there is evidence linking hematopoietic chimerism induction and solid organ transplant tolerance, the mechanistic requirements for chimerism-induced tolerance are not clearly elucidated. To address this, we used an MHC-defined primate model to determine the impact of impermanent, T cell-poor, mixed-chimerism on renal allograft survival. We compared two cohorts: one receiving a bone marrow and renal transplant ("BMT/renal") and one receiving only a renal transplant. Both cohorts received maintenance immunosuppression with CD28/CD40-directed costimulation blockade and sirolimus. As previously demonstrated, this transplant strategy consistently induced compartmentalized donor chimerism, (significant whole-blood chimerism, lacking T cell chimerism). This chimerism was not sufficient to prolong renal allograft acceptance: the BMT/renal mean survival time (MST, 76 days) was not significantly different than the renal transplant alone MST (85 days, p = 0.46), with histopathology documenting T cell mediated rejection. Flow cytometric analysis revealed significant enrichment for CD28-/CD95+ CD4+ and CD8+ Tem cells in the rejected kidney, suggesting a link between CD28-negative Tem and costimulation blockade-resistant rejection. These results suggest that in some settings, transient T cell-poor chimerism is not sufficient to induce tolerance to a concurrently placed renal allograft and that the presence of this chimerism per se is not an independent biomarker to identify tolerance.

Double negative Treg cells promote nonmyeloablative bone marrow chimerism by inducing T-cell clonal deletion and suppressing NK cell function

The establishment of immune tolerance and prevention of chronic rejection remain major goals in clinical transplantation. In bone marrow (BM) transplantation, T cells and NK cells play important roles for graft rejection. In addition, graft-versus-host-disease (GVHD) remains a major obstacle for BM transplantation. In this study, we aimed to establish mixed chimerism in an irradiation-free condition. Our data indicate that adoptive transfer of donor-derived T-cell receptor (TCR) αβ(+) CD3(+) CD4(-) CD8(-) NK1.1(-) (double negative, DN) Treg cells prior to C57BL/6 to BALB/c BM transplantation, in combination with cyclophosphamide, induced a stable-mixed chimerism and acceptance of C57BL/6 skin allografts but rejection of third-party C3H (H-2k) skin grafts. Adoptive transfer of CD4(+) and CD8(+) T cells, but not DN Treg cells, induced GVHD in this regimen. The recipient T-cell alloreactive responsiveness was reduced in the DN Treg cell-treated group and clonal deletions of TCRVβ2, 7, 8.1/2, and 8.3 were observed in both CD4(+) and CD8(+) T cells. Furthermore, DN Treg-cell treatment suppressed NK cell-mediated BM rejection in a perforin-dependent manner. Taken together, our results suggest that adoptive transfer of DN Treg cells can control both adoptive and innate immunities and promote stable-mixed chimerism and donor-specific tolerance in the irradiation-free regimen.

Heterogeneity within T Cell Memory: Implications for Transplant Tolerance

Adaptive immunity in both mouse and man results in the generation of immunological memory. Memory T cells are both friend and foe to transplant recipients, as they are intimately involved and in many cases absolutely required for the maintenance of protective immunity in the face immunosuppression, yet from the evidence presented herein they clearly constitute a formidable barrier for the successful implementation of tolerance induction strategies in transplantation. This review describes the experimental evidence demonstrating the increased resistance of memory T cells to many distinct tolerance induction strategies, and outlines recent advances in our knowledge of the ways in which alloreactive memory T cells arise in previously untransplanted individuals. Understanding the impact of alloreactive memory T cell specificity, frequency, and quality might allow for better donor selection in order to minimize the donor-reactive memory T cell barrier in an individual transplant recipient, thus allowing stratification of relative risk of alloreactive memory T cell mediated rejection, and conversely increase the likelihood of successful establishment of tolerance. However, further research into the molecular and cellular pathways involved in alloreactive memory T cell-mediated rejection is required in order to design new strategies to overcome the memory T cell barrier, without critically impairing protective immunity.

CD40 blockade combines with CTLA4Ig and sirolimus to produce mixed chimerism in an MHC-defined rhesus macaque transplant model

In murine models, T-cell costimulation blockade of the CD28:B7 and CD154:CD40 pathways synergistically promotes immune tolerance after transplantation. While CD28 blockade has been successfully translated to the clinic, translation of blockade of the CD154:CD40 pathway has been less successful, in large part due to thromboembolic complications associated with anti-CD154 antibodies. Translation of CD40 blockade has also been slow, in part due to the fact that synergy between CD40 blockade and CD28 blockade had not yet been demonstrated in either primate models or humans. Here we show that a novel, nondepleting CD40 monoclonal antibody, 3A8, can combine with combined CTLA4Ig and sirolimus in a well-established primate bone marrow chimerism-induction model. Prolonged engraftment required the presence of all three agents during maintenance therapy, and resulted in graft acceptance for the duration of immunosuppressive treatment, with rejection resulting upon immunosuppression withdrawal. Flow cytometric analysis revealed that upregulation of CD95 expression on both CD4+ and CD8+ T cells correlated with rejection, suggesting that CD95 may be a robust biomarker of graft loss. These results are the first to demonstrate prolonged chimerism in primates treated with CD28/mTOR blockade and nondepletional CD40 blockade, and support further investigation of combined costimulation blockade targeting the CD28 and CD40 pathways.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Belatacept: from rational design to clinical application

Gradually improved immunosuppression has contributed significantly to the progress achieved in transplantation medicine so far. Nevertheless, current drug regimens are associated with late graft loss--in particular as a result of immunologic damage or drug toxicity--and substantial morbidity. Recently, the costimulation blocker belatacept (marketed under the name Nulojix®) has been approved for immunosuppression in renal transplantation. Belatacept (a mutated version of CTLA4Ig) is a fusion protein rationally designed to block CD28, a critical activating receptor on T cells, by binding and saturating its ligands B7-1 and B7-2. In phase II and III trials, belatacept was compared with cyclosporine (in combination with basiliximab, MMF, and steroids). Advantages observed with belatacept include superior graft function, preservation of renal structure and improved cardiovascular risk profile. Concerns associated with belatacept are a higher frequency of cellular rejection episodes and more post-transplant lymphoproliferative disorder (PTLD) cases especially in EBV seronegative patients, who should be excluded from belatacept-based regimens. Thus, after almost three decades of calcineurin inhibitors as mainstay of immunosuppression, belatacept offers a potential alternative. In this article, we will provide an overview of belatacept's preclinical development and will discuss the available evidence from clinical trials.

Dipeptidyl peptidase IV (DPPIV/CD26) inhibition does not improve engraftment of unfractionated syngeneic or allogeneic bone marrow after nonmyeloablative conditioning

In order to develop minimally toxic bone marrow transplantation (BMT) protocols suitable for use in a wider range of indications, it is important to identify ways to enhance BM engraftment at a given level of recipient conditioning. CXCL12/stromal cell-derived factor-1α plays a crucial physiological role in homing of hematopoietic stem cells to BM. It is regulated by the ectopeptidase dipeptidyl peptidase IV (DPPIV; DPP4) known as CD26, which cleaves dipeptides from the N-terminus of polypeptide chains. Blocking DPPIV enzymatic activity had a beneficial effect on hematopoietic stem cell engraftment in various but very specific experimental settings. Here we investigated whether inhibition of DPPIV enzymatic activity through Diprotin A or sitagliptin (Januvia) improves BM engraftment in nonmyeloablative murine models of syngeneic (i.e., CD45-congenic) and allogeneic (i.e., Balb/c to B6) BMT (1 Gy total body irradiation, 10–15 × 10⁶ unseparated BM cells/mouse). Neither Diprotin A administered in vivo at the time of BMT and/or used for in vitro pretreatment of BM nor sitagliptin administered in vivo had a detectable effect on the level of multilineage chimerism (follow-up >20 weeks). Similarly, sitagliptin did not enhance chimerism after allogeneic BMT, even though DPPIV enzymatic activity measured in serum was profoundly inhibited (>98% inhibition at peak exposure). Our results provide evidence that DPPIV inhibition via Diprotin A or sitagliptin does not improve engraftment of unseparated BM in a nonmyeloablative BMT setting.

The role of natural killer T cells in costimulation blockade-based mixed chimerism

Distinct lymphocyte populations have been identified that either promote or impede the establishment of chimerism and tolerance through allogeneic bone marrow transplantation (BMT). Natural killer T (NKT) cells have pleiotropic regulatory properties capable of either augmenting or downmodulating various immune responses. We investigated in this study whether NKT cells affect outcome in mixed chimerism models employing fully mismatched nonmyeloablative BMT with costimulation blockade (CB). The absence of NKT cells had no detectable effect on chimerism or skin graft tolerance after conditioning with 3Gy total body irradiation (TBI), and a limited positive effect with 1Gy TBI. Stimulation of NKT cells with alpha-galactosylceramide (alpha-gal) at the time of BMT prevented chimerism and tolerance. Activation of recipient (as opposed to donor) NKT cells was necessary and sufficient for the alpha-gal effect. The detrimental effect of NKT activation was also observed in the absence of T cells after conditioning with in vivo T-cell depletion (TCD). NKT cells triggered rejection of BM via NK cells as chimerism and tolerance were not abrogated when NKT cells were stimulated in the absence of both NK cells and T cells. Thus, activation of NKT cells at the time of BMT overcomes the effects of CB, inhibiting the establishment of chimerism and tolerance.

Cell-based therapy in allergy

IgE-mediated allergy is an immunological disorder occurring in response to otherwise harmless environmental antigens (i.e., allergens). Development of effective therapeutic or preventive approaches inducing robust tolerance toward allergens remains an unmet goal. Several experimental tolerance approaches have been described. The therapeutic use of regulatory T cells (Tregs) and the establishment of molecular chimerism are two cell-based strategies that are of particular interest. Treg therapy is close to clinical application, but its efficacy remains to be fully defined. Recent proof-of-concept studies demonstrated that transplantation of syngeneic hematopoietic stem cells modified in vitro to express a major allergen leads to molecular chimerism and robust allergen-specific tolerance. Here we review cell-based tolerance strategies in allergy, discussing their potentials and limitations.

An MHC-defined primate model reveals significant rejection of bone marrow after mixed chimerism induction despite full MHC matching

In murine models, mixed hematopoietic chimerism induction leads to robust immune tolerance. However, translation to primates and to patients has been difficult. In this study, we used a novel MHC-defined rhesus macaque model to examine the impact of MHC matching on the stability of costimulation blockade-/sirolimus-mediated chimerism, and to probe possible mechanisms of bone marrow rejection after nonmyeloablative transplant. Using busulfan-based pretransplant preparation and maintenance immunosuppression with sirolimus, as well as CD28 and CD154 blockade, all recipients demonstrated donor engraftment after transplant. However, the mixed chimerism that resulted was compartmentalized, with recipients demonstrating significantly higher whole blood chimerism compared to T cell chimerism. Thus, the vast majority of T cells presenting posttransplant were recipient-rather than donor-derived. Surprisingly, even in MHC-matched transplants, rejection of donor hematopoiesis predominated after immunosuppression withdrawal. Weaning of immunosuppression was associated with a surge of antigen-experienced T cells, and transplant rejection was associated with the acquisition of donor-directed T cell alloreactivity. These results suggest that a reservoir of alloreactive cells was present despite prior costimulation blockade and sirolimus, and that the post-immunosuppression lymphocytic rebound may have lead to a phenotypic shift in these recipient T cells towards an activated, antigen-experienced phenotype, and ultimately, to transplant rejection.

Infusion of Lin- bone marrow cells results in multilineage macrochimerism and skin allograft tolerance in minimally conditioned recipient mice

Donor-specific immunological tolerance using high doses of donor bone marrow cells (BMC) has been demonstrated in mixed chimerism-based tolerance induction protocols; however, the development of graft versus host disease (GVHD) remains a risk. In the present study, we demonstrate that the infusion of low numbers of donor Lin(-) bone marrow cells (Lin(-) BMC) 7 days post allograft transplantation facilitates high level macrochimerism induction and graft tolerance. Full-thickness BALB/c skin allografts were transplanted onto C57BL/6 mice. Mice were treated with anti-CD4 and anti-CD8 mAbs on day 0, +2, +5, +7 and +14 along with low dose busulfan on day +5. A low dose of highly purified Lin(-) BMC from BALB/c donor mice was infused on day +7. Chimerism and clonal cell deletion were evaluated using flow cytometry. Donor-specific tolerance was tested by donor and third-party skin grafting and mixed leukocyte reaction (MLR). Lin(-) BMC infusion with minimal immunosuppression led to stable, mixed, multilineage macrochimerism and long-term allograft survival (>300 days). Mixed donor-recipient macrochimerism was observed. Donor-reactive T cells were clonally deleted and a 130% increase in CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) was observed in the spleen. Tolerant mice subsequently accepted second donor, but not third-party (C3H), skin grafts and recipient splenocytes failed to react with allogeneic donor cells indicating donor-specific immunological tolerance was achieved. We conclude that the infusion of donor Lin(-) BMC without cytoreductive recipient conditioning can induce indefinite survival of skin allografts via mechanisms involving the establishment of a multilineage macrochimeric state principally through clonal deletion of alloreactive T cells and peripherally induced CD4(+)Foxp3(+) Tregs.

GVHD after haploidentical transplantation: a novel, MHC-defined rhesus macaque model identifies CD28- CD8+ T cells as a reservoir of breakthrough T-cell proliferation during costimulation blockade and sirolimus-based immunosuppression

We have developed a major histocompatibility complex-defined primate model of graft-versus-host disease (GVHD) and have determined the effect that CD28/CD40-directed costimulation blockade and sirolimus have on this disease. Severe GVHD developed after haploidentical transplantation without prophylaxis, characterized by rapid clinical decline and widespread T-cell infiltration and organ damage. Mechanistic analysis showed activation and possible counter-regulation, with rapid T-cell expansion and accumulation of CD8(+) and CD4(+) granzyme B(+) effector cells and FoxP3(pos)/CD27(high)/CD25(pos)/CD127(low) CD4(+) T cells. CD8(+) cells down-regulated CD127 and BCl-2 and up-regulated Ki-67, consistent with a highly activated, proliferative profile. A cytokine storm also occurred, with GVHD-specific secretion of interleukin-1 receptor antagonist (IL-1Ra), IL-18, and CCL4. Costimulation Blockade and Sirolimus (CoBS) resulted in striking protection against GVHD. At the 30-day primary endpoint, CoBS-treated recipients showed 100% survival compared with no survival in untreated recipients. CoBS treatment resulted in survival, increasing from 11.6 to 62 days (P < .01) with blunting of T-cell expansion and activation. Some CoBS-treated animals did eventually develop GVHD, with both clinical and histopathologic evidence of smoldering disease. The reservoir of CoBS-resistant breakthrough immune activation included secretion of interferon-γ, IL-2, monocyte chemotactic protein-1, and IL-12/IL-23 and proliferation of cytotoxic T-lymphocyte-associated antigen 4 immunoglobulin-resistant CD28(-) CD8(+) T cells, suggesting adjuvant treatments targeting this subpopulation will be needed for full disease control.

Potential role of host effector memory CD8+ T cells in marrow rejection after mixed chimerism induction in cynomolgus monkeys

Mixed hematopoietic chimerism provides a powerful means of achieving transplantation tolerance. We investigated the efficacy of combined blockade of the CD40/CD154 and CD28/B7 costimulation pathways to induce sustained mixed chimerism in cynomolgus monkeys following major histocompatibility complex-mismatched bone marrow (BM) transplants. A nonmyeloablative conditioning regimen of busulfan, intravenous and intraosseous ifosfamide, and anti-thymocyte globulin was used. BM transplantation was followed by a one-week course of CTLA4-Ig/anti-CD154 monoclonal antibodies. Three recipients achieved a wide range of transient chimerism (10.8-79.8%). A rapid proliferation of host effector memory (CD28(low)CD95(high)) CD8(+) T cells was observed in conditioned animals whether or not they received allogeneic BM, and this expansion occurred concurrently with the loss of chimerism in BM recipients. CD8(+) T cells from the recipients had increased reactivity to donor stimulators vs. third-party stimulators. Additional immunosuppression with tacrolimus or deoxyspergualin after transplantation delayed post-transplant proliferation of effector memory CD8(+) T cells but did not promote chimerism. A one-month course of costimulatory blockade also did not prevent marrow rejection. These studies demonstrate that combined CD40/CD154 and CD28/B7 costimulatory blockade supports transient mixed chimerism induction following nonmyeloablative conditioning in primates, but is insufficient to overcome host immune resistance likely mediated by effector memory CD8(+) T cells.

Multiple costimulatory blockade in the peripheral nerve allograft

BACKGROUND

In the nerve allograft model, costimulation blockade has permitted good regeneration but is still inferior to the nerve isograft. We hypothesize that a short course of multiple costimulatory pathway blockade will be more effective in inhibiting the redundancy of the immune response and improve nerve regeneration through the nerve allograft.

METHODS

The murine sciatic nerve allograft model was used to reconstruct a 1 cm sciatic nerve gap. Treatment consisted of the inhibition of the CD40, CD28/B7 and ICOS pathways and was compared with only single or double costimulation blockade. Assessment methods included quantitative histomorphometry and ELISPOT assay to quantify the host immune response after 3 weeks post-operatively.

RESULTS

Triple costimulation blockade permitted regeneration through the nerve allograft that was equivalent to the nerve isograft. A short course of three doses was more effective than a single dose for all combinations tested. ELISPOT assay demonstrated minimal in vitro immune response with a short course of double or triple pathway-blocking agents.

CONCLUSION

Costimulation blockade, especially with the simultaneous inhibition of multiple pathways, remains a promising strategy to promote regeneration through the peripheral nerve allograft, and may be uniquely suited to the temporary immunosuppressive requirements of the peripheral nerve allograft.

Biomarkers to discern transplantation tolerance after allogeneic hematopoietic cell transplantation

Although it is commonly accepted that allogeneic hematopoietic cell transplant (HCT) recipients develop transplantation tolerance and can quickly discontinue all immunosuppressive drugs, existing data does not support this concept. Most patients will require a prolonged duration of immunosuppression, lasting commonly several years. This has even greater importance, as the majority of transplants are now performed utilizing peripheral blood mobilized stem cells, which are associated with an increased risk of chronic graft-versus-host disease (cGVHD) and prolonged duration of immunosuppression. Despite these challenges, the approach to liberation from immunosuppression after HCT is empiric, and biomarkers of operational tolerance after HCT are lacking. Conversely, investigators in solid organ allografting have begun to examine tolerance associated gene expression in renal and hepatic allograft recipients. Significant challenges in the design and interpretation of these studies potentially limit comparisons. However, a relatively unified model is beginning to emerge, which largely recapitulates previously established mechanisms of immune tolerance. This evidence supports a state of immune quiescence with reduced expression of costimulation and immune response genes, and upregulation of cell cycle control genes. Data indirectly supports the importance of tumor growth factor (TGF)-beta, supports the role of CD4(+)CD25(+) regulatory T cells, and offers new insights into the role of natural killer (NK) cells. Distinct in hepatic allograft tolerance, emerging evidence highlights the importance of gammadeltaT cells, and selection of the Vgammadelta1+ subtype among the gammadeltaT cell population. The deficiencies in the current understanding of transplantation tolerance after HCT, as well as the inadequacies evident in the current empiric approach to immunosuppressive medication (IS) management after HCT make clear the rationale for investigation aimed at elucidating tolerance associated biomarkers after HCT.

De novo recruitment of antigen-experienced and naive T cells contributes to the long-term maintenance of antiviral T cell populations in the persistently infected central nervous system

Mice infected with attenuated strains of mouse hepatitis virus, strain JHM, develop a chronic infection in the brain and spinal cord characterized by low levels of viral Ag persistence and retention of virus-specific CD4 and CD8 T cells at the site of infection. It is not known whether these cells are maintained by proliferation of T cells that entered the CNS during acute infection or are newly recruited from Ag-experienced or naive T cell pools. In this study, using adoptive transfer experiments and bone marrow chimeras, we show that at least some of these cells are recruited from the periphery, predominantly from the viral Ag-experienced T cell pool. Both virus-specific CD4 and CD8 T cells are functional, as assessed by cytokine expression and degranulation after peptide exposure. In addition, populations of virus-specific CD4 T cells undergo dynamic changes in the infected CNS, as previously shown for CD8 T cells, because ratios of cells responding to two CD4 T cell epitopes change by a factor of five during the course of persistence. Collectively, these results show that maintenance of T cell responses in the virus-infected CNS is a dynamic process. Further, virus-specific T cell numbers at this site of infection are maintained by recruitment from peripheral Ag-experienced and naive T cell pools.

Anti-CD40 monoclonal antibody synergizes with CTLA4-Ig in promoting long-term graft survival in murine models of transplantation

Blockade of the CD40/CD154 signaling pathway using anti-CD154 Abs has shown promise in attenuating the alloimmune response and promoting long-term graft survival in murine model systems, although side effects observed in humans have hampered its progression through clinical trials. Appropriately designed anti-CD40 Abs may provide a suitable alternative. We investigated two isoforms of a novel monoclonal rat anti-mouse CD40 Ab (7E1) for characteristics and effects mirroring those of anti-CD154: 7E1-G1 (an IgG1 isotype); and 7E1-G2b (an IgG2b isotype). In vitro proliferation assays to measure the agonist properties of the two anti-CD40 Abs revealed similar responses when plate bound. However, when present as a soluble stimulus, 7E1-G1 but not 7E1-G2b led to proliferation. 7E1-G2b was as effective as anti-CD154 when administered in vivo in concert with CTLA4-Ig in promoting both allogeneic bone marrow chimerism and skin graft survival, whereas 7E1-G1 was not. The protection observed with 7E1-G2b was not due to depletion of CD40-bearing APCs. These data suggest that an appropriately designed anti-CD40 Ab can promote graft survival as well as anti-CD154, making 7E1-G2b an attractive substitute in mouse models of costimulation blockade-based tolerance regimens.

Mitomycin C-treated antigen-presenting cells as a tool for control of allograft rejection and autoimmunity: from bench to bedside

Cells have been previously used in experimental models for tolerance induction in organ transplantation and autoimmune diseases. One problem with the therapeutic use of cells is standardization of their preparation. We discuss an immunosuppressive strategy relying on cells irreversibly transformed by a chemotherapeutic drug. Dendritic cells (DCs) of transplant donors pretreated with mitomycin C (MMC) strongly prolonged rat heart allograft survival when injected into recipients before transplantation. Likewise, MMC-DCs loaded with myelin basic protein suppressed autoreactive T cells of MS patients in vitro and prevented experimental autoimmune encephalitis in mice. Comprehensive gene microarray analysis identified genes that possibly make up the suppressive phenotype, comprising glucocorticoid leucine zipper, immunoglobulin-like transcript 3, CD80, CD83, CD86, and apoptotic genes. Based on these findings, a hypothetical model of tolerance induction by MMC-treated DCs is delineated. Finally, we describe the first clinical application of MMC-treated monocyte-enriched donor cells in an attempt to control the rejection of a haploidentical stem cell transplant in a sensitized recipient and discuss the pros and cons of using MMC-treated antigen-presenting cells for tolerance induction. Although many questions remain, MMC-treated cells are a promising clinical tool for controlling allograft rejection and deleterious immune responses in autoimmune diseases.

CTLA-4 on alloreactive CD4 T cells interacts with recipient CD80/86 to promote tolerance

Although the inhibitory receptor CTLA-4 (CD152) has been implicated in peripheral CD4 T-cell tolerance, its mechanism of action remains poorly defined. We analyzed mechanisms of CD4 cell tolerance in a model of tolerance induction involving establishment of mixed hematopoietic chimerism in recipients of fully MHC-mismatched allogeneic bone marrow cells with anti-CD154 mAb. Animals lacking CD80 and CD86 failed to achieve chimerism. We detected no T cell-intrinsic requirement for CD28 for chimerism induction. However, a CD4 T cell-intrinsic signal through CTLA-4 was shown to be essential within the first 48 hours of exposure to alloantigen for the establishment of tolerance and mixed chimerism. This signal must be provided by a recipient CD80/86(+) non-T-cell population. Donor CD80/86 expression was insufficient to achieve tolerance. Together, our findings demonstrate a surprising role for interactions of CTLA-4 expressed by alloreactive peripheral CD4 T cells with CD80/86 on recipient antigen-presenting cells (APCs) in the induction of early tolerance, suggesting a 3-cell tolerance model involving directly alloreactive CD4 cells, donor antigen-expressing bone marrow cells, and recipient antigen-presenting cells. This tolerance is independent of regulatory T cells and culminates in the deletion of directly alloreactive CD4 T cells.

Rapid deletional peripheral CD8 T cell tolerance induced by allogeneic bone marrow: role of donor class II MHC and B cells

Mixed chimerism and donor-specific tolerance are achieved in mice receiving 3 Gy of total body irradiation and anti-CD154 mAb followed by allogeneic bone marrow (BM) transplantation. In this model, recipient CD4 cells are critically important for CD8 tolerance. To evaluate the role of CD4 cells recognizing donor MHC class II directly, we used class II-deficient donor marrow and were not able to achieve chimerism unless recipient CD8 cells were depleted, indicating that directly alloreactive CD4 cells were necessary for CD8 tolerance. To identify the MHC class II(+) donor cells promoting this tolerance, we used donor BM lacking certain cell populations or used positively selected cell populations. Neither donor CD11c(+) dendritic cells, B cells, T cells, nor donor-derived IL-10 were critical for chimerism induction. Purified donor B cells induced early chimerism and donor-specific cell-mediated lympholysis tolerance in both strain combinations tested. In contrast, positively selected CD11b(+) monocytes/myeloid cells did not induce early chimerism in either strain combination. Donor cell preparations containing B cells were able to induce early deletion of donor-reactive TCR-transgenic 2C CD8 T cells, whereas those devoid of B cells had reduced activity. Thus, induction of stable mixed chimerism depends on the expression of MHC class II on the donor marrow, but no requisite donor cell lineage was identified. Donor BM-derived B cells induced early chimerism, donor-specific cell-mediated lympholysis tolerance, and deletion of donor-reactive CD8 T cells, whereas CD11b(+) cells did not. Thus, BM-derived B cells are potent tolerogenic APCs for alloreactive CD8 cells.

PD-1-dependent mechanisms maintain peripheral tolerance of donor-reactive CD8+ T cells to transplanted tissue

Peripheral mechanisms of self-tolerance often depend on the quiescent state of the immune system. To what degree such mechanisms can be engaged in the enhancement of allograft survival is unclear. To examine the role of the PD-1 pathway in the maintenance of graft survival following blockade of costimulatory pathways, we used a single-Ag mismatch model of graft rejection where we could track the donor-specific cells as they developed endogenously and emerged from the thymus. We found that graft-specific T cells arising under physiologic developmental conditions at low frequency were actively deleted at the time of transplantation under combined CD28/CD40L blockade. However, this deletion was incomplete, and donor-specific cells that failed to undergo deletion up-regulated expression of PD-1. Furthermore, blockade of PD-1 signaling on these cells via in vivo treatment with anti-PD-1 mAb resulted in rapid expansion of donor-specific T cells and graft loss. These results suggest that the PD-1 pathway was engaged in the continued regulation of the low-frequency graft-specific immune response and thus in maintenance of graft survival.

Murine mobilized peripheral blood stem cells have a lower capacity than bone marrow to induce mixed chimerism and tolerance

Allogeneic bone marrow transplantation (BMT) under costimulation blockade allows induction of mixed chimerism and tolerance without global T-cell depletion (TCD). The mildest such protocols without recipient cytoreduction, however, require clinically impracticable bone marrow (BM) doses. The successful use of mobilized peripheral blood stem cells (PBSC) instead of BM in such regimens would provide a substantial advance, allowing transplantation of higher doses of hematopoietic donor cells. We thus transplanted fully allogeneic murine granulocyte colony-stimulating factor (G-CSF) mobilized PBSC under costimulation blockade (anti-CD40L and CTLA4Ig). Unexpectedly, PBSC did not engraft, even when very high cell doses together with nonmyeloablative total body irradiation (TBI) were used. We show that, paradoxically, T cells contained in the donor PBSC triggered rejection of the transplanted donor cells. Rejection of donor BM was also triggered by the cotransplantation of unmanipulated donor T cells isolated from naïve (nonmobilized) donors. Donor-specific transfusion and transient immunosuppression prevented PBSC-triggered rejection and mixed chimerism and tolerance were achieved, but graft-versus-host disease (GVHD) occurred. The combination of in vivo TCD with costimulation blockade prevented rejection and GVHD. Thus, if allogeneic PBSC are transplanted instead of BM, costimulation blockade alone does not induce chimerism and tolerance without unacceptable GVHD-toxicity, and the addition of TCD is required for success.

Expansion of effector memory TCR Vbeta4+ CD8+ T cells is associated with latent infection-mediated resistance to transplantation tolerance

Therapies that control largely T cell-dependent allograft rejection in humans also possess the undesirable effect of impairing T cell function, leaving transplant recipients susceptible to opportunistic viruses. Prime among these opportunists are the ubiquitous herpesviruses. To date, studies are lacking that address the effect of viruses that establish a true latent state on allograft tolerance or the effect of tolerance protocols on the immune control of latent viruses. By using a mixed chimerism-based tolerance-induction protocol, we found that mice undergoing latent infection with gammaHV68, a murine gamma-herpesvirus closely related to human gamma-herpesviruses such as EBV and Kaposi's sarcoma-associated herpesvirus, significantly resist tolerance to allografts. Limiting the degree of virus reactivation or innate immune response did not reconstitute chimerism in latently infected mice. However, gammaHV68-infected mice showed increased frequency of CD8+ T cell alloreactivity and, interestingly, expansion of virus-induced, alloreactive, "effector/effector memory" TCR Vbeta4+CD8+ T cells driven by the gammaHV68-M1 gene was associated with resistance to tolerance induction in studies using gammaHV68-M1 mutant virus. These results define the viral gene and immune cell types involved in latent infection-mediated resistance to allograft tolerance and underscore the influence of latent herpesviruses on allograft survival.

Modulation of T cell homeostasis and alloreactivity under continuous FTY720 exposure

The immunomodulator FTY720 inhibits lymph node (LN) and thymic egress, thereby constraining T cell circulation and reducing peripheral T cell numbers. Here, we analyzed in mouse models the as yet scarcely characterized impact of long-term (up to 6 months) FTY720 exposure on T cell homeostasis and possible consequences for alloreactivity. In green fluorescent protein (GFP) hemopoietic chimeras, the turnover of (initially GFP(-)) peripheral T cell pools was markedly delayed under FTY720, while normal homeostatic differences between CD4 and CD8 T cell sub-populations were retained or amplified further. Homeostatic proliferation was enhanced, and within shrinking T cell pools, the proportions of effector memory phenotype CD4 T cells (CD4T(PEM)) increased in spleens and LNs and of central memory phenotype CD8 T cells (CD8T(PCM)) in LNs. By contrast, the fractions of CD8T(PEM) and CD4T(PCM) remained stably small under FTY720. The enrichment for CD4T(PEM) and CD8T(PCM) correlated with larger proportions of IFNgamma-producing T cells upon nonspecific but not allospecific stimulation. Splenic CD4 T cells from FTY720-treated mice proliferated more strongly upon transfer to semi-allogeneic hosts. However, heart allograft survival was not compromised in FTY720 pre-treated recipients. It correlated with reduced intra-graft CD8 T cells, and the longest surviving transplants contained the highest numbers of CD4 T cells. Thus, continuous FTY720 exposure reveals differential homeostatic responses by memory phenotype CD4 and CD8 T cell sub-populations, and it may enhance alloreactive CD4 T cell proliferation and tissue infiltration without accelerating allograft rejection.

Sustained expression of alpha1-antitrypsin after transplantation of manipulated hematopoietic stem cells

Inherited mutations in the human alpha(1)-antitrypsin (AAT) gene lead to deficient circulating levels of AAT protein and a predisposition to developing emphysema. Gene therapy for individuals deficient in AAT is an attractive goal, because transfer of a normal AAT gene into any cell type able to secrete AAT should reverse deficient AAT levels and attenuate progression of lung disease. Here we present an approach for AAT gene transfer based on the transplantation of lentivirally transduced hematopoietic stem cells (HSCs). We develop a novel dual-promoter lentiviral system to transfer normal human AAT cDNA as well as a fluorescent tracking "reporter gene" into murine HSCs. After transplantation of 3,000 transduced HSCs into irradiated mouse recipients, we demonstrate simultaneous and sustained systemic expression of both genes in vivo for at least 31 weeks. The stem cells transduced with this protocol maintain multipotency, self-renewal potential, and the ability to reconstitute the hematopoietic systems of both primary and secondary recipients. This lentiviral-based system may be useful for investigations requiring the systemic secretion of anti-proteases or cytokines relevant to the pathogenesis of a variety of lung diseases.

Transplantation tolerance: lessons from experimental rodent models

Immunological tolerance or functional unresponsiveness to a transplant is arguably the only approach that is likely to provide long-term graft survival without the problems associated with life-long global immunosuppression. Over the past 50 years, rodent models have become an invaluable tool for elucidating the mechanisms of tolerance to alloantigens. Importantly, rodent models can be adapted to ensure that they reflect more accurately the immune status of human transplant recipients. More recently, the development of genetically modified mice has enabled specific insights into the cellular and molecular mechanisms that play a key role in both the induction and maintenance of tolerance to be obtained and more complex questions to be addressed. This review highlights strategies designed to induce alloantigen specific immunological unresponsiveness leading to transplantation tolerance that have been developed through the use of experimental models.