Mixed chimerism and transplantation tolerance

Biomaterials-based immunomodulation enhances survival of murine vascularized composite allografts

Vascularized composite allotransplantation (VCA) is a restorative option for patients suffering from severe tissue defects not amenable to conventional reconstruction. However, the toxicities associated with life-long multidrug immunosuppression to enable allograft survival and induce immune tolerance largely limit the broader application of VCA. Here, we investigate the potential of targeted immunomodulation using CTLA4-Ig combined with a biological porcine-derived extracellular matrix (ECM) scaffold that elicits a pro-regenerative Th2 response to promote allograft survival and regulate the inflammatory microenvironment in a stringent mouse orthotopic hind limb transplantation model (BALB/c to C57BL/6). The median allograft survival time (MST) increased significantly from 15.0 to 24.5 days (P = 0.0037; Mantel-Cox test) after adding ECM to the CTLA4-Ig regimen. Characterization of the immune infiltration shows a pro-regenerative phenotype prevails over those associated with inflammation and rejection including macrophages (F4/80^hi+CD206^hi+MHCII^low), eosinophils (F4/80^lowSiglec-F⁺), and T helper 2 (Th2) T cells (CD4⁺IL-4⁺). This was accompanied by an increased expression of genes associated with a Type 2 polarized immune state such as Il4, Ccl24, Arg1 and Ym1 within the graft. Furthermore, when ECM was applied along with a clinically relevant combination of CTLA4-Ig and Rapamycin, allograft survival was prolonged from 33.0 to 72.5 days (P = 0.0067; Mantel-Cox test). These studies implicate the clinical exploration of combined regimens involving local application of pro-regenerative, immunomodulatory biomaterials in surgical wound sites with targeted co-stimulatory blockade to reduce adverse effects of immunosuppression and enhance graft survival in VCA.

Tregs and Mixed Chimerism as Approaches for Tolerance Induction in Islet Transplantation

Pancreatic islet transplantation is a promising method for the treatment of type 1 and type 3 diabetes whereby replacement of islets may be curative. However, long-term treatment with immunosuppressive drugs (ISDs) remains essential for islet graft survival. Current ISD regimens carry significant side-effects for transplant recipients, and are also toxic to the transplanted islets. Pre-clinical efforts to induce immune tolerance to islet allografts identify ways in which the recipient immune system may be reeducated to induce a sustained transplant tolerance and even overcome autoimmune islet destruction. The goal of these efforts is to induce tolerance to transplanted islets with minimal to no long-term immunosuppression. Two most promising cell-based therapeutic strategies for inducing immune tolerance include T regulatory cells (Tregs) and donor and recipient hematopoietic mixed chimerism. Here, we review preclinical studies which utilize Tregs for tolerance induction in islet transplantation. We also review myeloablative and non-myeloablative hematopoietic stem cell transplantation (HSCT) strategies in preclinical and clinical studies to induce sustained mixed chimerism and allograft tolerance, in particular in islet transplantation. Since Tregs play a critical role in the establishment of mixed chimerism, it follows that the combination of Treg and HSCT may be synergistic. Since the success of the Edmonton protocol, the feasibility of clinical islet transplantation has been established and nascent clinical trials testing immune tolerance strategies using Tregs and/or hematopoietic mixed chimerism are underway or being formulated.

Adipose-derived stromal cell therapy combined with a short course nonmyeloablative conditioning promotes long-term graft tolerance in vascularized composite allotransplantation

The risks of chronic immunosuppression limit the utility of vascularized composite allotransplantation (VCA) as a reconstructive option in complex tissue defects. We evaluated a novel, clinically translatable, radiation-free conditioning protocol that combines anti-lymphocyte serum (ALS), tacrolimus, and cytotoxic T-lymphocyte-associated protein 4 immunoglobulin (CTLA4-Ig) with adipose-derived stromal cells (ASCs) to allow VCA survival without long-term systemic immunosuppression. Full-mismatched rat hind-limb-transplant recipients received tacrolimus (0.5 mg/kg) for 14 days and were assigned to 4 groups: controls (CTRL) received no conditioning; ASC-group received CTLA4-Ig (10 mg/kg body weight i.p. postoperative day [POD] 2, 4, 7) and donor ASCs (1 × 10⁶ iv, POD 2, 4, 7, 15, 28); the ASC-cyclophosphamide (CYP)-group received CTLA4-Ig, ASC plus cyclophosphamide (50 mg/kg ip, POD 3); the ASC-ALS-group received CTLA4-Ig, ASCs plus ALS (500 µL ip, POD 1, 5). Banff grade III or 120 days were endpoints. ASCs suppressed alloresponse in vitro. Median rejection-free VCA survival was 28 days in CTRL (n = 7), 34 in ASC (n = 6), and 27.5 in ASC-CYP (n = 4). In contrast, ASC-ALS achieved significantly longer, rejection-free VCA survival in 6/7 animals (86%), with persistent mixed donor-cell chimerism, and elevated systemic and allograft skin T_regs , with no signs of acute cellular rejection. Taken together, a regimen comprised of short-course tacrolimus, repeated CTLA4-Ig and ASC administration, combined with ALS, promotes long-term VCA survival without chronic immunosuppression.

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.

A Comparative Examination of the Clinical Outcome and Histological Appearance of Cryopreserved and Fresh Split-Thickness Skin Grafts

The clinical use of frozen, human allogeneic skin grafts is considered a suitable alternative to freshly harvested allogeneic skin grafts when the latter are not available. However, limited functional and histological information exists regarding the effects of cryopreservation on allogeneic skin grafts, especially those across mismatched histocompatibility barriers. Thus, we performed a side-by-side comparative study of fresh vs frozen skin grafts, across both minor and major histocompatibility barriers, in a miniature swine model. Since porcine skin shares many physical and immunological properties with human skin, our findings have relevance to current clinical practices involving allogeneic grafting and may support future, temporary wound therapies involving frozen xenografts, comprised genetically modified porcine skin. Four miniature swine underwent harvest and grafting of split-thickness skin, with and without cryopreservation, in order to observe autologous grafts and grafts across minor and major histocompatibility barriers. A biopsy of the grafts was done at regular intervals for study of architecture, vascularization, and outcomes. All grafts vascularized without technical complications. Differences were noted in the early appearance of some fresh vs frozen grafts, but no significant difference was observed in overall survival times in any of the experimental groups. These results demonstrate that despite early observable differences in the healing process, cryopreservation and thawing does not significantly affect long-term graft survival or time to rejection, thus supporting the clinical and experimental use of fresh and frozen split-thickness skin grafts as comparable and interchangeable.

Combining Treg therapy with mixed chimerism: Getting the best of both worlds

Deliberate establishment of donor-specific immunologic tolerance is considered to be the "Holy Grail" in transplantation medicine, but clinical tolerance protocols for routine organ transplantation are still an unmet need. Mixed hematopoietic chimerism is an attractive tolerance strategy with considerable potential. Recent pilot trials provide proof-of-principle that mixed chimerism can induce tolerance in renal transplant recipients. Routine clinical translation, however, is impeded by the side effects of the cytotoxic recipient conditioning necessary for the transient engraftment of HLA-mismatched BM. In murine studies recently published in The American Journal of Transplantation, we demonstrated that the therapeutic application of polyclonal recipient regulatory T cells (Tregs) leads to engraftment of practicable doses of fully allogeneic BM and to donor-specific tolerance without any cytotoxic conditioning, thereby eliminating a major impediment for the clinical translation of the mixed chimerism strategy in the experimental setting. The background and the implications of these findings are discussed.

Donor-Specific Regulatory T Cells Acquired from Tolerant Mice Bearing Cardiac Allograft Promote Mixed Chimerism and Prolong Intestinal Allograft Survival

The induction of donor-specific transplant tolerance has always been a central problem for small bowel transplantation (SBT), which is thought to be the best therapy for end-stage bowel failure. With the development of new tolerance-inducing strategies, mixed chimerism induced by co-stimulation blockade has become most potent for tolerance of allografts, such as skin, kidney, and heart. However, a lack of clinically available co-stimulation blockers has hindered efficient application in humans. Furthermore, unlike those for other types of solid organ transplantation, strategies to induce robust mixed chimerism for intestinal allografts have not been fully developed. To improve current mixed chimerism induction protocols for future clinical application, we developed a new protocol using donor-specific regulatory T (Treg) cells from mice with heart allograft tolerance, immunosuppressive drugs which could be used clinically and low doses of irradiation. Our results demonstrated that donor-specific Treg cells acquired from tolerant mice after in vitro expansion generate stable chimerism and lead to acceptance of intestinal allograft. Increased intragraft Treg cells and clonal deletion contribute to the development of SBT tolerance.

Use of hematopoietic cell transplants to achieve tolerance in patients with solid organ transplants

The goals of tolerance in patients with solid organ transplants are to eliminate the lifelong need for immunosuppressive (IS) drugs and to prevent graft loss due to rejection or drug toxicity. Tolerance with complete withdrawal of IS drugs has been achieved in recipients of HLA-matched and mismatched living donor kidney transplants in 3 medical centers using hematopoietic cell transplants to establish mixed or complete chimerism.

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.

Bioengineering Thymus Organoids to Restore Thymic Function and Induce Donor-Specific Immune Tolerance to Allografts

One of the major obstacles in organ transplantation is to establish immune tolerance of allografts. Although immunosuppressive drugs can prevent graft rejection to a certain degree, their efficacies are limited, transient, and associated with severe side effects. Induction of thymic central tolerance to allografts remains challenging, largely because of the difficulty of maintaining donor thymic epithelial cells in vitro to allow successful bioengineering. Here, the authors show that three-dimensional scaffolds generated from decellularized mouse thymus can support thymic epithelial cell survival in culture and maintain their unique molecular properties. When transplanted into athymic nude mice, the bioengineered thymus organoids effectively promoted homing of lymphocyte progenitors and supported thymopoiesis. Nude mice transplanted with thymus organoids promptly rejected skin allografts and were able to mount antigen-specific humoral responses against ovalbumin on immunization. Notably, tolerance to skin allografts was achieved by transplanting thymus organoids constructed with either thymic epithelial cells coexpressing both syngeneic and allogenic major histocompatibility complexes, or mixtures of donor and recipient thymic epithelial cells. Our results demonstrate the technical feasibility of restoring thymic function with bioengineered thymus organoids and highlight the clinical implications of this thymus reconstruction technique in organ transplantation and regenerative medicine.

Depletion of foxp3(+) T cells abrogates tolerance of skin and heart allografts in murine mixed chimeras without the loss of mixed chimerism

The relative contribution of central and peripheral mechanisms to the generation and maintenance of allograft tolerance is of considerable interest. Here, we present new evidence that regulatory T cells (Foxp3(+) ) maintain skin and heart allograft tolerance in mixed hematopoietic chimeric mice. Transient depletion of both donor- and recipient-derived Foxp3(+) cells was necessary and sufficient to induce decisive rejection of long-accepted skin and heart allografts. In contrast, stable hematopoietic chimerism remained, and there was no detectable induction of donor-specific reactivity to hematopoietic cells. Foxp3(+) cell depletion did not result in the rejection of skin grafts of only MHC-disparate donors (B6.C-H2(d) /bByJ), indicating that MHC antigens were not the target in the graft. We conclude that two different mechanisms of tolerance are present in mixed chimeras. Hematopoietic chimerism, resistant to Foxp3(+) depletion, is probably due to deletional tolerance to MHC antigens, as supported by previous studies. In contrast, regulatory tolerance mechanisms involving Foxp3(+) cells are required to control reactivity against non-MHC antigens not present on hematopoietic lineages.

Immunosuppression and monitoring of rejection in hand transplantation

Advances in vascularized composite allotransplantation over the last decade have achieved significant milestones in basic science and translational research, as well as clinically with highly encouraging functional and immunologic outcomes. However, certain immunologic challenges remain. In particular, although tolerance has been induced to nearly all components of a hand allograft in experimental models, the skin component may still be subject to acute rejection episodes. Currently, conventional immunosuppressive protocols have been successful at preventing allograft loss; however, they have not prevented episodes of acute skin rejection. Furthermore, the profound side effect profile of the life-long, high-dose, multidrug immunosuppression regimen that is necessary to maintain a viable graft alters the risk to benefit ratio of this non-life-saving procedure. Therefore, there must be a concerted effort in the scientific community to develop novel protocols to either minimize immunosuppression or to induce tolerance to the allograft to promote the widespread application of this life-changing procedure.

Myogenic-induced mesenchymal stem cells are capable of modulating the immune response by regulatory T cells

Cell therapy for patients who have intractable muscle disorders may require highly regenerative cells from young, healthy allogeneic donors. Mesenchymal stem cells are currently under clinical investigation because they are known to induce muscle regeneration and believed to be immune privileged, thus making them suitable for allogeneic applications. However, it is unclear whether allogeneic and myogenic-induced mesenchymal stem cells retain their immunomodulatory characteristics. Therefore, our aim was to evaluate the effects of mesenchymal stem cell differentiation on the immune characteristics of cells in vitro. We investigated the immunologic properties of mesenchymal stem cells after myogenic induction. Mesenchymal stem cells were obtained from C57BL/6 mice and the C3H/10T1/2 murine mesenchymal stem cell line. Two different 5-aza-2'-deoxycytidine doses (0.5 and 3 µM) were evaluated for their effects on mesenchymal stem cell skeletal myogenic differentiation potential, immune antigen expression, and mixed lymphocytic reactions. Using a mixed lymphocytic reaction, we determined the optimal splenocyte proliferation inhibition dose. The induction of regulatory T cells was markedly increased by the addition of 3 µM 5-aza-2'-deoxycytidine-treated mesenchymal stem cells. Myogenic-induced mesenchymal stem cells do not elicit alloreactive lymphocyte proliferative responses and are able to modulate immune responses. These findings support the hypothesis that myogenic-induced mesenchymal stem cells may be transplantable across allogeneic barriers.

Molecular chimerism in IgE-mediated allergy: B-and T-cell tolerance toward highly immunogenic exogenous antigens

Specific immunotherapy is the only curative treatment currently available for IgE-mediated allergy and preventive strategies are lacking altogether. We have recently reported that molecular chimerism induces durable tolerance in experimental models of allergy, thus potentially providing a new approach for the treatment and prevention of allergic diseases. Molecular chimerism is a gene-therapy approach for tolerance induction toward defined disease-causing antigens. In proof-of-concept studies, we introduced a clinically relevant grass pollen allergen into hematopoietic stem cells and transplanted those modified cells into preconditioned syngeneic mice. Long-lasting and robust tolerance toward the allergen was achieved. In our most recent studies published in Clinical and Experimental Allergy we demonstrated that milder, non-myeloablative conditioning is sufficient to induce tolerance. Our results revealed that, in contrast to other rodent models of chimerism, persistent microchimerism suffices to induce lasting tolerance at the T cell, B cell and effector cell levels in IgE-mediated allergy. This article addendum provides a summary of the recent paper and its implications.

Persistent molecular microchimerism induces long-term tolerance towards a clinically relevant respiratory allergen

BACKGROUND

Development of antigen-specific preventive strategies is a challenging goal in IgE-mediated allergy. We have recently shown in proof-of-concept experiments that allergy can be successfully prevented by induction of durable tolerance via molecular chimerism. Transplantation of syngeneic hematopoietic stem cells genetically modified to express the clinically relevant grass pollen allergen Phl p 5 into myeloablated recipients led to high levels of chimerism (i.e. macrochimerism) and completely abrogated Phl p 5-specific immunity despite repeated immunizations with Phl p 5.

OBJECTIVE

It was unclear, however, whether microchimerism (drastically lower levels of chimerism) would be sufficient as well which would allow development of minimally toxic tolerance protocols.

METHODS

Bone marrow cells were transduced with recombinant viruses integrating Phl p 5 to be expressed in a membrane-anchored fashion. The syngeneic modified cells were transplanted into non-myeloablated recipients that were subsequently immunized repeatedly with Phl p 5 and Bet v 1 (control). Molecular chimerism was monitored using flow cytometry and PCR. T cell, B-cell and effector-cell tolerance were assessed by allergen-specific proliferation assays, isotype levels in sera and RBL assays.

RESULTS

Here we demonstrate that transplantation of Phl p 5-expressing bone marrow cells into recipients having received non-myeloablative irradiation resulted in chimerism persisting for the length of follow-up. Chimerism levels, however, declined from transient macrochimerism levels to persistent levels of microchimerism (followed for 11 months). Notably, these chimerism levels were sufficient to induce B-cell tolerance as no Phl p 5-specific IgE and other high affinity isotypes were detectable in sera of chimeric mice. Furthermore, T-cell and effector-cell tolerance were achieved.

CONCLUSIONS AND CLINICAL RELEVANCE

Low levels of persistent molecular chimerism are sufficient to induce long-term tolerance in IgE-mediated allergy. These results suggest that it will be possible to develop minimally toxic conditioning regimens sufficient for low level engraftment of genetically modified bone marrow.

Murine anti-third-party central-memory CD8(+) T cells promote hematopoietic chimerism under mild conditioning: lymph-node sequestration and deletion of anti-donor T cells

Transplantation of T cell-depleted BM (TDBM) under mild conditioning, associated with minimal toxicity and reduced risk of GVHD, offers an attractive therapeutic option for patients with nonmalignant hematologic disorders and can mediate immune tolerance to subsequent organ transplantation. However, overcoming TDBM rejection after reduced conditioning remains a challenge. Here, we address this barrier using donorderived central memory CD8(+) T cells (Tcms), directed against third-party antigens. Our results show that fully allogeneic or (hostXdonor)F1-Tcm, support donor chimerism (> 6 months) in sublethally irradiated (5.5Gy) mice, without GVHD symptoms. Chimerism under yet lower irradiation (4.5Gy) was achieved by combining Tcm with short-term administration of low-dose Rapamycin. Importantly, this chimerism resulted in successful donor skin acceptance, whereas third-party skin was rejected. Tracking of host anti-donor T cells (HADTCs), that mediate TDBMT rejection, in a novel bioluminescence-imaging model revealed that Tcms both induce accumulation and eradicate HADTCs in the LNs,concomitant with their elimination from other organs, including the BM. Further analysis with 2-photon microcopy revealed that Tcms form conjugates with HADTCs, resulting in decelerated and confined movement of HADTCs within the LNs in an antigen-specific manner. Thus, anti-third-party Tcms support TDBMT engraftment under reduced-conditioning through lymph-node sequestration and deletion of HADTCs, offering a novel and potentially safe approach for attaining stable hematopoietic chimerism.

Tolerance to MHC class II disparate allografts through genetic modification of bone marrow

Induction of molecular chimerism through genetic modification of bone marrow is a powerful tool for the induction of tolerance. Here we demonstrate for the first time that expression of an allogeneic MHC class II gene in autologous bone marrow cells, resulting in a state of molecular chimerism, induces tolerance to MHC class II mismatched skin grafts, a stringent test of transplant tolerance. Reconstitution of recipients with syngeneic bone marrow transduced with retrovirus encoding H-2I-A^b (I-A^b) resulted the long-term expression of the retroviral gene product on the surface of MHC class II-expressing bone marrow derived cell types. Mechanistically, tolerance was maintained by the presence of regulatory T cells, which prevented proliferation and cytokine production by alloreactive host T cells. Thus, the introduction of MHC class II genes into bone marrow derived cells through genetic engineering results in tolerance. These results have the potential to extend the clinical applicability of molecular chimerism for tolerance induction.

Tolerance to vascularized composite allografts in canine mixed hematopoietic chimeras

BACKGROUND

Mixed donor-host chimerism, established through hematopoietic cell transplantation (HCT), is a reproducible strategy for the induction of tolerance toward solid organs. Here, we ask whether a nonmyeloablative conditioning regimen establishing mixed donor-host chimerism leads to tolerance of antigenic vascularized composite allografts.

METHODS

Stable mixed chimerism was established in dogs given a sublethal dose (1-2 Gy) total body irradiation before and a short course of immunosuppression after dog leukocyte antigen-identical marrow transplantation. Vascularized composite allografts from marrow donors were performed after a median of 36 months (range, 4-54 months) after HCT.

RESULTS

All marrow recipients maintained mixed donor-host hematopoietic chimerism and accepted vascularized composite allografts for periods ranging between 52 and 90 weeks; in turn, marrow donors rejected vascularized composite allografts from their respective marrow recipients within 18 to 29 days. Biopsies of muscle and skin of vascularized composite allografts from mixed chimeras showed few infiltrating cells compared with extensive infiltrates in biopsies of vascularized composite allografts from marrow donors. Elevated levels of CD3+ FoxP3+ T-regulatory cells were found in skin and muscle of vascularized composite allografts of mixed chimeras compared with normal tissues. In mixed chimeras, increased numbers of T-regulatory cells were found in draining compared with nondraining lymph nodes of vascularized composite allografts.

CONCLUSIONS

These data suggest that nonmyeloablative HCT may form the basis for future clinical applications of solid organ transplantation and that T-regulatory cells may function toward maintenance of the vascularized composite allograft.

Tolerance-inducing strategies in islet transplantation

Allogeneic islet transplantation is a promising approach for restoring normoglycemia in type 1 diabetic patients. Current use of immunosuppressive therapies for management of islet transplant recipients can be counterintuitive to islet function and can lead to complications in the long term. The induction of donor-specific tolerance eliminates the dependency on immunosuppression and allows recipients to retain responses to foreign antigens. The mechanisms by which tolerance is achieved involve the deletion of donor-reactive T cells, induction of T-cell anergy, immune deviation, and generation of regulatory T cells. This review will outline the various methods used for inducing donor-specific tolerance in islet transplantation and will highlight the previously unforeseen potential of tissue stromal cells in promoting islet engraftment.

Costimulatory pathways in transplantation

Secondary, so-called costimulatory, signals are critically required for the process of T cell activation. Since landmark studies defined that T cells receiving a T cell receptor signal without a costimulatory signal, are tolerized in vitro, the investigation of T cell costimulation has attracted intense interest. Early studies demonstrated that interrupting T cell costimulation allows attenuation of the alloresponse, which is particularly difficult to modulate due to the clone size of alloreactive T cells. The understanding of costimulation has since evolved substantially and now encompasses not only positive signals involved in T cell activation but also negative signals inhibiting T cell activation and promoting T cell tolerance. Costimulation blockade has been used effectively for the induction of tolerance in rodent models of transplantation, but turned out to be less potent in large animals and humans. In this overview we will discuss the evolution of the concept of T cell costimulation, the potential of 'classical' and newly identified costimulation pathways as therapeutic targets for organ transplantation as well as progress towards clinical application of the first costimulation blocking compound.

Hand allotransplantation

In the past decade, more than 100 different composite tissue allotransplantation (CTA) procedures have been performed around the world including more than 50 hand and 8 facial transplants with encouraging graft survival and excellent functional outcomes. Broader clinical application of CTA, however, continues to be hampered by requirement for long-term, high-dose, multidrug maintenance immunosuppression to prevent graft rejection mediated particularly by composite tissue allograft's highly immunogenic skin component. Medication toxicity could result in severe adverse events including metabolic and infectious complications or malignancy. Notably, unlike in solid organs, clinical success is dictated not only by graft acceptance and survival but also by nerve regeneration, which determines ultimate functional outcomes. Novel strategies such as cellular and biologic therapies that integrate the concepts of immune regulation with those of nerve regeneration have shown promising results in small and large animal models. Clinical translation of these insights to reconstructive transplantation and CTA could further minimize the need of immunosuppression and optimize functional outcomes. This will enable wider application of such treatment options for patients in need of complex reconstructive surgery for congenital deformities or devastating injuries that are not amenable to standard methods of repair.

Achievements and challenges in composite tissue allotransplantation

Overall, more than 60 hand/forearm/arm transplantations and 16 face transplantations have been performed in the past 12 years. In the European experience summarized here, three grafts have been lost in response to a vascular thrombosis (n = 1), rejection and incompliance with immunosuppression (n = 1) and death (n = 1). The overall functional and esthetic outcome is very satisfactory, but serious side effects and complications related to immunosuppression are challenges hindering progress in this field. The high levels of immunosuppression, skin rejection, nerve regeneration, donor legislation and the acceptance level need to be addressed to promote growth of this promising new field in transplantation and reconstructive surgery.

Immunodepletive anti-alpha/beta-TCR antibody in transplantation of composite tissue allografts: Cleveland Clinic research experience

The immunologic characteristics of composite tissue allografts (CTA), which contain skin, lymphoid elements and bone with bone marrow, raise new challenges for transplant immunologists. Owing to the heterogeneity of transplanted tissues in limb or face transplant models, researchers are focusing on the new tolerance-inducing strategies facilitating CTA acceptance. A number of immunosuppressive protocols have been designed to develop tolerance in experimental models; however, only a few protocols have been introduced to clinical transplantation. In this review, based on own experiences, we discuss the major strategies for tolerance induction in limb and face allograft models in experimental studies. This review is focused on tolerance induction strategies by establishment of donor-specific chimerism using different immunomodulatory protocols, including nonselective T-cell depletion with polyclonal antibody antilymphocyte serum and selective inhibition of alphabeta-T-cell receptors on the alloreactive T cells.

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.

Blocking costimulatory pathways: prospects for inducing transplantation tolerance

Tolerance induction to alloantigens is a major challenge in transplant immunology. Whereas conventional immunosuppression inhibits the immune system in a nonspecific way, thereby also undermining an appropriate immune response towards potentially harmful infectious organisms, tolerance in a transplantation setting is restricted to alloantigens, while protective immunity is preserved. Moreover, tolerance implies an immunological status that is preserved after withdrawal of the tolerance-inducing therapy. Among the most promising strategies to induce immunological tolerance are costimulation blockade and establishment of mixed chimerism. Despite significant advances, we still know little about the mechanisms responsible for such tolerance. In this article, we discuss tolerance induction to transplantation antigens by costimulation blockade.

Mesenchymal stromal cells inhibit graft-versus-host disease of mice in a dose-dependent manner

BACKGROUND AIMS

Graft-versus-host disease (GvHD) remains a major complication after allogeneic hematopoietic cell transplantation (HCT). Recent literature demonstrates a potential benefit of human mesenchymal stromal cells (MSC) for the treatment of refractory GvHD; however, the optimal dose remains uncertain. We set out to develop an animal model that can be used to study the effect of MSC on GvHD.

METHODS

A GvHD mouse model was established by transplanting C3H/he donor bone marrow (BM) cells and spleen cells into lethally irradiated BALB/c recipient mice. MSC were obtained from C3H/he mice and the C3H/10T1/2 murine MSC line.

RESULTS

The mRNA expression of Foxp3 in regional lymph nodes (LN) localized with T cells was markedly increased by the addition of C3H10T1/2 cells in a real-time polymerase chain reaction (PCR). Using a mixed lymphocyte reaction, we determined the optimal splenocyte proliferation inhibition dose (MSC:splenocyte ratios 1:2 and 1:1). Three different C3H10T1/2 cell doses (low, 0.5 x 10(6), intermediate, 1 x 10(6), and high, 2 x 10(6)) with a consistent splenocyte dose (1 x 10(6)) were evaluated for their therapeutic potential in an in vivo GvHD model. The clinical and histologic GvHD score and Kaplan-Meier survival rate were improved after MSC transplantation, and these results demonstrated a dose-dependent inhibition.

CONCLUSIONS

We conclude that MSC inhibit GvHD in a dose-dependent manner in this mouse model and this model can be used to study the effects of MSC on GvHD.

Induction of transplantation tolerance to fully mismatched cardiac allografts by T cell mediated delivery of alloantigen

Induction of transplantation tolerance has the potential to allow for allograft acceptance without the need for life-long immunosuppression. Here we describe a novel approach that uses delivery of alloantigen by mature T cells to induce tolerance to fully allogeneic cardiac grafts. Adoptive transfer of mature alloantigen-expressing T cells into myeloablatively conditioned mice results in long-term acceptance of fully allogeneic heart transplants without evidence of chronic rejection. Since myeloablative conditioning is clinically undesirable we further demonstrated that adoptive transfer of mature alloantigen-expressing T cells alone into mice receiving non-myeloablative conditioning resulted in long-term acceptance of fully allogeneic heart allografts with minimal evidence of chronic rejection. Mechanistically, tolerance induction involved both deletion of donor-reactive host T cells and the development of regulatory T cells. Thus, delivery of alloantigen by mature T cells induces tolerance to fully allogeneic organ allografts in non-myeloablatively conditioned recipients, representing a novel approach for tolerance induction in transplantation.

Treg-therapy allows mixed chimerism and transplantation tolerance without cytoreductive conditioning

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

Cryopreservation and the age of the allotransplant

For centuries, reconstructive surgeons have restored form and function with autografts. These techniques are highly effective, but they are associated invariably with donor site morbidity. To avoid this, surgeons have long dreamed of using cadaveric sources for reconstructive material. However, allografts have two major limitations: rejection and limited donor tissue. In order to limit rejection, the allograft must be rendered more tolerable to the host or the host must be rendered more tolerant of the allograft. Both strategies have been used with considerable success in recent years. As understanding of the human immune response increases, clinical immunosuppressive regimens will undoubtedly become less morbid, and the indications for allotransplantation will broaden. This will place an even greater burden on the already small donor pool. One way to relieve this burden would be through the development of strategies for the long-term preservation of donated tissues and organs. Cryopreservation has been used clinically for decades, and recent advances in the field have allowed the preservation of an ever widening array of tissues and organs. As cold storage has been shown to reduce the antigenicity of parts, cryopreservation may actually serve to improve the survival rate of transplanted parts, as well as increase their availability. As the era of autotransplantation gives way to the age of allotransplantation, cryopreservation will play an increasingly important role.

Induction of tolerance to bone marrow allografts by donor-derived host nonreactive ex vivo-induced central memory CD8 T cells

Enabling engraftment of allogeneic T cell-depleted bone marrow (TDBM) under reduced-intensity conditioning represents a major challenge in bone marrow transplantation (BMT). Anti-third-party cytotoxic T lymphocytes (CTLs) were previously shown to be endowed with marked ability to delete host antidonor T cells in vitro, but were found to be less effective in vivo. This could result from diminished lymph node (LN) homing caused by the prolonged activation, which induces a CD44(+)CD62L(-) effector phenotype, and thereby prevents effective colocalization with, and neutralization of, alloreactive host T cells (HTCs). In the present study, LN homing, determined by imaging, was enhanced upon culture conditions that favor the acquisition of CD44(+)CD62L(+) central memory cell (Tcm) phenotype by anti-third-party CD8(+) cells. These Tcm-like cells displayed strong proliferation and prolonged persistence in BM transplant recipients. Importantly, adoptively transferred HTCs bearing a transgenic T-cell receptor (TCR) with antidonor specificity were efficiently deleted only by donor-type Tcms. All these attributes were found to be associated with improved efficacy in overcoming T cell-mediated rejection of TDBM, thereby enabling high survival rate and long-term donor chimerism, without causing graft-versus-host disease. In conclusion, anti-third-party Tcms, which home to recipient LNs and effectively delete antidonor T cells, could provide an effective and novel tool for overcoming rejection of BM allografts.

Induction of allogeneic mixed chimerism by immature dendritic cells and bone marrow transplantation leads to prolonged tolerance to major histocompatibility complex disparate allografts

Mixed chimerism has been shown to lead to prolonged major histocompatibility complex (MHC) disparate allograft survival and immune-specific tolerance; however, traditional conditioning regimes often involve myeloablation, which may pose a significant safety risk. In this study we examined the use of donor C57BL/6 (H-2(b)) immature dendritic cells (imDCs) to tolerize the BALB/c (H-2(d)) recipient to bone marrow transplantation (BMT), allowing the induction of mixed chimerism without immunosuppression or myeloablation. We showed that successful mismatched bone marrow engraftment can be achieved using imDCs given up to 3 days prior to BMT and that mixed chimerism can be established and detected in excess of 100 days post-BMT without evidence of graft-versus-host disease. Furthermore, we showed that imDCs can suppress lymphocyte proliferation in response to mismatched MHC stimulation, leading to increased expression of interleukin (IL)-4 and IL-10 and decreased expression of IL-2 and interferon-gamma (IFN-gamma). The induction of stable chimeras through pre-conditioning of mice with donor imDCs followed by BMT led to tolerance, allowing the long-term survival (> 110 days) of mismatched cardiac allografts and the prolonged survival of mismatched skin allografts without the need for immunosuppression or myeloablation. Transplantation with third-party C3H allografts were rapidly rejected in this model, suggesting that immune-specific tolerance was achieved. The induction of immune-specific tolerance without the need for immunosuppression or myeloablation represents a significant advance in transplant immunology and may provide clinicians with a plausible alternative in combating organ rejection following transplantation.

Gene therapy and bone marrow stem-cell transfer to treat autoimmune disease

Current treatment of human autoimmune disease by autologous bone marrow stem-cell transfer is hampered by frequent disease relapses. This is most probably owing to re-emergent self-reactive lymphocytes. Gene therapy combined with bone marrow stem cells has successfully introduced genes lacking in immunodeficiences. Because the bone marrow compartment has a key role in establishing immune tolerance, this combination strategy should offer a rational approach to prevent re-emergent self-reactive lymphocytes by establishing solid, life-long immune tolerance to causative self-antigen. Indeed, we have recently demonstrated the success of this combination approach to prevent and cure an experimental autoimmune disease. We suggest that this combination strategy has the potential for translation to treat human autoimmune diseases in which causative self-antigens are known.

Reversing the Autoimmune Condition: Experience with Experimental Autoimmune Gastritis

Autoimmune diseases remain a significant health problem in our society, despite the best efforts to understand and treat these conditions. Current clinical treatments are aimed at alleviating the consequences of these diseases, with limited prospects for cure. Our studies with the experimental model of autoimmune gastritis have led us to explore potential curative strategies that can reverse the autoimmune condition. Using mouse models, we have shown that expression of the known gastric autoantigen in the thymus results in immunological tolerance and resistance to the induction of autoimmune gastritis. Also, induced tolerance in donor mice can be transferred to syngeneic recipient mice by bone marrow cells. Strategies based on these observations could lead to reversal of established disease. Transfer of ensuing knowledge to the cure of serious human autoimmune diseases is our ultimate goal.

Conditional deletion of STAT5 in adult mouse hematopoietic stem cells causes loss of quiescence and permits efficient nonablative stem cell replacement

Currently, there is a major need in hematopoietic stem cell (HSC) transplantation to develop reduced-intensity regimens that do not cause DNA damage and associated toxicities and that allow a wider range of patients to receive therapy. Cytokine receptor signals through c-Kit and c-Mpl can modulate HSC quiescence and engraftment, but the intracellular signals and transcription factors that mediate these effects during transplantation have not been defined. Here we show that loss of one allele of signal transducer and activator of transcription 5 (STAT5) in nonablated adult mutant mice permitted engraftment with wild-type HSC. Conditional deletion of STAT5 using Mx1-Cre caused maximal reduction in STAT5 mRNA (> 97%) and rapidly decreased quiescence-associated c-Mpl downstream targets (Tie-2, p57), increased HSC cycling, and gradually reduced survival and depleted the long-term HSC pool. Host deletion of STAT5 was persistent and permitted efficient donor long-term HSC engraftment in primary and secondary hosts in the absence of ablative conditioning. Overall, these studies establish proof of principle for targeting of STAT5 as novel transplantation conditioning and demonstrate, for the first time, that STAT5, a mitogenic factor in most cell types, including hematopoietic progenitors, is a key transcriptional regulator that maintains quiescence of HSC during steady-state hematopoiesis.

New strategies in immune tolerance induction

Induction of tolerance in clinical organ transplantation that will obviate the use of chronic immunosuppression and preserve host immune response to other antigens remains the goal of transplant research. The thymus plays a critical role in the ability of the immune system to discriminate between self- and nonself-antigens or harmful and harmless alloantigens. We now know that multiple factors determine how the immune system responds to a self-antigen or foreign antigen. These determinants include developmental stage of the host, stage of T-cell maturity, site of antigen encounter, type and maturity of antigen-presenting cells, and presence and type of costimulatory molecules. Our understanding of the mechanisms of T-cell interactions with peptide/ major histocompatibility complex in peripheral lymphoid organs has led to experiments that translate into peripheral T-cell tolerance. The induction of high-avidity peripheral alloreactive T cells in the early phase of organ transplantation makes it difficult to achieve long-term alloantigen-specific tolerance without the use of transient perioperative immunosuppression. Therefore, protocols that induce robust tolerance in rodent and nonhuman primate models involve the use of donor antigen combined with a short course of perioperative immunosuppression. These studies suggest that the underlying mechanisms of peripheral tolerance include deletion, anergy, immune deviation, and regulatory T cells. This review focuses on recent advances in tolerance induction in experimental animal models and discusses their relevance to the development of protocols for the induction and maintenance of clinical transplant tolerance.

Immune tolerance in orthotopic liver transplantation induced by allogenic bone marrow transplantation in rats

AIM: To establish the allogeneic animal bone marrow and orthotopic liver transplantation model in rats, and to investigate the feasibility and possible mechanism of immune tolerance induced by allogeneic animal bone marrow tansplantation.

METHODS: SD rats (donor, ♂) and Wistar rats (recipient, ♀) were randomly and averagely divided into three groups and conditioned by three methods. Group II Wistar rats (recipient, ♀) were induced with sublethal total body irradiation (TBI, 11 Gy) and group III with TBI (7 Gy), followed by infusion of SD (donor, ♂) rat bone marrow cells (8×10⁷) within 4 h, then rats in group III were administered intraperitoneally with cytoxan (CTX, 50 mg/kg) 2 days later. Then three groups were performed orthotopic liver transplantations using modified Kamada's two-cuff technique in rats 28 days later. According to the GenBank, the specific primer of rat SRY gene was designed. Recipient rats were detected for donor origin cells in the peripheral blood lymphocyte on day 10, 20 using polymerase chain reaction (PCR). PCR product was analyzed by electrophoresis. Delayed type hypersensitivity (DTH), survival time and the histopathologic changes in liver after liver transplantation in rats were compared.

RESULTS: Chimera of SD rats was found in the peripheral blood lymphocytes of the Wistar rats in Group II and III. DTH results showed that Wistar rats were specifically tolerant to the SD rats. The DTH in group II or III is obviously lower than that in group I (0.22 ± 0.028 mm, 0.23 ± 0.032 mm vs 0.71 ± 0.026 mm, P < 0.01). The rats in Group I died 4-5 days later. The survival rate in Group II or III were significantly higher than that in group I (8.14 ± 2.53 d, 8.33 ± 2.11 d vs 3.79 ±0.83 d, P < 0.01). According the Banff scores, the rats in group II and III showed lighter pathological changes than in group I.

CONCLUSION: Treatment of 7 Gy TBI and the injection of CTX (50 mg/kg) plus donor bone marrow transplantation (BMT) establishes a rat chimera model successfully and enhance the survival time of liver transplantation model in rats. Infusion of the bone marrow cells might be an effective method for maintaining the tolerant state and for prolonging survival time of liver transplantation model in rats.

Tolerance induction in experimental autoimmune encephalomyelitis using non-myeloablative hematopoietic gene therapy with autoantigen

Experimental autoimmune encephalomyelitis (EAE) constitutes a paradigm of antigen (Ag)-specific T cell driven autoimmune diseases. In this study, we transferred bone marrow cells (BMCs) expressing an autoantigen (autoAg), the peptide 40-55 of the myelin oligodendrocytic glycoprotein (MOG(40-55)), to induce preventive and therapeutic immune tolerance in a murine EAE model. Transfer of BMC expressing MOG(40-55) (IiMOG-BMC) into partially myeloablated mice resulted in molecular chimerism and in robust protection from the experimental disease. In addition, in mice with established EAE, transfer of transduced BMC with or without partial myeloablation reduced the clinical and histopathological severity of the disease. In these experiments, improvement was observed even in the absence of engraftment of the transduced hematopoietic cells, probably rejected due to the previous immunization with the autoAg. Splenocytes from mice transplanted with IiMOG-BMC produced significantly higher amounts of interleukin (IL)-5 and IL-10 upon autoAg challenge than those of control animals, suggesting the participation of regulatory cells. Altogether, these results suggest that different tolerogenic mechanisms may be mediating the preventive and the therapeutic effects. In conclusion, this study demonstrates that a cell therapy using BMC expressing an autoAg can induce Ag-specific tolerance and ameliorate established EAE even in a nonmyeloablative setting.

New directions for induction immunosuppression strategy in solid organ transplantation

BACKGROUND

Solid organ transplant centers are increasingly using induction immunosuppression strategies. Induction immunosuppression involves the use of intense therapy at the time of transplantation with the goal of preventing acute rejection and ultimately inducing a tolerogenic state. The objective of this review is to examine specialized induction agents currently in clinical use and highlight novel therapeutics on the horizon for induction immunosuppression.

METHODS

A literature search using the PubMed and MEDLINE databases identified salient basic science and clinical research articles on induction immunosuppression for solid organ transplantation.

CONCLUSIONS

While current induction immunosuppression agents have reduced the incidence of acute rejection, the goal of transplant tolerance has not been realized. Furthermore, the long-term allograft survival rate is not clearly influenced by the practice of induction immunosuppression. New approaches to tolerance induction, such as costimulatory-based therapy, mixed chimerism, and adoptive cellular transfer, hold promise for more effective induction immunosuppression in solid organ transplantation.

Transplantation of bone marrow transduced to express self-antigen establishes deletional tolerance and permanently remits autoimmune disease

Autoimmune diseases are incurable. We have hypothesized that these diseases can be cured by the transplantation of bone marrow (BM) stem cells that have been genetically engineered to express self-Ag. Here we have tested this hypothesis in experimental autoimmune encephalomyelitis (EAE) induced by the self-Ag myelin oligodendrocyte glycoprotein (MOG). We show that, in mice, transplantation of BM genetically modified to express MOG prevented the induction and progression of EAE, and combined with antecedent corticosteroid treatment, induced long-term remission of established disease. Mice remained resistant to EAE development upon subsequent rechallenge with MOG. Transfer of BM from these mice rendered recipients resistant to EAE. Splenocytes from these mice failed to proliferate or produce IL-17, IFN-gamma, and GM-CSF in response to MOG(35-55) peptide stimulation and they failed to produce MOG autoantibody. Mechanistically, we demonstrated in vivo reduction in development of CD4(+) MOG(35-55)-specific thymocytes, indicative of clonal deletion with no evidence for selection of Ag-specific regulatory T cells. These findings validate our hypothesis that transplantation of genetically modified BM expressing disease-causative self-Ag provides a curative approach by clonal deletion of disease-causative self-reactive T cells.

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.

Tolerization of a type I allergic immune response through transplantation of genetically modified hematopoietic stem cells

Allergy represents a hypersensitivity disease that affects >25% of the population in industrialized countries. The underlying type I allergic immune reaction occurs in predisposed atopic individuals in response to otherwise harmless Ags (i.e., allergens) and is characterized by the production of allergen-specific IgE, an allergen-specific T cell response, and the release of biologically active mediators such as histamine from mast cells and basophils. Regimens permanently tolerizing an allergic immune response still need to be developed. We therefore retrovirally transduced murine hematopoietic stem cells to express the major grass pollen allergen Phl p 5 on their cell membrane. Transplantation of these genetically modified hematopoietic stem cells led to durable multilineage molecular chimerism and permanent immunological tolerance toward the introduced allergen at the B cell, T cell, and effector cell levels. Notably, Phl p 5-specific serum IgE and IgG remained undetectable, and T cell nonresponsiveness persisted throughout follow-up (40 wk). Besides, mediator release was specifically absent in in vitro and in vivo assays. B cell, T cell, and effector cell responses to an unrelated control allergen (Bet v 1) were unperturbed, demonstrating specificity of this tolerance protocol. We thus describe a novel cell-based strategy for the prevention of allergy.

Differential promotion of hematopoietic chimerism and inhibition of alloreactive T cell proliferation by combinations of anti-CD40Ligand, anti-LFA-1, everolimus, and deoxyspergualin

Allogeneic bone marrow (BM) engraftment for chimerism and transplantation tolerance may be promoted by combinations of costimulation blocking biologics and small molecular weight inhibitors. We showed previously in a mouse model that anti-CD40Ligand (anti-CD40L, CD154) combined with anti-LFA-1 or everolimus (40-O-(2-hydroxyethyl)-rapamycin) resulted in stable chimerism in almost all BM recipients, whereas anti-LFA-1 plus everolimus conferred approximately 50% chimerism stability. Here, we investigated whether this lower incidence could be increased with deoxyspergualin (DSG) in place of or in addition to everolimus. However, DSG and everolimus were similarly synergistic with costimulation blockade for stable hematopoietic chimerism. This correlated with allospecific T cell depletion and inhibition of acute but not chronic skin allograft rejection. Different treatments were also compared for their inhibition of alloreactive T cell proliferation in vivo. While anti-CD40L did not impair T cell proliferation, anti-LFA-1 reduced both CD4 and CD8 T cell proliferation, and combining anti-LFA-1 with everolimus or DSG had an additive inhibitory effect on CD4 T cell proliferation. Thus, despite their strong inhibition of alloreactive T cell proliferation, combinations of anti-LFA-1 with everolimus or DSG did not reach the unique potency of anti-CD40L-based combinations to support stable hematopoietic chimerism in this system.