Biologics and donor bone marrow cells for targeted immunomodulation in vascularized composite allotransplantation: a translational trial in swine

Cell-Based Therapies Induce Tolerance of Vascularized Composite Allotransplants: A Systematic Review

INTRODUCTION

Vascularized composite allotransplantation (VCA) is the transplantation of multiple tissue types as a solution for devastating injuries. Despite the highly encouraging functional outcomes of VCA, the consequences of long-term immunosuppression remain the main obstacle in its application. In this review, we provide researchers and surgeons with a summary of the latest advances in the field of cell-based therapies for VCA tolerance.

METHODS

Four electronic databases were searched: PubMed, Scopus, Cumulative Index to Nursing and Allied Health Literature , and Web of Science. We used the Preferred Reporting Items for Systematic Reviews and Meta-Analysis as the basis of our organization.

RESULTS

Hematopoietic stem cells prolonged VCA survival. A combination of immature dendritic cells and tacrolimus was superior to tacrolimus alone. T cell Ig domain and mucin domain modified mature dendritic cells increased VCA tolerance. Bone marrow-derived mesenchymal stem cells prolonged survival of VCAs. A combination of adipose-derived mesenchymal stem cells, cytotoxic T-lymphocyte antigen 4 immunoglobulin, and antilymphocyte serum significantly improved VCA tolerance. Ex-vivo allotransplant perfusion with recipient's bone marrow-derived mesenchymal stem cells increased VCA survival. Recipient's adipose-derived mesenchymal stem cells and systemic immunosuppression prolonged VCA survival more than any of those agents alone. Additionally, a combination of peripheral blood mononuclear cells shortly incubated in mitomycin and cyclosporine significantly improved VCA survival. Finally, a combination of donor recipient chimeric cells, anti-αβ-T cell receptor (TCR), and cyclosporine significantly prolonged VCA tolerance.

CONCLUSIONS

Evidence from animal studies shows that cell-based therapies can prolong survival of VCAs. However, there remain many obstacles for these therapies, and they require rigorous clinical research given the rarity of the subjects and the complexity of the therapies. The major limitations of cell-based therapies include the need for conditioning with immunosuppressive drugs and radiation, causing significant toxicity. Safety concerns also persist as most research is on animal models. While completely replacing traditional immunosuppression with cell-based methods is unlikely soon, these therapies could reduce the need for high doses of immunosuppressants and improve VCA tolerance.

Biodistribution and Safety of Human Multi-Chimeric Cells After Systemic Intraosseous and Intravenous Administration in the Experimental Mouse Model

Cellular therapies provide promising options for inducing tolerance in transplantation of solid organs, bone marrow, and vascularized composite allografts. However, novel tolerance-inducing protocols remain limited, despite extensive research. We previously introduced and characterized a human multi-chimeric cell (HMCC) line, created through ex vivo fusion of human umbilical cord blood (UCB) cells derived from three unrelated donors. In this study, we assessed in vivo biodistribution and safety of HMCCs in the NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ NOD scid gamma (NSG) mouse model. Twenty-four NSG mice were randomly assigned to four groups (n = 6/group) and received intraosseous (IO.) or intravenous (IV.) injections of 0.6 × 106 donor UCB cells or fused HMCC: Group 1-UCB (IO.), Group 2-UCB (IV.), Group 3-HMCC (IO.), and Group 4-HMCC (IV.). Hematopoietic phenotype maintenance and presence of human leukocyte antigens (HLA), class I antigens, in the selected lymphoid and nonlymphoid organs were assessed by flow cytometry. Weekly evaluation and magnetic resonance imaging (MRI) assessed HMCC safety. Comparative analysis of delivery routes revealed significant differences in HLA class I percentages for IO.: 1.83% ± 0.79%, versus IV. delivery: 0.04% ± 0.01%, P < 0.01, and hematopoietic stem cell marker percentages of CD3 (IO.: 1.41% ± 0.04%, vs. IV.: 0.07% ± 0.01%, P < 0.05) and CD4 (IO.: 2.74% ± 0.31%, vs. IV.: 0.59% ± 0.11%, P < 0.01). Biodistribution analysis after IO. delivery confirmed HMCC presence in lymphoid organs and negligible presence in nonlymphoid organs, except for lung (IO.: 0.19% ± 0.06%, vs. IV.: 6.33% ± 0.56%, P < 0.0001). No evidence of tumorigenesis was observed by MRI at 90 days following IO. and IV. administration of HMCC. This study confirmed biodistribution and safety of HMCC therapy in the NSG mouse model, both following IO. and IV. administration. However, IO. delivery route confirmed higher efficacy of engraftment and safety profile, introducing HMCCs as a novel cell-based therapeutic approach with promising clinical applications in solid organ, bone marrow, and vascularized composite allotransplantation transplantation.

Tacrolimus before CTLA4Ig and rapamycin promotes vascularized composite allograft survival in MGH miniature swine

BACKGROUND

We evaluated the outcome of vertical rectus abdominus myocutaneous flap (VRAM) allotransplantation in a mini-pig model, using a combined co-stimulation blockade (Co-SB) and mechanistic target of rapamycin inhibition (mTORi)-based regimen, with or without preceding calcineurin inhibition (CNI).

MATERIALS AND METHODS

VRAM allotransplants were performed between SLA-mismatched MGH miniature swine. Group A (n = 2) was treated continuously with the mTOR inhibitor rapamycin from day -1 in combination with the Co-SB agent cytotoxic T lymphocyte antigen 4-Ig (CTLA4-Ig) from post-operative day (POD) 0. In group B (n = 3), animals received tacrolimus daily from POD 0 to POD 13, followed by rapamycin daily from POD 7 and CTLA4-Ig weekly from POD 7-28. Graft rejection was determined by Banff criteria and host cellular and humoral immunity monitored.

RESULTS

In group A, allografts developed grade-I acute rejection by POD 2 and POD 7, and reached grade-IV by POD 17 and POD 20, respectively. By contrast, in group B, two allografts demonstrated grade-I rejection on POD 30 and grade-IV on POD 74, while the third exhibited grade-I rejection starting on POD 50, though this animal had to be euthanized on POD 58 due to Pneumocystis jirovecii infection. Time-to-event incidence of grade-I rejection was significantly lower in group A compared to group B. During the first 3 weeks post-transplant, no significant differences in anti-donor immunity were observed between the groups.

CONCLUSION

A short course of CNI, followed by combined Co-SB and mTORi significantly delays acute rejection of VRAM allografts in SLA-mismatched miniature swine.

Large Animal Models of Vascularized Composite Allotransplantation: A Review of Immune Strategies to Improve Allograft Outcomes

Over the past twenty years, significant technical strides have been made in the area of vascularized composite tissue allotransplantation (VCA). As in solid organ transplantation, the allogeneic immune response remains a significant barrier to long-term VCA survival and function. Strategies to overcome acute and chronic rejection, minimize immunosuppression and prolong VCA survival have important clinical implications. Historically, large animals have provided a valuable model for testing the clinical translatability of immune modulating approaches in transplantation, including tolerance induction, co-stimulation blockade, cellular therapies, and ex vivo perfusion. Recently, significant advancements have been made in these arenas utilizing large animal VCA models. In this comprehensive review, we highlight recent immune strategies undertaken to improve VCA outcomes with a focus on relevant preclinical large animal models.

Costimulation Blockade in Vascularized Composite Allotransplantation

Vascular composite allotransplantation (VCA) is a field under research and has emerged as an alternative option for the repair of severe disfiguring defects that result from infections or traumatic amputation in a selected group of patients. VCA is performed in centers with appropriate expertise, experience and adequate resources to effectively manage the complexity and complications of this treatment. Lifelong immunosuppressive therapy, immunosuppression associated complications, and the effects of the host immune response in the graft are major concerns in VCA. VCA is considered a quality of life transplant and the risk-benefit ratio is dissimilar to life saving transplants. Belatacept seems a promising drug that prolongs patient and graft survival in kidney transplantation and it could also be an alternative approach to VCA immunosuppression. In this review, we are summarizing current literature about the role of costimulation blockade, with a focus on belatacept in VCA.

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.

Mechanisms of rejection in vascular composite allotransplantation

PURPOSE OF REVIEW

For patients with devastating injuries in whom standard reconstruction is not an option, vascularized composite allotransplantation (VCA) has become a viable means of restoring form and function. However, immunological rejection continues to be a problem in VCA and has not yet been fully characterized. As the field is relatively new, much of the data on rejection and immunosuppression have been extrapolated from that of solid organ transplantation. In this review, we cover the basic mechanisms of rejection as they relate specifically to VCA with analysis of recent literature and future directions.

RECENT FINDINGS

Recent clinical studies have supported previously postulated T-cell-mediated mechanism of acute rejection and have also made strides in differentiating rejection from inflammation from other skin conditions and with different treatment regimens. Antibody-mediated rejection has been described in recent cases as well as treatment of presensitized patients receiving VCAs. With more long-term grafts, chronic changes, including vasculopathy, are being reported.

SUMMARY

Clinically observed types of rejection in VCA include mainly cell-mediated, antibody-mediated and chronic rejection. Advances in diagnosis and treatment of rejection have been made, but there is still much to be learned about VCA-specific rejection.

Recipient-derived angiogenesis with short term immunosuppression increases bone remodeling in bone vascularized composite allotransplantation: A pilot study in a swine tibial defect model

Current vascularized composite allotransplantation (VCA) transplantation protocols rely upon life-long immune modulation to maintain tissue perfusion. Alternatively, bone-only VCA viability may be maintained in small animal models using surgical angiogenesis from implanted autogenous vessels to develop a neoangiogenic bone circulation that will not be rejected. This study tests the method's efficacy in a large animal model as a bridge to clinical practice, quantifying the remodeling and mechanical properties of porcine tibial VCAs. A segmental tibial defect was reconstructed in Yucatan miniature swine by transplantation of a matched tibia segment from an immunologically mismatched donor. Microsurgical repair of nutrient vessels was performed in all pigs, with simultaneous intramedullary placement of an autogenous arteriovenous (AV) bundle in Group 2. Group 1 served as a no-angiogenesis control. All received 2 weeks of immunosuppression. After 16 weeks, micro-CT and histomorphometric analyses were used to evaluate healing and remodeling. Axial compression and nanoindentation studies evaluated bone mechanical properties. Micro-CT analysis demonstrated significantly more new bone formation and bone remodeling at the distal allotransplant/recipient junction and on the endosteal surfaces of Group 2 tibias (p = 0.03). Elastic modulus and hardness were not adversely affected by angiogenesis. The combination of 2 weeks of immunosuppression and autogenous AV-bundle implantation within a microsurgically transplanted tibial allotransplant permitted long-term allotransplant survival over the study period of 16 weeks in this large animal model. Angiogenesis increased bone formation and remodeling without adverse mechanical effects. The method may allow future composite-tissue allotransplantation of bone without the risks associated with long-term immunosuppression. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1242-1249, 2017.

The Influence of Timing and Frequency of Adipose-Derived Mesenchymal Stem Cell Therapy on Immunomodulation Outcomes After Vascularized Composite Allotransplantation

BACKGROUND

Cellular therapies for immunomodulation in vascularized composite allotransplantation (VCA) have gained importance due to their potential for minimization of immunosuppression. Adipose-derived (AD) mesenchymal stem cells (MSCs) especially have shown encouraging potential. We investigated the influence of timing and frequency of AD-MSC treatment on immunologic and graft survival as well as graft vasculopathy outcomes after VCA.

METHODS

Lewis rats received full-mismatched Brown Norway rat hindlimb transplants. Recipient animals were assigned to groups receiving donor-derived AD-MSCs (10 cells/animal) either on postoperative day (POD) 1, POD 4, or repeatedly on POD 4, 8, and 15, and compared to untreated controls.

RESULTS

Although AD-MSC administration on POD 1 or POD 4, 8, and 15 resulted in 50% long-term graft acceptance, recipients treated on POD 4, and controls rejected before POD 50. All treated animals revealed peripheral blood chimerism (4 weeks), most pronounced after repetitive cell administration (12.92% vs 5.03% [POD 1] vs 6.31% [POD 4]; P < 0.05; all P < 0.01 vs control 1.45%). Chimerism was associated with the generation of regulatory T cells (CD4CD25FoxP3). In vitro mixed lymphocyte reactions revealed modulation of the recipient immune response after AD-MSC treatment. Graft arteries at end point revealed significant differences of arterial intimal thickness between rejecting and AD-MSC-treated animals (P < 0.01).

CONCLUSIONS

Taken together, our results point to the potential for repetitive AD-MSC administration in improving outcomes after VCA. Future studies are warranted into optimization of the dosing and frequency of AD-MSC therapy, either alone or used in, combination with other cell therapies (such as hematopoietic stem cells or bone marrow-derived MSC or dendritic cells) for optimization of appropriate conditioning or maintenance regimens.

Transplant Tolerance Induction in Newborn Infants: Mechanisms, Advantages, and Potential Strategies

Although several tolerance induction protocols have been successfully implemented in adult renal transplantation, no tolerance induction approach has, as yet, been defined for solid organ transplantations in young infants. Pediatric transplant recipients have a pressing demand for the elaboration of tolerance induction regimens. Indeed, since they display a longer survival time, they are exposed to a higher level of risks linked to long-term immunosuppression (IS) and to chronic rejection. Interestingly, central tolerance induction may be of great interest in newborns, because of their immunological immaturity and the important role of the thymus at this early stage in life. The present review aims to clarify mechanisms and strategies of tolerance induction in these immunologically premature recipients. We first introduce the discovery and mechanisms of neonatal tolerance in murine experimental models and subsequently analyze tolerance induction in human newborn infants. Hematopoietic mixed chimerism in neonates is also discussed based on in utero hematopoietic stem cell (HSC) transplant studies. Then, we review the recent advances in tolerance induction approaches in adults, including the infusion of HSCs associated with less toxic conditioning regimens, regulatory T cells/facilitating cells/mesenchymal stem cells transplantation, costimulatory blockade, and thymus manipulation. Finally, IS withdrawal in pediatric solid organ transplant is discussed. In conclusion, the establishment of transplant tolerance induction in infants is promising and deserves further investigations. Future studies could focus on the selection of patients, on less toxic conditioning regimens, and on biomarkers for IS minimization or withdrawal.

Immunomodulatory Strategies Directed Toward Tolerance of Vascularized Composite Allografts

BACKGROUND

Achieving tolerance of vascularized composite allografts (VCAs) would improve the risk-to-benefit ratio in patients who undergo this life-enhancing, though not lifesaving, transplant. Kidney cotransplantation along with a short course of high-dose immunosuppression enables tolerance of heart allografts across a full major histocompatibility complex (MHC) mismatch. In this study, we investigated whether tolerance of VCAs across full MHC disparities could be achieved in animals already tolerant of heart and kidney allografts.

METHODS

Miniature swine that were tolerant of heart and/or kidney allografts long term underwent transplantation of myocutaneous VCA across the same MHC barrier. Before VCA transplant, group 1 (n = 3) underwent class I-mismatched kidney transplantation; group 2 (n = 3) underwent 2 sequential class I-mismatched kidney transplantations; group 3 (n = 2) underwent haploidentical MHC-mismatched heart/kidney transplantation; and group 4 (n = 2) underwent full MHC-mismatched heart/kidney transplantation.

RESULTS

All 3 animals in group 1 and 2 of 3 animals in group 2 showed skin rejection within 85 days; 1 animal in group 2 showed prolonged skin survival longer than 200 days. Animals in groups 3 and 4 showed skin rejection within 30 days and regained in vitro evidence of donor responsiveness.

CONCLUSIONS

This is the first preclinical study in which hearts, kidneys, and VCAs have been transplanted into the same recipient. Despite VCA rejection, tolerance of heart and kidney allografts was maintained. These results suggest that regulatory tolerance of skin is possible but not generally achieved by the same level of immunomodulation that is capable of inducing tolerance of heart and kidney allografts. Achieving tolerance of skin may require additional immunomodulatory therapies.

Vascularized composite allotransplantation: towards tolerance and the importance of skin-specific immunobiology

PURPOSE OF REVIEW

Vascularized composite allotransplantation (VCA) is increasingly utilized in the restoration of complex injuries and tissue loss. Acute skin-targeted rejection episodes are common and concerns remain regarding the risks of conventional immunosuppression. We review current immunosuppressive regimens for VCA, progress with immunomodulatory and tolerance protocols, and highlight recent advances in cutaneous immunobiology which will have significant implications for future development in the field.

RECENT FINDINGS

Advances in induction protocols have demonstrated effective prevention of early graft loss in hand transplantation, although long-term outcomes are still pending. Furthermore, recent findings in leukocyte populations within the skin and their mechanisms of communication reveal that considerable numbers of resident T-effector memory cells, including a T-regulatory subset, exist, and that epidermal Langerhans' cells communicate with these cells, mediating both immunity and tolerance to maintain skin homeostasis.

SUMMARY

The majority of VCA centers utilize antibody-mediated induction, followed by double or triple-agent maintenance immunosuppression. A clinical trial of a minimal-immunosuppression protocol based on bone marrow infusion reports encouraging interim results, but long-term follow-up will be required. Skin remains the primary target of rejection in VCA. New data demonstrate extensive T-cell memory resident in skin, and complex interactions between these cells and epidermal Langerhans' cells will have implications for VCA rejection and tolerance, and warrant further investigation in the allogeneic setting.

A modified heterotopic swine hind limb transplant model for translational vascularized composite allotransplantation (VCA) research

Vascularized Composite Allotransplantation (VCA) such as hand and face transplants represent a viable treatment option for complex musculoskeletal trauma and devastating tissue loss. Despite favorable and highly encouraging early and intermediate functional outcomes, rejection of the highly immunogenic skin component of a VCA and potential adverse effects of chronic multi-drug immunosuppression continue to hamper widespread clinical application of VCA. Therefore, research in this novel field needs to focus on translational studies related to unique immunologic features of VCA and to develop novel immunomodulatory strategies for immunomodulation and tolerance induction following VCA without the need for long term immunosuppression. This article describes a reliable and reproducible translational large animal model of VCA that is comprised of an osteomyocutaneous flap in a MHC-defined swine heterotopic hind limb allotransplantation. Briefly, a well-vascularized skin paddle is identified in the anteromedial thigh region using near infrared laser angiography. The underlying muscles, knee joint, distal femur, and proximal tibia are harvested on a femoral vascular pedicle. This allograft can be considered both a VCA and a vascularized bone marrow transplant with its unique immune privileged features. The graft is transplanted to a subcutaneous abdominal pocket in the recipient animal with a skin component exteriorized to the dorsolateral region for immune monitoring. Three surgical teams work simultaneously in a well-coordinated manner to reduce anesthesia and ischemia times, thereby improving efficiency of this model and reducing potential confounders in experimental protocols. This model serves as the groundwork for future therapeutic strategies aimed at reducing and potentially eliminating the need for chronic multi-drug immunosuppression in VCA.

Syngeneic adipose-derived stem cells with short-term immunosuppression induce vascularized composite allotransplantation tolerance in rats

BACKGROUND AIMS

A clinically applicable tolerance induction regimen that removes the requirement for lifelong immunosuppression would benefit recipients of vascularized composite allotransplantation (VCA). We characterized the immunomodulatory properties of syngeneic (derived from the recipient strain) adipocyte-derived stem cells (ADSCs) and investigated their potential to induce VCA tolerance in rats.

METHODS

ADSCs were isolated from Lewis (LEW, RT1A(l)) rats; their immunomodulatory properties were evaluated by means of mixed lymphocyte reactions in vitro and VCAs in vivo across a full major histocompatibility complex mismatch with the use of Brown-Norway (BN, RT1A(n)) donor rats. Two control and four experimental groups were designed to evaluate treatment effects of ADSCs and transient immunosuppressants (anti-lymphocyte globulin, cyclosporine) with or without low-dose (200 cGy) total body irradiation. Flow cytometry was performed to quantify levels of circulating CD4(+)CD25(+)FoxP3(+) regulatory T cells (Tregs).

RESULTS

Cultured syngeneic ADSCs exhibited CD90.1(+)CD29(+)CD73(+)CD45(-)CD79a(-)CD11b/c(-) phenotype and the plasticity to differentiate to adipocytes and osteocytes. ADSCs dramatically suppressed proliferation of LEW splenocytes against BN antigen and mitogen, respectively, in a dose-dependent fashion, culminating in abrogation of allo- and mitogen-stimulated proliferation at the highest concentration tested. Accordingly, one infusion of syngeneic ADSCs markedly prolonged VCA survival in LEW recipients treated with transient immunosuppression; of these, 66% developed tolerance. Total body irradiation provided no additional VCA survival benefit. An important role for Tregs in tolerance induction/maintenance was suggested in vivo and in vitro.

CONCLUSIONS

Treatment comprising syngeneic ADSCs and transient immunosuppression (i) increased levels of circulating Tregs and (ii) induced tolerance in 66% of recipients of major histocompatibility complex-mismatched VCAs.

Induction of tolerance of vascularized composite allografts

Vascularized composite allotransplantation has become established as a clinical specialty since the first successful hand transplant was performed in 1998. Data now available indicate that hand and face transplants offer patients good functional outcomes and significant improvements in quality of life. Despite the debilitating nature of the injuries treated by such transplants, the defects are generally not life threatening, making it difficult for physicians to recommend life-long immunosuppression that can itself have grave consequences. One potential solution to this dilemma is the induction of immunologic tolerance of the tissue transplants because tolerance would eliminate the need for such immunosuppression. Transplant tolerance may also prevent chronic rejection, a significant source of late graft loss after organ transplantation.Induction of mixed hematopoietic chimerism is a robust approach to establishing such transplant tolerance, which recently led to the first clinical application of a tolerance induction protocol for kidney transplantation. In this manuscript, we review the current status of VCA and of research directed toward bringing a tolerance approach to the VCA field. We also discuss the potential clinical significance of these studies and outline the remaining obstacles to introduction of a tolerance induction protocol to clinical practice in hand or face transplantation.

Vascularized composite tissue allotransplantation--state of the art

Vascularized composite tissue allotransplantation is a viable treatment option for injuries and defects that involve multiple layers of functional tissue. In the past 15 yr, more than 150 vascularized composite allotransplantation (VCA) surgeries have been reported for various anatomic locations including - but not limited to - trachea, larynx, abdominal wall, face, and upper and lower extremities. VCA can achieve a level of esthetic and functional restoration that is currently unattainable using conventional reconstructive techniques. Although the risks of lifelong immunosuppression continue to be an important factor when evaluating the benefits of VCA, reported short- and long-term outcomes have been excellent, thus far. Acute rejections are common in the early post-operative period, and immunosuppression-related side effects have been manageable. A multidisciplinary approach to the management of VCA has proven successful. Reports of long-term graft losses have been rare, while several factors may play a role in the pathophysiology of chronic graft deterioration in VCA. Alternative approaches to immunosuppression such as cellular therapies and immunomodulation hold promise, although their role is so far not defined. Experimental protocols for VCA are currently being explored. Moving forward, it will be exciting to see whether VCA-specific aspects of allorecognition and immune responses will be able to help facilitate tolerance induction.

Animal models for basic and translational research in reconstructive transplantation

Reconstructive transplantation represents a bona fide option for select patients with devastating tissue loss, which could better restore the appearance, anatomy, and function than any other conventional treatment currently available. Despite favorable outcomes, broad clinical application of reconstructive transplantation is limited by the potential side effects of chronic multidrug immunosuppression. Thus, any reconstructive measures to improve these non-life-threatening conditions must address a delicate balance of risks and benefits. Today, several exciting novel therapeutic strategies, such as the implementation of cellular therapies including bone marrow or stem cells that integrate the concepts of immune regulation with those of nerve regeneration, are on the horizon. The development of reliable and reproducible small and large animal models is essential for the study of the unique immunological and biological aspects of vascularized composite allografts and to translate such novel immunoregulatory and tolerance-inducing strategies and therapeutic concepts from the bench to bedside. This review provides an overview of the multitude of small and large animal models that have been particularly designed for basic and translational research related to reconstructive transplantation.