Alexander Albritton

Vascularized Composite Allograft Tolerance Across MHC Barriers in a Large Animal Model

Vascularized composite allograft (VCA) transplantation can restore form and function following severe craniofacial injuries, extremity amputations or massive tissue loss. The induction of transplant tolerance would eliminate the need for long‐term immunosuppression, realigning the risk–benefit ratio for these life‐enhancing procedures. Skin, a critical component of VCA, has consistently presented the most stringent challenge to transplant tolerance. Here, we demonstrate, in a clinically relevant miniature swine model, induction of immunologic tolerance of VCAs across MHC barriers by induction of stable hematopoietic mixed chimerism. Recipient conditioning consisted of T cell depletion with CD3‐immunotoxin, and 100 cGy total body irradiation prior to hematopoietic cell transplantation (HCT) and a 45‐day course of cyclosporine A. VCA transplantation was performed either simultaneously to induction of mixed chimerism or into established mixed chimeras 85–150 days later. Following withdrawal of immunosuppression both VCAs transplanted into stable chimeras (n = 4), and those transplanted at the time of HCT (n = 2) accepted all components, including skin, without evidence of rejection to the experimental end point 115–504 days posttransplant. These data demonstrate that tolerance across MHC mismatches can be induced in a clinically relevant VCA model, providing proof of concept for long‐term immunosuppression‐free survival.

Lack of cross-sensitization between α-1,3-galactosyltransferase knockout porcine and allogeneic skin grafts permits serial grafting

Background The current standard of care for burns requiring operative treatment consists of early burn excision and autologous split-thickness skin grafting. However, in large burns, sufficient donor sites may not be available to achieve total coverage, necessitating temporary coverage with allogeneic human cadaver skin grafts or synthetic skin substitutes. A previous study from this laboratory demonstrated that skin grafts from alpha-1,3 galactosyltransferase knockout (GalT-KO) miniature swine enjoyed survival comparable to that of allogeneic skin grafts in baboons. Methods In the present study, we have evaluated the immune response against sequential GalT-KO and allogeneic skin grafts to determine whether such serial grafts could extend the period of temporary wound coverage before definitive grafting with autologous skin. Results We report that rejection of primary GalT-KO skin grafts led to an anti-xenogeneic humoral response with no evidence for sensitization to alloantigens nor acceleration of rejection of allogeneic skin grafts. Similarly, presensitization with allogeneic skin did not lead to accelerated rejection of xenogeneic skin. Conclusions These data suggest that GalT-KO skin grafts could provide an early first-line treatment in the management of severe burns that would not preclude subsequent use of allografts, and that serial grafting of GalT-KO skin and allogeneic skin could potentially be used to provide an extended period of temporary burn wound coverage.

Genetically modified porcine split-thickness skin grafts as an alternative to allograft for provision of temporary wound coverage: preliminary characterization

Temporary coverage of severely burned patients with cadaver allograft skin represents an important component of burn care, but is limited by availability and cost. Porcine skin shares many physical properties with human skin, but is susceptible to hyperacute rejection due to preformed antibodies to α-1,3-galactose (Gal), a carbohydrate on all porcine cells. Our preliminary studies have suggested that skin grafts from α-1,3-galactosyltransferase knock out (GalT-KO) miniature swine might provide temporary wound coverage comparable to allografts, since GalT-KO swine lack this carbohydrate. To further evaluate this possibility, eight non-human primates received primary autologous, allogeneic, GalT-KO, and GalT+xenogeneic skin grafts. Additionally, secondary grafts were placed to assess whether sensitization would affect the rejection time course of identical-type grafts. We demonstrate that both GalT-KO xenografts and allografts provide temporary coverage of partial- and full-thickness wounds for up to 11 days. In contrast, GalT+xenografts displayed hyperacute rejection, with no signs of vascularization and rapid avulsion from wounds. Furthermore, secondary GalT-KO transplants failed to vascularize, demonstrating that primary graft rejection sensitizes the recipient. We conclude that GalT-KO xenografts may provide temporary coverage of wounds for a duration equivalent to allografts, and thus, could serve as a readily available alternative treatment of severe burns.

The Effect of MHC Antigen Matching Between Donors and Recipients on Skin Tolerance of Vascularized Composite Allografts

The emergence of skin‐containing vascularized composite allografts (VCAs) has provided impetus to understand factors affecting rejection and tolerance of skin. VCA tolerance can be established in miniature swine across haploidentical MHC barriers using mixed chimerism. Because the deceased donor pool for VCAs does not permit MHC antigen matching, clinical VCAs are transplanted across varying MHC disparities. We investigated whether sharing of MHC class I or II antigens between donors and recipients influences VCA skin tolerance. Miniature swine were conditioned nonmyeloablatively and received hematopoietic stem cell transplants and VCAs across MHC class I (n = 3) or class II (n = 3) barriers. In vitro immune responsiveness was assessed, and VCA skin‐resident leukocytes were characterized by flow cytometry. Stable mixed chimerism was established in all animals. MHC class II–mismatched chimeras were tolerant of VCAs. MHC class I–mismatched animals, however, rejected VCA skin, characterized by infiltration of recipient‐type CD8+ lymphocytes. Systemic donor‐specific nonresponsiveness was maintained, including after VCA rejection. This study shows that MHC antigen matching influences VCA skin rejection and suggests that local regulation of immune tolerance is critical in long‐term acceptance of all VCA components. These results help elucidate novel mechanisms underlying skin tolerance and identify clinically relevant VCA tolerance strategies.

Skin grafts from genetically modified α-1,3-galactosyltransferase knockout miniature swine: A functional equivalent to allografts

Burn is associated with a considerable burden of morbidity worldwide. Early excision of burned tissue and skin grafting of the resultant wound has been established as a mainstay of modern burn therapy. However, in large burns, donor sites for autologous skin may be limited. Numerous alternatives, from cadaver skin to synthetic substitutes have been described, each with varying benefits and limitations. We previously proposed the use of genetically modified (alpha-1,3-galactosyl transferase knockout, GalT-KO) porcine skin as a viable skin alternative. In contrast to wild type porcine skin, which has been used as a biologic dressing following glutaraldehyde fixation, GalT-KO porcine skin is a viable graft, which is not susceptible to loss by hyperacute rejection, and undergoes graft take and healing, prior to eventual rejection, comparable to cadaver allogeneic skin. In the current study we aimed to perform a detailed functional analysis of GalT-KO skin grafts in comparison to allogeneic grafts for temporary closure of full thickness wounds using our baboon dorsum wound model. Grafts were assessed by measurement of fluid loss, wound infection rate, and take, and healed appearance, of secondary autologous grafts following xenograft rejection. Comparison was also made between fresh and cryopreserved grafts. No statistically significant difference was identified between GalT-KO and allogeneic skin grafts in any of the assessed parameters, and graft take and function was not adversely effected by the freeze-thaw process. These data demonstrate that GalT-KO porcine grafts are functionally comparable to allogeneic skin grafts for temporary closure of full thickness wounds, and support their consideration as an alternative to cadaver allogeneic skin in the emergency management of large burns.

Upper Extremity Transplantation in Non-Human Primates: An Orthotopic Model for Translational Research

Vascularized composite allotransplantation (VCA) offers unparalleled restoration of function and form following devastating musculoskeletal and soft tissue injury. However, the potential adverse effects of life-long immunosuppression remain a significant cause for concern. Therefore, while the surgical techniques necessary for VCA have developed rapidly, the immunological aspects of these procedures and the potential functional significance of immunological processes on vascularized composite allografts remain areas in which further research is required. The functional complexity of these procedures, combined with the preclinical nature of many of the research questions, necessitates the use of large animal models to most effectively address some of the outstanding hypotheses. Cynomolgus macaques are among the premier large animal models for immunological research. This manuscript describes development of an orthotopic model of upper extremity transplantation in cynomolgus macaques. Following study of the anatomy to determine feasibility, in vivo proof of concept was achieved by autologous amputation and replantation in two animals, following which a preliminary series of four allotransplants was performed. The anatomy encountered and techniques required for successful transplantation are closely comparable to those in clinical upper extremity transplantation. This is a technically challenging model, but offers a rigorous pre-clinical platform for translational research in transplant immunology, and is suitable for detailed study of the impact of immunologic processes on functional outcomes following VCA.

Topical Delivery of Immunosuppression to Prolong Xenogeneic and Allogeneic Split-Thickness Skin Graft Survival

Abstract Cadaveric skin allograft is the current standard of treatment for temporary coverage of large burn wounds. Porcine xenografts are viable alternatives but undergo α-1,3-galactose (Gal)–mediated hyperacute rejection and are lost by post-operative day (POD) 3 because of naturally occurring antibodies to Gal in primate recipients. Using baboons, we previously demonstrated that xenografts from GalT-KO swine (lacking Gal) provided wound coverage comparable with allografts with systemic immunosuppression. In this study, we investigate topical immunosuppression as an alternative to prolong xenograft survival. Full-thickness wounds in baboons were created and covered with xenogeneic and allogeneic split-thickness skin grafts (STSGs). Animals were treated with slow-release (TyroSphere-encapsulated) topical formulations (cyclosporine-A [CSA] or Tacrolimus) applied 1) directly to the STSGs only, or 2) additionally to the wound bed before STSG and 1). Topical CSA did not improve either xenograft or allograft survival (median: treated grafts = 12.5 days, control = 14 days; P = 0.27) with similar results when topical Tacrolimus was used. Pretreatment of wound beds resulted in a significant reduction of xenograft survival compared with controls (10 vs 14 days; P = 0.0002), with comparable results observed in allografts. This observation was associated with marked reduction of inflammation on histology with Tacrolimus and not CSA. Prolongation of allograft and xenograft survival after application to full-thickness wound beds was not achieved with the current formulation of topical immunosuppressants. Modulation of inflammation within the wound bed was effective with Tacrolimus pretreatment before STSG application and may serve as a treatment strategy in related fields.

MHC Class I Matching Between Donors and Recipients Influences the Skin Tolerance of Vascularized Composite Allografts

The development of VCA tolerance protocols would obviate the need for chronic immunosuppression and allow widespread use of this highly promising reconstructive modality. However, achieving robust tolerance of the skin component of VCAs has proven particularly challenging in pre-clinical large animal models. Our laboratory has recently reported acceptance of VCAs (>600 d), including the skin component, across a haploidentical MHC barrier in a miniature swine model using a mixed chimerism approach. In this study, we investigated whether sharing of MHC class I and/ or class II between donor and recipient influences skin tolerance.

Abstract P16: Detailed Characterization and Functional Analysis of Skin-Resident Leukocytes from Vascularized Composite Allografts in Tolerant Miniature Swine Mixed Chimeras

differences (TCD) as would be expected, they ended with similar final TCD measurements, and surprisingly, helmet therapy duration was similar despite the worse initial asymmetry. This greater rate of TCD change is not explained by the similar age at initiation of helmet therapy. In addition, infants with torticollis who received physical therapy did not have improved final transcranial differences as compared to those who did not receive physical therapy. These results suggest that although torticollis and deformational plagiocephaly often occur handinhand, once the decision to proceed with helmet therapy has been made, their outcomes appear to proceed independently. Therefore, treatment can remain independent, with helmeting for the deformational plagiocephaly, and physical therapy for the torticollis. P16 detailed characterization and Functional analysis of Skin-resident leukocytes from Vascularized composite allografts in tolerant Miniature Swine Mixed chimeras

LOP10: VASCULARIZED COMPOSITE ALLOGRAFT TRANSPLANTATION AND MIXED HEMATOPOIETIC CHIMERISM ACROSS A FULL MHC BARRIER IN SWINE: PRELIMINARY RESULTS

36 www.PRSJournal.com 37 VASCULARIZED COMPOSITE ALLOGRAFT TRANSPLANTATION AND MIXED HEMATOPOIETIC CHIMERISM ACROSS A FULL MHC BARRIER IN SWINE: PRELIMINARY RESULTS Presenter: Angelo A. Leto Barone, MD Authors: Leto Barone AA, Torabi R, Randolph MA, Leonard DA, Mallard C, Albritton A, Duran-Struuck R, Matar A, Crepeau R, Tang Y, Scalea JR, Wang Z, Tena A, Hawley RJ, Kurtz JM, Huang CA, Sachs DH, Cetrulo CL Harvard Medical School/Massachusetts General Hospital