PRINCIPLES, TECHNIQUES, AND BASIC SCIENCE
CHAPTER 6 TRANSPLANTATION BIOLOGY AND APPLICATIONS TO PLASTIC SURGERY
DAMON S. COONEY, JUSTIN M. SACKS, GERALD BRANDACHER, AND W. P. ANDREW LEE
The transplantation of tissue from one location to another is a fundamental concept in plastic surgery. It is not surprising that the first successful transplantation of tissue from one person to another in the form of a kidney transplant was performed by a plastic surgeon, Dr. Joseph E. Murray. Other pioneering plastic surgeons helped spawn the new field of allogeneic organ transplantation, and with the development of improved surgical techniques and modern immunosuppression, transplantation has become the treatment of choice for end-stage organ failure of the liver, heart, lung, pancreas, and kidney. It is fitting that transplantation has returned to the field of plastic surgery more than 50 years later with the development of reconstructive transplantation, the new era in transplant medicine. It is only within the last decade that transplantation of vascularized composite allografts (VCAs), such as hand and face transplants, has become a clinical reality. VCAs involve transplantation of composite structures for reconstructive surgery and thereby fulfill a prime mandate of plastic surgery: to replace and restore devastating tissue defects using “like-with-like.” The ability to transfer vascularized allografts through microvascular surgical techniques, restoring form and function for complex cutaneous and musculoskeletal defects, is revolutionizing the field of reconstructive surgery and has added another rung to the “reconstructive ladder.” Long-term allograft survival, however, can only be achieved, as for any solid organ transplant, through the use of systemic immunosuppression with its associated sequela of organ toxicity, opportunistic infections, and potential for malignancy. Current research on immunomodulation and induction of tolerance holds promise for reducing the need for long-term high-dose immunosuppression. Although reconstructive allotransplantation in humans is a relatively new area with small numbers of patients, there are reasons to think that new innovations in immunomodulation and tolerance may come from the field of VCAs. For example, reconstructive transplant patients usually do not suffer from life-threatening illness or comorbidities and therefore the impetus to minimize side effects from immunosuppressive medications is stronger. Also, the ability to directly and continuously observe transplanted tissue that includes a skin component allows for the use of novel experimental protocols as rejection is seen earlier and can potentially be reversed by topical agents. Finally, the presence of vascularized bone marrow in many VCA grafts raises the possibility of unique modulatory strategies, as will be discussed.
Human tissue transplantation is an ancient concept. According to legend from the fourth century AD, Saints Cosmos and Damian—twin brothers—replaced the gangrenous leg of a parishioner with the leg of a deceased Ethiopian Moor. The earliest reliable, documented outcomes of allogeneic and xenogeneic skin grafts in human recipients were published by Schone in 1912 and Lexer in 1914. Schone and Lexer demonstrated that these grafts did not survive more than 3 weeks after transplantation. Padgett provided further evidence in 1932, when he reported rejection of all skin allografts within 35 days in 40 patients. However, Padgett also demonstrated that skin grafts exchanged between identical twins survived indefinitely. World War II accelerated progress in allotransplantation. Gibson, a plastic surgeon at the Glasgow Royal Infirmary, described the accelerated rejection of skin grafts in pilots due to the presence of humoral antibodies after repeat exposure to the same donor, also known as the “second-set rejection.” Medawar joined Gibson to investigate this phenomenon and, in combination with Billingham and Brent, laid the foundation for modern immunology. In 1954, Dr. Joseph E. Murray and colleagues performed the first successful kidney transplant between identical twins. Furthermore, the introduction of a novel immunosuppressive drug, 6-mercaptopurine and its precursor azathioprine (AZT) by Charles Zukosi and Roy Calne in 1960, was responsible for improvements in the field of organ transplantation.
With the discovery of the first human leukocyte antigen (HLA) in 1958, the matching of tissue beyond simply matching blood types became possible. Knowledge of these antigens allowed the avoidance of graft-versus-host disease. The first successful human bone marrow transplant was performed in 1968. A 4-month-old boy who had Wiskott-Aldrich syndrome received a bone marrow transplant from his sibling that effectively restored his immune system, duplicating Medawar’s animal findings that had previously resulted in immune tolerance in chimeric mice. Medawar’s chimeric mice contained genetically distinct cells originating from separate and unique zygotic cells.
Coinciding with these first successful human bone marrow transplantations, all major components of human clinical allotransplantation including immunosuppression, tissue preservation and matching, and complex microvascular techniques were elucidated. Following the first kidney transplantation, other solid organs such as the heart, liver, lung, and pancreas were transplanted and nonspecific immunosuppressive agents such as cyclosporine A (CsA) and FK506 were developed.
In the last two decades, over 150 different VCAs including more than 80 upper extremities and 24 partial faces, as well as abdominal walls, larynx, tongue, uterus, penis, vascularized bone and joint, and individual tissue components like peripheral nerve, flexor tendon, and skin have been successfully transplanted using conventional immunosuppressive protocols. Of the upper extremity transplants performed to date, only one graft was lost while patients were on high-dose immunosuppression. In the combined American and European experience, the early to intermediate allograft survival is greater than 95% (Figure 6.1).
Current immunosuppression protocols developed within the last century have allowed these transplantation ideas to become a surgical reality. The risk/benefit ratio that must be optimized when transplanting a piece of tissue that optimizes form and function but does not preserve or prolong life poses an ethical dilemma. Exposure to life-long immunosuppression comes with risk that must be articulated to the patient. Current immunologic research focuses on ways to obviate the use of systemic immunosuppression for both solid organ and reconstructive transplantation procedures. The field of reconstructive transplantation will become ubiquitous once tolerance can be achieved. Currently, great strides are being made in large animal studies and in the first clinical trials, moving ever closer to elucidating the immunologic processes that will unlock these barriers and allow the next revolution in plastic surgery to begin.
FIGURE 6.1. A–C. International experience with reconstructive transplantation 1999 to 2012. A. The cumulative total for all types of reconstructive transplant by type of transplant (except upper extremity) as of 2012. B. The cumulative total for upper extremity transplants by region as of 2012. Upper extremity continues to be the most commonly performed vascularized composite allograft. C. The number of hand and face transplants performed per year in the United States since 1999 broken down by year. Note the dramatic increase in reconstructive procedures in the last half-decade.
Proper nomenclature will help clarify subsequent discussions. Certain terms such as transplant, flap, and graft are often used to refer to a VCA. However, these terms should be used carefully with their true meaning in mind to ensure clear communication.
Transplantation can be defined as the transfer of tissue or an organ to another person or to a different location in the same person. According to this definition, much of what reconstructive surgeons do can be classified as a type of transplant. However, in its usual medical usage, the term transplant is used to describe an allotransplant or tissue transferred from a living or deceased human donor to another genetically not identical human patient. Within this chapter, our discussion of transplantation will be focused mostly on the topic of allotransplantation, as the transfer of autologous tissue locally or distantly is covered elsewhere in the text.
A graft is tissue completely separated from its donor bed and moved to a separate recipient bed, its survival relying on ingrowth of new vessels from the surrounding recipient tissue (Chapter 1). Avascularized graft or flap either remains attached to its native blood supply or becomes revascularized via microvascular anastomoses to recipient vessels (aka free flap). An autograft refers to tissue transplanted from one location to another within the same individual. An isograft is tissue transplanted between genetically identical individuals, such as transplants between syngeneic mice or human monozygotic twins. An allograft or homograft is tissue transplanted between unrelated individuals of the same species. A xenograft or heterograft is tissue transplanted between different species.
Transplantation can also be described according to the site into which the tissue is transferred. An orthotopic transplant is transferred into an anatomically similar site, whereas a heterotopic transplant is transferred into a different site from its donor origin. The term reconstructive transplantation is used to differentiate the transfer of composite tissues such as the hand or face from more traditional solid organ transplants. During the development of reconstructive transplantation, this novel field was commonly referred to as composite tissue allotransplantation (CTA). Unfortunately, the use of this term has caused some confusion and has largely fallen out of favor. In particular, the use of the word tissue raised the concern that reconstructive transplantation could be confused by regulatory bodies (such as the FDA) with non-vascular tissue transplantation, with potentially negative regulatory consequences. Therefore, vascularized composite allograft (VCA) transplantation has supplanted the term CTA to avoid this confusion.
Transplantation of organs or tissues between genetically disparate individuals of the same species (allogeneic individuals) leads to recognition and rejection of the allogeneic tissue by the recipient’s immune system. This “alloimmune response” that discriminates between self- and non-self tissues remains the main barrier to successful transplantation. These immunologic responses are initiated by graft antigens that are genetically encoded polymorphic proteins. The result of this interaction determines the acceptance or rejection of allograft tissue. For this reason, learning how to suppress these responses is a major goal of transplant immunologists. Transplant tolerance, as discussed, can be mediated by central or peripheral mechanisms and can be acquired with the assistance of either immunosuppression or immunomodulation.
Antigens are cell surface glycoproteins that are encoded in the major histocompatibility complex (MHC), a multigene cluster located on chromosome 6 in humans. There are two classes of MHC molecules, class I and II, that differ in their structure, function, and tissue distribution. MHC class I antigens are expressed on all nucleated cells, whereas class II expression is restricted to antigen presenting cells (APCs), such as B lymphocytes, monocytes, macrophages, dendritic cells (DCs), endothelial cells, and activated human and rat T cells. In humans, the MHC antigens are known as HLA. Each individual has two MHC regions, one of paternal and one of maternal origin. Each MHC contains an inherited group of HLA genes or haplotypes: HLA class I genes known as HLA-A, -B, -C and HLA class II genes known as HLA-DR, -DP, and -DQ. The HLA antigens determine the compatibility of all organ and tissue transplants.
Billingham, Brent, and Medawar demonstrated in their seminal 1953 Nature article that neonatal mice and irradiated adult mice developed donor-specific tolerance to skin grafts subsequent to successful engraftment of splenic and bone marrow cells into the recipient. These animals were considered chimeras consisting of both donor and recipient T cells. This built the foundation for the concept that cell-based therapies in clinical transplantation could potentially modify the host immune system to allow minimization or even avoidance of pharmacological immunosuppression. Since then, immune tolerance has been the “Holy Grail” of transplantation research.
Rejection of transplanted tissue occurs through both cellular and humoral immune responses. These responses are generated when host APCs and T lymphocytes respond to the genetic differences in the MHC molecules expressed by the donor cells. T cells have fundamental roles in graft rejection, and their responses are rapid and vigorous and ultimately will lead to inflammation and tissue destruction. There are two main pathways by which host T cells recognize donor alloantigens. Following transplantation, donor APCs migrate toward host lymphoid tissues and can directly activate recipient T cells. Once T cells are activated, they become effector T cells and migrate to the graft and mediate rejection. This is aptly named the direct pathway of allorecognition. In contrast to the direct pathway, host APCs play a significant role in the indirect pathway of allorecognition where they present processed donor antigens to host T cells. Both pathways of allorecognition are important in mediating graft rejection; however, the indirect pathway is thought to be of greater significance in the physiology of chronic graft rejection.
DCs are the most efficient APCs and have the capacity to take up, process, and present antigens to T cells in vivo. DCs rapidly respond to inflammatory stimuli, microbial products, or alloantigens following transplantation and express high levels of MHC class II and costimulatory molecules essential for T-cell activation.
All allotransplant recipients require some form of immunosuppression. Without these immunosuppressive modalities, rejection would inevitably occur in individuals unless they were genetically identical (identical twins). The immunosuppression used for transplantation of VCAs has for the most part mirrored the regimens for solid organ transplantation.
Several pharmacological drugs are used to prevent and control graft rejection (Table 6.1). It is important to note that these drugs lack selectivity and cause generalized immunosuppression rendering transplant patients highly susceptible to opportunistic infections and certain types of malignancies. Based on their mode of action, there are four main groups of immunosuppressive drugs: (1) steroids, (2) cytotoxic/anti-proliferative drugs, (3) anti–T-cell agents (calcineurin inhibitors), and (4) induction agents (polyclonal and monoclonal antibodies). One of the first immunosuppressant used was steroids with broad anti-inflammatory actions (i.e., prednisone and prednisolone). These medications inhibit activation of several transcription factors, thus modifying gene transcription and inhibiting cellular activation and cytokine production. Prednisolone was one of the first pharmacological agents used in allogeneic organ transplantation. Despite the well-known side effects of long-term use, steroids are still widely used today in combination with other immunosuppressive agents in most solid organ and VCA protocols. Short courses of high-dose steroids continue to be the frontline treatment for acute rejection episodes in all types of transplantation. Cytotoxic/anti-proliferative drugs include cyclophosphamide, methotrexate, AZT, and mycophenolate mofetil (MMF). These medications interfere with DNA replication and kill/arrest proliferating lymphocytes that are activated by alloantigens. Earlier, nonspecific anti-proliferative agents such as AZT had many side effects and increased the risk of transplant- associated malignancy. MMF has replaced the other agents in many protocols due to its ability to block purine synthesis selectively in T and B cells, which dramatically decrease side effects. Agents that selectively inhibit the activation pathways of T cells are typically fungal or bacterial products (i.e., CsA, tacrolimus [FK506], and rapamycin [RAPA; sirolimus]). CsA and tacrolimus inhibit the signaling pathways of T-cell activation by interfering with calcineurin activation and interleukin (IL)-2 gene transcription (Figure 6.2). CsA is a metabolic extract from the fungusTolypocladium inflatum gamus described in 1976. Its discovery revolutionized the field of solid organ transplantation by significantly increasing the survival of kidney, heart, and liver allografts. CsA was shown to prolong limb allograft survival in experimental animal models and thereby encouraged clinicians to pursue VCA in the 1980s and 1990s. Tacrolimus is a macrolide lactone antibiotic isolated from soil fungus and also inhibits the calcineurin/IL-2 pathway of T-cell activation, although at a different point in the pathway from cyclosporine. It has a favorable side-effect profile as compared with CsA, with less transplant-associated malignancy, although it has significant nephrotoxicity when used for long periods of time. Tacrolimus has replaced cyclosporine in many protocols for solid organ transplantation and has been a mainstay in all the clinical reconstructive transplantation treatment regimen. RAPA is an inhibitor of the mammalian target of rapamycin (mTOR), which in turn inhibits multiple biochemical pathways critical for cellular proliferation with the main target being T cells. RAPA is an attractive alternative to CsA and FK506, having a significantly different side-effect profile in particular with regard to its nephrotoxicity, promoting tolerance in some circumstances, and having anti-proliferative and anti- neoplastic properties. It does, however, suffer from the drawback of having profound negative effects on wound healing that may preclude its use in the early postoperative period. These immunosuppressive agents have been reported to allow successful allogeneic transplantation in clinical solid organ transplantation and VCA such as extremity transplantation and face transplants with both high graft and patient survival rates.
FIGURE 6.2. Immunosuppressive targets of T-cell activation. Immunosuppressive medications interrupt T-cell activation at various pathways. Cyclosporine A and FK506 interrupt the T-cell receptor (TCR) signaling cascade by blocking calcineurin activation, the latter by interaction with FK binding proteins. Rapamycin blocks the mammalian target of rapamycin signaling from both interleukin 2 receptor activation and TCR coreceptors interrupting the “second signal” pathway. Steroids alter gene transcription by modulating the effects of transcription factors including several necessary for T-cell activation. Azathioprine and mycophenolate mofetil inhibit the production of nucleotides in lymphocytes blocking the cell cycle and proliferation.
In addition to the three classes of drugs used for maintenance immunosuppression, many protocols include the addition of an induction agent, which causes the depletion of T cells in the perioperative period. The goal of this therapy is to decrease the chance of acute rejection immediately after the transplant, but to allow the recovery of the immune system and repopulation of the T-cell compartment in the presence of the new graft and immunosuppressive medications. This may alter the T-cell population toward a more tolerant phenotype allowing less immunosuppression. Polyclonal anti-thymoglobulin and anti-lymphocyte sera have been used in several studies for the depletion of recipient T cells and prevention of graft rejection. Furthermore, studies using monoclonal antibodies against T-cell receptors (TCRs), anti-CD3 immunotoxin, an anti-CD3 monoclonal antibody conjugated to a mutant form of the diphtheria toxin protein, and Campath-1H (alemtuzumab), an anti-CD52 monoclonal antibody, have been used to deplete lymphocytes yielding beneficial results for prolongation of graft survival. Total body, thymic, and graft irradiation have all been used as induction treatments in the early days of solid organ transplantation in both clinical and experimental studies, although they are not commonly used in current protocols.
The ultimate success and further acceptance of the field of reconstructive transplantation will depend on developments that reduce immunosuppressive sequela. Immunologic tolerance and the abrogation of the need for chronic immunosuppression will be the ultimate refinement in this ongoing process.
The precise definition of transplantation tolerance, often discussed and rarely agreed upon, can be regarded as the lack of a destructive immune response toward the allograft in the absence of ongoing immunosuppressive therapy. However, implicit in this definition is that such a state must coexist with general immune competence, including normal immune responses to pathogens and cancer risks no different than the general population.
Acceptance or tolerance of one’s own tissues first develops in utero, along with an immunologic ability to recognize foreign tissue. The ability of the immune system to distinguish between self and foreign antigens is controlled by two mechanisms called central and peripheral tolerance. The thymus plays a major role in the maintenance of tolerance to self and also the induction of tolerance to alloantigens. The mechanism of T-cell tolerance in the thymus is based on the deletion of self or alloreactive T cells upon interaction with bone marrow–derived APCs. Since such clonal deletion causes the elimination of donor reactive T cells, it is considered one of the most robust mechanisms for tolerance induction. In experimental models, deletion of antigen-specific T cells can be induced by direct injection of donor antigens into the thymus. Following intrathymic injection, donor antigens will be presented by APCs to thymocytes, and this will allow for activation of alloreactive T cells in the thymus and their deletion. Although intrathymic injection has been successful in rodent models, it has been of limited efficacy in larger animals. Another widely researched approach for the induction of tolerance is the use of hematopoietic bone marrow transplantations to induce mixed chimerism. The term chimera is derived from the Greek mythological figure comprised of the parts of different animals. The chimeric animals develop an immune system that is tolerant of both donor and recipient antigens.
Immunologic tolerance is also controlled in the periphery. The mechanisms of peripheral tolerance include T-cell anergy (non-responsiveness), induction of T regulatory/suppressor cells, or T-cell deletion. The induction of T-cell anergy has been demonstrated by the blockade of costimulatory signals using monoclonal antibodies during T-cell activation. The induction of T regulatory/suppressor cells is another mechanism to induce T-cell tolerance specific to donor antigens. T regulatory cells play a key role in the maintenance of tolerance to both self and foreign antigens. Furthermore, studies in recent years have demonstrated the potential role for particular subtypes of DCs such as plasmacytoid to promote and maintain peripheral tolerance to transplantation antigens.
As discussed previously, immunologic tolerance is defined as specific unresponsiveness of the immune system to donor antigens. However, this definition does not allow for the differentiation of systemic tolerance from the clinical situation of immunosuppression-free long-term graft acceptance. Mononuclear cell infiltration and the induction of alloantibodies have been observed in long-term renal allograft rhesus monkey recipients that were weaned off immunosuppression. Based on this and similar observations, it is critical to define immunologic tolerance using stringent criteria as well as to develop assays and tools to monitor for donor-specific nonreactivity in operationally tolerant transplant recipients in the future.
CURRENT TRANSPLANTATION IN PLASTIC SURGERY
Skin allografts are the most commonly performed human allotransplant and were the basis of the first transplantation research. Pioneering experiments by Guiseppe Baronio on auto- and allografts in sheep laid the groundwork for research in the fields of both skin grafting and transplant immunology. Baronio’s contributions are considered fundamental to the field of plastic surgery. Experimental use of skin allografts helped to elucidate and define the process and mechanisms of allograft rejection. At first, a skin allograft is accepted just as any skin autograft. However, once the skin allograft is revascularized, the recipient immune system mounts a cellular immune response. Two to three weeks following placement, the allograft is rejected through an antigen-specific T cell–mediated response resulting in loss of the skin allograft, necessitating alternative wound coverage most typically with skin autograft.
The most frequent clinical use for skin allografts is in the treatment of extensive burns. The ability to place temporary skin grafts without jeopardizing limited autogenous donor sites in these patients has resulted in dramatic improvements in the survival of high total body surface area burn wounds (Chapter 16). In some patients who are immunosuppressed either through medications or through severe illness, skin allografts have been shown to be tolerated with continued immunosuppression. Anecdotal reports of patients who have undergone slow withdrawal of immunosuppression have not required re-grafting either through permanent tolerance of the graft or more likely through slow substitution of the allograft with recipient cells.
Skin allografts will continue to be one of the most common forms of human allotransplantation. These grafts are vital to the treatment of wounds and burns. However, the health status of patients presenting with these types of wounds precludes the use of long-term immunosuppression to achieve graft maintenance due to the additional risk of opportunistic infection. Future research developments that enable prolonged skin graft survival in the absence of immunosuppression would precipitate a significant paradigm shift in the treatment of severe wounds and burns.
Porcine xenograft has been used as a temporary dressing for large burns. It is applied with a technique similar to that used for human allograft, with seeding of autologous grafts beneath it. The application of xenogeneic dermis has also been found valuable in preparing a wound for subsequent grafting by stimulation of granulation tissue formation. Xenogeneic tissue has limited uses in skin grafting at present and its cellular components are susceptible to hyperacute rejection typical of all xenograft materials.
Reconstruction of large bony defects in the axial and peripheral skeleton with non-vascularized allogeneic bone has been widely practiced. Well-organized tissue banks and improved methods of bone sterilization and preservation have made this possible. Very few of the donor cells, if any, in the non-vascularized bone allograft survive. These donor cells express antigens similar to other allogeneic tissues and are rejected. The remaining bone acts as a scaffold for ingrowth of recipient mesenchymal stem cells (osteocyte precursors), which repopulates the donor by “creeping substitution.” Although technically an allogeneic tissue transplant, non-vascularized allografts are totally replaced by recipient cells once the healing process is complete and no immunosuppression is given. Due to slow union, long-term fixation is required of bone allograft, which is prone to stress fracture and loosening of fixation hardware. In studies of retrieved human allografts, however, union was seen at the graft–host interface.
Vascularized bone allograft on the other hand contains living donor cells and is susceptible to immunologic rejection. The humoral and cellular responses generated by the transplanted bone was found to be similar in intensity and timing as that generated by other vascularized allogeneic tissues such as the skin and muscle. Although individual bone cells express antigens, the predominant antigenic stimulus in a bone allograft is thought to be derived from the marrow. Removal of bone marrow by irradiation or replacement with recipient marrow has been shown experimentally to prolong allograft survival. Like any other allogeneic tissue, this rejection process can be ameliorated with immunosuppression, and long-term survival of orthotopic vascularized skeletal allograft has been achieved in animal models. However, the adverse effects of prolonged immunosuppression required for survival of a vascularized bone allograft preclude its clinical application currently as autologous sources of vascularized bone and non-vascularized allograft are usually sufficient to reconstruct most simple bone defects. A series of knee vascularized composite allotransplants have been performed with poor results. Allografts failed in five of the six patients, presumably due to rejection and the lack of the ability to adequately monitor the immunologic status of the graft without an externalized skin component.
Cartilage is composed of chondrocytes within lacunae dispersed throughout a water-laden matrix. The matrix is composed predominantly of proteoglycans and type II collagen. Water is important as cartilage has no intrinsic blood supply and relies on diffusion of nutrients and oxygen through this matrix. The combination of water and proteoglycans imparts the characteristic of viscoelasticity depending on the relative concentrations of both elements. The variable water content in the matrix causes a balanced tension within it and helps maintain its three-dimensional shape. The viscoelastic property of the matrix confers “memory” such that cartilage returns to its original shape after deformation. Surgical manipulation or scoring disrupts this equilibrium. In contrast to osteocytes, chondrocytes have little reparative ability and heal by forming fibrous scar tissue. There are histologically three types of cartilage: hyaline, elastic, and fibrocartilage.
Chondrocytes express HLA antigens on their surface and are thus immunogenic in isolation. Cartilage, however, is immunologically privileged due to the shielding of chondrocytes by its matrix, which is only weakly antigenic. Surgical scoring or dicing of cartilage allograft with the resultant exposure of allogeneic cells has been shown to hasten cartilage resorption.
Cartilage allografts have been used successfully for similar applications as autologous cartilage. Allogeneic cartilage can be either preserved or fresh. Preserved cartilage has the advantage of a more abundant supply and decreased risk of infection in comparison to fresh cartilage. Although immunologically privileged, cartilage allografts are still susceptible to loss of volume through resorption.Whether this is due to immunologic rejection or lack of viable cells following preservation is a matter of debate. It has also been noted that small allografts are less prone to volume loss than larger grafts.
Some authors have advocated the use of bovine-derived cartilage xenografts. However, both chondrocytes and matrix are subject to xenogeneic mechanisms of rejection with a generally poorer outcome in comparison to autologous or allogeneic cartilage grafts. Attempts to modify these xenogeneic responses by altering the graft’s immunologic stereotactic structure have been reported as being beneficial.
The best clinical outcome following nerve transection is achieved with primary repair. More extensive injuries or a delay in repair may result in a nerve gap following debridement of damaged nerves, and a nerve graft may be necessary to achieve neurorrhaphy without tension. The nerve graft undergoes the same degenerative process as in the distal nerve after division. The myelin sheath remains with Schwann cells that act as a biological conduit for the regenerating axons. Vascularized nerve grafts are theoretically advantageous, particularly in scarred beds. Other “conduits” used as nerve grafts have included autologous vein, silicone tubes seeded with Schwann cells, and freeze-fractured autologous muscle. Autologous nerve grafts with acceptable donor site morbidity are limited, and extensive nerve reconstruction may require other sources such as nerve allografts. Immunologic rejection of nerve allograft can be ameliorated experimentally with immunosuppressive drugs, and axons were found to traverse the allogeneic nerve graft in rodents. A similar result has also been demonstrated in primates. Immunosuppression was necessary during axonal regeneration but could be terminated afterward in some studies with satisfactory nerve functions. In the only clinical experience, Mackinnon reported return of motor and sensory functions in the upper or lower limbs of six out of seven patients following nerve allograft reconstruction.
Clinical Reconstructive Transplantation
Upper Limb Transplantation. The field of reconstructive transplantation has been led by hand transplantation (Figure 6.3). The first attempted hand transplantation occurred in 1964 in Ecuador by Dr. Robert Gilbert. Although the surgery was successful, the immunosuppressive regime available at that time was insufficient to prevent acute rejection and the transplanted hand was lost after only 3 weeks. This led to the conclusion that hand transplantation or any transplant containing skin was not immunologically feasible. This attitude prevailed until the late 1990s. The rapid growth of immunosuppressive medications and the remarkable success of solid organ transplantation in the 1980s and 1990s led to renewed interest in VCA. Through the pioneering work of Dr. Jean-Michel Dubernard in France (1998) and Dr. Warren Breidenbach in the United States (1999), hand transplantation was shown to be possible with highly encouraging immunologic and functional results. Furthermore, hand transplants could be maintained on conventional triple-drug immunosuppression at levels similar to that used in solid organ transplantation and patients had functional recovery similar to that seen with replantation. Although the French patient became noncompliant with medication therapy and subsequently required the removal of his transplant, the American patient has had almost a decade and a half of use from his transplanted hand and remains a vocal advocate of hand transplantation.
In the intervening decades since the beginning of the modern era of hand transplantation, there have been over 70 transplants performed worldwide in centers across the United States, Europe, and Asia. In general, the results have been excellent with very few grafts lost, good hand function, and relatively few side effects. Of the centers participating in the International Registry on Hand and Composite Tissue Transplantation, results from 1998 to 2010 included 49 transplanted hands in 33 patients. Of these patients, one patient died due to sepsis following combined hand and face transplantation and three additional grafts were lost: one from infection, one from patient noncompliance, and one from intimal hyperplasia possibly representing a form of chronic rejection. All patients who have maintained their grafts developed protective sensation, 82.3% had discriminative sensation, and 75% reported significant improvement in quality of life. Immunosuppressive side effects included opportunistic infection (i.e., cytomegalovirus reactivation), diabetes, avascular necrosis of the hip, and post-transplant malignancy including one case of post-transplant lymphoproliferative disease.
Although the risk/benefit ratio of placing a patient on long-term immunosuppressive medications must always be considered, advances such as minimization protocols using donor bone marrow infusion and monotherapy maintenance are currently being studied in humans and are anticipated to favorably alter this balance in the future. The senior author and his team have performed eight limb transplants in five patients using alemtuzumab induction therapy at the time of transplantation followed by an infusion of bone marrow cells collected from the vertebral bodies of the limb donor. In four of the five patients, the immunomodulation caused by the donor bone marrow infusion has reduced the need for systemic immunosuppression and allowed the use of single-agent therapy with tacrolimus.
FIGURE 6.3. Hand transplantation. Schematic diagram showing the technique of mid-forearm hand transplantation. All structures are prepared and labeled prior to transplantation and osseous fixation. Note the opposing, interdigitating skin flap design. Inset: photograph of allograft prepared for transplant.
With an ever-increasing number of centers performing hand transplantation worldwide and more than a decade of experience with the techniques, hand transplantation has become less novel. It is increasingly being seen as another alternative in the reconstructive armamentarium used to treat patients with upper extremity amputation (Chapter 90). Upper limb transplantation restores the structure and function of the hand in a way not possible with any other reconstructive technique with a reasonable level of safety. It appears that reconstructive transplantation will continue to play an increasing role in the treatment of patients with upper limb amputations. However, as always the risk/benefit ratio for the use of systemic immunosuppression, regardless of whether it is single drug or multidrug, must be considered when deciding whether to operate on patients with complex injuries of the upper extremity.
Facial Transplantation. No other area of reconstructive transplantation captures the imagination of patients, clinicians, and the public like face transplantation. Human beings’ perception of “self” is tied to one’s facial appearance; transferring these tissues from one person to another raises major questions. However, for some patients with severe and devastating injuries to the craniofacial skeleton and soft tissues of the face, there is no other acceptable, effective option using standard reconstructive techniques. For these patients, transplantation of allogeneic cadaveric facial structures may be the only way of regaining normal facial appearance and being able to reintegrate into society in a meaningful way.
Dr. Dubernard and Dr. Devauchelle in France performed the first transplantation of facial tissue in 2005. Between 2005 and 2012, eighteen partial or full-face transplantations have been performed worldwide. These were done by centers in the United States, France, Spain, and China. Transplanted grafts have consisted of soft tissue (the nose and lips) up to and including all facial soft tissue and portions of facial bones and the tongue (entire face, maxilla, anterior portion of the mandible, and tongue). Indications for these procedures have included ballistic trauma sustained by military personnel and civilians, animal bites, tumor resection, neurofibromatosis, and burns. All of the patients’ transplants have included either portions of the orbicularis oris or oculi; loss of the sphincter function of the mimetic muscles of the face is generally considered to be one of the indications for facial transplantation. In general, face patients have been maintained on immunosuppression similar to that used in solid organ transplantation, with all patients receiving induction therapy followed by triple-drug immunosuppression.
Overall, patient outcomes have been excellent. All patients receiving facial transplantation have dramatically improved their aesthetic appearance, allowing easier integration back into society. All patients for whom outcomes have been reported in the literature report nearly normal sensory recovery, with return of normal two-point discrimination between 3 and 8 months after transplantation. Motor recovery has been slower, but all patients have recovered some degree of motor function allowing for oral competence. Typical motor recovery begins at 3 months, with maximum recovery around 18 months after transplant. Unfortunately, there have been 2 deaths (a 15% mortality rate) associated with facial transplantation among these 13 patients. The first death was in a Chinese patient; however, this death has not been reported in the literature and so the etiology remains unclear. The second death occurred in a patient who received a combined bilateral hand and face transplant for the treatment of extensive burns. This patient reportedly succumbed to overwhelming infection following immunosuppression. No surviving patients have lost their grafts due to rejection to date.
Facial transplantation is following hand transplantation as the next success story of reconstructive transplantation. Although technically demanding and potentially dangerous, facial transplantation has the potential to reintegrate patients into society who have been injured so severely that they are simply unable to function in or contribute to society in their current state. As more and more centers perform this groundbreaking technique, the number of patients treated per year has steadily increased. There is a clear indication that for carefully selected patients, facial transplantation offers a procedure that, while not life saving, is potentially life restoring.
Other Areas of Reconstructive Transplantation
Developments in reconstructive transplantation have clearly been led by upper extremity and facial transplantation. However, there have been much smaller series of patients treated with transplantation of several other composite structures including vascularized knee joints, lower extremities, trachea, larynx, abdominal wall, and reproductive organs. With the exception of vascularized knee joints (which have failed in five of the six patients attempted), reconstruction with these varied types of transplants has met with some qualified success. In general, patients are maintained on standard types of immunosuppression (i.e., triple-drug therapy) with no more or less complications from these regimens than those of solid organ transplant patients. While these less common types of transplants continue to be highly experimental, they demonstrate the possibilities that reconstructive transplantation offers. Traditional plastic surgery techniques are unable to restore complex tissues and anatomical structures with the fidelity equal to that of reconstructive transplantation, as evidenced by recipients’ functional and aesthetic outcomes. As clinical experience with these techniques is accumulated and immunosuppressive and immunomodulatory protocols are optimized, the risk–benefit ratio of reconstructive transplantation will continue to shift in favor of these procedures, making these techniques an increasingly available and important part of plastic surgeons’ reconstructive options for treating these crippling defects.
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