title: Towards skin tolerance in vascularised composite allografts
Creators: name: Ng, Zhi Yang
Publisher: name: University of Glasgow
Year: 2022

description: Despite more than twenty years of clinical experience since the first successful human hand transplant in 1998, the mechanisms underlying acute rejection in such vascularised composite allografts (VCAs) remain poorly understood. To further compound the problem, the reporting of patient data, treatment protocols, and definition of VCA rejection remains in constant flux. On the other hand, chronic rejection, which was previously thought to not affect VCAs, has now been accepted and recognised as both a reality and likely eventuality for VCA patients, with no realistic exit plan beyond re-transplantation currently. Between 2015 and 2017, I served as a post-doctoral research fellow at the VCA Laboratory of the Center for Transplantation Sciences, at the Massachusetts General Hospital (MGH), a teaching affiliate of Harvard Medical School in Boston, Massachusetts, USA to work on the development of immunologic tolerance protocols to VCA in both swine and non-human primate (NHP) pre-clinical VCA models. During this time, I was also a member of the clinical VCA service at MGH where I participated in the conduction of clinical VCA trials. I have maintained research collaborations with the same group since. Data from the International Registry of Hand and Composite Tissue Transplantation (IRHCTT) have shown that more than 85% of VCA patients develop at least one episode of acute VCA rejection within the first year, compared to approximately 10% of kidney transplant patients despite the use of highly similar clinical immunosuppression regimens. These differing observations led me to hypothesise that acute rejection might develop within the dermis, due to the vicinity of the subdermal plexus and abundance of skin immune cells, before presenting clinically on the surface of VCA skin. Therefore, this thesis is based on the enclosed publications which span the gamut of clinical, basic science and translational VCA research. By targeting VCA dermis, the primary objective is successful avertion of acute rejection, with the secondary objective of allowing mixed chimerism to develop following tolerance induction through delayed donor bone marrow transplantation (DBMT) in a NHP VCA model, based on clinical trials in renal transplantation patients at MGH that successfully achieved immunosuppression withdrawal. The first two papers (#1 and #2) in this thesis provide the clinical backdrop to the most pressing problems in VCA at present – acute and chronic rejection – for which we as a field still do not have definitive solutions to. Naturally, topical immunosuppression rapidly emerged as an attractive means of addressing acute rejection in VCA given the exteriorised skin component as compared to the typical, intra-abdominal location of solid organ transplant (SOT) allografts. It was theorised that such an approach would not only allow earlier identification and treatment of acute rejection, which presumably would reduce the risk of progression to chronic rejection, but also potentially allow a reduction in the overall immunosuppressive load. In turn, the risk of systemic complications to the VCA recipient would also be lowered. The above approach was explored in NHPs in the next two papers, initially inallogenic skin grafts (#3) (i.e. neovascularised) followed by VCAs, which are primarily vascularised (#4). Concurrently, ongoing research from the VCA Laboratory at MGH using a swine model suggested that rather than transient mixed chimerism (which was sufficient for renal transplantation), VCAs would require stable mixed chimerism instead. This could potentially be achieved through the DTIP approach which was successful in NHP lung transplantation at MGH. As well, analysis of swine VCA skin correlated remarkably well with MGH’s clinical hand transplant patient following transplantation (#5). Therefore, the DTIP approach was trialed in a NHP VCA model next (#6) where chronological analysis of VCA skin both in the short- and long-term reflected our clinical observations (in Papers #1 and #2) but ultimately, tolerance was not achieved due to the persistence of acute rejection. With knowledge of results from #3 and #4, the topical approach was re-engineered for local immunosuppression delivery at the subcutaneous level through a collaboration with engineering colleagues from Rutgers University in New Jersey, USA. Subsequently, acute rejection was successfully mitigated when FK506-loaded discs were implanted subcutaneously during the VCA procedure, and mixed chimerism could be induced through DTIP using the same NHP VCA model. Further work is required however, due to the persistent, premature development of post-transplant lymphoproliferative disorder, which may be an experimental and/or NHP species-specific issue. Overall, this submission of seven papers has introduced the barriers in achieving tolerance to VCAs clinically, and described the laboratory efforts undertaken to reproduce, within ethical limits and animal welfare considerations, these immunological challenges in a robust, pre-clinical NHP VCA model. Most strikingly, the clinical observations and anticipated problems were accurately replicated in the NHP VCA studies. Potential mechanistic insight of acute rejection via VCA dermis was derived, and targeted through further studies to achieve successful proof-of-concept for the mitigation of acute rejection and ultimately, development of mixed chimerism.