Long-term viability of articular cartilage after microsurgical whole-joint transplantation and immunosuppression with rapamycin, mycophenolate mofetil, and tacrolimus

Challenges and opportunities in vascularized composite allotransplantation of joints: a systematic literature review

Background

Joint allotransplantation (JA) within the field of vascularized composite allotransplantation (VCA) holds great potential for functional and non-prosthetic reconstruction of severely damaged joints. However, clinical use of JA remains limited due to the immune rejection associated with all forms of allotransplantation. In this study, we aim to provide a comprehensive overview of the current state of JA through a systematic review of clinical, animal, and immunological studies on this topic.

Methods

We conducted a systematic literature review in accordance with the PRISMA guidelines to identify relevant articles in PubMed, Cochrane Library, and Web of Science databases. The results were analyzed, and potential future prospects were discussed in detail.

Results

Our review included 14 articles describing relevant developments in JA. Currently, most JA-related research is being performed in small animal models, demonstrating graft survival and functional restoration with short-term immunosuppression. In human patients, only six knee allotransplantations have been performed to date, with all grafts ultimately failing and a maximum graft survival of 56 months.

Conclusion

Research on joint allotransplantation has been limited over the last 20 years due to the rarity of clinical applications, the complex nature of surgical procedures, and uncertain outcomes stemming from immune rejection. However, the key to overcoming these challenges lies in extending graft survival and minimizing immunosuppressive side effects. With the emergence of new immunosuppressive strategies, the feasibility and clinical potential of vascularized joint allotransplantation warrants further investigation.

Delivery of Rapamycin Using In Situ Forming Implants Promotes Immunoregulation and Vascularized Composite Allograft Survival

Vascularized composite allotransplantation (VCA), such as hand and face transplantation, is emerging as a potential solution in patients that suffered severe injuries. However, adverse effects of chronic high-dose immunosuppression regimens strongly limit the access to these procedures. In this study, we developed an in situ forming implant (ISFI) loaded with rapamycin to promote VCA acceptance. We hypothesized that the sustained delivery of low-dose rapamycin in proximity to the graft may promote graft survival and induce an immunoregulatory microenvironment, boosting the expansion of T regulatory cells (T_reg). In vitro and in vivo analysis of rapamycin-loaded ISFI (Rapa-ISFI) showed sustained drug release with subtherapeutic systemic levels and persistent tissue levels. A single injection of Rapa-ISFI in the groin on the same side as a transplanted limb significantly prolonged VCA survival. Moreover, treatment with Rapa-ISFI increased the levels of multilineage mixed chimerism and the frequency of T_reg both in the circulation and VCA-skin. Our study shows that Rapa-ISFI therapy represents a promising approach for minimizing immunosuppression, decreasing toxicity and increasing patient compliance. Importantly, the use of such a delivery system may favor the reprogramming of allogeneic responses towards a regulatory function in VCA and, potentially, in other transplants and inflammatory conditions.

Surgical angiogenesis with short-term immunosuppression maintains bone viability in rabbit allogenic knee joint transplantation

BACKGROUND

Vascularized composite allotransplantation has the potential for reconstruction of joint defects but requires lifelong immunosuppression, with substantial risks. This study evaluates an alternative, using surgical angiogenesis from implanted autogenous vessels to maintain viability without long-term immunotherapy.

METHODS

Vascularized knee joints were transplanted from Dutch Belted donors to New Zealand White rabbit recipients. Once positioned and revascularized microsurgically, a recipient-derived superficial inferior epigastric fascial flap and a saphenous arteriovenous bundle were placed within the transplanted femur and tibia, respectively, to develop a neoangiogenic, autogenous circulation. There were 10 transplants in group 1. Group 2 (n = 9) consisted of no-angiogenesis controls with ligated flaps and arteriovenous bundles. Group 3 rabbits (n = 10) were autotransplants with patent implants. Tacrolimus was used for 3 weeks to maintain nutrient flow during angiogenesis. At 16 weeks, the authors assessed bone healing, joint function, bone and cartilage mechanical properties, and histology.

RESULTS

Group 1 allotransplants had more robust angiogenesis, better healing, improved mechanical properties, and better osteocyte viability than ligated controls (group 2). All three groups developed knee joint contractures and arthritic changes. Cartilage thickness and quality were poorer in allograft groups than in autotransplant controls.

CONCLUSIONS

Surgical angiogenesis from implanted autogenous tissue improves bone viability, healing, and material properties in rabbit allogenic knee transplants. However, joint contractures and degenerative changes occurred in all transplants, regardless of antigenicity or blood supply. Experimental studies in a larger animal model with improved methods to maintain joint mobility are needed before the merit of living joint allotransplantation can be judged.

Multiscale mechanobiology of de novo bone generation, remodeling and adaptation of autograft in a common ovine femur model

The link between mechanics and biology in the generation and the adaptation of bone has been studied for more than a century in the context of skeletal development and fracture healing. However, the interplay between mechanics and biology in de novo generation of bone in postnatal defects as well as healing of morcellized bone graft or massive cortical bone autografts is less well understood. To address this, here we integrate insights from our previously published studies describing the mechanobiology on both de novo bone generation and graft healing in a common ovine femoral defect model. Studying these effects in a common experimental model provides a unique opportunity to elucidate factors conducive to harnessing the regenerative power of the periosteum, and ultimately, to provide mechanistic insights into the multiscale mechanobiology of bone generation, remodeling and adaptation. Taken together, the studies indicate that, as long as adequate, directional transport of cells and molecules can be insured (e.g. with periosteum in situ or a delivery device), biological factors intrinsic to the periosteum suffice to bridge critical sized bone defects, even in the absence of a patent blood supply. Furthermore, mechanical stimuli are crucial for the success of periosteal bone generation and bone graft healing. Interestingly, areas of highest periosteal strain around defects correlate with greatest amounts albeit not greatest mineralization of newly generated bone. This may indicate a role for convection enhanced transport of cells and molecules in modulation of tissue generation by pluripotent cells that ingress into the defect center, away from the periosteum and toward the surface of the intramedullary nail that fills the medullary cavity. These insights bring us much closer to understanding the mechanobiological environment and stimuli that stimulate the proliferation and differentiation of periosteum-derived progenitor cells and ultimately drive the generation of new bone tissue. Furthermore, these insights provide a foundation to create virtual predictive computational models of bone mechanophysiology, to develop cell seeding protocols for scale up and manufacture of engineered tissues, to optimize surgical procedures, and to develop post-surgical therapies with the ultimate goal of achieving the best possible healing outcomes for treatment and/or reconstruction of postnatal bone defects.