Immunologic approaches to composite tissue allograft

Characterization, prophylaxis, and treatment of infectious complications in craniomaxillofacial and upper extremity allotransplantation: a multicenter perspective

BACKGROUND

Vascularized composite allotransplants consist of heterogeneous tissues from different germ layers, which include skin, muscle, bone, fat, nerves, and lymph nodes. The antigenic diversity of these tissues, particularly of the highly immunogenic skin component, necessitates potent immunosuppressive regimens similar to that of some solid organ transplants. Indeed, the lifelong, high-dose, multidrug immunosuppressive protocols expose vascularized composite allotransplant recipients to considerable risk of infectious, metabolic, and neoplastic sequelae. In this article, the authors review the infectious risk to patients after vascularized composite allotransplantation, with special attention to the somewhat limited experience with the prophylaxis and treatment of infections after this innovative reconstructive surgery.

METHODS

A review of the literature was undertaken to elucidate the characterization, prophylaxis, and treatment of all documented infectious complications.

RESULTS

Infections in face and hand vascularized composite allotransplants follow a course similar to that of solid organ transplants. Several differences exist, including the unique flora of craniomaxillofacial transplants, the increased risk of donor-derived infections, and the alteration of the risk-to-benefit ratio for cytomegalovirus infections.

CONCLUSIONS

The patient with a face or limb transplant has many of the same infectious risks as a lung transplant recipient, which include bacterial, viral, and fungal infections. Because of the anatomy, mucosal exposure, and differing donor flora, however, the face or limb transplant is susceptible to invasive diseases from a variety of microbes.

Stimulation of the follicular bulge LGR5+ and LGR6+ stem cells with the gut-derived human alpha defensin 5 results in decreased bacterial presence, enhanced wound healing, and hair growth from tissues devoid of adnexal structures

BACKGROUND

Discovery of leucine-rich repeat-containing G-protein-coupled receptors 5 and 6 (LGR5 and LGR6) as markers of adult epithelial stem cells of the skin and intestine permits researchers to draw on the intrinsic cellular fundamentals of wound healing and proliferation dynamics of epithelial surfaces. In this study, the authors use the intestine-derived human alpha defensin 5 to stimulate epithelial proliferation, bacterial reduction, and hair production in burn wound beds to provide the field with initial insight on augmenting wound healing in tissues devoid of adnexal stem cells.

METHODS

Murine third-degree burn wound beds were treated with (1) intestine-derived human alpha defensin 5, (2) skin-derived human beta defensin 1, and (3) sulfadiazine to determine their roles in wound healing, bacterial reduction, and hair growth.

RESULTS

The human alpha defensin 5 peptide significantly enhanced wound healing and reduced basal bacterial load compared with human beta defensin 1 and sulfadiazine. Human alpha defensin 5 was the only therapy to induce LGR stem cell migration into the wound bed. In addition, gene heat mapping showed significant mRNA up-regulation of key wound healing and Wnt pathway transcripts such as Wnt1 and Wisp1. Ex vivo studies showed enhanced cell migration in human alpha defensin 5-treated wounds compared with controls.

CONCLUSIONS

Application of human alpha defensin 5 increases LGR stem cell migration into wound beds, leading to enhanced healing, bacterial reduction, and hair production through the augmentation of key Wnt and wound healing transcripts. These findings can be used to derive gut protein-based therapeutics in wound healing.

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.

Hand allotransplantation

In the past decade, more than 100 different composite tissue allotransplantation (CTA) procedures have been performed around the world including more than 50 hand and 8 facial transplants with encouraging graft survival and excellent functional outcomes. Broader clinical application of CTA, however, continues to be hampered by requirement for long-term, high-dose, multidrug maintenance immunosuppression to prevent graft rejection mediated particularly by composite tissue allograft's highly immunogenic skin component. Medication toxicity could result in severe adverse events including metabolic and infectious complications or malignancy. Notably, unlike in solid organs, clinical success is dictated not only by graft acceptance and survival but also by nerve regeneration, which determines ultimate functional outcomes. Novel strategies such as cellular and biologic therapies that integrate the concepts of immune regulation with those of nerve regeneration have shown promising results in small and large animal models. Clinical translation of these insights to reconstructive transplantation and CTA could further minimize the need of immunosuppression and optimize functional outcomes. This will enable wider application of such treatment options for patients in need of complex reconstructive surgery for congenital deformities or devastating injuries that are not amenable to standard methods of repair.

Immunologic aspects and rejection in solid organ versus reconstructive transplantation

The immunosuppressive medications developed over the past 3 decades have paved the way for solid organ transplantation to become the treatment of choice for end-stage organ failure. At the end of the century, composite tissue transplantation in humans was performed with success using the same immunosuppressive medications and therapeutic principles. A decade later, experience from >100 cases of reconstructive transplantation have increased the knowledge, changed the view, and affected the therapeutic principles in this novel field. We herein portray the evolution of this novel type of transplant with particular reference to immunologic aspects, particularly differences between reconstructive and solid organ transplantation.

Transgene expression in a model of composite tissue allotransplantation

BACKGROUND

Composite tissue allografting may be an ideal solution to many problems requiring reconstructive surgery. Unfortunately, complications associated with chronic immunocompromise are major impediments to widespread use of composite tissue allografting. Current immunosuppressive and immunomodulatory paradigms focus on modification of the recipient through global immunosuppression or donor/recipient chimerism. Alternatively, modifying the allograft to block rejection or promote tolerance could confine deleterious immunosuppressive effects to the graft or eliminate graft rejection. However, a technique introducing genetic information into the transplant is needed. The authors demonstrate the first model for expressing a gene of interest locally in a hind-limb transplant.

METHODS

Using a rat hind-limb transplant model, the authors tested the ability of naked DNA infusion, cationic polymer/DNA complex transfection, and adenoviral vector transduction to introduce genetic material into the composite tissue allograft. The marker genes luciferase and green fluorescent protein were used to follow gene expression.

RESULTS

Recombinant adenovirus showed rapid, high-level expression of marker genes in the graft, with no detectable expression in recipient animals. Expression was detectable at 18 hours and peaked at 7 days. Levels of expression were lower but above baseline at 4 weeks.

CONCLUSIONS

Using an adenoviral vector system, the authors have selectively introduced a marker gene (luciferase) into a transplanted hind-limb rat model. Expression was rapid and seen in a variety of cell types. Adenovirus infection had no impact on limb rejection. This method may be a powerful tool for genetically modifying composite tissue allografts and may contribute to immune tolerance and more widespread use of composite tissue allograft surgery.