Rejection of the component tissues of limb allografts in rats immunosuppressed with FK-506 and cyclosporine

The contribution of the donor vascularised hand and face allograft in transplant rejection: An immunological perspective

Overcoming immunological rejection remains a barrier to the safe adoption of Vascularised Composite Allotransplantation (VCA). To mitigate this risk, clinical protocols have been derived from solid organ transplantation, targeting recipient immunomodulation, yet VCA is unique. Face and hand composite allografts are composed of multiple different tissues, each with their own immunological properties. Experimental work suggests that allografts carry variable numbers and populations of donor leukocytes in an organ specific manner. Ordinarily, these passenger leukocytes are transferred from the donor graft into the recipient circulation after transplantation. Whether alloantigen presentation manifests as acute allograft rejection or transplant tolerance is unknown. This review aims to characterise the immunological properties of the constituent parts of the donor face and hand, the potential fate of donor leukocytes and to consider theoretical graft specific interventions to mitigate early rejection.

Transcriptome profiling of immune rejection mechanisms in a porcine vascularized composite allotransplantation model

Background

Vascularized composite allotransplantation (VCA) offers the potential for a biological, functional reconstruction in individuals with limb loss or facial disfigurement. Yet, it faces substantial challenges due to heightened immune rejection rates compared to solid organ transplants. A deep understanding of the genetic and immunological drivers of VCA rejection is essential to improve VCA outcomes.

Methods

Heterotopic porcine hindlimb VCA models were established and followed until reaching the endpoint. Skin and muscle samples were obtained from VCA transplant recipient pigs for histological assessments and RNA sequencing analysis. The rejection groups included recipients with moderate pathological rejection, treated locally with tacrolimus encapsulated in triglycerol-monostearate gel (TGMS-TAC), as well as recipients with severe end-stage rejection presenting evident necrosis. Healthy donor tissue served as controls. Bioinformatics analysis, immunofluorescence, and electron microscopy were utilized to examine gene expression patterns and the expression of immune response markers.

Results

Our comprehensive analyses encompassed differentially expressed genes, Gene Ontology, and Kyoto Encyclopedia of Genes and Genomes pathways, spanning various composite tissues including skin and muscle, in comparison to the healthy control group. The analysis revealed a consistency and reproducibility in alignment with the pathological rejection grading. Genes and pathways associated with innate immunity, notably pattern recognition receptors (PRRs), damage-associated molecular patterns (DAMPs), and antigen processing and presentation pathways, exhibited upregulation in the VCA rejection groups compared to the healthy controls. Our investigation identified significant shifts in gene expression related to cytokines, chemokines, complement pathways, and diverse immune cell types, with CD8 T cells and macrophages notably enriched in the VCA rejection tissues. Mechanisms of cell death, such as apoptosis, necroptosis and ferroptosis were observed and coexisted in rejected tissues.

Conclusion

Our study provides insights into the genetic profile of tissue rejection in the porcine VCA model. We comprehensively analyze the molecular landscape of immune rejection mechanisms, from innate immunity activation to critical stages such as antigen recognition, cytotoxic rejection, and cell death. This research advances our understanding of graft rejection mechanisms and offers potential for improving diagnostic and therapeutic strategies to enhance the long-term success of VCA.

Potential Roles of miRNAs in Acute Rejection for Vascularized Composite Allotransplantation

Aim

The development of microsurgery has greatly advanced vascularized composite allotransplantation (VCA). However, like organ transplantation, VCA is also limited by acute rejection, and concerns regarding long-term survival and function of the transplanted graft. Therefore, it is necessary to elucidate the molecular mechanisms underlying acute rejection caused by VCA, in order to improve patient survival.

Methods

Firstly, we used Brown Norway rats and Lewis rats to construct animal model of VCA. Regularly record the appearance changes of all subjects. Specimens were collected for histological examination, microRNAs (miRNAs) sequencing and RT-qPCR verification when acute immune rejection occurred. Then, bioinformatics analysis was employed to predict miRNA related molecules and pathway information. Finally, differentially expressed miRNAs were tested and verified.

Results

MiRNAs are small non coding RNA transcripts that regulate gene expression at the post-transcriptional level. Studies have shown that miRNAs are involved in immune regulation and several miRNAs have been identified that are potential diagnostic and prognostic biomarkers of acute rejection. In this study, we found that the expression levels of rno-miR-21-5p, rno-miR-340-5p, rno-miR-1-3p and rno-miR-195-5p are significantly associated with acute rejection following VCA.

Conclusion

This miRNA signature can potentially an auxiliary diagnostic indicator of rejection, which can help clinicians adjust the immunosuppressive program in time during acute rejection.

TRFs and tiRNAs sequence in acute rejection for vascularized composite allotransplantation

Illumina tRFs & tiRNAs-seq analysis was used to characterize the whole transcriptomes of acute rejection caused by vascularized composite allotransplantation (VCA). tRFs & tiRNAs-seq information for muscle samples with VCA was obtained and compared with similar information for same age- and sex-matched healthy control subjects. The expression of 16 tRFs and tiRNAs, including 5 up-regulated target genes and 11 down-regulated target genes, were significantly different. According to bioinformatics analysis and reverse transcription quantitative polymerase chain reaction, we speculate that tiRNA-1-34-Glu-CTC-1 plays an important role in VCA-induced acute rejection by regulating the CACNA1D gene in the MAPK signaling pathway The findings provide the whole-transcriptome signatures of acute rejection for VCA, allowing further exploration of gene expression patterns/signatures associated with the various clinical symptoms of acute rejection for VCA.

Measurement(s)	gene expression
Technology Type(s)	RNA sequence
Factor Type(s)	RNA
Sample Characteristic - Organism	rn6
Sample Characteristic - Location	China

Thermosensitive Polyester Hydrogel for Application of Immunosuppressive Drug Delivery System in Skin Allograft

Tacrolimus (FK506) is a common immunosuppressive drug that is capable of suppressing acute rejection reactions, and is used to treat patients after allotransplantation. A stable and suitable serum concentration of tacrolimus is desirable for better therapeutic effects. However, daily drug administration via oral or injection routes is quite inconvenient and may encounter drug overdose or low patient compliance problems. In this research, our objective was to develop an extended delivery system using a thermosensitive hydrogel of poly ethylene glycol, D,L-lactide (L), and ϵ-caprolactone (CL) block copolymer, mPEG-PLCL, as a drug depot. The formulation of mPEG-PLCL and 0.5% PVP-dissolved tacrolimus was studied and the optimal formulation was obtained. The in vivo data showed that in situ gelling is achieved, a stable and sustained release of the drug within 30 days can be maintained, and the hydrogel was majorly degraded in that period. Moreover, improved allograft survival was achieved. Together, these data imply the potential of the current formulation for immunosuppressive treatments.

The Positive Impact of Donor Bone Marrow Cells Transplantation into Immunoprivileged Compartments on the Survival of Vascularized Skin Allografts

Using the vascularized skin allograft (VSA) model, we compared the tolerogenic effects of different allogeneic bone marrow transplantation (BMT) delivery routes into immunoprivileged compartments under a 7-day protocol immunosuppressive therapy. Twenty-eight fully MHC mismatched VSA transplants were performed between ACI (RT1^a) donors and Lewis (RT1¹) recipients in four groups of seven animals each, under a 7-day protocol of alfa/beta TCRmAb/CsA (alpha/beta-TCR monoclonal antibodies/Cyclosporine A therapy). Donor bone marrow cells (BMC) (100 × 106 cells) were injected into three different immunoprivileged compartments: Group 1: Control, without cellular supportive therapy, Group 2: Intracapsular BMT, Group 3: Intragonadal BMT, Group 4: Intrathecal BMT. In Group 2, BMC were transplanted under the kidney capsule. In Group 3, BMC were transplanted into the right testis between tunica albuginea and seminiferous tubules, and in Group 4, cells were injected intrathecally. The assessment included: skin evaluation for signs and grade of rejection and immunohistochemistry for donor cells engraftment into host lymphoid compartments. Donor-specific chimerism for MHC class I (RT1^a) antigens and the presence of CD4⁺/CD25⁺ T cells were assessed in the peripheral blood of recipients. The most extended allograft survival, 50–78 days, was observed in Group 4 after intrathecal BMT. The T cells CD4⁺/CD25⁺ in the peripheral blood were higher after intrathecal BMC injection than other experimental groups at each post-transplant time point. Transplantation of BMC into immunoprivileged compartments delayed rejection of fully mismatched VSA and induction of robust, donor-specific chimerism.

Evolution of the rat hind limb transplant as an experimental model of vascularized composite allotransplantation: Approaches and advantages

As clinical experience with surgical techniques and immunosuppression in vascularized composite allotransplantation recipients has accumulated, vascularized composite allotransplantation for hand and face have become standard of care in some countries for select patients who have experienced catastrophic tissue loss. Experience to date suggests that clinical vascularized composite allotransplantation grafts undergo the same processes of allograft rejection as solid organ grafts. Nonetheless, there are some distinct differences, especially with respect to the immunologic influence of the skin and how the graft is affected by environmental and traumatic insults. Understanding the mechanisms around these similarities and differences has the potential to not only improve vascularized composite allotransplantation outcomes but also outcomes for all types of transplants and to contribute to our understanding of how complex systems of immunity and function work together. A distinct disadvantage in the study of upper extremity vascularized composite allotransplantation recipients is the low number of clinical transplants performed each year. As upper extremity transplantation is a quality of life rather than a lifesaving transplant, these numbers are not likely to increase significantly until the risks of systemic immunosuppression can be reduced. As such, experimental models of vascularized composite allotransplantation are essential to test hypotheses regarding unique characteristics of graft rejection and acceptance of vascularized composite allotransplantation allografts. Rat hind limb vascularized composite allotransplantation models have been widely used to address these questions and provide essential proof-of-concept findings which can then be extended to other experimental models, including mice and large animal models, as new concepts are translated to the clinic. Here, we review the large body of rat hind limb vascularized composite allotransplantation models in the literature, with a focus on the various surgical models that have been developed, contrasting the characteristics of the specific model and how they have been applied. We hope that this review will assist other researchers in choosing the most appropriate rat hind limb transplantation model for their scientific interests.

Technical, Immunological, and Ethical Similarities and Differences Between Vascularized Composite Allotransplantation and Solid Organ Transplantation in Current Practice

Solid organ transplantation (SOT) is a surgical method used as the gold standard in end-term organ failure. Following SOT, successful results have also been achieved in vascularized composite tissue transplantation, which improves the quality of life with the success of solid organ transplants and the development of modern immunosuppressive regimens. This review discusses the preoperative, operative, postoperative, and immunological differences between vascularized tissue transplantation and SOT.

Bone vascularized composite allotransplantation model in swine tibial defect: Evaluation of surgical angiogenesis and transplant viability

INTRODUCTION

In prior small animal studies, we maintained vascularized bone allotransplant viability without long-term immunotherapy. Instead, an autogenous neoangiogenic circulation is created from implanted vessels, sufficient to maintain bone viability with only 2 weeks immunosupression. Blood flow is maintained despite rejection of the allogeneic vascular pedicle thereafter. We have previously described a large animal (swine) pre-clinical model, reconstructing tibial defects with vascularized tibial allotransplants. In this manuscript, autologous angiogenesis is evaluated in this model and correlated with bone viability.

MATERIALS AND METHODS

Allogeneic tibial segments were transplanted across a major swine leukocyte antigen mismatch. Microvascular repair of the bone VCA pedicle was combined with intraosseous implantation of an autogenous arteriovenous (AV) bundle. The bundle was ligated in group 1 (n = 4), and allowed to perfuse in group 2 (n = 4). Three-drug immunotherapy was given for 2 weeks. At 16 weeks micro-CT angiography quantified neoangiogenic vessel volume. Bone viability, rejection grade, and bone healing were analyzed.

RESULTS

A substantial neoangiogenic circulation developed from the implanted AV-bundle in group 2, with vessel density superior to ligated AV-bundle controls (0.11 ± 0.05 vs. 0.01 ± 0.01, P = .029). Bone allotransplant viability was also significantly enhanced by neoangiogenesis (78.7 ± 4.4% vs. 27.7 ± 5.8%, P = .028) with higher bone healing scores (21.4 ± 2.9 vs. 12.5 ± 3.7, P = .029). Ligated control tibias demonstrated disorganized bone morphology and higher local inflammation (P = .143).

CONCLUSION

Implantation of autogenous AV bundles into vascularized bone allotransplants resulted in the rapid formation of a neoangiogenic autogenous blood supply in a swine tibia model that maintained bone viability, improved bone healing, and minimized rejection.

The abdominal wall transplant as a sentinel skin graft

PURPOSE OF REVIEW

Abdominal wall transplantation is a technique used to achieve abdominal closure after intestinal and multivisceral transplantation. This review focuses on whether there are additional benefits for the skin component as an immune-monitoring tool.

RECENT FINDINGS

The largest series of abdominal wall transplants has recently been published. Alongside the physiological advantage gained in abdominal closure, the authors describe the immunological insight that the skin component can provide and how this contributes to the management of patients. The skin appears to develop a rash with early rejection, which facilitates early systemic treatment before significant visceral rejection occurs. It can also help in cases in which there is diagnostic doubt regarding the cause of bowel dysfunction such as in instances of intestinal infection. Despite the additional immunological burden of donor tissue, there appears to be no requirement for increased immunosuppressive therapy.

SUMMARY

The technical and immunological feasibility of abdominal wall transplantation has now been demonstrated by several centres. Skin transplanted as part of the abdominal wall or as a separate vascularized sentinel skin flap may aid in the diagnosis of rejection. This has the potential to improve graft survival and reduce immunosuppressive morbidity.

Surgical Angiogenesis in Porcine Tibial Allotransplantation: A New Large Animal Bone Vascularized Composite Allotransplantation Model

Segmental bone loss resulting from trauma, infection malignancy and congenital anomaly remains a major reconstructive challenge. Current therapeutic options have significant risk of failure and substantial morbidity. Use of bone vascularized composite allotransplantation (VCA) would offer both a close match of resected bone size and shape and the healing and remodeling potential of living bone. At present, life-long drug immunosuppression (IS) is required. Organ toxicity, opportunistic infection and neoplasm risks are of concern to treat such non-lethal indications. We have previously demonstrated that bone and joint VCA viability may be maintained in rats and rabbits without the need of long-term-immunosuppression by implantation of recipient derived vessels within the VCA. It generates an autogenous, neoangiogenic circulation with measurable flow and active bone remodeling, requiring only 2 weeks of IS. As small animals differ from man substantially in anatomy, bone physiology and immunology, we have developed a porcine bone VCA model to evaluate this technique before clinical application is undertaken. Miniature swine are currently widely used for allotransplantation research, given their immunologic, anatomic, physiologic and size similarities to man. Here, we describe a new porcine orthotopic tibial bone VCA model to test the role of autogenous surgical angiogenesis to maintain VCA viability. The model reconstructs segmental tibial bone defects using size- and shape-matched allogeneic tibial bone segments, transplanted across a major swine leukocyte antigen (SLA) mismatch in Yucatan miniature swine. Nutrient vessel repair and implantation of recipient derived autogenous vessels into the medullary canal of allogeneic tibial bone segments is performed in combination with simultaneous short-term IS. This permits a neoangiogenic autogenous circulation to develop from the implanted tissue, maintaining flow through the allogeneic nutrient vessels for a short time. Once established, the new autogenous circulation maintains bone viability following cessation of drug therapy and subsequent nutrient vessel thrombosis.

Could Sentinel Skin Transplants Have Some Utility in Solid Organ Transplantation?

Accurate diagnosis of allograft rejection can be hazardous and challenging. A strategy that has emerged from experience with vascularized composite allografts (VCAs) is the use of sentinel skin transplants (SSTs)-portions of donor skin transplanted synchronously to an allograft. Work in nonhuman animal models and experience with VCAs suggest concordance between rejection occurring in the primary allograft and the SST, and that appearance of rejection in the SST may precede rejection in the primary allograft, permitting early therapeutic intervention that may improve outcomes with lower rates of chronic rejection. The encouraging findings reported in VCA transplantation raise the possibility that SST may also be useful in solid organ transplantation. Some evidence is provided by experience with abdominal wall transplantation in some intestinal and multivisceral transplant recipients. Results from those reports raise the possibility that rejection may manifest in the skin component before emergence in the intestinal allograft, providing a "lead time" during which treatment of rejection of the abdominal wall could prevent the emergence of intestinal rejection. It is plausible that these findings may be extrapolated to other solid organ allografts, especially those for which obtaining an accurate diagnosis of acute rejection can be hazardous and challenging, such as the lung or pancreas. However, more data are required to support widespread adoption of this technique.

Angiosome-Based Allografts: Vascularized Composite Allotransplantation for Tailored Subunit Reconstruction with Volkmann Ischemic Contracture as a Case in Point

BACKGROUND

As we enter an age with new approaches to tissue reconstruction, the emphasis on the adage "like for like" has become even more relevant. This study illustrates the potential for several tailored vascularized composite allotransplantation reconstructive techniques and, in particular, for the management of Volkmann contracture.

METHODS

Twenty fresh cadaver dissections and 30 archival lead oxide radiographic studies were examined to (1) identify potential upper limb vascularized composite allotransplantation donor sites (i.e., elbow, forearm, and flexor tendon complex) and (2) demonstrate a "mock transplant" of the vascularized volar forearm allograft for a severe Volkmann ischemia defect. They were designed without skin to reduce antigenicity.

RESULTS

The elbow joint was supplied within the brachial angiosome and the flexor tendon complex of the flexor digitorum superficialis and flexor digitorum profundus by the superficial palmar arch of the ulnar angiosome. The forearm allograft of flexor muscles, median, ulnar, and anterior interosseous nerves, when harvested on the brachial vessels, was supplied within the radial, ulnar, and anterior interosseous angiosomes but could be based on the ulnar artery alone because of intramuscular connections with the other territories. A mock transplant was performed with a distal-to-proximal dissection of the allograft, facilitating the best and fastest technique.

CONCLUSIONS

This application of the angiosome concept highlights the anatomical feasibility of the volar forearm vascularized composite allotransplantation donor site focusing on a complex subunit problem in the upper limb-severe Volkmann ischemic contracture. It demonstrates the potential use and immunologic advantage of subdivided and modified nonskin variations of vascularized composite allotransplantation in reconstructive transplantation surgery.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, V.

Digital Joint Reconstruction with Osteotendinous Joint Allograft: Experimental Study in Rats

Background:

Fresh or frozen nonvascularized osteotendinous joint allografts (OTJA) have not been used previously, clinically or experimentally, for metacarpophalangeal joint reconstruction. Therefore, we evaluated the viability of OTJA for metatarsophalangeal joint (MTJ) reconstruction in rats.

Methods:

In the experimental group of 12 Lewis rats, we reconstructed the MTJ of the third digit of the hindlimb with a fresh, nonvascularized OTJA obtained from the same digit from 12 donor rats. In the control group of 6 Lewis rats, an autologous composite osteotendinous graft of the MTJ of the same digit was obtained and repositioned in situ as an auto-transplant. Weight, pain, edema, dehiscence, and wound infection were evaluated every 24 hours for 30 days postoperatively. At the end of 30 days, we evaluated digit position, flexion and extension, passive mobility, radiological bone healing, and histological grades of rejection.

Results:

We found no statistically different changes in weight, edema, pain, digit position, or radiological bone healing in either group. No wound dehiscence or infection was seen in any of the rats. Ten degrees of flexion and extension mobility were lost in the control group; the experimental group lost up to 30 degrees (P = 0.009). Histologically, 9 of the experimental group rats (9/12, 75%) showed rejection reactions compared with none of the controls (0%) (P = 0.009).

Conclusions:

Fresh nonvascularized OTJA caused an immune reaction without exposure of the graft, but with bone resorption. However, the rats maintained digital form and alignment with decreased passive flexion and extension of 10–30 degrees.

Vascularized composite allograft-specific characteristics of immune responses

Vascularized composite allograft (VCA) transplantation, or reconstructive transplantation, has revolutionized the treatment of complex tissue and functional defects. Despite arriving during an age in which the immunology of solid organ transplant rejection has been investigated in much detail, these transplants have offered new perspectives from which to explore the immunobiology of transplantation. VCAs have a number of unique molecular, cellular, and architectural features which alter the character and intensity of the rejection response. While much is yet to be clarified, an understanding of these distinct mechanisms affords new possibilities for the control of immune responses in an effort to improve outcomes after VCA transplantation.

Immunomodulatory Effects of Different Cellular Therapies of Bone Marrow Origin on Chimerism Induction and Maintenance Across MHC Barriers in a Face Allotransplantation Model

Many more patients would benefit from vascularized composite allotransplantation if less toxic and safer immunosuppressive protocols will become available. Tolerance induction protocols with donor cells co-transplantation are one of the promising pathways to reduce maintenance immunosupressive regimens. We investigated the role of donor bone marrow cells (BMC), mesenchymal stromal cells (MSC) and in vivo created chimeric cells (CC) used as supportive therapies in a fully MHC-mismatched rat face transplantation model. Twenty-four fully MHC-mismatched hemiface transplantations were performed between ACI (RT1(a)) donors and Lewis (RT1(l)) recipients under combined seven-day immunosuppressive regimen of anti-αβ-T-cell receptor (TCR) monoclonal antibody and cyclosporin A. We studied four experimental groups-group 1: no cellular therapy; group 2: supportive therapy with BMC; group 3: supportive therapy with MSC; group 4: supportive therapy with CC generated in a primary chimera. We evaluated clinical and histological rejection grades, transplanted cells migration, donor-specific chimerism in the peripheral blood and bone marrow compartments, and CD4(+)/CD25(+) T-cell levels. Face allograft rejection was observed at 26.8 ± 0.6 days post-transplant (PT) in the absence of cellular therapy, at 34.5 ± 1.1 days for group 2, 29.3 ± 0.8 days for group 3, and 30.3 ± 1.38 PT for group 4. The longest survival was observed in allografts supported by co-transplantation of BMC. All support in cellular therapies delayed face allograft rejection by chimerism induction and/or immunomodulatory properties of co-transplanted cells. Survival time was comparable between groups, however, further studies, with different cell dosages, delivery routes and delivery times are required.

The potential role for regulatory T-cell therapy in vascularized composite allograft transplantation

PURPOSE OF REVIEW

Vascularized composite allograft (VCA) transplantation restores defects to a degree not possible by conventional techniques. However, it is limited by the need for long-term immunosuppression and high rates of acute rejection directed against skin. There is therefore a need for a therapy that may shift the risk-benefit ratio in favour of VCA transplantation. Regulatory T cells (Tregs) are a subset of T cells with potent immunoregulatory properties and the potential to promote immunosuppression-free allograft survival. In this review, we consider the evidence for Treg therapy in VCA transplantation.

RECENT FINDINGS

CD4 Tregs are the best-studied immunoregulatory cell type, and a large amount of experimental and clinical data is emerging to endorse their use in VCA transplantation. Data from animal and humanized models are particularly encouraging and demonstrate the potent efficacy of Treg at preventing skin allograft rejection. Moreover, central tolerance induction techniques in VCA transplantation models are demonstrating a dependence on Tregs for graft survival.

SUMMARY

An improvement in outcomes after VCA transplantation has the potential to revolutionize the field. Several effective therapeutic strategies have demonstrated great promise experimentally, and there is now a need to assess their safety and efficacy in a clinical setting.

Fibroblast growth factor-2 and vascular endothelial growth factor mediated augmentation of angiogenesis and bone formation in vascularized bone allotransplants

We previously demonstrated recipient-derived neoangiogenesis to maintain viability of living bone allogeneic transplants without long-term immunosuppression. The effect of cytokine delivery to enhance this process is studied. Vascularized femur transplantation was performed from Dark Agouti to Piebald Virol Glaxo rats. Poly(d,l-lactide-co-glycolide) microspheres loaded with buffer (N = 11), basic fibroblast growth factor (FGF2) (N = 10), vascular endothelial growth factor (VEGF) (N = 11), or both (N = 11) were inserted intramedullarly alongside a recipient-derived arteriovenous bundle. FK-506 was administered for 2 weeks. At 18 weeks, bone blood flow, microangiography, histologic, histomorphometric, and alkaline phosphatase measurements were performed. Bone blood flow was greater in the combined group than control and VEGF groups (P = 0.04). Capillary density was greater in the FGF2 group than in the VEGF and combined groups (P < 0.05). Bone viability, growth, and alkaline phosphatase activity did not vary significantly between groups. Neoangiogenesis in vascularized bone allotransplants is enhanced by angiogenic cytokine delivery, with results using FGF2 that are comparable to isotransplant from previous studies. Further studies are needed to achieve bone formation similar to isotransplants.

Nerve regeneration in rat limb allografts: evaluation of acute rejection rescue

BACKGROUND

Successful nerve regeneration is critical to the functional success of composite tissue allografts. The present study was designed to characterize the effect of acute rejection on nerve regeneration and functional recovery in the setting of orthotopic limb transplantation.

METHODS

A rat orthotopic limb transplantation model was used to evaluate the effects of acute rejection on nerve regeneration and motor recovery. Continuous administration of FK506 (full suppression), administration of FK506 for the first 8 of 12 weeks (late rejection), or delayed administration of FK506/dexamethasone following noticeable rejection (early rejection) was used to preclude or induce rejection following limb transplantation. Twelve weeks postoperatively, nerve regeneration was assessed by means of histomorphometric analysis of explanted sciatic nerve, and motor recovery was assessed by means of evoked muscle force measurement in extensor digitorum longus muscle.

RESULTS

A single episode of acute rejection that occurs immediately or late after reconstruction does not significantly alter the number of regenerating axonal fibers. Acute rejection occurring late after reconstruction adversely affects extensor digitorum longus muscle function in composite tissue allografts.

CONCLUSIONS

Collected data reinforce that adequate immunosuppressant administration in cases of allogeneic limb transplantation ensures levels of nerve regeneration and motor functional recovery equivalent to that of syngeneic transplants. Prompt rescue following acute rejection was further demonstrated not to significantly affect nerve regeneration and functional recovery postoperatively. However, instances of acute rejection that occur late after reconstruction affect graft function. In total, the present study begins to characterize the effect of immunosuppression regimens on nerve regeneration and motor recovery in the setting of composite tissue allografts.

Vascularized composite allograft rejection is delayed by intrajejunal treatment with donor splenocytes without concomitant immunosuppressants

Background. Mucosal or oral tolerance, an established method for inducing low-risk antigen-specific hyporesponsiveness, has not been investigated in vascularized composite allograft (VCA) research. We studied its effects on recipient immune responses and VCA rejection. Methods. Lewis rats (n = 12; TREATED) received seven daily intrajejunal treatments of 5 × 10⁷ splenocytes from semiallogeneic Lewis-Brown-Norway rats (LBN) or vehicle (n = 11; SHAM). Recipients' immune responses were assessed by mixed lymphocyte reaction (MLR) against donor antigen and controls. Other Lewis (n = 8; TREATED/VCA) received LBN hindlimb VCA and daily intrajejunal treatments of 5 × 10⁷ LBN splenocytes, or LBN VCA without treatment (n = 5; SHAM/VCA), until VCAs rejected. Recipients' immune responses were characterised and VCAs biopsied for histopathology. Immunosuppressants were not used. Results. LBN-specific hyporesponsiveness was induced only in treated Lewis recipients. Treatment significantly reduced MLR alloreactivity, significantly reduced VCA rejection on histopathology, and significantly delayed clinical VCA rejection (P < 0.0005; TREATED/VCA mean 9.6 versus 6.0 days for SHAM/VCA). Treatment significantly increased immunosuppressive IL-10/IL-4/TGF-β production and significantly decreased proinflammatory IFN-γ/TNF-α. Conclusion. Jejunal exposure to antigen conferred donor specific hyporesponsiveness that delayed VCA rejection. This method may offer a low-risk adjunctive treatment option to help protect VCAs from rejection.

Mitomycin-C-treated peripheral blood mononuclear cells (PBMCs) prolong allograft survival in composite tissue allotransplantation

BACKGROUND

Composite tissue allotransplantation (CTA) was introduced as a potential treatment for complex reconstructive procedures and has become a clinical reality. Hand and face transplantation, the most widely recognized forms of CTA, have intensified immunological research in this emerging field of transplantation. Mitomycin C (MMC) is an alkylating agent that suppresses allogeneic T-cell responses. MMC-treated dendritic cells/PBMCs have been shown to induce donor-specific tolerance in solid organ allograft transplantations.

METHODS

Fully mismatched rats were used as hind limb donors [Lewis (RT1(1))] and recipients [Brown-Norway (RT1(n))]. Fifty-five allogeneic hind limb transplantations were accomplished in six groups. Group A (n = 10) received donor-derived MMC-treated PBMCs on transplantation day. Group B (n = 10) rats received no immunosuppression, group C (n = 10) received FK506 and prednisolon, group D consisted in isograft transplantation without immunosuppression, group E (n = 10) received non-treated PBMCs, and group F (n = 5) received PBS without any donor-derived cells. Rejection was assessed clinically and histologically.

RESULTS

In group A, the survival times of the allografts were prolonged to an average of 8.0 d. Rejection was significantly delayed compared with the averages of the corresponding control groups B, E, and F (5.5, 5.9, and 5.8 d). No rejection was seen in control groups C and D.

CONCLUSION

These results demonstrate that MMC-treated donor PBMCs significantly prolong allograft survival when administered systemically on the day of transplantation. However, the immunomodulatory effect is relatively modest with further research being required to clarify dose-effect relations, cell characteristics, and an optimized mechanism and timing for cell application.

A modified vascularized whole knee joint allotransplantation model in the rat

Previous papers have shown surgical neoangiogenesis to allow long-term bone allotransplant survival without immunosuppression. Whole joint composite tissue allotransplants (CTA) might be treated similarly. A novel rat knee CTA model is described for further study of the roles of neoangiogensis in joint allotransplant survival and adjustment of immunosuppression. Microvascular knee CTA was performed in nine rats across a major histocompatibility barrier with both pedicle repair and implantation of host-derived arteriovenous ("a/v") bundles. In the control group (N = 3), the pedicle was ligated. Immunosuppression was given daily. Joint mobility, weight-bearing, pedicle patency, bone blood flow, and sprouting from a/v bundles were assessed at 3 weeks. All but the nonrevascularized control knees had full passive motion and full weight bearing. One nutrient pedicle thrombosed prematurely. Blood flow was measurable in transplants with patent nutrient pedicles. Implanted a/v bundles produced new vascular networks on angiography. This new rat microsurgical model permits further study of joint allotransplantation. Patency of both pedicles and implanted a/v bundles was maintained, laying a foundation for future studies.

Living bone allotransplants survive by surgical angiogenesis alone: development of a novel method of composite tissue allotransplantation

BACKGROUND

Segmental bone defects pose reconstructive challenges. Composite tissue allotransplantation offers a potential solution but requires long-term immunosuppression with attendant health risks. This study demonstrates a novel method of composite-tissue allotransplantation, permitting long-term drug-free survival, with use of therapeutic angiogenesis of autogenous vessels to maintain circulation.

METHODS

Ninety-three rats underwent femoral allotransplantation, isotransplantation, or allografting. Group-1 femora were transplanted across a major histocompatibility complex barrier, with microsurgical pedicle anastomoses. The contralateral saphenous artery and vein (termed the AV bundle) of the recipient animal were implanted within the medullary canal to allow development of an autogenous circulation. In Group 2, allotransplantation was also performed, but with AV bundle ligation. Group 3 bones were frozen allografts rather than composite-tissue allotransplantation femora, and Group 4 bones were isotransplants. Paired comparison allowed evaluation of AV bundle effect, bone allogenicity (isogeneic or allogeneic), and initial circulation and viability (allotransplant versus allograft). Two weeks of immunosuppression therapy maintained blood flow initially, during development of a neoangiogenic autogenous blood supply from the AV bundle in patent groups. At eighteen weeks, skin grafts from donor, recipient, and third-party rats were tested for immunocompetence and donor-specific tolerance. At twenty-one weeks, bone circulation was quantified and new bone formation was measured.

RESULTS

Final circulatory status depended on both the initial viability of the graft and the successful development of neoangiogenic circulation. Median cortical blood flow was highest in Group 1 (4.6 mL/min/100 g), intermediate in Group 4 isotransplants (0.4 mL/min/100 g), and absent in others. Capillary proliferation and new bone formation were generally highest in allotransplants (15.0%, 6.4 μm³/μm²/yr) and isotransplants with patent AV bundles (16.6%, 50.3 μm³/μm²/yr) and less in allotransplants with ligated AV bundles (4.4%, 0.0 μm³/μm²/yr) or allografts (8.1%, 24.1 μm³/μm²/yr). Donor and third-party-type skin grafts were rejected, indicating immunocompetence without donor-specific tolerance.

CONCLUSIONS

In the rat model, microvascular allogeneic bone transplantation in combination with short-term immunosuppression and AV bundle implantation creates an autogenous neoangiogenic circulation, permitting long-term allotransplant survival with measurable blood flow.

Ex vivo-expanded human regulatory T cells prevent the rejection of skin allografts in a humanized mouse model

BACKGROUND

Composite tissue transplantation effectively reconstructs the most complex defects, but its use is limited because of harmful immunosuppression and the high susceptibility of skin to rejection. Development of tolerance is an ideal solution, and protocols using regulatory T cells (Tregs) to achieve this have been promising in experimental animal models. The aim of this study was to investigate the ability of human Tregs to regulate immune responses to a human skin allograft in vivo.

METHODS

We isolated and expanded naturally occurring CD127loCD25+CD4+ human Tregs from peripheral blood mononuclear cells (PBMCs) and examined their phenotype and suppressive activity in vitro. Using a clinically relevant chimeric humanized mouse system, we transplanted mice with human skin grafts followed by allogeneic populations of PBMCs with or without Tregs derived from the same PBMC donor.

RESULTS

Ex vivo-expanded Tregs maintain the appropriate Treg markers and retain suppressive activity against allostimulated and polyclonally stimulated autologous PBMCs in vitro. Mice receiving allogeneic PBMCs alone consistently reject human skin grafts, whereas those also receiving Tregs display stable long-term human skin transplant survival along with a reduction in the CD8+ human cellular graft infiltrate.

CONCLUSIONS

We show for the first time the unique ability of human Tregs to prevent the rejection of a skin allograft in vivo, highlighting the therapeutic potential of these cells clinically.

Augmentation of surgical angiogenesis in vascularized bone allotransplants with host-derived a/v bundle implantation, fibroblast growth factor-2, and vascular endothelial growth factor administration

We have previously shown experimental transplantation of living allogeneic bone to be feasible without long-term immunosuppression by development of a recipient-derived neoangiogenic circulation within bone. In this study, we examine the role of angiogenic cytokine delivery with biodegradable microspheres to enhance this process. Microsurgical femoral allotransplantation was performed from Dark Agouti to Piebald Virol Glaxo rats. Poly(D,L-lactide-co-glycolide) microspheres loaded with buffer, basic fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF), or both, were inserted intramedullarly along with a recipient-derived arteriovenous (a/v) bundle. FK-506 was administered daily for 14 days, then discontinued. At 28 days, bone blood flow was measured using hydrogen washout. Microangiography, histologic, and histomorphometric analyses were performed. Capillary density was greater in the FGF+VEGF group (35.1%) than control (13.9%) (p < 0.05), and a linear trend was found from control, FGF, VEGF, to FGF+VEGF (p < 0.005). Bone formation rates were greater with VEGF (p < 0.01) and FGF+VEGF (p < 0.05). VEGF or FGF alone increased blood flow more than when combined. Histology rejection grading was low in all grafts. Local administration of vascular and fibroblast growth factors augments angiogenesis, bone formation, and bone blood flow from implanted blood vessels of donor origin in vascularized bone allografts after removal of immunosuppression.

Hand transplantation and vascularized composite tissue allografts in orthopaedics and traumatology

Composite tissue allograft (CTA) is defined as heterologous transplantation of a complex comprising skin and subcutaneous, neurovascular and mesenchymal tissue. Such techniques allow complex reconstruction using matched tissue, without donor site morbidity. The potential indications in orthopaedics-traumatology could in the future be more frequent than the present indications of heart, lung, liver, kidney and pancreas transplantation. International clinical experience clearly demonstrates the feasibility of CTA, both surgically and immunologically. However, immunosuppression remains indispensable, exposing the patient to risks that are not acceptable for purely functional surgery, except in very particular indications. The main hope for the future lies in induction of graft-specific tolerance.

Composite tissue vasculopathy and degeneration following multiple episodes of acute rejection in reconstructive transplantation

Transplant vasculopathy has not been systematically investigated in composite tissue allotransplantation (CTA). The impact of multiple acute rejections (ARs) on long-term graft outcomes in reconstructive transplantation remains unknown. This study in a rat hind-limb allotransplantation model systematically analyzes vasculopathy and tissue-specific pathological changes secondary to multiple AR episodes. LEW rats were transplanted with BN rat hind limbs and treated as follows: Group 1 (Iso): isografts. Group 2 (CsA): Cyclosporine (CsA) qd; Group 3 (mult AR): CsA and dexamethasone only when AR was observed. No AR was observed in Groups 1 and 2. Multiple AR were observed in Group 3, and each episode was completely reversed (clinically) with pulsed CsA + dexamethasone treatment. Group 3 animals demonstrated significant vascular lesions along with skin and muscle atrophy, upregulation of profibrotic gene expression and fibrosis when compared to Groups 1 and 2. In addition, allograft bone was sclerotic, weak and prone to malunion and nonunion. Interestingly, vasculopathy was a late finding, whereas muscle atrophy with macrophage infiltration was seen early, after only a few AR episodes. Taken together, multiple AR episodes lead to vasculopathy and tissue-specific pathology in CTA. This is the first evidence of 'composite tissue vasculopathy and degeneration (CTVD)' in CTA.

Daily topical tacrolimus therapy prevents skin rejection in a rodent hind limb allograft model

BACKGROUND

Skin is the most immunogenic component of a composite tissue allograft. Topical immunotherapy is an attractive therapeutic modality with which to provide local immunosuppression, with minimal systemic toxicity. The present study was performed to investigate the potential of topical tacrolimus to prolong survival of the skin component of a composite tissue allograft.

METHODS

Wistar Furth-to-Lewis rat orthotopic hind limb transplants were performed. Group I consisted of rats treated with topical tacrolimus; group II, antilymphocyte serum plus 21 days cyclosporine; and group III, antilymphocyte serum plus 21 days of cyclosporine plus topical tacrolimus. In group IV, tacrolimus levels in blood, skin, and muscle were measured in an autograft control group.

RESULTS

All animals in group I (n = 8) developed grade III clinical rejection by postoperative day 9. In group II (n = 9), the median onset of grade III rejection was postoperative day 40 (range, postoperative days 34 to 44). In group III (n = 6), two animals developed focal grade III rejection on postoperative days 35 and 56. The remaining four animals reached the 100-day endpoint without grade III rejection. In group IV, tacrolimus levels were low or undetectable in blood, whereas skin levels were 100-fold higher than underlying muscle.

CONCLUSIONS

Topical tacrolimus therapy has the potential to prevent skin rejection in a composite tissue allograft. Preoperative depletion of T cells with antilymphocyte serum, along with a short course of systemic immunosuppression, prevents acute rejection, whereas topical tacrolimus inhibits immune cell function in the skin. Concentrations of tacrolimus are substantially higher in skin compared with underlying muscle and peripheral blood. Topical immunotherapy could reduce the morbidity associated with systemic immunosuppression in clinical composite tissue allografts.

Recipient damage after musculocutaneous transplant rejection

BACKGROUND

In the event of a composite allograft failure, damage to the recipient tissues may make retransplantation impossible. This study aimed to quantify the damage after composite allograft failure to assess whether retransplantation is feasible.

METHODS

Rats (n=6) in the group I received composite musculocutaneous flap allotransplants (WF-->Lew) with immunosuppression allowing healing-in of the allograft before being tapered allowing rejection. At full rejection, the recipient vascular pedicle was examined, biopsies of the recipient tissue bed were taken and graded for damage, and in vitro assays were performed. Groups II (allograft without immunosuppression, n=7), III (isograft with immunosuppression, n=5), and IV (isograft without immunosuppression, n=6) were included to attempt to identify the contributions of the rejection process, immunosuppression, and healing to recipient tissue damage.

RESULTS

The vascular pedicle was patent to within 1 mm of the anastomosis in all rejected allografts. Furthermore, it was possible to retransplant after full rejection. There was minimal damage to the recipient tissues at the time of full rejection in group I. In contrast, group II had significantly more damage (P<0.05) to recipient muscle and skin. This correlated with more severe immune reaction with more than 100 times antibody production in group II compared with group I. Groups III and IV had little recipient tissue damage.

CONCLUSIONS

These results suggest that it is possible to retransplant after rejection of a musculocutaneous transplant while on immunosuppression. Furthermore, the second transplant will not be limited in form or function by recipient tissue bed damage.

Alloantigen-pulsed host dendritic cells induce T-cell regulation and prolong allograft survival in a rat model of hindlimb allotransplantation

BACKGROUND

Composite tissue allotransplantation is restricted due to the risks presented by long-term therapeutic immunosuppression. This study is conducted to investigate whether treatment with recipient immature dendritic cells (DCs) pulsed with donor alloantigens can prolong allograft survival and induce T-cell regulation in a rodent model.

MATERIALS AND METHODS

Orthotopic hindlimb transplants from Brown-Norway (RT1(n)) to Lewis (RT1(1)) rats were performed (day 0). DCs were propagated from the recipient bone marrow and pulsed with the donor alloantigen lysate. Group 1 (control group) did not receive any treatment. Groups 2 and 3 received cyclosporine A (CsA) at a concentration of 10 and 16 mg.kg(-1).day(-1), respectively, on days 0-20 following composite tissue allotransplantation. Group 4 received antilymphocyte serum (i.p. administered 4 d before and 1 d after transplantation) therapy. Group 5 received combined treatment with CsA (10 mg.kg(-1).day(-1), days 0-20) and donor alloantigen-pulsed recipient DCs (i.v. administered on days 7, 14, and 21). Group 6 received combined treatment with CsA (10 mg.kg(-1).day(-1) on days 0-20), antilymphocyte serum (administered i.p. 4 d before and 1 d after transplantation), and DCs (administered i.v. on days 7, 14, and 21). Graft rejection was defined as epidermolysis/desquamation of the donor skin. The mixed lymphocyte reaction was performed to determine the donor T-cell reactivity. Tissue samples were biopsied to analyze the histological changes, and flow cytometry was performed to quantify the donor T-cells.

RESULTS

Allograft survival was significantly prolonged (>200 d) in Group 6 when compared with the other groups (P < 0.001). The mixed lymphocyte reaction performed for Group 6 revealed hyporesponsiveness of the T-cells to donor alloantigens. Flow cytometric analysis in Group 6 revealed a significant increase in the percentage of CD4(+)/CD25(+) and CD4(+)/foxP3(+) T-cells expression, and significant increase in the percentage of donor cells (RT1(n)) in the recipient peripheral blood. Immunohistochemical staining of allo-skin revealed a significant increase in the proportion of CD25(+) cells in the subcutaneous and dermis layers in Group 6, as compared to other groups.

CONCLUSION

Treatment with donor alloantigen-pulsed recipient immature DCs in combination with transient immunosuppression prolongs allograft survival and induced tolerance by inducing T-cell hyporesponsiveness to donor alloantigens and increasing the CD4(+)/CD25(+) T-cell population.

Long-term survival of limb allografts induced by pharmacologically conditioned, donor alloantigen-pulsed dendritic cells without maintenance immunosuppression

BACKGROUND

We showed recently that limb allograft survival could be enhanced by administration of alloantigen (Ag)-pulsed immature dendritic cells (DC) after transplantation. Since indefinite graft survival was not achieved, we have further modified the DC by pharmacologic (rapamycin; Rapa) conditioning and ascertained their influence on graft survival, without continued immunosuppressive therapy.

METHODS

We compared the ability of donor Ag-pulsed, Rapa-conditioned rat myeloid DC (Rapa DC) and control DC (CTR DC) to inhibit alloreactive T-cell responses after limb transplantation in antilymphocyte serum (ALS)-treated recipients given a short postoperative course of cyclosporine (CsA).

RESULTS

Both DC populations expressed similar levels of major histocompatibility complex (MHC) II, CD40 and CD54, but Rapa DC expressed lower CD86. After toll-like receptor activation, both populations produced minimal interleukin (IL)-12p70, but Rapa DC secreted lower levels of IL-6 and IL-10. The capacity of DCs to stimulate T-cell proliferation in mixed leukocyte reactions was very low. Pulsing of the DC with donor Ag did not alter their phenotype or function. Interestingly, posttransplant administration of donor Ag-pulsed Rapa DC to rats given perioperative ALS and 21 days CsA significantly delayed graft rejection and promoted long-term (>125 days) graft survival. AlloAg-pulsed Rapa DC induced T-cell hyporesponsiveness and promoted the generation of IL-10-secreting CD4 T cells upon ex vivo challenge.

CONCLUSIONS

Infusion of donor Ag-pulsed, Rapa-conditioned DC after composite tissue transplantation can prevent rejection of the grafts, including skin, across a full MHC mismatch and in the absence of continued immunosuppressive therapy.