Prolonged survival of rat leg allografts due to immunological enhancement

A systematic review of immunomodulatory strategies used in skin-containing preclinical vascularized composite allotransplant models

BACKGROUND

Acute rejection remains a vexing problem in vascularized composite allotransplantation (VCA). Available immunosuppressive regimens are successful at minimizing alloimmune response and allowing VCA in humans. However, repeated rejection episodes are common, and systemic side effects of the current standard regimen (Tacrolimus, MMF, Prednisone) are dose limiting. Novel immunomodulatory approaches to improve allograft acceptance and minimize systemic toxicity are continuously explored in preclinical models. We aimed to systematically summarize past and current approaches to help guide future research in this complex field.

METHODS

We conducted a systematic review of manuscripts listed in the MEDLINE and PubMed databases. For inclusion, articles had to primarily investigate the effect of a therapeutic approach on prolonging the survival of a skin-containing preclinical VCA model. Non-VCA studies, human trials, anatomical and feasibility studies, and articles written in a language other than English were excluded. We followed the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines.

RESULTS

The search retrieved 980 articles of which 112 articles were ultimately included. The majority of investigations used a rat model. An orthotopic hind limb VCA model was used in 53% of the studies. Cell and drug-based approaches were investigated 58 and 52 times, respectively. We provide a comprehensive review of immunomodulatory strategies used in VCA preclinical research over a timeframe of 44 years.

CONCLUSION

We identify a transition from anatomically non-specific to anatomical models mimicking clinical needs. As limb transplants have been most frequently performed, preclinical research focused on using the hind limb model. We also identify a transition from drug-based suppression therapies to cell-based immunomodulation strategies.

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.

Hand transplantation: the state-of-the-art

The feasibility of hand transplantation has been demonstrated, both surgically and immunologically. Levels of immunosuppression comparable to regimens used in solid organ transplantation are proving sufficient to prevent graft loss. Many patients have achieved discriminative sensibility and recovery of intrinsic muscle function. In addition to restoration of function, hand transplantation offers considerable psychological benefits. The recipient's pre-operative psychological status, his motivation and his compliance with the intense rehabilitation programme are key issues. While the induction of graft specific tolerance represents a hope for the future, immunosuppression currently remains necessary and carries significant risks. Hand transplantation should, therefore, only be considered a therapeutic option for a carefully selected group of patients.

Mixed allogeneic chimerism as a reliable model for composite tissue allograft tolerance induction across major and minor histocompatibility barriers

BACKGROUND

Although prolonged composite tissue allograft (CTA) survival is achievable in animals using immunosuppressive drugs, long-term immunosuppression of CTAs in the clinical setting may be unacceptable for most patients. The purpose of this study was to develop a model for reliable CTA tolerance induction in the adult rat across a major MHC mismatch without the need for long-term immunosuppression.

METHODS

Mixed allogeneic chimeras were prepared by using rat strains with strong MHC incompatibility [WF (RT1Au), ACI (RT1Aa)] WF + ACI-->WF, n=23. The bone marrow (BM) of recipient animals was pretreated with low-dose irradiation (500-700 cGy), followed by reconstitution with a mixture of T cell-depleted syngeneic (WF) and allogeneic (ACI) cells. Additionally, the recipient animals received a single dose of anti-lymphocyte serum (10 mg) 5 days before bone marrow transplantation (BMT) and tacrolimus (1 mg/kg/day) from the day before BMT to 10 days post-BMT. Hindlimb transplants were performed 12 months after BMT. Five animals received a limb allograft irradiated (1000 cGy) just before transplantation. Rat chimeras were characterized (percentage of donor cells present within the bloodstream) by flow cytometry at 3 and 12 months after BM reconstitution and after hindlimb transplantation.

RESULTS

Peripheral blood lymphocyte chimerism (WF/ACI) remained stable >12 months after BM reconstitution in 18/23 animals. Multi-lineage chimerism of both lymphoid and myeloid lineages was present, suggesting that engraftment of the pluripotent rat stem cell had occurred. In animals with donor chimerism >60% (n=18) no sign of limb rejection was present for the duration of the study. All animals with chimerism <20% (n=5) developed moderate signs of rejection clinically and histologically. Gross motor and sensory reinnervation (weight bearing, toe spread) developed at >60 days in 14/21 rats. Postoperative flow cytometry studies demonstrated stable chimerism in all animals studied (n=10). Five out of five animals with irradiated limb transplants showed no sign of GVHD at >100 days.

CONCLUSIONS

Stable mixed allogeneic chimerism can be achieved in a rat hindlimb model of composite tissue allotransplantation. Hindlimb allografts to mixed allogeneic chimeras exhibit prolonged, rejection-free survival. Partial functional return should be expected. The BM transplanted as part of the hindlimb allograft plays a role in the etiology of GVHD. Manipulating that BM before transplantation may influence the incidence of GVHD. This represents the first reliable rat hindlimb model demonstrating rejection-free CTA survival in an adult animal across a major MHC mismatch without the long-term need for immunosuppressive agents.

Past, present, and future research in the field of composite tissue allotransplantation

In September 1998, a surgical team in Lyon, France, performed the first successful hand transplant. After this historic event, in January 1999, the University of Louisville performed the first hand transplant in the United States. These events sparked interest and debate concerning the justification of performing limb allotransplantation. The field of composite tissue allotransplantation (CTA) has made significant advances in the past two decades, yet advancement of the applications of CTA into the clinical arena had been fairly limited to this point. The most inherent controversy in CTA involves the fact that the clinical applications for the most part involve restoration of function and/or structural integrity. These procedures are done essentially for quality-of-life concerns, not life-saving issues. Present concern involves subjecting CTA recipients to a lifetime of postoperative immunosuppressive therapy. We cannot fully understand where we stand at present and in what future directions the field is heading unless we have an understanding of where we have been in composite tissue transplantation. This article reviews the historical aspects of CTA, discusses the present state of CTA, and speculates on potential future applications of CTA.

Comparison of long-term immunosuppression for limb transplantation using cyclosporine, tacrolimus, and mycophenolate mofetil: implications for clinical composite tissue transplantation

This study compared the efficacy of long-term intermittent immunosuppression in preventing the rejection of a limb transplant across the strongest histocompatibility barrier in ACI --> Lewis rats using the conventional immunosuppressive agent cyclosporine-A and the newer immunosuppressive agents FK-506 (tacrolimus) and RS-61443 (mycophenolate mofetil). The recipient animals were immunosuppressed daily for 14 days postoperatively, followed by long-term intermittent, twice-weekly immunosuppression using cyclosporine 25 mg/kg, RS-61443 30 mg/kg, or FK-506 2 mg/kg. All three immunosuppressive agents were able to prolong the rejection of the skin component of a limb transplant compared with nonimmunosuppressed controls. Eight of nine animals receiving cyclosporine immunosuppression showed signs of rejection of the skin component of the limb transplant while continuing to receive long-term immunosuppression and had a mean rejection time of 61.6 days. Seven of 10 animals immunosuppressed with RS-61443 also showed signs of rejection while still receiving immunosuppression, with a mean rejection time of 43.6 days. Nine of 10 animals receiving FK-506 immunosuppression showed no signs of skin rejection, but died of bacterial pneumonia between 273 and 334 days after transplantation, with a mean rejection time of 296.1 days. There was no statistically significant difference between intermittent immunosuppression with cyclosporine and RS-61443, but FK-506 was significantly superior to both cyclosporine and RS-61443. The implication of this study is that FK-506, but not cyclosporine or RS-61443, is probably the only single immunosuppressive agent capable of preventing rejection of the skin component of a composite tissue transplant. Combination immunosuppression with FK-506 and RS-61443, therefore, may be required to allow composite tissue transplantation to become a predictable clinical reality in the future.

Limb transplantation in rats: immunosuppression with FK-506

The efficacy of a new immunosuppressive agent, FK-506, to prevent the rejection of limb transplants was investigated across a major histocompatibility barrier in rats and compared with Cyclosporine. We performed 129 limb transplants between donor ACI rats and recipient Lewis rats. The mean rejection time of the skin component of limb allografts without immunosuppression was 7 days. Animals receiving a 14-day course of Cyclosporine 25 mg/kg had a mean rejection time of 30 days. In contrast, animals receiving a 14-day course of FK506 1 mg/kg had a mean rejection time of 54 days and those receiving a 14-day course of FK-506 2 mg/kg had a mean rejection time of 122 days. Intermittent administration of FK-506 twice weekly further prolonged the mean rejection times to 149 days in animals receiving 1 mg/kg and to 296 days in animals receiving 2 mg/kg.

Vascularized bone allografts: review of current concepts

Advances in the field of microsurgery over the past two decades, together with the discovery of a powerful immunosuppressive, cyclosporin A, have brought vascularized osteochondral allografts closer to reality. In this review, the authors describe the history of laboratory research on vascularized osteochondral allografts using immunosuppression, as well as their experimental and preliminary clinical experiences.

Extremity transplantation: a review of its current state of development

The discovery of a powerful immunosuppressive, cyclosporin A, together with the development of a microsurgical free tissue transfer technique, has brought extremity transplantation closer to being a realistic possibility. In this review the authors describe the history of laboratory research into extremity transplantation before immunosuppression, with early immunosuppressive agents, with cyclosporin A, and with FK-506, one of a new generation of immunosuppressives.

Tissue transplants in primates for upper extremity reconstruction: a preliminary report

Recent advances in clinical transplantation surgery suggest that hand transplantation is no longer an unrealistic expectation. However, two questions must be answered. Can composite tissue transplants survive in a primate species? Does the required neural reinnervation occur under immunosuppression? Four hand transplants and seven neurovascular free flap transplants were done in baboons immunosuppressed with Cyclosporin A and steroids (methylprednisolone). Long-term survival occurred in nine. Electrophysiologic tests of sensory axons revealed reinnervation of transplanted skin as evidenced by well-defined, low threshold receptive fields in the donor tissue. Reinnervation of donor muscle was demonstrated by motor unit recruitment in stepwise fashion after electrical stimulation of the recipient's median and ulnar nerves. Afferent fibers serving the donor's joints and muscle spindles were also observed.

Efforts to enhance survival of limb allografts by prior administration of whole blood in rats using a new survival end-point

Injecting whole blood into the recipient before surgery can significantly prolong renal transplant survival in rats. Therefore, experiments were performed in rats to study the effects of prior administration of whole blood on the survival of limb allografts. Tests to quantitate survival of the allografts included monitoring the internal temperature of the leg, assaying serum creatine kinase levels, and testing for alloantibodies. Lewis recipients of (BN x LEW)F1 limb transplants that received 1 ml of BN or (BN x LEW)F1 whole blood before surgery had mean survival times that were longer compared with controls as measured by a 10 F change in temperature. In a test-retest experiment, decline of temperature proved to be a reliable quantitative determination of limb allograft survival since a difference of only 5.6% was observed in the mean number of days of graft survival between two separate groups of control Lewis recipients. Moreover, combined data demonstrated that control Lewis recipients of (BN x LEW)F1 limb allografts averaged 24.0 days of graft survival based on a 10 F decline in temperature with a 95% confidence interval of +/- 6.3 days. It is concluded that prior administration of whole blood can produce significant prolongation of survival in organ transplantation, but it is not as effective in enhancing survival of limb allografts. It is also concluded that internal temperature measurement of limb allografts is an easy, effective, and quantitative method of monitoring rejection.