Immunosuppression and immunomodulation

Jacques Pirenne
MD PhD
Professor
Abdominal Transplant Surgery
University Hospitals Gasthuisberg
Catholic University Leuven, Belgium

The intestine remains the most challenging abdominal organ to transplant. This is because of the severe immune response, mostly rejection, that occurs, and therefore the need for profound immunosuppression with its attendant complications (sepsis, lymphoma and direct drug toxicity). Unlike other organs, graft loss due to acute rejection can occur late after transplantation (more than one year post-transplant). With the actual – generally tacrolimus- based – immunosuppressive regimens, results have improved, but considerable experience in patient management is required to optimise outcome of those complex transplants that are permanently at risk of rejection and infection.

Intestinal transplantation (ITx) is a procedure that needs to be refined, and its application to patients doing well on total parenteral nutrition (TPN) warrants further research into understanding the rejection process, ­together with development of less toxic and more efficient immunosuppressive protocols, and development of immuno­modulatory strategies to better control rejection and thereby reduce the need for nonspecific immunosuppression.

International Registry on ITx
David Grant from Toronto, Canada, recently reported data from the “International Registry on Intestinal Transplantation” (www.lhsc.on.ca/itr).(1–3) Since 1985, 474 ITx have been performed in 446 patients. At the present time, 46 centres worldwide are involved in ITx and have performed an average of two ITx each. The sex ratio is 55% male to 45% female. Two-thirds of transplants were performed in children, although recently the number of adults referred for ITx has increased. At the time of the transplant 50% of the patients were at home and 50% were hospitalised. Isolated ITx was performed in 45%, combined liver and ITx in 40%, and multivisceral Tx in 15%. The high incidence of combined liver and ITx reflects, in part, the late referral of patients in whom TPN has already caused liver failure.

The majority of patients have received tacrolimus- and steroid-based immunosuppression plus induction therapy (OKT(3), ATG [antithymocyte globulin], ALG [antilymphocyte globulin], anti-interleukin [IL]-2). However, the longest survival after ITx is a two-year-old girl who received an isolated ITx in 1985 with cyclosporin A, and who remains well and TPN-free more than 15 years later.(4,5,6) Many programmes now use new immunosuppressants, such as anti-IL-2 receptor antibodies, rapamycin, mycophenolate mofetil (MMF) and topical steroids. Currently the registry does not show any effect of those new immunosuppressants, and the numbers are too small to draw definitive conclusions. In individual centres, particularly in Omaha and Miami, the use of anti-IL-2 receptor antibodies and/or rapamycin has been associated with a reduction in the incidence of rejection.(2) There is concern with the use of MMF in ITx due to its gastrointestinal toxicity. Rapamycin may be a promising agent to reduce chronic rejection and as a tacrolimus-sparing agent. Impaired wound healing, however, has been a concern with this agent (personal observation and comment from AN Langnas at the Rome International Tx meeting).(2)

Among the parameters that can predict outcome, only the era and the number of transplants performed per centre are significant risk factors. This clearly reflects that experience in the management of complex patients is crucial in optimising outcome. Overall, one-year patient survival rates are around 70%, but results during the last two years have improved and are approaching 80%, which reflects the existence of a learning curve in the management of these complex transplants. Donor pretreatment (with lymphocyte-depleting agents) and recipient age did not influence the results. The type of organs transplanted (liver–intestine, intestine alone, multivisceral graft) also did not influence graft outcome. Survival of patients receiving multivisceral grafts, however, was reduced compared with those receiving liver–intestine or intestine alone.

The major cause of death reported in the registry has been sepsis (55%). Post-transplant lymphoproliferative disease (PTLD) is a complication occurring in 8%, 14% and 15% of isolated ITx, combined liver–intestine Tx and multivisceral Tx respectively. This reflects, in part, the profound nonspecific immunosuppression that is required in these patients.

An important outcome from the registry is that 80% of long-term recipients (surviving more than six months) are nutritionally completely independent and off TPN. The mean time to TPN independence is usually longer after liver-ITx (about 6–7 weeks) than after isolated ITx (3–4 weeks). Feeding should be started early, but progressively, using first an elemental diet followed by a mixed elemental/fat diet and eventually the cautious introduction of a mixed protein/fat diet. Diarrhoea remains a problem. Loperamide derivatives can be used, but the best way to manage diarrhoea is to close the intestinal stoma after some period of graft stability, usually within the first year post-transplant.

Single-centre trials
The management of ITx patients is extraordinarily complex and time-consuming. Patients are highly susceptible to both rejection and infection and one must be permanently alert to complications. In addition, rejection is not easy to diagnose, since no simple biochemical marker is available and clinical signs of rejection are nonspecific. Pathological criteria include apoptosis of epithelial cells in the crypts, cryptitis, villous blunting, and exfoliation at a later stage.(1,2,7) The difficulty, however, is that these findings can sometimes be present with infectious enteritis. Adenovirus, rotavirus, Clostridium difficile, herpes, cytomegalovirus (CMV) and Epstein–Barr virus (EBV) must be excluded. Eventually the diagnosis of rejection and the decision to treat rejection should be based upon a combination of clinical, endoscopic and pathological data.(1)

Pittsburgh(1,2,8–14)
The largest single centre experience is from Pittsburgh, USA. During the period 1990–2000, 151 ITx were performed in 143 patients, 55% of who were paediatric patients. A total of 48% were combined liver and ITx, 38% were isolated ITx, and 13% were multivisceral Tx. The one-year graft survival rate was approximately 70%, and the five-year graft survival rate was over 50%. Results have improved recently for multifactorial reasons, including accumulated centre experience, pre-emptive treatment and better control of post-transplantation lymphoproliferative disease (PTLD),(15) improved control of CMV, absence of the entire colon in the graft, and recipient selection.

Omaha(1,2,16–18)
Langnas et al. recently pointed out the potential benefit of new immunosuppressants, anti-IL 2 antibodies and rapamycin, in reducing the rejection rate, to the importance of aggressive CMV prophylaxis which may allow the more liberal acceptance of CMV-positive donors. The monitoring of EBV polymerase chain reaction and the introduction of pre-emptive treatment has reduced the likelihood of PTLD. However, unlike liver transplant patients, in whom immunosuppression can be safely reduced, and even stopped in many cases, such a strategy can lead to rejection after ITx. Recently, treatment with anti-CD-20 antibody has been introduced. Chronic rejection remains a problem and needs to be recognised before catastrophic clinical consequences develop. Transplantectomy may be needed. Whether retransplantation should be attempted is not known.

Miami(1,2,19–27)
In Miami 88 ITx have been performed in 80 patients. The majority were paediatric patients (56%). Twenty-five received an isolated ITx, 26 a combined liver­–ITx and 37 a multivisceral graft. The results were significantly improved in the second phase of their experience, and this was attributed to the following factors:

• More intense search for rejection through frequent endoscopies of the graft and even the use of a high-power zoom video endoscope.(19)
• Use of induction treatment with anti-IL-2 receptor antibodies (particularly effective in reducing ­rejection in isolated ITx).(23)
• Profound anti-CMV prophylaxis not only with ganciclovir, but also with frequent administration of anti-CMV immunoglobulin.

The latter strategy could allow more liberal use of CMV- positive donors, a practice previously not recommended in ITx.

ITx using living donors
Gruessner et al. from Minneapolis described a technique to safely remove the part of the ileum just distal to the takeoff of the right colic artery in a living donor.(26,28) Benedetti from the University of Illinois, Chicago, has recently reported three cases of living-related ITx.(1,2) No donor morbidity and mortality were observed. These three grafts were technically successful and allowed nutritional autonomy of their recipients (patients off TPN).(29) Interestingly, no rejection has been observed in these three grafts. Ischaemia reperfusion injury can probably increase the immunogenicity of the intestinal graft and cause endotoxin translocation with its attendant immune and physiological negative effects.(30) Thus, one can speculate that the short ischaemia time in living donation may account for a reduced rejection rate. In addition, animal experiments have indicated that the major histocompatibility complex is critical to graft rejection in ITx.(30) Favourable HLA (human leukocyte antigen) matching in living-related ITx could also contribute to rejection reduction. However, data from the International Registry so far do not indicate a statistically significant difference in graft outcome according to the donor source, living-related versus cadaveric donor, although only 16 living-related grafts have been reported so far, and some were done in the early phase of ITx when the results were still poor.

Interestingly, ITx has been successfully performed between identical twins.(31,32) In those cases there was no need for immunosuppression. Both recipients have done well, confirming that segments of ileum can provide nutritional autonomy to the recipient. Application of this technique is not necessarily limited to adults and older children. At least two living-related ITx have been performed by the Kyoto group in small-sized children.(33) Further experience is required to better delineate the role and the results of living donation in ITx in both adults and children.(26,34)

Shortage of grafts
In contrast to the adult population, where there is no shortage of isolated intestinal grafts, the paediatric population is confronted with a dramatic shortage of size-matched donors. This has resulted in an exceedingly high mortality rate on the waiting list, particularly in children awaiting liver and ITx. Recently, de Ville de Goyet from Birmingham and Xenos from Miami described techniques to split or reduce liver­–bowel graft from larger donors, and to effectively use them in smaller recipients.(35,36) Reduction techniques have also been described for isolated ITx.(37) Preferential allocation of paediatric liver–bowel donors to paediatric ITx recipients, and possibly international exchange of those scarce grafts, should be encouraged.

Immunomodulatory strategies to ease intestinal engraftment
Although the results of ITx have improved, the procedure remains challenging due to biological obstacles, mainly the susceptibility of those patients to rejection, the need for profound immunosuppression, and infection. Better understanding of the rejection process and development of immunomodulatory strategies to better control rejection are needed.

Combined liver transplantation
The liver is an immunoprivileged organ that protects other simultaneously transplanted organs from rejection. Interestingly, one of the first successful ITx was a case of combined liver and ITx.(38) It has been proposed to perform liver plus ITx even in candidates for isolated ITx.(25) The normal liver of the first recipient could be used in a second recipient so that no organ is wasted (domino transplantation).(25) However, there is some controversy as to whether this protective effect of the liver also applies to the intestine. The team from Paris revisited their experience with rejection in liver–bowel versus bowel Tx alone. (1,2,4–6,39) Similar to the group from Pittsburgh, they found a protective effect of the liver on the intestine. In Paris, results of liver–bowel Tx are superior to those of isolated ITx. This is in contrast, however, with the results of the Omaha group, who got better results with isolated ITx. (1,2,16)

It may be that reduced rejection in the combined liver ITx group reflects the fact that those patients are usually “sicker” pretransplant and therefore less likely to reject, or they may even succumb to the procedure before they reject. Indeed, candidates for liver and ITx suffer from liver failure, a condition which by itself causes immunosuppression. In addition, there is a higher rate risk of mortality early post-transplant after liver plus ITx versus ITx alone, and this may have introduced a bias comparing the incidence of rejection in both groups. In any case, this protective effect of the liver seems to be insufficient to justify application of liver plus ITx in candidates with normal liver function.(40)

Intraportal donor-specific blood transfusion (DSBT)
This technique has been shown to reduce rejection in a clinically relevant large animal model of ITx under FK506 treatment.(41) Whether this strategy will be efficacious clinically is not known – it is currently under clinical investigation at our centre. One patient recently received a combined liver and small bowel graft and intraportal DSBT with donor blood at the time of transplantation. No overt rejection has been seen so far (one-year follow-up) in either the bowel or liver graft.(42)

Bone marrow augmentation
Experiments with small animals have shown that donor-specific bone marrow augmentation can induce tolerance. However, in large animal models and clinically, bone marrow augmentation has not substantially reduced rejection after ITx, and the issue as to whether chimerism plays a role in development and/or maintenance of tolerance continues to be debated.(43) A trial is now in progress in Pittsburgh testing the effect of bone marrow augmentation in association with irradiation of the intestinal graft. This is based on experimental work showing that bone marrow augmentation blocks rejection only when associated with intestinal graft irradiation.(2) Five patients have been enrolled in this trial, and none has needed treatment for rejection so far, which seems encouraging. Longer follow-up is required to determine the effectiveness of this procedure – there is potential concern regarding the graft enteritis that can be induced by irradiation and which could compromise graft function.

Conclusion
Among abdominal organ transplants, ITx has had a poor reputation. This is because the results of ITx have been inferior to those of liver, kidney and pancreas Tx (90% one-year graft survival rates can be achieved for these organs). However, the gap between ITx and other organ Tx is now reduced, and a 70–80% one-year patient/graft survival rate can now be reached in experienced centres. There is no doubt that the intestine – by virtue of its unique immunological, physiological and bacteriological nature – will remain the most difficult abdominal organ to transplant. The results of ITx must be compared with the natural prognosis of patients who are obligated to lifelong TPN. In those who develop life-threatening complications, such as liver failure, repeated sepsis and shortage of venous access, ITx represents a lifesaving option with a high rate of success (70–80%). But further progress in the control of rejection and in reducing the need for immunosuppression is required before ITx can be applied to patients free of TPN-related complications.

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Resource
European Society of Organ Transplantation
W:www.esot.org