Tolerance: is it achievable in pediatric solid organ transplantation?

Cell therapeutic approaches to immunosuppression after clinical kidney transplantation

Refinement of immunosuppressive strategies has led to further improvement of kidney graft survival in recent years. Currently, the main limitations to long-term graft survival are life-threatening side effects of immunosuppression and chronic allograft injury, emphasizing the need for innovative immunosuppressive regimens that resolve this therapeutic dilemma. Several cell therapeutic approaches to immunosuppression and donor-specific unresponsiveness have been tested in early phase I and phase II clinical trials in kidney transplantation. The aim of this overview is to summarize current cell therapeutic approaches to immunosuppression in clinical kidney transplantation with a focus on myeloid suppressor cell therapy by mitomycin C-induced cells (MICs). MICs show great promise as a therapeutic agent to achieve the rapid and durable establishment of donor-unresponsiveness in living-donor kidney transplantation. Cell-based therapeutic approaches may eventually revolutionize immunosuppression in kidney transplantation in the near future.

Pediatric transplantation and tolerance: past, present, and future

Solid organ transplantation is the treatment of choice in children with end-stage organ failure. With improving methods of transplant surgery and post-transplant care, transplantation is more frequently performed worldwide. However, lifelong and non-specific suppression of the recipient's immune system is a cause of significant morbidity in children, including infection, diabetes, and cancer. There is a great need to develop IS minimization/withdrawal and tolerance induction approaches.

Immune profile of pediatric renal transplant recipients following alemtuzumab induction

The incidence of developing circulating anti-human leukocyte antigen antibodies and the kinetics of T cell depletion and recovery among pediatric renal transplant recipients who receive alemtuzumab induction therapy are unknown. In a collaborative endeavor to minimize maintenance immunosuppression in pediatric renal transplant recipients, we enrolled 35 participants from four centers and treated them with alemtuzumab induction therapy and a steroid-free, calcineurin-inhibitor-withdrawal maintenance regimen. At 3 months after transplant, there was greater depletion of CD4(+) than CD8(+) T cells within the total, naive, memory, and effector memory subsets, although depletion of the central memory subset was similar for CD4(+) and CD8(+) cells. Although CD8(+) T cells recovered faster than CD4(+) subsets overall, they failed to return to pretransplant levels by 24 months after transplant. There was no evidence for greater recovery of either CD4(+) or CD8(+) memory cells than naïve cells. Alemtuzumab relatively spared CD4(+)CD25(+)FoxP3(+) regulatory T cells, resulting in a rise in their numbers relative to total CD4(+) cells and a ratio that remained at least at pretransplant levels throughout the study period. Seven participants (20%) developed anti-human leukocyte antigen antibodies without adversely affecting allograft function or histology on 2-year biopsies. Long-term follow-up is underway to assess the potential benefits of this regimen in children.

Analysis of clinical variables associated with tolerance in pediatric liver transplant recipients

Tolerance has been defined as stable graft function off IMS. We reviewed the data of 369 pediatric liver transplant patients to examine demographic differences that may have a PV of pediatric LT tolerance. Of the 369 patients, 280 patients were stable with detectable blood levels of IMS agents and with good graft function without biopsy proven REJ > 1 yr posttransplantation, 18 patients were noted to be TOL off IMS, 27 patients were taking MIS with drug levels below detectable range by standard laboratory parameters, and 44 patients developed one or more episodes of biopsy proven acute or chronic REJ > 1 yr post-transplantation. Variables, including percentage of biliary atresia, type of transplanted organ, history of EBV infection, patient and donor gender, and ABO blood type mismatch between recipient and donor did not have PV of tolerance. Average age in years was 1.37 ± 1.53 (0.3-4.9) for TOL, 1.14 ± 0.89 (0.4-3.1) for MIS and 3.35 ± 4.45 (0.3-16) for REJ. Age difference of TOL/MIS vs. REJ was significant (p =0.002) and TOL vs. REJ was significant (0.01). Age at the time of transplantation is an important predictor in the development of pediatric LT tolerance.

Heart and lung transplantation in children

During the past two decades, several advances have resulted in marked improvement in medium-term survival for infants and children undergoing heart transplantation. Unfortunately, progress has been less dramatic in the field of lung and heart-lung transplantation, where there is little evidence of improved outcomes. The procedures remain palliative and all transplant recipients are at risk for the adverse effects of non-specific immunosuppression, including infections, lymphoproliferative disorders, and non-lymphoid malignancies. In addition, current immunosuppressive agents have narrow therapeutic windows and exhibit a wide array of organ toxicities, posing special challenges for the young patient who must endure life-long immunosuppression. New immunosuppressive regimens have lowered the rates of acute rejection but appear to have had relatively little impact on the incidence of chronic rejection, the principal cause of late graft loss. The ultimate goal is to induce a state of donor-specific tolerance, wherein the recipient will accept the allograft indefinitely without the need for long-term immunosuppression. This quest is currently being realised in animal models of solid organ transplantation, and offers great hope for children undergoing heart and lung transplantation in the future.

Cyclosporine A withdrawal during follow-up after pediatric liver transplantation

It is unclear whether cyclosporine A (CsA) can be withdrawn safely during follow-up after pediatric liver transplantation. In our transplant program we have been using a strict protocol to withdraw CsA. The aim of this study was to retrospectively assess the effects of CsA withdrawal after pediatric liver transplantation on the incidence of rejection and renal function. Between 1986 and 2001, 91 children received CsA for at least 2 yr after liver transplantation. Specific criteria for eligibility to withdraw CsA were set. In 53 of the 91 children CsA was withdrawn. In 35 patients (66%) withdrawal of CsA did not cause rejection. In these patients the renal function improved compared with baseline values (glomerular filtration rate (GFR) at 1 yr, +16 mL/minute/1.73 m3, P < 0.001; at 2 yr, +10 mL/minute/1.73 m3, P < 0.05). After CsA withdrawal, 18 patients developed rejection (34%), which could be effectively treated by methylprednisolone and restarting CsA. Failure to withdraw CsA was not associated with increased incidence of graft loss. A body weight below 10 kg at the time of transplantation correlated significantly with successful withdrawal of CsA (<10 kg, 85% vs. > 10 kg, 60%; P < 0.05). In conclusion CsA can successfully be withdrawn in a major proportion of selected pediatric liver transplantation patients during follow-up. The success rate is the highest in children with a body weight below 10 kg at the time of transplantation. Successful withdrawal improves renal function, whereas failure to withdraw is not associated with graft loss or persisting morbidity.

Bone marrow-induced tolerance in the era of pancreas and islets transplantation

Enormous progress has been made in the field of solid organ adaptation recently because of the improvement in immunosuppression. Although powerful immunosuppressive drugs decrease the rate of acute rejection significantly, the long-term functional graft survival and tolerance induction remains poor. Chronic rejection is the main cause of graft failure. An electronic search was performed for articles on chimerism, tolerance, and immunologic perspectives of islet and pancreas transplantation along with referrals to our experience. Infusion of donor bone marrow-derived cells to create a chimeric state continue to be tested in clinical protocols intended to induce specific immunologic tolerance. The proposed mechanisms of immunologic engagement and the emergence of a tolerant state through mixed chimerism include central depletion of alloreactive cells, induction of T-cell anergy, and generation of suppressor cells by interactions between donor and host cells. In this setting, depletion of recipient T cells by different strategies and subsequent repopulation by donor hematopoietic cells after donor bone marrow infusion are prerequisites for tolerance induction. Many efforts have aimed to establish mixed chimerism along with tolerance in solid organ transplantation including pancreas and islets to facilitate engraftment. A review of the more important advances in the field and the future prospects combined with our experience to induce tolerance in the clinic and the laboratory is presented in this article.

A technique of bone marrow collection from vertebral bodies of cynomolgus macaques for transplant studies

BACKGROUND

Strategies to induce donor-specific allograft tolerance are best tested in preclinical models developed in nonhuman primates (NHPs). Most protocols prepare the recipient by infusing hematopoietic cells from the donor. We report here a procedure to isolate and characterize large numbers of bone marrow cells (BMCs) from cynomolgus monkeys (cynos) that can then successfully be transplanted into conditioned recipients.

MATERIALS AND METHODS

Vertebral columns of five cynos were excised en bloc and separated into individual vertebrae. The cancelous bone was extracted with a core puncher, fractionated, filtered, centrifuged, and resuspended in transplantation media before being analyzed by flow cytometry. In two instances, the collected BMCs were reinfused into allogeneic recipients preconditioned with a nonmyeloablative regimen. Chimerism was monitored using short-tandem repeat analysis.

RESULTS

The mean total BMCs yield was 25.5 x 10(9) (range of 4.00 x 10(9) to 59 x 10(9)) with mean cell viability of 93.4% (range: 90-96%). CD34+ cells and CD3+ cells averaged 0.34 and 3.91% of total BMCs, respectively. This resulted in absolute cell number yields of 1.02 x 10(8) and 1.15 x 10(9) for CD34+ and CD3+ cells, respectively. Graft-versus-host disease was absent in both bone marrow infused animals, and a maximum level of chimerism of 18% was detected at 3 weeks after BMCs infusion.

CONCLUSION

We present here the first detailed report of a procedure to retrieve and characterize large numbers of BMCs from vertebral bodies of cynos and demonstrate that cells collected with this technique have the capability of engrafting in allogenic recipients.

Donor bone marrow transplantation: chimerism and tolerance

Infusion of donor bone marrow (DBM)-derived cells continue to be tested in clinical protocols intended to induce specific immunologic tolerance. Central clonal deletion of donor-specific alloreactive cells associated with mixed chimerism reliably produced long-term graft tolerance. In this setting, depletion of recipient T cells by antilymphocyte antibodies and subsequent repopulation by donor hematopoietic cells after donor bone marrow infusion (DBMI) are prerequisites for tolerance induction. Major advances have been made in animal models and in pilot clinical trials and the key questions with the future perspectives are presented in this article.