BACKGROUND:CD154-specific antibodies have been shown to prevent acute rejection in many preclinical models including nonhuman primates (NHPs). However, they have been ineffective in pilot clinical trials, suggesting a need for more robust preclinical analysis. One factor affecting the disparate results may be related to the recipient's immune activation state. Specifically, adult humans have a high percentage of memory-phenotype T cells compared to young animals. Postdepletional homeostatic repopulation has been shown to enrich for memory-phenotype T cells and interfere with CD154-based therapies in rodents. METHODS:We developed a NHP model nonspecifically enriched for peripheral memory-phenotype T cells. Thymectomized cynomolgus macaques underwent depletion with polyclonal anti-thymocyte globulin followed by repopulation. Peripheral phenotype was serially determined using polychromatic flow cytometry. In vitro response to donor and environmental antigens was also confirmed before and after manipulation. We then tested a regimen previously successful in rhesus monkeys combining anti-CD154, sirolimus, and donor-specific blood transfusion (DST), in a second primate species with and without the provocation of increased peripheral homeostatic T-cell activation. RESULTS:Monkeys that were thymectomized (n=3) and depleted recovered via homeostatic repopulation with a repertoire enriched for cells with a memory surface phenotype compared to unmanipulated controls (n=3). Despite a repertoire markedly enriched for memory-phenotype cells, the regimen effectively prevented acute rejection for the duration of therapy. CONCLUSIONS:Cynomolgus monkeys can be rendered memory phenotype enriched using homeostatic repopulation. Despite a generally activated T-cell repertoire, anti-CD154, sirolimus, and DST effectively prevents rejection in cynomolgus monkeys.

CD154- specific antibody therapy prevents allograft rejection in many experimental transplant models. However, initial clinical transplant trials with anti-CD154 have been disappointing suggesting the need for as of yet undetermined adjuvant therapy. In rodents, donor antigen (e.g., a donor blood transfusion), or mTOR inhibition (e.g., sirolimus), enhances antiCD154 ' s efficacy. We performed renal transplants in major histocompatibility complex-(MHC) mismatched rhesus monkeys and treated recipients with combinations of the CD154- specific antibody IDEC- 131, and/ or sirolimus, and/ or a pre- transplant donor- specific transfusion (DST). Therapy was withdrawn after 3 months. Triple therapy prevented rejection during therapy in all animals and led to operational tolerance in three of five animals including donor- specific skin graft acceptance in the two animals tested. IDEC- 131, sirolimus and DST are highly effective in preventing renal allograft rejection in primates. This apparently clinically applicable regimen is promising for human renal transplant trials.

Passenger leukocytes have been suggested to be both pro-tolerant and immunogenic. The opportunity to evaluate the role of allogeneic passenger leukocytes in humans was presented by a 47-year-old man who donated bone marrow to his HLA-identical leukemic sister. Eleven years later he developed renal failure. The sister's marrow was noted to be 100% XY karyotype and free of malignancy. She donated a kidney to her brother. Immunosuppression was tapered following transplantation. After 6 months, the recipient was on monotherapy sirolimus, 1 mg every third day. A surveillance biopsy was normal and sirolimus was stopped. Eight weeks later, he presented with severe rejection that reversed with Thymoglobulin. Renal function returned to baseline and has been stable on conventional immunosuppression.

Significant advances have been made in the understanding of allograft rejection. There is growing awareness that allograft acceptance, or tolerance, is also an active process rather than a passive absence of rejection. Mechanistic awareness of this process has spawned many preclinical strategies for the prevention of allograft rejection without the need for chronic immunosuppression. These therapies are currently entering clinical trials. This article reviews the prevailing therapies that hold promise for future clinical application. In particular, their application in children is discussed, as are biologic aspects of childhood immunity that may play a role in the success or failure of these strategies.

Significant advances have been made in the understanding of allograft rejection. There is growing awareness that allograft acceptance, or tolerance, is also an active process rather than a passive absence of rejection. Mechanistic awareness of this process has spawned many preclinical strategies for the prevention of allograft rejection without the need for chronic immunosuppression. These therapies are currently entering clinical trials. This article reviews the prevailing therapies that hold promise for future clinical application. In particular, their application in children is discussed, as are biologic aspects of childhood immunity that may play a role in the success or failure of these strategies.

BACKGROUND:We reported that rabbit anti-thymocyte globulin (RATG) induction followed by maintenance immunosuppression with sirolimus supports human kidney allograft survival and asked if this combination would promote islet allograft survival in our primate model.METHODS:Using intra-arterial streptozotocin infusion, we rendered four cynomolgus primates diabetic with undetectable C-peptide levels. Animals were maintained on insulin therapy for at least 1 month, and then islets from mixed lymphocyte reaction mismatched primates were infused into the portal vein. Immediately before the islet allotransplant and for 6 additional days, primates were infused with RATG (20 mg/kg) and given a sirolimus dose to achieve a 24-hr trough level of 8 to 14 ng/mL.RESULTS:The regimen resulted in profound peripheral and lymph node lymphocyte depletion for up to 1 month. Repopulation was gradual thereafter. One primate remained insulin-independent for 169 days and rejected after a sirolimus-dose reduction. Two primates died on day 23 while insulin independent because of wound dehiscence, and a third died on day 30 with high sirolimus levels. Liver sections revealed well-vascularized islets with no signs of inflammation.CONCLUSION:Using a nonhuman primate islet transplant model, RATG plus sirolimus supports islet survival as long as proper sirolimus levels are maintained, but the therapy is limited by sirolimus toxicity. Our findings suggest that RATG is not toxic for islets and thus may be considered in future clinical trails while recognizing that sirolimus monotherapy, with its difficult-to-achieve therapeutic dosing, may not be sufficient to maintain long-term islet allograft function in an autoimmune environment.

Anti-CD154 variably prolongs allograft survival in nonhuman primates. Rodent studies suggest that adding pretransplant donor-specific transfusion (DST) and/or rapamycin to anti-CD154 improves survival. The CD154-specific Ab IDEC-131 was tested alone and in combination with rapamycin for its ability to inhibit rhesus MLRs. The ability of the Ab to block endothelial activation was also assessed. IDEC-131 was then tested alone and in combination with DST and/or rapamycin for its ability to prevent rejection of full-thickness, MHC-mismatched rhesus skin allografts. Animals were monitored for donor-specific hyporesponsiveness by MLR and alloantibody determination. IDEC-131 modestly inhibited rhesus MLRs and inhibited CD154-dependent endothelial cell activation. Rapamycin combined with IDEC-131 additively inhibited MLRs. IDEC-131 modestly prolonged allograft survival when compared with no treatment, rapamycin alone, or DST plus rapamycin. Adding DST to IDEC-131 did not prolong survival beyond IDEC-131 alone. IDEC-131 plus rapamycin was effective in prolonging graft survival, although animals had episodes of acute rejection before graft demise. Therapy with IDEC-131, rapamycin, and DST induced long-term allograft survival without intermittent acute rejection. However, no evidence for MLR inhibition was seen, and most animals eventually developed alloantibody. All animals ultimately rejected their grafts after drug withdrawal. IDEC-131 modestly prolongs rhesus skin allograft survival. Rapamycin and rapamycin plus DST improves the efficacy of IDEC-131 in prolonging allograft survival. IDEC-131, rapamycin, and DST are a promising combination for clinical evaluation in allotransplantation.

Allotransplantation has improved dramatically in the past 2 decades, mostly as a result of more potent immunosuppression and increasingly effective prophylaxis against opportunistic infections. It is likely, however, that the field has entered a period of diminishing returns, such that increased immunosuppression will minimally increase graft survival and markedly augment morbidity. The authors suggest that gains in graft and patient survival will best be achieved by pursuing strategies promoting immune tolerance and selecting therapies that enhance physiologic immune adaptation rather than those that indiscriminately suppress immune function. Principles of tolerance as they relate to allotransplantation are discussed. The authors introduce the concept of context-based therapy. This approach places an emphasis on preserving specificity during immune intervention by augmenting or avoiding interference with antigen receptor function. Graft acceptance is fostered by altering contextual signals influencing the outcome of antigen recognition such as intrinsic indicators of cell injury, costimulatory signals, the activation state of antigen presenting cells, or the local environment of immune engagement. The manipulation of immune thresholds is also introduced as an additional concept influencing tolerance. Supportive data from nonhuman primate and human trials are presented.