Book Description

Organ transplantation is often the best therapeutic approach for end-stage organ failure. Graft rejection is the major obstacle to successful transplantation because transplantation is most frequently carried out between genetically distinct individuals. Transplanted tissues or organs are usually recognized by the immune system as foreign and are rapidly destructed without immune interventions. A series of approaches have thus been applied in clinic to inhibit the allogenic immune responses and in turn increase organ transplant acceptance. Immunosuppressive drugs are the key players in the "war" against immune cell-mediated rejection of allogenic transplants. Currently, the use of calcineurin inhibitors (CNIs) has dramatically decreased the risk of acute transplant rejection and improved the short-term outcomes of organ transplantation, but the long-term outcomes are still unsatisfied. One of the main reasons causing unsatisfied long-term outcomes is that current immunosuppressive drugs do not specifically target immune cells that cause transplant rejection. These immunosuppressive agents, especially CNIs, are the risk factors for late graft loss and death with functioning graft (DWFG) due to drug toxicity, over-immunosuppression, and other side-effects. Thus there is an urgent need for seeking novel therapies that can selectively eliminate the alloreactive immune responses while preserving the integrity of the remainder of the host immune system. It has been known that T cells play a central role in initiating allograft rejection. Full activation of T cells requires at least two collaborative but distinct signals. The first signal is generated by the interaction between T cell receptor (TCR) and MHC-peptide complex. The second signal, termed costimulatory signal, is delivered through the engagement of costimulatory receptors by their ligands. Importantly, TCR engagement in the absence of costimulatory signals can result in T cell anergy, a state of T cell unresponsiveness. Costimulatory molecules thus gain attention as potential therapeutic targets. Because these molecules are primary expressed on T cells and/or antigen-presenting cells, targeting costimulatory signaling pathway offers the oppertumities to modulate immune system in a more selective way relative to conventional immunosuppressive agents. To date, numerous costimulatory molecules have been identified and some have been tested as therapeutic targets in organ transplant models. CD28-CD80/86 and CD40-CD40L axis are two important and the most well known costimulatory signaling pathways. Belatacept, a variant of cytotoxic T lymphocyte antigen 4-immunoglobulin G (CTLA4-Ig) that blocks CD28-CD80/86 signaling pathway, is the only costimulation blocker to be approved for clinical use in organ transplantation. Compared to CNI-based regimen for kidney transplant recipients, belatacept-based regimens show similar patient and graft survival rate, superior graft function, and improved cardiovascular risk profile. However, belatacept is also associated with higher rates of acute rejection and posttransplant lymphoproliferative disorder (PTLD). Disruption of CD40-CD40L interaction with anti-CD40L mAbs has also been demonstrated to be a reliable approach for preventing acute rejection and for prolonging allograft survival. Unfortunately, unexpected thromboembolic complications in preclinical studies and clinical trials discontinued the development of anti-CD40L mAbs. The main objective of this thesis is to identify the optimal T cell costimulation blockers that can show improved safety and non-inferior efficacy in promoting allograft acceptance relative to current costimulatory blocking agents. Anti-CD40 mAbs are an alternative approach to block CD40-CD40L costimulatory pathway. CD40 is not involved in the stability of platelet thrombi and anti-CD40 mAbs are expected to not cause thromboembolic events. ASKP1240 is a novel fully human anti-CD40 mAb. In this study, the efficacy of ASKP1240 in the prevention of renal allograft rejection was evaluated in cynomolgus monkey kidney transplantation model. When ASKP1240 was administered alone, it dose-dependently reduced the incidence of acute rejection and prolonged renal allograft survival. Renal allograft acceptance was further improved in the monkey which received regiments using ASKP1240 combined with conventional immunosuppressive drugs tacrolimus (sub-therapeutic dose) or mycophenolate mofetil (MMF). Acute allograft rejection was totally eliminated in these animals and the kidney allograft median survival times (MST) of these groups were significantly longer than those of monotherapy groups. ASKP1240 administration did not induce cytokine release. This agent was well tolerated in monkey kidney transplant recipients during the 180-day treatment period. There were no obvious side effects including drug-related thromboembolic complications. Next, we tested the hypothesis that a CD86-selective variant of CTLA4-Ig might show non-inferior efficacy on the prevention of allograft rejection and prolongation of graft survival in comparison with belatacept. CD86 is the dominating ligand between the two natural ligands for CD28 in alloimmune response. Improvements in CD86 binding affinity of CTLA4-Igs confer increased immunosuppressive potency. A CD86-selective CTLA4-Ig variant may reach therapeutic levels of CD86 occupancy while occupies substantially less CD80 ligand than non-CD86-selective CTLA4-Igs. Preservation of CD80 signaling may be favoring to retain protective immunity and to improve safety of CTLA4-Ig therapy. This thesis investigated the effects of a novel CD86-selective CTLA4-Ig variant, ASP2409, on renal allograft rejection and survival in cynomolgus monkeys. ASP2409 possesses 14-fold higher in vitro CD86 binding affinity than belatacept and improved immunosuppressive potency. Compared to no-treatment control, low-dose (0.3 mg/kg) ASP2409 monotherapy significantly prolonged renal allograft survival (MST = 5 and 26 days, respectively) but did not decrease the incidence of acute rejection (8/8). The rate of acute renal allograft rejection was reduced in the group of high-dose ASP2409 monotherapy (2/7) and, correspondingly, a much longer MST (>91 days) was shown in this group. Combination therapy of low-dose ASP2409 and tacrolimus (sub-therapeutic dose) completely eliminated acute allograft rejection and notably prolonged renal allograft survival (MST >91 days). The MSTs of renal allografts in the animals receiving high-dose ASP2409-, belatacept-, and therapeutic dose tacrolimus-based immunosuppressive regimens were same (>91 days). High-dose ASP2409-based regimens exhibited better histopathological results than belatacept-based regimen. Furthermore, higher frequencies of FoxP3+ Tregs in renal allografts were observed in ASP2409- and belatacept-based regimens compared to tacrolimus-based regimen. ASP2409 was also demonstrated to be safe for animals with the doses to be tested. There were no serious side effects related to ASP2409 to be found in term of 91-day study. This study demonstrates that ASKP1240 is as effective as anti-CD40L mAbs on inhibition of acute rejection and prolongation of renal allograft survival, and do not cause thromboembolic complications. Previous studies indicated that CNIs could diminish the effects of anti-CD40L mAbs. In our study, ASKP1240 in combination with tacrolimus or MMF shows improved efficiency. Furthermore, the effects of ASP2409 on promoting renal allograft acceptance are not inferior to belatacept. These results collectively suggest that ASKP1240 and ASP2409 both are the promising immunosuppressive agents for solid organ transplantation.