Co-signals in organ transplantation. Curr Opin Organ Transplant. 2010;15:474-480. [PMID: 20631615 DOI: 10.1097/mot.0b013e32833c1369]

Update on the treatment of focal segmental glomerulosclerosis in renal transplantation

Focal segmental glomerulosclerosis (FSGS) represents one of the most severe glomerular diseases, with frequent progression to end-stage renal disease and a high rate of recurrence in renal allografts (30%-50%). Recurrent FSGS portends a negative outcome, with the hazard ratio of graft failure being two-fold higher then that of other glomerulonephritis. Two patterns of clinical presentations are observed: Early recurrence, which is characterized by massive proteinuria within hours to days after implantation of the renal graft, and late recurrence, which occurs several months or years after the transplantation. Many clinical conditions have been recognized as risk factors for recurrence, including younger age, rapid progression of the disease to end-stage renal disease on native kidneys, and loss of previous renal allografts due to recurrence. However, much less is known about the incidence and risk factors of the so-called “de novo” type of FSGS, for which sufferers are transplanted patients without disease on native kidneys; but, rapid development of allograft failure is frequently observed. Management of both forms is challenging, and none of the approaches proposed to date have been demonstrated as consistently beneficial or effective. In the present review we report an update on the available therapeutic strategies for FSGS in renal transplantation within the context of a critical overview of the current literature.

Lack of association between CD40 polymorphisms and acute rejection in German liver transplant recipients

CD40 and its ligand, CD154, are major costimulatory molecules whose interactions are important in alloreactive transplant rejection. The aim of this study was to examine the association of CD40 polymorphisms with the susceptibility to acute rejection episodes in liver transplantation. In total, 112 liver transplant recipients with biopsy proven acute rejections (BPAR), 97 without BPAR (WBPAR), and 112 healthy control individuals were enrolled in the study. Two single nucleotide polymorphisms (SNPs) of CD40 gene (rs1883832 and rs4810485) were genotyped by polymerase chain reaction-allele specific restriction enzyme analysis (PCR-ASRA). Both SNPs has been tested for a recessive and a dominant model. No significant differences were found in the genotype and allele frequencies of the SNPs rs1883832 and rs4810485 between BPAR liver recipients and WBPAR recipients. Our results do not suggest an important role of tested CD40 SNPs in the susceptibility to acute liver transplant rejection in a Caucasian population.

Association of cytokine/costimulatory molecule polymorphism and allograft rejection: a comparative review

One reason for genetic variations among human individuals is SNP which may confer diverse disease susceptibility or resistance in a population. Genetic variations in a key immunoregulatory agent can manifest various immunological responses, such as graft rejection. In fact, the outcome of organ transplantation can be impacted by several genetic causes including polymorphisms in genes encoding cytokines and costimulatory molecules in the donor or recipient. Thus, it can be helpful to contemplate the SNPs relating to these immunological determinants in order to achieve an improved transplantation therapy.

Fundamental immunology of skin transplantation and key strategies for tolerance induction

Transplantation of allogeneic or xenogeneic skin grafts can evoke strong immune responses that lead to acute rejection of the graft tissues. In this process, donor-derived dendritic cells play crucial roles in the triggering of such immune responses. Both the innate and acquired host immune systems participate in graft rejection. At present, the rejection of skin grafts cannot be well-controlled by ordinary systemic immunosuppression therapy. Although several strategies for the long-term survival of allogeneic or xenogeneic skin grafts have been demonstrated in animal models, the induction of long-term tolerance to skin grafts is still a great challenge in clinical settings. In this article, we review the progress in the understanding of immune responses to skin grafts and discuss the possible methods that can decrease the immunogenicity of graft tissues and improve the survival of skin grafts, especially those included in preoperative pre-treatments.

Fates of CD4+ T cells in a tolerant environment depend on timing and place of antigen exposure

In experimental organ transplantation, tolerance is induced by administration of anti-CD40L mAb in conjunction with donor-specific splenocyte transfusion. Multiple, sometimes conflicting mechanisms of action resulting from this treatment have been reported. To resolve these issues, this study assessed the fates of graft reactive cells at different times and locations in the tolerant environment. Alloantigen-specific CD4(+) T cells transferred at time of tolerance induction (7 days before transplantation) became activated, expressed CD69 and CD44, and proliferated. Importantly, a large subset of this population became Foxp3(+) , more so in the lymph nodes than spleen, indicative of differentiation to a regulatory phenotype. In contrast, graft reactive CD4(+) T cells transferred to tolerogen-treated recipients at the time of transplantation failed either to proliferate or to differentiate, and instead were deleted via apoptosis. In untreated rejecting recipients graft reactive CD4(+) T cells became activated, proliferated and differentiated mainly in the spleen, and many of these cells were eventually deleted. These data resolve many apparent contradictions in the literature by showing that the timing of antigen exposure, the immunologic status of the recipients and secondary lymphoid organ location act together as key factors to determine the fate of graft reactive CD4(+) T cells.

Relative efficiency of porcine and human cytotoxic T-lymphocyte antigen 4 immunoglobulin in inhibiting human CD4+ T-cell responses co-stimulated by porcine and human B7 molecules

α1,3-Galactosyltransferase gene-knockout pigs transgenic for porcine cytotoxic T-lymphocyte antigen 4 immunoglobulin (pCTLA4-Ig) have been produced to reduce T-cell-mediated rejection following xenotransplantation. The level of soluble pCTLA4-Ig in their blood was greatly in excess of the therapeutic level in patients, rendering the pigs immune-incompetent. Soluble pCTLA4-Ig produced by these transgenic pigs was evaluated for binding to porcine and human (h) B7 molecules, and for its inhibitory effect on allogeneic and xenogeneic human T-cell responses. Porcine CTLA4-Ig-expressing peripheral blood mononuclear cells (PBMCs) and aortic endothelial cells (AECs) were evaluated for their direct inhibitory effect on hCD4+ T-cell responses. Soluble pCTLA4-Ig and purified hCTLA4-Ig showed similar binding to pB7 molecules, but pCTLA4-Ig showed significantly less binding to hB7 molecules. The pCTLA4-Ig and hCTLA4-Ig inhibited the response of hCD4+ T cells to pAECs equally, but pCTLA4-Ig was less successful in inhibiting the human allogeneic response. The hCD4+ T-cell response to PBMCs from pCTLA4-Ig pigs was significantly lower than that of non-pCTLA4-Ig pigs. Although pCTLA4-Ig was detected in the cytoplasm of pCTLA4-Ig-expressing pAECs, only a minimal level of soluble pCTLA4-Ig was detected in the supernatant during culture, and pCTLA4-Ig-expressing pAECs did not inhibit the xenogeneic direct human T-cell response. High-level tissue-specific production of pCTLA4-Ig may be required for sufficient immunosuppression for organ or cell (e.g., islets) transplantation.

Costimulatory pathways in transplantation

Secondary, so-called costimulatory, signals are critically required for the process of T cell activation. Since landmark studies defined that T cells receiving a T cell receptor signal without a costimulatory signal, are tolerized in vitro, the investigation of T cell costimulation has attracted intense interest. Early studies demonstrated that interrupting T cell costimulation allows attenuation of the alloresponse, which is particularly difficult to modulate due to the clone size of alloreactive T cells. The understanding of costimulation has since evolved substantially and now encompasses not only positive signals involved in T cell activation but also negative signals inhibiting T cell activation and promoting T cell tolerance. Costimulation blockade has been used effectively for the induction of tolerance in rodent models of transplantation, but turned out to be less potent in large animals and humans. In this overview we will discuss the evolution of the concept of T cell costimulation, the potential of 'classical' and newly identified costimulation pathways as therapeutic targets for organ transplantation as well as progress towards clinical application of the first costimulation blocking compound.

Steroid and calcineurin inhibitor-sparing protocols in kidney transplantation

Recent improvements in kidney transplantation have been driven largely by lower acute rejection rates attributed to better immunosuppressive agents. In an effort to reduce the long-term toxicities of immunosuppressant drugs, corticosteroid- and calcineurin inhibitor (CNI)-sparing immunosuppression protocols have become increasingly popular in managing kidney transplant recipients. Nevertheless, these strategies may increase the risk of acute and chronic allograft injury (CAI) that may worsen the fate of transplant recipients. This article focuses on steroid and CNI sparing protocols to elucidate their safety and efficacy in patients receiving a kidney transplant. Steroid avoidance protocols are rapidly and increasingly being used. Studies have shown that corticosteroids are not essential to achieve excellent short- and intermediate-term results. However, the role of steroid withdrawal is only marginally beneficial and very often benefit overstated. CNI-sparing strategies have been used to help maintain the balance between allograft survival and nephrotoxicity. Trials evaluating CNI minimization have shown reduced incidence of CAI and preservation of allograft function. CNI withdrawal within 3 to 12 months after kidney transplantation improved graft function despite increased risk of acute rejection. This approach may be feasible with adequate exposure and proper usage of mammalian target of rapamacin inhibitors. Late withdrawal or conversion did not show a clear benefit. Timing and degree of renal dysfunction are key determining factors. With regards to CNI avoidance, earlier trials, such as the Symphony study, did not support the use of a CNI-free regimen of low-dose sirolimus as initial immunosuppression. However, recent studies using costimulatory blockade-based immusouppression showed that CNI avoidance is possible. The best maintenance immunosuppressive with CNI- or steroid-sparing is a work in progress and awaits longer term follow-up. The availability of newer biologics for costimulatory blockade and new immunosuppressive agents with novel mechanisms of action have the potential of using CNI- and steroid-sparing protocols to minimize the incidence of CAI and improve long-term outcomes in kidney transplant recipients.