Advancing kidney transplantation

Keywords::

• allograft survival
• antibody-mediated rejection
• donor-specific antibodies
• end-stage renal disease
• immunosuppression
• kidney transplantation
• renal allograft

One of the most important therapeutic advancements of the 20th century was the evolution of kidney transplantation from a curiosity to a viable and robust therapy for treatment of end-stage renal disease [1]. From the very beginning, advances in transplantation reflected advancements in our understanding of the basic immunologic principles of T-cell activation and effector functions. This knowledge was rapidly translated to development of T-cell directed therapies aimed at reduction of the allograft rejection process. Early work by Sir Peter Medawar and colleagues demonstrated that tolerance to nonself antigens could be achieved by in utero exposure and could potentially insure the long-term acceptance of an allograft without rejection or the need for immunosuppressive therapies [2]. These investigations set in motion a quest for methodologies to induce immunologic tolerance to the allograft. This has become the ‘Holy Grail’ of transplant immunology, and has proven to be an elusive and often disappointing journey [2,3]. Despite our inability to achieve long-term tolerance without immunosuppression, a number of impressive advancements have occurred over the past decade and are notable for changing our view of the causes of long-term allograft failures and the emergence of B cells, antibodies and complement as major players in newly described forms of allograft rejection (antibody-mediated rejection [ABMR]) [4–10]. Another important advancement is the identification of regulatory immune cells as potential regulators of allograft directed immune responses [11]. Below, we will cover some of the most important advances of the past decade that will likely impact the course and outcomes of allografts.

B cells, donor-specific antibodies & complement as mediators of allograft injury

Over the past 2 decades, improvements in renal allograft survival for transplant recipients relates primarily to reduction in the incidence and consequences of cell-mediated rejection through advancements in immunosuppression. Despite these advances, data on long-term outcomes of allografts show no clear superior protocol suggesting that, despite good short-term outcomes, we still do not completely understand the immunologic events that may play a role in diminishing long-term allograft survival. This results in higher morbidity, mortality and costs [4–10]. Currently, approximately 5000 kidney allografts fail each year in the USA. The attribution of graft failure was felt to be due to long-term calcineurin inhibitor exposure (cyclosporine and tacrolimus), but recent data suggest the majority of late allograft failures are due to chronic immunologic injury to the allograft mediated primarily by donor-specific antibodies (DSAs), B cells and complement [5–8].

The increasing numbers of sensitized transplant recipients (DSA⁺) coupled with progress and refinements in the pathological diagnosis of ABMR [4,10,12] and development of more robust platforms for determination of DSAs have shifted the focus of the transplant community to defining the role of antibody and complement as mediators of allograft injury and loss. These findings have stimulated interest in the development of newer, more specific therapies for ABMR aimed at depletion of B cells, antibody and inhibition of complement effectors [4,5,10,13].

The pathophysiology of ABMR is still incompletely understood, but several distinct patterns of injury or accommodation have emerged. ABMR is initiated by DSA binding to HLA antigens or autoantigens (such as the angiotensin 1 receptor) [14] on endothelium of the allograft. Complement binding DSAs (IgG₁ & IgG₃), initiate activation of the classic complement pathway with generation of C3a and C3b. C3b enzymatically cleaves C5 to C5a and C5b. C5a is a powerful anaphylatoxin and interacts with C5a receptors on the endothelium to initiate neutrophil and monocyte infiltration, platelet activation and thrombosis. C5b interacts with C6–C9 to form the C5b–C9 membrane attack complex. These events, if untreated, result in rapid loss of the allograft through severe ABMR. Complement activation by DSAs also has the potential to enhance alloantigen presentation to T cells by increasing CD40/CD40L expression and IL-12 production. Antibody production by antigen-specific B cells is also enhanced by C3a/C5a [15,16]. DSAs can bind to endothelial cell targets and stimulate cell proliferation or induce antibody-dependent cell-mediated cytotoxicity (ADCC) with γ-IFN release. ADCC is likely more important for the development of chronic antibody-mediated injury which is dependent on NK cells more than complement [17,18]. Antibodies can also bind to HLA and other targets and incompletely activate the complement system (no C5b–C9 membrane attack complex generation) without causing injury. This process is referred to as accommodation [16]. Other important factors that contribute to accommodation include the expression of complement regulatory proteins on the endothelium of allografts, which include CD46, CD55, and especially CD59 (protectin).[16] In addition, poor ADCC activity may relate to IgG Fc polymorphisms which fail to activate NK cells through the FcγR (CD16) [17].

In this complex and rapidly evolving field, future advancements in immunologic analysis of graft-directed immune responses will be critical to identifying patients at risk for allograft injury and loss. Therapeutic approaches to prevent ABMR are aimed at antibody (DSA) reduction and inhibition of complement activation and injury. These therapies include: plasma exchange with low-dose intravenous immunoglobulin (IVIG); high-dose IVIG and rituximab for antibody reduction; and the use of high-dose IVIG for complement inhibition [4,5,10,13]. Other, more specific, inhibitors of complement (Eculizumab^®, anti-C5, Alexion Pharmaceuticals, Cheshire, UK, CT.) and inhibitors of C1 (C1INH) may be useful in the prevention and treatment of ABMR, both in the acute and chronic phase. Trials of complement inhibitors in human kidney transplant recipients are now underway (NCT01327573, NCT01147302, NCT01134510) [15,16].

Thus, B cells, antibodies (DSAs) and complement have emerged as major effectors responsible for chronic allograft injury and loss. Future therapeutics for prevention and treatment of antibody-mediated allograft injury will be aimed at modification of B cells, antibodies and complement and represent a significant departure from the T-cell-centric approach that has dominated new drug development in transplantation since its inception.

Increasing regulatory immune cells to reduce allograft rejection & improve long-term outcomes

It is well known that dendritic cells, B cells and T cells are participants in the allograft rejection process, but it is now known that specialized cells from each of these lineages can promote tolerance of allografts by secretion of regulatory cytokines (commonly IL-10) or through destruction of allograft reactive cells [11,19,20]. Recently, Wood et al. reported on advancements in defining leukocyte populations that promote allograft tolerance [11]. Initially, regulatory cells were felt to be primarily CD4⁺/CD25⁺/FoxP3⁺ T cells, but advancements in this area have identified other cell types that have tolerance inducing activity in models of allograft rejection. These include regulatory B cells, regulatory macrophages, myeloid-derived suppressor cells, regulatory macrophages and mesenchymal stromal cells. Although multiple mechanisms of action are proposed for regulatory immune cells, most regulatory cells utilize IL-10 as a pathway for suppressing allo-reactive cell populations [11,20].

With the knowledge that regulatory cell populations exist and can be expanded by in vivo and in vitro mechanisms [21–23], strategies have been developed to utilize immunosuppressive agents to expand regulatory cell populations for infusion at the time of transplantation. Current approaches felt to enhance regulatory cell populations include: early sparing of calcineurin inhibitors; induction therapy with T-cell depleting agents; and the use of rapamycin, IVIG and Belatacept [11,21–23]. Currently, there are no proven pathways for induction of tolerogenic regulatory cell populations, but a clinical trial (ONE study) (Phase I/II) will investigate the safety of infusing Treg cells and TR₁ cells after transplantation [11]. Expansion of Treg cells by low-dose IL-2 infusions has provided encouraging results in treating graft versus host disease in bone marrow transplant recipients [24]. Of course, IL-2/IL-2R interactions are critical for T-cell mediated rejection of allografts, thus the use of IL-2 in solid-organ transplant recipients for purposes of expanding Treg populations might induce rejection episodes. Other concerns about creating excessive Treg cells would be potential poor immune responses to viruses and tumors [11]. Thus, the utility of regulatory immune cells in transplantation for the induction of tolerance remains an interesting possibility with need of further refinements and study.

Achieving transplant tolerance: the last frontier

Maintaining allograft function without drug based immunosuppression is, and has been, the ultimate goal of transplant immunology. There are examples of what appear to be clinical tolerance in patients who have stopped their medications because of noncompliance or due to chronic infections or cancer without loss of the allograft. These examples are rare, however, as most patients who discontinue or reduce their medications to subtherapeutic levels ultimately have allograft rejection and loose their allografts. The most consistent observation is the development of DSAs with subtherapeutic immunosuppression [4,6–8]. Thus, innate tolerance to allografts is not the default response suggesting that active immunosuppression/modulation is needed to achieve operational tolerance. Recently, Leventhal et al. described eight patients who underwent hematopoietic stem cell transplantation (HSCT) followed by renal transplant from the same donor [25]. Of the eight patients treated, five have full replacement of the bone marrow by donor cells and apparent tolerance of renal allografts. That kidney transplant acceptance after HSCT with 100% donor marrow replacement was achieved is not new [26]. However, these investigators appear to have developed an ability to perform HSCTs in patients with significant immunologic barriers who are chemotherapy naïve without the risk of graft versus host disease. This methodology allows subsequent kidney transplantation and could dramatically improve tolerance induction protocols. Unfortunately, the nature of this protocol is proprietary and was not revealed in the publication [25,26]. This will make validation and subsequent independent analysis of the protocol difficult. It will be important to follow up the transplanted patients to insure they are truly tolerant and do not develop subclinical rejection episodes associated with DSAs. In summary, there is some encouraging news, but miles to go before we can truly envision the implementation of operational transplant tolerance protocols on a larger scale.

Financial & competing interests disclosure

S Jordan has research grants from CSL Behring and Genentech Inc. to conduct research in desensitization of highly-HLA sensitized patients and also owns a patent USP171,585,B1 titled “IVIG immunosuppression of highly-HLA sensitized patients”. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.