Acute renal allograft rejection: progress in understanding cellular and molecular mechanisms.SA Divate
Department of Pathology, Seth G. S. Medical College & K. E. M. Hospital, Parel - 400 012, Mumbai, India. , India
The application of molecular biology tools to investigate the molecular basis of acute allograft rejection has unravelled many complex mechanisms and improved immunosuppressive therapies leading to significant improvements in graft survival. The "indirect" pathway of antigen presentation has emerged as more important, than the traditional "direct" pathway, for allorecognition by T cells. The recognition that CD28 costimulation is essential for allorecognition has provided novel targets for immunotherapy such as CTLA-Immunoglobulin. Understanding the role of Th1 and Th2 subsets of T helper cells, the cytokine network and cell adhesion molecules in the mediation or prevention of graft rejection has opened new avenues for research into therapeutic modalities. The ideal objective would be to identify the mechanisms of graft destruction and design specific inhibitors. This review highlights recent advances in the understanding of acute renal allograft rejection which may have future potential for rational design of new immunosuppressive strategies.
Keywords: Cell Adhesion Molecules, immunology,Cytokines, immunology,Graft Rejection, Human, Immunosuppressive Agents, therapeutic use,Kidney Transplantation, immunology,T-Lymphocytes, immunology,Transplantation, Homologous,
Potent new immunosuppressive drugs, that have emerged in recent years, particularly Cyclosporine, have greatly improved survival rates of renal transplants. However, a small proportion of grafts continue to be lost due to acute rejection and long-term survival poses some problem. The understanding of the mechanisms of rejection and devising strategies to facilitate long-term graft acceptance are major areas of active research. Rapid advances in molecular and cellular biology which have occurred in the last two decades have unravelled some of the complexities in the processes involved in allograft rejection. This in turn has facilitated the design of newer, more specific and more effective immunosuppressive therapies.
Acute rejection appears to be, predominantly, a cell mediated process with CD4+ T lymphocytes playing a central role. In recent years research has been focussed on understanding the mechanisms of allorecognition, role of co-stimulatory molecules and cytokines, relative importance of Th1 and Th2 cells and effector mechanisms of graft destruction. The present review summarises some of the important developments that have occurred in the past decade in this field.
The immunological event that initiates graft rejection is recognition of the foreign antigen. This involves an interaction between the donor derived peptide and major histocompatibility complex (MHC) molecules on antigen presenting cells (APCs) and the T cell receptor on CD4+ recipient T cells. In contrast to other -situations, in the allograft, both donor and recipient derived APCs are available.
Allorecognition was traditionally considered to occur by “direct recognition” by recipient T cells of intact allo-MHC molecules on donor APCs i.e. donor interstitial dendritic cells that are the “passenger leucocytes” within the graft. This belief was based on three sets of observations; that direct stimulation is very potent in primary allogenic mixed lymphocyte reactions, that depletion of donor APCs can sometimes prolong allograft survival and that matching of MHC antigens is important for graft survival.
Lechler and Batchelor first suggested the “indirect” pathway of allo-antigen recognition in which recipient T cells recognize allogenic peptides presented by MHC molecules on self-APCs. The evidences have been well discussed in some recent reviews.,, It has been shown that rejection can be initiated by the indirect pathway alone. Graft survival can be prolonged by introducing only the peptides of donor MHC antigens rather than intact molecules. Donor MHC molecules are presumed to be shed from the allograft into the recipients circulation, phagocytosed by recipient APCs, processed into peptides and presented on the cell surface by self MHC Class II molecules. The recipient CD4+ T cells recognize this foreign peptide-self MHC combination and get activated. These activated T cells proliferate, differentiate and secrete cytokines for activation of various effector cells which in turn destroy the graft. It is pertinent at this stage to point out that the “indirect pathway” is the natural mechanism for recognition of exogenous microorganisms and the term “indirect” is rather misleading. “Direct” recognition of alloantigen on graft cells (Direct presentation) and recognition of alloantigen after host APCs process & present allogenic peptides to T cells (Indirect presentation) are illustrated in [Figure - 1].
Recent reports also implicate the indirect pathway in late acute rejection (LAR) since APCs of donor origin get depleted with time. T cell recognition of donor-derived allopeptides presented by self APCs becomes more important in the initiation of LAR episodes and at this stage the mixed lymphocyte reaction is markedly reduced. Thus, while donor APCs, which mediate direct recognition and are potent stimulators of rejection, are possibly important for the strength of early graft rejection, further rejection activity is progressively dependent on the indirect pathway.
The actual site of activation of recipient T cells is still debated. The more popular view is that APCs sensitise T cells locally. However, recent studies show that T cell activation occurs in recipient lymphoid organs than in the graft.
For complete recognition of alloantigens, T cells require at least two signals. The first is delivered when the T cell receptor (TCR) binds to the MHC-allopeptide complex on APCs. The second signal termed “costimulation” involves ligation of specific molecules on the surface of T cells to molecules on APCs. B7-CD28 is the most important and markedly enhances T cell proliferation and production of cytokines, even in the presence of suboptimal TCR stimulation. The interaction between costimulatory ligands CD28 and B7 in the process of allorecognition is diagrammatically depicted in [Figure - 1].
CD28 has been shown to bind CD80 & CD86 molecules on APCs. These molecules belong to the B7 family of ligands. A prolonged state of antigen-specific unresponsiveness results in the absence of co-stimulation. Krummel and Allison have shown that the Cytotoxic T lymphocyte activation molecule (CTLA-4) binds to CD80 & CD86 with a 100-fold higher affinity as compared to CD28. They propose that by virtue of this property, CTLA-4 is a negative regulator of T cell activation.
Blockade of costimulatory molecules e.g. by CTLA-Immunoglobulin (CTLA-Ig) has been evaluated as a therapeutic modality to achieve graft-specific tolerance. These studies have been elegantly reviewed by Alegre M-L12 as also Dai and Lakkis.
Mossmann etal first reported that naive CD4+ T cells differentiate into two distinct T helper cell subsets, Th1 cells or Th2 cells, which have distinct profiles of cytokine production and thus mediate distinct functions. Th1 cells are mainly involved in cell mediated immunity whereas Th2 cells are associated with humeral immunity. The nature of the antigen and the pattern of cytokines in the microenvironment are the most important factors which dictate whether Th1 or Th2 cells predominate and Th1 and Th2 cells cross regulate each other through their respective cytokines. A simplified representation of these complex series of processes has been depicted in [Figure:2].
Tolerance to an allograft may be achieved by differentiation towards the Th2 phenotype. Induction of long-term tolerance to an allograft depends on the blockade of both Th1 and Th2 responses rather than a mere modulation towards a Th2 phenotype.
A variety of cytokines, each with numerous functions, are involved in an immune response. While IL-12 facilitates differentiation towards the Th1 phenotype and IL-4 towards the Th2 phenotype, other cytokines such as IFN-? and IL-2 secreted by the Th1 cells promote cell mediated immune responses and IL-4, IL-6 and IL-7 released by Th2 cells are important in B cell maturation.
The cytokines IL-2 and IFN-? play critical roles in graft rejection. They are crucial for proliferation, activation and recruitment of various leucocytes, for the induction or upregulation of cell adhesion molecules and MHC molecules and for mediating communication between leucocytes and parenchymal cells. Hence, IL-2, has been studied as a potential target for suppression of graft rejection. The complexity of the cytokine network, particularly the plethora of cytokines and their overlapping functions, is a major obstacle in achieving this objective.,
Cell adhesion molecules, particularly ICAM-1, VCAM-1 are key regulatory molecules in immune responses. They are important in migration and localization of leucocytes into tissues as well as in a variety of cell to cell interactions which include signalling between cells and even in cell mediated cytotoxicity.
Studies have shown that there is an enhanced and inducible expression of these cell adhesion molecules during allograft rejection., The expression of these molecules, which occurs in a sequential fashion, is important for orchestrating the various steps in graft rejection, though the actual sequence of events and their underlying mechanism is not yet clear. A few experimental studies did observe that antibodies against ICAM-1 were immunosuppressive and it is a promising target for immunosuppressive therapy. However recent clinical trials found that short-term treatment with anti-ICAM-1 monoclonal antibody did not reduce the rate of acute rejection.
A multitude of cell types, cytotoxic T lymphocytes, B cells, macrophages and natural killer cells which are recruited by activated CD4+ T cells are seen in a graft undergoing rejection. The three main mechanisms implicated for graft destruction are cell-mediated cytotoxicity, delayed type hypersensitivity and antibody-dependent cellular cytotoxicity.
Of these, selective cell-mediated cytotoxicity by the perform/granzyme B pathway is the most important. Direct antibody mediated damge and development of donor-specific allo-antibodies that can cause rejection have been found and lends support to the importance ot anubodies in graft rejection. We have, ourselves, observed that the pre- and post-transplant IgG antibodies against donor epithelium and/or endothelium may predict patients at risk of rejection. Thus humoral mechanisms appear to play a supplemental role at least.
Newer insights into the molecular basis of organ allograft rejection has provided novel targets for immunosuppressive therapy. Appreciation of the mechanisms underlying T cell recognition of alloantigen has given scope for the use of synthetic peptides in prevention of graft rejection. Elucidation of the role of costimulatory molecules, particularly CD28 has given rise to newer therapies such as CTLA-lg for the induction of transplantation tolerance. A better understanding of the importance of adhesion molecules in transplant rejection has encouraged the introduction of trials of anti-ICAM-1 monoclonal antibodies for the prevention of acute graft rejection. Hopefully, continuing rapid progress in unravelling the molecular mechanisms of graft rejection will facilitate further refinements in immunosuppressive protocols and soon achieve the ultimate goal of long-term acceptance of organ allografts.
[Figure - 1][Table - 1]