Thus far, we have described the molecular basis of alloantigen recognition and the cells involved in the recognition of and responses to allografts. We now turn to a consideration of the effector mechanisms responsible for the immunologic rejection of allografts. In different experimental models and in clinical transplantation, alloreactive CD4+ and CD8+ T cells and alloantibodies all have been shown to be capable of mediating allograft rejection. These different immune effectors cause graft rejection by different mechanisms, and all three effectors may contribute to rejection concurrently.
For historical reasons, graft rejection is classified on the basis of the time course of rejection after transplantation and histopathologic features rather than by immune effector mechanisms. Based on the experience of renal transplantation, the histopathologic types of rejection are called hyperacute, acute, and chronic. It is helpful to consider rejection in the context of these types because the immune mechanism of rejection correlates well with the pattern. Our discussion of these types of rejection will emphasize the underlying mechanisms rather than the pathologic or clinical features.
Hyperacute Rejection
Hyperacute rejection is characterized by thrombotic occlusion of the graft vasculature that begins within minutes to hours after host blood vessels are anastomosed to graft vessels and is mediated by preexisting antibodies in the host circulation that bind to endothelial antigens in the graft (Fig. 1A). Binding of antibody to endothelium activates complement, and antibody and complement products together induce a number of changes in the graft endothelium that promote intravascular thrombosis. Complement activation leads to endothelial cell injury and exposure of subendothelial basement membrane proteins that activate platelets. The endothelial cells are stimulated to secrete high-molecular-weight forms of von Willebrand factor, which causes platelet adhesion and aggregation. Both endothelial cells and platelets undergo membrane vesiculation, leading to shedding of lipid particles that promote coagulation. Endothelial cells lose the cell surface heparan sulfate proteoglycans that normally interact with antithrombin III to inhibit coagulation. These processes contribute to thrombosis and vascular occlusion (Fig. 1B), and the grafted organ suffers irreversible ischemic necrosis.
Fig1. Hyperacute rejection. (A) In hyperacute rejection, preformed antibodies reactive with vascular endothelium activate complement and trigger rapid intravascular thrombosis and necrosis of the vessel wall. (B) Hyperacute rejection involving a glomerulus in a kidney allograft. Typical features include endothelial damage, thrombi, and leukocytic infiltration. B, Courtesy Dr. Helmut Rennke, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts.
In the early days of transplantation, hyperacute rejection was often mediated by preexisting IgM alloantibodies specific for the carbohydrate ABO blood group antigens that are expressed on red blood cells and endothelial cells. These natural antibodies are present in most individuals. Hyperacute rejection by anti-ABO antibodies is extremely rare now because donor and recipient pairs are selected so that they have compatible ABO types. Hyperacute rejection caused by natural antibodies, specific for a variety of antigens that differ among species, is a major barrier to xenotransplantation and limits the use of animal organs for human transplantation.
Currently, the rare instances of hyperacute rejection of allografts that do occur are mediated by IgG antibodies directed against protein alloantigens, such as donor MHC molecules, or against less defined alloantigens expressed on vascular endothelial cells. Such antibodies generally arise as a result of previous exposure to alloantigens through blood transfusion, previous transplantation, or multiple pregnancies. If the level of these alloreactive antibodies is low, hyperacute rejection may develop slowly, over several days, but the onset is still earlier than that typical for acute rejection. As we will discuss later, patients in need of allografts are routinely screened before grafting for the presence of antibodies that bind to blood cells of a potential organ donor to avoid the use of organs that will likely suffer hyperacute rejection.
In unusual cases in which grafts have to be done between ABO-incompatible donors and recipients, graft survival may be improved by rigorous depletion of antibodies and B cells. Sometimes, if the graft is not rapidly rejected, it survives even in the presence of an antigraft antibody. One possible mechanism of this resistance to hyperacute rejection is increased expression of complement regulatory proteins on graft endothelial cells, a beneficial adaptation of the tissue called accommodation.
Acute Rejection
Acute rejection is a process of injury to the graft parenchyma and blood vessels mediated by alloreactive T cells and antibodies. Before modern immunosuppression, acute rejection would often begin several days to a few weeks after transplantation. The time of onset of acute rejection reflects the time needed to generate alloreactive effector T cells and antibodies in response to the graft. In current clinical practice, episodes of acute rejection may occur at much later times, even years after transplantation, if immunosuppression is reduced for any number of reasons. Although the patterns of acute rejection are divided into acute T-cell–mediated (cellular) rejection and acute antibody-mediated (humoral) rejection, both often coexist in an organ undergoing acute rejection.
Acute Cellular Rejection
The principal mechanisms of acute cellular rejection are CTL-mediated killing of graft parenchymal cells and endothelial cells and inflammation caused by cytokines produced by T cells (Fig. 2A). On histologic examination of kidney allografts, where this type of rejection is best characterized, there are infiltrates of lymphocytes and macrophages. In kidney allografts, the infiltrates may involve the tubules (called tubulitis; Fig. 2B), with associated tubular necrosis, and blood vessels (called endotheliitis; Fig. 2C), with necrosis of the walls of capillaries and small arteries. The cellular infiltrates present in grafts undergoing acute cellular rejection include both CD4+ helper T cells and CD8+ CTLs specific for graft alloantigens, and both types of T cells may cause parenchymal cell and endothelial injury. The helper T cells are mainly Th1 cells that produce interferon-γ (IFN-γ) and tumor necrosis factor (TNF). They cause macrophage and endothelial activation and inflammatory damage to the organ. Experimentally, adoptive transfer of alloreactive CD4+ helper T cells or CD8+ CTLs can cause acute cellular graft rejection in recipient mice.
Fig2. Acute cellular rejection. (A) In acute cellular rejection, CD4+ and CD8+ T lymphocytes reactive with alloantigens on endothelial cells in blood vessels and parenchymal cells mediate damage to these cell types. (B) Acute cellular rejection of a kidney with inflammatory cells in the interstitium around the tubules and between tubular epithelial cells. (C) Inflammation of the endothelial layer of a blood vessel (endotheliitis) in acute cellular rejection, with inflammatory cells damaging endothelium (arrow). APC, Antigen-presenting cell. B, Courtesy Dr. Helmut Rennke, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts. C, Courtesy Dr. Zoltan Laszik, Department of Pathology, University of California, San Francisco.
Acute Antibody-Mediated (Humoral) Rejection
Alloantibodies cause acute rejection by binding to alloantigens, mainly MHC molecules, on vascular endothelial cells, leading to endothelial injury and intravascular thrombosis that result in graft destruction (Fig. 3A). The binding of the alloantibodies to the endothelial cell surface triggers local complement activation, which causes lysis of the cells, recruitment and activation of neutrophils, and thrombus formation. Alloantibodies may also engage Fc receptors on neutrophils and NK cells, which then kill the endothelial cells. In addition, alloantibody binding to the endothelial surface may directly alter endothelial function by inducing intracellular signals that enhance surface expression of proinflammatory and procoagulant molecules.
Fig3. Acute antibody-mediated rejection. (A) Alloreactive antibodies formed after engraftment may contribute to parenchymal and vascular injury. (B) Acute antibody-mediated rejection of a kidney allograft with inflammatory cells in peritubular capillaries. (C) Complement C4d deposition in capillaries in acute anti body-mediated rejection, revealed by immunohistochemistry as brown staining. MHC, Major histocompatibility complex. B and C, Courtesy Dr. Zoltan Laszik, Department of Pathology, University of California, San Francisco.
The histologic hallmarks of acute antibody-mediated rejection of renal allografts are acute inflammation of glomeruli and peritubular capillaries with focal capillary thrombosis (Fig. 3B). Immunohistochemical identification of the C4d complement fragment in capillaries of renal allografts is used clinically as an indicator of activation of the classical complement pathway and humoral rejection (Fig. 3C).
Chronic Rejection
As therapy for acute rejection has improved, the major cause of the failure of vascularized organ allografts has become chronic rejection. Since 1990, 1-year survival of kidney allografts has been better than 90%, but the 10-year survival has remained approximately 60% despite advances in immunosuppressive therapy. Chronic rejection develops insidiously during months or years and may or may not be preceded by clinically recognized episodes of acute rejection. Chronic rejection of different transplanted organs is associated with distinct pathologic changes. In the kidney and heart, chronic rejection results in vascular occlusion and interstitial fibrosis. Lung transplants undergoing chronic rejection show thickened small airways (called bronchiolitis obliterans); liver transplants show fibrotic and nonfunctional bile ducts.
A dominant lesion of chronic rejection in vascularized grafts is arterial occlusion as a result of the proliferation of intimal smooth muscle cells, and the grafts eventually fail mainly because of the resulting ischemic damage (Fig. 4). The arterial changes are called graft vasculopathy or accelerated graft arteriosclerosis (see Fig. 14B). Graft vasculopathy is frequently seen in failed cardiac and renal allografts and can develop in any vascularized organ transplant within 6 months to a year after transplantation. The likely mechanisms underlying the occlusive vascular lesions of chronic rejection are activation of alloreactive T cells and secretion of IFN-γ and other cytokines that stimulate proliferation of vascular smooth muscle cells. As the arterial lesions of graft arteriosclerosis progress, blood flow to the graft parenchyma is compromised, and the parenchyma is slowly replaced by nonfunctioning fibrous tissue (see Fig. 4C). The interstitial fibrosis seen in chronic rejection also may be a repair response to parenchymal cell damage caused by repeated bouts of acute antibody-mediated or cellular rejection, perioperative ischemia, toxic effects of immunosuppressive drugs, and even chronic viral infections. Chronic rejection leads to congestive heart failure or arrhythmias in cardiac transplant patients or loss of glomerular and tubular function and renal failure in kidney transplant patients.
Fig4. Chronic rejection. (A) In chronic rejection with graft arteriosclerosis, injury to the vessel wall leads to intimal smooth muscle cell proliferation and luminal occlusion. This lesion may be caused by a chronic inflammatory reaction to alloantigens in the vessel wall. (B) Chronic rejection in a kidney allograft with graft arteriosclerosis. The vascular lumen is replaced by an accumulation of smooth muscle cells and connective tissue in the vessel intima. (C) Fibrosis and loss of tubules in a kidney with chronic rejection (lower left) adjacent to relatively normal kidney tissue (upper right). The blue area shows fibrosis, and an artery with graft arteriosclerosis is present (bottom right). APC, Antigen-presenting cell. B, Courtesy Dr. Helmut Rennke, Department of Pathology, Brigham and Women’s Hospital, Boston, Massachusetts. C, Courtesy Dr. Zoltan Laszik, Department of Pathology, University of California, San Francisco.
