Immunosuppression — The Promise of Specificity
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《新英格兰医药杂志》
The success of organ transplantation owes much to improvements in the immunosuppressive regimens that prevent or suppress allograft rejection. Nevertheless, the potent immunosuppressive drugs that are now in general use increase susceptibility to infection and cancer and can also have adverse effects not directly related to immunosuppression.
Conventional immunosuppressive agents affect not only immune cells but also other cells. Glucocorticoids, for example, can cause a myriad of side effects,1 but these harmful actions are often minimized by combining glucocorticoids with other immunosuppressive medications. In use since the early days of transplantation, small-molecule immunosuppressive drugs act by targeting DNA or proteins within the cell. The antimetabolite azathioprine, the earliest widely used immunosuppressive drug in transplantation, interferes with DNA synthesis and also causes apoptosis.2 The combination of azathioprine with glucocorticoids made renal transplantation a realistic option for the treatment of end-stage renal disease. For the past 15 years, the mainstay of immunosuppression has been the calcineurin antagonists cyclosporine and tacrolimus. Their immunosuppressive effects can be unpredictable, making monitoring of drug levels essential.3,4 Furthermore, nephrotoxicity caused by calcineurin inhibitors is a major problem; it can lead to a decrease in the glomerular filtration rate, chronic allograft nephropathy, and graft failure. The antiproliferative agents mycophenolate mofetil5 and sirolimus (formerly known as rapamycin),6 introduced in the 1990s, have helped decrease the rate of acute rejection.4 The lymphocyte-depleting antibodies antilymphocyte globulin, muromonab-CD3, and antithymocyte globulin have been used for decades, but the combination of these immunosuppressive antibodies with calcineurin antagonists is associated with an increased risk of post-transplantation lymphoproliferative disorders, including malignant lymphoma.
For all these reasons, there is a pressing need for selective, specific, and relatively nontoxic immunosuppressive agents. The immune system itself holds a clue to what may be a new era in immunosuppression. To understand what lies ahead, it is important to understand that many different molecules and signaling networks cooperate to activate and regulate T cells when they encounter a display of immunogenic peptides by antigen-presenting cells (Figure 1).
Figure 1. The Two-Signal Hypothesis.
In signal 1, the major histocompatibility complex (MHC) on the antigen-presenting cell interacts with the T-cell receptor. B7-1 (CD80) and B7-2 (CD86) on the antigen-presenting cell interact with their respective ligands, CD28 and CTLA4 (CD152). CD28 and CTLA4 share common motifs (MYPPY) that are essential for binding B7-1 and B7-2. Regulatory molecules, such as programmed death 1 (PD-1) and inducible costimulator (ICOS), which have different motifs, enter the regulatory cycle by affecting experienced T cells. CD28 and CTLA4 have similar structures but opposite functions. CD28 activates T cells, whereas CTLA4 inhibits them. PD-L1 denotes programmed death ligand 1, and PD-L2 programmed death ligand 2. Adapted from Sharpe and Freeman.7
That more than one signal is needed to activate the immune system was first advanced in 1970 by Peter Bretscher and Melvin Cohn,8 who proposed that activation of the immune system by an antigen requires not only the antigen itself (signal 1), but also a signal from a less specific molecule (signal 2). Bretscher and Cohn further postulated that if a lymphocyte received just the antigen-specific signal, it would not only fail to respond but also fall into a state of inactivity, now termed "anergy."8 In the ensuing 35 years, delineating the molecules and pathways leading to activation or dormancy of the immune system — the core of the two-signal hypothesis — has been at the epicenter of research in immunology. Although the immunologic jargon of 1970 differs from that of today, all the experimental evidence supports the basic tenets of the two-signal hypothesis. Moreover, work on the regulation of immune responses by positive and negative signals has culminated in promising clinical applications for the treatment of autoimmune diseases and for use in transplantation.
The two-signal system, a complex array of regulatory molecules called costimulatory molecules, is no more intricate than could be expected for a network that has two vital functions: continuously informing the immune system that an antigen is foreign or self and slowing down the clonal expansion of T cells that follows contact with a foreign antigen. The regulatory molecules of this system occur mainly on antigen-presenting cells (macrophages and dendritic cells) and T cells (Figure 1). The principal regulatory molecules of antigen-presenting cells are members of the B7 family — namely, B7-1 (CD80) and B7-2 (CD86). T cells display the ligands for these B7 molecules, CD28, and cytotoxic T-lymphocyte–associated antigen 4 (CTLA4). The B7–CD28 system regulates T cells mainly during their initial encounter with antigen, whereas recently discovered regulatory molecules, such as programmed death 1 (PD-1) and inducible costimulator (ICOS), enter the regulatory cycle by affecting experienced T cells. CD28 and CTLA4 have similar structures but opposite functions. CD28 activates T cells, whereas CTLA4 inhibits them.4,8
The four main steps in the series of activation events that follow the display of an immunogenic peptide on the surface of an antigen-presenting cell are up-regulation of B7 (B7-2 initially and then B7-1), presentation of the peptide to the T cell's antigen receptor, joining of B7 molecules with the T cell's CD28 molecules, and enhancement of the production of interleukin-2 (a T cell–stimulatory cytokine) and T-cell proliferation by signals from the B7–CD28 complex.4,8 Inhibition of the system ensues when the activated T cell displays CTLA4. This inhibitory molecule probably acts by competing with CD28 for B7 and winning out because it has a higher affinity for B7. The capture of B7 by CTLA4 into a zipper-like structure closes down B7–CD28 signaling, thereby inhibiting the production of interleukin-2 and T-cell proliferation. In effect, the entire system is a closed circuit of self-regulation: presentation of antigenic peptides, production of activating signals, T-cell proliferation, and finally, a new balance induced by inhibitory molecules. Some have also called the cytokine interaction that activates the mammalian target of rapamycin, leading to cell proliferation, a third signal, adding further to the complexity of the system.4 Clearly, these signaling mechanisms and other aspects of the immune response, such as T-cell transport, are targets for the development of selective immunosuppressive compounds. An important example is CTLA4, which has been made into a pharmacologically useful soluble form by fusing it to the constant region of the heavy chain of IgG1 (CTLA4Ig). This fusion molecule has already been shown to have beneficial effects in patients with rheumatoid arthritis and psoriasis. In this issue of the Journal, Vincenti et al. report their results with belatacept, a variant of CTLA4Ig.9
Belatacept (LEA29Y) was developed in response to early data indicating that CTLA4Ig (abatacept) often had unpredictable effects.10 To improve on abatacept, two amino acid substitutions were introduced into the CTLA4 domain of the fusion protein. This new protein, belatacept, has 10 times the binding avidity for CD80 and CD86 as the standard CTLA4Ig.10
In their multicenter study,9 Vincenti et al. compared the ability of belatacept and cyclosporine to prevent rejection of renal allografts. Initially, all patients received two doses of basiliximab, an anti–interleukin-2 receptor antibody, followed by adjunctive maintenance therapy with mycophenolate mofetil and glucocorticoids. Participants were randomly assigned to receive cyclosporine or an intensive or less-intensive regimen of belatacept. The belatacept was administered in two phases, a regimen that reflects the increased risk of rejection during the first 90 days after transplantation. The first phase involved a dose of belatacept of 10 mg per kilogram of body weight and lasted six months in the intensive regimen and three months in the less-intensive regimen. In the second phase, 5 mg of belatacept per kilogram was given every four weeks in the intensive regimen and every eight weeks in the less-intensive regimen. The investigators found that both regimens of belatacept were not inferior to cyclosporine in the prevention of episodes of acute rejection in patients with renal allografts. They also found evidence that glomerular filtration may be preserved, suggesting that belatacept may be less nephrotoxic than cyclosporine.
The types and severity of side effects appeared to be similar in the three groups. After one year of treatment, there appeared to be a lower incidence of both the metabolic syndrome and type 2 diabetes in the belatacept groups than in the cyclosporine group, but these results are preliminary. The rates of infections and neoplasms were similar in all three groups, but there were three cases of post-transplantation lymphoproliferative disorder in the belatacept groups. After four years of follow-up, no further cases of post-transplantation lymphoproliferative disorder have been reported; nevertheless, the occurrence of this complication warns us that belatacept, despite its specific point of action, may not be as specific an immunosuppressant as we would hope. Moreover, whether the introduction of belatacept will change clinical practice in renal transplantation is unknown. Its ability to prevent graft rejection seems similar to that of cyclosporine, although a major advantage is the absence of nephrotoxicity. We will need much more experience with belatacept before recommending it as a routine replacement for cyclosporine.
Will belatacept and some of the other new immunosuppressive agents being developed improve the outcomes of organ transplantation? It is too early to tell, but it is obvious that specific immunosuppressive reagents or manipulations that lead the immune system down the pathway toward immunologic tolerance of tissue antigens in the graft would go far in giving transplant recipients a normal life.
References
Karin M. New twists in gene regulation by glucocorticoid receptor: is DNA binding dispensable? Cell 1998;93:487-490.
Elion GB. The George Hitchings and Gertrude Elion Lecture: the pharmacology of azathioprine. Ann N Y Acad Sci 1993;685:400-407.
Clipstone NA, Crabtree GR. Identification of calcineurin as a key signalling enzyme in T-lymphocyte activation. Nature 1992;357:695-697. [CrossRef][ISI][Medline]
Halloran PF. Immunosuppressive drugs for kidney transplantation. N Engl J Med 2004;351:2715-2729.
Sollinger HW. Mycophenolate mofetil for the prevention of acute rejection in primary cadaveric renal allograft recipients. Transplant 1995;60:225-32.
Vezina C, Kudelski A, Sehgal SN. Rapamycin (AY-22,989), a new antifungal antibiotic. I. Taxonomy of the producing streptomycete and isolation of the active principle. J Antibiot (Tokyo) 1975;28:721-726.
Sharpe AH, Freeman GJ. The B7-CD28 superfamily. Nat Rev Immunol 2002;2:116-126.
Bretscher P, Cohn M. A theory of self-nonself discrimination. Science 1970;169:1042-1049.
Vincenti F, Larsen C, Durrbach A, et al. Costimulation blockade with belatacept in renal transplantation. N Engl J Med 2005;353:770-781.
Larsen CP, Pearson TC, Adams AB, et al. Rational development of LEA29Y (belatacept), a high-affinity variant of CTLA4-Ig with potent immunosuppressive properties. Am J Transplant 2005;5:443-453.(Julie R. Ingelfinger, M.D)
Conventional immunosuppressive agents affect not only immune cells but also other cells. Glucocorticoids, for example, can cause a myriad of side effects,1 but these harmful actions are often minimized by combining glucocorticoids with other immunosuppressive medications. In use since the early days of transplantation, small-molecule immunosuppressive drugs act by targeting DNA or proteins within the cell. The antimetabolite azathioprine, the earliest widely used immunosuppressive drug in transplantation, interferes with DNA synthesis and also causes apoptosis.2 The combination of azathioprine with glucocorticoids made renal transplantation a realistic option for the treatment of end-stage renal disease. For the past 15 years, the mainstay of immunosuppression has been the calcineurin antagonists cyclosporine and tacrolimus. Their immunosuppressive effects can be unpredictable, making monitoring of drug levels essential.3,4 Furthermore, nephrotoxicity caused by calcineurin inhibitors is a major problem; it can lead to a decrease in the glomerular filtration rate, chronic allograft nephropathy, and graft failure. The antiproliferative agents mycophenolate mofetil5 and sirolimus (formerly known as rapamycin),6 introduced in the 1990s, have helped decrease the rate of acute rejection.4 The lymphocyte-depleting antibodies antilymphocyte globulin, muromonab-CD3, and antithymocyte globulin have been used for decades, but the combination of these immunosuppressive antibodies with calcineurin antagonists is associated with an increased risk of post-transplantation lymphoproliferative disorders, including malignant lymphoma.
For all these reasons, there is a pressing need for selective, specific, and relatively nontoxic immunosuppressive agents. The immune system itself holds a clue to what may be a new era in immunosuppression. To understand what lies ahead, it is important to understand that many different molecules and signaling networks cooperate to activate and regulate T cells when they encounter a display of immunogenic peptides by antigen-presenting cells (Figure 1).
Figure 1. The Two-Signal Hypothesis.
In signal 1, the major histocompatibility complex (MHC) on the antigen-presenting cell interacts with the T-cell receptor. B7-1 (CD80) and B7-2 (CD86) on the antigen-presenting cell interact with their respective ligands, CD28 and CTLA4 (CD152). CD28 and CTLA4 share common motifs (MYPPY) that are essential for binding B7-1 and B7-2. Regulatory molecules, such as programmed death 1 (PD-1) and inducible costimulator (ICOS), which have different motifs, enter the regulatory cycle by affecting experienced T cells. CD28 and CTLA4 have similar structures but opposite functions. CD28 activates T cells, whereas CTLA4 inhibits them. PD-L1 denotes programmed death ligand 1, and PD-L2 programmed death ligand 2. Adapted from Sharpe and Freeman.7
That more than one signal is needed to activate the immune system was first advanced in 1970 by Peter Bretscher and Melvin Cohn,8 who proposed that activation of the immune system by an antigen requires not only the antigen itself (signal 1), but also a signal from a less specific molecule (signal 2). Bretscher and Cohn further postulated that if a lymphocyte received just the antigen-specific signal, it would not only fail to respond but also fall into a state of inactivity, now termed "anergy."8 In the ensuing 35 years, delineating the molecules and pathways leading to activation or dormancy of the immune system — the core of the two-signal hypothesis — has been at the epicenter of research in immunology. Although the immunologic jargon of 1970 differs from that of today, all the experimental evidence supports the basic tenets of the two-signal hypothesis. Moreover, work on the regulation of immune responses by positive and negative signals has culminated in promising clinical applications for the treatment of autoimmune diseases and for use in transplantation.
The two-signal system, a complex array of regulatory molecules called costimulatory molecules, is no more intricate than could be expected for a network that has two vital functions: continuously informing the immune system that an antigen is foreign or self and slowing down the clonal expansion of T cells that follows contact with a foreign antigen. The regulatory molecules of this system occur mainly on antigen-presenting cells (macrophages and dendritic cells) and T cells (Figure 1). The principal regulatory molecules of antigen-presenting cells are members of the B7 family — namely, B7-1 (CD80) and B7-2 (CD86). T cells display the ligands for these B7 molecules, CD28, and cytotoxic T-lymphocyte–associated antigen 4 (CTLA4). The B7–CD28 system regulates T cells mainly during their initial encounter with antigen, whereas recently discovered regulatory molecules, such as programmed death 1 (PD-1) and inducible costimulator (ICOS), enter the regulatory cycle by affecting experienced T cells. CD28 and CTLA4 have similar structures but opposite functions. CD28 activates T cells, whereas CTLA4 inhibits them.4,8
The four main steps in the series of activation events that follow the display of an immunogenic peptide on the surface of an antigen-presenting cell are up-regulation of B7 (B7-2 initially and then B7-1), presentation of the peptide to the T cell's antigen receptor, joining of B7 molecules with the T cell's CD28 molecules, and enhancement of the production of interleukin-2 (a T cell–stimulatory cytokine) and T-cell proliferation by signals from the B7–CD28 complex.4,8 Inhibition of the system ensues when the activated T cell displays CTLA4. This inhibitory molecule probably acts by competing with CD28 for B7 and winning out because it has a higher affinity for B7. The capture of B7 by CTLA4 into a zipper-like structure closes down B7–CD28 signaling, thereby inhibiting the production of interleukin-2 and T-cell proliferation. In effect, the entire system is a closed circuit of self-regulation: presentation of antigenic peptides, production of activating signals, T-cell proliferation, and finally, a new balance induced by inhibitory molecules. Some have also called the cytokine interaction that activates the mammalian target of rapamycin, leading to cell proliferation, a third signal, adding further to the complexity of the system.4 Clearly, these signaling mechanisms and other aspects of the immune response, such as T-cell transport, are targets for the development of selective immunosuppressive compounds. An important example is CTLA4, which has been made into a pharmacologically useful soluble form by fusing it to the constant region of the heavy chain of IgG1 (CTLA4Ig). This fusion molecule has already been shown to have beneficial effects in patients with rheumatoid arthritis and psoriasis. In this issue of the Journal, Vincenti et al. report their results with belatacept, a variant of CTLA4Ig.9
Belatacept (LEA29Y) was developed in response to early data indicating that CTLA4Ig (abatacept) often had unpredictable effects.10 To improve on abatacept, two amino acid substitutions were introduced into the CTLA4 domain of the fusion protein. This new protein, belatacept, has 10 times the binding avidity for CD80 and CD86 as the standard CTLA4Ig.10
In their multicenter study,9 Vincenti et al. compared the ability of belatacept and cyclosporine to prevent rejection of renal allografts. Initially, all patients received two doses of basiliximab, an anti–interleukin-2 receptor antibody, followed by adjunctive maintenance therapy with mycophenolate mofetil and glucocorticoids. Participants were randomly assigned to receive cyclosporine or an intensive or less-intensive regimen of belatacept. The belatacept was administered in two phases, a regimen that reflects the increased risk of rejection during the first 90 days after transplantation. The first phase involved a dose of belatacept of 10 mg per kilogram of body weight and lasted six months in the intensive regimen and three months in the less-intensive regimen. In the second phase, 5 mg of belatacept per kilogram was given every four weeks in the intensive regimen and every eight weeks in the less-intensive regimen. The investigators found that both regimens of belatacept were not inferior to cyclosporine in the prevention of episodes of acute rejection in patients with renal allografts. They also found evidence that glomerular filtration may be preserved, suggesting that belatacept may be less nephrotoxic than cyclosporine.
The types and severity of side effects appeared to be similar in the three groups. After one year of treatment, there appeared to be a lower incidence of both the metabolic syndrome and type 2 diabetes in the belatacept groups than in the cyclosporine group, but these results are preliminary. The rates of infections and neoplasms were similar in all three groups, but there were three cases of post-transplantation lymphoproliferative disorder in the belatacept groups. After four years of follow-up, no further cases of post-transplantation lymphoproliferative disorder have been reported; nevertheless, the occurrence of this complication warns us that belatacept, despite its specific point of action, may not be as specific an immunosuppressant as we would hope. Moreover, whether the introduction of belatacept will change clinical practice in renal transplantation is unknown. Its ability to prevent graft rejection seems similar to that of cyclosporine, although a major advantage is the absence of nephrotoxicity. We will need much more experience with belatacept before recommending it as a routine replacement for cyclosporine.
Will belatacept and some of the other new immunosuppressive agents being developed improve the outcomes of organ transplantation? It is too early to tell, but it is obvious that specific immunosuppressive reagents or manipulations that lead the immune system down the pathway toward immunologic tolerance of tissue antigens in the graft would go far in giving transplant recipients a normal life.
References
Karin M. New twists in gene regulation by glucocorticoid receptor: is DNA binding dispensable? Cell 1998;93:487-490.
Elion GB. The George Hitchings and Gertrude Elion Lecture: the pharmacology of azathioprine. Ann N Y Acad Sci 1993;685:400-407.
Clipstone NA, Crabtree GR. Identification of calcineurin as a key signalling enzyme in T-lymphocyte activation. Nature 1992;357:695-697. [CrossRef][ISI][Medline]
Halloran PF. Immunosuppressive drugs for kidney transplantation. N Engl J Med 2004;351:2715-2729.
Sollinger HW. Mycophenolate mofetil for the prevention of acute rejection in primary cadaveric renal allograft recipients. Transplant 1995;60:225-32.
Vezina C, Kudelski A, Sehgal SN. Rapamycin (AY-22,989), a new antifungal antibiotic. I. Taxonomy of the producing streptomycete and isolation of the active principle. J Antibiot (Tokyo) 1975;28:721-726.
Sharpe AH, Freeman GJ. The B7-CD28 superfamily. Nat Rev Immunol 2002;2:116-126.
Bretscher P, Cohn M. A theory of self-nonself discrimination. Science 1970;169:1042-1049.
Vincenti F, Larsen C, Durrbach A, et al. Costimulation blockade with belatacept in renal transplantation. N Engl J Med 2005;353:770-781.
Larsen CP, Pearson TC, Adams AB, et al. Rational development of LEA29Y (belatacept), a high-affinity variant of CTLA4-Ig with potent immunosuppressive properties. Am J Transplant 2005;5:443-453.(Julie R. Ingelfinger, M.D)