- Email: [email protected]
What can a worm teach us about autoimmunity?
for the tacrolimus and cyclosporin arms, respectively. Similarly, reanalysis of freedom from rejection after restoration of all censored categories did not change the results, in both the original report6 or in the Viewpoint’s reanalysis. By comparison with cyclosporin, 7% more tacrolimus-treated patients were free from rejection at 1 year in both the original report6 and in the reanalysis presented in this week’s Viewpoint. The same general conclusions about the relative immunosuppressive efficacies of tacrolimus-based and cyclosporin-based therapies apply to the data from the Pittsburgh randomised single-centre controlled trials5 and from the US6 and European’ randomised multicentre trials. The superiority of tacrolimus measured by freedom from rejection was substantially greater, however, in the Pittsburgh trial than in the multicentre trials. In the Pittsburgh and US studies, the immunosuppressive
superiority of tacrolimus over cyclosporin-based therapy nearly the same when the reduced need for OKT3 determined. Compared with the cyclosporin was was
cumulative oral’ or intravenous5°’ less in patients treated with tacrolimus. optimum dosing and blood concentration monitoring regimens are used, one can conclude from all three randomised controlled trials5,7,1O that an increased incidence of severe toxicity is not necessarily a penalty for the reduced incidence and severity of liver allograft rejection afforded by treatment with tacrolimus. In view of the overall agreement of the general conclusions derived from the Pittsburgh, US, and European controlled trials, are we now compelled to condemn the randomised multicentre trials as "cruel" and "unethical"? In retrospect, perhaps an argument could have been made for rapid approval of tacrolimus as rescue therapy, since tacrolimus seems to have increased patient survival.’ Early approval of tacrolimus for rescue therapy, however, could have encouraged its use for non-approved indications. Misuse of tacrolimus by those less experienced than the Pittsburgh investigators might have led to its withdrawal from use and the end of its commercial development. The narrow therapeutic index of tacrolimus seems to us to have warranted experience beyond its use for rescue therapy. The Pittsburgh randomised controlled primary prevention trial in liver allograft patients provided valuable information on the efficacy and safety of lowered tacrolimus doses. This single-site trial, however, enrolled a relatively small number of patients, did not require that biopsy-proven evidence define rejection, and was unfinished when the multicentre trials began. In our opinion, these considerations provided an acceptable and even persuasive rationale for proceeding with the multicentre trials. We should remember that, without the perseverance by the Pittsburgh group in the face of claims about the toxicity of tacrolimus, we would not have the existing luxury of current availibility. We hope that concepts raised in the Viewpoint and by us will help those who accept the challenge of designing and conducting clinical treatment
steroid When
Starzl TE. Closing remarks. Transplant Proc 1991; 22: 113. Kahan BD. New immunosuppressive drugs—pharmacologic approaches to alter immunoregulation. Ther Immunol 1994; 1: 33-44. 3 Nossal GJV. Summary of the first international FK 506 congress: perspectives and prospects. Transplant Proc 1991; 23: 3371-75. 4 Starzl TE, Todo S, Fung J, Demetris AJ, Venkataramman R, Jain A. FK 506 for liver, kidney, and pancreas transplantation. Lancet 1989; ii: 1000-04. 5 Fung J, Abu-Elmagd K, Jain A, et al. A randomized trial of primary liver transplantation under immunosuppression with FK 506 vs cyclosporine. Transplant Proc 1991; 23: 2977-83. 6 The US Multicentre FK 506 Liver Study Group. A comparison of tacrolimus (FK 506) and cyclosporine for immunosuppression in liver transplantation. N Engl J Med 1994; 331: 1110-15. 7 European FK506 Liver Study Group. Randomised trial comparing tacrolimus (FK506) and cyclosporin in prevention of liver allograft rejection. Lancet 1994; 344: 423-28. 8 Todo S, Fung JJ, Tzakis AJ, et al. One hundred ten consecutive primary orthotopic liver transplants under FK 506 in adults. Transplant Proc 1991; 23: 1397-402. 9 Steinmuller DR. FK506 and organ transplantation. Austin, TX: RG Landes Co, 1994: 1-111. 10 Klintmalm G. A review of FK506: a new immunosuppressive agent for the prevention and rescue of graft rejection. Transplant Rev 1994; 8: 53-63. 1
2
groups,
use was
trials of newer
immunosuppressants. Randall E Morris, Byron Wm Brown Jr Laboratory for Transplantation Immunology, Department
of
Cardiothoracic Surgery; and Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California, USA
What
can a worm
teach
us
about
autoimmunity? and potentially important insight into the pathogenesis of autoimmunity has emerged from a study by Gallin and colleagues’ of a filarial worm responsible for blindness and skin inflammation-Onchocerca volvulus. Gallin et al have identified a targeted autoantigen from neutrophils, the cells that are naturally drawn in to clear the infection. The subsequent cascade of molecular and cellular events may lead to the chronic granulomatous skin inflammation associated with this disease. But why are the antibodies generated to the self-antigen in the first
A novel
place? The involve
pathogenesis of several diseases is thought
to
autoimmune process initiated as a result of exposure to an infectious organism. In many cases the identity of the organism is not entirely clear and the evidence of a link is circumstantial and based on sequence homology between proteins of the organism and putative target autoantigens.’ For other diseases, the initiating organism may be known but the immunological mechanisms leading to autoimmunity have not been resolved. Thus rheumatic fever follows a group A streptococcal infection but the initiating bacterial antigen and the target autoantigen are not defined. Here, T cells3 and antibodies4 are thought to have important pathogenic roles. Of great interest in all these diseases is the mechanism that leads to the generation of an immune response to self-antigens. Most self-reactive clones of T and B cells are efficiently eliminated during ontogeny. However, some self-antigens or epitopes may never be presented to the immune system, and, while potentially self-reactive an
lymphocytes
occur,
they
never
encounter
self-epitopes.
This is referred to as immunological "ignorance". For other antigens, there may be tolerance of self-reactive T cells but not B cells.6 Since specific T cells are required to help B cells make high-affinity antibody, autoantibodies are never produced or are of low affinity and biologically irrelevant. In both these situations tolerance can be "broken". In the first example this may occur by
upregulated
or
inappropriate antigen processing
or
1311
presentation,leading to exposure of mature lymphocytes to self-antigens or epitopes (the antigenic determinants within an antigen) that were not available for presentation to the lymphocytes during their development. In the second situation, autoantibodies may be produced after exposure to a foreign organism that mimics the region of the antigen (epitope) recognised by B cells. If the foreign antigen can also stimulate helper T cells, then these helper T lymphocytes specific for the foreign antigen can provide help for B cells specific for a self-epitope, resulting in the production of autoantibodies. This latter mechanism may explain the production of human autoantibodies to the neutrophil antimicrobial peptide, defensin, as a result of infection with 0 volvulus.’ This parasite, which is transmitted by a black fly, infects millions of people in Africa and Central and South America, causing skin and ocular lesions ("river blindness"). All patients with one form of this disease, sowda, characterised by severe dermatitis with granulomas, vasculitis, and eosinophilia, have antibodies to defensin, compared with 16-52% of individuals with exposure or with other forms of the disease.’ Whether these antibodies are directly responsible for the disease manifestations is unclear, but antibodies to other neutrophil proteins may contribute to the pathogenesis of the closely related group of ANCA (anti-neutrophil cytoplasmic antibodies) diseases,8 by blocking the function of the antimicrobial peptides and thereby promoting an accumulation of inflammatory cells. In sowda, defensin is closely associated with the surface of the parasite; this observation suggests that macromolecular complexes may have formed between defensin and parasite antigens. If so, helper T cells specific for parasite antigen may provide help for B cells specific for defensin. Much seems to depend on the stability of such complexes since T and B cell epitopes must be taken up by the same B cell in order for it to receive specific T-cell help and so produce antibody. 0 volvulus had already delivered some insights into disease mechanisms in autoimmunity. Mimicry by an antigen of the worm of an antigen from retinal pigment epithelium may, via a similar mechanism, lead to immunological cross-reactivity and a pathogenic response in the eye contributing to the blindness associated with this infection.9 In this case, the inciting antigen is a parasite-encoded antigen, which is unlikely for the sowda
example. 17ULIr-IlLN
mu SVWUi;t mavc a
llt::IgllLt::l1t::U IllllllUlluIuglLal
responsiveness to parasiteantigens andcorrespondingly
responsiveness parasite antigens and correspondingly have very few microfilarial parasites in their skin. By contrast, endemic controls (immune individuals) have good cell-mediated immune responses but no detectable parasites, and individuals with the generalised form of onchocerciasis have many parasites but immunological hyporesponsiveness. While the pathology of the sowda form of the disease is associated with immunological hyperresponsiveness, the factors dictating this outcome are unknown. In various systems, high antigen dose is associated with low cell-mediated immunity and low antigen dose with a heightened cell-mediated immunity. For example, high-dose but not low-dose lymphocytic choriomeningitis virus infection of mice can exhaust or tolerise the specific T-cell response," and we have made similar observations with malaria in a rodent model two
1312
(unpublished). In all these diseases, the T-cell response is associated with abnormalities and a high antigen dose could have the potential to stimulate T cells excessively to the detriment of the host. However, T-cell cytokines" and nitric oxidel2 are capable of self-regulating the T-cell response, and the anergy or immunological hyporesponsiveness associated with excessive antigen may reflect self-regulation as a result of these or other factors. Such self-regulation also occurs in onchocerciasis, but in individuals with sowda, the heightened immune response can almost, but not quite, eliminate the infection. Gallin and colleagues’ may have identified an important factor contributing to this immunological lesion. Perhaps the development of anti-defensin antibodies as a result of tolerance-breaking parasite-defensin complexes impedes the effector arm of the immune response enough to allow the maintenance of low numbers of parasites, a consequent heightened immune response, and ensuing disease.
Michael F Good Queensland Institute of Medical Research, Brisbane, Australia
2
Gallin MY, Jacobi AB, Buttner DW, Schonberger O, Marti T, Erttmann KD. Human autoantibody to defensin: disease association with hyperreactive onchocerciasis (sowda). J Exp Med 1995; 182: 41-47. Oldstone MBA. Molecular mimicry and autoimmune disease. Cell
3
Pruksakon S, Currie B, Brandt E,
1
1987; 50: 819-20. et
al. Identification of T-cell
autoepitopes that cross-react with the carboxylterminal segment of the M protein of group A streptococci. Int Immunol 1994; 6: 1235-44. 4 Bronze MS, Beachey EH, Dale JB. Protective and heart-crossreactive epitopes located within the NH2 terminus of type 19 streptococcal M protein. J Exp Med 1988; 167: 1849-59. 5 Nossal GJV. Cellular mechanisms of immunologic tolerance. 6
7
Annu Rev Immunol 1983; 1: 33-62. Pombo D, Maloy WL, Berzofsky JA, Good MF. Neonatal exposure to immunogenic peptides: differential susceptibility to tolerance induction of helper T cells and B cells reactive to malaria circumsporozoite peptide epitopes. J Immunol 1988; 140: 3594-98. Lanzavecchia A. How can cryptic epitopes trigger autoimmunity?
J Exp Med 1995; 181: 1945-48. Gross WL, Schmitt WH, Csernok E. ANCA and associated diseases: and pathogenetic aspects. Clin Exp Immunol 1993; 91: 1-12. 9 Braun G, McKechnie NM, Connor V, et al. Immunological crossreactivity between a cloned antigen of Onchocerca volvulus and a component of the retinal pigment epithelium. J Exp Med 1991; 174; 169-77. 10 Moskophidis D, Lechner F, Pircher H, Zinkernagel RM. Virus persistence in acutely infected immunocompetent mice by exhaustion of antiviral cytotoxic effector T cells. Nature 1993; 362: 758-61. 11 Seder RA, Paul WE. Acquisition of lymphokine-producing phenotype by CD4+ T cells. Annu Rev Immunol 1994; 12: 635-73. 12 Taylor-Robinson AW, Liew FY, Severn A, et al. Regulation of the immune response by nitric oxide differentially produced by T helper type 1 and T helper type 2 cells. Eur J Immunol 1994; 24: 980-84. 8
immunodiagnostic
Herodotus and the
multidisciplinary clinic
scientific analysis, decisiveness, and compassion are the supposed hallmarks of the modem physician. Yet the image may be at odds with the facts when it comes to providing care for patients with complex medical problems. We know a lot, to be sure, much of it derived from clinical trials. But information learnt from groups, whatever the statistical certainty, is not always readily translated to the individual. For example, the randomised trial experience of 120 000 women with breast cancer does not make for particularly straightforward recommendations for the 50-year-old patient with four positive nodes and lupus nephritis, or Clear-minded
superiority of tacrolimus over cyclosporin-based therapy nearly the same when the reduced need for OKT3 determined. Compared with the cyclosporin was was
cumulative oral’ or intravenous5°’ less in patients treated with tacrolimus. optimum dosing and blood concentration monitoring regimens are used, one can conclude from all three randomised controlled trials5,7,1O that an increased incidence of severe toxicity is not necessarily a penalty for the reduced incidence and severity of liver allograft rejection afforded by treatment with tacrolimus. In view of the overall agreement of the general conclusions derived from the Pittsburgh, US, and European controlled trials, are we now compelled to condemn the randomised multicentre trials as "cruel" and "unethical"? In retrospect, perhaps an argument could have been made for rapid approval of tacrolimus as rescue therapy, since tacrolimus seems to have increased patient survival.’ Early approval of tacrolimus for rescue therapy, however, could have encouraged its use for non-approved indications. Misuse of tacrolimus by those less experienced than the Pittsburgh investigators might have led to its withdrawal from use and the end of its commercial development. The narrow therapeutic index of tacrolimus seems to us to have warranted experience beyond its use for rescue therapy. The Pittsburgh randomised controlled primary prevention trial in liver allograft patients provided valuable information on the efficacy and safety of lowered tacrolimus doses. This single-site trial, however, enrolled a relatively small number of patients, did not require that biopsy-proven evidence define rejection, and was unfinished when the multicentre trials began. In our opinion, these considerations provided an acceptable and even persuasive rationale for proceeding with the multicentre trials. We should remember that, without the perseverance by the Pittsburgh group in the face of claims about the toxicity of tacrolimus, we would not have the existing luxury of current availibility. We hope that concepts raised in the Viewpoint and by us will help those who accept the challenge of designing and conducting clinical treatment
steroid When
Starzl TE. Closing remarks. Transplant Proc 1991; 22: 113. Kahan BD. New immunosuppressive drugs—pharmacologic approaches to alter immunoregulation. Ther Immunol 1994; 1: 33-44. 3 Nossal GJV. Summary of the first international FK 506 congress: perspectives and prospects. Transplant Proc 1991; 23: 3371-75. 4 Starzl TE, Todo S, Fung J, Demetris AJ, Venkataramman R, Jain A. FK 506 for liver, kidney, and pancreas transplantation. Lancet 1989; ii: 1000-04. 5 Fung J, Abu-Elmagd K, Jain A, et al. A randomized trial of primary liver transplantation under immunosuppression with FK 506 vs cyclosporine. Transplant Proc 1991; 23: 2977-83. 6 The US Multicentre FK 506 Liver Study Group. A comparison of tacrolimus (FK 506) and cyclosporine for immunosuppression in liver transplantation. N Engl J Med 1994; 331: 1110-15. 7 European FK506 Liver Study Group. Randomised trial comparing tacrolimus (FK506) and cyclosporin in prevention of liver allograft rejection. Lancet 1994; 344: 423-28. 8 Todo S, Fung JJ, Tzakis AJ, et al. One hundred ten consecutive primary orthotopic liver transplants under FK 506 in adults. Transplant Proc 1991; 23: 1397-402. 9 Steinmuller DR. FK506 and organ transplantation. Austin, TX: RG Landes Co, 1994: 1-111. 10 Klintmalm G. A review of FK506: a new immunosuppressive agent for the prevention and rescue of graft rejection. Transplant Rev 1994; 8: 53-63. 1
2
groups,
use was
trials of newer
immunosuppressants. Randall E Morris, Byron Wm Brown Jr Laboratory for Transplantation Immunology, Department
of
Cardiothoracic Surgery; and Department of Health Research and Policy, Stanford University School of Medicine, Stanford, California, USA
What
can a worm
teach
us
about
autoimmunity? and potentially important insight into the pathogenesis of autoimmunity has emerged from a study by Gallin and colleagues’ of a filarial worm responsible for blindness and skin inflammation-Onchocerca volvulus. Gallin et al have identified a targeted autoantigen from neutrophils, the cells that are naturally drawn in to clear the infection. The subsequent cascade of molecular and cellular events may lead to the chronic granulomatous skin inflammation associated with this disease. But why are the antibodies generated to the self-antigen in the first
A novel
place? The involve
pathogenesis of several diseases is thought
to
autoimmune process initiated as a result of exposure to an infectious organism. In many cases the identity of the organism is not entirely clear and the evidence of a link is circumstantial and based on sequence homology between proteins of the organism and putative target autoantigens.’ For other diseases, the initiating organism may be known but the immunological mechanisms leading to autoimmunity have not been resolved. Thus rheumatic fever follows a group A streptococcal infection but the initiating bacterial antigen and the target autoantigen are not defined. Here, T cells3 and antibodies4 are thought to have important pathogenic roles. Of great interest in all these diseases is the mechanism that leads to the generation of an immune response to self-antigens. Most self-reactive clones of T and B cells are efficiently eliminated during ontogeny. However, some self-antigens or epitopes may never be presented to the immune system, and, while potentially self-reactive an
lymphocytes
occur,
they
never
encounter
self-epitopes.
This is referred to as immunological "ignorance". For other antigens, there may be tolerance of self-reactive T cells but not B cells.6 Since specific T cells are required to help B cells make high-affinity antibody, autoantibodies are never produced or are of low affinity and biologically irrelevant. In both these situations tolerance can be "broken". In the first example this may occur by
upregulated
or
inappropriate antigen processing
or
1311
presentation,leading to exposure of mature lymphocytes to self-antigens or epitopes (the antigenic determinants within an antigen) that were not available for presentation to the lymphocytes during their development. In the second situation, autoantibodies may be produced after exposure to a foreign organism that mimics the region of the antigen (epitope) recognised by B cells. If the foreign antigen can also stimulate helper T cells, then these helper T lymphocytes specific for the foreign antigen can provide help for B cells specific for a self-epitope, resulting in the production of autoantibodies. This latter mechanism may explain the production of human autoantibodies to the neutrophil antimicrobial peptide, defensin, as a result of infection with 0 volvulus.’ This parasite, which is transmitted by a black fly, infects millions of people in Africa and Central and South America, causing skin and ocular lesions ("river blindness"). All patients with one form of this disease, sowda, characterised by severe dermatitis with granulomas, vasculitis, and eosinophilia, have antibodies to defensin, compared with 16-52% of individuals with exposure or with other forms of the disease.’ Whether these antibodies are directly responsible for the disease manifestations is unclear, but antibodies to other neutrophil proteins may contribute to the pathogenesis of the closely related group of ANCA (anti-neutrophil cytoplasmic antibodies) diseases,8 by blocking the function of the antimicrobial peptides and thereby promoting an accumulation of inflammatory cells. In sowda, defensin is closely associated with the surface of the parasite; this observation suggests that macromolecular complexes may have formed between defensin and parasite antigens. If so, helper T cells specific for parasite antigen may provide help for B cells specific for defensin. Much seems to depend on the stability of such complexes since T and B cell epitopes must be taken up by the same B cell in order for it to receive specific T-cell help and so produce antibody. 0 volvulus had already delivered some insights into disease mechanisms in autoimmunity. Mimicry by an antigen of the worm of an antigen from retinal pigment epithelium may, via a similar mechanism, lead to immunological cross-reactivity and a pathogenic response in the eye contributing to the blindness associated with this infection.9 In this case, the inciting antigen is a parasite-encoded antigen, which is unlikely for the sowda
example. 17ULIr-IlLN
mu SVWUi;t mavc a
llt::IgllLt::l1t::U IllllllUlluIuglLal
responsiveness to parasiteantigens andcorrespondingly
responsiveness parasite antigens and correspondingly have very few microfilarial parasites in their skin. By contrast, endemic controls (immune individuals) have good cell-mediated immune responses but no detectable parasites, and individuals with the generalised form of onchocerciasis have many parasites but immunological hyporesponsiveness. While the pathology of the sowda form of the disease is associated with immunological hyperresponsiveness, the factors dictating this outcome are unknown. In various systems, high antigen dose is associated with low cell-mediated immunity and low antigen dose with a heightened cell-mediated immunity. For example, high-dose but not low-dose lymphocytic choriomeningitis virus infection of mice can exhaust or tolerise the specific T-cell response," and we have made similar observations with malaria in a rodent model two
1312
(unpublished). In all these diseases, the T-cell response is associated with abnormalities and a high antigen dose could have the potential to stimulate T cells excessively to the detriment of the host. However, T-cell cytokines" and nitric oxidel2 are capable of self-regulating the T-cell response, and the anergy or immunological hyporesponsiveness associated with excessive antigen may reflect self-regulation as a result of these or other factors. Such self-regulation also occurs in onchocerciasis, but in individuals with sowda, the heightened immune response can almost, but not quite, eliminate the infection. Gallin and colleagues’ may have identified an important factor contributing to this immunological lesion. Perhaps the development of anti-defensin antibodies as a result of tolerance-breaking parasite-defensin complexes impedes the effector arm of the immune response enough to allow the maintenance of low numbers of parasites, a consequent heightened immune response, and ensuing disease.
Michael F Good Queensland Institute of Medical Research, Brisbane, Australia
2
Gallin MY, Jacobi AB, Buttner DW, Schonberger O, Marti T, Erttmann KD. Human autoantibody to defensin: disease association with hyperreactive onchocerciasis (sowda). J Exp Med 1995; 182: 41-47. Oldstone MBA. Molecular mimicry and autoimmune disease. Cell
3
Pruksakon S, Currie B, Brandt E,
1
1987; 50: 819-20. et
al. Identification of T-cell
autoepitopes that cross-react with the carboxylterminal segment of the M protein of group A streptococci. Int Immunol 1994; 6: 1235-44. 4 Bronze MS, Beachey EH, Dale JB. Protective and heart-crossreactive epitopes located within the NH2 terminus of type 19 streptococcal M protein. J Exp Med 1988; 167: 1849-59. 5 Nossal GJV. Cellular mechanisms of immunologic tolerance. 6
7
Annu Rev Immunol 1983; 1: 33-62. Pombo D, Maloy WL, Berzofsky JA, Good MF. Neonatal exposure to immunogenic peptides: differential susceptibility to tolerance induction of helper T cells and B cells reactive to malaria circumsporozoite peptide epitopes. J Immunol 1988; 140: 3594-98. Lanzavecchia A. How can cryptic epitopes trigger autoimmunity?
J Exp Med 1995; 181: 1945-48. Gross WL, Schmitt WH, Csernok E. ANCA and associated diseases: and pathogenetic aspects. Clin Exp Immunol 1993; 91: 1-12. 9 Braun G, McKechnie NM, Connor V, et al. Immunological crossreactivity between a cloned antigen of Onchocerca volvulus and a component of the retinal pigment epithelium. J Exp Med 1991; 174; 169-77. 10 Moskophidis D, Lechner F, Pircher H, Zinkernagel RM. Virus persistence in acutely infected immunocompetent mice by exhaustion of antiviral cytotoxic effector T cells. Nature 1993; 362: 758-61. 11 Seder RA, Paul WE. Acquisition of lymphokine-producing phenotype by CD4+ T cells. Annu Rev Immunol 1994; 12: 635-73. 12 Taylor-Robinson AW, Liew FY, Severn A, et al. Regulation of the immune response by nitric oxide differentially produced by T helper type 1 and T helper type 2 cells. Eur J Immunol 1994; 24: 980-84. 8
immunodiagnostic
Herodotus and the
multidisciplinary clinic
scientific analysis, decisiveness, and compassion are the supposed hallmarks of the modem physician. Yet the image may be at odds with the facts when it comes to providing care for patients with complex medical problems. We know a lot, to be sure, much of it derived from clinical trials. But information learnt from groups, whatever the statistical certainty, is not always readily translated to the individual. For example, the randomised trial experience of 120 000 women with breast cancer does not make for particularly straightforward recommendations for the 50-year-old patient with four positive nodes and lupus nephritis, or Clear-minded