- Email: [email protected]
And the beat goes on: Discovery of “new” autoantigens
400
EDITORIALS
References 1. Rutgeerts P, Geboes K, Vantrappen G, Kerremans R, Coenegrachts JL, Coremans G. Natural history of recurrent Crohn’s disease at the ileocolonic anastomosis after curative surgery. Gut 1984;25:665–672. 2. Pallone F, Boirivant M, Stazi MA, Cosintino R, Prantera C, Torsoli A. Analysis of clinical course of postoperative recurrence in Crohn’s disease of distal ileum. Dig Dis Sci 1992;37:215–219. 3. Sachar DB, Subramani K, Mauer K, Rivera-MacMurray S, Turtel P, Bodian CA, Greenstein AJ. Patterns of postoperative recurrence in fistulizing and stenotic Crohn’s disease. A retrospective cohort study of 71 patients. J Clin Gastroenterol 1996;22:114–116. 4. Rutgeerts P, Geboes K, Vantrappen G, Beyls J, Kerremans R, Hiele M. Predictability of the postoperative course of Crohn’s disease. Gastroenterology 1990;99:956–963. 5. Tytgat GNJ, Mulder CJJ, Brummelkamp WH. Endoscopic lesions in Crohn’s disease early after ileocecal resection. Endoscopy 1988; 20:260–262. 6. Rutgeerts P, Goboes K, Peeters M, Hiele M, Penninckx F, Aerts R, Kerremans R, Vantrappen G. Effect of faecal stream diversion on recurrence of Crohn’s disease in the neoterminal ileum. Lancet 1991;338:771–774. 7. D’Haens GR, Geboes K, Peeters M, Baert F, Penninckx F, Rutgeerts P. Early lesions of recurrent Crohn’s disease caused by infusion of intestinal contents in excluded ileum. Gastroenterology 1998;114:262–267. 8. Sartor RB. Enteric microflora in IBD: pathogens or commensals? Inflam Bowel Dis 1997;3:230–235. 9. Burman JH, Thompson H, Cooke WT, Williams JA. The effects of diversion of intestinal contents on the progress of Crohn’s disease of the large bowel. Gut 1971;12:11–15. 10. Harper PH, Truelove SC, Lee ECG, Kettlewell MGW, Jewell DP. Split ileostomy and ileocolostomy for Crohn’s disease of the colon and ulcerative colitis: a 20 year survey. Gut 1983;24:106– 113. 11. Harper PH, Lee ECG, Kettlewell MGW, Bennett MK, Jewell DP. Role of the faecal stream in the maintenance of Crohn’s colitis. Gut 1985;26:279–284. 12. Fasoli R, Kettlewell MGW, Mortensen N, Jewell DP. Response to faecal challenge in defunctioned colonic Crohn’s disease: prediction of long-term course. Br J Surg 1990;77:616–617. 13. Rutgeerts P, Hiele M, Geboes K, Peeters M, Penninckx F, Aerts R, Kerremans R. Controlled trial of metronidazole treatment for prevention of Crohn’s recurrence after ileal resection. Gastroenterology 1995;108:1617–1621. 14. Sadlack B, Merz H, Schorle H, Schimpl A, Feller AC, Horak I. Ulcerative colitis–like disease in mice with a disrupted interleukin-2 gene. Cell 1993;75:253–261.
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15. Rath HC, Herfarth HH, Ikeda JS, Grenther WB, Hamm T, Balish E, Taurog JD, Hammer RE, Wilson KH, Sartor RB. Normal luminal bacteria, especially Bacteroides species, mediate chronic colitis, gastritis, and arthritis in HLA-B27/b2 microglobulin transgenic rats. J Clin Invest 1996;98:945–953. 16. Dianda L, Hanby AM, Wright NA, Sebesteny A, Hayday AC, Owen MJ. T cell receptor-ab–deficient mice fail to develop colitis in the absence of a microbial environment. Am J Pathol 1997;150:91– 97. 17. Keighley MR, Arabi Y, Dimock F, Burdon DW, Allan RN, AlexanderWilliams J. Influence of inflammatory bowel disease on intestinal microflora. Gut 1978;19:1099–1104. 18. Powell JJ, Ainley CC, Mason IM, Kendall MD, Sankey EA, Dhillon AP, Thompson RP. Characterization of inorganic microparticles in pigment cells of human gut associated lymphoid tissue. Gut 1996;38:390–395. 19. Duchmann R, Kaiser I, Hermann E, Mayet W, Ewe K, Mayer zum Buschenfelde H-H. Tolerance exists towards resident intestinal flora but is broken in active inflammatory bowel disease (IBD). Clin Exp Immunol 11995;102:448–455. 20. Mizoguchi A, Mizoguchi E, Chiba C, Bhan AK. Role of appendix in the development of inflammatory bowel disease in TCR-a mutant mice. J Exp Med 1996;184:707–715. 21. Rath HC, Ikeda JS, Wilson KH, Sartor RB. Varying cecal bacterial loads influences colitis and gastritis in HLA-B27 transgenic rats (abstr). Gastroenterology 1997;112:A938. 22. Scammell B, Ambrose MS, Alexander-Williams J, Allan RN, Keighley MRB. Recurrent small bowel Crohn’s disease is more frequent after subtotal colectomy and ileorectal anastomosis than proctocolectomy. Dis Colon Rectum 1985;28:770–771. 23. Cameron JL, Hamilton SR, Coleman J, Sitzmann JV, Bayless TM. Patterns of ileal recurrence in Crohn’s disease. A prospective randomized study. Ann Surg 1992;215:546–551. 24. Desreumaux P, Brandt E, Gambiez L, Emilie D, Geboes K, Klein O, Ectors N, Cortot A, Capron M, Colombel JF. Distinct cytokine patterns in early and chronic ileal lesions of Crohn’s disease. Gastroenterology 1997;113:118–126.
Address requests for reprints to: R. Balfour Sartor, M.D., Division of Digestive Diseases, Department of Medicine, University of North Carolina, CB 7080, Burnett–Womack Building, Chapel Hill, North Carolina 27599-7080. e-mail: [email protected]; fax: (919) 9666842. r 1998 by the American Gastroenterological Association 0016-5085/98/$3.00
And the Beat Goes On: Discovery of ‘‘New’’ Autoantigens See articles on pages 324 and 329.
n autoimmune response results from a failure in immunologic tolerance, the continuing process that ensures that each successive generation of lymphocytes can discriminate between self and nonself. Events result-
A
ing in the induction of autoimmunity still remain a mystery, but recent advances in molecular immunology have helped to better describe the details of the effector response against autoantigens. For example, for several diseases we now know in some detail the identity of the autoantigen and the epitopes on these molecules that are the target of T and B responses.1,2 Additionally, we have
February 1998
learned more about the phenotype of the immune cells involved in mediating the response, including profiles of cytokine release by T cells and the usage of variable region genes in antigen receptors. However, we still do not understand how such cells first became stimulated. This immunologic conundrum is compounded for the organ-specific autoimmune diseases, a group of diseases in which the distribution of the antigen is ubiquitous, i.e., found in all nucleated cells, but the immune destruction is targeted to relatively few tissues. Diseases of this type include Sjo¨gren’s syndrome, polymyositis and mixed connective tissue disease, and autoimmune liver diseases such as autoimmune hepatitis (AIH) and primary biliary cirrhosis (PBC). For example, in PBC, the autoantigens are predominantly components of normal mitochondria, although, less commonly, components of the cell nucleus and cytoplasm are also recognized. How a mitochondrial antigen, physically sheltered from the immune system by two membrane barriers, can elicit an immune response or, once tolerance is broken, how this ‘‘self-reactive’’ immunity causes tissue damage in only some cells is still unknown. Undeniably, however, in PBC, immune damage is almost exclusively directed against the intrahepatic biliary epithelium. Although the pathogenic mechanisms underlying the development and progression of AIH are still unclear, several characteristics of this disease suggest that it is an immunologically mediated disease process. The presence of a number of autoantibodies against intracellular structures such as nuclear and microsomal antigens, cytoskeleton, and cytosol in the sera of such patients are used as diagnostic markers for AIH.3 It has also been shown that one such microsomal antigen, cytochrome P450IID6, is expressed on the surface and is functional in the plasma membrane of human hepatocytes. In addition, patients with AIH with anti–cytochrome P450 recognize epitopes expressed on the outer hepatocyte surface.4 Thus, this is evidence that at least one intracellular autoantigen can be found on the surface of cells that are targets for destruction in an autoimmune disease. Similar surface expression of an intracellular autoantigen, pyruvate dehydrogenase E2, has also been suggested for PBC.5–7 In the two studies entitled ‘‘Two Cytochromes P450 Are Major Hepatocellular Autoantigens in Autoimmune Polyglandular Syndrome Type 1’’ by Clemente et al.8 and ‘‘Members of Glutathione S-Transferase Gene Family Are Antigens in Autoimmune Hepatitis’’ by We¸sierskaGa¸dek et al.,9 the investigators report the identification of two novel autoantigens. Both of these autoantigens, cytochrome P450 (CYP) 2A6 (a liver-kidney microsomal
EDITORIALS
401
autoantibody antigen) and glutathione S-transferase (GST) (a soluble liver antigen [SLA] antigen), are found in the cytosol of cells and are thus considered intracellular antigens. In addition, both are highly conserved enzymes with isoforms found in multiple organs. Clemente et al. describe the association of CYP2A6 with autoimmune polyglandular syndrome type 1 (APS-1) in 3 patients, some of whom had associated liver disease. However, the clinical manifestations associated with the presence of anti-CYP2A6 autoantibodies ranged from asymptomatic to moderate or fulminant hepatitis. It is interesting to note that patients with APS-1 have multiple autoantibodies directed against adrenal glands and gonads, two of the affected organs in this disease. The adrenal gland autoantigens described thus far are all cytochrome P450 enzymes involved in adrenal steroidosynthesis.10 The discovery of an additional CYP450 autoantigen, found in liver and kidney microsomal preparations and associated with the presence of liver disease in a small group of these patients, is important for two reasons. First, it suggests a possible relationship between a tissue-specific CYP450 autoantigen and the targeting of that organ for an autoimmune response. Second, it provides strong evidence for a phenomenon that is increasingly associated with autoimmune diseases: determinant spreading. The concept of determinant spreading suggests that a particular antigenic or autoantigenic determinant that is cryptic or hidden during an initial immune response can become immunogenic over the course of disease. Developing T cells potentially directed against cryptic self determinants escape tolerance induction in the thymus and thereby enrich the T-cell repertoire. However, when cryptic self-peptides become expressed at inflammatory tissue sites, leading to the engagement of this repertoire, induction and/or perpetuation of autoimmune reactivity could occur.11,12 This concept is further substantiated by a study by We¸sierska-Ga¸dek et al. As discussed earlier, AIH is a chronic liver disease of unknown etiology that is associated with several autoantibodies, including antibodies to cytochrome P450IID6 (type II AIH).3 We¸sierska-Ga¸dek et al. discuss the identification of a new autoantigen associated with a third type of AIH characterized by antibodies to a cytosolic SLA. SLA was originally described as a non–species-, non–organ-specific antigen, although it is found in its highest concentration in kidney and liver.13 In their current study, We¸sierskaGa¸dek et al. showed that at least three distinct subunits of the GST superfamily of multifunctional enzymes are present in SLA. The GSTs are multifunctional proteins
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EDITORIALS
important for the mediation of hepatic detoxification of cytotoxic compounds and as carrier proteins in biliary secretion.14 The importance of these enzymes in the function of healthy liver makes them an interesting target for a liver disease–specific autoantigen. As the investigators suggest, it would be of interest to know whether the anti-GST antibodies inhibit enzyme function. Perhaps these enzymes become exposed, or there is an alteration in production, upon liver tissue damage during the initiation of disease. This study identified at least three distinct subunits of GST as target antigens. It was also noted that the sera from these patients with AIH reacted differentially with each of these subunits, and as the investigators suggest, the autoantibodies can distinguish epitopes specific for each variant of the enzyme. These data further suggest evidence for determinant spreading rather that epitope cross-reactivity. How does an organ become a specific target for an autoimmune disease? In rheumatic fever, infection with Streptococcus precedes cardiac damage by weeks. Perhaps this is one extreme of the pendulum. Conceivably, some infections could precede the manifestation of disease by years (the so-called hit-and-run diseases), making it difficult, if not impossible, to identify the etiological agent. In such a case, perhaps there is no such thing as pure autoimmunity (except for rare instances in which trauma releases autoantigens). One hypothesis used to explain how human autoantibodies to self-proteins arise, break tolerance, and lead to autoimmune disease is the idea of molecular mimicry between host autoantigens and unrelated exogenous proteins, such as a bacteria or virus.15 For example, amino acid sequence homologies have been observed between p70 (U1)RNP and topoisomerase I16 with the p30gag retroviral nucleoprotein,16,17 the 60-kilodalton Ro/SS-A with the nuclear capsid of vesicular stomatitis virus,18 and fibrillarin with the nuclear protein encoded by Epstein–Barr virus.19 On the other hand, the presence of multiple B-cell epitopes strongly supports the thesis that the autoimmune response is antigen driven. These two hypotheses are not necessarily contradictory, because the possibility of molecular mimicry at the level of a single epitope as the initiating event cannot be ruled out. For instance, McNeilage et al.20 reported that in Sjo¨grens syndrome, in the very early stage of the anti–SS-B/La response, only the N-terminal 107 aa are recognized, and with time, the response broadens to include other epitopes. Interestingly, this region (aa 88–101) contains remarkable homology with an amino acid sequence in the gag protein of feline sarcoma virus.21
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Cross-recognition of determinants shared by unrelated antigens, at either the B-cell or T-cell level, is of great immunologic interest. It has been suggested that T-cell cross-reactive epitopes can exist in proteins with little more than random levels of sequence homology.22 The potential of these epitopes for cross-sensitization may play a role in the maintenance of T-cell memory, in the pathogenesis of autoimmune diseases, and possibly in a wider range of host immune responses to infectious pathogens. Studies of the evergrowing repertoire of autoantigens, such as those discussed here, will provide information regarding such questions and will aid in the future clarification of the pathogenesis of autoimmunity. JUDY VAN DE WATER M. ERIC GERSHWIN Division of Rheumatology, Allergy and Clinical Immunology University of California Davis, School of Medicine Davis, California
References 1. Coppel RL, McNeilage LJ, Surh CD, Van de Water J, Spithill TW, Whittingham S, Gershwin ME. Primary structure of the human M2 mitochondrial autoantigen of primary biliary cirrhosis: dihydrolipoamide acetyltransferase. Proc Natl Acad Sci USA 1988;85: 7317–7321. 2. Leung PS, Van de Water J, Coppel RL, Gershwin ME. Molecular characterization of the mitochondrial autoantigens in primary biliary cirrhosis. Immunol Res 1991;10:518–527. 3. Vergani D, Mieli-Vergani G. Autoimmune hepatitis. Ann Ital Med Int 1996;11:119–124. 4. Loeper J, Descatoire V, Maurice M, Beaune P, Belghiti J, Houssin D, Ballet F, et al. Cytochromes P450 in human hepatocyte plasma membrane: recognition by several autoantibodies. Gastroenterology 1993;104:203–216. 5. Van de Water J, Gerson LB, Ferrell LD, Lake JR, Coppel RL, Batts KP, Wiesner RH, et al. Immunohistochemical evidence of disease recurrence following liver transplantation for primary biliary cirrhosis. Hepatology 1996;24:1079–1084. 6. Joplin R, Gershwin ME. Ductular expression of autoantigens in primary biliary cirrhosis. Semin Liver Dis 1997;17:97–103. 7. Joplin R, Lindsay JG, Johnson GD, Strain A, Neuberger J. Membrane dihydrolipoamide acetyltransferase (E2) on human biliary epithelial cells in primary biliary cirrhosis. Lancet 1992;339:93– 94. 8. Clemente MG, Meloni A, Obermayer-Straub P, Frau F, Manns MP, De Virgiliis S. Two cytochromes P450 are major hepatocellular autoantigens in autoimmune polyglandular syndrome type 1. Gastroenterology 1998;114:324–328. 9. We¸sierska-Ga¸dek J, Grimm R, Hitchman E, Penner E. Members of glutathione S-transferase gene family are antigens in autoimmune hepatitis. Gastroenterology 1998;114:329–335. 10. Uibo R, Aavik E, Peterson P, Perheentupa SA, Pelkonen R, Krohn K. Autoantibodies to cyotchrome P450 enzymes P450ssc, P450c17, P450c21 in autoimmune polyglandular disease types I and II and in isolated Addison’s disease. J Clin Endocrinol Metab 1994;78:323–328. 11. Moudgil KD, Sercarz EE. The T cell repertoire against cryptic self determinants and its involvement in autoimmunity and cancer. Clin Immunol Immunopathol 1994;73:283–289.
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EDITORIALS
12. Lehmann PV, Forsthuber T, Miller A, Sercarz EE. Spreading of T-cell autoimmunity to cryptic determinants of an autoantigen. Nature 1992;358:155–157. 13. Manns M, Gerken G, Kyriatsoulis A, Staritz M, Meyer zum Bu¨shenfelde KH. Characterization of a new subgroup of autoimmune chronic active hepatitis by autoantibodies against a soluble liver antigen. Lancet 1987;1:292–294. 14. Hayes JD, Pulford DJ. The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance. Crit Rev Biochem Mol Biol 1995;30:445–600. 15. Horsfall AC. Molecular mimicry and autoantigens in connective tissue diseases. Mol Biol Rep 1992;16:139–147. 16. Maul GG, Jiminez SA, Riggs E, Ziemnicka-Kotula D. Determination of an epitope of the diffuse systemic sclerosis marker antigen DNA topoisomerase I: sequence similarities with retroviral p30gag protein suggests a possible cause for autoimmunity in systemic sclerosis. Proc Natl Acad Sci USA 1989;86:8492–8496. 17. Query CC, Keene JD. A human autoimmune protein associated with U1 RNA containing a region of homology that is crossreactive with retroviral p30gag antigen. Cell 1987;51:211–220. 18. Scofield RH, Harley JB. Autoantigenicity of Ro/SSA antigen related to a nucleocapsid protein of vesicular stomatitis virus. Proc Natl Acad Sci USA 1991;88:3343–3347.
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19. Kasturi KN, Hatakeyama A, Spiera H, Bonsa CA. Antifibrallin autoantibodies present in systemic sclerosis and other connective diseases interact with similar epitopes. J Exp Med 1995;181: 1027–1036. 20. McNeilage LJ, Macmillan EM, Whittingham SF. Mapping of epitopes on the La(SS-B) autoantigen of primary Sjo¨gren’s syndrome: identification of a cross-reactive epitope. J Immunol 1990;145:3829–3835. 21. Kohsaka H, Yamamoto K, Fuji H, Miura H, Nishioka K, Miyamoto T. Fine epitope mapping of the human SS-B(La) protein. Identification of a distinct autoepitope homologous to a viral gag polypeptide. J Clin Invest 1990;85:1566–1574. 22. Harris DP, Vordermeier HM, Singh M, Moreno C, Jurcevic S, Ivanyi J. Cross-recognition by T cells of an epitope shared by two unrelated mycobacterial antigens. Eur J Immunol 1995;25:3173– 3179.
Address correspondence to: Judy Van de Water, M.D., Division of Rheumatology, Allergy and Clinical Immunology, University of California Davis, School of Medicine, Davis, California 95616. r 1998 by the American Gastroenterological Association 0016-5085/98/$3.00
Methionine Adenosyltransferase and Liver Disease: It’s All About SAM See article on page 364.
ethionine adenosyltransferase (MAT, also known as S-adenosylmethionine synthetase and AdoMet synthetase) is the enzyme responsible for the synthesis of S-adenosyl-L-methionine (SAM) using methionine and adenosine triphosphate (ATP).1 SAM is the principal biological methyl donor, the precursor of aminopropyl groups used in polyamine biosynthesis; in the liver, SAM is also a precursor of glutathione through its conversion to cysteine via the transsulfuration pathway.1 SAM contains a sulfonium ion that makes it a high-energy reagent and can easily transfer its methyl group to a large variety of acceptor substrates including nucleic acids, proteins, phospholipids, biological amines, and a long list of small molecules.2 Given the critical role of methylation in determining various cellular processes ranging from gene expression to membrane fluidity, any alteration in the availability of SAM may have profound effects on cellular growth, differentiation, and function. In mammals, two different genes, MAT1A and MAT2A, encode for two homologous MAT catalytic subunits, a1 and a23–5 (see Kotb et al.5 for a consensus
M
nomenclature of the mammalian MAT genes and gene products). MAT1A is expressed only in the liver and encodes the a1 subunit found in two native MAT isozymes, which are either a dimer (MATIII) or tetramer (MATI) of this single subunit.5 MAT2A encodes for a catalytic subunit (a2) found in a native MAT isozyme (MATII), which is widely distributed.3,5 MAT2A and its gene product also predominate in the fetal liver and is progressively replaced by MAT1A during development.6,7 In hepatocarcinogenesis, there is a switch in the gene expression from MAT1A to MAT2A, suggesting that the expression of MAT2A in liver correlates with more rapid cell growth.8,9 Different isoforms of MAT differ in kinetic and regulatory properties. Of the two liver-specific MAT isoforms, MATI, the tetramer, exhibits much lower Michaelis constant (Km) for its substrates (the highaffinity Km for methionine, 36 mmol/L; Km for ATP, 300 mmol/L) than MATIII, the dimer (Km for methionine, 700 mmol/L; Km for ATP, 1 mmol/L).2,10 Because the hepatic concentration of methionine is approximately 50–80 mmol/L,1,11 the specific activity of MATI is likely to be tenfold higher than that of MATIII under physiological conditions.11
EDITORIALS
References 1. Rutgeerts P, Geboes K, Vantrappen G, Kerremans R, Coenegrachts JL, Coremans G. Natural history of recurrent Crohn’s disease at the ileocolonic anastomosis after curative surgery. Gut 1984;25:665–672. 2. Pallone F, Boirivant M, Stazi MA, Cosintino R, Prantera C, Torsoli A. Analysis of clinical course of postoperative recurrence in Crohn’s disease of distal ileum. Dig Dis Sci 1992;37:215–219. 3. Sachar DB, Subramani K, Mauer K, Rivera-MacMurray S, Turtel P, Bodian CA, Greenstein AJ. Patterns of postoperative recurrence in fistulizing and stenotic Crohn’s disease. A retrospective cohort study of 71 patients. J Clin Gastroenterol 1996;22:114–116. 4. Rutgeerts P, Geboes K, Vantrappen G, Beyls J, Kerremans R, Hiele M. Predictability of the postoperative course of Crohn’s disease. Gastroenterology 1990;99:956–963. 5. Tytgat GNJ, Mulder CJJ, Brummelkamp WH. Endoscopic lesions in Crohn’s disease early after ileocecal resection. Endoscopy 1988; 20:260–262. 6. Rutgeerts P, Goboes K, Peeters M, Hiele M, Penninckx F, Aerts R, Kerremans R, Vantrappen G. Effect of faecal stream diversion on recurrence of Crohn’s disease in the neoterminal ileum. Lancet 1991;338:771–774. 7. D’Haens GR, Geboes K, Peeters M, Baert F, Penninckx F, Rutgeerts P. Early lesions of recurrent Crohn’s disease caused by infusion of intestinal contents in excluded ileum. Gastroenterology 1998;114:262–267. 8. Sartor RB. Enteric microflora in IBD: pathogens or commensals? Inflam Bowel Dis 1997;3:230–235. 9. Burman JH, Thompson H, Cooke WT, Williams JA. The effects of diversion of intestinal contents on the progress of Crohn’s disease of the large bowel. Gut 1971;12:11–15. 10. Harper PH, Truelove SC, Lee ECG, Kettlewell MGW, Jewell DP. Split ileostomy and ileocolostomy for Crohn’s disease of the colon and ulcerative colitis: a 20 year survey. Gut 1983;24:106– 113. 11. Harper PH, Lee ECG, Kettlewell MGW, Bennett MK, Jewell DP. Role of the faecal stream in the maintenance of Crohn’s colitis. Gut 1985;26:279–284. 12. Fasoli R, Kettlewell MGW, Mortensen N, Jewell DP. Response to faecal challenge in defunctioned colonic Crohn’s disease: prediction of long-term course. Br J Surg 1990;77:616–617. 13. Rutgeerts P, Hiele M, Geboes K, Peeters M, Penninckx F, Aerts R, Kerremans R. Controlled trial of metronidazole treatment for prevention of Crohn’s recurrence after ileal resection. Gastroenterology 1995;108:1617–1621. 14. Sadlack B, Merz H, Schorle H, Schimpl A, Feller AC, Horak I. Ulcerative colitis–like disease in mice with a disrupted interleukin-2 gene. Cell 1993;75:253–261.
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15. Rath HC, Herfarth HH, Ikeda JS, Grenther WB, Hamm T, Balish E, Taurog JD, Hammer RE, Wilson KH, Sartor RB. Normal luminal bacteria, especially Bacteroides species, mediate chronic colitis, gastritis, and arthritis in HLA-B27/b2 microglobulin transgenic rats. J Clin Invest 1996;98:945–953. 16. Dianda L, Hanby AM, Wright NA, Sebesteny A, Hayday AC, Owen MJ. T cell receptor-ab–deficient mice fail to develop colitis in the absence of a microbial environment. Am J Pathol 1997;150:91– 97. 17. Keighley MR, Arabi Y, Dimock F, Burdon DW, Allan RN, AlexanderWilliams J. Influence of inflammatory bowel disease on intestinal microflora. Gut 1978;19:1099–1104. 18. Powell JJ, Ainley CC, Mason IM, Kendall MD, Sankey EA, Dhillon AP, Thompson RP. Characterization of inorganic microparticles in pigment cells of human gut associated lymphoid tissue. Gut 1996;38:390–395. 19. Duchmann R, Kaiser I, Hermann E, Mayet W, Ewe K, Mayer zum Buschenfelde H-H. Tolerance exists towards resident intestinal flora but is broken in active inflammatory bowel disease (IBD). Clin Exp Immunol 11995;102:448–455. 20. Mizoguchi A, Mizoguchi E, Chiba C, Bhan AK. Role of appendix in the development of inflammatory bowel disease in TCR-a mutant mice. J Exp Med 1996;184:707–715. 21. Rath HC, Ikeda JS, Wilson KH, Sartor RB. Varying cecal bacterial loads influences colitis and gastritis in HLA-B27 transgenic rats (abstr). Gastroenterology 1997;112:A938. 22. Scammell B, Ambrose MS, Alexander-Williams J, Allan RN, Keighley MRB. Recurrent small bowel Crohn’s disease is more frequent after subtotal colectomy and ileorectal anastomosis than proctocolectomy. Dis Colon Rectum 1985;28:770–771. 23. Cameron JL, Hamilton SR, Coleman J, Sitzmann JV, Bayless TM. Patterns of ileal recurrence in Crohn’s disease. A prospective randomized study. Ann Surg 1992;215:546–551. 24. Desreumaux P, Brandt E, Gambiez L, Emilie D, Geboes K, Klein O, Ectors N, Cortot A, Capron M, Colombel JF. Distinct cytokine patterns in early and chronic ileal lesions of Crohn’s disease. Gastroenterology 1997;113:118–126.
Address requests for reprints to: R. Balfour Sartor, M.D., Division of Digestive Diseases, Department of Medicine, University of North Carolina, CB 7080, Burnett–Womack Building, Chapel Hill, North Carolina 27599-7080. e-mail: [email protected]; fax: (919) 9666842. r 1998 by the American Gastroenterological Association 0016-5085/98/$3.00
And the Beat Goes On: Discovery of ‘‘New’’ Autoantigens See articles on pages 324 and 329.
n autoimmune response results from a failure in immunologic tolerance, the continuing process that ensures that each successive generation of lymphocytes can discriminate between self and nonself. Events result-
A
ing in the induction of autoimmunity still remain a mystery, but recent advances in molecular immunology have helped to better describe the details of the effector response against autoantigens. For example, for several diseases we now know in some detail the identity of the autoantigen and the epitopes on these molecules that are the target of T and B responses.1,2 Additionally, we have
February 1998
learned more about the phenotype of the immune cells involved in mediating the response, including profiles of cytokine release by T cells and the usage of variable region genes in antigen receptors. However, we still do not understand how such cells first became stimulated. This immunologic conundrum is compounded for the organ-specific autoimmune diseases, a group of diseases in which the distribution of the antigen is ubiquitous, i.e., found in all nucleated cells, but the immune destruction is targeted to relatively few tissues. Diseases of this type include Sjo¨gren’s syndrome, polymyositis and mixed connective tissue disease, and autoimmune liver diseases such as autoimmune hepatitis (AIH) and primary biliary cirrhosis (PBC). For example, in PBC, the autoantigens are predominantly components of normal mitochondria, although, less commonly, components of the cell nucleus and cytoplasm are also recognized. How a mitochondrial antigen, physically sheltered from the immune system by two membrane barriers, can elicit an immune response or, once tolerance is broken, how this ‘‘self-reactive’’ immunity causes tissue damage in only some cells is still unknown. Undeniably, however, in PBC, immune damage is almost exclusively directed against the intrahepatic biliary epithelium. Although the pathogenic mechanisms underlying the development and progression of AIH are still unclear, several characteristics of this disease suggest that it is an immunologically mediated disease process. The presence of a number of autoantibodies against intracellular structures such as nuclear and microsomal antigens, cytoskeleton, and cytosol in the sera of such patients are used as diagnostic markers for AIH.3 It has also been shown that one such microsomal antigen, cytochrome P450IID6, is expressed on the surface and is functional in the plasma membrane of human hepatocytes. In addition, patients with AIH with anti–cytochrome P450 recognize epitopes expressed on the outer hepatocyte surface.4 Thus, this is evidence that at least one intracellular autoantigen can be found on the surface of cells that are targets for destruction in an autoimmune disease. Similar surface expression of an intracellular autoantigen, pyruvate dehydrogenase E2, has also been suggested for PBC.5–7 In the two studies entitled ‘‘Two Cytochromes P450 Are Major Hepatocellular Autoantigens in Autoimmune Polyglandular Syndrome Type 1’’ by Clemente et al.8 and ‘‘Members of Glutathione S-Transferase Gene Family Are Antigens in Autoimmune Hepatitis’’ by We¸sierskaGa¸dek et al.,9 the investigators report the identification of two novel autoantigens. Both of these autoantigens, cytochrome P450 (CYP) 2A6 (a liver-kidney microsomal
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401
autoantibody antigen) and glutathione S-transferase (GST) (a soluble liver antigen [SLA] antigen), are found in the cytosol of cells and are thus considered intracellular antigens. In addition, both are highly conserved enzymes with isoforms found in multiple organs. Clemente et al. describe the association of CYP2A6 with autoimmune polyglandular syndrome type 1 (APS-1) in 3 patients, some of whom had associated liver disease. However, the clinical manifestations associated with the presence of anti-CYP2A6 autoantibodies ranged from asymptomatic to moderate or fulminant hepatitis. It is interesting to note that patients with APS-1 have multiple autoantibodies directed against adrenal glands and gonads, two of the affected organs in this disease. The adrenal gland autoantigens described thus far are all cytochrome P450 enzymes involved in adrenal steroidosynthesis.10 The discovery of an additional CYP450 autoantigen, found in liver and kidney microsomal preparations and associated with the presence of liver disease in a small group of these patients, is important for two reasons. First, it suggests a possible relationship between a tissue-specific CYP450 autoantigen and the targeting of that organ for an autoimmune response. Second, it provides strong evidence for a phenomenon that is increasingly associated with autoimmune diseases: determinant spreading. The concept of determinant spreading suggests that a particular antigenic or autoantigenic determinant that is cryptic or hidden during an initial immune response can become immunogenic over the course of disease. Developing T cells potentially directed against cryptic self determinants escape tolerance induction in the thymus and thereby enrich the T-cell repertoire. However, when cryptic self-peptides become expressed at inflammatory tissue sites, leading to the engagement of this repertoire, induction and/or perpetuation of autoimmune reactivity could occur.11,12 This concept is further substantiated by a study by We¸sierska-Ga¸dek et al. As discussed earlier, AIH is a chronic liver disease of unknown etiology that is associated with several autoantibodies, including antibodies to cytochrome P450IID6 (type II AIH).3 We¸sierska-Ga¸dek et al. discuss the identification of a new autoantigen associated with a third type of AIH characterized by antibodies to a cytosolic SLA. SLA was originally described as a non–species-, non–organ-specific antigen, although it is found in its highest concentration in kidney and liver.13 In their current study, We¸sierskaGa¸dek et al. showed that at least three distinct subunits of the GST superfamily of multifunctional enzymes are present in SLA. The GSTs are multifunctional proteins
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important for the mediation of hepatic detoxification of cytotoxic compounds and as carrier proteins in biliary secretion.14 The importance of these enzymes in the function of healthy liver makes them an interesting target for a liver disease–specific autoantigen. As the investigators suggest, it would be of interest to know whether the anti-GST antibodies inhibit enzyme function. Perhaps these enzymes become exposed, or there is an alteration in production, upon liver tissue damage during the initiation of disease. This study identified at least three distinct subunits of GST as target antigens. It was also noted that the sera from these patients with AIH reacted differentially with each of these subunits, and as the investigators suggest, the autoantibodies can distinguish epitopes specific for each variant of the enzyme. These data further suggest evidence for determinant spreading rather that epitope cross-reactivity. How does an organ become a specific target for an autoimmune disease? In rheumatic fever, infection with Streptococcus precedes cardiac damage by weeks. Perhaps this is one extreme of the pendulum. Conceivably, some infections could precede the manifestation of disease by years (the so-called hit-and-run diseases), making it difficult, if not impossible, to identify the etiological agent. In such a case, perhaps there is no such thing as pure autoimmunity (except for rare instances in which trauma releases autoantigens). One hypothesis used to explain how human autoantibodies to self-proteins arise, break tolerance, and lead to autoimmune disease is the idea of molecular mimicry between host autoantigens and unrelated exogenous proteins, such as a bacteria or virus.15 For example, amino acid sequence homologies have been observed between p70 (U1)RNP and topoisomerase I16 with the p30gag retroviral nucleoprotein,16,17 the 60-kilodalton Ro/SS-A with the nuclear capsid of vesicular stomatitis virus,18 and fibrillarin with the nuclear protein encoded by Epstein–Barr virus.19 On the other hand, the presence of multiple B-cell epitopes strongly supports the thesis that the autoimmune response is antigen driven. These two hypotheses are not necessarily contradictory, because the possibility of molecular mimicry at the level of a single epitope as the initiating event cannot be ruled out. For instance, McNeilage et al.20 reported that in Sjo¨grens syndrome, in the very early stage of the anti–SS-B/La response, only the N-terminal 107 aa are recognized, and with time, the response broadens to include other epitopes. Interestingly, this region (aa 88–101) contains remarkable homology with an amino acid sequence in the gag protein of feline sarcoma virus.21
GASTROENTEROLOGY Vol. 114, No. 2
Cross-recognition of determinants shared by unrelated antigens, at either the B-cell or T-cell level, is of great immunologic interest. It has been suggested that T-cell cross-reactive epitopes can exist in proteins with little more than random levels of sequence homology.22 The potential of these epitopes for cross-sensitization may play a role in the maintenance of T-cell memory, in the pathogenesis of autoimmune diseases, and possibly in a wider range of host immune responses to infectious pathogens. Studies of the evergrowing repertoire of autoantigens, such as those discussed here, will provide information regarding such questions and will aid in the future clarification of the pathogenesis of autoimmunity. JUDY VAN DE WATER M. ERIC GERSHWIN Division of Rheumatology, Allergy and Clinical Immunology University of California Davis, School of Medicine Davis, California
References 1. Coppel RL, McNeilage LJ, Surh CD, Van de Water J, Spithill TW, Whittingham S, Gershwin ME. Primary structure of the human M2 mitochondrial autoantigen of primary biliary cirrhosis: dihydrolipoamide acetyltransferase. Proc Natl Acad Sci USA 1988;85: 7317–7321. 2. Leung PS, Van de Water J, Coppel RL, Gershwin ME. Molecular characterization of the mitochondrial autoantigens in primary biliary cirrhosis. Immunol Res 1991;10:518–527. 3. Vergani D, Mieli-Vergani G. Autoimmune hepatitis. Ann Ital Med Int 1996;11:119–124. 4. Loeper J, Descatoire V, Maurice M, Beaune P, Belghiti J, Houssin D, Ballet F, et al. Cytochromes P450 in human hepatocyte plasma membrane: recognition by several autoantibodies. Gastroenterology 1993;104:203–216. 5. Van de Water J, Gerson LB, Ferrell LD, Lake JR, Coppel RL, Batts KP, Wiesner RH, et al. Immunohistochemical evidence of disease recurrence following liver transplantation for primary biliary cirrhosis. Hepatology 1996;24:1079–1084. 6. Joplin R, Gershwin ME. Ductular expression of autoantigens in primary biliary cirrhosis. Semin Liver Dis 1997;17:97–103. 7. Joplin R, Lindsay JG, Johnson GD, Strain A, Neuberger J. Membrane dihydrolipoamide acetyltransferase (E2) on human biliary epithelial cells in primary biliary cirrhosis. Lancet 1992;339:93– 94. 8. Clemente MG, Meloni A, Obermayer-Straub P, Frau F, Manns MP, De Virgiliis S. Two cytochromes P450 are major hepatocellular autoantigens in autoimmune polyglandular syndrome type 1. Gastroenterology 1998;114:324–328. 9. We¸sierska-Ga¸dek J, Grimm R, Hitchman E, Penner E. Members of glutathione S-transferase gene family are antigens in autoimmune hepatitis. Gastroenterology 1998;114:329–335. 10. Uibo R, Aavik E, Peterson P, Perheentupa SA, Pelkonen R, Krohn K. Autoantibodies to cyotchrome P450 enzymes P450ssc, P450c17, P450c21 in autoimmune polyglandular disease types I and II and in isolated Addison’s disease. J Clin Endocrinol Metab 1994;78:323–328. 11. Moudgil KD, Sercarz EE. The T cell repertoire against cryptic self determinants and its involvement in autoimmunity and cancer. Clin Immunol Immunopathol 1994;73:283–289.
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12. Lehmann PV, Forsthuber T, Miller A, Sercarz EE. Spreading of T-cell autoimmunity to cryptic determinants of an autoantigen. Nature 1992;358:155–157. 13. Manns M, Gerken G, Kyriatsoulis A, Staritz M, Meyer zum Bu¨shenfelde KH. Characterization of a new subgroup of autoimmune chronic active hepatitis by autoantibodies against a soluble liver antigen. Lancet 1987;1:292–294. 14. Hayes JD, Pulford DJ. The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance. Crit Rev Biochem Mol Biol 1995;30:445–600. 15. Horsfall AC. Molecular mimicry and autoantigens in connective tissue diseases. Mol Biol Rep 1992;16:139–147. 16. Maul GG, Jiminez SA, Riggs E, Ziemnicka-Kotula D. Determination of an epitope of the diffuse systemic sclerosis marker antigen DNA topoisomerase I: sequence similarities with retroviral p30gag protein suggests a possible cause for autoimmunity in systemic sclerosis. Proc Natl Acad Sci USA 1989;86:8492–8496. 17. Query CC, Keene JD. A human autoimmune protein associated with U1 RNA containing a region of homology that is crossreactive with retroviral p30gag antigen. Cell 1987;51:211–220. 18. Scofield RH, Harley JB. Autoantigenicity of Ro/SSA antigen related to a nucleocapsid protein of vesicular stomatitis virus. Proc Natl Acad Sci USA 1991;88:3343–3347.
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19. Kasturi KN, Hatakeyama A, Spiera H, Bonsa CA. Antifibrallin autoantibodies present in systemic sclerosis and other connective diseases interact with similar epitopes. J Exp Med 1995;181: 1027–1036. 20. McNeilage LJ, Macmillan EM, Whittingham SF. Mapping of epitopes on the La(SS-B) autoantigen of primary Sjo¨gren’s syndrome: identification of a cross-reactive epitope. J Immunol 1990;145:3829–3835. 21. Kohsaka H, Yamamoto K, Fuji H, Miura H, Nishioka K, Miyamoto T. Fine epitope mapping of the human SS-B(La) protein. Identification of a distinct autoepitope homologous to a viral gag polypeptide. J Clin Invest 1990;85:1566–1574. 22. Harris DP, Vordermeier HM, Singh M, Moreno C, Jurcevic S, Ivanyi J. Cross-recognition by T cells of an epitope shared by two unrelated mycobacterial antigens. Eur J Immunol 1995;25:3173– 3179.
Address correspondence to: Judy Van de Water, M.D., Division of Rheumatology, Allergy and Clinical Immunology, University of California Davis, School of Medicine, Davis, California 95616. r 1998 by the American Gastroenterological Association 0016-5085/98/$3.00
Methionine Adenosyltransferase and Liver Disease: It’s All About SAM See article on page 364.
ethionine adenosyltransferase (MAT, also known as S-adenosylmethionine synthetase and AdoMet synthetase) is the enzyme responsible for the synthesis of S-adenosyl-L-methionine (SAM) using methionine and adenosine triphosphate (ATP).1 SAM is the principal biological methyl donor, the precursor of aminopropyl groups used in polyamine biosynthesis; in the liver, SAM is also a precursor of glutathione through its conversion to cysteine via the transsulfuration pathway.1 SAM contains a sulfonium ion that makes it a high-energy reagent and can easily transfer its methyl group to a large variety of acceptor substrates including nucleic acids, proteins, phospholipids, biological amines, and a long list of small molecules.2 Given the critical role of methylation in determining various cellular processes ranging from gene expression to membrane fluidity, any alteration in the availability of SAM may have profound effects on cellular growth, differentiation, and function. In mammals, two different genes, MAT1A and MAT2A, encode for two homologous MAT catalytic subunits, a1 and a23–5 (see Kotb et al.5 for a consensus
M
nomenclature of the mammalian MAT genes and gene products). MAT1A is expressed only in the liver and encodes the a1 subunit found in two native MAT isozymes, which are either a dimer (MATIII) or tetramer (MATI) of this single subunit.5 MAT2A encodes for a catalytic subunit (a2) found in a native MAT isozyme (MATII), which is widely distributed.3,5 MAT2A and its gene product also predominate in the fetal liver and is progressively replaced by MAT1A during development.6,7 In hepatocarcinogenesis, there is a switch in the gene expression from MAT1A to MAT2A, suggesting that the expression of MAT2A in liver correlates with more rapid cell growth.8,9 Different isoforms of MAT differ in kinetic and regulatory properties. Of the two liver-specific MAT isoforms, MATI, the tetramer, exhibits much lower Michaelis constant (Km) for its substrates (the highaffinity Km for methionine, 36 mmol/L; Km for ATP, 300 mmol/L) than MATIII, the dimer (Km for methionine, 700 mmol/L; Km for ATP, 1 mmol/L).2,10 Because the hepatic concentration of methionine is approximately 50–80 mmol/L,1,11 the specific activity of MATI is likely to be tenfold higher than that of MATIII under physiological conditions.11