Immunologic and Genetic Factors in Type 1 Diabetes (2002)

Under normal physiologic conditions, the entry of glucose into beta cells triggers the secretion of insulin (1Bell G.I. Polonsky K.S. Nature. 2001; 414: 788-791Crossref PubMed Scopus (405) Google Scholar) (Fig.1). The released insulin is carried in the blood to peripheral tissues where it binds to insulin receptors, which are members of the receptor tyrosine kinase family. This initiates a cascade of transmembrane events resulting in the uptake of glucose by cells and its metabolism into energy or storage as glycogen (2Saltiel A.R. Kahn C.R. Nature. 2001; 414: 799-806Crossref PubMed Scopus (3813) Google Scholar). Defects anywhere along the beta cell-peripheral tissue pathway can result in hyperglycemia, but at the clinical level there are two major forms of diabetes: type 1 diabetes, previously known as juvenile or insulin-dependent diabetes, and type 2 diabetes, previously known as adult or non-insulin-dependent diabetes. Type 1 diabetes is caused by an absolute deficiency in the production of insulin as a result of destruction of pancreatic beta cells. About one million people in the United States suffer from this form of the disease. Type 2 diabetes is the more common form of the disease and afflicts about 16 million people. It is the result of a double defect: inadequate or inappropriate secretion of insulin by beta cells and resistance to the action of insulin in both peripheral tissues (e.g. muscle, adipose tissues) and beta cells. Insulin resistance occurs at the postinsulin receptor level and may be the result of a defect in any one of several genes or pathways as recently demonstrated in transgenic and knock-out mice (1Bell G.I. Polonsky K.S. Nature. 2001; 414: 788-791Crossref PubMed Scopus (405) Google Scholar, 2Saltiel A.R. Kahn C.R. Nature. 2001; 414: 799-806Crossref PubMed Scopus (3813) Google Scholar). The precise defect(s) in the human, however, is still not known. Over the last few years, evidence has accumulated that type 1 diabetes is an autoimmune disease or at least has a major autoimmune component. The evidence comes from three sources: the presence of an inflammatory infiltrate (insulitis) in the islets; a strong linkage between type 1 diabetes and certain alleles of the major histocompatibility complex (MHC) 1The abbreviations used are: MHC, major histocompatibility complex; APC, antigen-presenting cell; GAD65, glutamic acid decarboxylase; HLA, major histocompatibility complex of humans; IA-2, islet-associated antigen 2; ICA, islet cell autoantibodies; LCM, lymphocytic choriomeningitis; PTP, protein-tyrosine phosphatase 1The abbreviations used are: MHC, major histocompatibility complex; APC, antigen-presenting cell; GAD65, glutamic acid decarboxylase; HLA, major histocompatibility complex of humans; IA-2, islet-associated antigen 2; ICA, islet cell autoantibodies; LCM, lymphocytic choriomeningitis; PTP, protein-tyrosine phosphatase; and autoantibodies that react with islet cell autoantigens. The purpose of this article on type 1 diabetes is to review some of this evidence with emphasis on the immunological and genetic factors involved in prediction of disease and destruction of beta cells. In humans, the MHC is known as the HLA complex and contains over 200 genes (3Klein J. Sato A. N. Engl. J. Med. 2000; 343: 702-709Crossref PubMed Scopus (543) Google Scholar). It is located on chromosome 6 and encodes HLA class I and class II molecules. The main function of these molecules, which are heterodimers made up of α and β chains, is to present antigens that have been processed into peptides to antigen-specific receptors on CD4+ and CD8+ T lymphocytes. Class I molecules, expressed on most nucleated cells, are encoded by genes within the HLA-A, -B, and -C loci, whereas class II molecules, expressed primarily on antigen-presenting cells (e.g. macrophages and dendritic cells), are encoded by genes within the HLA-DP, -DQ, and -DR loci. HLA class I and II genes are highly polymorphic and consist of many different alleles (4She J.X. Immunol. Today. 1996; 17: 323-329Abstract Full Text PDF PubMed Scopus (307) Google Scholar). In type 1 diabetes, certain HLA class II alleles or combinations of alleles (haplotypes) show a strong association with the development of diabetes, whereas other haplotypes show a weak or even protective association. For example, individuals with the HLA haplotype DRB1*0302-DQA1*0301, especially when combined with DRB1*0201-DQA1*0501, are highly susceptible (10–20-fold increase) to type 1 diabetes. In contrast, individuals with the haplotype DRB1*0602-DQA1*0102 rarely develop type 1 diabetes. Many other high and low risk haplotypes have been identified, and the frequency of specific haplotypes differs among ethnic groups. Other genes within the HLA complex, particularly class I genes, also have been linked to type 1 diabetes, but the strongest linkage by far is with the DQ and DR class II genes. Experimental support for the importance of class II genes in the development of diabetes comes from a variety of sources including the deletion of specific MHC loci in mice and their replacement with human HLA homologs. Although the linkage of HLA class II molecules with type 1 diabetes is now well established and the binding of peptides to pockets within the groove of the HLA class II molecule understood, why the binding of peptides to certain HLA class II molecules, and not to others, is associated with autoimmune type 1 diabetes remains unresolved. Regardless of mechanism, HLA typing has proved useful in population screening for identification and follow-up of individuals at high risk for disease. The initial evidence for autoimmunity in patients with type 1 diabetes came from immunofluorescence studies, which showed that a high percentage of sera from newly diagnosed type 1 patients reacted with pancreatic islets. This led to an intensive search for the actual autoantigens with which the islet cell autoantibodies (ICA) reacted. Three major autoantigens now have been identified. The first is an isoform of glutamic acid decarboxylase (GAD65) (5Baekkeskov S. Aanstoot H.J. Christgau S. Reetz A. Solimena M. Cascalho M. Folli F. Richter-Olesen H. DeCamilli P. Camilli P.D. Nature. 1990; 347: 151-156Crossref PubMed Scopus (1477) Google Scholar). GAD65 is a protein of 585 amino acids with a molecular weight of 65,000 encoded by a gene on chromosome 10p11. It is expressed in neuroendocrine cells, including pancreatic islets, and is located within neuron-like small vesicles. The function of GAD65 in the islets is not known. Between 60 and 80% of newly diagnosed type 1 diabetes patients have autoantibodies to GAD65. These antibodies are directed primarily to the middle and C-terminal portions of the molecule and recognize conformational epitopes. The second major autoantigen, IA-2 (also known as ICA512), is a member of the transmembrane protein-tyrosine phosphatase (PTP) family (6Lan M.S. Wasserfall C. Maclaren N.K. Notkins A.L. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 6367-6370Crossref PubMed Scopus (217) Google Scholar). It is 979 amino acids in length, has a molecular weight of 106,000, and is encoded by a gene on chromosome 2q35. Because of a critical amino acid replacement at position 911 (Asp for Ala), which is required for enzymatic activity, IA-2 is catalytically inactive. IA-2 is a transmembrane protein found in the secretory vesicles of both endocrine and neuronal cells (7Solimena M. Dirkx Jr., R. Hermel J.M. Pleasic-Williams S. Shapiro J.A. Caron L. Rabin D.U. EMBO J. 1996; 15: 2102-2114Crossref PubMed Scopus (228) Google Scholar). The function of this molecule has remained a mystery, but recent knock-out experiments suggest that it may play a role in insulin secretion (8Saeki K. Zhu M. Kubosaki A. Xie J. Lan M. Notkins A.L. Diabetes. 2002; 51: 1842-1850Crossref PubMed Scopus (100) Google Scholar). Nearly the same percentage (60–70%) of newly diagnosed patients with type 1 diabetes have autoantibodies against IA-2 as GAD. These autoantibodies are directed exclusively to the intracellular domain of IA-2 and recognize primarily conformational epitopes. Both IA-2 and the closely related protein IA-2β (also known as phogrin), which is encoded by a gene on chromosome 7q36, belong to the IA-2 subgroup of the PTP family, and homologs have been found in mice, rats, cows, macaques, zebrafish, Caenorhabditis elegans and Drosophila (9Notkins A.L. Lernmark A. J. Clin. Invest. 2001; 108: 1247-1252Crossref PubMed Scopus (212) Google Scholar, 10Lu J., Li, Q. Xie H. Chen Z.J. Borovitskaya A.E. Maclaren N.K. Notkins A.L. Lan M.S. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 2307-2311Crossref PubMed Scopus (198) Google Scholar). The third major autoantigen is insulin. This protein is only 51 amino acids in length, and its gene is on chromosome 11p15. As with IA-2 and GAD65, the majority of the autoantibodies recognize conformational epitopes, mainly on the B chain of insulin. Autoantibodies against insulin are among the first autoantibodies to appear in the prediabetic state and are usually found in very young children. Between 30 and 50% of young children with type 1 diabetes have autoantibodies against insulin (11Atkinson M.A. Eisenbarth G.S. Lancet. 2001; 358: 221-229Abstract Full Text Full Text PDF PubMed Scopus (1125) Google Scholar). The frequency of autoantibodies against insulin is substantially lower in individuals who develop type 1 diabetes at an older age. The identification of these autoantigens and the ability to prepare them in recombinant form has made it possible to develop rapid, sensitive, and reproducible radioimmunoassays, especially for IA-2 and GAD65, which have been standardized by a series of international workshops. Tests using these recombinant molecules have replaced the ICA immunofluoresence assay for most routine studies. By use of these tests, it is now estimated that up to 90% of newly diagnosed Caucasian subjects with type 1 diabetes have autoantibodies to one or more of the three major autoantigens. At first, the autoantibodies to IA-2, GAD65, and insulin were used for diagnosis and classification of patients with type 1 diabetes. It soon became apparent, however, that these autoantibodies appeared many months or years before the onset of clinical disease. This made it clear that type 1 diabetes is not an acute but a long term chronic disease. Both prospective and retrospective studies demonstrated that the presence of these autoantibodies could be used to predict individuals at risk of developing clinical disease. In fact, the number of autoantibodies, rather than the titer of the autoantibodies proved to be the key in predicting type 1 diabetes. Estimates based on first degree relatives of diabetic patients showed that the likelihood of developing type 1 diabetes within 5 years was ∼10% in the presence of one autoantibody, ∼50% in the presence of two autoantibodies, and 60–80% in the presence of three autoantibodies (9Notkins A.L. Lernmark A. J. Clin. Invest. 2001; 108: 1247-1252Crossref PubMed Scopus (212) Google Scholar). By screening a population for individuals who are positive for two or more of these autoantibodies, it is now possible to readily select subjects at high risk of developing type 1 diabetes for entry into therapeutic intervention trials long before their beta cell reserve is depleted. Autoantibodies also have provided new insight into adults classified as having type 2 diabetes. Five to ten percent of these individuals have autoantibodies to GAD65, 2–4% have autoantibodies to IA-2, and 1% or less have autoantibodies to insulin (12Turner R. Stratton I. Horton V. Manley S. Zimmet P. Mackay I.R. Shattock M. Bottazzo G.F. Holman R. Lancet. 1997; 350: 1288-1293Abstract Full Text Full Text PDF PubMed Scopus (661) Google Scholar). Based on these findings, GAD65 appears to be the predominant autoantigen in the older age group. Thus, either a number of patients classified as having type 2 diabetes have been misclassified and really have type 1 diabetes or some of these individuals have a combination of type 1 and type 2 diabetes. Although these numbers seem small, the issue is not trivial. Because about 16 million people in the United States have type 2 diabetes, if only 5% of these have been misclassified, the number of patients with type 1 diabetes would be almost doubled. Although autoantibodies have turned out to be excellent diagnostic and predictive markers for type 1 diabetes, it is generally thought that they play only a minor role, if any, in the actual pathogenesis of the disease. Instead, the cell-mediated immune response is believed to be responsible for beta cell killing. Inflammatory cells are found in and around the pancreatic islets. However, in some individuals these inflammatory cells are present for years without clinical symptoms. In fact, some individuals with autoantibodies and insulitis do not go on to develop clinical disease. The outcome appears to be related to the amount of beta cell destruction. It is estimated from animal studies that between 80 and 90% of the beta cells must be destroyed before the diabetes becomes clinically apparent. In humans, however, the temporal and quantitative relationships between inflammatory cells, beta cell damage, and clinical diabetes have been difficult to determine because in the United States pancreatic biopsies are not performed. As a consequence, much of our information about cell-mediated immune pathogenesis and beta cell killing comes from animal models, particularly NOD mice and BB rats. These animals spontaneously develop an autoimmune disease similar, although not identical, to human autoimmune type 1 diabetes (13Atkinson M.A. Leiter E.H. Nat. Med. 1999; 5: 601-604Crossref PubMed Scopus (511) Google Scholar). In mice and humans, there are two major classes of T lymphocytes: CD8+ cytotoxic lymphocytes, which recognize processed antigens (i.e. peptides) bound to MHC class I molecules on the surface of cells (e.g. beta cells), and CD4+helper lymphocytes, which recognize processed antigens bound to MHC class II molecules on the surface of antigen-presenting macrophages and dendritic cells (APCs). In type 1 diabetes, the direct (cell-to-cell) interaction between antigen-specific CD8+ cytotoxic T lymphocytes and autoantigens on beta cells results in beta cell killing (Fig. 2 A). In contrast, antigen-specific CD4+ helper T lymphocytes do not recognize autoantigens on beta cells because beta cells do not express MHC class II molecules. Instead, they act by recognizing autoantigens that have been picked up and processed by APCs expressing class II molecules. This indirect mechanism results in the release of a variety of effector molecules and is known as bystander killing (Fig. 2 B). Both direct and indirect killing (14Horwitz M.S. Ilic A. Fine C. Rodriguez E. Sarvetnick N. J. Clin. Invest. 2002; 109: 79-87Crossref PubMed Scopus (119) Google Scholar, 15Kay T.W. Thomas H.E. Harrison L.C. Allison J. Trends Endocrinol. Metab. 2000; 11: 11-15Abstract Full Text Full Text PDF PubMed Scopus (61) Google Scholar, 16Mathis D. Vence L. Benoist C. Nature. 2001; 414: 792-798Crossref PubMed Scopus (752) Google Scholar) are thought to occur by apoptosis following activation of caspases, but necrosis also might play some role. Based on animal models, it is now generally believed that multiple effector molecules and pathways are involved in beta cell killing. A critical question, independent of the mechanism by which the immune response kills beta cells, is what actually triggers the autoimmune cascade. Immunologic, genetic, and environmental factors have been implicated. Normally an individual's T lymphocytes are immunologically anergic or tolerant to self-antigens (3Klein J. Sato A. N. Engl. J. Med. 2000; 343: 702-709Crossref PubMed Scopus (543) Google Scholar). T lymphocyte education and selection takes place in the thymus. T cells that do not receive a signal from an HLA-autoantigen complex die by neglect. T cells that receive a signal from an HLA-autoantigen complex that is too strong die by apoptosis. However, T cells that receive a weak, low affinity signal from an HLA-autoantigen complex are positively selected. These positively selected autoantigen-specific T cells, generally present in very low numbers, escape from the thymus and migrate to peripheral organs throughout the body including the pancreas. Under ordinary circumstances they remain dormant and are kept under strict regulatory control by still poorly defined regulatory mechanisms (e.g. CD4+CD25+ and/or NK T lymphocytes) (17Piccirillo C.A. Shevach E.M. J. Immunol. 2001; 167: 1137-1140Crossref PubMed Scopus (612) Google Scholar, 18Kukreja A. Cost G. Marker J. Zhang C. Sun Z. Lin-Su K. Ten S. Sanz M. Exley M. Wilson B. Porcelli S. Maclaren N. J. Clin. Invest. 2002; 109: 131-140Crossref PubMed Scopus (608) Google Scholar). If, however, these antigen-specific T cells come in contact with cognate autoantigens presented by beta cells or APCs in the pancreas and if the regulatory controls fail, these dormant, antigen-specific T cells will be activated and the autoimmune cascade of beta cell killing will be initiated. Thus, immune dysregulation may serve as one of the triggers for autoimmunity. Genetic and environmental factors also have been implicated as possible initiating triggers (19Todd J.A. Bioessays. 1999; 21: 164-174Crossref PubMed Scopus (110) Google Scholar, 20Jun H.S. Yoon J.W. Diabetologia. 2001; 44: 271-285Crossref PubMed Scopus (111) Google Scholar). The fact that in identical twins the concordance rate for type 1 diabetes is less than 50% argues for a genetic predisposition upon which an environmental is than genes have been identified, but most of them have only a weak and in many the association has been difficult to (19Todd J.A. Bioessays. 1999; 21: 164-174Crossref PubMed Scopus (110) Google Scholar, Diabetologia. 2002; PubMed Scopus Google Scholar). The one is the HLA genes, which are thought to as much as 50% of the genetic risk for type 1 diabetes Diabetologia. 2002; PubMed Scopus Google Scholar). Although HLA genes may be by they seem not to be to the disease. a genetic of the evidence to type 1 diabetes as a complex disease a combination of several different genes. However, it may be that there is specific Instead, there may be the of normal genes (i.e. that at the level of the beta as apoptosis or antigen and in may an autoimmune In of environmental have been the In animal experiments have been to diabetes H.S. Yoon J.W. Diabetologia. 2001; 44: 271-285Crossref PubMed Scopus (111) Google Scholar). as act by in and pancreatic beta cells. A amino acid in the by its binding to beta cells, or not diabetes H.S. Notkins A.L. Yoon J.W. J. 1997; PubMed Google Scholar). The on the other diabetes not by beta cells but by the of the H.S. Yoon J.W. Diabetologia. 2001; 44: 271-285Crossref PubMed Scopus (111) Google Scholar). other are thought to or the autoimmune response molecular J. PubMed Google Scholar) or by autoantigens from beta cells. In a transgenic autoimmune N. D. M. Rodriguez F. S. J. 2000; PubMed Scopus Google the of lymphocytic to transgenic animals expressing in their beta cells results in diabetes, but the same not diabetes in In humans, and studies H.S. Yoon J.W. Diabetologia. 2001; 44: 271-285Crossref PubMed Scopus (111) Google Scholar) suggest that particularly may play a role, but most as a in individuals who have some autoimmune beta cell For the majority of the of type 1 diabetes in humans, a has not been the recent in the and of type 2 diabetes E. J.M. N. Engl. J. Med. 2002; PubMed Scopus Google Scholar) that and weight could the of clinical disease by the for most patients with type 1 diabetes, of has remained the same for the last 80 Based on the that type 1 diabetes is caused by immunologically destruction of beta cells, new for therapeutic intervention are and 2001; Scholar). These on of the autoimmune response by replacement of beta cells by and of insulin by genetic In to the far only in mice and rats, an a insulin under control of a was into diabetic animals K.S. Yoon J.W. Nature. 2000; PubMed Scopus (217) Google Scholar). The insulin gene and became in cells, blood glucose to normal for at least 6 This the for to of beta cells and also the autoimmune disease because beta cell is not Although some of these new therapeutic intervention the of type 1 diabetes, remains out of and identification of the genetic and/or environmental factors that actually the initial autoimmune Type 1 diabetes is caused by autoimmune destruction of pancreatic beta cells. Although still not in in humans, studies in animals that the destruction of beta cells is the result of a T immune response and that more than one effector pathway is At least different genes have been linked to type 1 diabetes, but with the of HLA genes, most of these have been studies, however, have three major GAD65, IA-2, and insulin. Autoantibodies against these autoantigens appear years before clinical and are markers for individuals at high risk of developing clinical disease. a very this has made type 1 diabetes a and autoantibodies now are used to high risk subjects for into therapeutic intervention the from type 1 diabetes in to autoantibodies as of disease may be to many of the 30 or more other chronic autoimmune as a autoimmune are the third of and disease and For some of these it is known that specific autoantibodies appear long before clinical symptoms. in the prospective studies the predictive of these autoantibodies, high for these autoantibodies will almost an of the routine D. P. Notkins A.L. J. Clin. Invest. 2001; 108: PubMed Scopus Google Scholar). I for and in this