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Insulin-Dependent Diabetes Mellitus
An Autoimmune Disease
Insulin-dependent diabetes mellitus (IDDM), commonly referred to as type-I diabetes, is a condition caused when an autoimmune response induces the death of insulin-secreting b cells in the pancreas. Insulin (see Figure 1) is the protein that is responsible for transporting glucose (secreted by pancreatic a cells) from the blood into the cells, where the glucose is metabolized for energy. When b cells are killed the body has no way of producing insulin, so glucose levels in the blood are unable to be controlled, leading to hyperglycemia, or high blood glucose level. Longterm complications of hyperglycemia include cardiovascular, kidney, and eye diseases, as well as various nervous system disorders. (diabetes.com, 2000)
IDDM tends to run in families, and there is substantial evidence that genetics plays a significant role in causing the disease. However, studies have shown that the concordance rate for IDDM among monozygotic twins is less than 50 percent, meaning that environmental factors must also play a significant role. (Tisch et al., 1996). The fact that both environment and genetics contribute to IDDM has led to the assumption that the autoimmune response causing insulitis and autoantibody release (see pathogenesis below) is generally triggered by an environmental stimulus, but occurs primarily in those who are genetically predisposed to the disease.
Genetic susceptibility to IDDM, as with most autoimmune diseases, is believed to be primarily related to the MHC genotype. The MHC II genes that appear to play the most significant role in IDDM susceptibility are found at the HLA-DR locus, which is linked tightly to HLA-DQ. The two alleles HLA-DR3 and HLA-DR4 seem to be the strongest genetic causes of IDDM, since nearly all diabetics express at least one, or more often both of these alleles. Conversely, a third allele, HLA-DR2, is commonly expressed in non-diabetic individuals, but very seldom appears among diabetics, which suggests that this allele helps prevent IDDM. The HLA-A, HLA-B, and HLA-C genes encoding MHC I are also believed to be associated with IDDM. Both CD8 and CD4 T cells have been shown to play significant roles in mediating the IDDM autoimmune response, so it is reasonable that the genetic source is associated with genes encoding MHC I and MHC II, the molecules with which CD8 and CD4 T cells interact, respectively. (Janeway et al., 1999) A figure showing a map of the human MHC complex, with labeled IDDM-associated loci, can be found at <http://www.harrisonsonline.com/marketing/sample/entrypages/public/ch334/334_fig1.htm>.
Although IDDM susceptibility has an apparent genetic origin, what exactly is responsible for stimulating the autoimmune response remains unclear. The most common examples of the various possible external factors that have been shown to trigger a response leading to IDDM include:
Viruses—Strong correlations between IDDM and exposure to certain viruses such as those causing German measles, mumps, and certain variants of Polio, especially at a young age, suggest that these viruses can stimulate a b cell autoimmune response. It is possible that these viruses induce activation and proliferation of T cells specific for a viral epitope that mimics a protein unique to b cells, therefore causing armed T cells to respond to the b cell autoantigens. (diabetes.com, 2000) Drugs and chemicals—Pyriminil (rat poison) and pentamidine, a drug used to treat pneumonia, are among several synthetic chemicals and drugs that have been shown to induce IDDM. (diabetes.com, 2000) Cow’s milk—Although it remains controversial, one theory suggests that exposure to cow’s milk during infancy can induce an autoantibody response to p69, a protein often expressed by b cells. Expression of p69 on b cells
can be induced by IFN-g, which could be present due to a viral infection. This theory therefore works in conjunction with the virus theory, claiming that infant exposure to cow's milk elicits antibodies capable of attacking b cells later in life following a viral infection. (Harrison's Online, 2000)
The two distinctive features of IDDM are the infiltration of pancreatic islets by macrophages and lymphocytes, a condition known as insulitis (see Figure 2), and the presence of autoantibodies in the serum. (Haskins et al., 1996) Both of these events serve as markers for the prediabetic phase of IDDM.
More than a dozen islet-cell proteins that elicit antibody responses in type-I diabetics have been identified, and the presence of these autoantigens (and/or their corresponding autoantibodies) serve as important diagnostic tools for early identification of IDDM. Among the apparently more significant autoantigens are:
Insulin and GAD are the two evidently self-reactive proteins that have been studied most in depth since both of these autoantigens consistently indicate autoimmune activity in the pancreatic islets. (Nepom, 1995) However, although the identification of autoantibodies in the serum has been valuable to the study of which proteins are involved in b cell autoimmunity, the precise role of these antibodies in the pathogenesis of IDDM remains unclear. Therefore, a greater emphasis has been placed on studying the cell-mediated (T cell) rather than the humoral (antibody) response. This shift in focus has led to the discovery that many of these autoantigens, including GAD and insulin, are targeted not only by autoantibodies, but also by T cells. (Haskins et al., 1996)
- 37k antigen--The antibodies to this autoantigen are found in over half of all type-I diabetics, and these autoantibodies are unique to diabetes and are not found in patients suffering from other endocrine-related autoimmune diseases.
- b-granule antigen--This autoantigen has not been completely identified yet, but it is known the be the target for the Th1 T cell clones derived from NOD mice.
- GAD--Glutamic acid decarboxylase (GAD) exists in two isoforms of 65kD (GAD65) and 67kD (GAD67).
Insulitis, also referred to as isletitus, occurs when macrophages and activated T cells invade the islets of
Langerhans in the pancreas and attack the insulin-secreting b cells. Insulitis occurs during the prediabetic phase, ultimately leading to the complete depletion of b cells which is characteristic of IDDM. Analysis of pancreatic islets during this stage has revealed the presence of both CD4 and CD8 T cells, and tests have indicated that both are necessary to induce b cell autoimmunity, but the role of CD4 is more clearly understood. One relatively new strategy used to examine the respective roles of CD4 and CD8 T cells in insulitis has been to create T cell clones of T cell lines derived from the islets of nonobese diabetic (NOD) mice. Through adoptive-transfer experiments it has been possible to identify specific T cells as diabetogenic, or able to induce diabetes in healthy animals. Studies using this method concluded that although CD8 T cells are generally present and active in the b cell autoimmune response, these cytotoxic T cells usually failed to induce diabetes in the absence of CD4, specifically Th1 T cells. CD4 T cells generally appeared to be responsible for initiating the autoimmune response and recruiting other cells, including CD8 T cells, to the pancreas, which explains the dependency of cytotoxic T cells on Th1 cells. In some cases, the particular antigen specificities of the T cell clones have been identified, making it possible to study the diabetogenicity of T cell clones specific to proteins such as GAD and insulin, as well as other pancreatic proteins. (Haskins et al., 1996) In one study by Zekzer et al., a line of CD4 Th1 T cell clones specific to GAD65 were derived from NOD mice that developed diabetes in response to purified GAD injection. These Th1 cells, called 5A T cells, were shown to induce insulitis spontaneously in NOD mice through adoptive transfer. (Zekzer et al., 1998) Another related study by Tian et al. demonstrated that a single intranasal administration of GAD65 peptides to NOD mice elicited a Th2 rather than Th1 response. The fact that these NOD mice did not develop IDDM suggests that the Th1 autoimmune response was inhibited by stimulating a dominantly Th2 GAD-specific T cell population. (Tian et al., 1996) Although these studies provide strong evidence that GAD-reactive Th1 cells are capable of inducing b cell autoimmunity in genetically predisposed NOD mice, there is no evidence establishing that autoreactive Th1 cells are limited to GAD specificity. (Zekzer et al., 1998)
Several studies have demonstrated an important role of many apoptotic pathways, specifically Fas/Fas Ligand (FasL), perforin, and TNF-a pathways, in the T cell-induced apoptosis of b cells. A study by Su et al., using a population of mutated Fas-deficient NOD mice (NOD-lpr/lpr), demonstrated that NOD mice that do not express Fas fail to develop IDDM. This study also showed that NOD-lpr/lpr mice were also resistant to adoptive transfer of diabetogenic T cells, suggesting that the Fas/FasL apoptotic pathway is the primary method of killing b cells. (Su et al., 2000) A related study suggested that the cytokines IL-a, IL-1b, and IFN-g somehow prime b cells to be destroyed by CD4 T cells. Amrani et al. identified a diabetogenic b cell-specific T cell receptor (4.1-TCR) capable of inducing apoptosis of b cells via the Fas/FasL pathway, and demonstrated the importance of these cytokines, as well as Fas, by observing that cytokine-treated NOD-lpr/lpr mice introduced with 4.1-TCR T cells failed to develop IDDM, as did untreated NOD mice. Cytokine-treated NOD mice, on the other hand, quickly developed IDDM. The study suggests that the Fas/FasL pathway is the initial process of inducing b cell apoptosis, but is followed later by perforin-mediated CD8 cytotoxicity. (Amrani et al., 2000) This suggestion is consistent with the earlier mentioned studies which demonstrated that CD4 T cell infiltration into the islets preceded that of CD8 T cells.
Despite the vast amount of research that has been done exploring the mechanisms which lead to IDDM, we have only reached the tip of the iceberg. The fact that so many different proteins and pathways have been demonstrated to be relevant to the development of IDDM suggests that it may take significantly more time and effort until a clear understanding of the pathogenesis of this disease. The complexity of the disease is further amplified by the likelihood that there are many different causes possibly leading to a multitude of various mechanisms by which autoimmunity is induced and b cells are killed.
Since it remains unclear what causes the autoimmune response leading eventually to IDDM, and it is almost certain that there are numerous such causes, we are still quite far away from developing effective means of preventing the onset of IDDM. However, recent studies with pancreatic stem cells have shown a lot of promise to providing a possible cure for type-I diabetic individuals with fully depleted b cell populations. Experiments have been conducted in which pancreatic islet stem cells were removed from NOD mice that had not yet developed IDDM. These stem cells were cultured and differentiated into a population of mature, insulin-producing islets of Langerhans. The cells, when inserted into fully diabetic NOD mice, seemed to reverse diabetes since these mice continued to be able to produce insulin in response to glucose after being gradually weaned off of insulin injections. A similar process could be used for humans, in which pancreatic tissue taken from cadavers could be cultured and transplanted into type-I diabetic patients. (Berger, 2000) This possibility, however, brings up the issue of possible graft rejection. Transplantation creates the need for immunosuppressant drugs, such as cyclosporin, to prevent rejection, and there is significant controversy over whether it is worthwhile to compromise the effectiveness of the immune system in general to attain good blood glucose control. IDDM is presently treated very effectively with regular routine of insulin injections (usually 1-2 injections per day), blood glucose monitoring, and balanced diet. These daily routine treatments allow type-I diabetics to lead a fairly normal lifestyle, and when blood glucose levels are closely monitored and controlled with insulin injections, the risk of complications caused by hyperglycemia is greatly reduced. Transplantation may present a cure, but there is reason to believe that immunosuppresants may present more health risks than well-controlled IDDM, especially as methods for maintaining normal blood glucose levels continue to improve. (Harrison's Online, 2000)
Figure 1: Chime figure of an R6 Human Insulin Hexamer. The figure shows 6 insulin proteins, each consisting of two peptide chains linked by 2 disulfide bonds. Image obtained from the National Center for Biotechnology Information (NCBI) <http://www.ncbi.nlm.nih.gov/Structure/>
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Figure 2: This is an insulitis of an islet of Langerhans in a patient who will eventually develop type I diabetes mellitus. The presence of the lymphocytic infiltrates in this edematous islet suggests an autoimmune mechanism for this process. The destruction of the islets leads to an absolute lack of insulin that characterizes type I diabetes mellitus. Image and text used by permission from Edward C. Klatt, MD. <http://www-medlib.med.utah.edu/WebPath/ENDOHTML/ENDO040.html>
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-Amrani A, Verdaguer J, Thiessen S, Bou S, Santamaria P. 2000. IL-1a, IL-1b, and IFN-g mark b cells for Fas-dependent destruction by diabetogenic CD4+ T lymphocytes. 105:459-468.
-Berger A. 2000. Transplanted pancreatic stem cells can reverse diabetes in mice. BMJ 320:736.
-Diabetes.com: What is Diabetes? <http://www.diabetes.com/health_library/articles/l3t100117.html> Accessed 2000 April 18.
-The Internet Pathology Laboratory for Education: Endocrine Pathology: Image of an Insulitis of the Islet of Langerhans. <http://www-medlib.med.utah.edu/WebPath/ENDOHTML/ENDO040.html> Accessed 2000 April 18.
-Harrison’s Online. Chapter 334: Diabetes Mellitus
<http://www.harrisonsonline.com/marketing/sample/entrypages/public/ch334/334_index.htm> Accessed 2000 April 18.
-Haskins K, Wegmann, D. 1996. Diabetogenic T-Cell Clones. Diabetes 45:1299-1405.
-Janeway CA, Travers P, Walport M, Capra JD. Immunobiology: The Immune System in Health and Disease. New York: Garland Publishers, Fourth Edition, 1999.
-Nepom GT. 1995. Glutamic acid decarboxylase and other autoantigens in IDDM. Curr Opin Immunol 7:825-830.
-Su X, Hu Q, Kristan JM, Costa C, Shen Y, Gero D, Matis LA, Wang Y. 2000. Significant Role for Fas in the Pathogenesis of Autoimmune Diabetes. J Immunol 164:2523-2532.
-Tian J, Atkinson MA, Clare-Salzler M, Herschenfeld A, Forsthuber T, Lehmann PV, Kaufman DL. 1996. Nasal Administration Of Glutamate Decarboxylase (GAD65) Peptides Induces Th2 Responses and Prevents Murine Insulin-dependent Diabetes. J Exp Med 183:1561-1567.
-Tisch R, McDevitt H. 1996. Insulin-Dependent Diabetes Mellitus. Cell 85:291-297.
-Zekzer D, Wong FS, Ayalon O, Millet I, Altieri M, Shintani S, Solimena M, Sherwin RS. 1998. GAD-reactive CD4+ Th1 Cells Induce Diabetes in NOD/SCID Mice. J Clin Invest 101:68-73.
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