[MUSIC] Let us now first consider how inflammatory beta cell destruction comes about in Type 1 diabetes. To understand how the immune system accomplishes this, we first need to discuss how the immune system is activated. The first step in the immune activation is engaging the innate system. And the most important cells here are the dendritic cells and the resident macrophage. These cells are residing in all tissues. Upon the recognition of dangerous signals, or in case of the release and uptake of antigen, this will activate these cells and they become so called antigen presenting cells. This process involves the uptake of the antigen, the processing of it, and then presentation of the antigen on the surface of the cell in the context of MHC-class 2 molecules. At the same time these cells activated to secrete inflammatory mediators that will both produce local harm to the tissue, but also serve as recruitment signals to the adaptive immune system. And so these compounds are called chemokines because they recruit cells of the adaptive immune system to the site of inflammation. Very important cell of the adapted immune system is the T-helper cell. This cell is able to recognize the presented antigen in the context of MHC class 2 by means of the t cell receptor complex. And when receiving a number of different ghost imagery signals this cell will be activated, secrete further mediators, such as cytokines and chemokines, that will both recruit further cells to the site of inflammation, but also activate these cells. T-helper cells are helping other immune cells to achieve their differentiated functions. For example, cytotoxic t-cells that can kill target cells. And other inflammatory T-cells, such as the TH 2 and TH 17 cell, that have differentiated functions in relation to antibody production and inflammatory functions in general, respectively. The T-helper cell, via the secretion of interferon gamma, will also activate in a feedback fashion the dendritic cells and activated macrophages in the target tissue and this leads eventually to a build up of inflammatory infiltrate inside the tissue and thereby to harmful effects on the target tissue. Now we have now discussed how the infiltrates build up. But this does not say anything about the actual molecular mechanisms of beta cell destruction. And these are still a matter of debate in the scientific community. There are two main schools of thought. From this figure on the top panel, you can see one school of thought. That indicate that simply that beta cell killing is a result of contact dependent killing, by contact of the cytotoxic t cell that recognizes the antigen now in context of the class one MHC and that will then trigger classical cytotoxic t-cell infecto systems such as those that are dependent upon the perforin grand sign and fast ligen, fast interactions. The other model is a bit more complicated because it indicates that there's no need of contact between the effector cells, and the pancreatic beta cells. In this case, the cytotoxic T-cells, and the helper-T cells, they act as cells that focus the infiltrate to the pancreatic islet. But then they orchestrate inflammation by activating the inflammatory infiltrate, and by orchestrating the secretion of inflammatory mediators, and it is those mediators that eventually cause the destruction of the pancreatic beta cells. It is believed that only the beta cells are destroyed because they are particularly susceptible to mechanisms elicited by these cytokines. Now how does beta cell failure occur in type 2 diabetes? Let us recapitulate the causes of type 2 diabetes. The etiology of type 2 diabetes implies both environmental and genetic causes. The most important environmental predisposing factor for type 2 diabetes is obesity. In terms of genetic susceptibility genome wide association scans have shown that most of the genes that are associated with the risk of developing type 2 diabetes when you become obese are in fact not encoding for insulin resistance. But for beta cell dysfunction and apoptosis. In addition to these classical genetic susceptibility factors, it is becoming increasingly evident that so-called epigenetic mechanisms, that is, heritable mechanisms that are not due to variations in DNA, playing an important role. For example, in the neonatal and intrauterine programming of a beta cell dysfunction and failure later on in life. And it's the combination of insulin resistance and failure to compensate for that, in terms of beta cell function, that leads to type-two diabetes. As in Type 1 diabetes, there are two main schools of thought regarding the molecular mechanisms underlying the development of insulin resistance in insulin-sensitive target tissues, and the betacell dysfunction. The classical school of thought suggests that as you become insulin-resistant, and as your levels of free fatty acids increase in circulation, this will negatively impact mitochondrial function leading to oxidative stress in fat cells, liver cells, muscle cells, even nerve cells, and also the pancreatic beta cell. And that it is this mechanism that leads to insulin resistance and beta cell dysfunction and even also contributes to vascular damage which is one of the hallmarks of type 2 diabetes late complications and the courses of action vascular end points. The other school of thought suggests that this is mainly an effect of inflammatory events occurring in insulin sensitive tissues and in the pancreatic islets. In this case, excess glucose, as diabetes develops and progresses as well as elevated free fatty acids in the metabolic syndrome in the pre-diabetic state, will lead to the liberation of inflammatory mediators, in particular from fat, but also from other cells, in insulin sensitive target tissues. And it is these inflammatory mediators that cause the molecular events inside the cells that lead to insulin resistance and beta cell dysfunction. So since the inflammatory school of thought implicates that inflammatory mediators can act on, for example, beta cells via surface receptors, what is then the evolutionary advantage of beta cells being capable of sensing these potentially dangerous inflammatory signals? The reason for the beta cell capability of sensing acute inflammatory stress is that it contributes to homeostasis. In states have, for example, bacterial infection, there will be low concentrations of circulating bacterial constituents that combine to so-called toll-like receptors on the surface of the beta cell, of which the beta cell express and ancly numbers of these toll-like receptors. And in addition, the activation of these bacterial constitutents of the immune system will lead to the secretion of inflammatory mediators such as TNF and Interleukin-1. And again, the beta cell expresses receptors for these molecules that lead to a fine tuning, if you wish of insulin secretion that will enable the body to compensate for the increased insulin needs during states of fever, of tissue damage, and also stress. However, evolution did not envisage that islets could becomes the site of immune cell infiltration and as you will remember, in type 1 diabetes, the islets are the site of a dense mononuclear cell infiltrate as part of the autoimmune reaction against the beta cells. And as I've also told you, in type 2 diabetes, we now know that there are inflammatory cells in the vicinity of the beta cells. A more discrete infiltration, but definitely a pathological infiltration of immune cells in the eye lid micro-environment. So in this case, beta cell sensing of a chronic inflammatory stress as that produced by high local concentrations and inflammatory mediators will lead to homeostatic failure. That is, for example, the activated resident macrophages producing interleukin-1 can both induce beta cell apoptosis and inhibit beta cell secretory function. In addition to that, we now know that the metabolic hallmarks of type two diabetes, elevated glucose, elevated free fatty acids, and even the secretion of the islet amyloid polypeptide, which is co-secreted with insulin, will activate both resident macrophages and beta cells to become inflammatory active cells. And this occurs via the NLRP3 inflammasome that is important for the processing of pro-AL1-beta, pro-AL18, and pro-AL33 into mature bioactive cytokines. It's this combination of metabolic stress and inflammatory stress that eventually causes the demise of the insulin secretory function and the viability of these cells. Apparently, the homeostatic and the pathogenic inflammatory signals also use distinct signaling pathways in the beta cell. We know that the homeostatic stimulation of inflammatory mediators of insulin secretion are dependent on the PLD and protein kinase C pathways, whereas the damaging pathways that are elicited by insulitis are dependent by classical inflammatory stress pathways such as those controlled NFkB and mitogen-activated protein kinase (MAPK) pathways. In that sense, the beta cell becomes a moving target of inflammatory mediators. Low concentrations, short-term exposures for these inflammatory mediators are beneficial to the body because it helps the beta cell to sense the increased need of insulin and thereby helps it to secrete more insulin to compensate for insulin resistance. But in the case of a chronic lasting infiltrate there is a gradual decrease of function eventually leading to beta cell destruction and apoptosis. So in that way we can now envisage a model where the beta cell of apoptotic processes in type 1 and type 2 diabetes are actually the result of a convergence on interleuken-1 as a common pathway. So just to summarize, in type 1 diabetes, the infiltrating immune cells will secrete cytokines such is interleukin-1 and interferon gamma. This can induce on the surface of the beta cell the Fas receptor, which can either ligate with the Fas ligand constitutively expressed by the beta cell, and this can induce the apoptotic program in the beta cell. Alternatively, activated T-cells that express the Fas ligand can also engage with a Fas receptor resulting also in the induction of the apoptotic program. And thereby, as you can clearly see, cytokine exposure of beta cells are important to the sensitization of the beta cell to killing by the adaptive immune cells. There are also independent pathways of Fas that lead to killing of beta cells via cytokines and those are elicited by the NFkB and MAP kinase pathways, in particular the C-Jun N-terminal kinase pathway. In type 2 diabetes, glucose, lipids, islet amyloid polypeptide as well as adipocytokines secreted from adipose tissue can induce resident macrophages to produce interleukin-1, but could also induce that production in the beta cell itself. And of course, this will lead to an increasing level of intra-islet interleukin-1 concentrations that can elicit the same death pathways that we have just been discussing, in terms of type 1 diabetes. So having introduced Interleukin 1 as a central converging mechanism for beta cell failure and destruction in type 1 and type 2 diabetes, let us now consider a bit more, what is Interleukin 1? So, interleukin 1 are seven kilodalton proteins, they are several family members. And they're expressed as 32 kilodalton precursors in response to pathogen- associated molecular patterns, such as LPS or other bacterial products, and many cytokines including R1 itself. The precursor form of Interleukin 1, the pro R1 molecule is process by protease called kaspase-1 that cleaves proteins between a cystine and aspartate residue upon activation of a multimeric protein complex, the so-called inflammasome. And the activation of this is caused by a co-called danger-associated molecular pattern. One important danger-associated molecular pattern is ATP. As you can imagine, this important energy source is not supposed to be outside the cells. So, if ATP is outside cells, it's a sign of cellular destruction and thereby cellular danger. There are receptors for ATP and these receptors can convey signals into the cells that are conveyed to the inflammasome. Most nuclear cells both produce and can respond to interleukin-1 via expression of interleukin-1 receptors. One is the protypic mediate of inflammation, fever, the acute phase response, and the innate defensive responses to microbes, trauma and stress. Interleukin-1 is in fact the cause that you feel so badly when you have an infectious disease like influenza, because it causes both the need of sleeping, it causes the headache and the muscle pain and it also causes the temperature rise characteristic of these infectious diseases. Apart from helping the body in coping with infectious diseases, interleukin-1 is also an effect of tissue destruction and fibrosis. Excessive effects of interleukin-1 in tissues can lead to functional failure and even destruction and scarring of the tissues after the eventual stress of the tissue has been cleared. So for these reasons, of course, interleukin-1 is the target of clinical treatment in many inflammatory diseases. Let me now introduce you to the interleukin-1 family proteins. The most important human interleukin-1 is interleukin-1 beta, which is a secreted protein, but there's also an interleukin alpha which is bound to the cellular membrane. And these two interleukins both activate the interleukin-1 receptor. There's also a naturally occurring interleukin-1 receptor antagonist that competitively binds and thereby displaces the interleukin agonist from the r1 receptor. There are two forms of interleukin-1 receptors. One is the interleukin-1 type 1 receptor which is a sickling receptor because it has a full transmembrane and introcytoplasmic domain. This receptor requires, apart from binding interleukin-1 alpha or beta, also the docking of the interleukin-1 receptor accessory protein, or R1 RAcp. In addition to the Type 1 receptor, there is a so-called decoy receptor, the interleukin-1 Receptor Type 2, which cannot transmit a signal, because it is truncated on the intracellular domain. And thereby serve as an inhibitor of interleukin-1 action, by serving as a sink that will attract interleukin-1 agonism away from the Type 1 receptor. The production of interleukin-1 depends on two different processes in interleukin-1 producing cells. First of all we need to have the interleukin-1 messenger-RNA transcribed and translated into pro-interleukin-1. But as I mentioned, this form in interleuken-1 is not biologically active. So it requires a second process, which is the inflammasome dependent processing that occurs via a caspase-1 activation that cleaves away the pro piece of interleukin-1, liberating the fully mature biologically active interleukin-1. [MUSIC]
