type 1 diabetes ( t1d ) is an autoimmune disease characterized by the progressive destruction of insulin - producing cells in the pancreatic islets by autoreactive t cells , which eventually leads to hyperglycemia . the disease accounts for about 10% of all cases of diabetes , occurs most commonly in people of european descent , and affects two million people in europe and north america . there is a marked geographic variation in the incidence , probably because the different populations vary in genetic susceptibility / resistance factors or in exposure to environmental triggers . for instance , a child in finland ( northern europe ) is about 80 times more likely to acquire the disease than a child in china ( eastern asia ) . the current global increase in incidence of 3% per year is well established [ 3 , 4 ] , and this rapid rise strongly suggests that environmental factors should be acting on susceptibility genes and contributing to the evolving epidemiology of t1d . in patients with t1d , daily delivery of insulin by injection or however , this exogenous insulin delivery can not achieve physiological control of blood glucose concentrations and also has the risk of causing hypoglycemic episodes . moreover , a significant proportion of patients suffers chronic and degenerative complications , such as nephropathy , retinopathy , and vascular and heart disease [ 5 , 6 ] . the appropriate treatment to achieve insulin independence for t1d is replacement of the -cell mass , currently being accomplished through whole pancreas transplantation and islet transplantation . transplantation of the whole pancreas is a standard treatment for diabetic patients , which can achieve insulin independence with a single donor . pancreatic islet transplantation is a safer and less invasive method than whole - organ transplant therapy , which causes thrombosis , pancreatitis , and peritonitis . however , the major drawback of islet transplantation compared with pancreas transplantation is the greater requirement for donors and the lower 5-year insulin independence rate . transplantation therapy should provide a better quality of life than current therapies and should help avoid complications . unfortunately , immune - mediated destruction and inadequate numbers of donor organs for transplantation are the major obstacles to achieving insulin independence and long - term survival of grafts in this therapy . to circumvent those problems , genetically modifying islets to enhance their resistance to immune attack and explorations of alternative sources of insulin - secreting cells are being investigated intensively . in this paper , we will summarize the current knowledge regarding immunomodulatory therapy in islet transplantation and examine alternative sources of insulin - secreting cells for cell replacement therapy . the process of islet isolation also triggers a cascade of stressful events in the cells involving the induction of apoptosis or necrosis and production of proinflammatory molecules that negatively influence islet viability and function . transplantation procedures such as collagenase - based islet isolation trigger proinflammatory cytokine and chemokine production by the islets . proinflammatory cytokines such as interleukin- ( il- ) 1 and tumor necrosis factor- ( tnf- )  produced by islet - resident macrophages are toxic to islets and can induce the local production of reactive oxygen species ( ros ) [ 9 , 10 ] . by contrast , chemokine receptors such as chemokine ( c - c motif ) receptor ( ccr)2 , ccr5 , and c - x - c chemokine receptor ( cxcr)3 , and their ligands are crucial to generate acute islet allograft rejection . taken together , the inflammatory cytokines , chemokines , and ros contribute to the first line of attack to the islets , which can cause apoptosis and loss of function . previous reports have demonstrated that th1 cells , type 1 cytotoxic cd8 t cells , and th1-type cytokines such as interferon ( ifn)- and il-2 are commonly associated with graft rejection [ 1215 ] . th1 responses initiate allograft rejection by promoting cytotoxic t - cell activities and ifn--mediated delayed - type hypersensitivity reactions , whereas th2 responses cause allograft damage through the recruitment of eosinophils induced by il-4 and il-5 . moreover , cytokines produced by non - t - cell sources from the graft microenvironment , such as il-7 produced by stromal cells and il-15 produced by activated macrophages and endothelial cells , further support the idea of alloreactive t - cell proliferation [ 14 , 16 ] . recently , the role of subsets of th cells in graft rejection has been reexamined after the identification and thorough characterization of th17 cells . several lines of evidence have demonstrated that th17 cells have the capacity to cause rejection of cardiac allografts [ 1719 ] . however , it is not clear whether th1 and th17 cells work synergistically or sequentially to cause graft rejection . in our laboratory , we have investigated the relative contribution of th1 and th17 cells in the autoimmune - mediated rejection of islets in a nonobese diabetic ( nod ) mouse model . we have demonstrated that th1 cells play a crucial role in the destruction of the islet graft , whereas th17 cells constitute a much smaller population in the islet graft and might play only a minor destructive role in this model of autoimmune islet transplantation ( chou et al . , manuscript in revision ) . in summary , understanding the inflammatory factors that attack islets during the early phase and dissecting the role of the effector t - cell subsets in the rejection responses might contribute to developing target therapies to protect islets from inflammatory insults and to modulate t - cell responses . in autoimmune diabetes , pancreatic  cells suffer from inflammatory stress following t - cell - mediated destruction . macrophages and/or dendritic cells in the islet microenvironment produce proinflammatory cytokines and free radicals , which induce -cell damage . activated t cells express death receptors and release cytotoxic molecules including granzyme b , perforin , or cytokines to further activate other immune cells and exacerbate -cell death . several preventive and therapeutic approaches have been demonstrated to protect  cells from immune attack , including the modulation of t - cell activity and inhibition of inflammatory responses in the islet microenvironment . to protect islets from immune attack , many gene targets that exhibit strong immunoregulatory effects and antiapoptotic effects have been introduced to the islets through different approaches : generation of transgenic mice using islet - specific promoters to carry genes of interest , delivery of genes into islets by viral vectors or transfection , and the administration of recombinant proteins and drugs . genetically manipulating islets by transgenic techniques was originally designed for study of the immunopathogenesis of autoimmune diabetes . this could help in dissecting the roles of different cytokines , death receptors , and major histocompatibility complex ( mhc)/costimulation molecules in -cell destruction ( reviewed in ) . among the transgenic - mouse models that have been generated , some molecules display strong immunoregulatory functions and cytoprotective effects , which could be applied further in islet transplantation therapy ( table 1 ) . it is well established that proinflammatory cytokines and th1-type cytokines are toxic to islets whereas il-4 and transforming growth factor- ( tgf- )  are postulated to be protective . transgenic expression of il-4 in  cells under the control of the insulin promoter in nod mice suppresses insulitis and diabetes ; however , islet expression of il-4 is incapable of preventing islet rejection in diabetic recipients . in other studies , expression of tgf- driven by an insulin promoter   or a glucagon promoter   protected islets from autoimmune destruction in nod mice . however , the -cell - specific expression of tgf- changes the pancreatic architecture , and this tgf--expressing pancreatic tissue fails to inhibit allograft rejection . in other aspects , the inhibition of toxic cytokine signaling in islets represents an attractive strategy in designing therapies to prevent islet destruction . islets with transgenic expression of suppressor of cytokine signaling 1 ( socs1 ) show delayed allograft rejection but can not circumvent destruction of the islets by the autoimmune destruction . t - cell activation occurs through two important signals : one is the t - cell receptor recognizing a specific peptide mhc complex and the other is a costimulatory signal . upon the t - cell activation , the programmed death ( pd)-1 and cytotoxic t - lymphocyte antigen ( ctla)-4 are two important negative costimulatory molecules expressed on t cells , which control their effector functions , tolerance , and autoimmunity . we have demonstrated that transgenic expression of pd - l1 ( ligand of pd-1 )   or a membrane - bound , agonistic single - chain anti - ctla-4 fv antibody ( anti - ctla-4 scfv )   on islets in nod mice reduces the severity of insulitis and suppresses the development of diabetes . in an islet transplantation study , transgenic anti - ctla-4 scfv prolonged islet graft survival and reduced the th1 cell counts in islet grafts after transplantation into spontaneous diabetic nod mice . however , the expression of pd - l1 on islets could not prolong graft survival . the role of pd - l1 in the regulation of t - cell tolerance to islets needs to be further investigated because the transgenic expression of pd - l1 on islets in mice with a b6 background induced t - cell - mediated spontaneous diabetes , and the islets from transgenic mice displayed accelerated rejection in an allogeneic transplantation model . inflammatory cytokines such as il-1 , tnf- , and ifn- sensitize  cells to fas - dependent and/or other death receptor - mediated apoptosis   and induce ros formation in  cells . because islets produce very low levels of antioxidative enzymes and are very sensitive to oxidative stress , the reduction of ros levels in islets is crucial for maintaining the function and viability of islets . others and we have demonstrated that -cell - specific expression of the antiapoptotic and anti - inflammatory proteins , thioredoxin ( trx )   or heme oxygenase-1 ( ho-1 ) , prevented autoimmune diabetes in nod mice . moreover , the islets from ho-1 transgenic mice survived longer in diabetic recipients , indicating that control of the initial inflammatory responses can promote graft survival . the roles of ros scavengers in islet transplantation have also been investigated in transgenic mouse models . thus , several -cell - specific transgenic mice with different antioxidant enzymes have been generated ( reviewed in ) . in general , islets with transgenic antioxidative genes ( e.g. , catalase , glutathione peroxidase , metallothionein , copper / zinc superoxide dismutase , and manganese superoxide dismutase ) are resistant to oxidative stress induced by chemicals [ 4042 ] or hypoxia ; however , they are still sensitive to proinflammatory cytokines induced cytotoxicity . among these transgenic mice , islets from metallothionein transgenic mice showed prolonged survival of islet grafts in an allogeneic transplantation model . in summary , most ex vivo studies have shown that overexpression of antioxidative genes in islets protects them from oxidative injury ; however , the in vivo function and survival of these genetically modified islets in diabetic recipients have not produced overt success . the direct delivery of protective and therapeutic genes to islet grafts can overcome many problems ; for example , the therapeutic agents can not be targeted locally and might have effects on other organs or tissues , causing unexpected side effects . by using gene therapy , islets can be manipulated by any vector system ex vivo without exposing the recipient to the vectors . moreover , graft - specific gene therapy can provide prolonged , safe , and locally controlled gene expression . in this regard , ex vivo manipulation of islets by gene transfer systems becomes an attractive approach to protect grafts from immune attack . nonimmunogenic vectors that can not activate the host 's immune response are used for long - term gene expression . many strategies have been proven to improve the function of islet grafts and protect grafts from immune attack ( table 2 ) . these approaches include blockade of co - stimulation signals by ctla-4-ig [ 45 , 46 ] ; downregulation of th1 responses by overexpression of galectin-9 ( chou et al . , manuscript in revision ) ; overexpression of antiapoptotic and antioxidative molecules such as b - cell lymphoma ( bcl)-2 , trx , and superoxide dismutases ( sods ) ; blockade of inflammatory cytokine signaling by overexpression of il-1 receptor antagonist protein ; overexpression of anti - inflammatory cytokines such as tgf- , il-10 , and il-4 . in summary , these strategies significantly reduce apoptosis in islet grafts and prolong graft survival in diabetic recipients . however , the application of these protective genes to transplantation therapy has not been successful . in general , therapeutic targets that have paracrine actions would exert more marked biological effects than membrane - bound or intracellular molecules . moreover , the efficiency of gene delivery to islets and the expression levels of target proteins in the microenvironment of grafts are closely linked to the protective effect in the grafts . therefore , better results might have been obtained by using a  cocktail  therapy , for example , combining antiapoptotic and anti - inflammatory genes , which could display synergistic protective effects . although islet transplantation is seen as a  cure  therapy for diabetes , this procedure is hampered by the limited number of donors for isolating islets . many alternative approaches that can be applied to obtain insulin - secreting cells are being investigated intensively . these include the following : ( 1 ) the production of surrogate cells by genetically modifying nonendocrine cells to secrete insulin in response to glucose challenge , ( 2 ) the transdifferentiation of nonendocrine stem / progenitor cells or mature cells to glucose - responsive adult tissues [ 60 , 61 ] , ( 3 ) the regulated differentiation of islet stem / progenitor cells to produce large numbers of mature , functional islets [ 62 , 63 ] , ( 4 ) the in vitro differentiation of stem cells to become insulin - secreting cells , and ( 5 ) the in vitro differentiation of induced pluripotent stem cells ( ipscs ) derived from patients to form pancreatic -like cells . stem cells can reproduce themselves ( self - renew ) and can differentiate into many cell types . besides , stem cells have strong immunosuppressive effects and can secrete many trophic factors that promote the regeneration of damaged tissues . there are many stem cell types available as a potential source for the generation of insulin - producing cells , including embryonic stem cells ( escs ) , adult stem cells , and , most recently , the ipscs . previous reports have demonstrated that escs can be induced to become insulin - secreting tissue with structures similar to pancreatic islets [ 64 , 65 ] , however , these cells are often unresponsive to glucose or produce lower levels of insulin compared with the endogenous  cells , which is insufficient to control normoglycemia in diabetic recipients in a mouse model . escs have not yet been used therapeutically for treating diabetes mellitus in humans because the animal experiments have not progressed sufficiently to justify this approach ; for example , the positive insulin staining of esc - derived pancreatic - like tissue probably occurs by uptake of insulin from the culture medium , and the intermediate stages involved in the differentiation pathway are complicated and not fully understood . in addition , the clinical applications of human escs are limited by ethical concerns , as well as the potential for teratoma formation . in addition to the escs produced from embryos , mesenchymal stromal cells ( mscs ) and ipscs with fewer limitations or restrictions in ethical concerns in the research and clinical settings have become ideal cell types for regeneration therapies . previous reports have demonstrated that mscs can be differentiated into islet - like cell clusters , and these cell clusters can reverse the diabetic status after transplant into streptozotocin- ( stz- ) induced diabetic recipients . several lines of evidence have demonstrated that cotransplantation of islets and mscs produces superior outcomes to islet transplantation alone . in cotransplantation therapy , besides , mscs also express immunoregulatory molecules , which might help to reestablish peripheral tolerance in diabetic recipients and to prevent allorejection . recently , ipscs generated from reprogrammed somatic cells have become an exciting alternative source of cells with pluripotent characteristics similar to escs . this concept has been tested further using human skin fibroblasts   or cells from skin biopsies from patients with t1d   to generate ipsc - derived islet - like clusters . these cells can secrete c - peptide and respond to high concentrations of glucose , suggesting that functional  cells might eventually be generated from ipscs . in further studies , transplantation of ipsc - derived pancreatic beta - like cells reversed hyperglycemia in diabetic mouse models . these data provide an initial proof of principle for the potential clinical application of ipscs . besides , if ipscs could be generated from patients with diabetes , they would have the same genotype as the recipient , avoiding the problem of immune rejection . unexpectedly and surprisingly , autologous ipscs reprogrammed from fetal fibroblasts by viral or nonviral genetic approaches elicit t - cell - dependent immune reactions in genetically identical mice , resulting in their rejection . this is likely because of the abnormal expression of antigens in the ipscs , leading to a breakdown of peripheral tolerance . . clinical trials of islet transplantation are showing remarkable success since the edmonton protocol was developed , and this glucocorticoid - free immunosuppressive protocol was replicated successfully . however , the long - term success of this procedure is limited by the effects of allograft rejection and recurrent autoimmunity . moreover , the scarcity of organ donors also frustrates the treatment of t1d . to overcome these problems , researchers have developed many strategies to modulate the detrimental immune responses and have also explored the alternative sources of insulin - secreting cells . gene therapy offers a powerful tool to engineer islet grafts to become resistant to inflammation - induced apoptosis , as well as modifying islets to produce immunosuppressive molecules to attenuate t - cell response . the use of stem cells in the generation of renewable and functional  cells is now a promising reality . moreover , based on current knowledge about genomic cell reprogramming , it should be possible to develop patient - specific , autologous cell replacement therapy by using ipsc - derived pancreatic -like cells . although these proofs of concepts for potential preclinical applications show a big breakthrough in this field , some issues need to be considered : ( 1 ) the duration and expression levels of targeted genes in islets , ( 2 ) the use of viral vectors for direct gene therapy raises the possibility of insertional mutagenesis ( retroviruses and lentiviruses ) and host immunogenicity ( adenoviruses ) , and ( 3 ) the efficiency of differentiation of insulin - secreting cells from stem cells . in conclusion , further investigations are required to develop the most potent graft - specific immunoregulatory therapies and to generate safe and stable sources of insulin - secreting cells for clinical islet transplantation or cell replacement treatments .
islet transplantation is a promising therapy for patients with type 1 diabetes that can provide moment - to - moment metabolic control of glucose and allow them to achieve insulin independence . 
 however , two major problems need to be overcome : ( 1 ) detrimental immune responses , including inflammation induced by the islet isolation / transplantation procedure , recurrence autoimmunity , and allorejection , can cause graft loss and ( 2 ) inadequate numbers of organ donors . 
 several gene therapy approaches and pharmaceutical treatments have been demonstrated to prolong the survival of pancreatic islet grafts in animal models ; however , the clinical applications need to be investigated further . 
 in addition , for an alternative source of pancreatic -cell replacement therapy , the ex vivo generation of insulin - secreting cells from diverse origins of stem / progenitor cells has become an attractive option in regenerative medicine . 
 this paper focuses on the genetic manipulation of islets during transplantation therapy and summarizes current strategies to obtain functional insulin - secreting cells from stem / progenitor cells .