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Our understanding of the genetics, aetiology and pathogenesis of Type 1 Diabetes (T1D) was propelled by the discovery of animal models of T1D in the late 1970s and early 1980s, particularly the non-obese diabetic (NOD) mouse. Since then, transgenic and gene-targeting technologies allowed the generation of many models with reduced genetic and pathogenic complexity. These models allowed researchers to zoom in on specific aspects of this complex disease. In this review, we provide an overview of currently available mouse models for T1D.
Type I diabetes mellitus, also called juvenile diabetes or insulin-dependent diabetes, is a chronic autoimmune disorder that precipitates in genetically susceptible individuals by environmental factors. The body's own immune system attacks the β-cells in the islets of Langerhans of the pancreas, destroying or damaging them sufficiently to reduce and eventually eliminate insulin production. When >80–90% of the β-cells have been destroyed, the production of glucagon by the neighboring α-cells is de-repressed. The metabolic consequences of insulin insufficiency and glucagon excess are hyperglycemia and ketoacidosis. Type 1 diabetes is distinguished from type 2 (non-insulin-dependent) diabetes by the presence of autoantibodies, the genetic link, the insulin dependence and by insulitis, which is characterized by activated T lymphocytes.
A widely used virus-induced diabetes model is the LCMV infection of RIP-GP or R/HIP-NP on the C57Bl/6, Balb/c, or NOD background. RIP-GP mice are transgenic mice expressing the lymphocytic choriomeningitis virus glycoprotein (LCMV-GP) under control of the rat insulin promoter (RIP), resulting in expression of LCMV-GP in pancreatic β-cells [1,2]. There are two strains of RIP-GP mice: the RIP-GP Berlin mice express the LCMV-GP from LCMV strain WE , and the RIP-GP Arm mice express the LCMV-GP from LCMV strain Armstrong . Naïve RIP-GP mice do not develop diabetes. However, after infection with LCMV, LCMV-specific T cells recognize virus-infected cells as well as GP expressed by pancreatic β-cells. The immune attack leads to destruction of these cells resulting in a very rapid development of T1D (within 1–2 weeks) [1,3]. The advantages of using this RIP-GP system is that the whole autoantigen (LCMV-GP/NP) as well as the CD4 and CD8 epitopes are defined, and that TCR transgenic mice specific for the (neo)autoantigen are available.
Our lab has set up a non-viral version of this model that harnesses both the innate and the adaptive components of the immune system and reliably induces T1D in RIP-GP mice within 2 weeks. The injections schedules for transfer of P14 cells, gp-peptide, CpG and poly(I:C) is described in detail elsewhere .
The similarities with human autoimmune diabetes have led to the use of the NOD mouse as the mainstay of preclinical diabetes research . The timeline of diabetes development starts at 5 weeks of age with an initial insulitis that manifests by means of leukocytic aggregates at the perimeter of the islets and a lymphocytic infiltration into the pancreatic islets. The dominant initial population consists of CD4+ T cells followed by infiltration of CD8+ T cells, macrophages and B cells in smaller numbers . The majority of female NOD mice develop diabetes by 40 weeks, but incidence in males is dramatically lower . The specific diabetes incidence of any colony also critically depends on the “cleanliness” status of the colony .
T cells isolated from the pancreatic islets of NOD mice are insulin reactive after stimulation ex vivo , but other specificities have been found . Islet reactive T cell clones have been established from NOD spleen, pancreatic islets and lymph nodes for both CD4+ [11,12] and CD8+ T cells [13,14]. Diabetes induction is possible with many of these clones in the absence of B cell help or other T cell lines. Transgenic SCID or RAG-knockout mice expressing such TCRs develop diabetes. It has recently been demonstrated that there is a progressive decrease in the ratio of Treg cell:Teff cell in inflamed islets, but not in the pancreatic lymph nodes . This is probably due to increased apoptosis of the regulatory T cells, as administration of low doses of IL-2 promotes regulatory T cell survival and protects NOD mice from diabetes.
While it is accepted that T1D in NOD is primarily T cell-mediated, NOD mice also develop autoantibodies to insulin first detectable at 6 weeks of age and peaking between 8 and 16 weeks. These antibodies themselves do not cause diabetes, but they contribute significantly to the rate of disease progression [16,17].
The double transgenic models are characterized by the transgenic expression of an antigen X on the pancreatic β-cells via an insulin promoter (RIP/HIP/MIP, see above) in combination with a transgenic TCR specific for that antigen X. Current double-Tg models are based on the model antigen OVA, influenza haemagglutinin (HA), or LCMV glycoprotein (GP).
In the DO11.10 TCR-Tg×RIP-mOVA model , Ovalbumin is expressed as a self-Ag in the pancreas. Co-expression of the DO11.10 TCR permits development of autoreactive OVA-specific CD4+ T cells. These mice develop diabetes by age 10 weeks, with 100% penetrance by week 20. The advantages of this autoreactive CD4+ T cell model are its shorter time frame (compared to NOD), the singularity of the T cell under study, and the availability of an antibody to detect the specific T cells.
Viral antigen-based models are useful for the study of “autoreactive” CD8+ T cell responses. The P14/RIP-GP model combines expression of LCMV-GP under the control of RIP on pancreatic β-islet cells, with LCMV-gp(33–41)-specific CD8+ T cells . In this model, LCMV-GP-specific CD8+ T cells are not tolerized, but remain immunologicaly unaware of the presence of their antigen. When stimulated under the appropriate conditions, e.g. LCMV infection, P14 are fully capable of responding to LCMV-GP, either in virus-infected cells or on GP-expressing β-cell. This model was used to examine how gp33-induced tolerance can be switched to autoimmunity . A similar setup is the TCR-HA/Ins-HA mouse, H-2d mice expressing both the influenza virus HA as a transgene-encoded protein on β-cells (Ins-HA) , as well as the Clone 4 CD8+ T cell, specific for the dominant H-2Kd-restricted HA epitope. These mice develop spontaneous autoimmune diabetes and die within 2 weeks of age .
The BDC2.5 line allows one to bypass events involved in the generation and expansion of the repertoire of autoimmune T cells, which are preformed in the Tg mice, and to focus on the later steps of diabetes development. Gonzalez et al. compared BDC2.5/NOD and BDC2.5/B6g7 to study the genetic control of diabetes progression . The BDC2.5//B6g7 was generated by crossing the BDC2.5 to the MHC-congenic line B6.H2g7, which carries the g7 haplotype of the MHC but has all other genes from the B6 strain. As such, the final strain contains BDC2.5 TCR transgenes together with selecting H2g7 MHC alleles but different alleles at all other loci . One advantage of the model is that the background is C57Bl/6. Also, the diabetes incidence in this strain reaches a stable 60% at 10 weeks, allowing both increased or decreased diabetes incidence and time of onset to be studied within a reasonable amount of time.
Transfer of splenocytes from diabetic NOD females induces diabetes in NOD.CB17-Prkdcscid (NOD-scid) which are immunodeficient. Using this transfer approach, Miller and Wicker  first demonstrated that both CD4+ and CD8+ cells were required for diabetes onset, which was later confirmed using β2-microglobulin knockout (CD8 deficient), or MHCII knockout  and CIITA-deficient (CD4 paucity) . Also, islet-infiltrating lymphocytes from prediabetic NOD mice rapidly transfer diabetes to NOD-scid mice . By comparing diabetes incidence after cotransfers of combinations of gene-KO/WT CD4 and CD8, this transfer system allows determination of the defective T cell subset in gene-KO NOD mice that are resistant to diabetes (e.g. ). Insufficient purity of the subsets being co-transferred can hamper unambiguous interpretation of the results.
The BDC2.5 TCR transgenic line carries the rearranged TCR genes from a CD4+ T cell clone that is specific for an (unknown) pancreatic islet β-cell antigen, is restricted by the Ag7, and is diabetogenic . Using transfer of Th1 or Th2 BDC2.5 CD4+ T cells to NOD neonates (age <11 days), Katz et al. showed that both Th1 and Th2 induce insulitis, but only Th1 induce diabetes in the recipient mice within 2 weeks [29,30]. Cotransfer of regulatory T cells, natural or induced, can be employed to determine suppressive capacity of cell types on the islet-directed CD4+ T cell response. This model allows tracking T cells by tetramers, distributed by NIH , or by the inbreeding of an allelic marker (Thy1, or Ly5).
The LCMV-induced models of T1D (see above) allow monitoring of virus/neoantigen-specific responses by means of tetramer staining and/or in vitro LCMV-peptide restimulation for cytokine production. However, it does not allow tracing of a population that was manipulated before, or transgenically altered. This is solved by transfer of P14 cells (CD8+ T cells specific for LCMV gp33–41 on H-2Db, ) or the SMARTA cells (CD4+ T cells specific for the LCMV gp61–80 on I-Ab, ) a day before LCMV infection of the RIP-GP mice. These cells will respond to LCMV and subsequently migrate to the pancreas where the LCMV-glycoprotein is expressed on β-cells. Breeding in an allelic marker (CD45.1/2, CD90.1/2) or fluorochromes (GFP, DsRed), or labeling with fluorescent dyes (e.g. CFSE, SNARF, PKH) provide means to trace the P14 or SMARTA cells accurately during their response, even in immunohistochemistry or 2-photon microscopy studies. A similar system can be set up using TCR-HA  into RIP-HA mice .
The need for experimental studies on human haematopoietic and immune systems has propelled the development of appropriate and effective animal models that do not put individuals at risk. NOD-scid mice have been the "gold standard" for studies of human haematolymphoid engraftment in small animal models over the last 10 years. Now, immunodeficient mice bearing a targeted mutation in the IL-2 receptor common gamma chain (NOD-scid/IL-2Rγ−/− mice) allow superior engraftment and function of human haematolymphoid cells as compared with NOD-scid mice and are very useful for studying human lymphocytes in vivo. NOD-scid/IL-2Rγ−/− mice are also used in studies on human haematopoiesis, innate and adaptive immunity, autoimmunity, infectious diseases, cancer biology, and regenerative medicine, as reviewed elsewhere .
Alloxan and streptozotocin (STZ) are toxic glucose analogues that preferentially accumulate in pancreatic β-cells via the GLUT2 glucose transporter . Alloxan and high doses of STZ selectively kill the insulin-producing β-cells, whereas multiple low doses of STZ (MLD-STZ: 5 daily i.p. doses of 40mg/kg/day) generate H2O2 and induces expression of glutamic acid decarboxylase (GAD) autoantigens. GAD is a strong trigger of β-cell-specific autoimmunity, both in humans and in experimental models of diabetes, and has been shown to require a Th1-dependent inflammatory reaction. This model offers an opportunity to study the effects of compounds on synchronized groups of rodent animals within 20–30 days.
Temporary and reversible β-cell ablation can be achieved in the PANIC-ATTAC (pancreatic islet beta-cell apoptosis through targeted activation of caspase-8) mouse model . This model can be used for the identification of β-cell precursors, evaluation of glucotoxicity effects in diabetes, and examination of pharmacological interventions.
Given the complexity of T1D, a single inbred animal model of diabetes is unlikely to unravel everything there is to know about (all forms of) autoimmune diabetes (that might exist) in the human population. These animal models can however help in the identification of many of the genetic, signalling and immune pathways involved in diabetes. Differing models have differing strengths and weaknesses, and before embarking on a clinical trial, several experimental animal models need to be consulted. The therapeutic target should direct the choice of model(s) in the experimental design that should include complimentary and/or confirming approaches. In our opinion, confirmation of those findings in NOD mice is not a strict requirement for the validity of the results. However, it is conceivable that translational potential to a complex disease as T1D increases with the broader applicability of a therapy or phenomenon in multiple model systems, each with their own emphasis and angle.
So far, research of human T1D has been limited to blood draws of diabetic versus healthy or genetically susceptible individuals, in a cross-sectional or sometimes longitudinal study. However, the study of the diabetic (or unaffected) pancreas is mostly restricted to the use of cadaveric donors. An ethical use of living humans in research is possible through noninvasive β-cell imaging to quantify the remaining functional β-cell mass in diagnosed T1D patients or in autoantibody positive individuals, but also prove important as follow-up tool after therapy. Modern diagnostic equipment can provide very high sensitivity, e.g. positron emission tomography (PET), single photon computed tomography (SPECT), but also spatial resolution, e.g. magnetic resonance imaging (MRI), or a combination of both by PET/CT. These imaging techniques rely on a specific molecular marker to enable differentiation between scattered islets or single β-cells, or surrounding tissue. Currently, IC2 seems to be the only useful marker for noninvasive functional imaging of native β-cells and thus in vivo β-cell mass quantification . Vesicular monoamine transporter 2 (VMAT2) is present on β-cells, but was also found on pancreatic polypeptide cells . Obviously, experimental animal models might benefit from these noninvasive techniques as well.
Apart from anti-CD3 therapy , translation of promising preclinical therapies has not demonstrated reliable predictability and/or efficacy towards patient treatments. This is most likely because of the inherent differences between the two species that make identical responses to all immunological situations impossible. Recent years have seen a boom in the application of informatics techniques to immunological data and the creation of in silico models based on this data. To aid the transition from successful mouse therapy to successful human therapy, the biotech company Entelos has created a virtual NOD mouse platform. This platform enables in silico investigation of critical pathways that regulate autoimmunity and/or β cell destruction. In silico animal models have the advantage of quickly and quantitatively linking observed animal behaviors with predicted human response and Entelos hopes researchers will be able to combine their own work with the benefits of in silico research. Cellular models of diabetes progression, T cell function and immunoregulation have also been created. This will allow hypothesis-driven research at the cellular level without the actual labor-intensive and expensive cell and animal work. Most importantly, these in silico platforms allow the testing of multiple parameters over large numbers of virtual immunological situations which, when performed in conjunction with traditional immunology experimentation, can help answer questions that current methodologies are not yet capable of alone. The virtual NOD mouse represents yet another valuable tool in the diabetes researcher’s arsenal and the impact of this platform will increase as their sophistication - and thus predictive power - improve, both as stand-alone in silico models and as an integrated part of type 1 diabetes research.
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