Mouse Models of Inflammatory Bowel Disease
Choosing a Mouse Model for Inflammatory Bowel Disease Studies
Choosing a mouse model for inflammatory bowel disease (IBD) preclinical studies isn’t easy. There’s a vast array of different model types and subtypes available (with key models shown in the table below), each with their own pros and cons.
Commonly Used IBD Mouse Models
|Model Examples||Location of Response||Prevalent Type of Response|
|Chemically Induced||DSS||Colon||Epithelial damage|
|TNBS||Colon||Epithelial damage Immune-mediated|
|Oxazolone||Colon||Epithelial damage Immune-mediated|
|Adoptive T Cell Transfer||CD4+CD45RBhi||Colon||Immune-mediated|
|Microbiome Induced||Germ-free IL-10-/- +/- microbial transfer||Colon||Immune-mediated|
The different models reproduce different IBD features (to varying extents) for histology, therapeutic response, disease location, etc. This post reviews the main model types available, looking at their unique characteristics and potential uses.
Chemically Induced Mouse Models
The most widely used IBD preclinical mouse models are those with chemically induced disease e.g. by dextran sulfate sodium (DSS), trinitrobenzene sulfonic acid (TNBS), oxazolone etc. The main advantages of chemically induced IBD models are that they are relatively cheap, and quick and easy to develop. Different models in this class have specific uses based upon their different disease induction methods.
DSS-Induced Colitis Model
DSS is a negatively charged sulfated polysaccharide which damages epithelial cells when administered to mice. Innate immune cells then release cytokines causing inflammation in the colon, characterized by ulcers and granulocyte infiltration.
Common uses for the DSS-induced colitis model include studying how the innate immune system is involved in intestinal inflammation, and also for looking at factors that maintain or reestablish epithelium integrity during/after injury.
DSS-induced colitis mouse models also respond to cyclosporine A, providing a relevant model to assess new agents which target the same immune mechanisms, e.g. new immunosuppressants.
TNBS-Induced Colitis Model
TNBS is a haptenating agent. It’s a small molecule which is not antigenic by itself, but causes an immune response when it binds to host proteins. TNBS administration results in a preclinical mouse model replicating clinical Crohn's disease (CD). The immune response generated is Th1-mediated, characterized by infiltration of CD4+ T cells, neutrophils, and macrophages. Transversely-spreading inflammation develops, resulting in transmural colitis.
TNBS-induced colitis models are ideal to study the immunologic aspects of CD, and also to test the efficacy of potential new immunotherapies.
Oxazolone-Induced Colitis Model
Oxazolone is also a haptenating agent, but induces a different kind of inflammation to TNBS. This results in a model more like clinical ulcerative colitis, including similarities in immunopathogenesis. The immune response induced is Th2-mediated, resulting in diffuse colonic inflammation.
This model was historically used to study delayed-type hypersensitivity reactions in the skin, and is also used to assess agents targeting Th2-mediated mechanisms.
Chemically Induced Model Limitations
When using any chemically induced IBD model there are several variables to take into account. The same protocols should always be used, and to make sure studies are reproducible you should closely monitor chemical batch, strain, gender, animal source, chemical supplier, dosing level, frequency, and duration. This type of model can also be severe, with TNBS showing increased severity over DSS models.
Spontaneous Mutation Mouse Models
This group of models develop IBD due to spontaneously occurring mutations. This is not due to transgenic overexpression or gene suppression. Commonly used spontaneous mutation models include SAMP1/Yit and C3H/HeJBir, with both representing chronic intestinal inflammation.
Mice of the SAMP1/Yit substrain are used to model clinical CD based on disease location, histological features, and treatment response. Spontaneous inflammation of the terminal ileum (which is the primary location of clinical CD lesions) is seen, with immense infiltration of activated CD4+ and CD8α+TCRαβ+ T cells into the lamina propria.
IBD development and disease progression follow a well-defined time course in the SAMP1/Yit substrain, providing a good model to assess new therapies for different disease stages. This substrain also responds to anti-TNF treatment in a similar way to clinical patients, showing usefulness and translatability for assessing similar new agents.
Mice of the C3H/HeJBir substrain develop a predominantly right-sided colitis characterized by acute and chronic inflammation. The substrain provides a valuable model for studies on IBD immuno-pathogenesis, revealing increased B and T cell reactivity to antigens in the enteric bacterial flora and high levels of serum IgG antibodies against select bacterial antigens.
Spontaneous Mutation Model LImitations
One major limitation of spontaneous IBD models is the time needed for full disease penetrance. For example, for the SAMP1/Yit substrain, 100% penetrance takes approximately 30 weeks. This can result in long and costly study timelines.
Adoptive T Cell Transfer Mouse Model
The adoptive T cell transfer model induces chronic small bowel and colonic inflammation, which resembles some key aspects of human IBD. To generate the model, CD4+CD45RBhi T cells (which are CD25-) are sorted and isolated from donor BALB/c splenocytes. Cell transfer to a syngeneic immunodeficient SCID or RAG2-/- recipient generates a model with primary inflammation in the colon.
This inflammation is attributed to a lack of Treg cells in the naïve T cell population. The adoptive T cell transfer model is therefore used to study the role of pathogenic T cells in mucosal inflammation, and a lot of Tregs and other T cell population research has been performed in this model.
Adoptive T Cell Transfer Model Limitations
As immunodeficient mice are used to generate this model, it should be noted that a full overview of colitis development is not possible
Genetically Engineered Mouse Models of IBD
Genetically engineered mouse models of IBD spontaneously develop colitis and/or ileitis. Many of the models harbor susceptibility genes identified in human IBD. The most well-known model of this type is the constitutive knockout IL-10-/- mouse, still used in research studies 25 years after first being developed.
IL-10-/- Knockout Mouse
Spontaneous colitis develops in the IL-10 KO mouse, as the Treg cells of these animals have an inability to produce IL-10. Specifically, colonic inflammation is seen, characterized by an inflammatory infiltrate of lymphocytes, macrophages, and neutrophils. Disease severity can be modulated by the background strain used, with more severe disease observed in BALB/c compared to C57BL/6 mice.
Colitis develops over 24 weeks, and results in models useful for studying different immune mechanisms of IBD. Tregs can also be studied as they do keep some function, obviously with the caveat that IL-10 is removed.
IL-10-/- Knockout Mouse Limitations
One limitation of this model is that substantial variability in colitis development can occur between facilities. This is due to the model being highly dependent on microbiome differences. There’s also the long disease development time, though colitis onset can be accelerated and synchronized by using piroxicam. This itself needs to be carefully validated with regards to dose, formulation, and the age and microbial status of the mice.
There’s also no weight loss observed in this model. You can monitor other clinical indicators instead, but some of them are only seen with severe disease. A more efficient, sensitive, and validated method to monitor inflammation levels is to measure lipocalin 2 in the feces.
Microbiome-Induced Mouse Models
Intestinal bacteria are implicated in immune-mediated intestinal inflammation in IBD models. For example, in IL-10 knockout mice (and other IBD models), stimulation of the mucosal immune system by the microbiome is critical for colitis development.
We still don’t know the precise pathogenic role of the microbiome in human IBD, partly because the microbiome is analyzed once IBD has already developed. However, the importance of intestinal symbiotic bacteria and flora disorders in IBD pathogenesis has been widely investigated.
There is evidence that antibiotics result in improvements and attenuated mucosal inflammation in IBD patients. In addition, a novel fecal microbiota transplant (FMT) strategy has recently been under preclinical and clinical investigation for the treatment of IBD. FMT aims to reverse disease by adjusting intestinal flora disturbances and restoring the homeostasis between the host and intestinal microorganisms.
Introducing select bacteria (e.g. Helicobacter hepaticus, alone or in combination with other bacteria) or microbiota cocktails to germ-free IL-10-/- animals results in accelerated development of colitis in 2-4 weeks. Microbiome-induced mouse models therefore provide a useful tool for evaluating antimicrobial therapeutics, and studying microbes or microbial communities that can be beneficial or pathogenic.
Microbiome-Induced Model Limitations
Limitations of microbiome-induced IBD models include that colonization is variable and that the germ-free conditions needed before induction are hard to control. Germ-free mice can also have issues with food digestion pre model induction which needs taken into account during model development.
IBD mouse models are an invaluable tool for preclinical research and drug development. While no single model is fully clinically relevant, different models are used to study different and specific disease mechanisms. By understanding the differences, pros, and cons across the available range of IBD models, researchers can select the most appropriate mouse model to use on a study by study basis.