| Abstract|| |
Gastro-intestinal (GI) lesions are common outcome to diverse etiological agents affecting the GI tract. It requires significant expertise to accurately diagnose the fundamental cause and treat accordingly. A better understanding of the immunological underpinning of these lesions is of great importance to ensure their successful management. Availability of specific animal models allows us to understand the subtle differences among diverse disease conditions and help decide upon the treatment trajectories. Since murine models are best suited for studying the immunopathogenesis of any disease, we will restrict our discussions here to the available murine models and their applications to study gastrointestinal lesions. In this review, we have systematically examined and compared the variety of mice models that are routinely used to study Inflammatory Bowel disease (IBD) and also how they can be leveraged to address specific questions relating to IBD.
Keywords: IBD, murine model, non-human primates
|How to cite this article:|
Jain N, Sharma P, Kumar D. Murine models for studying immunopathogenesis in gastrointestinal lesions: How to go about it. Indian J Pathol Microbiol 2021;64, Suppl S1:58-62
|How to cite this URL:|
Jain N, Sharma P, Kumar D. Murine models for studying immunopathogenesis in gastrointestinal lesions: How to go about it. Indian J Pathol Microbiol [serial online] 2021 [cited 2021 Sep 21];64, Suppl S1:58-62. Available from: https://www.ijpmonline.org/text.asp?2021/64/5/58/317926
GI tract is continuously attacked and invaded by different microbes and chemicals. Its abrasion makes it prone to damages leading to perpetual repair and maintenance. This damage is presented clinically most often in the form of lesions in the GI tract. Therefore, understanding its etiology requires studying the underlying path-physiology. Among the GI tract disorders, lesions mediated by IBD have been on a steep rise across the world in the 20th century. IBD has been further categorized into Ulcerative Colitis (UC) and Crohn's Disease. UC is limited to chronic inflammation of the colonic internal mucosal epithelial layer only while CD affects complete GI tract transversing all layers.
IBD is a complex disease as it is multifactorial and is governed by plethora of underlying biological mechanisms. This complexity ensues due to broad clinical manifestation in humans, unpredictable outcome of therapeutic intervention and long disease timeline. Major factors influencing the disease outcome include genetic predisposition (Gene regulation, gene-gene interaction, and SNPs), immune system dysfunction (cytokines, chemokines, reactive species with their dynamic interaction), gut microbiota alterations (trillions of interacting bacteria) and external environmental factors (vary with human behavior and evolution), which together form a complex network. Additionally, similarity with other intestinal inflammatory disorders like Intestinal tuberculosis in clinical manifestation makes it further complicated to understand the underlying causes and clinical course of action. Considering the magnitude of intricacies, deciding on the treatment option often becomes a daunting task, and requires a strategic treatment plan.
To understand such a complex disease, in vivo models have become paramount as they could replicate the disease manifestation allowing in-depth dissection of pathways involved and thereby unraveling new therapeutic interventions. Diverse in vivo models were developed and studied to address scientific questions like underlying pathogenesis, preclinical efficacy, and drug pharmacokinetics. Most of the in vivo models exploit the murine system, which has advantages like being simplistic, less expensive, shorter breeding time and ease of genetic manipulation. However, apart from mice models, nematodes (C. elegans), insects (Drosophila melanogaster) rodents (rat) and mammalian (pig and NHP) models are also used to study IBD. There are some other less frequently used animal models employed in intestinal inflammation studies like gnotobiotic juvenile beagles, german shepherd dogs, sheep, bovine (Mycobacterium avium subspecies paratuberculosis (Map) infection), rabbit and guinea pigs. Among all the models studied, NHP models (Spontaneous colitis in Cotton-top Tamarins and DSS-induced colitis in Rhesus macaques) resemble closest to the human IBD pathology, however their maintenance, ethical issues and timeline limits their usage. In this review, we will mostly focus on the murine models, which are summarized in the table below [Table 1] highlighting the key features of these IBD mice models.
As can be appreciated from the above table, no one single experimental IBD mice model closely resembles human IBD. However, taking advantage of strength of different individual IBD models, many facets of IBD can be studied for addressing specific question and desired translational relevance. The decision on which mice model should be used must combine the research question under consideration and the IBD subtype to achieve the desired outcomes. Selection of mice strain strongly affects the severity of colitis, thus affecting experimental outcome. For e.g., C57BL/6 mice develop more severe colitis than BALB/c mice in DSS induced IBD, whereas among the IL-10 knockouts BALB/c strain develops faster and more severe colitis than C57BL/6 strain. In adoptive transfer models, recipient BALB/SCID mice have more severe and faster disease phenotype than C57BL6/RAG-/- mice.
[Table 2] highlights the IBD models that can be used for studying different biological responses as per the requirement:
| Humanized Mice Models|| |
Despite the progress and utilization of different IBD mice models to assess the efficacy and toxicity of different therapeutic agents, numerous preclinical studies failed to translate effectively in humans. Prominent reason behind these failures is the existence of significant differences between mouse and human immune system. The last decade has seen a surge in the development of humanized mice models using immune-deficient mice, such as NOD/SCID, NOD/RAG-/-, NOD/SCID IL-2rγ-/- (NSG and NOG, NOD/RAG-/- IL-2rγ-/- (eg, NRG). These immuno-deficient mice are engrafted with human PBMCs (Hu-PBMC) and human CD34+ HSCs (Hu- HSCs). Since, NSG mice support engraftment of larger numbers of human HSCs and promote T cell development to a greater extent, they have emerged as the preferred choice for humanized mouse models to study IBD.
Curiously, while a large number of preclinical studies are done in mice models, only few studies have attempted to study IBD in the humanized mice models. Nolte et al. investigated oxazalone-induced IBD in a Hu-PBMCs-NSG mouse model. NSG mice were engrafted with human PBMCs taken from healthy donors, individuals suffering from UC or atopic dermatitis. Mice developed colonic inflammation that consisted of fibrosis, edema and infiltration of T cells into the lamina propria. Similarly, Goettel et al. used transgenic NSG mice that expressed human HLADR1 (called NSGAb0DR1 mice) as a model for inducing acute colitis. HLA-matched CD4+ T cells were engrafted into NSGAb0DR1 mice followed by dermal-sensitization and rectal-challenge with TNBS in ethanol resulting in clinical and histological evidence of intestinal inflammation in treated mice. In yet another study by Weigmann et al. a mouse model of allergen-induced intestinal inflammation was reported where PBMCs were engrafted into NSG mice collected from donors with allergy to grass, birch pollen or hazelnut along with their respective allergen. After 21 days of engraftment, mice were challenged orally or rectally with their specific allergen. Afterwards, mice developed colonic inflammation that consisted primarily of human lymphocytes and neutrophils. While the humanized mice models could be developed for specific disease conditions of IBD, advantages of using these models over the wild type models remains very sparsely understood.
Ideas for an integrated approach of IBD mouse models
Though preclinical colitis models are complex, they are excellent tools for asking mechanistic questions, which are needed for a deeper understanding of IBD biology and associated pathways. It is essential to look at these models as a tool to address specific mechanistic question rather than as a model that can recapitulate all aspects of IBD pathophysiology.
It is certain that all future therapeutic interventions against IBD will have to undergo experimental validations in the murine models of IBD for safety and efficacy. However, there are dramatic discordance in the expected response when it comes to extending the results from pre-clinical murine models to the clinical settings. Two quick examples being: IL17 blockage, which was efficient in mice but ineffective in human IBD and anti-TNF treatment, which show significant improvement in humans but fails in chemically induced mice colitis models. Surprisingly, anti-TNF treatment succeeds in preventing disease occurrence in IL10-/- murine model. In view of the above, few important points are worth noticing. Depending on the mechanism being studied, it will be crucial to select more than one kind of murine models for any study to obtain more reliable inferences. The selection of model will be guided by the specific question, which is being addressed.
Finally, one critical point that seems to be missing from most drug studies is the impact of microbiome on IBD. Multiple studies show the contrasting drug response as a consequence of diversity in the gut microbiota. To minimize the effect of such variation, it is highly recommended to practice sourcing of animal, animal diets and other resources from a consistent vendor, and practice cohousing or mixed bedding to improve consistency in results. This may improve IBD presentation and clinical correlation between murine models and humans. Further, exploring humanized mice along with its responsiveness vis-à-vis gut microbiome needs to be studied which can significantly enhance the concurrence between murine models and clinical settings of IBD.
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Conflicts of interest
There are no conflicts of interest.
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Cellular Immunology Group, ICGEB, New Delhi
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2]