1. Introduction

Crohn's disease (CD) is a complex and debilitating inflammatory bowel disease (IBD) that affects millions of individuals worldwide. Despite extensive research, the precise etiology and pathogenesis of CD remain elusive, making it challenging to develop effective therapies. Animal models play a crucial role in advancing our understanding of CD, allowing researchers to investigate various aspects of the disease, from its immunological basis to potential therapeutic interventions. The researchers explore the key animal models used in CD research, their contributions to our understanding of the disease, and their limitations.

2. Mouse Models

Mouse models are among the most widely used animal models for CD research due to their genetic tractability, relatively low cost, and similarity in immune system development to humans. Several different mouse models have been developed to mimic different aspects of CD pathology. Notable mouse models include:

2.1. Dextran Sodium Sulfate (DSS)-Induced Colitis:

DSS-induced colitis is a commonly used acute model of colitis in mice. Administration of DSS in drinking water causes epithelial damage, inflammation, and clinical symptoms resembling CD. This model is particularly useful for studying the early stages of inflammation and testing potential therapeutic agents ^[1].

2.2. TNF-α Overexpression Models:

Transgenic mice overexpressing TNF-α develop spontaneous chronic intestinal inflammation, providing insights into the role of TNF-α in CD pathogenesis. These models have been instrumental in the development of anti-TNF therapies ^[2].

2.3. IL-10 Deficiency Models:

IL-10 is an immunoregulatory cytokine, and IL-10-deficient mice develop spontaneous colitis resembling human CD. These models have contributed to our understanding of the role of immune dysregulation in CD ^[3].

2.4. Adoptive T Cell Transfer Models:

These models involve transferring CD4+ T cells from genetically susceptible donor mice into immunocompromised recipients. This approach allows researchers to investigate the specific contribution of T cells to CD pathology ^[4].

While mouse models have provided valuable insights into CD, they also have limitations. They do not fully recapitulate the heterogeneity of human CD and lack certain aspects of the human intestinal immune system. Additionally, the rapid progression of inflammation in some mouse models may not mirror the chronic nature of CD in humans.

3. Rat Models

Rat models have also been used in CD research, offering some advantages over mouse models. The rat gastrointestinal tract is anatomically and physiologically more similar to humans, making them suitable for certain aspects of CD research. Key rat models include:

3.1. TNBS-Induced Colitis:

Intrarectal administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) induces colitis in rats, characterized by transmural inflammation. This model has been valuable for studying the immunological basis of CD and testing potential therapeutic agents ^[4].

3.2. Rat IL-10 Deficiency Models:

Similar to the mouse IL-10 deficiency models, rats deficient in IL-10 develop spontaneous colitis, aiding in the exploration of immune dysregulation in CD ^[5].

3.3. Chemically Induced Models:

Rats have been used in various chemically induced models of colitis, such as acetic acid-induced colitis and oxazolone-induced colitis, to mimic different aspects of CD pathology.

Rat models offer advantages in terms of their larger size, which facilitates surgical procedures and allows for more extensive tissue sampling. However, they are less genetically tractable than mice, and the availability of specific rat strains may be more limited ^[5].

4. Non-Human Primate Models

Non-human primate models, particularly rhesus macaques, have been employed to gain insights into CD due to their genetic and physiological similarity to humans. These models allow researchers to study CD in a system more closely resembling the human condition. They have been instrumental in ^[6]:

4.1. Inflammation and Immunology Studies:

Non-human primate models have been used to investigate the complex interactions between the immune system and the gut in CD, shedding light on the role of specific immune cells and cytokines.

4.2. Mucosal Healing:

Researchers have examined the process of mucosal healing in non-human primates, a critical aspect of CD management.

4.3. Therapeutic Testing:

Non-human primate models have been used to test novel therapeutic approaches, including biologics and cell-based therapies.

Despite their advantages, non-human primate models are expensive, ethically challenging, and have limited availability, which restricts their use in CD research.

5. Zebrafish Models

Zebrafish (Danio rerio) models have gained popularity in CD research due to their optical transparency during early development, genetic tractability, and conserved innate immune system. While zebrafish do not have a true digestive system analogous to humans, they have a simplified gut structure that can be used to study inflammation and immune responses. Researchers have developed zebrafish models of intestinal inflammation, including those with genetic mutations related to CD, providing insights into the early stages of CD pathogenesis ^[7].

6. Porcine Models

Pigs share anatomical and physiological similarities with humans, making them a valuable large animal model for CD research. Researchers have utilized porcine models to investigate various aspects of CD, including ^[8]:

6.1. Mucosal Healing

The porcine model allows for the study of mucosal healing processes, which are essential in CD management.

6.2. Microbiota Interactions

Pigs have a gut microbiota composition closer to humans than rodents, making them suitable for studying the interplay between the microbiome and CD.

6.3. Surgical Interventions

Due to their size, pigs are particularly useful for testing surgical interventions and evaluating postsurgical outcomes in CD.

One limitation of porcine models is their higher cost and logistical challenges compared to smaller animal models.

7. Humanized Mouse Models

To bridge the gap between animal models and human CD, humanized mouse models have been developed. These models involve the engraftment of human immune cells or tissues into immunodeficient mice, allowing researchers to study human-specific immune responses in the context of CD. Humanized mice have been used to investigate the efficacy of novel therapeutics, including those targeting specific immune pathways ^[9].

8. Conclusion

Animal models have significantly contributed to our understanding of Crohn's disease, shedding light on its immunological basis, potential therapeutic interventions, and the role of genetics and environment. Each type of animal model has its advantages and limitations, and researchers must carefully choose the model that best suits their specific research questions and goals. Integrating findings from different models and, where possible, validating results in human studies, is crucial for advancing our understanding of this complex and debilitating disease. As research continues to evolve, the insights gained from animal models will play a pivotal role in developing more effective treatments and ultimately improving the quality of life for individuals living with Crohn's disease.