INTRODUCTION

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CROHN'S DISEASE (CD), one of the chronic inflammatory bowel diseases (IBD), is a debilitating disorder of unknown etiology whose incidence is on the rise worldwide (21). Although it is well recognized that the disease may involve genetic as well as environmental factors, the precise initiating events leading to CD are unknown. Therefore, therapeutic approaches for the treatment of CD are aimed at alleviating symptoms, are often nonspecific, and do not affect the natural history of the disease. One approach to investigating the pathogenic mechanisms involved in a complex disease such as CD is the use of appropriate animal models. Such an approach offers several advantages compared with direct clinical investigation of the affected patient population. For example, the time course of disease phenotype as well as specific pathophysiological events occurring before disease onset can be studied. In addition, modern technology has made it possible to perform genetic and immunologic manipulations of relevant genes that may be involved in the pathogenesis of the human disease (2). The relative contribution of bacterial and dietary antigens can also be assessed using germ-free animals, monoassociation studies with specific bacteria, as well as defined dietary regimens. Finally, specifically targeted interventions as well as experimental treatment modalities can be successfully tested in the appropriate animal model system. Ideally, an appropriate animal model for investigating IBD, particularly CD, should display an intestinal phenotype that develops spontaneously without gene targeting or immunologic manipulations (1) and should closely resemble the specific human disease. The present article briefly reviews currently available animal models of IBD and discusses in detail two novel genetically engineered murine models of experimental CD.

	OVERVIEW OF ANIMAL MODELS OF IBD

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Several animal models of intestinal inflammation/IBD have been developed and extensively studied (3). These models are based on chemical induction, gene targeting, or immune-cell transfer manipulations that all result in gut inflammation. Their characteristics are described in Table 1. One limiting factor with such models is that, with few exceptions, they do not possess the hallmark features of IBD in regards to disease location, histological features, or clinical course of the human condition. However, these models may prove useful in characterizing a specific biological mechanism of gut inflammation. Examples include the characterization of the interleukin (IL)-1/IL-1 receptor antagonist balance in the rabbit formalin-immune complex colitis model (5), the role of protective factors in response to intestinal epithelial injury in the dextran sodium sulfate (DSS) colitis model (16), the key function of regulatory T cells in the CD45RB^high adoptive transfer model (6), and the importance of neurogenic gut inflammation in the dinitrobenzenesulfonic acid (DNBS) colitis model (23). These models of intestinal inflammation/IBD, however, may have little value in defining the precise etiopathogenesis of the human disease.

Chemically induced models. This group of animal models requires the administration of an exogenous chemical agent for the induction of colitis. With few exceptions, the resulting intestinal inflammation is acute and persists as long as the chemical agent is present in the gut lumen. Examples include the acetic acid model of colitis (14), the 2,4,6-trinitrobenzenesulfonic acid (TNBS) model of colitis (19), the DSS model of colitis, and the formalin-immune complex colitis model (7). These models have each generated important information on specific biochemical pathways of inflammation and have provided proof of concept for specific therapeutic interventions for the treatment of IBD. However, they are primarily characterized by acute mucosal injury (29) and clearly differ from human IBD in many aspects, including the initiating event and the clinical course of gut inflammation.

Immunologic models. Immunologically mediated models are defined as models of adoptively transferred T cells or bone marrow precursors, which are introduced into immunodeficient recipients, usually mice. The two best examples are the CD45RB^high transfer model (20) and the bone marrow chimera transfer model (8). This group of animal models has provided very important information on the role of regulatory T cells in controlling mucosal immunity and intestinal inflammation and offers strong evidence that Th1 polarization may play a key role in the pathogenesis of CD (22). However, the profound immunological abnormality in the recipient mice, who lack a competent immune system, make these models unlikely to be suitable for investigating the precise factors involved in human CD.

Genetic models. The advent of transgenic and knockout methodologies have revolutionized the field of animal models of IBD after the discovery that a variety of genetically manipulated mouse models develop chronic intestinal inflammation. Examples include the IL-2 (26), T cell receptor (TCR)/ (18), IL-10 (13), and Gi2 (25) knockout models. With the exception of IL-10-deficient mice, which possess some features of human CD (Table 2), the majority of these models display features more similar to ulcerative colitis than CD. These models have contributed greatly to our current understanding of the role of key immune-related molecules and genes, including specific cytokines, in the pathogenesis of chronic intestinal inflammation. Collectively, these models have clearly demonstrated the requirement for strict regulation of the mucosal immune response and have allowed identification of key components involved in gut immune regulation. However, it is unlikely that these mutations represent the underlying defect in human CD. Therefore, the use of these models is limited, particularly when they are directed at investigating causal factors of human CD.

Spontaneous models. This group represents the most attractive model system of intestinal inflammation/IBD because gut disease occurs without any apparent exogenous manipulations, similar to human disease. Two animal models have been investigated and characterized in this group. The first is represented by the cotton-top tamarin model, which shows features similar to human ulcerative colitis in regards to clinical course, location of disease, histological features, increased risk of colon cancer, and response to conventional treatments (15). The second model is the C3H/HeJBir model of colitis characterized by a spontaneous and chronic focal inflammation localized to the right colon and cecal region (27). Interestingly, the colitis occurs in young mice and tends to resolve with age, without recurrence. Similar to severely immunocompromised mice, as well as some of the knockout mice, there appears to be a correlation between Helicobacter infection and the onset of colitis in the C3H/HeJBir model. However, both the C3H/HeJBir and cotton-top tamarin models of gut inflammation have similarities to human ulcerative colitis and do not involve the small intestine. Until recently, no models of spontaneous ileitis were reported.

	NOVEL MOUSE MODELS OF CROHN'S DISEASE

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In the last three years, our group has characterized two new murine models of experimental CD that closely resemble many of the characteristics of the human disease. These models provide an exciting opportunity to attempt to define the precise etiology of CD and to begin to develop a cure for this devastating disease.

TNF ARE model of ileitis. A large body of evidence supports the concept that tumor necrosis factor (TNF) may play a key role in the pathogenesis of chronic inflammatory diseases, including IBD (9). The most compelling evidence for the importance of TNF in mediating human CD is the ability of drugs that modulate TNF production to markedly decrease symptoms and severity of inflammatory lesions in patients with active CD (28). On the basis of these observations, we recently reported (10) a unique model of TNF overexpression that develops an impressive CD-like phenotype, which is localized primarily to the small intestine and is characterized by chronic transmural ileitis with features very similar to human CD (Table 2).

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SAMP1/Yit model of ileitis. Another model of experimental CD that shows great promise to identify characteristics of the human disease is the SAMP1/Yit model of ileitis. This mouse line was generated by genetic manipulation involving brother-sister mating for >20 generations of the original senescence-accelerated mouse (SAM) line (17). This particular model has the unique feature of developing spontaneously without either gene targeting or immunologic manipulations. The spontaneous ileitis develops at ~20 wk of age and reaches virtually 100% penetrance after 30 wk of age. Interestingly, the ileitis characteristic of this model does not resolve with age but tends to progress in severity, up to 80 wk (unpublished results). The histological features of this model closely resemble human terminal ileitis, with segmental inflammation localized primarily to the terminal ileum. In addition, transmural inflammation, presence of mucosal and submucosal granulomas, changes in epithelial cell morphology and architecture, and muscular and neural hypertrophy are all pathologies observed in the SAMP1/Yit model (Table 2). Therefore, one of the most attractive features of this model is that all the aforementioned characteristics are hallmark events typical of human CD. Minor differences do exist, however, including the presence of sporadic macroscopic ulceration as well as the absence of pyloric metaplasia, which are usually included in the histopathological diagnosis of the human disease. Similar to other models of IBD, the chronic ileitis can be adoptively transferred by CD4⁺ T cells into severe combined immunodeficient (SCID) mice (11). Interestingly, the resulting disease is once again localized to the terminal ileum, suggesting that the activated CD4⁺ T cells may recognize antigens specifically localized to the terminal ileum.

View larger version (42K): [in this window] [in a new window]   Fig. 1.   Working hypothesis for pathogenesis of Crohn's disease (CD). The disease is initiated and perpetuated by a dysregulated immune response to unknown antigens in a genetically susceptible host. CD4+ T cells and Th1 polarizing cytokines, such as tumor necrosis factor (TNF), interleukin (IL)-12, and IL-18, may play an important role in this process. Leukocyte adhesion and trafficking is mediated by specific adhesion molecules, including intercellular adhesion molecule 1 (ICAM-1) and vascular cell adhesion molecule 1 (VCAM-1), which are expressed on endothelial cells. In this context, intestinal epithelial cells may play a key role in regulating pro- and anti-inflammatory events characteristic of CD. IFN-, interferon-; M, macrophage; ROS, reactive oxygen species; APC, antigen-presenting cell.