Why study animal models of IBD?
Article first published online: 24 SEP 2008
Copyright © 2008 Crohn's & Colitis Foundation of America, Inc.
Inflammatory Bowel Diseases
Supplement: A Clinician's Guide to Common Questions in IBD
Volume 14, Issue Supplement 2, pages S129–S131, October 2008
How to Cite
Strober, W. (2008), Why study animal models of IBD?. Inflamm Bowel Dis, 14: S129–S131. doi: 10.1002/ibd.20667
- Issue published online: 24 SEP 2008
- Article first published online: 24 SEP 2008
In the last 10–15 years the study of animal (rodent/murine) models of mucosal inflammation has led to many significant advances in our understanding of the human inflammatory bowel diseases (IBD).1, 2 In essence, this research has enabled a dual approach to the study of these diseases characterized by a highly productive back-and-forth mouse–human study dialog concerning disease pathogenesis and treatment. Here we examine where model studies need to tend in the years ahead, so as to propel further advances. Our approach will be to look back at research utilizing models through the formulation of a series of “principles” of mucosal inflammation that have been derived from the models and that therefore serve as a platform for future research.
Principle 1: Mucosal inflammation in murine models can arise from a host of genetic defects that converge on a limited number of final common pathways of disease pathogenesis.
Over the years a great number of murine models of mucosal inflammation have been described, many of them characterized by a specific immunologic defect such as an inability to produce a major cytokine (e.g., IL-2 or IL-10) or a major downstream signaling molecule (e.g., Stat3).1 The remarkable fact, however, is that despite this genetic verisimilitude, the end result, inflammation of the colon or intestine, resolves itself into 1 of several pathogenic pathways. This situation in mouse inflammation strongly suggests that even though its clinical and pathologic manifestations are relatively constant in any give pool of patients, IBD also arises from a number of different genetic defects. A related point concerning the multiple genes giving rise to murine models is the fact that a given genetic defect has a differing capacity to produce inflammation in different mouse strains; thus, even disease genes capable of causing major immune abnormalities require the presence of additional modifying genes.3 One implication of the genetic complexity found in mouse models is that insight into the way disease genes relevant to human disease function and interact with one another to produce disease will almost surely require introduction and analysis of these genes in a mouse model, since only the mouse model will be likely to unveil how these genes lead to one of the final common inflammatory pathways and which modifying genes are present that facilitate a particular pathogenic pathway.
Principle 2: Intestinal inflammation in murine models is driven by components of the resident mucosal microflora.
A constant characteristic of the murine models is that inflammation fails to develop if the disease-susceptible mice are raised in a germ-free environment.1, 4 This tells us that, regardless of its underlying genetic basis, mucosal inflammation consists of an inappropriate response to antigens in the normal mucosal microflora rather to a mucosal pathogen. This has direct relevance for human disease because it helps establish that the human disease is also not driven by a pathogen and we need to put aside the now out-dated quest to find a pathogen. As to the future, there is still much to be learned about which elements in the microflora are most relevant to disease pathogenesis. In this context, these elements could conceivably function as a bacterial or viral component that induces inflammation via the innate immune system or as a conventional antigen inducing an adoptive immune response. This dual possibility is nicely illustrated by recent studies showing that bacterial flagellin may be particularly important in IBD pathogenesis, but it is not at all clear if this product is acting as a Toll-like receptor (TLR) ligand or as an antigen.5 Only further mouse studies are likely to clarify this issue.
Principle 3: Mucosal inflammation in murine models is usually driven by a polarized Th1–Th17- or Th2- T-cell-mediated process.
The convergence of genetic abnormalities to a final pathway of disease pathogenesis in murine models takes tangible form in the third principle that holds that, by and large, mucosal inflammation in animal models is due to either a Th1/Th17- or Th2- T-cell-mediated pathogenesis.1, 2, 6 This is a major organizing principle that has had and will continue to have a great impact on our notions of IBD pathogenesis and our approaches to IBD therapy. Thus, the knowledge that most murine models of inflammation are due to a Th1/Th17-T-cell-mediated inflammation coupled with mounting knowledge (in part, driven by studies of models) that Crohn's disease (CD) is also a Th1/Th17-T-cell-mediated inflammation allows one to focus attention on the various checkpoints of the Th1 or Th17 pathway and how they can be targeted to thwart the inflammation of CD.2, 7 Similarly, the knowledge that some models of mucosal inflammation are Th2-mediated inflammations has led to the awareness that Th2 processes can also be the cause of human IBD. This notion has had its greatest impact with the demonstration that oxazolone-colitis is due to NKT cells producing IL-13 and that ulcerative colitis (UC) is likely to be due to a similar process.8, 9 These observations in mice and humans should lead to therapeutic options that focus on the prevention of IL-13 and NKT cell activity. All in all, the evidence derived from the study of murine models and the studies of humans which these model studies have spawned has led to the ever more accepted concept that CD is a Th1/Th17-T-cell-driven process and that UC is a Th2-like T-cell-driven process. The main task of future studies of models in this arena will be the discovery of new approaches to these respective types of inflammations.
Principle 4: Mucosal inflammation can result from a loss of mucosal immune tolerance.
A fourth principle is one that has been fully developed in murine models but is only now being productively applied to human IBD. It is that IBD can be viewed as a defect in oral tolerance, the process by which the mucosal immune system limits responses to gut constituents while at the same time preserving the ability to respond to pathogens. The relation of oral tolerance to mucosal inflammation was first clearly delineated in the study of hapten (trinitrobenzene sulfonic acid, TNBS)-induced colitis wherein mice administered TNBS intrarectally (along with ethanol) do not develop colitis if they are pre-fed TNBS. In addition, in these studies it was shown that prevention of colitis by oral feeding is due to a manifestation of oral tolerance, the development of regulatory T cells.10, 11 Additional and more definitive studies of such regulatory T-cell function in mucosal inflammation have been carried out using the so-called transfer colitis model wherein mice develop colitis upon transfer of naïve T cells unless the latter are transferred along with mature cell populations that contain regulatory T-cell subsets.12 This model has proven to be extraordinarily useful in identifying the origin and properties of regulatory T cells that affect mucosal inflammation, such as natural CD25+ (Foxp3+) regulatory T cells and induced IL-10-producing Tr1 cells. In particular, this model has been used to show that mucosal dendritic cells, after receiving signals from mucosal epithelial cells, produce agents that induce Foxp3+ regulatory T cells, TGF-β1, and retinoic acid.13 As a result of the findings using the aforementioned and other models, much research in human IBD is now centered on whether or not deficiencies or defects of regulatory T cells is a basis of disease, at least in some patients. Unraveling this question, however, will surely require further work with murine models of inflammation, since only such work will provide further clues as to how to identify regulatory T cells in inflamed human tissues and how to gauge the activity of such cells once they are identified.
Principle 5: Defects in innate immunity contribute to the development of mucosal inflammation.
One of the newer developments in the study of murine models is encompassed by the fifth principle, that innate immune factors are likely to influence the development of the mucosal immune response underlying IBD. This was nicely demonstrated in the colitis induced by administration of TNBS mentioned above wherein it was shown that only those strains of mice that express a colitis susceptibility locus on chromosome 11 are susceptible to colitis.14 In studies relevant to this conclusion it was shown that chromosome 11 encodes a gene causing increased responses to LPS, and therefore elevated IL-12p70 responses upon exposure to TNBS (when administered with ethanol). Another and perhaps more important relation of innate immunity to IBD arises from the recent discovery that some 15% of Caucasian patients with CD bear disease-associated polymorphisms in NOD2, an intracellular protein that responds to muramyl dipeptide (a component of bacterial wall peptidoglycan) with the activation of NF-κB. Studies of murine models in which the mice lack this gene reveal the presence of cytokine production abnormalities and thus, for the first time, providing a specific molecular link between an innate response to a bacterial component and the development of inflammation.15 In particular, they show that innate immune responses have both positive and negative effects on mucosal responses to the commensal microflora and that the NOD2 polymorphisms associated with CD lead to a loss in the negative (downregulatory) effects ordinarily mediated by NOD2 activation and signaling.16 Overall, these studies of NOD2 in animal models alert us to the possibility that additional defects in innate immunity leading to inflammation can be discovered with the use of mouse models, or that such defects identified in studies of humans can be elucidated with such models.
Principle 6: Epithelial barrier function acting as a gatekeeper for the exposure of the mucosal immune system to the mucosal microflora can effect the development of gut inflammation.
In several models of mucosal inflammation, including those in which the tight junctions between epithelial cells are grossly defective17 or in other models in which more subtle defects of barrier function exist, such as the SAMP1/Yit mouse (Cominelli F, pers. commun, 2008). a disturbance of the barrier function of the epithelial lining has been shown to be the main cause of inflammation. The existence of such models emphasizes in a different way the importance of the mucosal microflora in disease pathogenesis, since they show that excessive exposure to the microflora can cause inflammation even in otherwise immunologically normal mice. This relates to human IBD because of studies that show that patients as well as relatives of these patients manifest epithelial permeability defects, suggesting a genetic basis of the abnormality. Finally, it has recently been shown that the overproduction of IL-13 in UC may lead to decreased barrier function due to the effects of IL-13 on epithelial cells.18 Given the fact that primary molecular studies of epithelial barrier function are only possible in the study of murine models, the latter become an important component of the study of this principle of gut inflammation as well.
In summary, it is now clear that the studies of murine models briefly reviewed above have led to the establishment of a number of principles that now guide current thinking concerning the pathogenesis and treatment of human IBD. In particular, these principles form a major part of the evidential base for the current consensus view that IBD is due to a defective response to components of the normal microbial microflora. Further support for this view, or indeed other views, will surely rely on studies of murine models, since the latter have become a kind of alternative universe of IBD research where new ideas can be studied and old ideas can be put to the test.