JOURNAL OF PATHOLOGY, VOL. 172 313-316 (1994)

REVIEW ARTICLE

ANIMAL MODELS OF INFLAMMATORY BOWEL DISEASE

B. F. WARREN AND P. E. WATKINS

School of Medical Sciences, University of Bristol, University Walk, Bristo!, BS8 I TD, U K.

Accepted 4 January 1994

INTRODUCTION

Despite many advances in the study of inflam- matory bowel disease, fundamental gaps remain in our knowledge. The causes of ulcerative colitis and Crohn’s disease are still unknown. Medical therapy for these conditions relies on steroids and 5-aminosalicylic acid compounds, which treat attacks but do not provide a cure. Surgical man- agement, again rarely curative, is reserved for patients in whom medical treatment has failed and for the treatment of complications. Added to this is the ever-increasing problem of colonoscopic screening to prevent carcinoma in long-standing extensive ulcerative colitis, which is of uncertain cost benefit.’ Furthermore, the extra-intestinal manifestations of inflammatory bowel disease develop unpredictably and may themselves cause considerable morbidity.

Animal models of other diseases have, to date, provided a great deal of information relevant to the study of human disease. One of the problems in inflammatory bowel disease research has been the lack of a suitable animal model. Although a review of the literature reveals descriptions of a large number of animal models of inflammatory bowel disease, their suitability is called into question by closer scrutiny.2

In broad terms, the available models of inflam- matory bowel disease fall into two main categories, those which are experimentally induced and those which occur spontaneously. Both need

Addressee for correspondence: Dr B. F. Warren, Lecturer in Pathology, Department of Pathology and Microbiology, University of Bristol, School of Medical Sciences, University Walk, Bristol, BS8 ITD, U.K.

CCC 0022-34 17/94/0403 13-04 0 1994 by John Wiley & Sons, Ltd.

careful characterization before they can be of any value.

INDUCED MODELS

Intestinal inflammation has been induced by a variety of methods in experimental animals. These have ranged from use of infectious agents to immunological manipulations. Chiodini et have described a model in infant goats fed a mycobacterium isolated from a case of human Crohn’s disease. These animals developed segmen- tal granulomatous bowel disease. Mycobacterium Linda has since been identified by in situ hybridiz- ation. These findings, along with the application of the polymerase chain reaction, have led to a re- surgence of interest in the role of mycobacteria in the causation of Crohn’s d i ~ e a s e . ~ , ~

More recently, lesions similar to those seen in Crohn’s disease have been induced by subserosal injection of peptidoglycan polysaccharide com- ponents of bacterial cell walls into the rat caecum. In a genetically susceptible rat species (Lewis rat but not in Buffalo or Fischer rats), this produces chronic transmural inflammation in the form of lymphoid follicles and also results in multiple hepatic granulomas.6 This model of Sartor is unique in providing a model of genetic suscepti- bility. It also develops fat wrap ing a unique

One of the oldest models of colonic inflam- mation occurs in the guinea pig fed on degraded carrageenan. The induction of a granulomatous, usually patchy colitis depends on the use of degraded carrageenan’ in a colon with normal flora since lesions will not develop in a germ-free

feature of Crohn’s disease in man. ? -

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This reproducible model shows some histological overlap between Crohn's disease and ulcerative colitis and may indeed tell us something of the relationship between the two diseases and raises a possible role for bacteria in inflammatory bowel disease. Interestingly, colon cancer has been reported to complicate this model."

More recently, a model of granulomatous ileitis has been developed by injectin live BCG directly into the ileum of guinea pigs.

Research by Wakefield et has focused on ischaemia as a possible cause for Crohn's disease. This has led to the production of a model of multifocal intestinal ischaemia in the ferret, in- volving occlusion of the mesenteric vasculature with glass spherules. l3 Dextran sulphate sodium in the drinking water of mice produces ulcer- ation, ulcer healing by crypt and surface epi- thelial regeneration. It also develops focal chronic inflammation.

Most models of ulcerative colitis have been induced by perfusing the large bowel of rodents or rabbits with irritant ~olu t ions '~ including acetic acid,' ' trinitrobenzyl sulphonic acid,I6 and Formalin with and without prior sensitization has been used.I7 The Arthus reaction produces a colitis in this way in rabbits," but a more realis- tic model is produced by the Auer modification of this r ea~ t ion . '~ In this case, inflammation arises at the site of a previous mild inflammation induced by the rectal instillation of formalin, on parenteral administration of egg albumin to an immunized animal.17 This model has been used to study the effects of anti-inflammatory drugs.20 The colitis becomes chronic if the rabbit is pre- viously sensitized to enterobacterial common antigen.I4 This model demonstrates that although once the immune defences are weakened, colitis may be maintained by allergic reaction to intra- luminal antigens. This finding led workers to postulate that colitis may be caused by a humoral response to colonic ba~ter ia , '~ although experi- mental generation of colonic autoantibodies by Escherichia coli does not consistently produce ~o l i t i s . ' ~ Colitis has also been induced in both rabbits and guinea pigs by sensitization with dinitrochlorobenzene. This substance forms haptedprotein complexes which are recognized as foreign by the host T lymphocytes. Recently the role of nitric oxide has been studied in inflamma- tory bowel disease by Rachmilewitz by induction of chronic inflammation with peroxynitrite.21 This model exemplifies the most productive

I F

use of induced models to study an individual component in the pathogenesis of intestinal inflammation.

Whether genetic 'knock out' mice will offer the same advantage is as yet unknown. T cell receptor mutant mice,22 IL 2,231L and TGF beta,25 knock out mice all develop a chronic colitis. Although patients with inflammatory bowel dis- ease sometimes have low levels of some of these cytokines,26 they are not born with a congenital absence of them. It has yet to be determined whether these mice are developing a colitis specific to each deficient cytokine or whether this is a non specific effect of immunosuppression with or with- out bacterial flora. Mouse models whatever the cause cannot be followed by sequential endoscopy and biopsy which limits their usage in therapeutic trials.

While induced models of colonic inflammation serve as useful and reproducible models for study- ing mechanisms of inflammation, they do not resemble human ulcerative colitis. They are known to differ markedly in their histological features and response to treatment, and they do not develop any of the recognized complications of the human diseases. We believe that it is more appropriate to consider these as induced models of 'colonic inflammation' and not as 'inflammatory bowel disease'. Rather than trying to induce a complete model of human inflammatory bowel disease, we should perhaps be devising experimental models to study a particular feature of these conditions, such as the role of cytokines, the mechanisms of inflammation, mucin depletion, uleceration, ulcer healing, and fat wrapping.

SPONTANEOUSLY OCCURRING MODELS

Spontaneously occurring models of inflamma- tory bowel disease have the advantage of bearing a close similarity to human disease, but their major drawback is that they are often less readily avail- able for study than laboratory-induced models.

Spontaneous granulomatous bowel disorders resembling Crohn's disease are often due to in- fection. Johne's disease of ruminants is due to Mycobacterium paratuberculosis and bears some resemblance to Crohn's disease.27 Granulomatous terminal ileitis in the Syrian hamster really bears a closer resemblance to Yersinia infection than to Crohn's disease.28 Many spontaneously occurring models of colitis have an underlying bacterial

REVIEW ARTICLE 315

infection. These include swine dysentery in pigs and equine colitis in horses.29

Increasingly, lymphocytic plasmacytic colitis is recorded in dogs, but its aetiology remains unclear. In addition, boxer dogs develop a steroid respon- sive and usually granulomatous total colitis. l4

Recently a steroid responsive colitis has been observed in cats in Germany.30

Spontaneous models of colitis in rodents differ from human inflammatory bowel disease in a number of features and, in most cases, the under- lying aetiology is infectious. Japanese waltzing mice suffer from a predominantly right-sided colitis caused by Bacillus ~yli formis,’~ whereas Swiss Webster mice develop a distal colitis due to Citrobacter freundii which proceeds to rectal pro- lapse and death.14 Transgenic rats with HLA B27 develop a spontaneous colitis which may in the future provide a useful route for investigating the link between HLA and ulcerative colitis.”

Primates offer most hope as a spontaneous model of human inflammatory bowel disease. A number of New World monkeys develop a colitis which bears a very close resemblance to human ulcerative colitis, clinically, endoscopicall histo- logically, and in its response to treatment.’This is particularly true of the cotton top tamarin, which also develops the complications seen in human ulcerative ~ o l i t i s . ~ ~ , ~ ~ The cotton top tamarin is a small (450 g) native of the Columbian rain-forest. Unfortunately its natural habitat is under threat. The suitability of this animal as a model for colitis research has been discussed for some time.29

One of the problems has been that organisms (Cumpylobacter) have been isolated from some cases of cotton top tamarin colitis and not from others and that some animals have responded to steroids whereas others have not. The cotton top tamarin has remained in a shadow of doubt as a model of ulcerative colitis until recent work clari- fied this situation.2 From clinical, bacteriological, endoscopic, histological, and treatment studies, the cotton top tamarin seems to develop not one, but five types of colitis. One is an infective colitis due to Cumpylobacter jejuni, another is a rare neonatal infection due to Klebsiella pneurnoniae, and a third resembles human pseudomembranous colitis. The majority of cases, however, are like human inflam- matory bowel disease, a very small fraction of which bear a closer similarity to Crohn’s disease than to ulcerative colitis.

Cotton tops with Cumpylobacter colitis have extremely offensive stools, a high peripheral blood

white cell count, and endoscopy reveals a total or patchy colitis. The histological appearance is of a classical infective Here there is superficial, predominantly acute inflammation with the forma- tion of neutrophil collections in the lamina propria and eccentric, superficial crypt abscesses. This condition responds well to oral erythromycin.2

Klebsiella pneumoniae colitis is very rare and is the only colitis in the cotton top which does not have a direct human counterpart. The histological appearance is of gross mucosal oedema, in which many organisms may be seen.2

Pseudomembranous colitis follows antibiotic therapy and is an endoscopically patchy colitis with a histological appearance of intercrypt erosions with volcanic eruptions of neutrophils and fibrin on the surface. This again is often fatal. We have seen four cases of cotton top tamarin colitis which resemble Crohn’s colitis more closely than ulcerative colitis. Here the endoscopic and histological inflammation were patchy and in two out of four cases mucosal microgranulomas were seen.2 They responded to the same treatment as that outlined for ulcerative colitis.

The vast majority of cases of cotton top tamarin colitis resemble human ulcerative colitis. Although this disease in some animals is more or less con- tinuous, in most it follows a pattern of relapse and remission. Endoscopically, there is diffuse colitis and histologically, there is a diffuse mixed inflam- matory cell infiltrate limited to the mucosa. This is associated with crypt distortion and mucin de- pletion. When the disease is active, neutrophil numbers increase and crypt abscesses form. Stain- ing for mucin with high iron diaminelalcian blue36 shows a change from normal colonic type sulpho- mucin to small bowel type sialomucin with the development of colitis, as seen in man. We have treated this condition successfully with the 5-aminosalicylic acid compound Olsalazine (Phar- macia). If the colitis is mild, this is the only treatment needed. However, if the colitis is more severe (as judged endoscopically), treatment with prednisolone and Olsalazine is instituted, provided the stool culture is negative for Campylobucter.

The complications of cotton top tamarin colitis make it unique. We have known for some time that cotton tops develop colon cancer as a sequel to ulcerative colitis. Like the carcinomas compli- cating human ulcerative colitis, these tumours are right-sided, flat, and often multiple. The only two differences are that these tumours are invariably mucinous and that they have not yet been shown

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to metastasize to the liver. More recently, liver disease has been recognized in cotton tops with ulcerative including periportal chronic inflammation and a histological appearance which resembles human sclerosing cholangitis.

CONCLUSION

In conclusion, the range of animal models of colonic inflammation is very wide, but few act as complete models of human inflammatory bowel disease, It is hoped that with the increasing recog- nition of the advantages of the cotton top tamarin as a model, it can be used increasingly in biomedi- cal research to clarify some of the fundamental gaps in our knowledge of ulcerative colitis.

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