Role of commensal bacteria in chronic experimental colitis: lessons from the HLA-B27 transgenic rat

Rats on Lewis or Fischer background, transgenic for human HLA-B27 and beta(2)-microglobulin genes spontaneously develop colitis, gastritis, arthritis, dermatitis, orchitis, epididymitis, carditis, alopecia and nail changes. Disease susceptibility correlates with the gene copy number and is influenced by the genetic background. The pathomechanism in this model is still not completely understood. Cell transfer experiments indicate an essential role of HLA-B27 expression in bone marrow-derived cells. On Fischer background the onset of colitis occurs at 2 months of age, peaks at 3 months of age, and plateaus. Histologic findings include inflammatory cell infiltration, mostly limited to the mucosa, crypt hyperplasia, reduction of goblet cells, occasionally crypt abscesses and early ulcers. There is evidence that normal luminal bacteria play an essential role in initiating and perpetuating chronic colitis and gastritis in HLA-B27 transgenic rats: Transgenic rats raised under germ-free conditions do not develop gastrointestinal disease, whereas transgenic littermates exposed to specific pathogen-free bacteria develop colitis and gastritis within 2-4 weeks. Obligate anaerobic bacteria, especially Bacteroides spp., may play a predominant role since metronidazole prevents colitis and transgenic germ-free rats contaminated with a cocktail of six obligate and facultative anaerobic bacteria develop colitis and gastritis only in the presence of Bacteroides vulgatus. Luminal bacteria may also be involved in trafficking and homing of inflammatory cells into remote organs, since varying cecal bacterial composition does not only alter local inflammation but also influences gastritis. Lymphocyte transfer experiments indicate a specific response to luminal bacteria. In summary, this animal model is suitable for investigating the influence of normal luminal bacteria on the cellular immune mechanism in chronic intestinal inflammation.