Inflammatory bowel disease (IBD) is a common indication for intestinal resection. It has been estimated that upwards of 75% of patients suffering from Crohn’s disease will require surgical intervention at some point for complications of their disease [1
]. As many of these patients will ultimately undergo multiple bowel resections for recurrent disease during their lifetime, they are at risk of developing intestinal failure due to short bowel syndrome. Clinically, the most frequent site of disease onset involves the ileum and proximal right colon, making an ileo-cecal resection (ICR) the most common intestinal surgical procedure in patients with Crohn’s disease [3
]. Multiple studies in animal resection models have demonstrated that the remaining intestine undergoes a process of adaptive growth to compensate for the loss of bowel [4
]. We have recently developed and characterized a murine model of ICR in wild type (WT) mice to study adaptive growth response of the small intestine and colon after this operative procedure [9
]. Data from our initial characterization of the ICR model in WT mice demonstrated marked early increases in the presence of crypt fission after resection that was temporally associated with expansion of intestinal stem cells (ISC) leading to sustained long-term adaptive growth. Several other published studies have also associated the incidence of crypt fission and the expansion of ISC, notably during development [10
], following chemo-radiation [11
] and in pre-malignant conditions [12
]. Despite the high incidence of intestinal surgery required to treat the complications of Crohn’s disease and the extensive research that has focused on the adaptive growth of the small intestine, surprisingly little is known about the effect of active inflammation on the adaptive response of the remaining small bowel after resection.
The ideal animal model to study the surgical effects of Crohn’s disease currently does not exist as the etiology has not been well established. We therefore sought to identify a murine model that develops reproducible, spontaneous, transmural small intestinal inflammation. Unfortunately, to date the vast majority of IBD models predominantly develop spontaneous colitis without small intestinal inflammation. Although a few models of ileitis have been described [13
], there is not one that reproducibly involves the small intestine in a healthy animal that can undergo surgical resection. In this study we utilized the IL-10 null mouse as it provides a well characterized, reproducible model of colitis that develops only when exposed to microbiota and not when housed under germ free (GF) conditions [13
]. The small bowel in un-operated or sham-operated IL-10 null mice does not become inflamed following conventionalization (CONV), thus bacteria alone are insufficient to induce small bowel inflammation in IL-10 null mice. We have recently demonstrated that ICR results in chronic small intestinal inflammation at the anastomosis and importantly within the distal jejunum remote from the anastomosis that persists following resection [23
], thus providing a valuable model of post-surgical small intestinal inflammation in a widely used animal model of IBD. Importantly, IL-10 null mice maintained in a GF environment remain disease free after resection, providing genetically matched controls to assess the effects of small bowel inflammation on the adaptive response to ICR.
The effect of microbiota on intestinal adaptation following resection has been previously examined. Juno et al demonstrated that a small but significant increases in both villus height and proliferation occurred within the ileum in GF rats after massive proximal small bowel resection when compared to CONV rats [4
]. As regional differences in luminal bacteria and adaptation following resection exist when comparing the distal versus proximal small bowel [4
], we have recently developed and characterized the effect of distal small bowel resection (ICR) in mice [9
]. In addition, we have created a surgical isolator in our gnotobiotic facility that allows us to perform ICR and maintain GF status in mice post-operatively [24
]. An important observation from studies in GF and CONV WT C57BL6 mice was that baseline differences in intestinal morphology and homeostasis exist when comparing un-operated GF and CONV animals. These differences have been described in a few animal models including the pig where GF status has been shown to result in significantly larger villi and smaller crypts when compared to CONV pigs [25
], but have not been well characterized in the small intestine from WT or IL-10 null mice. In our previous study in WT C57BL6 mice, when we corrected for these baseline differences, no obvious differences in the magnitude of small intestinal adaptive responses occurred in GF versus CONV mice after ICR. However, bacteria-dependent up-regulation of intestinal bile acid binding protein (IBABP), and colonic adaptation occurred in CONV mice when compared to GF following ICR. In the current study, we sought to use our ICR model in GF and CONV IL-10 null mice to determine the early effects of post-surgical small intestinal inflammation on adaptive growth in the small intestine and colon. We hypothesized that bacterially-mediated intestinal inflammation will enhance the adaptive response after ICR in this genetic model of IBD.