Cohousing of Non-Inflamed MDR Mice With Ileocolitic MDR Mice Alters Their Colonic Microbiota and Attenuates Ileitis in Ileocolitic Mice, Without Inducing Either Phenotype in Non-Inflamed Counterparts

Abstract Background/Aims Crohn’s disease involvement may be restricted to the terminal ileum (ileitis), colon (colitis) or both (ileocolitis). Mutations in the NOD2 gene, and development of antibacterial antibodies are more prevalent in ileal CD suggesting that ileitis may represent a distinct disease entity. The determinants of this regional distribution are not known but are likely bacterial in origin. MDR1a-/- housed at our facility develop not only colitis but also ileitis. However, the colony from which they are derived at Taconic does not develop ileitis or colitis. Here, we characterize the time course of ileocolitis in MDR1a-/- mice at UCSD and examine the effect of cohousing with non-colitic MDR1α−/− mice from Taconic on ileocolitis severity and ileal and colonic microbial communities. METHODS The severity of ileal and colonic histopathological involvement was analyzed at 5, 12, 20, and 30 weeks old by a pathologist in a blinded fashion. Relevant cellular subsets of mesenteric lymph nodes (MLN) in inflamed and non-inflamed mice were examined via mass cytometry. Mice originating from colonies with and without colitis were cohoused, after which their ilea and colon were harvested for histopathologic evaluation. Metagenomic analyses (shotgun) were performed in ileum contents and stool before and after cohousing. RESULTS Mice develop ileitis between 5 and 12 weeks (n 7 for all time points, p 0.01), and its severity remains relatively stable afterwards. Colitis was first observed at 12-weeks-of age, remained stable through 20 weeks and worsened in 30-week-old mice (p 0.01). Cellular composition of MLN showed expansion of central memory (TCM (CD44+/CD62L+) CD8+ T cells) in inflamed mice from our colony compared with non-colitic mice from Taconic (n = 4, p 0.005). Cohousing experiments showed attenuation of ileitis in ileocolitic mice (UCSD colony, n = 4, p 0.05), when co-housed with non-colitic mice from the Taconic colony, which were not affected. Cohousing did not trigger ileitis or colitis in mice from Taconic. After cohousing, Taconic mice had a loss of verrucomicrobiales and increased campylobacterales, whereas in UCSD mice, desulfovibrionales increased along with overall diversity and lactobacillales decreased. Evaluation of the ileal microbiota is ongoing. CONCLUSIONS MDR1α -/- mice at our vivarium develop not only colitis but also ileitis (ileocolitis) between 5 and 12 weeks-of age with expansion of TCM in their MLN. Cohousing of ileocolitic mice (UCSD) with non-inflamed mice from Taconic carrying an identical MDR1α mutation led to attenuation of ileitis but not colitis. Thus, mice at Taconic appear to have transmissible beneficial (anti-inflammatory) elements in their microbiota able to attenuate ileitis in their ileocolitic MDR1α -/- counterparts at our facility.