Markedly increased expression of interleukin-8 in the colorectal mucosa of inflammatory colorectal polyps in miniature dachshunds

Inflammatory colorectal polyps (ICRPs) in miniature dachshunds were recently recognized as a major cause of large bowel diarrhea in this dog breed in Japan. ICRPs are characterized by the formation of multiple small polyps and/or space-occupying large polyps in the colorectal area and are thought to be a novel form of inflammatory bowel disease (IBD). To explore key mediators in the pathogenesis of ICRPs, we analyzed several pro-inflammatory cytokine (IL-1β, IL-6, TNF-α, IL-8, IL-12p35, IL-12/23p40, and IL-23p19) mRNA expressions in colorectal polyps in ICRP dogs by quantitative PCR. Among these cytokines, IL-8 mRNA expression was markedly up-regulated in large polyps. To examine IL-8 protein expression, we analyzed IL-8 protein level and its location in colorectal mucosal specimens of ICRP dogs by ELISA and immunofluorescence microscopy. IL-8 protein was significantly increased in large polyps and serum in dogs with ICRPs compared to controls. By immunofluorescence microscopy, IL-8 was only localized in macrophages, but not in mucosal epithelial cells or neutrophils. IL-8-positive macrophages were significantly increased in large polyps compared to controls. These results suggest that IL-8 is produced mainly by macrophages and may induce neutrophil infiltration in the colorectal area of ICRP dogs.