Abstract
Ulcerative colitis (UC) is caused by an excessive immune response to gut microbiota. A recent study reported that the population of IgG-coated gut microbes increases with disease severity in patients with UC, but the role of these IgG-coated microbes in UC pathology is unclear. Serum, feces and colonoscopic lavage fluids (CLFs) were collected from pediatric UC (n = 13) and non-inflammatory bowel disease (IBD) patients (n = 15). Gut microbes were isolated from feces. Serum IgG reactivity to microbial cells and CLF-derived proteins was evaluated by Western blotting. Complement activation by the bacteria-IgG complexes was also assessed. Serum IgG reactivity to gut microbial extracts was highly variable in patients with active UC and increased with mucosal inflammation. IgG reactivity and clinical condition were inversely associated depending on disease activity. Non-IBD patients showed a similar degree of serum IgG response as that seen for patients whose UC was in remission. Lactobacillaceae bound higher amounts of IgG than other gut microbes tested and absorbed IgG to other bacteria. Lacticaseibacillus paracasei suppressed complement activation by Escherichia coli-IgG immune complexes. Appropriate IgG responses to luminal microbes might play a key role in gut microbiota stability by reducing excessive mucosal inflammation.