Melatonin Mitigates Oxazolone-Induced Colitis in Microbiota-Dependent Manner

Levels of type 2 cytokines are elevated in the blood and intestinal tissues of ulcerative colitis (UC) patients in the active phase; this phenomenon indicates the participation of type 2 immune response in UC progression. The beneficial effects of melatonin in dextran sodium sulfate (DSS) and 2,4,6-trinitrobenzene sulfonic acid (TNBS) colitis models have been illustrated, but its role in the oxazolone (Oxa)-induced colitis model (driven by type 2 immune response) remains relatively unknown. We investigated the relationship between melatonin concentration and the severity of UC, revealing a significantly negative correlation. Subsequently, we investigated the effects of melatonin in Oxa-induced colitis mice and the potential underlying mechanisms. Administration of melatonin significantly counteracted body weight loss, colon shortening, and neutrophil infiltration in Oxa-induced colitis mice. Melatonin treatment mitigated Oxa-induced colitis by suppressing type 2 immune response. In addition, melatonin attenuated intestinal permeability by enhancing the expression of ZO-1 and occludin in colitis mice. Interestingly, the protective effect of melatonin was abolished when the mice were co-housed, indicating that the regulation of gut microbiota by melatonin was critical in alleviating Oxa-induced colitis. Subsequently, 16S rRNA sequencing was performed to explore the microbiota composition. Decreased richness and diversity of intestinal microbiota at the operational taxonomic unit (OTU) level resulted from melatonin treatment. Melatonin also elevated the abundance of Bifidobacterium, a well-known probiotic, and reduced proportions of several harmful bacterial genera, such as Desulfovibrio, Peptococcaceae, and Lachnospiraceae. Fecal microbiota transplantation (FMT) was used to explore the role of microbiota in the function of melatonin in Oxa-induced colitis. Microbiota transplantation from melatonin-treated mice alleviated Oxa-induced colitis, suggesting that the microbiome participates in the relief of Oxa-induced colitis by melatonin. Our findings demonstrate that melatonin ameliorates Oxa-induced colitis in a microbiota-dependent manner, suggesting the therapeutic potential of melatonin in treating type 2 immunity-associated UC.