Inflammatory bowel diseases (IBDs), including ulcerative colitis (UC) and Crohn's disease (CD), are chronic debilitating gastrointestinal disorders with increasing incidence. The role of mast cells (MCs) in UC pathogenesis is debated; while MCs and their mediators are implicated in various inflammatory disorders, their specific mechanism in UC remains unclear. FcεRI, a high-affinity IgE receptor on MCs, is crucial for MC activation, and its role in UC is controversial despite elevated serum IgE levels observed in some UC patients. The gut microbiota is also a key factor in UC development, and its interaction with MCs may be significant. This study aimed to investigate the relationship between FcεRla deficiency, gut microbiota composition, and DSS-induced colitis in mice, exploring potential diagnostic biomarkers and mechanisms.

Studies have suggested a role for basophils and MCs in various immune and inflammatory disorders, including some indications of MC involvement in UC. The mechanism by which MCs exacerbate DSS-induced colitis, however, requires further investigation. FcεRI, a key MC receptor, plays a central role in allergic cascades via IgE binding. Although elevated serum IgE is sometimes reported in UC patients, the precise role of IgE and FcεRI in UC pathogenesis remains uncertain. A growing body of evidence underscores the importance of gut microbial dysbiosis in UC development. The interplay between MCs, their mediators, and the gut microbiota's metabolites warrants deeper exploration to elucidate UC pathogenesis.

The study involved both clinical and animal experiments. Clinical trials recruited 62 UC patients and 75 healthy controls to assess serum IgE, anti-IgE, and anti-FcεRI levels. Receiver operating characteristic (ROC) analysis was performed to evaluate the diagnostic value of anti-FcεRI. Immunofluorescence and mRNA expression of FcεRIα were also examined in colonic tissue samples. In the animal experiments, FcεRla knockout (FC) mice and wild-type (WT) mice were subjected to DSS-induced colitis. Colon length, body weight, disease activity index (DAI), histopathological scores, and serum cytokine levels (IL-1β, IL-6, TNF-α, IL-10) were measured. Flow cytometry assessed Th17 and Treg cell frequencies. 16S rRNA gene sequencing analyzed gut microbiota composition. Fecal microbiota transplantation (FMT) and cohousing experiments further investigated the role of the microbiota. RNA sequencing of the gut identified differentially expressed genes in FC and WT mice with DSS-induced colitis. In vitro and in vivo experiments using *Lactobacillus plantarum* (AR113) and a lactate dehydrogenase (LDH)-deficient variant (AR113Δ) assessed the role of lactic acid production in the protective effects of *Lactobacillus*. Statistical analyses included unpaired two-tailed Student's t-tests, Kruskal-Wallis tests, PERMANOVA, LEfSe, Wilcoxon rank-sum tests, and Spearman's correlation analysis.

Clinical trials revealed significantly elevated serum anti-FcεRI levels in UC patients compared to healthy controls, suggesting its potential as a diagnostic biomarker, even surpassing ANCA and GAB in diagnostic accuracy. FcεRla knockout (FC) mice exhibited significantly reduced colitis severity compared to WT mice, as indicated by improved colon length, weight retention, lower DAI scores, and attenuated histopathological damage. FC mice also showed reduced pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) and increased anti-inflammatory cytokine (IL-10) levels, along with a restored Th17/Treg balance. Gut microbiota analysis revealed increased α-diversity in FC mice and significant differences in microbiota composition compared to WT mice, particularly a notable increase in *Lactobacillus* abundance. FMT and cohousing experiments confirmed the critical role of microbiota in the protective effect of FcεRla deficiency. RNA sequencing identified enrichment of the NLRP6 inflammasome pathway in FC mice, with increased expression of NLRP6 inflammasome components, MUC2, and AMPs. *L. plantarum* supplementation protected against colitis, but this effect was absent in the LDH-deficient variant, highlighting the importance of lactic acid production. Correlation analysis showed a strong negative correlation between *Lactobacillus* abundance and pro-inflammatory cytokines, and a strong positive correlation with goblet cell numbers and IL-10.

The study's findings demonstrate a strong link between FcεRI signaling, gut microbiota composition, and colitis severity. Elevated anti-FcεRI in UC patients points towards a potential diagnostic application. FcεRla deficiency ameliorates colitis, likely through microbiota-dependent mechanisms, primarily the increase of *Lactobacillus*. The protective effects of *Lactobacillus* appear to involve NLRP6 inflammasome activation, enhanced mucus production (MUC2), and AMPs secretion. Lactic acid production by *Lactobacillus* seems crucial for its protective effects. These findings provide new insights into the complex interplay between allergic reactions, the gut microbiome, and the pathogenesis of UC, suggesting potential therapeutic strategies targeting mast cell signaling pathways or the gut microbiota.

This research establishes anti-FcεRI as a potential diagnostic marker for UC. FcεRla deficiency protects against DSS-induced colitis, a protective effect significantly associated with increased *Lactobacillus* abundance. *Lactobacillus*'s protective effects likely stem from NLRP6 inflammasome activation and lactic acid production. Future studies could explore the therapeutic potential of *Lactobacillus* or its metabolites in UC treatment, focusing on specific mechanisms of NLRP6 inflammasome activation and the interaction between mast cells and the gut microbiota.

The study used a murine model of colitis, which may not fully capture the complexities of human UC. While the results strongly suggest a role for *Lactobacillus*, further investigation is needed to confirm its therapeutic potential in humans. The mechanism of *Lactobacillus*'s impact on the NLRP6 pathway requires further clarification. The study focused primarily on *Lactobacillus*; the influence of other differentially abundant bacterial species on colitis deserves further investigation.