Growing evidence suggests that immune activation and gut microbiota are involved in the pathophysiology of IBS. We have recently demonstrated that a diet low in fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAP) improved symptoms in IBS patients and this was associated with lower urinary histamine and changes in the microbiota composition. A high FODMAP diet in germ-free mice colonized with IBS patient microbiome modulated motility, permeability, and visceral sensitivity. However, the underlying mechanisms are unclear. To investigate the interactions of host’s immune system and intestinal microbiota in response to FODMAPs in the humanized IBS mouse model. Germ-free NIH Swiss mice were colonized with fecal microbiota from two IBS patients (non-constipation) with a high (HH) or low (LH) urinary histamine (n=24 mice/patient), and one healthy control (HV) (n=8). SPF mice (n=8) were included as additional controls. Mice were assigned to a custom-made low or high FODMAP diet (LF and HF, respectively). Microbiota composition was analysed by 16S rRNA gene sequencing. At sacrifice, intestinal tissues were collected for immunohistochemistry (c-kit staining) and immunofluorescence (tryptase, PGP9.5, and Substance P) detection of mast cell and nerves. HH colonized mice receiving HF diet had increased number of mast cells (c-kit positive cells) in both small intestine and colon compared to HH mice on a LF diet (Effect of diet x microbiota: p=0.041, F(1,32)=4.51 for small intestine, and p=0.029, F(1,30)=5.25 for colon). No differences were observed in the colon and small intestine of LH colonized mice. Immunofluorescent staining of colonic and small intestinal sections revealed high number of tryptase positive mast cells in HH colonized mice on HF diet. Moreover, the majority of the mast cells were in close proximity of nerve fibers (PGP9.5 positive) (Fig 1). In HH colonized mice, colonic mast cell numbers were negatively correlated with the relative abundance of Adlercreutzia spp, Oscillospira and Ruminococcus spp. Similarly, in the small intestine of HH colonized mice, mast cell numbers negatively correlated with the relative abundance of Blautia, Ruminococcus, and Lactobacillus spp. A high fermentable diet alters intestinal microbiota composition only in mice colonized with microbiota from a patient with high urinary histamine. These changes are associated with increased numbers of mast cells in both the colon and small intestine. Furthermore, mast cells seem to closely interact with neurons, likely contributing to the functional changes observed in these mice. CIHRNIH