2569-P: Dynamic Impact of Gut-Microbiota Colonization on Epididymal Adipose Tissue in Germ-Free Mice

Introduction and Objective: The gut microbiota plays a central role in host energy metabolism, and its disruption is implicated in obesity. Clinically, frequent antibiotic exposure during childhood is associated with an increased risk of obesity later in life. Although germ-free (GF) mice resist weight gain and insulin resistance, the short-term effects of microbial colonization on adipose tissue remain unclear. This study examined how microbiota colonization from a microbiota-depleted state affects adipose tissue expansion and remodeling. Methods: Eight-week-old GF mice underwent fecal microbiota transplantation (FMT), or conventional mice were treated with antibiotics followed by FMT. Body weight, glucose metabolism, and epididymal white adipose tissue (eWAT) were assessed. Molecular and cellular changes in eWAT were analyzed by bulk RNA sequencing, RT-PCR, flow cytometry, and immunohistochemistry. Results: Within two weeks of colonization, body weight was unchanged, whereas eWAT, inguinal white adipose tissue, and the liver exhibited rapid lipid accumulation with an approximately twofold increase in tissue mass and impaired glucose tolerance. Transcriptomic analysis showed that eWAT from chow-fed colonized mice resembled that of high-fat diet-fed mice. Immune-related pathways were upregulated, and among 1,620 differentially expressed genes, signatures of adipocytes, adipocyte progenitor cells (APs), and M2 macrophages (M2Ms) were enriched. Flow cytometry and immunohistochemistry demonstrated rapid expansion of DPPIV+ newly recruited APs (2.5-fold), DPPIV+ ICAM1+ young APs (3.5-fold), and an increase in F4/80+ CD206+ M2Ms from 18.6% to 25.3%. Similar changes were observed after microbiota transfer in antibiotic-treated conventional mice. Conclusion: Gut microbiota colonization rapidly drives AP and M2M expansion, implicating post-antibiotic microbial recovery in early adipose remodeling and obesity risk. Disclosure S. Fujisaka: None. M. Aslam: None. M. Yokoyama: None. Y. Watanabe: None. T. Kado: None. A. Nishimura: None. M. Bilal: None. H. Honoki: None. S. Inagawa: None. M. Kato: Speaker's Bureau; Ended; AbbVie Inc., Eli Lilly and Company. K. Tobe: None.