Abstract Background Vedolizumab (VDZ) targets the gut-homing receptor α4β7. Despite its widespread use as a frontline therapy for ulcerative colitis (UC), its mode of action (MOA) remains unclear. Previously, we documented that response to VDZ is associated with size loss of gut-associated lymphoid tissues (GALT). To further understand drug MOA, here we aimed at defining the relationship between GALT attrition and resolution of inflammation. Methods Tissue immunofluorescence (IF) was performed on two independent UC cohorts (n=60, n=21) studied longitudinally pre- and post-VDZ. Peripheral blood mononuclear cells from UC patients (n=56) were examined at week 0 (pre-VDZ) and 14 (post-VDZ) by flow cytometry, while intestinal biopsies were analyzed in a subset of patients. Transcriptomic analyses were performed in a third cohort of 401 UC patients and 243 controls where bulk RNA-sequencing was performed on intestinal biopsies. Finally, data from a fourth validation cohort (n=31) was examined pre- and post-VDZ. Results Using IF, we examined follicular naïve B (FB) cells (IgD+), germinal center (GC) B cells (BCL6+CD3-) and total T cells (CD3+), and identified three GALT types: organized aggregates (demarcated B/T cell zones) with GC, organized aggregates without GC, and poorly organized aggregates (Fig1A). When comparing pre- and post-VDZ, Responders (R) had a significant reduction in GALT organization, while no change was seen in Non-Responders (NR). The number of FB cells was significantly reduced post-VDZ in R but not in NR (Fig1B), with no change in T cells (Fig1C). The reduction in FB was validated using transcriptomic data (Fig1D). To explore potential consequences of GALT attrition, we analyzed circulating β7+ (GALT-generated) and β7- plasmablasts (PB). The frequencies of β7+IgG+ and β7+IgA+ PB were significantly reduced post-VDZ in R, but not in NR (Fig1E). No change was observed in β7- PB post-VDZ (Fig1F). Among β7+ PB, there was a greater reduction in IgG+ compared to IgA+ (Fig1G). Furthermore, colonic IgG+ plasma cells (PC) were significantly reduced post-VDZ in R but not in NR (Fig1H). To evaluate IgG-mediated proinflammatory function, we studied the expression of FcγR-pathway genes in colonic biopsies. Importantly, the FcγR-pathway was enhanced in inflamed tissues of UC patients (Fig1I-J) and was significantly reduced in R (but not in NR) post-VDZ (Fig1K). Conclusion Our work demonstrates a novel effect of anti-α4β7 blockade that results in intestinal lymphoid aggregate attrition through impaired recruitment of naïve B cells, leading to the suppression of IgG-driven immune responses and proinflammatory FcγR signaling. These data highlight the targeting of GALT as a novel therapeutic paradigm in IBD.