Abstract TAC241: Renin-Angiotensin System Deficiency Induces Tertiary Lymphoid Tissue Formation and Promotes Inflammation-Driven Fibrosis in the Kidney

Background: Tertiary lymphoid tissues (TLTs) are ectopic lymphoid tissues formed in response to chronic inflammation. TLTs comprise a hematopoietic compartment—mainly proliferating T and B cells—and stromal components, including fibroblasts and fibroblast-derived follicular dendritic cells. These structures produce local cytokines, chemokines, and antibodies. TLTs have been reported to have both beneficial and detrimental effects depending on the disease context. We have previously observed that mice with renin-angiotensin system (RAS) deficiency, such as Ren1 c knockout (KO) mice and those treated with RAS inhibitors, develop concentric hypertrophy of the renal arteries and arterioles and striking aggregates of perivascular inflammatory cells. Whether these inflammatory aggregates are true TLTs of functional relevance is unclear. Objective: To determine whether the inflammatory cell infiltrates observed in Ren1 c KO mice represent TLT and clarify their potential pathological effects. Methods: Ren1 c-/- mice were analyzed at six months of age. Kidneys were analyzed for TLT features by immunostaining for CD3, B220, CD21, Ki67, and CD138. Periodic acid-Schiff staining was used to quantify TLT areas. Quantitative PCR was performed to measure Tnfa and Il6 expression levels. In addition, VISIUM HD Spatial Gene Expression analysis was performed to assess gene expression profiles. Results: The inflammatory cell aggregates consisted of CD3+ T cells and B220+ B cells, with the presence of CD21+ and Ki67+ cells, confirming their TLT identity. CD138+ plasma cells were also detected within the TLTs. qPCR showed significantly increased in Tnfa and IL6 expression in Ren1 c KO groups (8.2 ± 8.6 and 25.5 ± 21.7-fold, respectively; p<0.005), which positively correlated with TLT area (R 2 =0.71, 0.84, respectively; p<0.001). Spatial transcriptomic analysis revealed upregulation of plasma cell-related genes, including Mzb1 , as well as increased expression of complement and fibrosis-associated genes. Conclusion: These findings indicate that the inflammatory cell infiltrates in RAS-deficient mouse kidneys represent TLTs. TLTs are likely sites of local cytokine production and autoantibody generation, contributing to complement activation and promote fibrotic remodeling, thereby exacerbating the arterial and interstitial disease. Our results highlight a link between RAS deficiency, TLT formation, and inflammation-driven fibrosis in the kidney.