Meher, A. K.; Huang, W.; Yamaguchi, D. J.; Yin, C.; Swyers, M. M.; Dubis, G.; Yang, L. V.; Mannie, M. D.; McNamara, C. A.

Objective: Perivascular Adipose Tissue (PVAT), tertiary lymphoid structures (TLS), and the B lymphocytes they harbor have been implicated in regulating vascular diseases, including abdominal aortic aneurysm (AAA). Yet, the relationship of adipose tissue to TLS and B cell responses is poorly understood. Approach and Results: Here, we used a multi-omic approach to rigorously define the landscape of human AAA and characterize the adipocyte and TLS microenvironment with regard to B-cell responses to vascular antigens in human AAA and control non-aneurysmal abdominal aortas. Single-nucleus and single-cell RNA sequencing identified transcripts for lipogenesis, lipolysis, extracellular remodeling, haptotactic and chemotactic factors, with potential to recruit CD44+ leukocytes within AAAs. Immunohistology and Visium HD spatial transcriptomics revealed that at least two adipocyte types exist: one in the PVAT and another as isolated clusters in the medial-adventitial region in human AAAs but not in control aortas. Further characterization of these medial-adventitial adipocytes in the AAAs showed close association with TLSs. Mature TLSs recapitulated the organization of functional secondary lymphoid structures, with multiple B-cell follicles clustered to form a nodular structure partly covered by a layer of smooth muscle cells. The TLSs harbored isotype-switched B cells expressing IgG1, IgG3, or IgA, with dominant kappa chain usage and abundant IgG antibodies to aortic wall. Myeloid cells in AAA were implicated in promoting B cell effects, as they were the primary producers of B-cell activating factor (BAFF), a cytokine that promotes B cell infiltration, survival, and function. Conclusion: Altogether, our results support a novel model in which new adipocytes in the medial-adventitial layer produce factors that promote early TLS formation, and BAFF produced by myeloid cells promotes the survival and maturation of vascular wall-antigen-exposed B cells, leading to mature TLS formation, production of antibodies to AAA antigens, fueling inflammation and AAA growth.