Virus-induced pulmonary TLSs are supported by a FAP-derived stromal cell network. (A and B) The proportion of tdTomato⁺ cells expressing PDGFRα, CD31, and/or CD45 (A), the proportion of each population that is tdTomato⁺, and tdTomato mean fluorescence intensity (MFI; B) were determined in naive FC^Tomato lungs. Statistical significance was determined by one-way ANOVA, comparing all groups to CD45⁺: *, P < 0.01; **, P < 0.0001; ns, not significant; SSC, side scatter. (C–F) tdTomato⁺ cells in naive FC^Tomato lung sections showing CD31 (C), αSMA (D), PDGFRα (E), and PDGFRβ (F) expression. The fidelity of transgenic system is demonstrated in C^Tomato littermates (C). (G–I) TLS development in IAV-infected FC^Tomato mice was determined by staining for B cells and the presence of peripheral node addressin⁺ high endothelial venules (G), T cells (H), and CD35⁺ FDC-like cells (I). Arrows in I denote CD35⁺tdTomato⁺ reticular cells. Images in G–I are z-stack projections. Scale bars, 100 µm (C, D, and G–I) or 50 µm (E and F). Data represent at least two independent experiments with three to five mice.