Abstract 6408: Stromal cell PD-L1 potentiates epitope spreading in the germinal center

Abstract Background: Although immune checkpoint therapy targeting PD-1/PD-L1 can induce durable responses in cancer patients, it is also associated with epitope spreading and adverse events that can limit cancer treatment. Epitope spreading may occur in the germinal center (GC), where T follicular helper (TFH) and T follicular regulatory (TFR) cells interact with B cells to generate humoral immunity. Despite high expression of PD-1 by TFH and TFR cells, the function of its ligand PD-L1 in the GC remains unknown. Methods: We characterized the stromal cell compartment in secondary lymphoid organs by performing single cell RNA sequencing on human tonsils and mouse Peyer’s Patches and integrating these data with publicly available datasets. To investigate the functional significance of follicular dendritic cell (FDC) expression of PD-L1, we crossed Cxcl13Cre mice with Pdl1fl/fl mice to generate mice with conditional deletion of PD-L1 in FDCs. To visualize and quantify cellular dynamics in vivo, we adoptively transferred fluorescently labeled lymphocytes into mice and performed intravital multiphoton microscopy on skin draining lymph nodes. Results: Integrated analyses of human and mouse secondary lymphoid organs revealed that FDCs are heterogenous across tissues and species. FDCs also express PD-L1 and this expression is increased in autoantibody disease, although is not correlated with other changes in FDC gene expression. FDC specific deletion of PD-L1 decreases GC formation and prevents loss of B cell tolerance measured by entry of naïve B cells into autoreactive GCs. Autoreactive B cells also express increased PD-L1 and B cell specific deletion of PD-L1 similarly hinders GC responses. Intravital multiphoton microscopy demonstrated that FDC PD-L1 controls TFH and GC B cells by promoting stable T-B contacts. Single cell analyses of TFH cells revealed that PD-1 expression is highly correlated with Sostdc1 expression, which has been shown to promote TFR formation. Mechanistically, PD-L1 limits TFH production of Sostdc1, thereby suppressing TFR differentiation and enabling the GC response. Conclusion: We show that PD-L1 tunes T-B collaboration in the GC to promote humoral immunity. These findings uncover an immunogenic non-cell autonomous role of PD-L1 that is orthogonal to its well-established role in suppression of cell mediated immunity. We propose a model in which levels of PD-L1 on FDCs and bystander B cells tune the GC response by manipulating the follicular T cell compartment. Citation Format: Elliot Akama-Garren, Yingying Zhang, Balthasar Heesters, Padraic Fallon, Michael Carroll. Stromal cell PD-L1 potentiates epitope spreading in the germinal center [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 6408.