Background/Purpose: Germinal center (GC) responses generate humoral immunity through coordinated interactions between B cells and T follicular helper (TFH) and T follicular regulatory (TFR) cells. Despite high expression of PD-1 by TFH and TFR cells, the function of its ligand PD-L1 in the GC remains unknown. We hypothesized that PD-L1 has an orthogonal function in the GC, where it directly modulates TFH and TFR functionality via PD-1 to tune availability of T cell help.

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 Cxcl13^Cre mice with Pdl1^fl/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. In contrast, mixed bone marrow chimeras and competitive transfer experiments revealed an immunosuppressive cell-autonomous function of PD-L1 in GC B cells, suggesting that PD-L1 has a unique non-cell autonomous function on FDCs and naïve B cells. 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: Here, 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.

ACR Meeting Abstracts - https://acrabstracts.org/abstract/follicular-dendritic-cell-pd-l1-expression-promotes-autoreactive-germinal-center-formation/