P. gingivalis infection upregulates PD-L1 expression on dendritic cells, suppresses CD8+ T cell responses, and aggravates oral cancer.

Accumulating evidence shows that PD-L1 expression on dendritic cells (DCs) is critical for cancer immunotherapy and that Porphyromonas gingivalis (Pg) colonization aggravates progression of upper gastrointestinal cancers. However, the effects of Pg infection on PD-L1 expression on DCs and related immune consequences in the infection milieu of oral cancer remains unexplored. Here, we found that Pg infection robustly enhanced PD-L1 expression on DCs in a gingipain-dependent manner in cultured cell and systemic infection assays. Pg infection suppressed antigen-specific CD8+ T cells through upregulation of PD-L1 expression on ovalbumin (OVA)-pulsed DCs. This suppression was manifested by decreased IFN, perforin, granzyme B, and CD107a. Further analysis showed that Pg drastically reduced CD8+ T cells' ability to lyse OVA-pulsed target cells. Additionally, Pg infection increased phosphorylation of Akt and STAT3, leading to a significant increase of PD-L1 expression. This was substantiated by using siRNA, overexpression plasmids, and pharmacological inhibitors. Consistent with the in vitro observations, in a syngeneic mouse oral cancer model, Pg infection significantly enhanced PD-L1 expression on DCs from intratumoral tissues and cervical lymph nodes and exacerbated oral cancer progression, whereas a Pg lysine-specific, gingipain-defective mutant failed to do so. These influences of Pg were largely diminished when tumor cells were pretreated with antibiotics or a STAT3 inhibitor. Therefore, we demonstrated that Pg infection upregulates PD-L1 expression on DCs through Akt-STAT3 signaling, suppresses CD8+ T-cell cytotoxicity, and aggravates oral cancer growth, suggesting targeting Pg, and/or its-mediated signaling, could be a therapeutic strategy to improve the efficacy of checkpoint blockade immunotherapy.