Porphyromonas Gingivalis Upregulates DC-PD-L1, Suppresses T Cell Cytotoxicity, and Aggravates OSCC

Objectives: Accumulating evidence shows that PD-L1 expression on dendritic cells (DCs) is critical for cancer immunotherapy and 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 remain unexplored.
Methods: Bone marrow derived dendritic cells were challenged with P. gingivalis or ΔKgp mutant, then co-cultured with splenic cells from OT-I mice. DC-PD-L1, CD8 + T cell cytotoxicity and its effectors were measured by flow cytometry. A syngeneic mouse model was also generated by inoculation of MOC1 cells in mice tongues to examine the effect of P. gingivalis on DC-PDL1 expression and tumor growth.
Results: 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. Moreover, Pg infection suppressed ovalbumin (OVA)-pulsed DCs to activate antigen-specific CD8 + T cells, represented by the decreases of IFNγ, 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 remarkably 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. However, these influences of Pg were largely diminished when the tumor cells were pretreated with antibiotics or STAT3 inhibitor.
Conclusions: We have demonstrated that Pg infection upregulated PD-L1 expression on DCs through Akt-STAT3 signaling, suppressed CD8 + T cell cytotoxicity, and aggravated oral cancer growth, suggesting targeting Pg and/or its-mediated signaling could be a therapeutic strategy to improve the efficacy of checkpoint blockade immunotherapy.