Background: Expression of programmed death ligand-1 (PD-L1) is related to the prognosis of many solid malignancies, including oral squamous cell carcinoma (OSCC), but the mechanism of PD-L1 induction remains obscure. In this study, we examined the expression of PD-L1 and partial epithelial–mesenchymal transition (pEMT) induced by bacterial lipopolysaccharide (LPS) in OSCC. Methods: The expression of Toll-like receptor 4 (TLR4) recognizing LPS in OSCC cell lines was analyzed. Moreover, the induction of PD-L1 expression by Porphyromonas gingivalis (P.g) or Escherichia coli (E. coli) LPS and EMT was analyzed by western blotting and RT-PCR. Morphology, proliferation, migration, and invasion capacities were examined upon addition of LPS. PD-L1 within EXOs was examined. Results: PD-L1 expression and pEMT induced by LPS of P.g or E. coli in TLR4-expressing OSCC cell lines were observed. Addition of LPS did not change migration, proliferation, or cell morphology, but increased invasive ability. Moreover, higher expression of PD-L1 was observed in OSCC EXOs with LPS. Conclusion: Oral bacterial LPS is involved in enhanced invasive potential in OSCC cells, causing PD-L1 expression and induction of pEMT. The enhancement of PD-L1 expression after addition of LPS may be mediated by EXOs.

Tumor budding (TB) occurs at the deepest site of tumor invasion and is a significant prognostic indicator of cervical metastasis in oral squamous cell carcinoma (OSCC). The mechanism of TB, however, remains unclear. This study investigated the roles of the tumor microenvironment and partial epithelial–mesenchymal transition (p-EMT) in TB expression using molecular and cellular physiological analyses. We established oral metachronous carcinoma cell lines (gingival carcinoma: 020, tongue carcinoma with high TB expression: 020G) from two cancers with pathologically different TB in the same patient and subjected them to exome analysis to detect gene mutations related to carcinogenesis and malignancy. Differences in EMT expression induced by transforming growth factor-β (TGF-β) between 020 and 020G were analyzed by Western blotting and reverse transcription polymerase chain reaction, and TGF-β-induced changes in cell morphology, proliferation, migration, and invasive ability were also examined. TGF-β expression was observed in the deepest tumor invasion microenvironment. TGF-β also induced the expression of several p-EMT markers and increased the migration and invasive abilities of 020G compared with 020 cells. In conclusion, TGF-β in the deep-tumor microenvironment can induce p-EMT in tumor cells, expressed as TB.