Fusobacterium nucleatum: Effects on Cancer and Oral Epithelial Cell PD-L1 Expression and Cell Viability

Abstract

This study investigated the impact of heat-killed Fusobacterium nucleatum (F. nucleatum) strains on PD-L1 expression and cell viability in three distinct cell types: SCC-25 (oral squamous cell carcinoma), CL-11 (colorectal cancer), and primary human gingival keratinocytes (PHGK). Our results demonstrated that heat-killed F. nucleatum strains modulate PD-L1 expression in a strain- and cell type-dependent manner, with notable differences in protein and mRNA expression. Additionally, the effects of bacterial exposure on cell viability varied over time, further emphasizing the complexity of F. nucleatum-host interactions. In SCC-25 cells, all three F. nucleatum strains (ATCC 49256, ATCC10953, and ATCC 25586) significantly upregulated PD-L1 protein expression, particularly at 24 hours. However, mRNA expression did not consistently correlate with these changes, suggesting that PD-L1 regulation may involve post-transcriptional mechanisms. Cl-11 cells exhibited a similar trend, with ATCC 10953 inducing the most pronounced PD-L1 upregulation, followed by ATCC 49256 and ATCC 25586 at later time points. In contrast, PHGK cells exhibited a more limited response to bacterial stimulation, ATCC 49256 did not significantly alter PD-L1 expression, while ATCC 10953 and ATCC 25586 induced modest upregulation at later time points. The effects of heat-killed F. nucleatum on cell viability were also highly dynamic and strain-dependent. In SCC-25 cells, bacterial exposure initially promoted viability at early time points, but prolonged exposure (48 hours) resulted in a dose-dependent decrease, particularly at higher multiplicity of infection (MOI). This biphasic response was most pronounced in ATCC 49256 and ATCC 10953, whereas ATCC 25586 exhibited a weaker effect. In CL-11 cells, F. nucleatum predominatly exerted an inhibitory effect on viability, with significant reductions observed in specific treatment groups, particularly at 24 and 48 hours. However, some conditions, notably higher bacterial doses, showed signs of viability recovery at later time points, particularly for ATCC10953. In PHGK cells, viability responses varied depending on the strain and exposure duration. ATCC 49256 and ATCC 10953 initially enhanced viability at 4 hours but induced significant reductions at 24 and 48 hours, whereas ATCC 25586 had minimal effects on PHGK viability. Overall, these findings highlight the strain-specific and time- dependent effects of heat-killed F. nucleatum on immune modulation and cellular responses. The differential induction of PD-L1 suggests that distinct bacterial components may contribute to immune evasion, while the observed viability changes indicate that heat-killed F. nucleatum interactions with host cells are complex and dynamic. Further research is needed to elucidate the molecular mechanisms underlying these responses and to determine the potential implications for progression and therapeutic interventions.