Polyphenol Supplementation Enhances the Efficacy of PD-1/PD-L1 Inhibitors Against Cancer: A Meta-Analysis of Animal Studies

<bold>Background: </bold>This study performed a meta-analysis to evaluate the combined effects of polyphenols and anti-programmed cell death-1 (PD-1)/programmed cell death ligand-1 (PD-L1) inhibitors.<bold>Methods: </bold>Relevant studies were collected from electronic databases. Standardized mean differences (SMDs) or hazard ratio (HR) was calculated by Stata 15.0 software.<bold>Results: </bold>Sixteen preclinical studies were included. The overall meta-analysis showed that, compared to anti-PD-1/PD-L1 alone, polyphenol combined therapy significantly reduced the tumor volume (SMD = -3.28), weight (SMD = -2.18), number (SMD = -2.17), and prolonged the survival (HR = 0.45) of mice (all P < 0.001). Pooled analysis of mechanism studies indicated polyphenol combined therapy could increase the number of cytotoxic CD8(+) T cells (SMD = 3.88; P < 0.001), IFN-gamma(+) CD8(+) T cells (SMD = 2.38; P < 0.001), decrease the number of myeloid-derived suppressor cells (SMD = -2.52; P = 0.044) and T-reg cells (SMD = -4.00; P = 0.004) and suppress PD-L1 expression in tumors (SMD = -13.41; P < 0.001). Subgroup analyses demonstrated curcuminoids, flavonoids, and stilbene changed the tumor volume, the percentage of CD8(+) T cells, IFN-gamma(+)CD8(+) T cells, and PD-L1 expression.<bold>Conclusion: </bold>Polyphenol supplementation may be a promising combined strategy for patients with poor response to anti-PD-1/PD-L1 monotherapy.