METTL3 Inhibition Restores PD-L1 Expression and CD8+ T-cell Cytotoxic Function in Immunotherapy-Treated Gastric Cancer

The efficacy of immunotherapy targeting PD-1/PD-L1 in gastric cancer depends on PD-L1 expression levels and the infiltration of immune cells within the tumor microenvironment (TME). Although methyltransferase-like 3 (METTL3) plays a role in the development and progression of gastric cancer, its mechanism of regulating the TME in gastric cancer remains unclear. In this study, we demonstrated that the expression of PD-L1 is regulated by METTL3. We found that METTL3 mediated N6-methyladenosine (m6A) modification of PDL1 mRNA in the 3' untranslated region and induced mRNA degradation through an m6A/YTHDF2-dependent pathway in human gastric cancer cells. METTL3 knockdown or inhibition in gastric cancer cells significantly enhanced Jurkat cell migration and cytotoxic activity. In clinical gastric cancer tissue, a negative correlation was observed between the expression levels of PD-L1 and those of METTL3 or YTHDF2. In vivo, combination treatment with the METTL3 inhibitor STM2457 and PD-1 mAb resulted in a significant reduction in tumor growth, enhanced PD-L1 expression, and increased infiltration of CD8+ T cells. Finally, lower METTL3 expression in tumors correlated with improved sensitivity to anti-PD-1 immunotherapy in patients. Our findings revealed that METTL3-mediated m6A modification of PDL1 mRNA levels represents an epigenetic mechanism regulating antitumor immunity in gastric cancer, and inhibiting METTL3 during PD-1 mAb treatment reshaped the TME, thereby establishing a promising treatment approach for enhancing immunotherapy efficacy in patients with gastric cancer.