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 (GC) depends on PD-L1 expression level and infiltration of immune cells within the tumor microenvironment (TME). While methyltransferase-like 3 (METTL3) plays a role in the development and progression of GC, its mechanism of regulating the TME in GC remains unclear. In this study, we demonstrated that expression of PD-L1 is regulated by METTL3. We found that METTL3 mediated m6A modification of PDL1 mRNA in the 3'-untranslated region (UTR) and induced mRNA degradation through an m6A/YTHDF2-dependent pathway in human GC cells. METTL3-knockdown or inhibition in GC cells significantly enhanced Jurkat cell migration and cytotoxic activity. In clinical GC 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 monoclonal antibody (mAb) resulted in a significant reduction in tumor growth, enhanced PD-L1 expression, and increased the 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 anti-tumor immunity in GC, and inhibiting METTL3 during PD-1 mAb treatment reshaped the TME, thereby establishing a promising treatment approach for enhancing immunotherapy efficacy in GC patients.