Nuclear glycine decarboxylase suppresses STAT1-dependent MHC-I and promotes cancer immune evasion.

Inadequate antigen presentation by MHC-I in tumor microenvironment (TME) is a common immune escape mechanism. Here, we show that glycine decarboxylase (GLDC), a key enzyme in glycine metabolism, functions as an inhibitor of MHC-I expression in EGFR-activated tumor cells to induce immune escape by a mechanism independent of its enzymatic activity. Upon EGFR activation, GLDC is phosphorylated by SRC and subsequently translocated to the nucleus in human NSCLC cells. Nuclear GLDC sequesters STAT1 co-activator SMARCE1, inhibiting STAT1-dependent transcription of the inflammatory genes IRF1 and NLRC5. Further, GLDC recruits DNMT1 to the IRF1/NLRC5 promoter inducing DNA hypermethylation, suppressing transcription of downstream MHC-I genes. Inhibition of GLDC restores MHC-I levels in tumor cells, improves tumor-specific CD8(+) T cells functions in the TME, and rescues anti-tumor effects of PD-1 blockade therapy in mice. Our findings reveal a non-enzymatic nuclear function for GLDC in the suppression of MHC-I antigen presentation, suggesting new strategies for ICB-based combination immunotherapy.