Propionate promotes MHC class I antigen presentation and anti-tumor immunity.

Defective tumor antigen presentation promotes immune evasion and resistance to immune checkpoint blockade. Here, we identified the short-chain fatty acid propionate as a potent enhancer of major histocompatibility complex class I (MHC class I) antigen presentation. Propionate stabilizes MHC class I on tumor cells, promotes CD8+ T cell infiltration and activation, and suppresses tumor growth. Mechanistically, the E3 ubiquitin (Ub) ligase STUB1 directly interacts with human leukocyte antigen (HLA)-A to mediate K168/K170 polyubiquitination and subsequent degradation. Propionate directly binds STUB1 through hydrogen bonds involving K30 and N34, residues that overlap with the MHC class I-binding interface. This interaction blocks STUB1-mediated MHC class I polyubiquitination, thereby stabilizing MHC class I, enhancing T cell receptor (TCR) engagement and T cell activation, and restraining tumor growth. Importantly, propionate or the propionate-producing probiotic Akkermansia muciniphila markedly enhanced anti-PD-1 efficacy. These findings uncover a non-metabolic moonlighting function of short-chain fatty acids in regulating tumor immunity and provide a rationale for leveraging dietary propionate or propionate-producing probiotics to enhance immune checkpoint therapy.