Abstract
Background
Short-chain fatty acids (SCFAs) produced by the gut microbiota are increasingly recognized as regulators of host immunity, yet the role of individual SCFAs in shaping the tumor immune microenvironment in colorectal cancer (CRC) remains poorly defined. This study aims to investigate the role of propionate in antitumor immunity and immunotherapy responsiveness in CRC.
Methods
SCFA levels, systemic inflammatory cytokine levels, and immune cell subsets were quantified in peripheral blood and tumor tissues from patients with CRC and healthy controls. Syngeneic microsatellite instability-high (MSI-H) and microsatellite stable (MSS) murine CRC models were used to evaluate the therapeutic efficacy of propionate alone and in combination with anti-programmed death-ligand 1 (PD-L1) blockade. CD8
+
T-cell infiltration, activation status, and cytotoxic function were assessed by flow cytometry, immunohistochemistry, and single-cell RNA sequencing. Mechanistic investigations included acetylation-dependent protein modification analyses. Molecular analyses of signaling pathways were performed to delineate the regulatory axis underlying propionate-mediated immune modulation.
Results
Propionate levels were inversely associated with systemic inflammation and positively correlated with intratumoral CD8
+
T-cell infiltration. Propionate supplementation enhanced CD8
+
T-cell cytotoxicity, promoted CD8
+
T-cell tumor infiltration and suppressed tumor growth in both MSI-H and MSS mouse models. Notably, propionate significantly potentiated the efficacy of PD-L1 blockade and restored immunotherapy sensitivity in otherwise resistant MSS tumors. Mechanistically, propionate induced acetyl-CoA acetyltransferase 1-dependent acetylation and proteasomal degradation of argininosuccinate synthase 1 (ASS1), resulting in activation of signal transducer and activator of transcription 1 signaling and upregulation of the T cell-recruiting chemokine C-X-C motif chemokine ligand 9. In clinical samples, low ASS1 expression was associated with high CD8
+
T-cell infiltration.
Conclusions
This study revealed the role of propionate in regulating the tumor immune microenvironment in CRC, suggesting propionate as a promising strategy to overcome immune exclusion and enhance the efficacy of immunotherapy.