Dickkopf 1 Impairs Sensitivity to PD-1 Blockade through CD8+ T Cell Inactivation in dMMR/MSI Colorectal Cancer

Dickkopf 1 (DKK1) could promote tumor progression by suppressing immunity. Therefore, we investigated whether DKK1 influence prognosis and sensitivity to PD-1 blockade in colorectal cancers (CRCs) with defective DNA mismatch repair genes (dMMR) or microsatellite instability (MSI). We found that elevated DKK1 expression was associated with recurrence and dismissed CD8+ T cell infiltrations, and patients with high serum DKK1 had poor anti-PD-1 response. RNA interference or neutralization of DKK1 in CRCs enhanced CD8+ T cell cytotoxicity, and down-regulation of T-bet and E2F1 following GSK3ß activation was detected in DKK1-treated CD8+ T cells. In organoid-lymphocyte co-culture model, apoptosis proportions were elevated after individual neutralization of both PD-1 and DKK1, and the combined neutralization resulted in further increases. In conclusion, DKK1 suppresses tumor immunity in dMMR/MSI CRCs by inactivating CD8+ T cells through GSK3ß/E2F1/T-bet axis. DKK1 neutralization may improve the sensitivity to PD-1 blockade in dMMR/MSI CRCs. Overall design: We obtained tumor tissues and serum of dMMR/MSI CRCs. DKK1 expression and immune status of 80 cases were evaluated by immunohistochemistry, while serum DKK1 of another 43 cases receiving PD-1 blockade were measured by ELISA. Control and DKK1 knockdown CT26 cells were grafted in BALB/c mice, or co-cultured with T cells in vitro. Organoids of dMMR CRCs were co-cultured with T cells, treated with DKK1, anti-DKK1 and mock. PD-1 neutralizations were given to some of those models. To assess cytotoxicity of T cells, apoptosis assays and flow cytometrys were used. RNA sequencing on CD8+ T cells were performed to investigate downstream of DKK1 and underlying mechanism.