CRISPR Screen identifies regulators for T cells activities in pancreatic cancer

Most human pancreatic ductal adenocarcinoma (PDAC) are not infiltrated with cytotoxic T cells and are highly resistant to immunotherapy. Over 90% of PDAC have oncogenic KRAS mutations, and phosphoinositide 3-kinases (PI3Ks) are direct effectors of KRAS. Our previous study demonstrated that ablation of Pik3ca in KPC (KrasG12D; Trp53R172H; Pdx1-Cre) pancreatic cancer cells induced host T cells to infiltrate and completely eliminate the tumors in a syngeneic orthotopic implantation mouse model. This study now shows that implantation of Pik3ca-/- KPC (named αKO) cancer cells induces clonal expansion of cytotoxic T cells infiltrating the pancreatic tumors in this mouse model. To identify potential molecules that can regulate the activity of these anti-tumor T cells, we conducted an in vivo genome-wide gene-deletion screen using αKO cells implanted in the mouse pancreas. The result shows that deletion of propionyl-CoA carboxylase subunit B gene (Pccb) in αKO cells (named p-αKO) leads to immune evasion, tumor progression and death of host mice. Surprisingly, p-αKO tumors are still infiltrated with T cells but they are inactive against tumor cells. However, blockade of PD-L1/PD1 interaction reactivated clonally expanded T cells infiltrating p-αKO tumors, leading slower tumor progression and improve survival of host mice. These results indicate that Pccb can modulate the activity of cytotoxic T cells infiltrating some pancreatic tumors and this understanding may help to improve immunotherapy for PDAC. aKO cells were transfected with the genome-wide CRISPR screen library (GeCKOv2) that target 20,611 genes in the mouse genome with 3 sgRNA for each gene, resulting in 1 gene KO in each cell. After impanting the transfected aKO cells to the pancreas of B6 mice, tumor progression was monitored. Tumor tissues were harvested from mice at week 7 and week 13, followed by genomic DNA extraction and PCR amplification of the sgRNAs. The amplified library was then sequenced with amplicon seq.