MLLT6 maintains PD-L1 expression and mediates tumor immune resistance

We present a CRISPR-Cas9 screen in human cancer cells to identify genes that confers tumors with the ability to evade the cytotoxic effects of the immune system. We discovered that the transcriptional regulator MLLT6 (AF17) is required for PD-L1 cell surface presentation and protein expression in multiple cancer types and MLLT6 depletion alleviates suppression of CD8+ cytotoxic T cell-mediated cytolysis. Furthermore, cancer cells lacking MLLT6 exhibit impaired STAT1 signaling and are insensitive to interferon gamma induced stimulation of IDO1, GBP5, CD74 and MHC class II genes, while maintaining MHC class I gene expression. Collectively, our findings establish MLLT6 as a regulator of oncogenic and interferon gamma associated immune resistance Overall design: Pooled CRISPR-Cas9 based screen to identify genes regulating PD-L1. Cells that expressed low levels of PD-L1 were subjected to deep sequencing to determine frequencies of sgRNAs and their target genes that were enriched, IFNG treated and untreated samples from 3 different time points were used for studying the sgRNA expression profiles by deepsequencing, RNA-seq transcriptome profiling of U2OS wildtype and MLLT6 knockout cells treated with and without IFNG. Samples of each condition was submitted in triplicates for deep sequencing.