Translation control of the immune checkpoint in cancer and its therapeutic targeting.

Cancer cells develop mechanisms to escape immunosurveillance, among which modulating the expression of immune suppressive messenger RNAs is most well-documented. However, how this is molecularly achieved remains largely unresolved. Here, we develop an in vivo mouse model of liver cancer to study oncogene cooperation in immunosurveillance. We show that MYC overexpression (MYC) synergizes with KRAS to induce an aggressive liver tumor leading to metastasis formation and reduced mouse survival compared with KRAS alone. Genome-wide ribosomal footprinting of MYC;KRAS tumors compared with KRAS revealed potential alterations in translation of mRNAs, including programmed-death-ligand 1 (PD-L1). Further analysis revealed that PD-L1 translation is repressed in KRAS tumors by functional, non-canonical upstream open reading frames in its 5' untranslated region, which is bypassed in MYC;KRAS tumors to evade immune attack. We show that this mechanism of PD-L1 translational upregulation was effectively targeted by a potent, clinical compound that inhibits eIF4E phosphorylation, eFT508, which reverses the aggressive and metastatic characteristics of MYC;KRAS tumors. Together, these studies reveal how immune-checkpoint proteins are manipulated by distinct oncogenes at the level of mRNA translation, which can be exploited for new immunotherapies.