RNAi nanomaterials targeting immune cells as an anti-tumor therapy: the missing link in cancer treatment?

siRNA delivery targeting tumor cells and cancer-associated immune cells has been gaining momentum in the last few years. A combinatorial approach for silencing crucial factors essential for tumor progression in cancer-associated immune cells and in cancer cells simultaneously can effectively shift the tumor microenvironment from pro-oncogenic to anti-tumoral. Gene-therapy using RNAi nanomaterials can help shift this balance; however, fully utilizing the potential of RNAi relies on effective and specific delivery. RNAi nanomaterials can act as a Trojan horse which delivers siRNAs against immunosuppressive factors and reverses the regulatory activity of tumor immune cells residing in the tumor microenvironment. Here we review potential RNAi targets, means to activate and control the immune response, as well as ways to design delivery nanovehicles for successful RNAi immunotherapy.