We present here an integrated nanotechnology/biology strategy for cancer immunotherapy that uses arginine nanoparticles (ArgNPs) to deliver CRISPR-Cas9 gene editing machinery into cells to generate SIRP-α knockout macrophages. The NP system efficiently codelivers single guide RNA (sgRNA) and Cas9 protein required for editing to knock out the "don't eat me signal" in macrophages that prevents phagocytosis of cancer cells. Turning off this signal increased the innate phagocytic capabilities of the macrophages by 4-fold. This improved attack and elimination of cancer cells makes this strategy promising for the creation of "weaponized" macrophages for cancer immunotherapy.