Targeting the innate immune system with nanoparticles for cancer immunotherapy

Various cancer therapies have advanced remarkably over the past decade. Unlike the direct therapeutic targeting of tumor cells, cancer immunotherapy is a new strategy that boosts the host's immune system to detect specific cancer cells for efficient elimination. Unfortunately, the efficacy of these treatments has been limited to a fraction of patients within a subset of tumor types, and further studies are still needed to clarify these mechanisms and develop novel approaches to improve the efficacy of cancer immunotherapy. Emerging data suggest that the innate immune system also plays a key role in tumor immunosurveillance and generation of antitumor immune responses. Nanoparticles incorporating immunomodulatory agents can activate immune cells and modulate the tumor microenvironment to enhance antitumor immunity. Such nanoparticle-based cancer immunotherapies have received considerable attention and have been extensively studied in recent years. In this review, we will discuss the anticancer activities of nanoparticles designed to target innate immune pathways, including Toll-like receptor, nucleotide-binding oligomerization domain-like receptor, and retinoic acid-inducible gene-I-like receptor pathways, as well as DNA sensing pathways. In addition, nanoparticles that target key innate immune cell types, such as macrophages, myeloid-derived suppressor cells, dendritic cells, natural killer cells, and neutrophils, also will be investigated. In summary, although further research and clinical studies are still needed to solve the safety concerns and improve the efficacy of nanoplatform-based cancer immunotherapy, the recent studies presented in this review prove that nanoparticle-incorporated cancer immunotherapy is a highly promising treatment for cancer patients.