Immunotherapeutic Implications of Toll-like Receptors Activation in Tumor Microenvironment

Toll-like receptors (TLRs) play an important role between innate and adaptive immunity as one of the pattern recognition receptors (PRRs). Both immune cells and tumor cells express TLRs, and the same TLR molecule is expressed in different cells with different roles. TLR activation in the tumor microenvironment mostly has a dual role in tumor progression during chronic inflammation. Clinically, the therapeutic efficacy of most cancer immunotherapy strategies is restricted by the suppressive immune infiltrative environment within the tumor. Therefore, activation of TLRs in innate immune cells has the potential to eradicate tumors lacking T-cell infiltration. TLR agonists have served as important immunomodulators of cancer immunotherapy through immune responses and reprogramming the tumor suppressive microenvironment. Meanwhile, considering the complex interaction of TLRs with the tumor microenvironment, a combined approach of cancer immunotherapy and nanotechnology has been adopted to improve cancer immunotherapy not only by combining multiple drug combinations, but also by targeting the tumor microenvironment using nanoparticles. Many clinical trials are underway to improve antitumor activity through combination with other immunotherapies. In this review, we provide a comprehensive and detailed overview of the immunotherapeutic implications of TLRs activation in tumor microenvironment, highlighting its great potential to be an important tool for cancer immunotherapy.