Targeting Toll-Like Receptors for Cancer Therapy

Marc J. Braunstein, John Kucharczyk, Sylvia Adams

The immune system encompasses a broad array of defense mechanisms against foreign threats, including invading pathogens and transformed neoplastic cells. Toll-like receptors (TLRs) are critically involved in innate immunity, serving as pattern recognition receptors whose stimulation leads to additional innate and adaptive immune responses. Malignant cells exploit the natural immunomodulatory functions of TLRs, expressed mainly by infiltrating immune cells but also aberrantly by tumor cells, to foster their survival, invasion, and evasion of anti-tumor immune responses. An extensive body of research has demonstrated context-specific roles for TLR activation in different malignancies, promoting disease progression in certain instances while limiting cancer growth in others. Despite these conflicting roles, TLR agonists have established therapeutic benefits as anti-cancer agents that activate immune cells in the tumor microenvironment and facilitate the expression of cytokines that allow for infiltration of anti-tumor lymphocytes and the suppression of oncogenic signaling pathways. This review focuses on the clinical application of TLR agonists for cancer treatment. We also highlight agents that are undergoing development in clinical trials, including investigations of TLR agonists in combination with other immunotherapies.