Dendritic cells (DCs) comprise a complex lineage and subset of bone marrow-derived antigen-presenting cells. DCs not only directly participate in the defense of microbial pathogens in the innate immune system; they also transmit the information of the invading pathogens and present pathogen antigens to T cells of the adaptive immune system and modulate subsequent immune responses. One critical factor for DCs to discriminate antigens derived from dead tissue cells or pathogens is the co-presence of certain pathogen-associated molecular patterns. DCs express several pattern-recognition receptors including the toll-like receptors. DCs use different toll-like receptors to simultaneously detect several features of the pathogens. Signals generated from various toll-like receptors coordinately regulate the production of distinct cytokines and determine the nature of the resulting immune responses. Thus, the ligands of toll-like receptors or toll-like receptor agonists are potent stimulators of the immune system. Several pharmaceutical companies are now exploring the application of toll-like receptor agonists in the treatment of several diseases including cancers. Preliminary results suggest that this kind of strategy may provide a new direction of immunotherapy.
