Toll-like receptors: cost or benefit for cancer?

The function of the Toll-like receptor (TLR) family members has been extensively studied in the recent decades. The TLR family is generally involved in the defense against microbial infections. TLRs are expressed mainly on macrophages and dendritic cells (DCs) and activate these cells upon ligand binding. The activation of TLRs basically initiates innate immune response, but can also induce adaptive immune response. TLRs have also been found on epithelial and tumor cells, but their role on tumor cells is still unclear. In some tumor types TLRs promote tumor proliferation and survival, while in others TLR2, -3 and -9 have been shown to be directly involved in apoptosis. Therefore, they seem to be promising candidates for the development of new, effective therapeutic options. It is however necessary to conduct comprehensive studies to assess the significance of these receptors in neoplastic cells. TLR ligands can also be used as immunostimulatory molecules to boost immune system in anticancer treatment. In this respect TLRs have been used in numerous preclinical and clinical studies. However, adjuvants can evoke distinct immune responses, either beneficial or deleterious in the neoplastic setting. Moreover, neoplastic processes may also subvert different signaling pathways and thereby advance cancer progression. From both points of view careful selection of adjuvants is a necessary prerequisite for cancer patient's treatment. Thus, TLRs have a dual role, when used as a target for immunostimulation, as well as when used directly to kill the cancer cell.