Molecular biology of toll-like receptors.

Toll-like receptors (TLRs) play important roles in host resistance to infections, but also act as mediators of pathologies in autoimmunity, septic shock, metabolic disease and cancer. TLRs are expressed in sentinel cells of the immune system (most notably dendritic cells and macrophages) and are key sensors of bacteria, viruses, fungi and protozoa (O'Neill 2006). TLRs also recognize endogenous ligands present in tissues and cells in the absence of infection (Kawai and Akira 2005; Nizet 2006; Kim et al. 2009). In this review, the TLRs are characterized at the cellular level with emphasis on TLR-mediated signaling pathways along with their negative regulation, including specificity of the response(s) at the promoter level.