The role of Toll-like receptor signaling in human immunodeficiencies

Through the last decade, clinical immunology has witnessed a considerable progress in understanding the role of the innate immunity in human host defense, with Toll-like receptors (TLRs) being the most extensively innate immune receptors investigated. Growing literature documents the relevance of TLR signaling pathways to human disease, revealing a small, but expanding, group of new monogenic primary immunodeficiencies, in patients with various infectious diseases, previously considered as of unexplained "idiopathic" origin. Herein, we review these recently described deficiencies. Autosomal recessive IRAK-4 and myeloid differentiation factor 88 deficiencies were reported in 2003 and 2008, respectively, conferring predisposition to pyogenic bacterial infections, and autosomal recessive UNC93B1 and autosomal dominant TLR3 deficiencies were reported in 2006 and 2007, respectively, conferring predisposition to herpes simplex encephalitis. Furthermore, we highlight the published data associating TLR polymorphism with an altered susceptibility to infectious diseases.