Roles of toll-like receptors in natural interferon-producing cells as sensors in immune surveillance

Natural IFN-alpha/beta producing cells (IPCs) play a central role in innate immunity against microbial infections. In primary immune responses, toll-like receptors (TLRs), as major pattern-recognition receptors, are essential for IPCs as well as other antigen presenting cell (APC) subsets to recognize microbes. IPCs unequivocally express TLR7 and TLR9, and can respond to the respective ligand to produce IFN-alpha/beta and to rapidly differentiate into dendritic cells (DCs). Thereby, IPCs can not only activate innate immune system but also provoke T cell responses. Thus, IPCs link innate and adaptive immunity through TLR system. In addition, recent work has revealed the regulatory system of DC subsets in response to microbial invasion. In this context, by the different but complementary expression profile of TLRs, IPCs together with myeloid APC subsets constitute a rational system of immune surveillance that can cover a wide variety of pathogens and enlarge immune adjuvant effects.