Oral bacteria and periodontitis, with special reference to innate immune system in oral mucosa

Innate immune system is an ubiquitous system from insects to human and responsible for initial host defense against invasive pathogens, where pathogen-associated molecular patterns (PAMPs) are recognized by the pattern recognition molecules of hosts. Representative human receptors for PAMPs are nine Toll-like receptors (TLRs) and intracellular NOD molecules which recognize peptidoglycans of bacterial cell-walls. Oral epithelial cells express various TLRs and NODs so far examined, and these cells in normal condition produce antibacterial factors, but not proinflammatory factors, in response to various PAMPs to prevent bacterial invasion without excessive inflammatory responses.