Our results revealed the impaired ability to develop endotoxin tolerance resulted from aging, which might have an influence on the development of periodontitis in old individuals. In addition, this impaired ability might be related to the agedassociated changes in TLR2, 4. The primary etiologic factor of periodontitis is bacterial biofilm. Accumulating evidence indicates that specific microorganisms in subgingival plaque, including P. gingivalis, P. intermedia,F. nucleatum and A. actinomycetemcomitans, initiate the disease. LPS is one of the most important virulence factors of gram-negative bacteria, and plays an essential role in triggering periodontal inflammation. Endotoxin tolerance induced by repeated LPS stimulations could lead to the reprogramming of the immune system, such as the downregulation of TNF-a and IL-1b, and the preservation of IL-10. It could play a protective role against inflammatory tissue destruction and might have an effect on the development of periodontitis. Aging, which is characterized by the gradual decline in immune function, might also be associated with the prevalence and severity of periodontitis, at least in part. However, the influences of aging on endotoxin tolerance induced by periodontal pathogens and their underlying mechanisms still remain poorly characterized. This is the first report on the effects of aging on endotoxin tolerance induced by LPS derived from periodontal bacteria. Our results provided evidence that the ability to develop tolerance in response to the repeated stimulation with LPS from both periodontal bacteria and non-periodontal bacteria was impaired in peritoneal macrophages from middle-aged mice. In addition, the different sensitivity to repeated LPS exposure in the cells from young and middle-aged mice might be partly associated with the different expressions levels of TLR2, 4. In this present study, endotoxin tolerance was induced by P. gingivalis LPS and E.coli LPS. Although there are many common grounds on the biochemical and immunobiological properties of LPS from different gram-negative bacteria, differences in biological potency and pathogenicities still exist. It is not surprising that there are quantitative and/or qualitative differences in triggering TLRs and developing endotoxin tolerance between E.coli LPS and P. gingivalis LPS. E.coli LPS represents the BAY 43-9006 284461-73-0 classic LPS derived from gram-negative bacteria and is the optimal TLR4 agonist. LPS from many periodontopathic bacteria, such as F. nucleatum and A. actinomycetemcomitans, can also activate TLR4, and there are some similarities in triggering inflammation between E. coli LPS and these periodontopathic bacteria LPS. P. gingivalis LPS is an unusual pattern recognition receptor ligand for the innate defense system and expresses a low level of endotoxic activity relative to E. coli LPS. The protein structure of P. gingivalis LPS lacks heptose and 2-keto-3-deoxyoctonate, which are unique to enterobacterial LPS. Moreover, its lipid A exhibits a phosphorylation and acylation pattern, and contains branched and relatively longer fatty acids.