We investigated counteracting interactions between the lipopolysaccharides (LPS) from Escherichia coli (Ec-LPS) and Porphyromonas gingivalis (Pg-LPS), which induce cellular activation through Toll-like receptor 4 (TLR4) and TLR2, respectively. We found that Ec-LPS induced tolerance in THP-1 cells to subsequent tumor necrosis factor alpha (TNF-alpha) and interleukin 1 beta (IL-1beta) induction by Pg-LPS, though the reverse was not true, and looked for explanatory differential effects on the signal transduction pathway. Cells exposed to Pg-LPS, but not to Ec-LPS, displayed persisting expression of IL-1 receptor-associated kinase without apparent degradation, presumably allowing prolonged relay of downstream signals. Accordingly, cells pretreated with Pg-LPS, but not with Ec-LPS, were effectively activated in response to subsequent exposure to either LPS molecule, as evidenced by assessing nuclear factor (NF)-kappaB activity. In fact, Pg-LPS primed THP-1 cells for enhanced NF-kappaB activation and TNF-alpha release upon restimulation with the same LPS. This was a dose-dependent effect and correlated with upregulation of surface TLR2 expression. Furthermore, we observed inhibition of NF-kappaB-dependent transcription in a reporter cell line pretreated with Ec-LPS and restimulated with Pg-LPS (compared to cells pretreated with medium only and restimulated with Pg-LPS), but not when the reverse treatment was made. Although Pg-LPS could not make cells tolerant to subsequent activation by Ec-LPS, Pg-LPS inhibited Ec-LPS-induced TNF-alpha and IL-6 release when the two molecules were added simultaneously into THP-1 cell cultures. Pg-LPS also suppressed P. gingivalis FimA protein-induced NF-kappaB-dependent transcription in the 3E10/huTLR4 reporter cell line, which does not express TLR2. This rules out competition for common signaling intermediates, suggesting that Pg-LPS may block component(s) of the TLR4 receptor complex. Interactions between TLR2 and TLR4 agonists may be important in the regulation of inflammatory reactions.