Differential effects of Gram‐positive versus Gram‐negative bacteria on NOSII and TNF α in macrophages: role of TLRs in synergy between the two

1 Gram‐negative and Gram‐positive bacteria are sensed by Toll‐like receptor (TLR)4 and TLR2, respectively. TLR4 recruits MyD88 and TRIF, whereas TLR2 recruits MyD88 without TRIF. NOSII and TNF α are central genes in innate immunity and are thought to be differentially regulated by the MyD88 versus TRIF signalling pathways. Here, we have used Gram‐positive Staphylococcus aureus , Gram‐negative Escherichia coli and highly selective TLR ligands to establish the precise relationship between TLR2, TLR1, TLR6 and TLR4 for NOSII versus TNF α induction. 2 In murine macrophages at 24 h, E. coli or LPS (TLR4) induced NO and TNF α release. In contrast, S. aureus (TLR2/TLR1/TLR6) or Pam 3 CSK4 (TLR2/TLR1), or FSL‐1 and LTA (TLR2/TLR6) induced TNF α without an effect on NO. 3 At later time points (48–72 h), S. aureus induced NO release. The ability of S. aureus , but not E. coli or LPS, to induce NO release was inhibited by anti‐TNF α ‐binding antibodies. 4 At 24 h, LPS synergised with TLR2 ligands to induce NO release and NOSII protein expression. LPS also induced the expression of TLR2 gene expression without affecting levels of TLR4. 5 Using cells from TLR2 −/− or TLR4 −/− mice, the ability of LPS to synergise with S. aureus or Pam 3 CSK4 was found to be dependent on both TLR2 and TLR4. 6 These observations are the first to clearly delineate the role of separately activating TLR2 and TLR4 in the induction of NOSII and TNF α genes compared with their coinduction when both receptor pathways are activated.British Journal of Pharmacology (2006) 148 , 1067–1075. doi: 10.1038/sj.bjp.0706815