Differential roles of Toll-like receptors in the elicitation of proinflammatory responses by macrophages

Background-Mammalian Toll-like receptor (TLR) proteins are pattern recognit ion receptors for a diverse array of bacterial and viral products. Gram neg ative bacterial lipopolysaccharide (LPS) activates cells through TLR4, wher eas the mycobacterial cell wall glycolipids, lipoarabinomannan (LAM) and ma nnosylated phosphatidylinositol (PIM), activate cells through TLR2. Further more, short term culture filtrates of M tuberculosis bacilli contain a TLR2 agonist activity, termed soluble tuberculosis factor (STF), that appears t o be PIM. It was recently shown that stimulation of RAW264.7 murine macroph ages by LPS, LAM, STF, and PIM rapidly activated NF-kappaB, AP1, and MAP ki nases. Results-This study shows that signalling by TLR2 and TLR4 also activates th e protein kinase Akt, a downstream target of phosphatidylinositol-3'-kinase (PI-3-K). This finding suggests that activation of PI-3-K represents an ad ditional signalling pathway induced by engagement of TLR2 and TLR4. Subsequ ently, the functional responses induced by the different TLR agonists were compared. LPS, the mycobacterial glycolipids, and the OspC lipoprotein (a T LR2 agonist) all induced macrophages to secrete tumour necrosis factor alph a (TNF alpha), whereas only LPS could induce nitric oxide (NO) secretion. H uman alveolar macrophages also exhibited a distinct pattern of cellular res ponse after stimulation with TLR2 and TLR4 agonists. Specifically, LPS indu ced TNF alpha, MIP-1 beta, and RANTES production in these cells, whereas th e TLR2 agonists induced only MIP-1 beta production. Conclusion-Together, these data show that different TLR proteins mediate th e activation of distinct cellular responses, despite their shared ability t o activate NF-kappaB, AP1, MAP kinases, and PI-3-K.