Different Toll-like receptor agonists induce distinct macrophage responses

We previously reported that gram-negative bacterial lipopolysaccharide (LPS) activates cells via Toll-like receptor (TLR) 4, whereas the mycobacterial cell wall glycolipid lipoarabinomannan (LAM) activates cells via TLR2. We also identified a secreted TLR2 agonist activity in short-term culture filtrates of Mycobacterium tuberculosis bacilli, termed soluble tuberculosis factor (STF). Here we show that STF contains mannosylated phosphatidylinositol (PIM) and that purified PIM possesses TLR2 agonist activity. Stimulation of RAW 264.7 macrophages by LPS, LAM, STF, and PIM rapidly activated nuclear factor (NF)-kappaB, activator protein-1 (AP-1), and mitogen-activated protein (MAP) kinases. These TLR agonists induced similar levels of NF-kappaB and AP-1 DNA-binding activity, as well as trans-activation function. Unexpectedly, these TLR agonists induced tumor necrosis factor alpha secretion, whereas only LPS was capable of inducing interleukin-1beta and nitric oxide secretion. Thus, different TLR proteins are still capable of activating distinct cellular responses, in spite of their shared capacities to activate NF-kappaB, AP-1, and MAP kinases.