An Essential Role for ras in LPS Mediated Amplification of TREM-1 Induced Neutrophil Activation.

The triggering receptor expressed on myeloid cells 1 (TREM-1) is constitutively expressed on polymorphonuclear neutrophils (PMN), monocytes and macrophage subsets and has been recently identified as an important player in the innate inflammatory response to microbial infections and sepsis. Expression of TREM-1 and activation of PMN and monocytes is regulated in concert with microbial products via the recognition of Toll-like receptor (TLR) ligands such as bacterial lipopolysaccaride (LPS). However, it is currently unclear how the amplification of the inflammatory responses by TREM-1 and TLR agonists is mediated on an intracellular level. To this end, we were interested to identify signalling events leading to the synergistic activation of PMN upon TREM-1 and TLR ligation. Using pharmacologic inhibitors and western blot analyses we demonstrate that PI3 kinase, PLC-g and p38 MAP kinase are essential for the initiation of the respirator burst as an important and immediate effector mechanism of human PMN. Furthermore, the phosphorylation of PKB/Akt and MAP kinase ERK show a biphasic pattern upon TREM-1/TLR4 coligation indicating distinct activation mechanisms for these receptors. Interestingly, the TLR4, but not TREM-1 mediated respiratory burst was dependent on the activation of ras leading downstream to an augmented calcium flow and subsequently synergistic burst activity. Taken together, we provide a new mechanism how TREM-1 and TLR interact in PMN mediating enhanced activation. These results shed a new light on our understanding how the innate inflammatory responses are regulated and might contribute to the development of future concept for the treatment of a dysregulated immune response in severe inflammatory conditions such as sepsis.