Mechanisms of Synergy Between Toll-Like Receptor 4 and Triggering Receptor Expressed on Myeloid Cells-1 in Human Neutrophils

The triggering receptor expressed on myeloid cells 1 (TREM-1) is an important player in the innate inflammatory response to microbial infections. Activation and expression of TREM-1 by polymorphonuclear neutrophils (PMN) occurs in concert with Toll-like receptors (TLR) such as TLR4 for bacterial lipopolysaccharide. However, it is currently unclear how this is mediated on a molecular level. Using pharmacologic inhibitors and western blot analysis we demonstrate that phosphatidyl inositide 3-kinase, phospholipase C and the mitogen activated kinase p38 are essential for the TREM-1 and TLR4 mediated respiratory burst of human PMN. The down stream phosphorylation of protein kinase B and extracellular signal related kinase show characteristic phosphorylation patterns upon single or co-ligation indicating individual activation pathways of both receptors. TREM-1, but not TLR4 mediated respiratory burst depended on calcium flow via store operated calcium entry channels, while transient receptor potential channels were important for TLR and TREM-1. Taken together, we provide new insights on the mechanisms how TREM-1 and TLR interact creating synergistic activation in PMN. These results shed a new light on our understanding how the innate inflammatory responses are regulated and might contribute to the development of future concepts for the treatment of severe inflammatory conditions such as sepsis.