Role for IRAK-4 and p38 in Neutrophil Signaling in Response to Bacterial Lipoproteins from Staphylococcus aureus

Neutrophils, polymorphonuclear leukocytes (PMN), express numerous pattern recognition receptors, including TLRs, capable of recognizing a wide variety of pathogens. Receptor engagement initiates a cascade of PMN responses with some occurring in seconds, and some requiring de novo protein synthesis over the course of many hours. Although numerous species of bacteria and bacterial products have been shown to activate PMN via TLRs, the signaling intermediates required for distinct PMN responses have not been well-defined in human PMN. Given the potential for host tissue damage by overexuberant PMN activity, a better understanding of neutrophil signaling is needed to generate effective therapies. We hypothesized that PMN responses to a lipoprotein-containing cell membrane preparation from methicillin-resistant S. aureus (MRSA-CMP) would activate signaling via IRAK4 and p38, with potentially distinct pathways for early vs. late responses. Using human PMN we investigated MRSA-CMP-elicited reactive oxygen species (ROS) production, elastase activity, NET formation, IL-8 production, and the role of IRAK4 and p38 activation. MRSA-CMP elicited ROS in a concentration and lipoprotein-dependent manner. MRSA-CMP elicited phosphorylation of p38 MAPK, and MRSA-CMP-elicited ROS production was partially dependent on p38 MAPK and IRAK4 activation. Inhibition of IRAK4 resulted in a reduction of p38 phosphorylation. MRSA-CMP-elicited elastase activity and NET formation was partially dependent on p38 MAPK activation, but independent of IRAK4 activation. MRSA-CMP-elicited IL-8 production required both p38 and IRAK4 activation. In conclusion, MRSA-CMP elicits PMN responses via distinct signaling pathways. There is potential to target components of the neutrophil inflammatory response without compromising critical pathogen-specific immune functions.