How does HS-activated PMN enhance TLR4 upregulation of TLR2 in AMφ? We found that reactive oxygen species (ROS) derived from PMN NAD(P)H oxidase play an important role in amplifying the TLR2 upregulation (116) . Studies have also shown that lack of endogenous NAD(P)H oxidase in the AMφ caused a decrease in TLR2 expression in response to LPS stimulation; however, the decrease was restored when the AMφ was coincubated with PMN isolated from wild-type mice subjected to HS. These results indicate that although the endogenous NAD(P)H oxidase in AMφ is also involved in the signaling, the exogenous oxidants from PMN NAD(P)H oxidase are essential for inducing amplified TLR2 expression in AMφ in response to LPS.