Inhibitory effect of particulate matter on toll-like receptor 9 stimulated dendritic cells by downregulating mitogen-activated protein kinase and NF-kappaB pathway.

Ambient particulate matter (PM) is associated with adverse health consequences. However, the influence of PM on the innate immune system is poorly understood. The aim of the present study was to examine the effect of diesel particulate matter 2.5 mum (PM2.5, SRM1650b) on dendritic cells. PM2.5 significantly reduced cytokine levels of interleukin (IL)-12 p40, IL-6 and TNF-alpha levels in CpG-DNA (TLR9 ligand)-stimulated dendritic cells. To determine the mechanisms underlying this observed inhibition induced by PM2.5, western blot analysis was conducted. PM2.5 was found to downregulate ERK1/2, JNK1/2, p38 MAPKs, and NF-kappaB pathways. PM2.5 exposure decreased TLR9-dependent NF-kappaB and activator protein (AP-1) reporter luciferase activities. Our findings demonstrate that PM2.5 reduced the production of cytokines which may be associated with inhibition of MAPK and NF-kappaB signaling pathway. Further, data suggest the immunosuppressive effect of PM2.5 on the innate immune cells may lead to serious damage to the host immune system.