Rationale: Chronic exposure to ambient air-borne particulate matter of < 2.5 μm (PM₂.₅) increases cardiovascular risk. The mechanisms by which inhaled ambient particles are sensed and how these effects are systemically transduced remain elusive.
Objective: To investigate the molecular mechanisms by which PM₂.₅ mediates inflammatory responses in a mouse model of chronic exposure.
Methods and results: Here, we show that chronic exposure to ambient PM₂.₅ promotes Ly6C(high) inflammatory monocyte egress from bone-marrow and mediates their entry into tissue niches where they generate reactive oxygen species via NADPH oxidase. Toll-like receptor (TLR)4 and Nox2 (gp91(phox)) deficiency prevented monocyte NADPH oxidase activation in response to PM₂.₅ and was associated with restoration of systemic vascular dysfunction. TLR4 activation appeared to be a prerequisite for NAPDH oxidase activation as evidenced by reduced p47(phox) phosphorylation in TLR4 deficient animals. PM₂.₅ exposure markedly increased oxidized phospholipid derivatives of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphorylcholine (oxPAPC) in bronchioalveolar lavage fluid. Correspondingly, exposure of bone marrow-derived macrophages to oxPAPC but not PAPC recapitulated effects of chronic PM₂.₅ exposure, whereas TLR4 deficiency attenuated this response.
Conclusions: Taken together, our findings suggest that PM₂.₅ triggers an increase in oxidized phospholipids in lungs that then mediates a systemic cellular inflammatory response through TLR4/NADPH oxidase-dependent mechanisms.