Air particulate matter SRM 1648a primes macrophages to hyperinflammatory response after LPS stimulation

Exposure to air particulate matter (PM) is associated with chronic inflammatory and autoimmune diseases. Macrophages are responsible for the regulation of chronic inflammation. However, whether PM affects macrophage polarization remains unclear. The aim of this study was to evaluate whether nontoxic concentrations of urban PM are able to prime macrophages to altered inflammatory response upon LPS challenge. We used two forms of the urban particulate matter SRM 1648a, intact PM and PM deprived of organic compounds (PMa dagger C). Peritoneal murine macrophages were exposed to different concentrations of PM for 24 h and then challenged with LPS. Production of inflammatory mediators by macrophages was measured to test immunostimulatory/priming capacity of PM. Particulate matter used at non-cytotoxic concentrations induced a dose-dependent production of proinflammatory cytokines (TNF-alpha, IL-6, IL-12p40). By contrast, PMa dagger C were not able to stimulate macrophages. However, macrophages primed with both forms of PM show proinflammatory response upon LPS challenge. Our data indicate that exposure of macrophages to low concentrations of PM may prime the cells to hyperinflammatory response upon contact with LPS. Further studies are necessary to explain whether the exposure of patients suffering from chronic inflammatory diseases to particulate matter is responsible for the exacerbation of clinical symptoms during bacterial infections.