The inflammatory effects of a repeated exposure of human macrophages to PM2.5 from house dust-SRM 2585 are partially reversible and disrupt the LPS/CD14 signaling pathways

Abstract

Objectives

To evaluate whether repeated exposures to house dust PM_2.5, used at nontoxic concentrations, activate human macrophages after one- and seven-days post-treatment, and whether such PM_2.5 exposure may alter their response to LPS.

Methods

Human monocyte-derived macrophages (MDMs) were repeatedly (4 consecutive days) exposed to 1 and 10 μg/cm² house dust PM_2.5 fraction of SRM 2585®. MDM phenotype and functions were evaluated by flow cytometry and ELISA one- and seven-days after the last PM_2.5 exposure with or without LPS priming during the last 24 h of exposure.

Results

PM_2.5 used at non cytotoxic concentrations induced a pro-inflammatory phenotype characterized by an increased expression of M1 macrophagic markers (CD40, CD80, CD86) and pro-inflammatory cytokines (IL-6, IL-8, TNFα) and a decreased expression of M2 macrophagic markers (CD163, CD204) and of the anti-inflammatory cytokine IL-10, however depending of post-exposure time and the concentration. PM_2.5-induced polarization profile of MDM was reversible when they were exposed to 1 μg/cm² but not to 10 μg/cm². PM_2.5-primed MDMs and then exposed to LPS showed a pro-inflammatory phenotype characterized by altered secretion of IL-8 and IL-10, a decreased expression of CD14, scavenger receptors (SR)-A1 and interferon responsive genes (CXCL9/10, ISG15) and a decrease of endocytosis and phagocytosis of bacteria.

Conclusions

Repeated exposures, even to low concentration of house dust PM_2.5, primed MDMs to a pro-inflammatory phenotype that is only partly reversible seven days post-exposure. The altered response of PM_2.5-pre-exposed MDMs to LPS might be due to, at least in part, impaired endocytosis crucial for the TLR4/CD14 pathway.