Pro-inflammatory and oxidative responses to burn pit relevant desert particulate matter in macrophages: A role for TLR2 signaling

Abstract

Particulate matter (PM) exposure is linked to many respiratory diseases such as asthma, yet the underlying mechanisms remain unclear. A large number of warfighters deployed to Afghanistan and Iraqi have developed deployment related respiratory disease. This study investigates the effects of burn pit-relevant desert PM from Afghanistan (APM) and comparative desert PM from China Lake, California (CPM) on oxidative stress and pro-inflammatory responses in macrophages. Using mouse monocyte cell line and primary bone marrow-derived macrophages (BMDMs) with or without Toll-like Receptor 2 (TLR2) deficiency, we assessed nitric oxide (NO^.), hydrogen peroxide (H₂O₂), and cytokine production using biochemical assays, enzyme-linked immunosorbent assay (ELISA), pharmacological and genetic modulation, and bulk RNA sequencing. APM was more cytotoxic than CPM in monocytes. Both PMs increased H₂O₂ levels, with acellular conditions generating higher H₂O₂ levels, which was catalase-sensitive and attenuated by metal chelation. APM induced stronger NO^. and cytokine C-X-C motif chemokine ligand 1 (CXCL1) responses than CPM, with NO^. production attenuated by the nitric oxide synthase (NOS) inhibitor L-N^G-nitroarginine methyl ester (LNAME), which also prevented cytotoxicity. TLR2 activation via agonist Pam3CSK4 enhanced NO^. and CXCL1, while inhibition with antagonist C29 or TLR2 knockout partially suppressed APM-induced responses. Bulk RNA sequencing revealed that APM upregulated M1 pro-inflammatory polarization markers, many of which were significantly reduced in TLR2-deficient BMDMs. These findings demonstrate that APM exposure of monocytes/macrophages elicits stronger oxidative and pro-inflammatory responses than CPM, partially mediated through TLR2 signaling. This study provides mechanistic insight into deployment-related PM exposure and identifies potential therapeutic targets to mitigate inflammation in affected individuals.