Exposure to air pollution fine particulate matter (PM2.5) is associated with increased incidence and mortality from cardiovascular diseases (CVDs), including stroke and ischaemic heart disease, and accelerates the development of cardiovascular risk factors such as obesity. Alveolar macrophage PM2.5 uptake orchestrates inflammatory responses in the lungs that impact secondary target organs such as adipose tissue, thereby promoting adiposity, impaired glucose homeostasis, and insulin resistance through mechanisms that remain incompletely understood. In this context, our research focuses on elucidating the crosstalk between macrophages and adipocytes driving cardiometabolic derangements following PM2.5 exposure. In a biologically relevant real-life animal model, C57BL/6 mice breathing polluted urban air (27±8 µg/m3 PM2.5) for 16 weeks exhibited increased weight gain, impaired glucose homeostasis, and visceral white adipose tissue (WAT) inflammation, together with altered metabolic and thermogenic gene expression (i.e., Adrb3, Pparg, Elovl3, Ucp1) in brown adipose tissue (BAT); control mice were handled in parallel and breathed filtered air (2±1 µg/m3 PM2.5). In an acute animal model, mice receiving a PM2.5 surrogate (Residual Oil Fly Ash, ROFA) via intranasal instillation (1 mg/kg body weight) displayed a biphasic inflammatory response in the lungs, with neutrophils peaking at 6 hours and macrophages at 72 hours post-exposure, accompanied by increased pro-inflammatory gene expression and cytokine production (i.e., IL-1β, TNF-α, IL-6, CCL2) in bronchoalveolar lavage (BAL) fluid and plasma; control mice were handled in parallel and received the same volume of sterile saline solution. Furthermore, bulk mRNA sequencing of sorted alveolar macrophages revealed a pro-inflammatory transcriptional signature in those from PM2.5-exposed mice. Metabolic cage analyses using the CLAMS system demonstrated significantly reduced heat production in PM2.5-exposed mice, indicative of impaired thermogenesis. To specifically investigate macrophage-adipocyte interactions, we designed an in vitro Transwell co-culture system in which primary brown adipocytes were cultured in the lower compartment and bone marrow-derived macrophages (BMDMs) in the upper inserts, separated by a 0.4 µm pore membrane. Co-culture with PM2.5-exposed BMDMs (100 µg/mL for 24 h) resulted in significantly reduced metabolic and thermogenic gene expression in primary brown adipocytes (i.e., Adrb3, Pparg, Elovl3, Ucp1), along with decreased glucose uptake. Seahorse analysis further revealed reduced mitochondrial and uncoupling protein 1 (UCP1)-dependent respiration in primary brown adipocytes, and mitochondrial ultrastructural alterations were confirmed by transmission electron microscopy (TEM). Together, these findings provide novel insight into how innate immune responses triggered by PM2.5 exposure contribute to metabolic dysfunction and the development of cardiovascular risk factors.
