Abstract
BACKGROUND: Air pollution particles exacerbate allergic asthma and can enhance inflammatory responses to allergen exposure, but the cellular mechanisms involved remain incompletely defined. We examined how diesel exhaust particles (DEP) enhance house-dust-mite (HDM) inflammatory responses within the lung and characterised potential mechanisms that may contribute to enhanced type 2 (T2) inflammatory responses. RESULTS: In mice subjected to repeated intranasal exposures, DEP alone had modest effects, whereas DEP + HDM markedly increased type-2 inflammatory indicators (Serum IgE; Airway Il13, Il4 & Tslp) and eosinophilia alongside expansion of Th2 cells. Bulk transcriptomics showed far stronger differential expression in luminal airway cells than tissue, with a DEP + HDM-specific signature enriched for mast cells, alternatively activated macrophages (AAM), and B-cells in the lumen. Combined single-cell proteomic and transcriptomic profiling identified an expanded Cd11c⁺, SiglecF⁻, Apoe⁺, Gpnmb⁺ monocyte-derived macrophage subset (RM.Gp2), which showed increased type 2 chemokines Ccl8 and Ccl24 with DEP + HDM compared to HDM alone. Trajectory analysis placed RM.Gp2 downstream of Ccr2⁺ monocyte derived population, and protein/mRNA data supported a Ccl2-Ccr2-dependent influx that enlarges the RM.Gp2 pool. High-content imaging confirmed increased RM.Mo and RM.Gp2 numbers and higher total luminal Ccl8/Ccl24. F4/80⁺ luminal airway macrophages isolated from DEP pre-treated mice, demonstrated enhanced upregulation of Ccl8 and Ccl24 mRNA in response to ex vivo Il-4/Il-13 treatment, compared to macrophages isolated from control mice. Examination of an additional particle type (CeO(2) Nanoparticles) in the same exposure model, revealed a shared luminal transcriptomic response and AAM/chemokine programme as with DEP. CONCLUSIONS: Our data suggests that pollutant particles such as DEP may contribute to enhanced HDM induced type 2 inflammation by expanding Ccr2-dependent monocyte-derived macrophages into the airway lumen and licensing a Th2-cytokine-responsive chemokine programme (Ccl8- Ccr8 to recruit Th2 cells; Ccl24-Ccr3 to recruit eosinophils). These findings identify luminal recruited macrophages as important targets in allergic inflammation within the lung, providing insight into potential mechanisms from which exposure and disease mitigation strategies may be developed.