Air pollutants elicit type 3 immune response in asthma through Ly6C⁺ monocyte-derived macrophages

Diesel exhaust particles (DEP) have been implicated in reducing lung function and exacerbating asthma. However, precise mechanisms remain unclear. This study aimed to investigate the impact of DEP exposure on airway innate immune cells, focusing on macrophages/monocytes, and their role in asthma exacerbation. Using a murine asthma model, we sensitized and challenged 6-week-old BALB/c mice with ovalbumin (OVA). These mice underwent 10 repeated OVA inhalations over three weeks, with and without concurrent DEP inhalation. Airway hyperresponsiveness and airway inflammation were assessed. We characterized immune cell populations and their crosstalk, focusing on mouse Ly6C⁺ monocyte-derived macrophages (MoMs) and human CD14⁺CD16^- MoMs. DEP exposure worsened histological scores in the asthmatic lungs and altered the OVA-induced type 2 inflammation profile, leading to increased type 3 inflammation. In terms of lung macrophages, DEP inhalation resulted in the depletion of SiglecF⁺CD11c⁺CD11b^- resident alveolar macrophages and an increase in SiglecF^-CD11b⁺ MoMs, particularly Ly6C⁺ MoMs. DEP-exposed Ly6C⁺ MoMs isolated from the OVA model (DEP-Ly6C⁺ MoMs^OVA) heightened neutrophil chemotaxis and the expression of fibrosis-related genes. DEP-Ly6C⁺ MoMs^OVA also promoted the differentiation of innate lymphoid cells (ILCs) and T helper (Th) cells toward ILC3s and Th17 cells, respectively. Ex vivo DEP-Ly6C⁺ MoMs^OVA transfer also induced significant type 3 inflammation with SiglecF⁺ neutrophils. In vitro DEP-treated human CD14⁺CD16^- MoMs upregulated neutrophil chemotaxis-related genes and exacerbated non-type 2 inflammation. In conclusion, DEP exposure exacerbates asthma by inducing mixed type 2 and type 3 inflammation in the asthmatic airways through the modulation of Ly6C⁺ MoMs, which enhance type 3 immunity.