Trajectory of neutrophilic responses in a mouse model of pollutant-aggravated allergic asthma

Experimental studies suggest that neutrophils could contribute to allergic asthma pathogenesis, that is mainly driven by type 2 immunity. Inhalation of diesel exhaust particles (DEP) is implicated in both exacerbation and development of asthma. Since exposure to DEP is associated with a neutrophilic component, we aimed to investigate how exposure to the combination of allergens and DEP modulates neutrophilic responses. Human bronchial epithelial cells (HBEC) were exposed to house dust mite (HDM), DEP or HDM + DEP in vitro to determine the expression of neutrophil-recruiting chemokines. Female (C57BL/6 J) mice were intranasally instilled with saline, DEP, HDM or combined HDM + DEP for 3 weeks (subacute) or 6 weeks (chronic). The neutrophilic responses were determined in lung tissue and bronchoalveolar lavage fluid (BALF). Simultaneous exposure to HDM + DEP resulted in increased CXCL1 and CXCL8 mRNA expression by HBEC in vitro. In mice, subacute exposure to HDM + DEP induced a strong mixed eosinophilic/neutrophilic inflammation in BALF and lung and was associated with higher expression of neutrophil-attracting chemokines and NET formation compared to the sole exposures. After chronic HDM + DEP exposure, a similar neutrophilic response was observed, however the NET formation was less pronounced. Interestingly, the increase of BALF eosinophils was also significantly attenuated after chronic HDM + DEP exposure compared to the subacute exposure. Subacute and chronic HDM + DEP exposure induced goblet cell hyperplasia and airway hyperresponsiveness. Our data suggest a role for neutrophils and NETs in pollutant-aggravated eosinophilic allergic asthma. Moreover, subacute exposure to HDM + DEP induces a mixed eosinophilic/neutrophilic response whereas upon chronic HDM + DEP exposure there is a shift in inflammatory response with a more prominent neutrophilic component.