Neutrophils are actively involved in the regulation of inflammation and the maintenance of surfactant homeostasis during cigarette smoke exposure

Abstract Rationale Exposure to cigarette smoke induces inflammation marked by rapid and sustained neutrophil infiltration as well as pulmonary surfactant damage. However, the extent to which neutrophils contribute to the clearance of surfactant and the inflammatory response to cigarette smoke exposure is not fully understood. Objective We aim 1) to assess the capacity of neutrophils to internalise surfactant and 2) to determine whether neutrophil depletion has an effect on surfactant homeostasis and the inflammatory response induced by cigarette smoke exposure. Methods/Results 1) By incubating cells isolated from the bronchoalveolar lavage of cigarette smoke exposed mice with fluorescently labelled surfactant (NBD-phosphatidylcholine), we observed via microscopy significant in vitro surfactant internalisation by neutrophils. Similarly, neutrophils absorbed labelled surfactant delivered intranasally in vivo as measured by flow cytometry. 2) Neutrophil depletion during cigarette smoke exposure via injection of depleting antibodies (anti-Ly6G or anti-Gr-1) led to exaggerated pro-inflammatory cytokine release (IL-1α, CCL2, GM-CSF and G-CSF) and increased macrophage protease expression (Mmp12). Depletion of neutrophils also led to increased surfactant protein levels (SP-A and SP-D). Conclusions Our results show that neutrophils recruited to the lungs during cigarette smoke exposure internalise pulmonary surfactant and that their absence leads to exacerbation of the pro-inflammatory response and alterations to surfactant composition. Therefore, neutrophils seem to play a crucial role in the maintenance of pulmonary homeostasis following cigarette smoke exposure.