CX3CR1-fractalkine axis drives kinetic changes of monocytes in fibrotic interstitial lung diseases

Circulating immune cell populations have been shown to contribute to interstitial lung disease (ILD). In this study, we analysed circulating and lung resident monocyte populations, and assessed their phenotype and recruitment from the blood to the lung in ILD. Flow cytometry analysis of blood samples for quantifying circulating monocytes was performed in 105 subjects: 83 with ILD (n=36, 28, and 19 for non-specific interstitial pneumonia, hypersensitivities pneumonitis, and connective-tissue disease-associated ILD, respectively), as well as 22 controls. Monocyte localisation and abundance were assessed by immunofluorescence and flow cytometry of lung tissue. Monocyte populations were cultured either alone or with endothelial cells to assess fractalkine-dependent transmigration pattern. We show that circulating classical monocytes (CM) were increased in ILD compared with controls, while non-classical monocytes (NCM) were decreased. CM abundance inversely correlated, while NCM abundance positively correlated, with lung function. Both CCL2 and CX3CL1 concentrations were increased in plasma and blood of ILD patients. Fractalkine co-localised with ciliated bronchial epithelial cells, thereby creating a chemoattractant gradient towards the lung. Fractalkine enhanced endothelial transmigration of NCM in ILD samples only. Immunofluorescence, as well as flow cytometry, showed an increased presence of NCM in fibrotic niches in ILD lungs. Moreover, NCM in the ILD lungs expressed increased CX3CR1, M2-like, and phagocytic markers. Taken together, our data support that in ILD, fractalkine drives the migration of CX3CR1+ NCM to the lungs, thereby perpetuating the local fibrotic process.