Myeloid cell-specific deletion of inducible nitric oxide synthase protects against smoke-induced pulmonary hypertension in mice

Pulmonary hypertension (PH) is a common complication of chronic obstructive pulmonary disease (COPD), associated with increased mortality and morbidity. Intriguingly, pulmonary vascular alterations have been suggested to drive emphysema development. We previously identified inducible nitric oxide synthase (iNOS) as an essential enzyme for development and reversal of smoke-induced PH and emphysema, and showed that iNOS expression in bone-marrow-derived cells drives pulmonary vascular remodelling, but not parenchymal destruction. In this study, we aimed to identify the iNOS-expressing cell type driving smoke-induced PH and to decipher pro-proliferative pathways involved.To address this question we used 1) myeloid cell-specific iNOS knockout mice in chronic smoke exposure, 2) co-cultures of macrophages and pulmonary artery smooth muscle cells (PASMC) to decipher underlying signalling pathways.Myeloid cell-specific iNOS knockout prevented smoke-induced PH but not emphysema in mice. Moreover, iNOS deletion in myeloid cells ameliorated the increase in expression of CD206, a marker of M2 polarisation, on interstitial macrophages. Importantly, the observed effects on lung macrophages were hypoxia-independent, as these mice developed hypoxia-induced PH. In vitro, smoke-induced PASMC proliferation in co-cultures with M2-polarised macrophages could be abolished by iNOS deletion in phagocytic cells, as well as by ERK inhibition in PASMC. Crucially, CD206-positive and iNOS-positive macrophages accumulated in proximity of remodelled vessels in the lungs of COPD patients, as shown by immunohistochemistry.In summary, our results demonstrate that iNOS deletion in myeloid cells confers protection against PH in smoke-exposed mice and provide evidence for an iNOS-dependent communication between M2-like macrophages and PASMC in underlying pulmonary vascular remodelling.