A Time- and Compartment-Specific Activation of Lung Macrophages in Hypoxic Pulmonary Hypertension

Abstract

Studies in various animal models suggest an important role for pulmonary macrophages in the pathogenesis of pulmonary hypertension (PH). Yet, the molecular mechanisms characterizing the functional macrophage phenotype relative to time and pulmonary localization/compartmentalization remain largely unknown. Here, we utilized a hypoxic murine model of PH in combination with flow cytometry assisted cell sorting (FACS) to quantify and isolate lung macrophages from two compartments over time and characterized their programing via RNA sequencing (RNAseq) approaches. In response to hypoxia, we found an early increase in macrophage number that was restricted to the interstitial/perivascular compartment, without recruitment of macrophages to the alveolar compartment or changes in the number of resident alveolar macrophages. Principle component analysis demonstrated significant differences in overall gene expression between alveolar (AMs) and interstitial macrophages (IMs) at baseline and after 4 and 14 days hypoxic exposure. AM’s at both day 4 and 14 and IM’s at day 4 shared a conserved “hypoxia program” characterized by mitochondrial dysfunction, pro-inflammatory gene activation and mTORC1 signaling, while IM’s at day 14 demonstrated a unique anti-inflammatory/ pro-reparative programming state. We conclude that the pathogenesis of vascular remodeling in hypoxic PH involves an early compartment independent activation of lung macrophages towards a conserved hypoxia program, with the development of compartment specific programs later on in the course of disease. Thus, harnessing time and compartment specific differences in lung macrophage polarization needs to be considered in the therapeutic targeting of macrophages in hypoxic PH and potentially other inflammatory lung diseases. (word count 243)

Extracellular Superoxide Dismutase Regulates Early Vascular Hyaluronan Remodeling in Hypoxic Pulmonary Hypertension

“…It is also possible that this peripheral HA breakdown could be driven by other adventitial cell types. For instance, lung interstitial macrophages are recruited to the exterior vessel wall after just 4 days of hypoxia 60 and express high levels of Hyal2 in a model of LPS-induced lung injury 37 . HYAL2 and HA fragmentation have been implicated in matrix destruction, lung inflammation, and loss of lung function associated with COPD 61 .…”

Pulmonary Arterial Hypertension and Chronic Thromboembolic Pulmonary Hypertension: An Immunological Perspective

“…Pulmonary macrophages can classically be divided into interstitial and alveolar macrophages, although recent single-cell RNA sequencing and lineage tracing studies have defined multiple pulmonary macrophage subtypes in mice [24][25][26]. In a hypoxia-driven PH mouse model, specifically interstitial macrophages-but not alveolar macrophages-were increased [27]. They might be involved in PH development through their production of cytokines [28][29][30].…”

Macrophage Immunomodulation: The Gatekeeper for Mesenchymal Stem Cell Derived-Exosomes in Pulmonary Arterial Hypertension?

“…The “reprogramming” of monocytes over the course of injury (initiation/progression) results in a cell with a different epigenomic and transcriptomic landscape that can differentially localize to specific areas of the lung. Recent, elegant studies in preclinical models stress the effects of the microenvironment on monocytic differentiation and address the diversity of temporal and compartmental macrophage phenotypes in relation to lung homeostasis and disease [ 26 ]. Arguably, the classic ‘M1/M2’ binary approach is an over simplification that is perhaps considered obsolete [ 27 ].…”