CRTH2 Mediates Pro-fibrotic Macrophage Differentiation and Promotes Lung Fibrosis

A dual role for CRTH2 in acute lung injury

Acute respiratory distress syndrome (ARDS) is a life-threatening clinical condition defined by rapid-onset respiratory failure following acute lung injury (ALI). The high mortality rate and rising incidence of ARDS due to COVID-19 make it an important research priority. Here we sought to investigate the role of chemoattractant receptor-homologous molecule expressed on Th2 cells (CRTH2) in ARDS. CRTH2 is a G protein-coupled receptor best studied in the context of type 2 immunity, but it also exerts effects on neutrophilic inflammation. To evaluate its role in mouse models of ARDS, we first examined its expression pattern on murine neutrophils. We found it is expressed on neutrophils, but only after extravasation into the lung. Next, we showed that CRTH2 expression on extravasated lung neutrophils promotes cell survival, as genetic deletion of CRTH2 and pharmacologic inhibition of CRTH2 using fevipiprant both led to increased apoptosis in vitro. We then evaluated the role of CRTH2 in vivo using a murine model of LPS-induced ALI. In line with the pro-inflammatory effects of CRTH2 in vitro, we observed improvement of lung injury in CRTH2-deficient mice in terms of vascular leak, weight loss and survival after LPS administration. However, neutrophilic inflammation was elevated, not suppressed in the CRTH2 KO. This finding indicated a second mechanism offsetting the pro-survival effect of CRTH2 on neutrophils. Bulk RNAseq of lung tissue indicated impairments in type 2 immune signaling in the CRTH2 KO, and qPCR and ELISA confirmed downregulation of IL-4, which is known to suppress neutrophilic inflammation. Thus, CRTH2 may play a dual role in ALI, directly promoting neutrophil cell survival, but indirectly suppressing neutrophil effector function via IL-4.

Osteopontin: an essential regulatory protein in idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a progressive, chronic lung disease characterized by abnormal proliferation and activation of fibroblasts, excessive accumulation of extracellular matrix (ECM), inflammatory damage, and disrupted alveolar structure. Despite its increasing morbidity and mortality rates, effective clinical treatments for IPF remain elusive. Osteopontin (OPN), a multifunctional ECM protein found in various tissues, has been implicated in numerous biological processes such as bone remodeling, innate immunity, acute and chronic inflammation, and cancer. Recent studies have highlighted the pivotal role of OPN in the pathogenesis of IPF. This review aims to delve into the involvement of OPN in the inflammatory response, ECM deposition, and epithelial-mesenchymal transition (EMT) during IPF, and intends to lay a solid theoretical groundwork for the development of therapeutic strategies for IPF.

Macrophage Implication in IPF: Updates on Immune, Epigenetic, and Metabolic Pathways

Idiopathic pulmonary fibrosis (IPF) is a lethal interstitial lung disease of unknown etiology with a poor prognosis. It is a chronic and progressive disease that has a distinct radiological and pathological pattern from common interstitial pneumonia. The use of immunosuppressive medication was shown to be completely ineffective in clinical trials, resulting in years of neglect of the immune component. However, recent developments in fundamental and translational science demonstrate that immune cells play a significant regulatory role in IPF, and macrophages appear to be among the most crucial. These highly plastic cells generate multiple growth factors and mediators that highly affect the initiation and progression of IPF. In this review, we will provide an update on the role of macrophages in IPF through a systemic discussion of various regulatory mechanisms involving immune receptors, cytokines, metabolism, and epigenetics.

Biology of lung macrophages in health and disease

Tissue-resident alveolar and interstitial macrophages and recruited macrophages are critical players in innate immunity and maintenance of lung homeostasis. Until recently, assessing the differential functional contributions of tissue-resident versus recruited macrophages has been challenging because they share overlapping cell surface markers, making it difficult to separate them using conventional methods. This review describes how scRNA-seq and spatial transcriptomics can separate these subpopulations and help unravel the complexity of macrophage biology in homeostasis and disease. First, we provide a guide to identifying and distinguishing lung macrophages from other mononuclear phagocytes in humans and mice. Second, we outline emerging concepts related to the development and function of the various lung macrophages in the alveolar, perivascular, and interstitial niches. Finally, we describe how different tissue states profoundly alter their functions, including acute and chronic lung disease, cancer, and aging.

Chitinase 3-like-1 contributes to the development of pulmonary vascular remodeling in pulmonary hypertension

Chitinase 3-like 1 (CHI3L1) is the prototypic chitinase-like protein mediating inflammation, cell proliferation, and tissue remodeling. Limited data suggests CHI3L1 is elevated in human pulmonary arterial hypertension (PAH) and is associated with disease severity. Despite its importance as a regulator of injury/repair responses, the relationship between CHI3L1 and pulmonary vascular remodeling is not well understood. We hypothesize that CHI3L1 and its signaling pathways contribute to the vascular remodeling responses that occur in pulmonary hypertension (PH). We examined the relationship of plasma CHI3L1 levels and severity of PH in patients with various forms of PH, including Group 1 PAH and Group 3 PH, and found that circulating levels of serum CHI3L1 were associated with worse hemodynamics and correlated directly with mean pulmonary artery pressure and pulmonary vascular resistance. We also used transgenic mice with constitutive knockout and inducible overexpression of CHI3L1 to examine its role in hypoxia-, monocrotaline-, and bleomycin-induced models of pulmonary vascular disease. In all 3 mouse models of pulmonary vascular disease, pulmonary hypertensive responses were mitigated in CHI3L1 null mice and accentuated in transgenic mice that overexpress CHI3L1. Finally, CHI3L1 alone was sufficient to induce pulmonary arterial smooth muscle cell proliferation, inhibit pulmonary vascular endothelial cell apoptosis, induce the loss of endothelial barrier function, and induce endothelial-to-mesenchymal transition. These findings demonstrate that CHI3L1 and its receptors play an integral role in pulmonary vascular disease pathobiology and may offer a novel target for the treatment PAH and PH associated with fibrotic lung disease.