Myeloid- and Epithelial-derived Heparin-Binding Epidermal Growth Factor-like Growth Factor Promotes Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a poorly understood, progressive lethal lung disease with no known cure. In addition to alveolar epithelial cell (AEC) injury and excessive deposition of extracellular matrix proteins, chronic inflammation is a hallmark of IPF. Literature suggests that the persistent inflammation seen in IPF primarily consists of monocytes and macrophages. Recent work demonstrates that monocyte-derived alveolar macrophages (moAMs) drive lung fibrosis, but further characterization of critical moAM cell attributes is necessary. Heparin-binding epidermal growth factor-like growth factor (HB-EGF) is an important epidermal growth factor receptor ligand that has essential roles in angiogenesis, wound healing, keratinocyte migration, and epithelial-mesenchymal transition. Our past work has shown HB-EGF is a primary marker of profibrotic M2 macrophages, and this study seeks to characterize myeloid-derived HB-EGF and its primary mechanism of action in bleomycin-induced lung fibrosis using Hbegff/f;Lyz2Cre+ mice. Here, we show that patients with IPF and mice with pulmonary fibrosis have increased expression of HB-EGF and that lung macrophages and transitional AECs of mice with pulmonary fibrosis and humans all express HB-EGF. We also show that Hbegff/f;Lyz2Cre+ mice are protected from bleomycin-induced fibrosis and that this protection is likely multifactorial, caused by decreased CCL2-dependent monocyte migration, decreased fibroblast migration, and decreased contribution of HB-EGF from AEC sources when HB-EGF is removed under the Lyz2Cre promoter.

M2 macrophages have unique transcriptomes but conditioned media does not promote profibrotic responses in lung fibroblasts or alveolar epithelial cells in vitro

Macrophages are critical regulators of pulmonary fibrosis. Their plasticity, proximity, and ability to cross talk with structural cells of the lung make them a key cell type of interest in the regulation of lung fibrosis. Macrophages can express a variety of phenotypes, which have been historically represented through an "M1-like" to "M2-like" delineation. In this classification, M1-like macrophages are proinflammatory and have increased phagocytic capacity compared with alternatively activated M2-like macrophages that are profibrotic and are associated with wound healing. Extensive evidence in the field in both patients and animal models aligns pulmonary fibrosis with M2 macrophages. In this study, we performed RNA sequencing (RNAseq) to fully characterize M1- vs. M2-skewed bone marrow-derived macrophages (BMDMs) and investigated the profibrotic abilities of M2 BMDM conditioned media (CM) to promote fibroblast migration and proliferation, alveolar epithelial cell (AEC) apoptosis, and mRNA expression of key fibrotic genes in both fibroblasts and AECs. Although M2 CM-treated fibroblasts had increased migration and M2 CM-treated fibroblasts and AECs had increased expression of profibrotic proteins over M1 CM-treated cells, all differences can be attributed to M2 polarization reagents IL-4 and IL-13 also present in the CM. Collectively, these data suggest that the profibrotic effects associated with M2 macrophage CM in vitro are attributable to effects of polarization cytokines rather than additional factors secreted in response to those polarizing cytokines.

Alternate-day feeding leads to improved glucose regulation on fasting days without significant weight loss in genetically obese mice

Alternate-day fasting (ADF) is effective for weight loss and increases insulin sensitivity in diet-induced obese rodents. However, the efficacy of ADF in genetic models of obesity has not been comprehensively studied. Mice that are deficient in leptin ( ob/ob mice) are obese, diabetic, and prone to deep bouts of torpor when fasted. We tested the hypotheses that an ADF protocol in ob/ob mice would result in 1) induction of torpor on fasted days, 2) minimal body weight loss if the mice experienced torpor, and 3) no improvement in glucose control in the absence of weight loss. Female ob/ob mice and littermate controls were assigned to 1) an ad libitum regimen or 2) an ADF regimen, consisting of fasting every other day with ad libitum feeding between fasts. Over a 19-day period, littermate control mice on the ADF regimen consumed the same amount of food as littermate control mice on the ad libitum regimen, whereas the ADF ob/ob mice consumed 37% less food than ad libitum ob/ob mice. Fasting days, but not fed days, led to torpor in both genotypes. Fasting days, but not fed days, led to weight loss in both genotypes relative to ad libitum controls. Fasting days, but not fed days, produced enhanced insulin sensitivity in both genotypes and normalized circulating glucose in ob/ob mice. These data demonstrate improved glucose control on fasting days with the use of ADF in a genetic model of obesity in the face of minimal weight loss.

A pathologic two-way street: how innate immunity impacts lung fibrosis and fibrosis impacts lung immunity

Lung fibrosis is characterised by the accumulation of extracellular matrix within the lung and is secondary to both known and unknown aetiologies. This accumulation of scar tissue limits gas exchange causing respiratory insufficiency. The pathogenesis of lung fibrosis is poorly understood, but immunologic‐based treatments have been largely ineffective. Despite this, accumulating evidence suggests that innate immune cells and receptors play important modulatory roles in the initiation and propagation of the disease. Paradoxically, while innate immune signalling may be important for the pathogenesis of fibrosis, there is also evidence to suggest that innate immune function against pathogens may be impaired, leading to dysregulated and/or impaired host defence. This review summarises the evidence for this pathologic two‐way street, highlights new concepts of pathogenesis and recommends future directions for research emphasis.

Loss of myeloid-specific protein phosphatase 2A enhances lung injury and fibrosis and results in IL-10-dependent sensitization of epithelial cell apoptosis

Protein phosphatase 2A (PP2A), a ubiquitously expressed Ser/Thr phosphatase is an important regulator of cytokine signaling and cell function. We previously showed that myeloid-specific deletion of PP2A (LysM^crePP2A^-/-) increased mortality in a murine peritoneal sepsis model. In the current study, we assessed the role of myeloid PP2A in regulation of lung injury induced by lipopolysaccharide (LPS) or bleomycin delivered intratracheally. LysM^crePP2A^-/- mice experienced increased lung injury in response to both LPS and bleomycin. LysM^crePP2A^-/- mice developed more exuberant fibrosis in response to bleomycin, elevated cytokine responses, and chronic myeloid inflammation. Bone marrow-derived macrophages (BMDMs) from LysM^crePP2A^-/- mice showed exaggerated inflammatory cytokine release under conditions of both M1 and M2 activation. Notably, secretion of IL-10 was elevated under all stimulation conditions, including activation of BMDMs by multiple Toll-like receptor ligands. Supernatants collected from LPS-stimulated LysM^crePP2A^-/- BMDMs induced epithelial cell apoptosis in vitro but this effect was mitigated when IL-10 was also depleted from the BMDMs by crossing LysM^crePP2A^-/- mice with systemic IL-10^-/- mice (LysM^crePP2A^-/- × IL-10^-/-) or when IL-10 was neutralized. Despite these findings, IL-10 did not directly induce epithelial cell apoptosis but sensitized epithelial cells to other mediators from the BMDMs. Taken together our results demonstrate that myeloid PP2A regulates production of multiple cytokines but that its effect is most pronounced on IL-10 production. Furthermore, IL-10 sensitizes epithelial cells to apoptosis in response to myeloid-derived mediators, which likely contributes to the pathogenesis of lung injury and fibrosis in this model.