CCR2+ monocytic myeloid-derived suppressor cells (M-MDSCs) inhibit collagen degradation and promote lung fibrosis by producing transforming growth factor-ß1.

Monocytes infiltrating scar tissue are predominantly viewed as progenitor cells. Here, we show that tissue CCR2+ monocytes have specific immunosuppressive and profibrotic functions. CCR2+ monocytic cells are acutely recruited to the lung before the onset of silica-induced fibrosis in mice. These tissue monocytes are defined as monocytic myeloid-derived suppressor cells (M-MDSCs) because they significantly suppress T-lymphocyte proliferation in vitro. M-MDSCs collected from silica-treated mice also express transforming growth factor (TGF)-ß1, which stimulates lung fibroblasts to release tissue inhibitor of metalloproteinase (TIMP)-1, an inhibitor of metalloproteinase collagenolytic activity. By using LysMCreCCR2loxP/loxP mice, we show that limiting CCR2+ M-MDSC accumulation reduces the pulmonary contents of TGF-ß1, TIMP-1 and collagen after silica treatment. M-MDSCs do not differentiate into lung macrophages, granulocytes or fibrocytes during pulmonary fibrogenesis. Collectively, our data indicate that M-MDSCs contribute to lung fibrosis by specifically promoting a non-degrading collagen microenvironment. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

IL-1α induces CD11b(low) alveolar macrophage proliferation and maturation during granuloma formation.

Macrophages play a central role in immune and tissue responses of granulomatous lung diseases induced by pathogens and foreign bodies. Circulating monocytes are generally viewed as central precursors of these tissue effector macrophages. Here, we provide evidence that granulomas derive from alveolar macrophages serving as a local reservoir for the expansion of activated phagocytic macrophages. By exploring lung granulomatous responses to silica particles in IL-1-deficient mice, we found that the absence of IL-1α, but not IL-1ß, was associated with reduced CD11b(high) phagocytic macrophage accumulation and fewer granulomas. This defect was associated with impaired alveolar clearance and resulted in the development of pulmonary alveolar proteinosis (PAP). Reconstitution of IL-1α(-/-) mice with recombinant IL-1α restored lung clearance functions and the pulmonary accumulation of CD11b(high) phagocytic macrophages. Mechanistically, IL-1α induced the proliferation of CD11b(low) alveolar macrophages and differentiated these cells into CD11b(high) macrophages which perform critical phagocytic functions and organize granuloma. We newly discovered here that IL-1α triggers lung responses requiring macrophage proliferation and maturation from tissue-resident macrophages.

PDGF-D expression is down-regulated by TGFß in fibroblasts.

Transforming growth factor-ß (TGFß) is a key mediator of fibrogenesis. TGFß is overexpressed and activated in fibrotic diseases, regulates fibroblast differentiation into myofibroblasts and induces extracellular matrix deposition. Platelet-derived growth factor (PDGF) is also a regulator of fibrogenesis. Some studies showed a link between TGFß and PDGF in certain fibrotic diseases. TGFß induces PDGF receptor alpha expression in scleroderma fibroblasts. PDGF-C and -D are the most recently discovered ligands and also play a role in fibrosis. In this study, we report the first link between TGFß and PDGF-D and -C ligands. In normal fibroblasts, TGFß down-regulated PDGF-D expression and up-regulated PDGF-C expression at the mRNA and protein levels. This phenomenon is not limited to TGFß since other growth factors implicated in fibrosis, such as FGF, EGF and PDGF-B, also regulated PDGF-D and PDGF-C expression. Among different kinase inhibitors, only TGFß receptor inhibitors and the IκB kinase (IKK) inhibitor BMS-345541 blocked the effect of TGFß. However, activation of the classical NF-κB pathway was not involved. Interestingly, in a model of lung fibrosis induced by either bleomycin or silica, PDGF-D was down-regulated, which correlates with the production of TGFß and other fibrotic growth factors. In conclusion, the down-regulation of PDGF-D by TGFß and other growth factors may serve as a negative feedback in the network of cytokines that control fibrosis.

Critical role of aquaporins in interleukin 1ß (IL-1ß)-induced inflammation.

Rapid changes in cell volume characterize macrophage activation, but the role of water channels in inflammation remains unclear. We show here that, in vitro, aquaporin (AQP) blockade or deficiency results in reduced IL-1ß release by macrophages activated with a variety of NLRP3 activators. Inhibition of AQP specifically during the regulatory volume decrease process is sufficient to limit IL-1ß release by macrophages through the NLRP3 inflammasome axis. The immune-related activity of AQP was confirmed in vivo in a model of acute lung inflammation induced by crystals. AQP1 deficiency is associated with a marked reduction of both lung IL-1ß release and neutrophilic inflammation. We conclude that AQP-mediated water transport in macrophages constitutes a general danger signal required for NLRP3-related inflammation. Our findings reveal a new function of AQP in the inflammatory process and suggest a novel therapeutic target for anti-inflammatory therapy.

Dysregulated proinflammatory and fibrogenic phenotype of fibroblasts in cystic fibrosis.

Morbi-mortality in cystic fibrosis (CF) is mainly related to chronic lung infection and inflammation, uncontrolled tissue rearrangements and fibrosis, and yet the underlying mechanisms remain largely unknown. We evaluated inflammatory and fibrosis responses to bleomycin in F508del homozygous and wild-type mice, and phenotype of fibroblasts explanted from mouse lungs and skin. The effect of vardenafil, a cGMP-specific phosphodiesterase type 5 inhibitor, was tested in vivo and in culture. Responses of proinflammatory and fibrotic markers to bleomycin were enhanced in lungs and skin of CF mice and were prevented by treatment with vardenafil. Purified lung and skin fibroblasts from CF mice proliferated and differentiated into myofibroblasts more prominently and displayed higher sensitivity to growth factors than those recovered from wild-type littermates. Under inflammatory stimulation, mRNA and protein expression of proinflammatory mediators were higher in CF than in wild-type fibroblasts, in which CFTR expression reached similar levels to those observed in other non-epithelial cells, such as macrophages. Increased proinflammatory responses in CF fibroblasts were reduced by half with submicromolar concentrations of vardenafil. Proinflammatory and fibrogenic functions of fibroblasts are upregulated in CF and are reduced by vardenafil. This study provides compelling new support for targeting cGMP signaling pathway in CF pharmacotherapy.

CD4+ T lymphocytes in lung fibrosis: diverse subsets, diverse functions.

The discovery of several subsets of CD4(+) Th lymphocytes has contributed to refine and to challenge our understanding of the roles of CD4(+) T cells in the pathogenesis of fibrotic lung diseases. Here, we review recent findings, indicating that CD4(+) T subpopulations possess contrasting pro- and antifibrotic activities in human and experimental lung fibrosis. Special attention is given to delineate the activity of the newly discovered CD4(+) T lymphocyte subsets (Tregs, Th22, and Th9) on fibroblast function and matrix deposition through the release of growth factors, cytokines, and eicosanoids. It appears that the function of a CD4(+) T lymphocyte subset or of a cytokine can differ with the disease stage (acute vs. chronic), pulmonary localization (bronchial vs. alveolar), cellular level (epithelial cell vs. fibroblast), or immune environment (inflammatory or immunosuppressive). Integrating our recent understanding of the contrasting functions of T lymphocyte subsets in fibrosis provides new insights and opportunities for improved treatment strategies.

Platelet-derived growth factor-producing CD4+ Foxp3+ regulatory T lymphocytes promote lung fibrosis.

RATIONALE: There is evidence that CD4(+) effector T lymphocytes (T eff) participate in the development of lung fibrosis, but the role of their CD4(+) regulatory T-cell (T reg) counterparts remains to be determined. OBJECTIVES: To elucidate the contribution of T reg cells in a mouse model of lung fibrosis induced by silica (SiO(2)) particles. METHODS: Lung T reg and T eff cells purified from SiO(2)-treated Foxp3-GFP transgenic mice were cocultured with naive lung fibroblasts or transferred to the lungs of healthy mice. DEREG mice, which express the diphtheria toxin receptor under the control of the foxp3 gene, were used to deplete T reg cells during fibrogenesis. MEASUREMENTS AND MAIN RESULTS: CD4(+) Foxp3(+) T reg cells were persistently recruited in the lungs in response to SiO(2). T reg accumulation paralleled the establishment of pulmonary immunosuppression and fibrosis. T reg cells highly expressed platelet-derived growth factor (PDGF)-B via a TGF-ß autocrine signaling pathway, directly stimulated fibroblast proliferation in vitro, and increased lung collagen deposition upon transfer in the lung of naive mice. The direct profibrotic effects of T reg cells were abolished by the inhibitor of the PDGF-B/TGF-ß signaling pathway, imatinib mesylate. Neutralization of T reg-immunosuppressive activity resulted in enhanced accumulation of T eff cells and IL-4-driven pulmonary fibrogenesis, further demonstrating that T reg cells control T eff cell functions during inflammatory fibrosis. CONCLUSIONS: Our study indicates that T reg cells contribute to lung fibrosis by stimulating fibroblasts through the secretion of PDGF-B in noninflammatory conditions and regulate detrimental T eff cell activities during inflammation-related fibrosis.

Lung fibrosis induced by crystalline silica particles is uncoupled from lung inflammation in NMRI mice.

Previous studies in rats have suggested a causal relationship between progressive pulmonary inflammation and lung fibrosis induced by crystalline silica particles. We report here that, in NMRI mice, the lung response to silica particles is accompanied by a mild and non progressive pulmonary inflammation which is dispensable for the development of lung fibrosis. We found that glucocorticoid (dexamethasone) dramatically reduced lung injury, cellular inflammation and pro-inflammatory cytokine expression (TNF-α, IL-1ß and KC) but had no significant effect on silica-induced lung fibrosis and expression of the fibrogenic and suppressive cytokines TGF-ß and IL-10 in mice. Other anti-inflammatory molecules such as the COX inhibitor piroxicam or the phosphodiesterase 5 inhibitor sildenafil also reduced lung inflammation without modifying collagen, TGF-ß or IL-10 lung content. Our findings indicate that the development of lung fibrosis in silica-treated NMRI mice is not driven by inflammatory lung responses and suggest that suppressive cytokines may represent critical fibrotic factors and potential therapeutic targets in silicosis.

Type I interferon signaling contributes to chronic inflammation in a murine model of silicosis.

Lung disorders induced by inhaled inorganic particles such as crystalline silica are characterized by chronic inflammation and pulmonary fibrosis. Here, we demonstrate the importance of type I interferon (IFN) in the development of crystalline silica-induced lung inflammation in mice, revealing that viruses and inorganic particles share similar signaling pathways. We found that instillation of silica is followed by the upregulation of IFN-beta and IRF-7 and that granulocytes (GR1(+)) and macrophages/dendritic cells (CD11c(+)) are major producers of type I IFN in response to silica. Two months after silica administration, both IFNAR- and IRF-7-deficient mice produced significantly less pulmonary inflammation and chemokines (KC and CCL2) than competent mice but developed similar lung fibrosis. Our data indicate that type I IFN contributes to the chronic lung inflammation that accompanies silica exposure in mice. Type I IFN is, however, dispensable in the development of silica-induced acute lung inflammation and pulmonary fibrosis.

IL-17A-producing gammadelta T and Th17 lymphocytes mediate lung inflammation but not fibrosis in experimental silicosis.

IL-17-producing T lymphocytes play a crucial role in inflammation, but their possible implication in fibrosis remains to be explored. In this study, we examined the involvement of these cells in a mouse model of lung inflammation and fibrosis induced by silica particles. Upregulation of IL-17A was associated with the development of experimental silicosis, but this response was markedly reduced in athymic, gammadelta T cell-deficient or CD4(+) T cell-depleted mice. In addition, gammadelta T lymphocytes and CD4(+) T cells, but not macrophages, neutrophils, NK cells or CD8 T cells, purified from the lungs of silicotic mice markedly expressed IL-17A. Depletion of alveolar macrophages or neutralization of IL-23 reduced upregulation of IL-17A in the lung of silicotic mice. IL-17R-deficient animals (IL-17R(-/-)) or IL-17A Ab neutralization, but not IL-22(-/-) mice, developed reduced neutrophil influx and injury during the early lung response to silica. However, chronic inflammation, fibrosis, and TGF-beta expression induced by silica were not attenuated in the absence of IL-17R or -22 or after IL-17A Ab blockade. In conclusion, a rapid lung recruitment of IL-17A-producing T cells, mediated by macrophage-derived IL-23, is associated with experimental silicosis in mice. Although the acute alveolitis induced by silica is IL-17A dependent, this cytokine appears dispensable for the development of the late inflammatory and fibrotic lung responses to silica.

Azithromycin reduces exaggerated cytokine production by M1 alveolar macrophages in cystic fibrosis.

Macrophages phagocyte pathogenic microorganisms and orchestrate immune responses by producing a variety of inflammatory mediators. The cystic fibrosis (CF) transmembrane conductance regulator chloride channel has been reported to be of pivotal importance for macrophage functions. The exact phenotype and role of macrophages in CF is still unknown. Alveolar and peritoneal macrophages were monitored in CF mice homozygous for the F508 del mutation and in wild-type control animals. Classical (M1) and alternative (M2) macrophage polarization and responses to LPS from Pseudomonas aeruginosa were investigated, and the effect of azithromycin was examined in both cell populations. We show that alveolar macrophage counts were 1.7-fold higher in CF as compared with wild-type mice. The macrophage-related chemokine, chemokine C-C motif ligand (CCL)-2, was found to be at least 10-fold more abundant in the alveolar space of mutant mice. Cell count and CCL-2 protein levels were also increased in the peritoneal cavity of CF mice. Both M1 and M2 macrophage polarization were significantly enhanced in alveolar and peritoneal cells from F508del-CF mice as compared with control animals. LPS-stimulated expression of proinflammatory mediators, such as nitric oxide synthase-2, IL-1beta, and CCL-2, was increased, whereas anti-inflammatory IL-10 expression was decreased in CF macrophages. Azithromycin, added to cell cultures at 1 mg/liter, significantly reduced proinflammatory cytokine expression (IL-1beta, CCL-2, TNF-alpha) in M1-induced CF and wild-type alveolar macrophages. Our findings indicate that CF macrophages are ubiquitously accumulated, and that these cells are polarized toward classical and alternative activation status. Azithromycin down-regulates inflammatory cytokine production by M1-polarized CF alveolar macrophages.