Abstract
GM-CSF promotes the functional maturation of lung alveolar macrophages (A-MØ), whose differentiation is dependent on the peroxisome proliferator-activated receptor gamma (PPARγ) transcription factor. In fact, blockade of GM-CSF-initiated signaling or deletion of the PPARγ-encoding gene PPARG leads to functionally defective A-MØ and the onset of pulmonary alveolar proteinosis. In vitro, macrophages generated in the presence of GM-CSF display potent proinflammatory, immunogenic and tumor growth-limiting activities. Since GM-CSF upregulates PPARγ expression, we hypothesized that PPARγ might contribute to the gene signature and functional profile of human GM-CSF-conditioned macrophages. To verify this hypothesis, PPARγ expression and activity was assessed in human monocyte-derived macrophages generated in the presence of GM-CSF [proinflammatory GM-CSF-conditioned human monocyte-derived macrophages (GM-MØ)] or M-CSF (anti-inflammatory M-MØ), as well as in ex vivo isolated human A-MØ. GM-MØ showed higher PPARγ expression than M-MØ, and the expression of PPARγ in GM-MØ was found to largely depend on activin A. Ligand-induced activation of PPARγ also resulted in distinct transcriptional and functional outcomes in GM-MØ and M-MØ. Moreover, and in the absence of exogenous activating ligands, PPARγ knockdown significantly altered the GM-MØ transcriptome, causing a global upregulation of proinflammatory genes and significantly modulating the expression of genes involved in cell proliferation and migration. Similar effects were observed in ex vivo isolated human A-MØ, where PPARγ silencing led to enhanced expression of genes coding for growth factors and chemokines and downregulation of cell surface pathogen receptors. Therefore, PPARγ shapes the transcriptome of GM-CSF-dependent human macrophages (in vitro derived GM-MØ and ex vivo isolated A-MØ) in the absence of exogenous activating ligands, and its expression is primarily regulated by activin A. These results suggest that activin A, through enhancement of PPARγ expression, help macrophages to switch from a proinflammatory to an anti-inflammatory polarization state, thus contributing to limit tissue damage and restore homeostasis.
Highlights
Tissue-resident macrophages in homeostasis, as well as monocytederived macrophages within inflamed tissues, exhibit a huge functional diversity which derives from their exquisite sensitivity to extracellular cues [1, 2]
To initially assess the PPARγ activation-dependent transcriptional profile of GM-MØ and M-MØ, both human macrophage subtypes were exposed for 24 h to the PPARγ agonist GW7845 and the expression of the GM-MØ-specific “Proinflammatory gene set” and M-MØ-specific “Anti-inflammatory gene set” [11, 12] was determined
PPARγ activation alters the expression of known PPARγ targets in both GM-MØ and M-MØ, it promotes human macrophage subtype-dependent transcriptional changes because the expression of CCR2, IL10, CCL2, and HAMP is downregulated by GW7845 only in proinflammatory GM-MØ
Summary
Tissue-resident macrophages in homeostasis, as well as monocytederived macrophages within inflamed tissues, exhibit a huge functional diversity which derives from their exquisite sensitivity to extracellular cues [1, 2]. Besides its role in myeloid cell differentiation, GM-CSF is a central mediator of tissue inflammation [7] and its neutralization has been proposed as a therapeutic strategy for inflammatory disorders [8] As a consequence, both colony-stimulating factors promote the generation of functionally distinct macrophages [9]: GM-CSF-conditioned human monocyte-derived macrophages (GM-MØ) produce large amounts of proinflammatory cyto kines in response to stimulation, whereas M-CSF-dependent monocyte-derived macrophages (M-MØ) primarily produce anti-inflammatory factors upon activation [9,10,11]. Since GM-CSF-conditioned monocytederived human macrophages exhibit potent proinflammatory functions upon stimulation [9, 11], and in spite of the intrinsic anti-inflammatory functions of PPARγ, we hypothesized that PPARγ might contribute to the gene signature and functional profile of human GM-CSF-conditioned macrophages. The activin A-dependent expression of PPARγ in GM-MØ and in A-MØ suggests a role for activin A in promoting inflammation resolution
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