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Updated: Oct 26, 2025
Isolation and In Vitro Culture of Murine and Human Alveolar Macrophages
Published on: April 20, 2018
Evidence for reprogramming of monocytes into reparative alveolar macrophages in vivo by targeting PDE4b
Ian Rochford1, Jagdish Chandra Joshi1, Sheikh Rayees1
1Department of Pharmacology and Regenerative Medicine and Centre for Lung and Vascular Biology, University of Illinois, College of Medicine, Chicago, Illinois.
Abstract:
Increased lung vascular permeability and neutrophilic inflammation are hallmarks of acute lung injury. Alveolar macrophages (AMϕ), the predominant sentinel cell type in the airspace, die in massive numbers while fending off pathogens. Recent studies indicate that the AMϕ pool is replenished by airspace-recruited monocytes, but the mechanisms instructing the conversion of recruited monocytes into reparative AMϕ remain elusive. Cyclic AMP (cAMP) is a vascular barrier protective and immunosuppressive second messenger in the lung. Here, we subjected mice expressing GFP under the control of the Lysozyme-M promoter (LysM-GFP mice) to the LPS model of rapidly resolving lung injury to address the impact of mechanisms determining cAMP levels in AMϕ and regulation of mobilization of the reparative AMϕ-pool. RNA-seq analysis of flow-sorted Mϕ identified phosphodiesterase 4b (PDE4b) as the top LPS-responsive cAMP-regulating gene. We observed that PDE4b expression markedly increased at the time of peak injury (4 h) and then decreased to below the basal level during the resolution phase (24 h). Activation of transcription factor NFATc2 was required for the transcription of PDE4b in Mϕ. Inhibition of PDE4 activity at the time of peak injury, using intratracheal rolipram, increased cAMP levels, augmented the reparative AMϕ pool, and resolved lung injury. This response was not seen following conditional depletion of monocytes, thus establishing airspace-recruited PDE4b-sensitive monocytes as the source of reparative AMϕ. Interestingly, adoptive transfer of rolipram-educated AMϕ into injured mice resolved lung edema. We propose suppression of PDE4b as an effective approach to promote reparative AMϕ generation from monocytes for lung repair.
Insights
In acute lung injury, inhibiting phosphodiesterase 4b (PDE4b) boosts reparative alveolar macrophages (AMϕ) from monocytes, promoting lung repair. This strategy enhances cyclic AMP (cAMP) signaling for effective recovery.
Area of Science:
- Pulmonary immunology
- Cellular signaling in inflammation
- Lung injury and repair mechanisms
Background:
- Acute lung injury (ALI) is characterized by increased lung vascular permeability and neutrophilic inflammation.
- Alveolar macrophages (AMϕ) are crucial sentinel cells in the lungs but undergo massive cell death during ALI.
- The replenishment of AMϕ by circulating monocytes is critical for lung repair, yet the underlying mechanisms are not fully understood.
Purpose of the Study:
- To investigate the role of cyclic AMP (cAMP) regulatory mechanisms in AMϕ during lung injury.
- To identify key genes and pathways involved in the generation of reparative AMϕ from monocytes.
- To explore therapeutic strategies targeting cAMP signaling for promoting lung repair in ALI.
Main Methods:
- Utilized the LPS-induced lung injury model in LysM-GFP mice to study AMϕ and monocyte dynamics.
- Performed RNA-sequencing on flow-sorted macrophages to identify cAMP-regulating genes.
- Administered intratracheal rolipram (PDE4 inhibitor) to modulate cAMP levels and assessed its impact on lung injury and AMϕ populations.
Main Results:
- Phosphodiesterase 4b (PDE4b) was identified as a top LPS-responsive gene regulating cAMP levels in lung macrophages.
- PDE4b expression peaked during injury and decreased during resolution, with its transcription dependent on NFATc2 activation.
- Inhibition of PDE4b activity with rolipram increased cAMP, expanded the reparative AMϕ pool derived from monocytes, and resolved lung injury.
Conclusions:
- Airspace-recruited monocytes are the primary source of reparative AMϕ in resolving lung injury.
- PDE4b activity in monocytes critically regulates the generation of reparative AMϕ and subsequent lung repair.
- Suppression of PDE4b represents a promising therapeutic approach to enhance AMϕ-mediated lung repair.
