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Acute lung injury (ALI) is a lethal inflammatory lung disorder whose incidence is on the rise. Alveolar macrophages normally act to resolve inflammation, but when dysregulated they can provoke ALI. We demonstrate that monocyte-derived macrophages (CD11b+macrophages) recruited into the airspace upregulate the anti-inflammatory function of alveolar macrophages by suppressing their stimulator of type 1 interferon gene (STING) signaling. Depletion of CD11b+macrophages in mice (macrophagedepmice) after endotoxin or after Pseudomonas aeruginosacauses expansion of the inflammatory alveolar macrophage population, leading to neutrophil accumulation, irreversible loss of lung vascular barrier function, and lethality. We show that CD11b+macrophages suppress alveolar macrophage-STING signaling via sphingosine kinase-2 (SPHK2) generation of sphingosine-1-phosphate (S1P). Thus, adoptive transfer of wild-type (WT) or STING−/−, but not SPHK2−/−, CD11b monocytes from murine bone marrow into injured macrophagedepmice rescue anti-inflammatory alveolar macrophages and reverse lung vascular injury. SPHK2-induced S1P generation in CD11b+macrophages has the potential to educate alveolar macrophages to resolve ALI.

Alveolar macrophages (AMs), upon sensing pathogens, trigger host defense by activating toll-like receptor 4 (TLR4), but the counterbalancing mechanisms that deactivate AM inflammatory signaling and prevent lethal edema, the hallmark of acute lung injury (ALI), remain unknown. Here, we demonstrate the essential role of AM protease-activating receptor 2 (PAR2) in rapidly suppressing inflammation to prevent long-lasting injury. We show that thrombin, released during TLR4-induced lung injury, directly activates PAR2 to generate cAMP, which abolishes Ca2+entry through the TRPV4 channel. Deletion of PAR2 and thus the accompanying cAMP generation augments Ca2+entry via TRPV4, causing sustained activation of the transcription factor NFAT to produce long-lasting TLR4-mediated inflammatory lung injury. Rescuing thrombin-sensitive PAR2 expression or blocking TRPV4 activity in PAR2-null AMs restores their capacity to resolve inflammation and reverse lung injury. Thus, activation of the thrombin-induced PAR2-cAMP cascade in AMs suppresses TLR4 inflammatory signaling to reinstate tissue integrity.