Extracellular adenosine has been implicated as anti-inflammatory signaling molecule during acute lung injury (ALI). The main source of extracellular adenosine stems from a coordinated two-step enzymatic conversion of precursor nucleotides via the ecto-apyrase (CD39) and the ecto-5'-nucleotidase (CD73). In the present study, we hypothesized a critical role of CD39 and CD73 in mediating pulmonary neutrophil (PMN) transmigration during lipopolysaccharide (LPS) -induced lung injury. Initial studies revealed that pulmonary CD39 and CD73 transcript levels were elevated following LPS exposure in vivo. Moreover, LPS-induced accumulation of PMN into the lungs was enhanced in cd39(-/-) or cd73(-/-) mice, particularly into the interstitial and intra-alveolar compartment. Such increases in PMN trafficking were accompanied by corresponding changes in alveolar-capillary leakage. Similarly, inhibition of extracellular nucleotide phosphohydrolysis with the nonspecific ecto-nucleoside-triphosphate-diphosphohydrolases inhibitor POM-1 confirmed increased pulmonary PMN accumulation in wild-type, but not in gene-targeted mice for cd39 or cd73. Finally, treatment with apyrase or nucleotidase was associated with attenuated pulmonary neutrophil accumulation and pulmonary edema during LPS-induced lung injury. Taken together, these data reveal a previously unrecognized role for CD39 and CD73 in attenuating PMN trafficking into the lungs during LPS-induced lung injury and suggest treatment with their soluble compounds as a therapeutic strategy.

Extracellular adenosine and adenosine receptors are critically involved in various inflammatory pathways. Adenosine receptor A1 (A1AR) has been implicated in mediating transmigration of leukocytes to sites of inflammation. This study was designed to characterize the role of A1AR in a murine model of LPS-induced lung injury. LPS-induced transmigration of polymorphonuclear cells (PMNs) and microvascular permeability was elevated in A1AR−/− mice. Pretreatment of wild-type mice with the specific A1AR agonist 2′Me–2-chloro-N6-cyclopentyladenosine attenuated PMN accumulation in the interstitium and alveolar space as well as microvascular permeability. Lower PMN counts in the lungs of pretreated wild-type mice were associated with reduced amounts of the chemotactic cytokines TNF-α, IL-6, and CXCL2/3 in the bronchoalveolar lavage. Pretreatment was only effective when A1AR was expressed on hematopoietic cells as demonstrated in chimeric mice. These findings were confirmed by in vitro transmigration assays demonstrating that chemokine-induced transmigration of PMNs was reduced when PMNs but not when pulmonary endothelial or alveolar epithelial cells were pretreated. 2′Me–2-chloro-N6-cyclopentyladenosine prevented pulmonary endothelial but not epithelial cells from LPS-induced cellular remodeling and cell retraction. Our data reveal what we believe to be a previously unrecognized distinct role of A1AR for PMN trafficking and endothelial integrity in a model of acute lung injury.

Adenosine receptor A(3) (A(3)) regulates directed movement of polymorphonuclear cells (PMNs) to sites of inflammation and has been implicated as a relevant mediator in models of inflammatory diseases. Here, we sought to characterize the role of A(3) in a murine model of lung inflammation. Initial studies revealed that pulmonary A(3) transcript levels were elevated following LPS exposure in vivo. In addition, inhalation of LPS increased the accumulation of PMNs in wild-type and A(3)(-/-) mice in all lung compartments. Pretreatment with the specific A(3)-agonist Cl-IB-MECA significantly decreased migration of PMNs into lung interstitium and alveolar air space of wild-type mice but not of A(3)(-/-) mice. Lower PMN counts were associated with reduced levels of TNF-α and IL-6 in the alveolar space of wild-type mice that received Cl-IB-MECA. In addition, Cl-IB-MECA attenuated LPS-induced microvascular permeability in wild-type mice as assessed by the extravasation of Evans blue. In pulmonary microvascular endothelial cells, Cl-IB-MECA reduced LPS-induced cytoskeletal remodeling and cell retraction, consistent with a specific role of A(3) for maintaining endothelial integrity. Migratory activity of human PMNs across an endothelial or epithelial monolayer was reduced when A(3) was activated on PMNs. Studies in chimeric mice, however, revealed that Cl-IB-MECA required A(3) on both hematopoietic and nonhematopoietic cells to reduce transmigration in vivo. Together, our results shed new light on the role of A(3) in LPS-induced PMN trafficking in the lung and suggest pharmacological modulation of A(3)-dependent pathways as a promising approach in lung inflammation.