Ablation of PDE4A Exacerbates Pseudomonas aeruginosa-Induced Acute Lung Injury in Mice

Type 4 cyclic nucleotide phosphodiesterases (PDE4s) comprise a family of four isoenzymes, PDE4A to D, that hydrolyze the second messenger cAMP. Three of these enzymes, PDE4A, PDE4B and PDE4D, are widely expressed throughout pulmonary and immune cell types, and their concurrent inhibition with non/PAN-selective PDE4 inhibitors is well-known to exert potent anti-inflammatory benefits. To determine if PDE4 inhibition may represent a therapeutic approach for acute lung injury resulting from severe pneumonia, we explored the effect of genetic ablation of individual PDE4 subtypes, or their concurrent inhibition with PAN-PDE4 inhibitors, in a model of acute intra-tracheal Pseudomonas aeruginosa (PA) infection in mice. We have shown recently that selective ablation of PDE4B is sufficient to protect mice from lung injury in this model by reducing pulmonary and systemic levels of pro-inflammatory cytokines, as well as pulmonary vascular leakage and mortality. With the present study we explored whether selective inactivation of PDE4A or treatment with PAN-PDE4 inhibitors may provide additional therapeutic benefits. METHODS: Mice deficient in PDE4A (PDE4A-KO) and their wildtype (WT) littermate controls were infected intratracheally with the PA lab strain PA01 and subjected 16 h later to bronchoalveolar lavage (BAL), cardiac blood draws, and tissue extractions. Alternatively, vascular/alveolar leakage was assessed by measuring Evans Blue Dye extravasation. To test the effect of PAN-PDE4 inhibition, mice were treated with the PDE4 inhibitor Piclamilast (5 mg/kg, i.p.) at 2 h prior to PA01 infection. RESULTS: Compared to WT controls, PDE4A-KO mice displayed a hyper-inflammatory phenotype characterized by increased levels of pro-inflammatory cytokines, immune cells (neutrophils) in BAL fluid, and vascular leakage. Despite these hyperinflammatory responses, the bacterial load in lungs was also increased in PDE4A-KO mice compared to WT controls. Treatment with a PAN-PDE4 inhibitor produced incongruent effects, with an increased bacterial load mirroring the effect of PDE4A ablation, whereas reduced levels of inflammatory cytokines such as TNFα mimicked the effect of PDE4B ablation. CONCLUSIONS: Ablation of PDE4A produces detrimental effects in this model of acute PA-lung infection. Current studies aim to determine whether an impaired bacterial killing, and the resulting increase in bacterial load, are responsible for the hyperinflammation, or whether these are independent effects of PDE4A ablation. PAN-PDE4 inhibition mimics some of the beneficial effects of selective PDE4B ablation, but also some of the detrimental effects of PDE4A ablation. These findings suggest that selective inactivation of PDE4B may be a more effective therapeutic approach, compared to PAN-PDE4 inhibition, in settings of acute PA-lung infection. © FASEB.