(418) Ablation of two distinct PDE4 subtypes produces diametrical (diametrically opposite) effects in an acute model of Pseudomonas aeruginosa-induced lung injury in mice

Background: Type 4 cyclic nucleotide phosphodiesterases (PDE4s), a group of isoenzymes that hydrolyze and inactivate the second messenger cAMP, are promising targets for CF therapy. Non-selective PDE4 inhibition promotes the cAMP/PKA-mediated activation of CFTR and is well established to exert potent anti-inflammatory effects. However, non-selective PDE4 inhibitors are also associated with dose-limiting side effects, including nausea, emesis, diarrhea, and weight loss, that are counterindicated for a CF therapeutic. The PDE4 family comprises four subtypes, PDE4A to D, of which three (PDE4A, PDE4B and PDE4D) are highly expressed in lung and immune cells. Each of the PDE4 subtypes is thought to play unique and non-overlapping physiological roles. Thus, targeting individual PDE4 subtypes may serve to separate therapeutically beneficial from the side effects of the non-selective PDE4 inhibitors available to date. Towards this end, we have shown that selective ablation of PDE4B is sufficient to protect mice from lung injury in a model of acute intratracheal Pseudomonas aeruginosa (PA) infection in mice 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 may provide separate and/or 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, or to measurement of vascular/alveolar Evans Blue Dye extravasation.

Results: Compared to WT controls, PDE4A-KO mice presented with a hyper-inflammatory phenotype characterized by increased levels of pro-inflammatory cytokines, a higher number of immune cells (neutrophils) in BAL fluid, and increased vascular leakage. In addition, the bacterial load in lungs was also increased in PDE4A-KO mice compared to WT controls. Preliminary data, assessing the ability of granulocytes to kill bacteria in vitro, suggest that PDE4A deficiency may impair bacterial killing.

Conclusions: Ablation of PDE4A is detrimental in this model of acute PA-lung infection. Current studies aim to determine whether impaired bacterial killing, and a resulting increase in bacterial load, are responsible for the observed hyperinflammatory responses, or whether these are independent effects of PDE4A ablation. Our findings suggest that selective inactivation of PDE4B may be the more effective therapeutic approach in settings of PA-lung infection, compared to treatment with currently available non-selective PDE4 inhibitors, because not all effects of global PDE4 inactivation (e.g. inactivation of PDE4A) are therapeutically beneficial.

Acknowledgements: This work was supported in part by funds from the NIH (HL076125, HL141473, HL066299) and the Cystic Fibrosis Foundation (BOYD22H0, SALEH19HO, FIEDLE22H0, RICHTE16GO).

References: