Neutrophil-expressed p21/waf1 Favors Inflammation Resolution in Pseudomonas aeruginosa Infection

Neutrophil-associated inflammation during Pseudomonas aeruginosa lung infection is a determinant of morbidity in cystic fibrosis (CF). Neutrophil apoptosis is a key factor in inflammation resolution and is controlled by cytosolic proliferating cell nuclear antigen (PCNA). p21/Waf1, a cyclin-dependent kinase inhibitor, is a partner of PCNA and its mRNA is upregulated in human neutrophils during lipopolysaccharide challenge. We here show that after 7 days of persistent infection with Pseudomonas aeruginosa, neutrophilic inflammation was more prominent in p21-/- compared to wild type mice. Notably, no intrinsic defect in the phagocytosis of apoptotic cells by macrophages was found in p21-/- compared to wild type mice. Inflammatory cells analysis in the peritoneal lavages after zymosan-induced peritonitis showed a significant increased number of neutrophils at 48h in p21-/- compared with wild type mice. In vitro analysis was consistent with a delayed neutrophil apoptosis in p21-/- compared with wild type mice. Ectopic expression of p21/waf1 in neutrophil-differentiated PLB985 cells potentiated apoptosis and reversed the prosurvival effect of PCNA. In human neutrophils, p21 mRNA was induced by TNF-α, G-CSF and LPS. Neutrophils isolated from CF patients showed enhanced survival, which was reduced after treatment with a carboxy-peptide derived from the sequence of p21/waf1. Notably, p21/waf1 was detected by immunohistochemistry in neutrophils within lung from CF patients. Our data reveals a novel role for p21/waf1 in the resolution of inflammation via its ability to control neutrophil apoptosis. This mechanism may be relevant in the neutrophil dominated inflammation observed in CF and other chronic inflammatory lung conditions.