Pseudomonas aeruginosa elastase down-regulates host inflammatory responses by degrading cytokines and chemokines: a non-healing wound perspective

Non-healing venous leg ulcers are characterized by dysfunctional wound healing and frequently exhibit an absence of classical inflammatory signs, despite substantial bacterial loads of the Gram-negative pathogen <i>Pseudomonas aeruginosa</i>. To investigate this clinical observation, we used a porcine wound infection model and complementary <i>in vitro</i> cell and enzymatic activity assays. <i>In vivo</i>, <i>P. aeruginosa</i> infected wounds resulted in attenuated inflammatory responses compared to those infected with <i>Staphylococcus aureus</i>. Protease activity was elevated in <i>P. aeruginosa</i>-infected wounds relative to uninfected controls, while pro-inflammatory cytokine levels decreased over time. <i>In vitro</i> analyses employing cell cultures, wildtype and mutant strains, and clinical isolates from venous ulcers and blood, revealed that <i>P. aeruginosa</i> elastase (LasB) degrades a range of pro-inflammatory cytokines (G-CSF, GM-CSF, IFN-γ, IL-1ra, IL-6, IL-12p40, IL-23, TNF-α) and chemokines (Gro-α, IL-8, IP-10, MCP-1, MIP-1α, MIP-1β) in the extracellular milieu, without impacting cell morphology, transcription factor activation, or subsequent intracellular cytokine production. Correspondingly, wound fluids from non-healing ulcers colonized/infected with <i>P. aeruginosa</i> degraded cytokines, whereas fluids from uninfected wounds did not. Collectively, our findings indicate that <i>P. aeruginosa</i> modulates host inflammation by degrading cytokines and chemokines.