Interferon gamma applied ex vivo restores function to neutrophils from critically ill patients.

INTRODUCTION: Critically ill patients commonly develop acquired neutrophil dysfunction, which increases susceptibility to intensive care unit-acquired infection (ICU-AI). This study aimed to assess whether interferon gamma (IFN-γ) can restore function in dysfunctional neutrophils from critically ill patients and to uncover potential underlying mechanisms. METHODS: This was an observational cohort study. Neutrophils were isolated from whole blood donated by critically ill patients (n=31) in four separate teaching hospital intensive care units (ICUs). Neutrophils were subsequently treated with recombinant human IFN-γ or vehicle for 1 hour following either Fc gamma receptor (FcγR) blockade, selective inhibition of the gamma isoform of phosphoinositide 3-kinase (PI3K-γ) or vehicle control for 30 min. Neutrophil phagocytosis, bacterial killing, superoxide generation, phagocytic receptor expression and small Rho GTPase activity were assessed. Neutrophil dysfunction was defined as <50% of cells ingesting 2 or more zymosan particles in a phagocytosis assay. RESULTS: IFN-γ significantly improved phagocytosis (control 36.5%, IFN-γ 56.0%), bacterial killing (control 31.6%, IFN-γ 82.1%) and superoxide generation (2.8-fold increase relative to control) in dysfunctional neutrophils. IFN-γ also increased the activity of the small GTPases, Rac and Cdc42 (2.4-fold and 1.5-fold increase relative to control, respectively) in dysfunctional neutrophils. Selective inhibition of PI3K-γ prevented the IFN-γ-mediated improvement of phagocytosis (IFN-γ 62.5%, with inhibitor 27.9%), bacterial killing (IFN-γ 82.1%, with inhibitor 30.5%) and superoxide generation (IFN-γ 2.8-fold change relative to control, 0.7 with inhibitor). The IFN-γ-mediated improvement of bacterial killing in dysfunctional neutrophils was also prevented by FcγR blockade (IFN-γ 82.1%, FcγR inhibition 28.7%). CONCLUSIONS: In critically ill patients with known acquired neutrophil dysfunction, ex vivo application of IFN-γ consistently improved a range of neutrophil effector functions.