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.