Gamma Interferon and Interleukin-17A Differentially Influence the Response of Human Macrophages and Neutrophils to Pseudomonas aeruginosa Infection

Macrophages are important orchestrators of inflammation during bacterial infection, acting as both effector cells and regulators of neutrophil recruitment and life span. Differently activated macrophage populations with distinct inflammatory and microbicidal potentials have been described. Our previous work unveiled a positive and a negative correlation between levels of gamma interferon (IFN-γ) and interleukin-17A (IL-17A), respectively, and lung function in cystic fibrosis, particularly in patients chronically infected with Pseudomonas aeruginosa. This study sought to define key parameters in human antibacterial immunity under Th1- and Th17-dominated inflammatory conditions; the final aim was to identify unique characteristics that could be fine-tuned therapeutically to minimize tissue damage while maximizing bacterial clearance. Toward this aim, neutrophils were incorporated into cultures of macrophages treated with IFN-γ or IL-17A and infected with P. aeruginosa. The intent of this design was to model (i) initiation of inflammation by infected macrophages and (ii) delayed arrival of neutrophils and their exposure to macrophage-derived cytokines. Under these conditions, IFN-γ decreased bacterial killing and promoted the production of monocyte chemoattractant protein 1 (MCP-1). In contrast, IL-17A promoted bacterial killing but did not affect MCP-1 production. The level of secretion of the pyrogen IL-1β was significantly lower in the presence of IFN-γ than in the presence of IL-17A and correlated with levels of the IL1B transcript in infected macrophages. These findings support the validity of this model to investigate human antibacterial immunity. Based on these observations, the protective and damaging roles of IFN-γ and IL-17A, respectively, during P. aeruginosa infection could be caused by their contrasting effects on IL-1β and MCP-1 production.