Interleukin-6 Concentration Regulates the Neutrophil Response to Diverse Bacteria

Neutrophils are critical mediators of the innate immune response, and their antimicrobial functions are tightly regulated by a myriad of cytokines. Interleukin-6 (IL-6) is known to be essential for an effective immune response; however, how varying IL-6 concentrations affect the neutrophil response remains poorly understood. Because IL-6 concentrations can vary greatly across different disease states, we investigated the concentration dependent effects of IL-6 on the neutrophil response to diverse bacterial pathogens using an infection-on-a-chip microfluidic device. We found that a high exogenous IL-6 concentration (100 ng/mL) reduced neutrophil extravasation, migration speed, and displacement compared to conditions without exogenous IL-6. In contrast, a lower exogenous IL-6 concentration (10 ng/mL) produced pathogen-specific effects on neutrophil extravasation: exogenous IL-6 increased neutrophil extravasation in response to Pseudomonas aeruginosa, did not change in response to Listeria monocytogenes, and decreased in response to Staphylococcus aureus relative to controls. We then determined the potential endothelial cell contributions to these responses. We found that increasing IL-6 concentration resulted in decreased VE-cadherin expression and that 100 ng/mL exogenous IL-6 resulted in lower ICAM-1 expression than 10 ng/mL exogenous IL-6 in an endothelium exposed to P. aeruginosa. Together, these results demonstrate that IL-6 exerts concentration- and pathogen-dependent effects on neutrophil recruitment and migration, supporting a dual role for IL-6 as both pro-inflammatory and anti-inflammatory, with higher IL-6 concentrations resulting in a more anti-inflammatory neutrophil response.