IL-27 Enhances LPS-Induced Proinflammatory Responses in Human Monocytes: Augmented Inflammasome Activity and IL-23 Expression

Abstract

Inflammation plays an important role in responding to injury and combating infections. In this thesis, I examine how inflammation is regulated by cytokines responsible for driving initial immune responses to combat infections. Toll-Like receptor (TLR)-mediated activation of monocytes, macrophages and dendritic cells can lead to the co-expression of proinflammatory cytokines including IL-1β, IL-23, and IL-27. IL-23 and IL-27 belong to the IL-12 cytokine family yet have distinct functions; IL-23, along with IL-1β, regulates TH17 cell differentiation, while IL-27 supports TH1 proliferation and inhibits TH17 differentiation. Our lab has previously demonstrated that IL-27 can modulate inflammasome activation, the multi-protein regulatory complex that produces bioactive IL-1β; however, the mechanism behind this is poorly understood. Similarly, the effect of IL-27 on IL-23 expression has not been well described. Using the CD14+ THP-1 monocytic cell line as a model system, I investigated the role of IL-27 on LPS-mediated inflammasome activation and IL-23 expression. To induce inflammasome activation, CD14+ THP-1 cells were treated with LPS and/or IL-27, followed by treatment with ATP. I demonstrated that IL-27-enhanced inflammasome activation, which is associated with increased surface expression of LPS and ATP receptors: TLR4 and P2X7 respectively. Furthermore, costimulation resulted in increased secretion of ATP from CD14+ THP-1 cells. Inhibition of ATP signaling and inflammasome activation significantly decreased secreted IL-1β, suggesting that an ATP autocrine feedback loop is driving IL-1β secretion. Moreover, LPS and IL-27 costimulation increased IL-23 expression concurrent with that of IL-1β and ATP secretion. Furthermore I showed that IL-23 secretion is dependent on inflammasome activation and IL-1β, and ATP signaling following IL-27 and LPS priming. My data point to a novel mechanism of IL-27 enhanced LPS-induced IL-1β and IL-23 secretion from CD14+ THP-1 cells through an ATP autocrine feedback loop.