SLPI Inhibits ATP-Mediated Maturation of IL-1β in Human Monocytic Leukocytes: A Novel Function of an Old Player

Anna Zakrzewicz,Katrin Richter,Dariusz Zakrzewicz,Alisa Agné,Ritva Tikkanen,Jelena Damm,Gabriela Krasteva-Christ,Walee Chamulitrat,Ivan Manzini,J Michael Mcintosh,Veronika Grau,Winfried Padberg,Kathrin Siebers,Sabina Janciauskiene,Andreas Hecker

doi:10.3389/fimmu.2019.00664

Abstract

Interleukin-1β (IL-1β) is a potent, pro-inflammatory cytokine of the innate immune system that plays an essential role in host defense against infection. However, elevated circulating levels of IL-1β can cause life-threatening systemic inflammation. Hence, mechanisms controlling IL-1β maturation and release are of outstanding clinical interest. Secretory leukocyte protease inhibitor (SLPI), in addition to its well-described anti-protease function, controls the expression of several pro-inflammatory cytokines on the transcriptional level. In the present study, we tested the potential involvement of SLPI in the control of ATP-induced, inflammasome-dependent IL-1β maturation and release. We demonstrated that SLPI dose-dependently inhibits the ATP-mediated inflammasome activation and IL-1β release in human monocytic cells, without affecting the induction of pro-IL-1β mRNA by LPS. In contrast, the ATP-independent IL-1β release induced by the pore forming bacterial toxin nigericin is not impaired, and SLPI does not directly modulate the ion channel function of the human P2X7 receptor heterologously expressed in Xenopus laevis oocytes. In human monocytic U937 cells, however, SLPI efficiently inhibits ATP-induced ion-currents. Using specific inhibitors and siRNA, we demonstrate that SLPI activates the calcium-independent phospholipase A2β (iPLA2β) and leads to the release of a low molecular mass factor that mediates the inhibition of IL-1β release. Signaling involves nicotinic acetylcholine receptor subunits α7, α9, α10, and Src kinase activation and results in an inhibition of ATP-induced caspase-1 activation. In conclusion, we propose a novel anti-inflammatory mechanism induced by SLPI, which inhibits the ATP-dependent maturation and secretion of IL-1β. This novel signaling pathway might lead to development of therapies that are urgently needed for the prevention and treatment of systemic inflammation.

Highlights

Interleukin-1β (IL-1β) is a potent, multifunctional, proinflammatory cytokine of the innate immune system that is essential for host-response and resistance to pathogens [1]
We hypothesized that Secretory leukocyte protease inhibitor (SLPI) might rapidly control the release of monocytic IL-1β and protect against detrimental effects caused by elevated systemic levels of this cytokine
We provided evidence for a novel antiinflammatory mechanism mediated by SLPI, which results in a dose-dependent inhibition of the BzATP-induced IL-1β release by human and murine mononuclear leukocytes

Summary

Introduction

Interleukin-1β (IL-1β) is a potent, multifunctional, proinflammatory cytokine of the innate immune system that is essential for host-response and resistance to pathogens [1]. To avoid severe adverse effects, the release of IL-1β is firmly controlled and most often requires two consecutive danger signals. Tolllike receptor agonists such as lipopolysaccharide (LPS) are prototypical first signals (signal 1) initiating the synthesis of the inactive IL-1β precursor (pro-IL-1β) via nuclear factor-κB (NF-κB)-dependent signal transduction [5]. ATP activates the purinergic P2X7 receptor (P2X7R), triggers the efflux of potassium ions out of the cells, the assembly of the cytoplasmic NLRP3 (NACHT, LRR, and PYD domains-containing protein 3) inflammasome, followed by the conversion of pro-caspase-1 into active caspase-1 that is required for the proteolytic cleavage of pro-IL-1β and release of mature IL-1β [6, 7]. Mechanisms controlling IL-1β maturation in the presence of both danger signals are of substantial clinical interest [8]

Methods

Results

Conclusion