Signal inhibitory receptor on leukocytes-1 (SIRL-1) negatively regulates the oxidative burst in human phagocytes.

ROS production is an important effector mechanism mediating intracellular killing of microbes by phagocytes. Inappropriate or untimely ROS production can lead to tissue damage, thus tight regulation is essential. We recently characterized signal inhibitory receptor on leukocytes-1 (SIRL-1) as an inhibitory receptor expressed by human phagocytes. Here, we demonstrate that ligation of SIRL-1 dampens Fc receptor-induced ROS production in primary human phagocytes. In accordance, SIRL-1 engagement on these cells impairs the microbicidal activity of neutrophils, without affecting phagocytosis. The inhibition of ROS production may result from reduced ERK activation, since co-ligation of Fc receptors and SIRL-1 on phagocytes inhibited phosphorylation of ERK. Importantly, we demonstrate that microbial and inflammatory stimuli cause rapid downregulation of SIRL-1 expression on the surface of primary neutrophils and monocytes. In accordance, SIRL-1 expression levels on neutrophils in bronchoalveolar lavage fluid from patients with neutrophilic airway inflammation are greatly reduced. We propose that SIRL-1 on phagocytes sets an activation threshold to prevent inappropriate production of oxygen radicals. Upon infection, SIRL-1 expression is downregulated, allowing microbial killing and clearance of the pathogen

Differential Signalling and Kinetics of Neutrophil Extracellular Trap Release Revealed by Quantitative Live Imaging

A wide variety of microbial and inflammatory factors induce DNA release from neutrophils as neutrophil extracellular traps (NETs). Consensus on the kinetics and mechanism of NET release has been hindered by the lack of distinctive methods to specifically quantify NET release in time. Here, we validate and refine a semi-automatic live imaging approach for quantification of NET release. Importantly, our approach is able to correct for neutrophil input and distinguishes NET release from neutrophil death by other means, aspects that are lacking in many NET quantification methods. Real time visualization shows that opsonized S. aureus rapidly induces cell death by toxins, while actual NET formation occurs after 90 minutes, similar to the kinetics of NET release by immune complexes and PMA. Inhibition of SYK, PI3K and mTORC2 attenuates NET release upon challenge with physiological stimuli but not with PMA. In contrast, neutrophils from chronic granulomatous disease patients show decreased NET release only in response to PMA. With this refined method, we conclude that NET release in primary human neutrophils is dependent on the SYK-PI3K-mTORC2 pathway and that PMA stimulation should be regarded as mechanistically distinct from NET formation induced by natural triggers

Immune checkpoints and rheumatic diseases : what can cancer immunotherapy teach us?

The recent success of immune checkpoint blockade in cancer therapy illustrates the importance of the inhibitory receptors cytotoxic T-lymphocyte-associated antigen 4 (CTLA4) and programmed cell death protein 1 (PD1) in the regulation of antitumour immune responses. However, blocking signalling by these inhibitory immune checkpoint receptors is also associated with substantial inflammatory effects that can resemble autoimmune responses, which is consistent with the role of these receptors in protecting the host from excessive inflammation. The human genome encodes over 300 inhibitory receptors, which represent as many opportunities to modulate inflammation in a disease-specific and tissue-specific manner. We argue that rheumatologists and oncologists should join forces to study these inhibitory immune molecules. An improved understanding of these immune checkpoints will enable both fields to make progress in exploiting inhibitory immune receptors therapeutically. In this Review, we discuss data from studies reporting the adverse inflammatory effects of cancer therapies that target immune checkpoints. We discuss the potential implications of these findings on the biological understanding of autoimmune rheumatic diseases and highlight therapeutic strategies that could be used to target inhibitory receptors for the treatment of these conditions