Salmonella typhimurium-induced IL-1 release from primary human monocytes requires NLRP3 and can occur in the absence of pyroptosis

Catherine E Diamond,David Brough,Keith Weng Kit Leong,Jack Rivers-Auty,Maurizio Vacca,Alessandra Mortellaro

doi:10.1038/s41598-017-07081-3

Abstract

Large molecular complexes known as inflammasomes regulate the release of IL-1β from immune cells in response to infection and injury. Salmonella typhimurium infection is reported to activate NLRP3 and NLRC4 inflammasomes which are subsequently involved in pyroptosis of the cell and pathogen clearance. However, the response to S. typhimurium in primary human monocytes has not been studied in detail. The aim of this study was to investigate the effect of S. typhimurium on inflammasomes in primary human monocytes. Much of the previous research in the field has been conducted in murine models and human THP-1 cells, which may not reflect the responses of primary human monocytes. Here, we report that inhibiting NLRP3 with the selective inhibitor MCC950, blocked release of IL-1β and the related cytokine IL-1α from primary human monocytes in response to S. typhimurium. Additionally, under these conditions S. typhimurium-induced IL-1 release occurred independently of pyroptosis. We propose that IL-1β release without pyroptosis may occur in early-recruited monocytes to regulate a maximal innate immune response to Salmonella infection, allowing a sustained inflammatory signal. This insight into the mechanisms involved in IL-1 release from primary human monocytes highlights major differences between immune cell types, and the defences they employ during bacterial infection.

Highlights

Salmonella enterica serovar Typhimurium (S. typhimurium) is a food and water-borne, Gram-negative bacteria that causes millions of cases of gastroenteritis, fever and septicaemia a year, in infants and young people
We sought to investigate the role of the NLRP3 inflammasome in both pyroptosis and IL-1 release in primary human monocytes during acute S. typhimurium infection
It has been shown that NLR containing card domain 4 (NLRC4) and NLRP3 can localise to the same inflammasome complex in response to S. typhimurium infection in non-LPS primed macrophages[16]

Summary

Introduction

Salmonella enterica serovar Typhimurium (S. typhimurium) is a food and water-borne, Gram-negative bacteria that causes millions of cases of gastroenteritis, fever and septicaemia a year, in infants and young people. In order for IL-1β to be secreted, the protease caspase-1 needs to be activated before it can cleave pro-IL-1β8, 9 This process requires the formation of large multi-molecular protein complexes known as inflammasomes. NLRP3 (NACHT, LRR and PYD domains containing protein 3) is the most extensively studied NLR and forms an inflammasome by associating with the adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD) This in turn recruits and induces the auto-activation of caspase-1, leading to the cleavage and secretion of IL-1β11. S. typhimurium-induced activation of caspase-1 causes an inflammatory form of programmed cell death known as pyroptosis in macrophages[19, 20] It is unknown if monocytes undergo pyroptosis in response to Salmonella infection. We and others previously reported important differences in the mechanisms of NLRP3 inflammasome activation between human monocytes and cell lines[14, 21], highlighting specific responses of different immune cell types

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Conclusion