An NLRP7-Containing Inflammasome Mediates Recognition of Microbial Lipopeptides in Human Macrophages

Abstract

SUMMARY Cytosolic pathogen- and damage-associated molecular patterns are sensed by pattern recognition receptors, including members of the nucleotide-binding domain and leucine-rich repeat containing gene family (NLR), which cause inflammasome assembly and caspase-1 activation to promote maturation and release of the inflammatory cytokines interleukin (IL)-1β and IL-18 and induction of pyroptosis. However, the contribution of most of the NLRs to innate immunity, host defense, inflammasome activation and their specific agonists, are still unknown. Here we describe identification and characterization of an NLRP7 inflammasome in human macrophages, which is induced in response to microbial acylated lipopeptides. Activation of NLRP7 promoted ASC-dependent caspase-1 activation, IL-1β and IL-18 maturation and restriction of intracellular bacterial replication, but not caspase-1-independent secretion of the pro-inflammatory cytokines IL-6 and tumor necrosis factor-α. Our study therefore increases our currently limited understanding of NLR activation, inflammasome assembly and maturation of IL-1β and IL-18 in human macrophages.

The Type II Secretion System of Legionella pneumophila Dampens the MyD88 and Toll-Like Receptor 2 Signaling Pathway in Infected Human Macrophages

“…During L. pneumophila infection of murine macrophages, activation of the inflammasome occurs through NAIP5/NLRC4, triggered by flagellin, as well as through an apoptosis-associated speck-like protein containing a CARD (ASC)-dependent pathway, resulting in the production of cytokines via an IL-1 autocrine loop (22,23,(53)(54)(55)(56). Humans lack the NAIP5 allele present in murine cells (57); however, the canonical ASC-dependent inflammasome is still activated upon L. pneumophila infection of human macrophages (58). Furthermore, it has been shown, via gene knockdown in human macrophages, that noncanonical inflammasome activation occurs through caspase-4, which serves as an intracellular lipopolysaccharide (LPS) receptor (59).…”

Mechanisms of Inflammasome Activation by Vibrio cholerae Secreted Toxins Vary with Strain Biotype

“…These POPs are not present in mouse cells (45). In addition, human and mouse macrophages differ in the expression of NLR and ALR genes (27,46). Thus, the difference in the system may reflect actual variation in the signaling of inflammasome activation, but this will need to be examined further.…”

“…Human PBMCs were isolated by Ficoll-Hypaque centrifugation (Sigma) from healthy donor blood after informed consent was obtained under a protocol approved by the Northwestern University Institutional Review Board. Monocytes were isolated from PBMCs by countercurrent centrifugal elutriation in the presence of 10 g/ml polymyxin B with a JE-6B rotor (Beckman Coulter) as described previously (27). Monocytes were washed in Hanks balanced salt solution and resuspended in RPMI medium supplemented with 20% FBS.…”

“…Monocytes were washed in Hanks balanced salt solution and resuspended in RPMI medium supplemented with 20% FBS. Monocytes were routinely phenotyped to ensure Ͼ85% purity, as determined by flow cytometry for CD45 and CD14 (27).…”

Expression regulation and function of NLRC5

“…Meanwhile, it also suppresses colon inflammation and tumorigenesis through negatively regulating the canonical or non-canonical NF-κB signaling (Zaki et al, 2011;Allen et al, 2012;Vladimer et al, 2012). NLRP7 forms infl ammasome during recognition of microbial lipopeptides in human macrophages (Khare et al, 2012). NLRP4 plays a negative role in the regulation of type I interferon signaling and autophagy (Jounai et al, 2011;.…”