Human NLRP3 inflammasome senses multiple types of bacterial RNAs

Abstract

Inflammasomes are multiprotein platforms that activate caspase-1, which leads to the processing and secretion of the proinflammatory cytokines IL-1β and IL-18. Previous studies demonstrated that bacterial RNAs activate the nucleotide-binding domain, leucine-rich-repeatcontaining family, pyrin domain-containing 3 (NLRP3) inflammasome in both human and murine macrophages. Interestingly, only mRNA, but neither tRNA nor rRNAs, derived from bacteria could activate the murine Nlrp3 inflammasome. Here, we report that all three types of bacterially derived RNA (mRNA, tRNA, and rRNAs) were capable of activating the NLRP3 inflammasome in human macrophages. Bacterial RNA's 5′-end triphosphate moieties, secondary structure, and double-stranded structure were dispensable; small fragments of bacterial RNA were sufficient to activate the inflammasome. In addition, we also found that 20-guanosine ssRNA can activate the NLRP3 inflammasome in human macrophages but not in murine macrophages. Therefore, human and murine macrophages may have evolved to recognize bacterial cytosolic RNA differently during bacterial infections.bacterial RNA | single-stranded RNA | NLRP3 inflammasome | innate immunity | primary macrophages

Mechanism and Regulation of NLRP3 Inflammasome Activation

“…Subsequently, a wide array of stimuli were identified to activate the NLRP3 inflammasome including multiple microbial products, endogenous molecules or particulate matter [13–15]. In subsequent studies, however, it became clear that most microbial stimuli, such as Toll-like receptor (TLR) ligands and muramyl dipeptide, do not directly activate the NLRP3 inflammasome, but they instead prime the NLRP3 inflammasome for activation [16].…”

TLR8 Senses Bacterial RNA in Human Monocytes and Plays a Nonredundant Role for Recognition of Streptococcus pyogenes

“…For stimulation experiments, bacterial RNA (1 mg/ml, unless indicated otherwise) was encapsulated with DOTAP at a ratio of 3 ml DOTAP per 1 mg RNA, according to the manufacturer's protocol. Unless otherwise indicated, bacterial RNA isolated from Streptococcus pneumoniae was used for experiments [transfection of highly purified bacterial RNA that is devoid of contaminating PAMPs induces comparable immune stimulation independently of the source from which the RNA was isolated (10,12,21)]. The absence of cell wall components within the RNA preparations that may trigger TLR2-dependent responses was confirmed using TLR2-deficient murine macrophages (data not shown).…”

“…Although the relevance of viral RNA and bacterial DNA recognition for initiation of innate immune responses has been intensively studied, bacterial RNA is a so far less well-characterized activator of innate immune responses and its physiological role in defense of infections is just beginning to be deciphered (1)(2)(3)(4)(5)(6). Apart from inducing proinflammatory mediators such as TNF, IL-6, and type I IFN (7)(8)(9), bacterial RNA has previously been identified as an important trigger of Nlrp3 inflammasome activation, resulting in caspase-1-mediated cleavage of pro-IL-1b into its biologically active form (10)(11)(12). Moreover, bacterial RNA has been suggested to play an important role as ligand that indicates immediate danger to the host by functioning as so-called Vita-PAMP that allows the host to discriminate between live and dead bacteria (13).…”

Bacterial RNA: An Underestimated Stimulus for Innate Immune Responses

“…Caspase-1 activation also can be achieved by intracellular delivery of total bacterial RNA derived from a variety of Gram-positive and Gram-negative bacteria (36,(50)(51)(52). Although RIG-I-dependent inflammasome activation by viral RNA requires sensing of 59-triphosphate termini (53), triphosphate moieties, as they can be found in bacterial mRNA, were dispensable for Nlrp3-dependent caspase-1 cleavage in response to bacterial RNA in both murine and human immune cells (35,36,54). Interestingly, intracellularly delivered bacterial RNA seems to serve as both signal 1 and signal 2 for inflammasome activation (Fig.…”