| 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-repeat-containing 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.The innate immune system is the first line of defense against microbial infections. Germ-line–encoded pattern-recognition receptors (PRRs) of the innate immune system recognize the presence of invariant evolutionarily conserved microbial components called “pathogen-associated molecular patterns” (1–3). In response to microbial infections, PRRs rapidly initiate signal-transduction pathways to induce type 1 IFN production, proinflammatory cytokine production, and inflammasome activation. The inflammasome is a cytosolic large caspase-1–containing multiprotein complex that enables autocatalytic activation of caspase-1. Once caspase-1 is activated, it starts to cleave prointerleukin-1β (pro–IL-1β) and prointerleukin-18 (pro–IL-18) proteolytically into bioactive IL-1β and IL-18 (4–7). The mature forms of IL-1β and IL-18 play roles in a variety of infectious and inflammatory processes.Cytosolic microbial nucleic acids are important activators of the innate immune system against both bacterial and viral infections, which induce type 1-IFN and proinflammatory cytokine responses as well as inflammasome activation. The role of microbial nucleic acids in inflammasome activation has been studied mostly in murine bone marrow-derived dendritic cells (BMDCs) or bone marrow-derived macrophages (BMDMs). AIM2 has been identified as a specific cytosolic dsDNA sensor that directly binds ASC (apoptosis-associated speck-like protein containing a carboxyl-terminal CARD-like domain) and forms inflammasome complexes in human and murine cells (8–11).Viral dsRNA was found to activate the nucleotide-binding domain, leucine-rich-repeat-containing family, pyrin domain-containing 3 (NLRP3) inflammasome in human and murine cells (12–15). Several groups have reported that cytosolic bacterial RNA activate the Nlrp3 inflammasome in murine macrophages (13, 16, 17). Our group also has reported that human THP-1–derived macrophages recognize cytosolic bacterial RNA and induce NLRP3 inflammasome activation (12). Bacterial RNA is composed of mRNA, tRNA, and three different sizes of rRNA (23s, 16s, and 5s). Sander et al. (18) reported that, of the different types of Escherichia coli RNA, only E. coli mRNA induced the secretion of IL-1β by murine BMDMs, but E. coli tRNA and E. coli rRNAs did not.We aimed to study (i) whether a variety of cytosolic bacterial RNAs could activate the inflammasome in human myeloid cells and (ii) what types of bacterial RNA activate the inflammasome in human and murine myeloid cells. Here, we demonstrate that a broad spectrum of cytosolic bacterial RNAs strongly induce the cleavage of caspase-1 and the secretion of IL-1β and IL-18 in human macrophages. Human macrophages can sense mRNA, tRNA, rRNAs, and small synthetic ssRNA through NLRP3, but murine macrophages can sense only the mRNA component. Bacterial RNA’s 5′-end triphosphate moieties, secondary structure, and double-stranded structure were dispensable, but small fragments of bacterial RNA were sufficient to activate the inflammasome. These findings suggest that upon bacterial infections the human and murine NLRP3 inflammasomes sense cytosolic bacterial RNAs differently. |
