Bacterial RNA is recognized by different sets of immunoreceptors

Abstract

Innate immunity recognizes microbial nucleic acids by endosomal TLR and cytosolic recognition systems. Despite increasing knowledge on the properties of nucleic acid recognition for viral RNA and bacterial DNA, little is known about the immunogenicity of prokaryotic RNA. Here we show that bacterial RNA is a potent trigger for type-I IFN secretion in human PBMC. Activation of human plasmacytoid dendritic cells was dependent on endosomal maturation and could be blocked by a TLR7-specific inhibitor. Murine plasmacytoid dendritic cells from TLR7-deficient mice were unresponsive to bacterial RNA. Surprisingly, in myeloid DC, TLR were dispensable for TNF-a and IL-12 induction by bacterial RNA. Even non-immune stroma cells were able to mount a NF-jB response upon triggering with bacterial RNA. Retinoic-acid inducible gene I and melanoma-differentiation-associated gene 5 could be ruled out to be responsible for this reactivity. Although the inflammasome adaptor protein, apoptosis-associated speck-like protein, and a functional type-I IFN receptor were necessary for IL-1b secretion in myeloid DC, these proteins were dispensable for TNF-a and IL-12 induction by cytosolic bacterial RNA. Our results show that besides of activation of TLR7 and inflammasomes, bacterial RNA activates additional cytosolic receptors similarly as has been reported for recognition of bacterial DNA.Key words: Host defense . Immunology . Inflammation IntroductionInnate immunity relies on germline-encoded PRR, which sense conserved microbial structures of non-host origin. The bestdescribed class of PRR is the TLR. Most TLR sense pathogen components on the cell surface but a subset of TLR (TLR3,7, 8 and 9) have been shown to detect nucleic acids in the endosomal compartment [1][2][3]. Besides unmethylated CpG-DNA, which is the ligand for TLR9 [4], viral RNA was identified as a ligand for nucleic acid detecting TLR. TLR3 detects double-stranded RNA [5]. RNA-oligonucleotides as well as viral RNA are also recognized by TLR7 [6,7], especially in plasmacytoid dendritic cells (pDC). The quality and the extent of nucleotide modifications are critical for recognition by these receptors [8]. Importantly, host-derived RNA shows a much higher frequency of RNA modifications than RNA derived from prokaryotic sources [8]. Nucleic acid modifications therefore represent a structural feature, which enables the immune system to distinguish self from non-self RNA. Additionally, GU-and U-rich sequence motives were identified to play a role in TLR7-and TLR8-mediated recognition [6,9,10]. In general, TLR triggering in innate immune cells results in a strong NF-kB response. Exceptionally, TLR 7 and 9 signaling in pDC also leads to an IFN-regulatory factor 7 (IRF7) activation, resulting in type-I IFN secretion [11].The cytosolic RNA helicases retinoic-acid inducible gene I (RIG-I) and melanoma-differentiation-associated gene 5 (MDA5) A third class of innate immune receptors is the cytosolic NODlike receptors. Particularly, NACHT-, LRR-and pyrin domaincontaining protein 3 (...

Bacterial RNA: An Underestimated Stimulus for Innate Immune Responses

“…pDCs are professional producers of type I IFN and are equipped with a limited set of innate immune receptors, including TLR7 and TLR9. Experiments with FLT3L-generated murine pDCs demonstrated that IFN-a production upon bacterial RNA stimulation was abolished in TLR7-deficient pDCs (28,34). Similar results were observed in human pDCs in which TLR7 expression was silenced by small interfering RNA (siRNA) (32).…”

“…Similarly, both human PBMCs and murine bone marrow-derived macrophages (BMDMs) responded to intracellular delivery of total bacterial RNA with TNF production (27)(28)(29)(30). In addition to inducing classical NF-kB-dependent cytokines, bacterial RNA has been identified as a strong trigger of IFN-a in plasmacytoid dendritic cells (pDCs) and of IFN-b in myeloid dendritic cells (mDCs) in an IRF-dependent manner (28,(30)(31)(32)(33)(34). Depending on the model system, the involvement of IRF1, IRF3, IRF5, or IRF7 has been implicated (30,31,33,35).…”

“…Two important points to consider are the release of RNA from bacteria and, thus, the mechanisms of how to get access to any innate immune receptor and second, a point that is often overlooked, the concentration necessary to trigger an immune response. From the studies using purified bacterial RNA, it seems obvious that bacterial RNA, in general, can activate innate immune cells, because Gram-positive and Gram-negative RNA preparations induced cytokine secretion (28,30,31,36,38,39,64). Yet the contribution of RNA to whole bacteria recognition has been less well analyzed.…”

Early Inhibition of IL-1β Expression by IFN-γ Is Mediated by Impaired Binding of NF-κB to the IL-1β Promoter but Is Independent of Nitric Oxide

“…Although the critical roles for bacterial DNA and viral RNA in the induction of innate immune responses have been acknowledged for long time, the importance of bRNA recognition has only recently started to be unraveled. It was demonstrated that intracellular delivery of bRNA induced secretion of type I IFNs, as well as proinflammatory cytokines, including TNF, IL-12p40, or IL-1b (10,(14)(15)(16)(17). In plasmacytoid DCs, bRNA-induced production of type I IFNs is mediated by TLR7 (15), whereas the murine receptor TLR13 was identified only recently to sense bRNA in both bone marrow-derived macrophages (BMDMs) and DCs by recognizing a unique nucleotide motif present in the bacterial 23S rRNA (18,19).…”

“…It was demonstrated that intracellular delivery of bRNA induced secretion of type I IFNs, as well as proinflammatory cytokines, including TNF, IL-12p40, or IL-1b (10,(14)(15)(16)(17). In plasmacytoid DCs, bRNA-induced production of type I IFNs is mediated by TLR7 (15), whereas the murine receptor TLR13 was identified only recently to sense bRNA in both bone marrow-derived macrophages (BMDMs) and DCs by recognizing a unique nucleotide motif present in the bacterial 23S rRNA (18,19). Activation of TLR13 triggers the recruitment of the common TLR adapter molecule MyD88, resulting in activation of NF-kB and transcription of NF-kB-dependent cytokines (19).…”