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The research target has been confirmed as mouse Toll-like receptor 13 (TLR13), encoded by the Tlr13 gene, corresponding to UniProt accession Q6R5N8. TLR13 belongs to the Toll-like receptor family and is present in mice but absent from the human genome, where TLR8 is considered the closest functional counterpart (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3). The protein contains characteristic domains including leucine-rich repeats (LRR) and a Toll/interleukin-1 receptor (TIR) domain, consistent with the pattern-recognition receptor architecture of the TLR family (jiang2020molecularcharacterizationand pages 15-16, jiang2020molecularcharacterizationand pages 13-15, moghaddam2024regulationofimmune pages 3-5).
TLR13 functions as a highly sequence-specific pattern-recognition receptor for bacterial ribosomal RNA. The receptor recognizes a conserved motif within bacterial 23S ribosomal RNA (23S rRNA), distinguishing it from other nucleic acid sensors through its remarkable substrate specificity (sato2025rnaset2deficiency pages 3-4, milillo2022microbialrnathe pages 4-6, sato2023microbiomessrnaas pages 4-6). Rather than responding indiscriminately to RNA, TLR13 detects a defined consensus sequence of approximately 13 nucleotides with the pattern 5′-XCGGAAAGACCXX-3′ (lind2022regulationofthe pages 8-9).
Recent structural studies have identified the precise sequence recognized by TLR13 as the 15-nucleotide motif 5′-ACGGAAAGACCCCGU-3′ within bacterial 23S rRNA (inujima2026bacterialremnantsin pages 13-19). Importantly, a minimal 12-nucleotide core sequence is sufficient to activate TLR13 signaling, demonstrating that ligand recognition depends on this conserved bacterial RNA element rather than requiring full-length ribosomal RNA (sato2025rnaset2deficiency pages 3-4). This sequence is conserved across diverse bacterial species, enabling TLR13 to serve as a broad bacterial sensor while maintaining specificity for prokaryotic rather than eukaryotic RNA (milillo2022microbialrnathe pages 4-6, sato2023microbiomessrnaas pages 4-6).
The substrate specificity of TLR13 positions it uniquely within the innate immune system. Unlike enzymatic receptors that catalyze chemical reactions, TLR13 functions as a molecular sensor that initiates immune signaling cascades upon detecting its bacterial RNA ligand (sato2025rnaset2deficiency pages 3-4, inujima2026bacterialremnantsin pages 13-19). This recognition mechanism allows the immune system to discriminate between self and non-self nucleic acids, as the target sequence is characteristic of bacterial but not mammalian ribosomes (milillo2022microbialrnathe pages 4-6, lind2022regulationofthe pages 8-9).
TLR13 carries out its function within intracellular endosomal and endolysosomal compartments, where it is positioned to encounter bacterial RNA released from phagocytosed or internalized microorganisms (jiang2020molecularcharacterizationand pages 13-15, moghaddam2024regulationofimmune pages 3-5, sato2025rnaset2deficiency pages 2-3). This endosomal localization is a defining feature of nucleic acid-sensing TLRs and serves an important regulatory function by restricting receptor activation to compartments containing foreign material while limiting exposure to host RNA in the cytoplasm or extracellular space (sato2023microbiomessrnaas pages 4-6, lind2022regulationofthe pages 2-3).
Immunofluorescence studies have demonstrated that TLR13 exhibits a punctate cytoplasmic distribution pattern consistent with association with endosomal vesicles, distinct from nuclear or plasma membrane localization (jiang2020molecularcharacterizationand pages 13-15). This compartmentalization is critical for preventing inappropriate activation by self-nucleic acids and maintaining immune homeostasis (lind2022regulationofthe pages 2-3).
The proper trafficking and function of TLR13 depends absolutely on UNC93B1, a 12-pass transmembrane chaperone protein that is essential for all nucleic acid-sensing TLRs (lind2022regulationofthe pages 2-3, sato2025rnaset2deficiency pages 2-3). UNC93B1 binds to TLR13 in the endoplasmic reticulum and mediates its export from the ER, stability during transit, and delivery to functional endosomal compartments (lind2022regulationofthe pages 2-3). Genetic deletion of Unc93b1 abolishes TLR13-dependent phenotypes, confirming the absolute requirement for this trafficking factor (sato2025rnaset2deficiency pages 2-3). Following synthesis in the ER, TLR13 transits through the classical secretory pathway to reach endosomes, where ligand engagement and signaling occur (lind2022regulationofthe pages 2-3).
TLR13 exhibits the canonical three-domain architecture characteristic of Toll-like receptors (jiang2020molecularcharacterizationand pages 15-16, jiang2020molecularcharacterizationand pages 13-15, moghaddam2024regulationofimmune pages 3-5). The protein consists of:
N-terminal ectodomain: Contains multiple leucine-rich repeat (LRR) motifs that form a horseshoe-shaped structure responsible for ligand recognition. The LRR domain forms the luminal, ligand-binding region of the receptor, with the specific arrangement and number of LRR motifs determining substrate specificity (jiang2020molecularcharacterizationand pages 15-16, jiang2020molecularcharacterizationand pages 13-15, lind2022regulationofthe pages 8-9).
Transmembrane domain: A single membrane-spanning helix that anchors the receptor in the endosomal membrane (jiang2020molecularcharacterizationand pages 13-15, moghaddam2024regulationofimmune pages 3-5).
C-terminal intracellular TIR domain: The Toll/interleukin-1 receptor (TIR) domain serves as the signaling module, recruiting adaptor proteins upon ligand-induced receptor dimerization (jiang2020molecularcharacterizationand pages 15-16, jiang2020molecularcharacterizationand pages 13-15, lind2022regulationofthe pages 8-9, moghaddam2024regulationofimmune pages 3-5).
The LRR ectodomain contains conserved cysteine residues that likely form disulfide bonds, stabilizing the extracellular structure required for precise RNA sequence recognition (jiang2020molecularcharacterizationand pages 15-16, jiang2020molecularcharacterizationand pages 13-15). Upon ligand binding, conformational changes promote receptor dimerization, bringing the intracellular TIR domains into proximity to initiate downstream signaling (lind2022regulationofthe pages 8-9).
TLR13 signals exclusively through the MyD88-dependent pathway, distinguishing it from TLR3, which uses TRIF as its primary adaptor (kim2023tolllikereceptor4 pages 9-10, moghaddam2024regulationofimmune pages 5-6, rodrigues2024tlr10anintriguing pages 1-2, wang2024toll‐likereceptorsin pages 2-5). Upon recognition of bacterial 23S rRNA, TLR13 dimerization triggers recruitment of the TIR domain-containing adaptor protein MyD88 to the receptor's cytoplasmic TIR domain (kim2023tolllikereceptor4 pages 9-10, moghaddam2024regulationofimmune pages 5-6). This initiates a well-characterized signaling cascade:
MyD88 recruitment: The TLR13 TIR domain directly engages MyD88 following ligand-induced dimerization (moghaddam2024regulationofimmune pages 5-6, wang2024toll‐likereceptorsin pages 2-5).
IRAK activation: MyD88 recruits and activates IRAK4 (IL-1 receptor-associated kinase 4), which in turn phosphorylates and activates IRAK1 and IRAK2 (moghaddam2024regulationofimmune pages 5-6, wang2024toll‐likereceptorsin pages 2-5, chen2025patternrecognitionreceptors pages 15-16).
TRAF6 engagement: Activated IRAK1 associates with TRAF6 (TNF receptor-associated factor 6), a ubiquitin ligase that serves as a critical signaling node (moghaddam2024regulationofimmune pages 5-6, wang2024toll‐likereceptorsin pages 2-5, chen2025patternrecognitionreceptors pages 15-16).
Transcription factor activation: TRAF6, together with TAB2 and other downstream components, activates the transcription factors NF-κB (nuclear factor kappa B), AP-1 (activator protein 1), and MAP kinases (mitogen-activated protein kinases) (kim2023tolllikereceptor4 pages 9-10, moghaddam2024regulationofimmune pages 3-5, moghaddam2024regulationofimmune pages 5-6, wang2024toll‐likereceptorsin pages 2-5).
This canonical pathway, designated the TLR13–MyD88–NF-κB signaling axis, drives expression of inflammatory and antimicrobial genes (kim2023tolllikereceptor4 pages 9-10).
Recent research has revealed that TLR13 signaling outcomes are highly context-dependent. In addition to classical inflammatory responses, TLR13 can activate tissue-protective programs in specific macrophage populations. In hepatic Ly6C-low macrophages exposed to microbiome-derived bacterial RNA, TLR13 activation induces the transcription factors LXRα (liver X receptor alpha) and MafB, which drive expression of tissue-clearance and immunoregulatory molecules including CD5L, C1qb, Axl, and Tgm2 (sato2023microbiomessrnaas pages 4-6). This tissue-protective program operates through Syk, ERK, mTOR, and β-catenin signaling components and is associated with resistance to liver injury rather than classical inflammation (sato2023microbiomessrnaas pages 4-6, sato2025rnaset2deficiency pages 4-5).
The specific inflammatory mediators produced following TLR13 activation vary by cell type and context:
Antibacterial inflammatory responses: In alveolar macrophages and microglia responding to pneumococcal infection, TLR13 contributes to production of neutrophil-attracting chemokines CXCL1 and CXCL2, as well as proinflammatory cytokines IL-1β, TNF-α, IL-12p70, and IFN-γ (fama2020nucleicacidsensingtolllike pages 11-13, fama2020nucleicacidsensingtolllike pages 9-11). These mediators promote neutrophil recruitment, phagocyte activation, and bacterial clearance (fama2020nucleicacidsensingtolllike pages 11-13, fama2020nucleicacidsensingtolllike pages 9-11).
Tissue-protective responses: In splenic and hepatic macrophages from RNase T2-deficient mice, TLR13 activation preferentially induces the anti-inflammatory cytokine IL-10, with minimal induction of TNF-α, IL-6, or type I interferons (IFNβ1) (sato2025rnaset2deficiency pages 6-7, sato2025rnaset2deficiency pages 4-5). This demonstrates that TLR13 can drive either inflammatory or immunoregulatory programs depending on cellular context and the nature of RNA ligand presentation (sato2025rnaset2deficiency pages 6-7, sato2025rnaset2deficiency pages 4-5).
TLR13 is expressed predominantly in myeloid immune cells. Expression has been documented in splenic macrophages, hepatic macrophages, Kupffer cells (liver-resident macrophages), Ly6C-high and Ly6C-low monocyte/macrophage populations, neutrophils, conventional dendritic cells, alveolar macrophages, and microglia (sato2025rnaset2deficiency pages 3-4, fama2020nucleicacidsensingtolllike pages 2-3, fama2020nucleicacidsensingtolllike pages 11-13). Notably, TLR13 expression is absent or minimal in lymphoid cells including T cells, B cells, and plasmacytoid dendritic cells (sato2025rnaset2deficiency pages 3-4). This expression pattern positions TLR13 as a sensor within the innate immune system's myeloid compartment.
TLR13 plays important roles in host defense against bacterial pathogens, particularly Gram-positive bacteria. Studies using knockout mice have demonstrated that TLR13 cooperates with other endosomal TLRs (TLR7 and TLR9) in detecting and responding to Streptococcus pneumoniae, the causative agent of pneumococcal pneumonia, meningitis, and sepsis (fama2020nucleicacidsensingtolllike pages 2-3, fama2020nucleicacidsensingtolllike pages 11-13). Mice lacking all three endosomal RNA/DNA-sensing receptors (TLR7/9/13 triple knockout) exhibited extreme susceptibility to pneumococcal lung infection, with impaired chemokine production by alveolar macrophages, defective neutrophil recruitment to the lungs, and rapid systemic bacterial dissemination (fama2020nucleicacidsensingtolllike pages 2-3, fama2020nucleicacidsensingtolllike pages 11-13). The phenotype of triple-knockout mice closely resembled that of MyD88-deficient mice, indicating that nucleic acid sensing through these endosomal TLRs accounts for much of the MyD88-dependent defense against pneumococci (fama2020nucleicacidsensingtolllike pages 2-3).
TLR13 has also been implicated in immune responses to other bacterial pathogens, including Klebsiella pneumoniae, Streptococcus pyogenes (group A streptococcus), and group B streptococci (dyckhoffshen2024combiningantibioticwith pages 15-16). The common feature linking these pathogens is that TLR13 detects conserved bacterial 23S rRNA sequences released when bacteria are phagocytosed and degraded within endolysosomal compartments (dyckhoffshen2024combiningantibioticwith pages 15-16, moghaddam2024regulationofimmune pages 3-5, kim2023tolllikereceptor4 pages 9-10).
A major recent advance has been the recognition that TLR13 senses RNA from the commensal microbiome, not just from invading pathogens. Bacterial 23S rRNA sequences derived from intestinal bacteria such as Escherichia coli, Lactobacillus helveticus, and Lactococcus lactis can be detected in the circulation and within hepatic macrophages (sato2023microbiomessrnaas pages 4-6, sato2025rnaset2deficiency pages 13-14). This microbiome-derived RNA continuously activates TLR13 in tissue-resident macrophages, particularly in the liver (sato2023microbiomessrnaas pages 4-6).
In mice deficient for the endosomal RNase T2, undegraded bacterial RNA accumulates in endosomes and causes enhanced TLR13 activation, leading to proliferation and accumulation of tissue-protective macrophages in the spleen and liver (sato2025rnaset2deficiency pages 3-4, sato2025rnaset2deficiency pages 13-14, sato2025rnaset2deficiency pages 2-3). This activation is antibiotic-sensitive, confirming that the ligand originates from living commensal bacteria (sato2023microbiomessrnaas pages 4-6, sato2025rnaset2deficiency pages 3-4). Rather than causing harmful inflammation, this chronic low-level TLR13 stimulation by microbiome RNA promotes a tissue-protective macrophage phenotype characterized by expression of CD5L, complement component C1qb, and the receptor tyrosine kinase Axl (sato2023microbiomessrnaas pages 4-6).
TLR13, together with the cell-surface receptor TLR2, has been identified as a master sensor of Streptococcus pneumoniae in the cerebrospinal fluid during bacterial meningitis (dyckhoffshen2024combiningantibioticwith pages 2-3). Mice lacking both TLR2 and TLR13 phenocopy the severe susceptibility and impaired immune responses observed in MyD88-deficient mice, indicating that these two receptors account for most of the MyD88-dependent recognition of pneumococci in the central nervous system (dyckhoffshen2024combiningantibioticwith pages 2-3).
This finding has important therapeutic implications. While TLR2/TLR13 activation is necessary for initial bacterial detection and immune cell recruitment, excessive signaling through these receptors also drives brain-destructive inflammation that contributes to neurological sequelae of meningitis (dyckhoffshen2024combiningantibioticwith pages 2-3). In preclinical mouse models, combining antibiotic therapy (ceftriaxone) with pharmacological inhibition of TLR2 (using neutralizing antibody T2.5) and endosomal RNA sensing (using chloroquine to block TLR13 and its human ortholog TLR8) significantly improved clinical outcomes compared to antibiotic treatment alone or antibiotics combined with dexamethasone, the current standard adjunctive therapy (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3). This demonstrates the potential for targeting TLR13 as a host-directed therapy to reduce immunopathology while antibiotics clear the infection (dyckhoffshen2024combiningantibioticwith pages 2-3, dyckhoffshen2024combiningantibioticwith pages 15-16).
Recent studies have revealed an unexpected protective role for TLR13 in the liver. Mice with genetically enhanced TLR13 signaling due to RNase T2 deficiency exhibit resistance to experimental acute liver injury induced by acetaminophen (APAP) overdose or by combined LPS and D-galactosamine administration (sato2023microbiomessrnaas pages 4-6, sato2023microbiomessrnaas pages 8-10, sato2023microbiomessrnaas pages 1-4, sato2025rnaset2deficiency pages 2-3). This protection is mediated by TLR13-dependent replenishment and activation of Kupffer cells and other hepatic macrophages that express tissue-clearance molecules and limit inflammatory damage (sato2025rnaset2deficiency pages 3-4, sato2023microbiomessrnaas pages 4-6, sato2025rnaset2deficiency pages 2-3).
In wild-type mice, repeated administration of TLR13 ligands (but not ligands for other TLRs) selectively expanded Kupffer cell populations expressing immunoregulatory genes, recapitulating the protective phenotype observed in RNase T2-deficient mice (sato2023microbiomessrnaas pages 4-6, sato2023microbiomessrnaas pages 8-10). These findings suggest that controlled stimulation of TLR13 could potentially be exploited therapeutically to enhance hepatic resilience against toxic or inflammatory injury (sato2023microbiomessrnaas pages 4-6, sato2023microbiomessrnaas pages 1-4).
An important consideration for translating mouse TLR13 biology to human medicine is that TLR13 is absent from the human genome (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3). However, human TLR8 appears to serve as the functional counterpart, recognizing bacterial and viral single-stranded RNA within endosomes (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3). Studies of pneumococcal sensing in human blood cells have shown that chloroquine, which inhibits endosomal acidification required for both TLR13 and TLR8, blocks responses to pneumococcal RNA in both mouse and human immune cells (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3). This cross-species functional analogy supports the relevance of mouse TLR13 research for understanding human immune responses and for developing therapies targeting endosomal RNA sensing (dyckhoffshen2024combiningantibioticwith pages 2-3).
The availability and presentation of TLR13 ligands are tightly regulated by endosomal RNases, particularly RNase T2 (sato2025rnaset2deficiency pages 3-4, sato2025rnaset2deficiency pages 13-14, sato2025rnaset2deficiency pages 2-3). RNase T2 functions in endolysosomes to degrade RNA, limiting the accumulation of TLR13 ligands and preventing excessive receptor activation (sato2025rnaset2deficiency pages 3-4, sato2025rnaset2deficiency pages 2-3). In the absence of RNase T2, bacterial 23S rRNA fragments accumulate in endosomal compartments and produce enhanced TLR13 responses (sato2025rnaset2deficiency pages 3-4, sato2025rnaset2deficiency pages 13-14, sato2025rnaset2deficiency pages 2-3). This regulatory mechanism ensures that TLR13 activation is proportional to the actual burden of bacterial material and prevents autoimmune or autoinflammatory responses to host RNA (sato2025rnaset2deficiency pages 3-4, sato2025rnaset2deficiency pages 2-3).
| Year | Key Finding / Development | Research Context | Citation |
|---|---|---|---|
| 2025 | Peer-reviewed work showed that loss of endosomal RNase T2 permits microbiota-derived 23S rRNA to accumulate and activate TLR13, driving replenishment of tissue-protective Kupffer cells and resistance to experimental acetaminophen- and LPS/D-galactosamine-induced liver injury. The study also confirmed sequence-dependent activation by bacterial 23S-rRNA fragments, including a minimal 12-nt stimulatory region. | Microbiome RNA sensing, lysosomal RNA homeostasis, hepatic macrophage biology, and acute liver injury | (sato2025rnaset2deficiency pages 3-4, sato2025rnaset2deficiency pages 13-14, sato2025rnaset2deficiency pages 2-3) |
| 2024 | Mouse TLR13 and TLR2 were identified as major sensors driving host responses and damaging inflammation in experimental pneumococcal meningitis. Combining ceftriaxone with TLR2 blockade and chloroquine-mediated inhibition of endosomal RNA sensing improved disease outcomes compared with antibiotic treatment alone or ceftriaxone plus dexamethasone. | Preclinical host-directed therapy for pneumococcal meningitis | (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3) |
| 2024 | Translational comparisons treated human TLR8 as the functional counterpart of murine TLR13 for sensing pneumococcal RNA. Combined TLR2 and endosomal RNA-sensor inhibition suppressed activation of both mouse and human primary immune cells exposed to antibiotic-treated pneumococci. | Cross-species interpretation of mouse TLR13 biology and human therapeutic relevance | (dyckhoffshen2024combiningantibioticwith pages 15-16, dyckhoffshen2024combiningantibioticwith pages 2-3) |
| 2023 | A preprint reported that circulating microbiome-derived 23S rRNAs—including sequences attributed to Escherichia coli and Lactobacillus helveticus/Lactococcus lactis—reach hepatic macrophages and activate TLR13. This induced LXRα–MafB-dependent expression of CD5L, C1qb, Axl, and other tissue-clearance genes; the work was subsequently developed into the 2025 peer-reviewed study. | Microbiome–liver communication and tissue-protective macrophage programming | (sato2023microbiomessrnaas pages 4-6, sato2023microbiomessrnaas pages 1-4) |
| 2022 | A major review consolidated the sequence specificity of TLR13: it recognizes a defined 13-nt consensus, 5′-XCGGAAAGACCXX-3′, within bacterial 23S rRNA, rather than responding indiscriminately to RNA. It also summarized UNC93B1-dependent delivery of nucleic-acid-sensing TLRs from the endoplasmic reticulum to endosomes. | Molecular specificity, receptor structure, and endosomal trafficking | (lind2022regulationofthe pages 2-3, lind2022regulationofthe pages 8-9) |
| 2020 | TLR13 was shown to cooperate with TLR7 and TLR9 in pulmonary defense against Streptococcus pneumoniae. Tlr7/9/13 triple-knockout mice were highly susceptible to pneumococcal pneumonia, with impaired macrophage production of CXCL1/CXCL2 and other mediators and defective neutrophil recruitment; single-receptor loss produced milder, tissue-dependent effects. | Antibacterial innate immunity, pneumonia, systemic spread, and CNS involvement | (fama2020nucleicacidsensingtolllike pages 2-3, fama2020nucleicacidsensingtolllike pages 11-13, fama2020nucleicacidsensingtolllike pages 9-11) |
Table: Chronological summary of major TLR13 advances from 2020–2025, spanning RNA-sequence specificity, pneumococcal disease, microbiome sensing, liver-protective macrophage programs, and comparison with human TLR8.
| Feature Category | Details | Key References |
|---|---|---|
| Ligand / substrate | Sequence-selective pattern-recognition receptor for single-stranded bacterial 23S rRNA. The reported consensus is 5′-XCGGAAAGACCXX-3′; an active 15-nt ligand is 5′-ACGGAAAGACCCCGU-3′, and a minimal 12-nt segment can activate TLR13. Recognition is therefore not a catalytic reaction or indiscriminate response to RNA. | (sato2025rnaset2deficiency pages 3-4, inujima2026bacterialremnantsin pages 13-19, lind2022regulationofthe pages 8-9) |
| Subcellular localization | Integral membrane receptor functioning in intracellular endosomal/endolysosomal compartments, where bacterial RNA delivered by uptake or phagocytosis becomes accessible. This compartmentalization limits exposure to extracellular or host RNA and helps control self-reactivity. | (moghaddam2024regulationofimmune pages 3-5, sato2025rnaset2deficiency pages 2-3, lind2022regulationofthe pages 2-3) |
| Structural features | Canonical TLR architecture: an N-terminal signal peptide and luminal, horseshoe-shaped leucine-rich-repeat (LRR) ectodomain for RNA recognition; one transmembrane helix; and a cytosolic Toll/IL-1 receptor (TIR) domain for adaptor recruitment. Ligand engagement promotes receptor dimerization and signaling. | (jiang2020molecularcharacterizationand pages 15-16, jiang2020molecularcharacterizationand pages 13-15, lind2022regulationofthe pages 8-9, moghaddam2024regulationofimmune pages 3-5) |
| Trafficking requirements | UNC93B1, a 12-pass transmembrane chaperone, binds nucleic-acid-sensing TLRs in the endoplasmic reticulum and is required for their ER export, stability, and delivery to signaling-competent endosomes. Genetic loss of Unc93b1 abrogates the TLR13-dependent phenotype associated with RNase T2 deficiency. | (lind2022regulationofthe pages 2-3, sato2025rnaset2deficiency pages 2-3) |
| Signaling pathway | TLR13 is MyD88-dependent. TIR-domain engagement recruits MyD88, followed by IRAK4 and IRAK1/2, TRAF6, and downstream NF-κB and MAPK activation. Context-specific macrophage programs can additionally involve Syk–ERK–mTOR–β-catenin and the transcription factors LXRα and MafB. Evidence does not establish TRIF as a TLR13 adaptor. | (kim2023tolllikereceptor4 pages 9-10, moghaddam2024regulationofimmune pages 5-6, sato2023microbiomessrnaas pages 4-6, wang2024toll‐likereceptorsin pages 2-5, sato2025rnaset2deficiency pages 4-5) |
| Downstream mediators | In antibacterial responses, TLR13-containing endosomal pathways contribute to CXCL1, CXCL2, IL-1β, TNF-α, IL-12p70, and IFN-γ, promoting neutrophil recruitment and microbial control. In RNase T2-deficient splenic macrophages, signaling instead favors IL-10 with little induction of TNF-α, IL-6, or IFN-β, demonstrating strong tissue and stimulus dependence. | (fama2020nucleicacidsensingtolllike pages 11-13, fama2020nucleicacidsensingtolllike pages 9-11, sato2025rnaset2deficiency pages 4-5) |
| Cell types | Demonstrated principally in myeloid cells: splenic and hepatic macrophages, Kupffer cells, Ly6C-high and Ly6C-low monocyte/macrophage populations, neutrophils, conventional dendritic cells, alveolar macrophages, and microglia. One recent mouse study detected little or no expression in T cells, B cells, or plasmacytoid dendritic cells. | (sato2025rnaset2deficiency pages 3-4, fama2020nucleicacidsensingtolllike pages 2-3, fama2020nucleicacidsensingtolllike pages 11-13) |
| Disease contexts and applications | Supports recognition and clearance of bacterial pathogens, including Streptococcus pneumoniae and other streptococci; TLR7/9/13 triple deficiency markedly compromises pneumococcal lung defense. Conversely, TLR2/TLR13-driven inflammation contributes to brain injury in pneumococcal meningitis, motivating combined antibiotic and receptor-inhibition strategies in mice. Microbiome-derived RNA can also drive TLR13-dependent replenishment of tissue-protective Kupffer cells and resistance to experimental acetaminophen- or LPS/D-galactosamine-induced liver injury. | (dyckhoffshen2024combiningantibioticwith pages 2-3, dyckhoffshen2024combiningantibioticwith pages 15-16, fama2020nucleicacidsensingtolllike pages 2-3, sato2023microbiomessrnaas pages 1-4, sato2025rnaset2deficiency pages 2-3) |
Table: This table integrates the experimentally supported ligand specificity, cellular location, architecture, trafficking, signaling outputs, expression pattern, and disease relevance of mouse TLR13. It also distinguishes canonical antibacterial inflammation from recently identified tissue-protective macrophage programs.
TLR13 is a specialized endosomal pattern-recognition receptor that detects a highly conserved sequence within bacterial 23S ribosomal RNA, serving as a sensitive and specific sensor of bacterial presence within phagocytic cells. Its function extends beyond simple pathogen detection to include both inflammatory responses for bacterial clearance and tissue-protective programs that support macrophage homeostasis and organ resilience. The receptor operates through MyD88-dependent signaling to activate NF-κB and MAPK pathways, with context-dependent transcriptional programs determined by cell type and the nature of RNA ligand presentation.
Current research highlights TLR13's dual roles in host defense against pneumococcal and other bacterial infections, and in microbiome sensing that maintains tissue-protective macrophage populations in the liver. Therapeutic targeting of TLR13 shows promise for reducing harmful inflammation in bacterial meningitis while preserving or enhancing its beneficial tissue-protective functions. The relationship between mouse TLR13 and human TLR8 provides a framework for translating these findings to human disease, with ongoing investigation of endosomal RNA-sensing pathways as targets for host-directed immunotherapy.
Key URLs and Publication Dates:
Major recent publications include:
- Sato et al. (2025), Journal of Experimental Medicine - RNase T2 deficiency and Kupffer cell protection (https://doi.org/10.1084/jem.20230647)
- Inujima et al. (2026), Communications Biology - Bacterial RNA sequence specificity (https://doi.org/10.1038/s42003-026-10452-0)
- Dyckhoff-Shen et al. (2024), JCI Insight - Pneumococcal meningitis therapy (https://doi.org/10.1172/jci.insight.165737)
- Lind et al. (2022), Nature Reviews Immunology - Regulation of nucleic acid-sensing TLRs (https://doi.org/10.1038/s41577-021-00577-0)
- Famà et al. (2020), mBio - Pneumococcal innate immunity (https://doi.org/10.1128/mbio.00415-20)
References
(dyckhoffshen2024combiningantibioticwith pages 15-16): Susanne Dyckhoff-Shen, Ilias Masouris, Heba Islam, Sven Hammerschmidt, Barbara Angele, Veena Marathe, Jan Buer, Stefanie Völk, Hans-Walter Pfister, Matthias Klein, Uwe Koedel, and Carsten J. Kirschning. Combining antibiotic with anti-tlr2/tlr13 therapy prevents brain pathology in pneumococcal meningitis. JCI Insight, Feb 2024. URL: https://doi.org/10.1172/jci.insight.165737, doi:10.1172/jci.insight.165737. This article has 9 citations and is from a domain leading peer-reviewed journal.
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(sato2025rnaset2deficiency pages 2-3): Ryota Sato, Kaiwen Liu, Takuma Shibata, Katsuaki Hoshino, Kiyoshi Yamaguchi, Toru Miyazaki, Ryosuke Hiranuma, Ryutaro Fukui, Yuji Motoi, Yuri Fukuda-Ohta, Yun Zhang, Tatjana Reuter, Yuko Ishida, Toshikazu Kondo, Tomoki Chiba, Hiroshi Asahara, Masato Taoka, Yoshio Yamauchi, Toshiaki Isobe, Tsuneyasu Kaisho, Yoichi Furukawa, Eicke Latz, Kohta Nakatani, Yoshihiro Izumi, Yunzhong Nie, Hideki Taniguchi, and Kensuke Miyake. Rnase t2 deficiency promotes tlr13-dependent replenishment of tissue-protective kupffer cells. The Journal of Experimental Medicine, Jan 2025. URL: https://doi.org/10.1084/jem.20230647, doi:10.1084/jem.20230647. This article has 8 citations.
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(sato2025rnaset2deficiency pages 4-5): Ryota Sato, Kaiwen Liu, Takuma Shibata, Katsuaki Hoshino, Kiyoshi Yamaguchi, Toru Miyazaki, Ryosuke Hiranuma, Ryutaro Fukui, Yuji Motoi, Yuri Fukuda-Ohta, Yun Zhang, Tatjana Reuter, Yuko Ishida, Toshikazu Kondo, Tomoki Chiba, Hiroshi Asahara, Masato Taoka, Yoshio Yamauchi, Toshiaki Isobe, Tsuneyasu Kaisho, Yoichi Furukawa, Eicke Latz, Kohta Nakatani, Yoshihiro Izumi, Yunzhong Nie, Hideki Taniguchi, and Kensuke Miyake. Rnase t2 deficiency promotes tlr13-dependent replenishment of tissue-protective kupffer cells. The Journal of Experimental Medicine, Jan 2025. URL: https://doi.org/10.1084/jem.20230647, doi:10.1084/jem.20230647. This article has 8 citations.
(fama2020nucleicacidsensingtolllike pages 11-13): Agata Famà, Angelina Midiri, Giuseppe Mancuso, Carmelo Biondo, Germana Lentini, Roberta Galbo, Maria Miriam Giardina, Giuseppe Valerio De Gaetano, Letizia Romeo, Giuseppe Teti, and Concetta Beninati. Nucleic acid-sensing toll-like receptors play a dominant role in innate immune recognition of pneumococci. Apr 2020. URL: https://doi.org/10.1128/mbio.00415-20, doi:10.1128/mbio.00415-20. This article has 33 citations and is from a domain leading peer-reviewed journal.
(fama2020nucleicacidsensingtolllike pages 9-11): Agata Famà, Angelina Midiri, Giuseppe Mancuso, Carmelo Biondo, Germana Lentini, Roberta Galbo, Maria Miriam Giardina, Giuseppe Valerio De Gaetano, Letizia Romeo, Giuseppe Teti, and Concetta Beninati. Nucleic acid-sensing toll-like receptors play a dominant role in innate immune recognition of pneumococci. Apr 2020. URL: https://doi.org/10.1128/mbio.00415-20, doi:10.1128/mbio.00415-20. This article has 33 citations and is from a domain leading peer-reviewed journal.
(sato2025rnaset2deficiency pages 6-7): Ryota Sato, Kaiwen Liu, Takuma Shibata, Katsuaki Hoshino, Kiyoshi Yamaguchi, Toru Miyazaki, Ryosuke Hiranuma, Ryutaro Fukui, Yuji Motoi, Yuri Fukuda-Ohta, Yun Zhang, Tatjana Reuter, Yuko Ishida, Toshikazu Kondo, Tomoki Chiba, Hiroshi Asahara, Masato Taoka, Yoshio Yamauchi, Toshiaki Isobe, Tsuneyasu Kaisho, Yoichi Furukawa, Eicke Latz, Kohta Nakatani, Yoshihiro Izumi, Yunzhong Nie, Hideki Taniguchi, and Kensuke Miyake. Rnase t2 deficiency promotes tlr13-dependent replenishment of tissue-protective kupffer cells. The Journal of Experimental Medicine, Jan 2025. URL: https://doi.org/10.1084/jem.20230647, doi:10.1084/jem.20230647. This article has 8 citations.
(fama2020nucleicacidsensingtolllike pages 2-3): Agata Famà, Angelina Midiri, Giuseppe Mancuso, Carmelo Biondo, Germana Lentini, Roberta Galbo, Maria Miriam Giardina, Giuseppe Valerio De Gaetano, Letizia Romeo, Giuseppe Teti, and Concetta Beninati. Nucleic acid-sensing toll-like receptors play a dominant role in innate immune recognition of pneumococci. Apr 2020. URL: https://doi.org/10.1128/mbio.00415-20, doi:10.1128/mbio.00415-20. This article has 33 citations and is from a domain leading peer-reviewed journal.
(sato2025rnaset2deficiency pages 13-14): Ryota Sato, Kaiwen Liu, Takuma Shibata, Katsuaki Hoshino, Kiyoshi Yamaguchi, Toru Miyazaki, Ryosuke Hiranuma, Ryutaro Fukui, Yuji Motoi, Yuri Fukuda-Ohta, Yun Zhang, Tatjana Reuter, Yuko Ishida, Toshikazu Kondo, Tomoki Chiba, Hiroshi Asahara, Masato Taoka, Yoshio Yamauchi, Toshiaki Isobe, Tsuneyasu Kaisho, Yoichi Furukawa, Eicke Latz, Kohta Nakatani, Yoshihiro Izumi, Yunzhong Nie, Hideki Taniguchi, and Kensuke Miyake. Rnase t2 deficiency promotes tlr13-dependent replenishment of tissue-protective kupffer cells. The Journal of Experimental Medicine, Jan 2025. URL: https://doi.org/10.1084/jem.20230647, doi:10.1084/jem.20230647. This article has 8 citations.
(sato2023microbiomessrnaas pages 8-10): Ryota Sato, Kaiwen Liu, Takuma Shibata, Katsuaki Hoshino, Kiyoshi Yamaguchi, Toru Miyazaki, Ryosuke Hiranuma, Ryutaro Fukui, Yuji Motoi, Yuri Fukuda-Ohta, Yun Zhang, Tatjana Reuter, Yuko Ishida, Toshikazu Kondo, Tomoki Chiba, Hiroshi Asahara, Masato Taoka, Yoshio Yamauchi, Toshiaki Isobe, Tsuneyasu Kaisho, Yoichi Furukawa, Eicke Latz, and Kensuke Miyake. Microbiome ssrna as an environmental cue to activate tlr13-dependent tissue-protective programs in cd5lhi hepatic macrophages. bioRxiv, May 2023. URL: https://doi.org/10.1101/2023.05.11.540294, doi:10.1101/2023.05.11.540294. This article has 0 citations.
(sato2023microbiomessrnaas pages 1-4): Ryota Sato, Kaiwen Liu, Takuma Shibata, Katsuaki Hoshino, Kiyoshi Yamaguchi, Toru Miyazaki, Ryosuke Hiranuma, Ryutaro Fukui, Yuji Motoi, Yuri Fukuda-Ohta, Yun Zhang, Tatjana Reuter, Yuko Ishida, Toshikazu Kondo, Tomoki Chiba, Hiroshi Asahara, Masato Taoka, Yoshio Yamauchi, Toshiaki Isobe, Tsuneyasu Kaisho, Yoichi Furukawa, Eicke Latz, and Kensuke Miyake. Microbiome ssrna as an environmental cue to activate tlr13-dependent tissue-protective programs in cd5lhi hepatic macrophages. bioRxiv, May 2023. URL: https://doi.org/10.1101/2023.05.11.540294, doi:10.1101/2023.05.11.540294. This article has 0 citations.