Recognition Mechanism of RNA by TLR13: Structural Insights and Implications for Immune Activation

RNA serves as a distinctive pathogen-associated molecular pattern (PAMP) that plays a critical role in innate immunity. However, the specific mechanisms of RNA recognition remain largely unexplored, especially given RNA's vulnerability to degradation and the absence of sequence specificity in most RNA recognition receptors. Notably, Toll-like receptor 13 (TLR13) is capable of detecting a conserved RNA sequence, RNA15 (2054-2068, ACG GAA AGA CCC CGU), within bacterial 23S rRNA, thereby triggering an immune response. To unravel the exact mechanism by which TLR13 recognizes RNA15, we combined experimental approaches with molecular dynamics simulations. Our results suggest that RNA15 adopts a stable hairpin structure in solution, protected from nuclease degradation by intramolecular interactions. TLR13 specifically recognizes this hairpin structure, leading to the dimerization of TLR13. This interaction further induces RNA15 to transition into a stem-loop-like conformation, thereby activating TLR13 downstream signaling. Additionally, our study indicates that TLR13 can form stable dimers in the membrane independently of ligand binding. Although the hairpin structure is the predominant form of RNA15 in solution, the temporary stem-loop-like structure can spontaneously bind to dimeric TLR13, initiating the immune response. These insights deepen our understanding of the complex recognition process of RNA15 by TLR13 and explore the complicated mechanisms governing innate immune system function.

Design, Synthesis, and Biological Evaluation of BODIPY-Caged Resiquimod as a Dual-Acting Phototherapeutic

Resiquimod, an imidazoquinoline scaffold, exhibits potent immunotherapeutic activity but is associated with off-target effects, limiting its clinical utility. To address this limitation, we developed a novel BODIPY-caged resiquimod that is responsive to red light, combining photocaging and photodynamic therapy functionalities. Molecular docking studies guided identification of the optimal caging site for resiquimod, effectively masking its immune activity. BODIPY-caged resiquimod remained inactive under dark conditions, effectively masking resiquimod's immunostimulatory effects. However, red light irradiation precisely uncaged resiquimod, inducing robust immune activation, even in the presence of N-acetyl cysteine as an antioxidant. Notably, the attachment of resiquimod to BODIPY reduced the dark toxicity typically associated with BODIPY as a photosensitizer. In 3D spheroid models of HeLa and A549 cells, BODIPY-caged resiquimod demonstrated spatiotemporal control over cytotoxicity, significantly enhancing cell death only upon irradiation. This dual-function therapeutic approach highlights a "win-win" strategy: precise, red-light-mediated control of immune activation and photodynamic efficacy with reduced collateral toxicity.

Discovery of Novel Isoxazole-Based Small-Molecule Toll-Like Receptor 8 Antagonists

Toll-like receptor 8 (TLR8) recognizes viral and bacterial RNA, initiating inflammatory responses that are crucial for innate immunity. Dysregulated TLR8 signaling contributes to autoimmune diseases, including systemic lupus erythematosus and rheumatoid arthritis, driving chronic inflammation and tissue damage. Therefore, targeting TLR8 has gained attention as a promising therapeutic strategy. We report a novel selective TLR8 antagonist scaffold identified through computational modeling and simulation. In silico-guided rational drug design and synthesis led to potent isoxazole-based compounds that were characterized by structure-activity relationships. The most active compounds inhibited TLR8-mediated signaling in cell lines and primary cells, reduced MyD88 recruitment, suppressed NF-κB- and IRF-dependent signaling, and decreased inflammatory responses. In silico and pharmacological analyses demonstrated competitive binding to the pocket of chemical ligands within the TLR8 dimerization interface. These highly selective and potent TLR8 antagonists possess favorable physicochemical properties, representing potential clinical candidates for TLR8-targeted therapy.

OpenMMDL - Simplifying the Complex: Building, Simulating, and Analyzing Protein-Ligand Systems in OpenMM

Molecular dynamics (MD) simulations have become an essential tool for studying the dynamics of biological systems and exploring protein-ligand interactions. OpenMM is a modern, open-source software toolkit designed for MD simulations. Until now, it has lacked a module dedicated to building receptor-ligand systems, which is highly useful for investigating protein-ligand interactions for drug discovery. We therefore introduce OpenMMDL, an open-source toolkit that enables the preparation and simulation of protein-ligand complexes in OpenMM, along with the subsequent analysis of protein-ligand interactions. OpenMMDL consists of three main components: OpenMMDL Setup, a graphical user interface based on Python Flask to prepare protein and simulation settings, OpenMMDL Simulation to perform MD simulations with consecutive trajectory postprocessing, and finally OpenMMDL Analysis to analyze simulation results with respect to ligand binding. OpenMMDL is not only a versatile tool for analyzing protein-ligand interactions and generating ligand binding modes throughout simulations; it also tracks and clusters water molecules, particularly those exhibiting minimal displacement from their previous coordinates, providing insights into solvent dynamics. We applied OpenMMDL to study ligand-receptor interactions across diverse biological systems, including LDN-193189 and LDN-212854 with ALK2 (kinases), nifedipine and amlodipine in Cav1.1 (ion channels), LSD in 5-HT2B (G-protein coupled receptors), letrozole in CYP19A1 (cytochrome P450 oxygenases), flavin mononucleotide binding the FMN-riboswitch (RNAs), ligand C08 bound to TLR8 (toll-like receptor), and PZM21 bound to MOR (opioid receptor), highlighting distinct functionalities of OpenMMDL. OpenMMDL is publicly available at https://github.com/wolberlab/OpenMMDL.

In silico design of a trivalent multi-epitope global-coverage vaccine-candidate protein against influenza viruses: evaluation by molecular dynamics and immune system simulation

Hemagglutinin (HA), a variable viral surface protein, is essential for influenza vaccine development. Annually, traditional trivalent vaccines containing influenza A/H1N1, A/H3N2 and B viruses are administered globally, which are not very effective for the mutations in HA protein. The aim of this study was to design a multi-epitope vaccine containing epitopes of the HA protein of H1N1, H3N2 and B viruses using immunoinformatics methods. The HA protein epitope prediction was performed using Immune Epitope Database. Toxicity, antigenicity and conservancy of the epitopes were evaluated using ToxinPred, VaxiJen and Epitope Conservancy Analysis tools, respectively. Then, nontoxic, antigenic and high conserved epitopes with high prediction scores were selected. Their binding affinity was evaluated against human and mouse MHC class I and II molecules using the HPEPDOCK tool. Physicochemical properties and post-translational modifications were evaluated using ProtParam, SOLpro and MusiteDeep tools, respectively. Top selected epitopes were joined using linkers to produce the best effective recombinant trivalent vaccine candidate to elicit cellular and humoral immune responses in mouse and human host models. These sequences were modeled and verified. By evaluating the results of various analyses of all models and the most similarity to the native HA protein, model 5 was selected as the best model. Finally, in silico cloning of this model as vaccine candidate was performed in pET21. This study was a computer-aided analysis for a multi-epitope trivalent recombinant vaccine candidate against influenza viruses. The efficiency of our best model of vaccine candidates should be validated using in vitro and in vivo studies. Communicated by Ramaswamy H. Sarma.

Implication of Toll-Like Receptors in growth and management of health and diseases: Special focus as a promising druggable target to Prostate Cancer

Toll-like receptors (TLRs) are protein structures belonging to the pattern recognition receptors family. TLRs have the great potential that can directly recognize the specific molecular structures on the surface of pathogens, damaged senescent cells and apoptotic host cells. Available evidence suggests that TLRs have crucial roles in maintaining tissue homeostasis through control of the inflammatory and tissue repair responses during injury. TLRs are the player of first line of defense against different microbes and activate the signaling cascades which help to induce the immune system and inflammatory responses by affecting various signaling pathways, including nuclear factor-κB (NF-κB), interferon regulatory factors, and mitogen-activated protein kinases (MAPKs). TLRs have been identified to be over-expressed in different types of cancers and play an important role in control of health and management of diseases. The current review provides updated knowledge on the implication of TLRs in growth and management of cancers including prostate cancer.

Bacteria-specific modified nucleoside is released and elevated in urine of patients with bacterial infections

Over 170 types of chemical modifications have been identified in cellular RNAs across the three domains of life. Modified RNA is eventually degraded to constituent nucleosides, and in mammals, modified nucleosides are released into the extracellular space. By contrast, the fate of modified nucleosides in bacteria remains unknown. In this study, we performed liquid chromatography-mass spectroscopy (LC-MS) analysis of modified nucleosides from the RNA of 23 pathogenic bacteria, revealing 2-methyladenosine (m2A) as a common bacteria-specific modified nucleoside detected in all bacterial RNAs. Under normal culture conditions, bacteria did not actively release most modified nucleoside species, but robustly released nucleosides, including m2A, following addition of antibiotics or immune cells. These results indicate that m2A is released following bacterial lysis. Intraperitoneal injection of mice with m2A increased detectable levels of m2A in the urine, indicating that mammals can effectively excrete m2A. Additionally, mice infected with wild-type E. coli showed higher levels of m2A in their urine than mice infected by m2A-deficient rlmN KO E. coli. This suggests that m2A from the infected bacteria is excreted in the urine. Lastly, clinical studies using urine samples from febrile patients revealed significantly elevated levels of m2A during bacterial infections, and these values did not correlate with inflammation severity markers, such as white blood count (WBC) and C-reactive protein (CRP). This study reports the mammalian metabolism of modified nucleosides derived from bacterial RNA, and the elevation of urinary m2A in patients with bacterial infections.

IMPORTANCE

This study reveals the differences in the fate and release of modified nucleosides in bacteria and mammals. Additionally, our study highlights that external bacteria-damaging factors, such as antibiotics and phagocytosis by host immune cells, promote the release of bacteria-specific modified nucleosides. Furthermore, we found that m2A was elevated in the urine from animal models of bacterial infection and the urine of patients with bacterial infections. Collectively, this work spans basic biology and clinical science, offering valuable insights into the fate of modified nucleosides in bacterial systems and their relevance to infectious diseases.

Bacterial RNA sensing by TLR8 requires RNase 6 processing and is inhibited by RNA 2'O-methylation

TLR8 senses single-stranded RNA (ssRNA) fragments, processed via cleavage by ribonuclease (RNase) T2 and RNase A family members. Processing by these RNases releases uridines and purine-terminated residues resulting in TLR8 activation. Monocytes show high expression of RNase 6, yet this RNase has not been analyzed for its physiological contribution to the recognition of bacterial RNA by TLR8. Here, we show a role for RNase 6 in TLR8 activation. BLaER1 cells, transdifferentiated into monocyte-like cells, as well as primary monocytes deficient for RNASE6 show a dampened TLR8-dependent response upon stimulation with isolated bacterial RNA (bRNA) and also upon infection with live bacteria. Pretreatment of bacterial RNA with recombinant RNase 6 generates fragments that induce TLR8 stimulation in RNase 6 knockout cells. 2'O-RNA methyl modification, when introduced at the first uridine in the UA dinucleotide, impairs processing by RNase 6 and dampens TLR8 stimulation. In summary, our data show that RNase 6 processes bacterial RNA and generates uridine-terminated breakdown products that activate TLR8.

Targeting toll-like receptor 7 as a therapeutic development strategy for systemic lupus erythematosus

Endosomal TLRs (TLR3/7/8/9) are highly analogous innate immunity sensors for various viral or bacterial RNA/DNA molecular patterns. Among them, TLR7, in particular, has been suggested to be a target for various inflammatory disorders and autoimmune diseases including systemic lupus erythematosus (SLE); but few small-molecule inhibitors with elaborated mechanism have been reported in literature. Here, we reported a well-characterized human TLR7-specific small-molecule inhibitor, TH-407b, with promising potency and negligible cytotoxicity through a novel binding mechanism. Notably, TH-407b not only effectively inhibited TLR7-mediated pro-inflammatory signaling in a variety of cultured cell lines but also demonstrated potent inflammation suppressing activities in primary peripheral blood mononuclear cells (PBMCs) derived from SLE patients. Furthermore, TH-407b showed prominent efficacy in vivo, improved survival rate and ameliorated symptoms of SLE model mice. To obtain molecular insights into the TH-407b derivatives' inhibition mechanism, we performed the structural analysis of TLR7/TH-407b complex using cryogenic electron microscopy (cryo-EM) method. As an atomistic resolution cryo-EM structure of the TLR family, it not only of value to facilitate structure-based drug design, but also shed light to methodology development of small proteins using EM. Significantly, TH-407b has unveiled an inhibition strategy for TLR7 via stabilizing its resting/inactivated state. Such a resting state could be generally applicable to all TLRs, rendering a useful method for targeting this group of important immunological receptors.

Mannose and Lactobionic Acid in Nasal Vaccination: Enhancing Antigen Delivery via C-Type Lectin Receptors

BACKGROUND/OBJECTIVES

Nasal vaccines are a promising strategy for enhancing mucosal immune responses and preventing diseases at mucosal sites by stimulating the secretion of secretory IgA, which is crucial for early pathogen neutralization. However, designing effective nasal vaccines is challenging due to the complex immunological mechanisms in the nasal mucosa, which must balance protection and tolerance against constant exposure to inhaled pathogens. The nasal route also presents unique formulation and delivery hurdles, such as the mucous layer hindering antigen penetration and immune cell access.

METHODS

This review focuses on cutting-edge approaches to enhance nasal vaccine delivery, particularly those targeting C-type lectin receptors (CLRs) like the mannose receptor and macrophage galactose-type lectin (MGL) receptor. It elucidates the roles of these receptors in antigen recognition and uptake by antigen-presenting cells (APCs), providing insights into optimizing vaccine delivery.

RESULTS

While a comprehensive examination of targeted glycoconjugate vaccine development is outside the scope of this study, we provide key examples of glycan-based ligands, such as lactobionic acid and mannose, which can selectively target CLRs in the nasal mucosa.

CONCLUSIONS

With the rise of new viral infections, this review aims to facilitate the design of innovative vaccines and equip researchers, clinicians, and vaccine developers with the knowledge to enhance immune defenses against respiratory pathogens, ultimately protecting public health.

Impact of Mygalin on Inflammatory Response Induced by Toll-like Receptor 2 Agonists and IFN-γ Activation

Several natural products are being studied to identify new bioactive molecules with therapeutic potential for infections, immune modulation, and other pathologies. TLRs are a family of receptors that play a crucial role in the immune system, constituting the first line of immune defense. They recognize specific products derived from microorganisms that activate multiple pathways and transcription factors in target cells, which are vital for producing immune mediators. Mygalin is a synthetic acylpolyamine derived from hemocytes of the spider Acanthoscurria gomesiana. This molecule negatively regulates macrophage response to LPS stimulation by interacting with MD2 in the TLR4/MD2 complex. Here, we investigated the activity of Mygalin mediated by TLR2 agonists in cells treated with Pam3CSK4 (TLR2/1), Pam2CSK4, Zymosan (TLR2/6), and IFN-γ. Our data showed that Mygalin significantly inhibited stimulation with agonists and IFN-γ, reducing NO and IL-6 synthesis, regardless of the stimulation. There was also a significant reduction in the phosphorylation of proteins NF-κB p65 and STAT-1 in cells treated with Pam3CSK4. Molecular docking assays determined the molecular structure of Mygalin and agonists Pam3CSK4, Pam2CSK4, and Zymosan, as well as their interaction and free energy with the heterodimeric complexes TLR2/1 and TLR2/6. Mygalin interacted with the TLR1 and TLR2 dimer pathway through direct interaction with the agonists, and the ligand-binding domain was similar in both complexes. However, the binding of Mygalin was different from that of the agonists, since the interaction energy with the receptors was lower than with the agonists for their receptors. In conclusion, this study showed the great potential of Mygalin as a potent natural inhibitor of TLR2/1 and TLR2/6 and a suppressor of the inflammatory response induced by TLR2 agonists, in part due to its ability to interact with the heterodimeric complexes.

Design and Optimization of Selectivity-Tunable Toll-like Receptor 7/8 Agonists as Novel Antibody-Drug Conjugate Payloads

Toll-like receptors 7 and 8 are involved in modulating the adaptive and innate immune responses, and their activation has shown promise as a therapeutic strategy in the field of immuno-oncology. While systemic exposure to TLR7/8 agonists can result in poor tolerance, combination therapies and targeted delivery through antibody-drug conjugates (ADCs) can help mitigate adverse effects. Described herein is the identification of a novel and potent series of pyrazolopyrimidine-based TLR7/8 agonists with tunable receptor selectivity. Representative agonists from this series were successfully able to induce the production of various proinflammatory cytokines and chemokines from human peripheral blood mononuclear cells. Anti-HER2-25 and anti-HER2-26 ADCs made from this class of payloads demonstrated mechanism-based activation of TLR7/8 in a THP1/N87 coculture system.

Discovery of Novel Small Molecule Dual Inhibitor Targeting Toll-Like Receptors 7 and 9

The aberrant secretion of proinflammatory cytokines by immune cells is the principal cause of inflammatory diseases, such as systemic lupus erythematosus and rheumatoid arthritis. Toll-like receptor 7 (TLR7) and TLR9, sequestered to the endosomal compartment of dendritic cells and macrophages, are closely associated with the initiation and progression of these diseases. Therefore, the development of drugs targeting dysregulated endosomal TLRs is imperative to mitigate systemic inflammation. Here, we applied the principles of computer-aided drug discovery to identify a novel low-molecular-weight compound, TLR inhibitory compound 10 (TIC10), and its potent derivative (TIC10g), which demonstrated dual inhibition of TLR7 and TLR9 signaling pathways. Compared to TIC10, TIC10g exhibited a more pronounced inhibition of the TLR7- and TLR9-mediated secretion of the proinflammatory cytokine tumor necrosis factor-α in a mouse macrophage cell line and mouse bone marrow dendritic cells in a concentration-dependent manner. While TIC10g slightly prevented TLR3 and TLR8 activation, it had no impact on cell surface TLRs (TLR1/2, TLR2/6, TLR4, or TLR5), indicating its selectivity for TLR7 and TLR9. Additionally, mechanistic studies suggested that TIC10g interfered with TLR9 activation by CpG DNA and suppressed downstream pathways by directly binding to TLR9. Western blot analysis revealed that TIC10g downregulated the phosphorylation of the p65 subunit of nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) and mitogen-activated protein kinases (MAPKs), including extracellular-signal-regulated kinase, p38-MAPK, and c-Jun N-terminal kinase. These findings indicate that the novel ligand, TIC10g, is a specific dual inhibitor of endosomal TLRs (TLR7 and TLR9), disrupting MAPK- and NF-κB-mediated proinflammatory gene expression.

Erythroid-intrinsic activation of TLR8 impairs erythropoiesis in inherited anemia

Inherited non-hemolytic anemia is a group of rare bone marrow disorders characterized by erythroid defects. Although concerted efforts have been made to explore the underlying pathogenetic mechanisms of these diseases, the understanding of the causative mutations are still incomplete. Here we identify in a diseased pedigree that a gain-of-function mutation in toll-like receptor 8 (TLR8) is implicated in inherited non-hemolytic anemia. TLR8 is expressed in erythroid lineage and erythropoiesis is impaired by TLR8 activation whereas enhanced by TLR8 inhibition from erythroid progenitor stage. Mechanistically, TLR8 activation blocks annexin A2 (ANXA2)-mediated plasma membrane localization of STAT5 and disrupts EPO signaling in HuDEP2 cells. TLR8 inhibition improves erythropoiesis in RPS19+/- HuDEP2 cells and CD34+ cells from healthy donors and inherited non-hemolytic anemic patients. Collectively, we identify a gene implicated in inherited anemia and a previously undescribed role for TLR8 in erythropoiesis, which could potentially be explored for therapeutic benefit in inherited anemia.

Structure-Activity Relationships toward the Identification of a High-Potency Selective Human Toll-like Receptor-7 Agonist

Toll-like receptor (TLR)-7 agonists are immunostimulatory vaccine adjuvants. A systematic structure-activity relationship (SAR) study of TLR7-active 1-benzyl-2-butyl-1H-imidazo[4,5-c]quinolin-4-amine led to the identification of a potent hTLR7-specific p-hydroxymethyl IMDQ 23 with an EC50 value of 0.22 μM. The SAR investigation also resulted in the identification of TLR7 selective carboxamide 12 with EC50 values of 0.32 μM for hTLR7 and 18.25 μM for hTLR8. In the vaccination study, TLR7-specific compound 23 alone or combined with alum (aluminum hydroxide wet gel) showed adjuvant activity for a spike protein immunogen in mice, with enhanced anti-spike antibody production. Interestingly, the adjuvant system comprising carboxamide 12 and alum showed prominent adjuvant activity with high levels of IgG1, IgG2b, and IgG2c in immunized mice, confirming a balanced Th1/Th2 response. In the absence of any apparent toxicity, the TLR7 selective agonists in combination with alum may make a suitable vaccine adjuvant.

Molecule engineering strategy of toll-like receptor 7/8 agonists designed for potentiating immune stimuli activation

Toll-like receptor 7/8 (TLR-7/8) agonists serve as a promising class of pattern recognition receptors that effectively evoke the innate immune response, making them promising immunomodulatory agents for tumor immunotherapy. However, the uncontrollable administration of TLR-7/8 agonists frequently leads to the occurrence of severe immune-related adverse events (irAEs). Thus, it is imperative to strategically design tumor-microenvironment-associated biomarkers or exogenous stimuli responsive TLR-7/8 agonists in order to accurately evaluate and activate innate immune responses. No comprehensive elucidation has been documented thus far regarding TLR-7/8 immune agonists that are specifically engineered to enhance immune activation. In this feature article, we provide an overview of the advancements in TLR-7/8 agonists, aiming to enhance the comprehension of their mechanisms and promote the clinical progression through nanomedicine strategies. The current challenges and future directions of cancer immunotherapy are also discussed, with the hope that this work will inspire researchers to explore innovative applications for triggering immune responses through TLR-7/8 agonists.

ACE mRNA (Additional Chimeric Element incorporated IVT mRNA) for Enhancing Protein Expression by Modulating Immunogenicity

The development of in vitro transcribed mRNA (IVT mRNA)-based therapeutics/vaccines depends on the management of IVT mRNA immunogenicity. IVT mRNA, which is used for intracellular protein translation, often triggers unwanted immune responses, interfering with protein expression and leading to reduced therapeutic efficacy. Currently, the predominant approach for mitigating immune responses involves the incorporation of costly chemically modified nucleotides like pseudouridine (Ψ) or N1-methylpseudouridine (m1Ψ) into IVT mRNA, raising concerns about expense and the potential misincorporation of amino acids into chemically modified codon sequences. Here, an Additional Chimeric Element incorporated mRNA (ACE mRNA), a novel approach incorporating two segments within a single IVT mRNA structure, is introduced. The first segment retains conventional IVT mRNA components prepared with unmodified nucleotides, while the second, comprised of RNA/DNA chimeric elements, aims to modulate immunogenicity. Notably, ACE mRNA demonstrates a noteworthy reduction in immunogenicity of unmodified IVT mRNA, concurrently demonstrating enhanced protein expression efficiency. The reduced immune responses are based on the ability of RNA/DNA chimeric elements to restrict retinoic acid-inducible gene I (RIG-I) and stimulator of interferon genes (STING)-mediated immune activation. The developed ACE mRNA shows great potential in modulating the immunogenicity of IVT mRNA without the need for chemically modified nucleotides, thereby advancing the safety and efficacy of mRNA-based therapeutics/vaccines.

Emerging evidence for endogenous neurosteroid modulation of pro-inflammatory and anti-inflammatory pathways that impact neuropsychiatric disease

This mini-review presents emerging evidence that endogenous neurosteroids modulate both pro- and anti-inflammatory signaling by immune cells and brain cells that contribute to depression, alcohol use disorders, and other inflammatory conditions. We first review the literature on pregnenolone and allopregnanolone inhibition of proinflammatory neuroimmune pathways in the periphery and the brain - effects that are independent of GABAergic mechanisms. We follow with evidence for neurosteroid enhancement of anti-inflammatory and protective pathways in brain and immune cells. These studies draw clinical relevance from a large body of evidence that pro-inflammatory immune signaling is dysregulated in many brain disorders and the fact that neurosteroids inhibit the same inflammatory pathways that are activated in depression, alcohol use disorders and other inflammatory conditions. Thus, we describe evidence that neurosteroid levels are decreased and neurosteroid supplementation has therapeutic efficacy in these neuropsychiatric conditions. We conclude with a perspective that endogenous regulation of immune balance between pro- and anti-inflammatory pathways by neurosteroid signaling is essential to prevent the onset of disease. Deficits in neurosteroids may unleash excessive pro-inflammatory activation which progresses in a feed-forward manner to disrupt brain networks that regulate stress, emotion and motivation. Neurosteroids can block various inflammatory pathways in mouse and human macrophages, rat brain and human blood and therefore provide new hope for treatment of intractable conditions that involve excessive inflammatory signaling.

Interface Gain-of-Function Mutations in TLR7 Cause Systemic and Neuro-inflammatory Disease

TLR7 recognizes pathogen-derived single-stranded RNA (ssRNA), a function integral to the innate immune response to viral infection. Notably, TLR7 can also recognize self-derived ssRNA, with gain-of-function mutations in human TLR7 recently identified to cause both early-onset systemic lupus erythematosus (SLE) and neuromyelitis optica. Here, we describe two novel mutations in TLR7, F507S and L528I. While the L528I substitution arose de novo, the F507S mutation was present in three individuals from the same family, including a severely affected male, notably given that the TLR7 gene is situated on the X chromosome and that all other cases so far described have been female. The observation of mutations at residues 507 and 528 of TLR7 indicates the importance of the TLR7 dimerization interface in maintaining immune homeostasis, where we predict that altered homo-dimerization enhances TLR7 signaling. Finally, while mutations in TLR7 can result in SLE-like disease, our data suggest a broader phenotypic spectrum associated with TLR7 gain-of-function, including significant neurological involvement.

Endosomal Toll-Like Receptors as Therapeutic Targets for Autoimmune Diseases

Nucleic acid (NA)-sensing Toll-like receptors (TLRs) reside in the endosomal compartment of innate immune cells, such as macrophages and dendritic cells. NAs transported to the endosomal compartment are degraded by DNases and RNases. Degradation products, including single-stranded DNA, oligoRNA, and nucleosides, are recognized by TLR7, TLR8, and TLR9 to drive the defense responses against pathogens. NA degradation influences endosomal TLR responses by generating and degrading TLR ligands. TLR ligand accumulation because of impaired NA degradation causes constitutive TLR activation, leading to autoinflammatory and autoimmune diseases. Furthermore, some genes associated with these diseases promote endosomal TLR responses. Therefore, endosomal TLRs are promising therapeutic targets for TLR-mediated inflammatory diseases, and novel drugs targeting TLRs are being developed.

Small molecule modulators of immune pattern recognition receptors

Pattern recognition receptors (PRRs) represent a re-emerging class of therapeutic targets for vaccine adjuvants, inflammatory diseases and cancer. In this review article, we summarize exciting developments in discovery and characterization of small molecule PRR modulators, focusing on Toll-like receptors (TLRs), NOD-like receptors (NLRs) and the cGAS-STING pathway. We also highlight PRRs that are currently lacking small molecule modulators and opportunities for chemical biology and therapeutic discovery.

Toll-Like Receptors 7/8: A Paradigm for the Manipulation of Immunologic Reactions for Immunotherapy

The innate immune system recognizes conserved features of viral and microbial pathogens through pattern recognition receptors (PRRs). Toll-like receptors (TLRs) are one type of PRR used by the innate immune system to mediate the secretion of proinflammatory cytokines and promote innate and adaptive immune responses. TLR family members TLR7 and TLR8 (referred to as TLR7/8 from herein) are endosomal transmembrane receptors that recognize purine-rich single-stranded RNA (ssRNA) and bacterial DNA, eliciting an immunologic reaction to pathogens. TLR7/8 were discovered to mediate the secretion of proinflammatory cytokines by activating immune cells. In addition, accumulating evidence has indicated that TLR7/8 may be closely related to numerous immune-mediated disorders, specifically several types of cancer, autoimmune disease, and viral disease. TLR7/8 agonists and antagonists, which are used as drugs or adjuvants, have been identified in preclinical studies and clinical trials as promising immune stimulators for the immunotherapy of these immune-mediated disorders. These results provided reasoning to further explore immunotherapy for the treatment of immune-mediated disorders. Nevertheless, numerous needs remain unmet, and the therapeutic effects of TLR7/8 agonists and antagonists are poor and exert strong immune-related toxicities. The present review aimed to provide an overview of the TLR family members, particularly TLR7/8, and address the underlying molecular mechanisms and clinical implications of TLR7/8 in immune-mediated disorders. The aim of the work is to discuss the underlying molecular mechanisms and clinical implications of TLR7/8 in immune-mediated disorders.

A novel Toll-like receptor 7/8-specific antagonist E6742 ameliorates clinically relevant disease parameters in murine models of lupus

The sensing of self RNA by the endosomal Toll-like receptors (TLRs) 7 and 8 initiates pathogenic mechanisms underlying the autoimmune disease lupus. A blockade of the TLR7/8 signals may, therefore, be a novel therapeutic intervention for lupus. To test the hypothesis, a novel compound E6742 that blocks TLR7/8 activation was identified. The mode of action of E6742 was investigated by analysis of the tertiary structure of TLR7 and 8 in complex with E6742. The in vitro activities of the compound were examined in cellular systems and its therapeutic potential was evaluated in murine lupus models. Tertiary structures of the extracellular domain of TLR7 and 8 in complex with E6742 showed that E6742 binds specifically and non-covalently to the hydrophobic pocket located at the interface of TLR7 or TLR8 homodimers. E6742 potently and selectively inhibited several TLR7/8-mediated cytokine responses in human PBMC. In two mouse models of lupus, oral dosing of E6742 after the onset of disease suppressed increase in autoantibodies and blocked the advance of organ damage. Collectively, the data show that TLR7/8 activation contributes to disease progression and its blocking by E6742 has potential as a therapeutic intervention for lupus.

Toll-like receptor-guided therapeutic intervention of human cancers: molecular and immunological perspectives

Toll-like receptors (TLRs) serve as the body's first line of defense, recognizing both pathogen-expressed molecules and host-derived molecules released from damaged or dying cells. The wide distribution of different cell types, ranging from epithelial to immune cells, highlights the crucial roles of TLRs in linking innate and adaptive immunity. Upon stimulation, TLRs binding mediates the expression of several adapter proteins and downstream kinases, that lead to the induction of several other signaling molecules such as key pro-inflammatory mediators. Indeed, extraordinary progress in immunobiological research has suggested that TLRs could represent promising targets for the therapeutic intervention of inflammation-associated diseases, autoimmune diseases, microbial infections as well as human cancers. So far, for the prevention and possible treatment of inflammatory diseases, various TLR antagonists/inhibitors have shown to be efficacious at several stages from pre-clinical evaluation to clinical trials. Therefore, the fascinating role of TLRs in modulating the human immune responses at innate as well as adaptive levels directed the scientists to opt for these immune sensor proteins as suitable targets for developing chemotherapeutics and immunotherapeutics against cancer. Hitherto, several TLR-targeting small molecules (e.g., Pam3CSK4, Poly (I:C), Poly (A:U)), chemical compounds, phytocompounds (e.g., Curcumin), peptides, and antibodies have been found to confer protection against several types of cancers. However, administration of inappropriate doses of such TLR-modulating therapeutics or a wrong infusion administration is reported to induce detrimental outcomes. This review summarizes the current findings on the molecular and structural biology of TLRs and gives an overview of the potency and promises of TLR-directed therapeutic strategies against cancers by discussing the findings from established and pipeline discoveries.

Structural basis for plastic glycolipid recognition of the C-type lectin Mincle

Mincle (macrophage-inducible C-type lectin, CLEC4E) is a C-type lectin immune-stimulatory receptor for cord factor, trehalose dimycolate (TDM), which serves as a potent component of adjuvants. The recognition of glycolipids by Mincle, especially their lipid parts, is poorly understood. Here, we performed nuclear magnetic resonance analysis, revealing that titration of trehalose harboring a linear short acyl chain showed a chemical shift perturbation of hydrophobic residues next to the Ca-binding site. Notably, there were split signals for Tyr201 upon complex formation, indicating two binding modes for the acyl chain. In addition, most Mincle residues close to the Ca-binding site showed no observable signals, suggesting their mobility on an ∼ ms scale even after complex formation. Mutagenesis study supported two putative lipid-binding modes for branched acyl-chain TDM binding. These results provide novel insights into the plastic-binding modes of Mincle toward a wide range of glycol- and glycerol-lipids, important for rational adjuvant development.

Surface toll-like receptor 9 on immune cells and its immunomodulatory effect

Toll like receptor 9 (TLR9) has been considered as a crucial intracellular pattern recognition receptor in the immune system, which can directly or indirectly mediate innate and adaptive immune responses by recognizing CpG DNA in endosomes to initiate its downstream signaling. However, TLR9 can also be expressed on the membrane surface of some immune and non-immune cells, called surface TLR9 (sTLR9), which covers the TLR9 and its immunomodulatory role with a mysterious veil. In this review, we mainly focus on the sTLR9 expressed on neutrophils, B cells and erythrocytes, and its immunomodulatory roles displayed alone or in coordination with endosomal TLR9 (eTLR9), providing a theoretical reference for the application of its modulators.

Imidazoquinolines with improved pharmacokinetic properties induce a high IFNα to TNFα ratio in vitro and in vivo

TLR Agonists have promising activity in preclinical models of viral infection and cancer. However, clinical use is only in topical application. Systemic uses of TLR-ligands such as Resiquimod, have failed due to adverse effects that limited dose and thus, efficacy. This issue could be related to pharmacokinetic properties that include fast elimination leading to low AUC with simultaneously high c_max at relevant doses. The high c_max is associated with a sharp, poorly tolerated cytokine pulse, suggesting that a compound with a higher AUC/c_max-ratio could provide a more sustained and tolerable immune activation. Our approach was to design TLR7/8-agonist Imidazoquinolines intended to partition to endosomes via acid trapping using a macrolide-carrier. This can potentially extend pharmacokinetics and simultaneously direct the compounds to the target compartment. The compounds have hTLR7/8-agonist activity (EC50 of the most active compound in cellular assays: 75-120 nM hTLR7, 2.8-3.1 µM hTLR8) and maximal hTLR7 activation between 40 and 80% of Resiquimod. The lead candidates induce secretion of IFNα from human Leukocytes in the same range as Resiquimod but induce at least 10-fold less TNFα in this system, consistent with a higher specificity for human TLR7. This pattern was reproduced in vivo in a murine system, where small molecules are thought not to activate TLR8. We found that Imidazoquinolines conjugated to a macrolide or, substances carrying an unlinked terminal secondary amine, had longer exposure compared with Resiquimod. The kinetics of pro-inflammatory cytokine release for these substances in vivo were slower and more extended (for comparable AUCs, approximately half-maximal plasma concentrations). Maximal IFNα plasma levels were reached 4 h post application. Resiquimod-treated groups had by then returned to baseline from a peak at 1 h. We propose that the characteristic cytokine profile is likely a consequence of altered pharmacokinetics and, potentially, enhanced endosomal tropism of the novel substances. In particular, our substances are designed to partition to cellular compartments where the target receptor and a distinct combination of signaling molecules relevant to IFNα-release are located. These properties could address the tolerability issues of TLR7/8 ligands and provide insight into approaches to fine-tune the outcomes of TLR7/8 activation by small molecules.

Small-Molecule Modulators Targeting Toll-like Receptors for Potential Anticancer Therapeutics

Toll-like receptors (TLRs) are key components of the innate immune system and serve as a crucial link between innate and acquired immunity. In addition to immune function, TLRs are involved in other important pathological processes, including tumorigenesis. TLRs have dual regulatory effects on tumor immunity by activating nuclear factor κ-B signaling pathways, which induce tumor immune evasion or enhance the antitumor immune response. Therefore, TLRs have become a popular target for cancer prevention and treatment, and TLR agonists and antagonists offer considerable potential for drug development. The TLR7 agonist imiquimod (1) has been approved by the U.S. Food and Drug Administration as a treatment for malignant skin cancer. Herein, the structure, signaling pathways, and function of the TLR family are summarized, and the structure-activity relationships associated with TLR selective and multitarget modulators and their potential application in tumor therapy are systematically discussed.

A nanoadjuvant that dynamically coordinates innate immune stimuli activation enhances cancer immunotherapy and reduces immune cell exhaustion

Although conventional innate immune stimuli contribute to immune activation, they induce exhausted immune cells, resulting in suboptimal cancer immunotherapy. Here we suggest a kinetically activating nanoadjuvant (K-nanoadjuvant) that can dynamically integrate two waves of innate immune stimuli, resulting in effective antitumour immunity without immune cell exhaustion. The combinatorial code of K-nanoadjuvant is optimized in terms of the order, duration and time window between spatiotemporally activating Toll-like receptor 7/8 agonist and other Toll-like receptor agonists. K-nanoadjuvant induces effector/non-exhausted dendritic cells that programme the magnitude and persistence of interleukin-12 secretion, generate effector/non-exhausted CD8⁺ T cells, and activate natural killer cells. Treatment with K-nanoadjuvant as a monotherapy or in combination therapy with anti-PD-L1 or liposomes (doxorubicin) results in strong antitumour immunity in murine models, with minimal systemic toxicity, providing a strategy for synchronous and dynamic tailoring of innate immunity for enhanced cancer immunotherapy.

The architecture of transmembrane and cytoplasmic juxtamembrane regions of Toll-like receptors

Toll-like receptors (TLRs) are the important participants of the innate immune response. Their spatial organization is well studied for the ligand-binding domains, while a lot of questions remain unanswered for the membrane and cytoplasmic regions of the proteins. Here we use solution NMR spectroscopy and computer simulations to investigate the spatial structures of transmembrane and cytoplasmic juxtamembrane regions of TLR2, TLR3, TLR5, and TLR9. According to our data, all the proteins reveal the presence of a previously unreported structural element, the cytoplasmic hydrophobic juxtamembrane α-helix. As indicated by the functional tests in living cells and bioinformatic analysis, this helix is important for receptor activation and plays a role, more complicated than a linker, connecting the transmembrane and cytoplasmic parts of the proteins.

Bioorthogonal-Activated In Situ Vaccine Mediated by a COF-Based Catalytic Platform for Potent Cancer Immunotherapy

Personalized tumor vaccines have become a promising modality for cancer immunotherapy. However, in situ personalized tumor vaccines generated from immunogenic cancer cell death (ICD) and adjuvants are mired by toxic side effects and unsatisfactory efficiency. Herein, by functionalizing the reticular structure to optimize the catalytic activity of the materials, a series of biocompatible covalent organic framework (COF)-based catalysts have been designed and screened for establishing a bioorthogonal-activated in situ cancer vaccine in an efficient and safe way. Especially, pro-doxorubicin (pro-DOX) could be bioorthogonally activated in situ by the COF-based Fe(II) catalysts, which elicited ICD and released tumor-associated antigens (TAAs). This in situ prodrug activation strategy could minimize drug side effects and maximize treatment effects. More importantly, the system could also catalytically activate pro-imiquimod (pro-IMQ, a TLR7/8 immune agonist), which served as an adjuvant to amplify the antitumor immunity. Notably, this bioorthogonal-activated in situ cancer vaccine not only facilitated a strong antitumor immune response but also prevented the dose-dependent side effects of chemotherapeutic drugs, including systemic inflammation caused by the random distribution of adjuvants. To the best of our knowledge, it is the first time to devise an efficient catalytic platform for generating an in situ bioorthogonal-activated cancer vaccine, which would provide a paradigm for achieving secure and robust immunotherapy.

DSP-0509, a systemically available TLR7 agonist, exhibits combination effect with immune checkpoint blockade by activating anti-tumor immune effects

TLR7 is an innate immune receptor that recognizes single-stranded RNAs, and its activation leads to anti-tumor immune effects. Although it is the only approved TLR7 agonist in cancer therapy, imiquimod is allowed to be administered with topical formulation. Thus, systemic administrative TLR7 agonist is expected in terms of expanding applicable cancer types. Here, we demonstrated the identification and characterization of DSP-0509 as a novel small-molecule TLR7 agonist. DSP-0509 is designed to have unique physicochemical features that could be administered systemically with a short half-life. DSP-0509 activated bone marrow-derived dendritic cells (BMDCs) and induced inflammatory cytokines including type I interferons. In the LM8 tumor-bearing mouse model, DSP-0509 reduced tumor growth not only in subcutaneous primary lesions but also in lung metastatic lesions. DSP-0509 inhibited tumor growth in several syngeneic tumor-bearing mouse models. We found that the CD8⁺ T cell infiltration of tumor before treatment tended to be positively correlated with anti-tumor efficacy in several mouse tumor models. The combination of DSP-0509 with anti-PD-1 antibody significantly enhanced the tumor growth inhibition compared to each monotherapy in CT26 model mice. In addition, the effector memory T cells were expanded in both the peripheral blood and tumor, and rejection of tumor re-challenge occurred in the combination group. Moreover, synergistic anti-tumor efficacy and effector memory T cell upregulation were also observed for the combination with anti-CTLA-4 antibody. The analysis of the tumor-immune microenvironment by using the nCounter assay revealed that the combination of DSP-0509 with anti-PD-1 antibody enhanced infiltration by multiple immune cells including cytotoxic T cells. In addition, the T cell function pathway and antigen presentation pathway were activated in the combination group. We confirmed that DSP-0509 enhanced the anti-tumor immune effects of anti-PD-1 antibody by inducing type I interferons via activation of dendritic cells and even CTLs. In conclusion, we expect that DSP-0509, a new TLR7 agonist that synergistically induces anti-tumor effector memory T cells with immune checkpoint blockers (ICBs) and can be administered systemically, will be used in the treatment of multiple cancers.

Novel TLR7/8 agonists promote activation of HIV-1 latent reservoirs and human T and NK cells

Antiretroviral therapy can successfully suppress HIV-1 replication to undetectable levels but fails to eliminate latent and persistent HIV-1 reservoirs. Recent studies have focused on the immunomodulatory agents such as Toll-like receptor 7 and 8 (TLR7 and TLR8) capable of activating, thereby rendering the reservoir susceptible to antiretroviral inhibition and immune recognition and elimination. In this context, this study focused on generating a diverse repertoire of TLR7/8 agonists to identify more potent candidates for activating latent HIV-1 and immune cells' response. Through combinational strategies of computer-aided design and biological characterization, 159 pyrido [3,2-d] pyrimidine and pyridine-2-amine-based derivatives were synthesized. Of which, two TLR7/8 dual and one TLR8-specific agonists with exceptionally high potency in activating HIV-1 latent reservoirs in cell lines and PBMCs of patients with persistent and durable virologic controls were identified. Particularly, these agonists appeared to enhance NK and T cells activity, which were correlated with the degree of surface activation markers. The outcome of this study highlights the remarkable potential of TLR7/8 agonists in simultaneously activating HIV-1 from the latently infected cells and augmenting immune effector cells.

Selecting Therapeutic Antisense Oligonucleotides with Gene Targeting and TLR8 Potentiating Bifunctionality

A growing body of preclinical evidence has led to the hypothesis that human Toll-like receptor 8 (hTLR8) activation in the tumor microenvironment (TME) could have potent anticancer effects through its action on monocytes, myeloid dendritic cells (mDCs), and natural killer (NK) cells. This has motivated the initiation of several clinical trials for chemical hTLR8 agonists in a variety of cancers. Concurrently, a growing number of synthetic antisense oligonucleotides (ASOs) are being developed as cancer therapeutics. We have recently reported that 2'-O-methyl (2'OMe)-modified ASOs can potentiate sensing of hTLR8 chemical agonists in a sequence-dependent manner. This suggests that select gene-targeting ASOs with anticancer activity may synergize with low-dose hTLR8 agonists in the TME. Here, we provide a detailed protocol to rapidly screen and identify such synthetic bifunctional oligonucleotides with synergistic activity on hTLR8 sensing.

Targeting toll-like receptor 7/8 for immunotherapy: recent advances and prospectives

Toll-like receptors (TLRs) are a large family of proteins that are expressed in immune cells and various tumor cells. TLR7/8 are located in the intracellular endosomes, participate in tumor immune surveillance and play different roles in tumor growth. Activation of TLRs 7 and 8 triggers induction of a Th1 type innate immune response in the highly sophisticated process of innate immunity signaling with the recent research advances involving the small molecule activation of TLR 7 and 8. The wide range of expression and clinical significance of TLR7/TLR8 in different kinds of cancers have been extensively explored. TLR7/TLR8 can be used as novel diagnostic biomarkers, progression and prognostic indicators, and immunotherapeutic targets for various tumors. Although the mechanism of action of TLR7/8 in cancer immunotherapy is still incomplete, TLRs on T cells are involved in the regulation of T cell function and serve as co-stimulatory molecules and activate T cell immunity. TLR agonists can activate T cell-mediated antitumor responses with both innate and adaptive immune responses to improve tumor therapy. Recently, novel drugs of TLR7 or TLR8 agonists with different scaffolds have been developed. These agonists lead to the induction of certain cytokines and chemokines that can be applied to the treatment of some diseases and can be used as good adjutants for vaccines. Furthermore, TLR7/8 agonists as potential therapeutics for tumor-targeted immunotherapy have been developed. In this review, we summarize the recent advances in the development of immunotherapy strategies targeting TLR7/8 in patients with various cancers and chronic hepatitis B.

SARS-CoV-2-Associated ssRNAs Activate Human Neutrophils in a TLR8-Dependent Fashion

COVID-19 disease is characterized by a dysregulation of the innate arm of the immune system. However, the mechanisms whereby innate immune cells, including neutrophils, become activated in patients are not completely understood. Recently, we showed that GU-rich RNA sequences from the SARS-CoV-2 genome (i.e., SCV2-RNA1 and SCV2-RNA2) activate dendritic cells. To clarify whether human neutrophils may also represent targets of SCV2-RNAs, neutrophils were treated with either SCV2-RNAs or, as a control, R848 (a TLR7/8 ligand), and were then analyzed for several functional assays and also subjected to RNA-seq experiments. Results highlight a remarkable response of neutrophils to SCV2-RNAs in terms of TNFα, IL-1ra, CXCL8 production, apoptosis delay, modulation of CD11b and CD62L expression, and release of neutrophil extracellular traps. By RNA-seq experiments, we observed that SCV2-RNA2 promotes a transcriptional reprogramming of neutrophils, characterized by the induction of thousands of proinflammatory genes, similar to that promoted by R848. Furthermore, by using CU-CPT9a, a TLR8-specific inhibitor, we found that SCV2-RNA2 stimulates neutrophils exclusively via TLR8-dependent pathways. In sum, our study proves that single-strand RNAs from the SARS-CoV-2 genome potently activate human neutrophils via TLR8, thus uncovering a potential mechanism whereby neutrophils may contribute to the pathogenesis of severe COVID-19 disease.

Bifurcation of signalling in human innate immune pathways to NF-kB and IRF family activation

The human innate immune response can be activated through a wide range of stimuli. This multi-faceted system can be triggered by a range of immunostimulants including pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). These stimuli drive intracellular signalling pathways that branch off downstream to activate several distinct transcription factors. The two most impactful of which in innate immune outcomes are the NF-κB and the IRF family members. Both transcription factor families play defining roles in driving inflammation as well as the antiviral response. Pathways leading to their simultaneous activation share common upstream components but eventually distinct regulators which directly facilitate their activation. This review will discuss the current state of knowledge about what is known about how these pathways bifurcate to activate NF-κB and IRF family members.

Molecular modelling on multiepitope-based vaccine against SARS-CoV-2 using immunoinformatics, molecular docking, and molecular dynamics simulation

The pandemic of COVID-19 caused by SARS-CoV-2 has made a worldwide health emergency. Despite the fact that current vaccines are readily available, several SARSCoV-2 variants affecting the existing vaccine are to be less effective due to the mutations in the structural proteins. Furthermore, the appearance of the new variants cannot be easily predicted in the future. Therefore, the attempts to construct new vaccines or to modify the current vaccines are still pivotal works for preventing the spread of the virus. In the present investigation, the computational analysis through immunoinformatics, molecular docking, and molecular dynamics (MD) simulation is employed to construct an effective vaccine against SARS-CoV2. The structural proteins of SARS-CoV2 are utilized to create a multiepitope-based vaccine (MEV). According to our findings presented by systematic procedures in the current investigation, the MEV construct may be able to trigger a strong immunological response against the virus. Therefore, the designed MEV could be a potential vaccine candidate against SARS-CoV-2, and also it is expected to be effective for other variants.

Structure and dynamics of Toll immunoreceptor activation in the mosquito Aedes aegypti

Aedes aegypti has evolved to become an efficient vector for arboviruses but the mechanisms of host-pathogen tolerance are unknown. Immunoreceptor Toll and its ligand Spaetzle have undergone duplication which may allow neofunctionalization and adaptation. Here we present cryo-EM structures and biophysical characterisation of low affinity Toll5A complexes that display transient but specific interactions with Spaetzle1C, forming asymmetric complexes, with only one ligand clearly resolved. Loop structures of Spaetzle1C and Toll5A intercalate, temporarily bridging the receptor C-termini to promote signalling. By contrast unbound receptors form head-to-head homodimers that keep the juxtamembrane regions far apart in an inactive conformation. Interestingly the transcriptional signature of Spaetzle1C differs from other Spaetzle cytokines and controls genes involved in innate immunity, metabolism and tissue regeneration. Taken together our results explain how upregulation of Spaetzle1C in the midgut and Toll5A in the salivary gland shape the concomitant immune response.

Development, Optimization, and In Vivo Validation of New Imidazopyridine Chemotypes as Dual TLR7/TLR9 Antagonists through Activity-Directed Sequential Incorporation of Relevant Structural Subunits

Undesirable activation of endosomal toll-like receptors TLR7 and TLR9 present in specific immune cells in response to host-derived ligands is implicated in several autoimmune diseases and other contexts of autoreactive inflammation, making them important therapeutic targets. We report a drug development strategy identifying a new chemotype for incorporating relevant structural subunits into the basic imidazopyridine core deemed necessary for potent TLR7 and TLR9 dual antagonism. We established minimal pharmacophoric features in the core followed by hit-to-lead optimization, guided by in vitro and in vivo biological assays and ADME. A ligand-receptor binding hypothesis was proposed, and selectivity studies against TLR8 were performed. Oral absorption and efficacy of lead candidate 42 were established through favorable in vitro pharmacokinetics and in vivo pharmacodynamic studies, with IC₅₀ values of 0.04 and 0.47 μM against TLR9 and TLR7, respectively. The study establishes imidazopyridine as a viable chemotype to therapeutically target TLR9 and TLR7 in relevant clinical contexts.

Host-pathogen protein-nucleic acid interactions: A comprehensive review

Recognition of pathogen-derived nucleic acids by host cells is an effective host strategy to detect pathogenic invasion and trigger immune responses. In the context of pathogen-specific pharmacology, there is a growing interest in mapping the interactions between pathogen-derived nucleic acids and host proteins. Insight into the principles of the structural and immunological mechanisms underlying such interactions and their roles in host defense is necessary to guide therapeutic intervention. Here, we discuss the newest advances in studies of molecular interactions involving pathogen nucleic acids and host factors, including their drug design, molecular structure and specific patterns. We observed that two groups of nucleic acid recognizing molecules, Toll-like receptors (TLRs) and the cytoplasmic retinoic acid-inducible gene (RIG)-I-like receptors (RLRs) form the backbone of host responses to pathogen nucleic acids, with additional support provided by absent in melanoma 2 (AIM2) and DNA-dependent activator of Interferons (IFNs)-regulatory factors (DAI) like cytosolic activity. We review the structural, immunological, and other biological aspects of these representative groups of molecules, especially in terms of their target specificity and affinity and challenges in leveraging host-pathogen protein-nucleic acid interactions (HP-PNI) in drug discovery.

Cyprinid-specific duplicated membrane TLR5 senses dsRNA as functional homodimeric receptors

Membrane-embedded Toll-like receptor 5 (TLR5) functions as a homodimer to detect bacterial flagellin. Cyprinid grass carp (Ctenopharyngodon idella) encodes two TLR5 genes, CiTLR5a and CiTLR5b. Here, we show that cyprinid TLR5a and TLR5b homodimers unexpectedly bind the dsRNA analog poly(I:C) and regulate interferon (IFN) response in early endosomes and lysosomes. Although TLR5 homodimers also bind flagellin, an immune response to flagellin is only triggered by TLR5a/b heterodimer. Moreover, we demonstrate that two TLR5 paralogs have opposite effects on antiviral response: CiTLR5a slightly promotes and powerfully maintains, whereas CiTLR5b remarkably inhibits virus replication. We show that the ectodomain of CiTLR5 is required for dsRNA-induced IFN signaling, and we map the key poly(I:C) binding sites to G240 for CiTLR5a and to N547 for CiTLR5b. Furthermore, we reveal that differential N-glycosylation of CiTLR5a/b affects dsRNA-IFN signaling but has no role in flagellin-mediated NF-κB induction, with paralog-specific roles for CiTLR5a-T101 and corresponding CiTLR5b-I99. Moreover, we provide evidence that the ability to sense dsRNA represents a neofunctionalization specific for membrane-bound TLR5 in cyprinid, bridging viral and bacterial immune responses.

Inhibition of human macrophage activation via pregnane neurosteroid interactions with toll-like receptors: Sex differences and structural requirements

We recently discovered that (3α,5α)3-hydroxypregnan-20-one (allopregnanolone) inhibits pro-inflammatory toll-like receptor (TLR) activation and cytokine/chemokine production in mouse macrophage RAW264.7 cells. The present studies evaluate neurosteroid actions upon TLR activation in human macrophages from male and female healthy donors. Buffy coat leukocytes were obtained from donors at the New York Blood Center (http://nybloodcenter.org/), and peripheral blood mononuclear cells were isolated and cultured to achieve macrophage differentiation. TLR4 and TLR7 were activated by lipopolysaccharide (LPS) or imiquimod in the presence/absence of allopregnanolone or related neurosteroids and pro-inflammatory markers were detected by ELISA or western blotting. Cultured human monocyte-derived-macrophages exhibited typical morphology, a mixed immune profile of both inflammatory and anti-inflammatory markers, with no sex difference at baseline. Allopregnanolone inhibited TLR4 activation in male and female donors, preventing LPS-induced elevations of TNF-α, MCP-1, pCREB and pSTAT1. In contrast, 3α,5α-THDOC and SGE-516 inhibited the TLR4 pathway activation in female, but not male donors. Allopregnanolone completely inhibited TLR7 activation by imiquimod, blocking IL-1-β, IL-6, pSTAT1 and pIRF7 elevations in females only. 3α,5α-THDOC and SGE-516 partially inhibited TLR7 activation, only in female donors. The results indicate that allopregnanolone inhibits TLR4 and TLR7 activation in cultured human macrophages resulting in diminished cytokine/chemokine production. Allopregnanolone inhibition of TLR4 activation was found in males and females, but inhibition of TLR7 signals exhibited specificity for female donors. 3α,5α-THDOC and SGE-516 inhibited TLR4 and TLR7 pathways only in females. These studies demonstrate anti-inflammatory effects of allopregnanolone in human macrophages for the first time and suggest that inhibition of pro-inflammatory cytokines/chemokines may contribute to its therapeutic actions.

N-glycosylation of UNC93B1 at a Specific Asparagine Residue Is Required for TLR9 Signaling

Toll-like receptors (TLRs) play critical roles in the first line of host defense against pathogens through recognition of pathogen-associated molecular patterns and initiation of the innate immune responses. The proper localization of TLRs in specific subcellular compartments is crucial for their ligand recognition and downstream signaling to ensure appropriate responses against pathogens while avoiding erroneous or excessive activation. Several TLRs, including TLR7 and TLR9 but not TLR4, depend on UNC93B1 for their proper intracellular localization and signaling. Accumulating evidence suggest that UNC93B1 differentially regulates its various client TLRs, but the specific mechanisms by which UNC93B1 controls individual TLRs are not well understood. Protein N-glycosylation is one of the most frequent and important post-translational modification that occurs in membrane-localized or secreted proteins. UNC93B1 was previously shown to be glycosylated at Asn251 and Asn272 residues. In this study, we investigated whether N-glycosylation of UNC93B1 affects its function by comparing wild type and glycosylation-defective mutant UNC93B1 proteins. It was found that glycosylation of Asn251 and Asn272 residues can occur independently of each other and mutation of neither N251Q or N272Q in UNC93B1 altered expression and localization of UNC93B1 and TLR9. In contrast, CpG DNA-stimulated TLR9 signaling was severely inhibited in cells expressing UNC93B1(N272Q), but not in cells with UNC93B1(N251Q). Further, it was found that glycosylation at Asn272 of UNC93B1 is essential for the recruitment of MyD88 to TLR9 and the subsequent downstream signaling. On the other hand, the defective glycosylation at Asn272 did not affect TLR7 signaling. Collectively, these data demonstrate that the glycosylation at a specific asparagine residue of UNC93B1 is required for TLR9 signaling and the glycosylation status of UNC93B1 differently affects activation of TLR7 and TLR9.

Toll-Like Receptor Signaling in Severe Acute Respiratory Syndrome Coronavirus 2-Induced Innate Immune Responses and the Potential Application Value of Toll-Like Receptor Immunomodulators in Patients With Coronavirus Disease 2019

Toll-like receptors (TLRs) are key sensors that recognize the pathogen-associated molecular patterns (PAMPs) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) to activate innate immune response to clear the invading virus. However, dysregulated immune responses may elicit the overproduction of proinflammatory cytokines and chemokines, resulting in the enhancement of immune-mediated pathology. Therefore, a proper understanding of the interaction between SARS-CoV-2 and TLR-induced immune responses is very important for the development of effective preventive and therapeutic strategies. In this review, we discuss the recognition of SARS-CoV-2 components by TLRs and the downstream signaling pathways that are activated, as well as the dual role of TLRs in regulating antiviral effects and excessive inflammatory responses in patients with coronavirus disease 2019 (COVID-19). In addition, this article describes recent progress in the development of TLR immunomodulators including the agonists and antagonists, as vaccine adjuvants or agents used to treat hyperinflammatory responses during SARS-CoV-2 infection.

TIRAP/Mal Positively Regulates TLR8-Mediated Signaling via IRF5 in Human Cells

Toll-like receptor 8 (TLR8) recognizes single-stranded RNA of viral and bacterial origin as well as mediates the secretion of pro-inflammatory cytokines and type I interferons by human monocytes and macrophages. TLR8, as other endosomal TLRs, utilizes the MyD88 adaptor protein for initiation of signaling from endosomes. Here, we addressed the potential role of the Toll-interleukin 1 receptor domain-containing adaptor protein (TIRAP) in the regulation of TLR8 signaling in human primary monocyte-derived macrophages (MDMs). To accomplish this, we performed TIRAP gene silencing, followed by the stimulation of cells with synthetic ligands or live bacteria. Cytokine-gene expression and secretion were analyzed by quantitative PCR or Bioplex assays, respectively, while nuclear translocation of transcription factors was addressed by immunofluorescence and imaging, as well as by cell fractionation and immunoblotting. Immunoprecipitation and Akt inhibitors were also used to dissect the signaling mechanisms. Overall, we show that TIRAP is recruited to the TLR8 Myddosome signaling complex, where TIRAP contributes to Akt-kinase activation and the nuclear translocation of interferon regulatory factor 5 (IRF5). Recruitment of TIRAP to the TLR8 signaling complex promotes the expression and secretion of the IRF5-dependent cytokines IFNβ and IL-12p70 as well as, to a lesser degree, TNF. These findings reveal a new and unconventional role of TIRAP in innate immune defense.

Molecular dynamics simulations reveal the selectivity mechanism of structurally similar agonists to TLR7 and TLR8

TLR7 and TLR8 are key members of the Toll-like receptor family, playing crucial roles in the signaling pathways of innate immunity, and thus become attractive therapeutic targets of many diseases including infections and cancer. Although TLR7 and TLR8 show a high degree of sequence homology, their biological response to small molecule binding is very different. Aiming to understand the mechanism of selective profiles of small molecule modulators against TLR7 and TLR8, we carried out molecular dynamic simulations on three imidazoquinoline derivatives bound to the receptors separately. They are Resiquimod (R), Hybrid-2 (H), and Gardiquimod (G), selective agonists of TLR7 and TLR8. Our MD trajectories indicated that in the complex of TLR7-R and TLR7-G, the two chains forming the TLR7 dimer tended to remain “open” conformation, while the rest systems maintained in the closed format. The agonists R, H, and G developed conformational deviation mainly on the aliphatic tail. Furthermore, we attempted to quantify the selectivity between TLR7 and TLR8 by binding free energies via MM-GBSA method. It showed that the three selected modulators were more favorable for TLR7 than TLR8, and the ranking from the strongest to the weakest was H, R and G, aligning well with experimental data. In the TLR7, the flexible and hydrophobic aliphatic side chain of H has stronger van der Waals interactions with V381 and F351 but only pick up interaction with one amino acid residue i.e. Y353 of TLR8. Unsurprisingly, the positively charged side chain of G has less favorable interaction with I585 of TLR7 and V573 of TLR8 explaining G is weak agonist of both TLR7 and TLR8. All three imidazoquinoline derivatives can form stable hydrogen bonds with D555 of TLR7 and the corresponding D543 of TLR8. In brief, the set of total 400ns MD studies sheds light on the potential selectivity mechanisms of agonists towards TLR7 and TLR8, indicating the van der Waals interaction as the driving force for the agonists binding, thus provides us insights for designing more potent and selective modulators to cooperate with the hydrophobic nature of the binding pocket.