Rationally Designed Small-Molecule Inhibitors Targeting an Unconventional Pocket on the TLR8 Protein-Protein Interface

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.

Modulation of IRAK enzymes as a therapeutic strategy against SARS-CoV-2 induced cytokine storm

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the cause of the current pandemic coronavirus disease 2019 (COVID-19). Dysregulated and excessive production of cytokines and chemokines, known as cytokine storm, is frequently seen in patients with severe COVID-19 disease and it can provoke a severe systematic inflammation in the patients. The IL-1R/TLRs/IRAKs signaling network is a key pathway in immune cells that plays a central role in regulating innate immunity and inflammatory responses via stimulating the expression and production of various proinflammatory molecules including cytokines. Modulation of IRAKs activity has been proposed to be a promising strategy in the treatment of inflammatory disorders. In this review, we highlight the biochemical properties of IRAKs and their role in regulating inflammatory molecular signaling pathways and discuss the potential targeting of IRAKs to suppress the SARS-CoV-2-induced cytokine storm in COVID-19 patients.

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.

Proteome-Wide and Protein-Specific Multi-Epitope Vaccine Constructs Against the Rift Valley Fever Virus Outbreak Using Integrated Omics Approaches

Rift Valley fever (RVF) is a viral disease caused by a member of the Bunyavirales family causing severe infections in humans. The RVF virus is an enveloped, negative-sense, single-stranded RNA virus that can infect both animals and humans. The symptoms associated with these infections span from minor (fever and headaches) to severe (meningoencephalitis and hemorrhagic fever syndrome) symptoms. Despite the outbreaks of the RVF virus being reported in different parts of the world, no effective therapy is available. Herein, the development of an efficient vaccine is critical for the control of infections associated with the RVF virus. Moreover, computational vaccine approaches are helpful in the design of specific, safe, and stable peptide-based designs when compared to the conventional methods of vaccine development. In this study, the whole proteome of the virus, comprising four proteins (NP, L, GP, and NSP), was screened to find putative vaccine epitope sequences (T cell, B cell, and HTL) specific for each protein. These shortlisted epitopes were then combined with flexible linkers to design protein-specific and proteome-wide immunogenic multi-epitope-based vaccine constructs. The results revealed that these multi-epitope vaccine constructs (MEVCs) are strongly antigenic and non-allergenic in nature. The efficacy of these constructs was further validated by docking with immune receptors, which revealed strong binding interactions with human TLR8. Using the MD simulation approach, the binding stability and residual flexibility of the best vaccine construct (proteome-wide) were confirmed, which revealed stable dynamic and favorable features. Furthermore, in-silico cloning and immune simulation analysis confirmed the expression and production of immune factors, that is, IgM, IgG, and IL-6, against the proposed vaccine designs. Additionally, 3D models of all the MEVC constructs have been developed and evaluated for potential immunization against the RVF virus. Finally, the proteome-wide vaccine candidate (MEVC-PW-RVFV) with the highest immune reinforcement potential provides new insights into the development of future vaccines against the emerging RVF virus.

Subtractive proteomics assisted therapeutic targets mining and designing ensemble vaccine against Candida auris for immune response induction

The emergence of variants and the reports of co-infection caused by Candida auris in COVID-19 patients adds a further complication to the global pandemic situation. To date, no effective therapy is available for C. auris infections. Thus, characterization of therapeutic targets and designing effective vaccine candidates using subtractive proteomics and immune-informatics approaches is useful tool in controlling the emerging infections associated with SARS-CoV-2. In the current study, subtractive proteomics-assisted annotation of the vaccine targets was performed, which revealed seven vaccine targets. An immunoinformatic-driven approach was then employed to map protein-specific and proteome-wide immunogenic peptides (CTL, B cell, and HTL) for the design of multi-epitope vaccine candidates (MEVCs). The results demonstrated that the vaccine candidates possess strong antigenic features (>0.4 threshold score) and are classified as non-allergenic. Validation of the designed MEVCs through molecular docking, in-silico cloning, and immune simulation further demonstrated the efficacy of the vaccines by producing immune factor titers (ranging from 2500 to 16000 au/mL) i.e., IgM, IgG, IL-6, and Interferon-α. In conclusion, the current study provides a strong impetus in designing anti-fungal strategies against Candida auris.

Further hit optimization of 6-(trifluoromethyl)pyrimidin-2-amine based TLR8 modulators: Synthesis, biological evaluation and structure-activity relationships

Toll-like receptor 8 (TLR8) is an endosomal TLR that has an important role in the innate human immune system, which is involved in numerous pathological conditions. Excessive activation of TLR8 can lead to inflammatory and autoimmune diseases, which highlights the need for development of TLR8 modulators. However, only a few small-molecule modulators that selectively target TLR8 have been developed. Here, we report the synthesis and systematic investigation of the structure-activity relationships of a series of novel TLR8 negative modulators based on previously reported 6-(trifluoromethyl)pyrimidin-2-amine derivatives. Four compounds showed low-micromolar concentration-dependent inhibition of TLR8-mediated signaling in HEK293 cells. These data confirm that the 6-trifluoromethyl group and two other substituents on positions 2 and 4 are important structural elements of pyrimidine-based TLR8 modulators. Substitution of the main scaffold at position 2 with a methylsulfonyl group or para hydroxy/hydroxymethyl substituted benzylamine is essential for potent negative modulation of TLR8. Our best-in-class TLR8-selective modulator 53 with IC₅₀ value of 6.2 μM represents a promising small-molecule chemical probe for further optimization to a lead compound with potent immunomodulatory properties.

Toll-like receptors and toll-like receptor-targeted immunotherapy against glioma

Glioma represents a fast proliferating and highly invasive brain tumor which is resistant to current therapies and invariably recurs. Despite some advancements in anti-glioma therapies, patients’ prognosis remains poor. Toll-like receptors (TLRs) act as the first line of defense in the immune system being the detectors of those associated with bacteria, viruses, and danger signals. In the glioma microenvironment, TLRs are expressed on both immune and tumor cells, playing dual roles eliciting antitumoral (innate and adaptive immunity) and protumoral (cell proliferation, migration, invasion, and glioma stem cell maintenance) responses. Up to date, several TLR-targeting therapies have been developed aiming at glioma bulk and stem cells, infiltrating immune cells, the immune checkpoint axis, among others. While some TLR agonists exhibited survival benefit in clinical trials, it attracts more attention when they are involved in combinatorial treatment with radiation, chemotherapy, immune vaccination, and immune checkpoint inhibition in glioma treatment. TLR agonists can be used as immune modulators to enhance the efficacy of other treatment, to avoid dose accumulation, and what brings more interests is that they can potentiate immune checkpoint delayed resistance to PD-1/PD-L1 blockade by upregulating PD-1/PD-L1 overexpression, thus unleash powerful antitumor responses when combined with immune checkpoint inhibitors. Herein, we focus on recent developments and clinical trials exploring TLR-based treatment to provide a picture of the relationship between TLR and glioma and their implications for immunotherapy.

Photoactivation of Innate Immunity Receptor TLR8 in Live Mammalian Cells by Genetic Encoding of Photocaged Tyrosine

The effectiveness of innate immune responses relies on an intricate balance between activation and regulation. TLR8, a member of the Toll-like receptor (TLR) family, plays a fundamental role in host defense by sensing viral single-stranded RNAs (ssRNAs). However, the molecular recognition and regulatory mechanism of TLR8 is not fully understood, especially in a whole-cell environment. Here, we engineer the first light-controllable TLR8 model by genetically encoding a photocaged tyrosine, NBY, into specific sites of TLR8. In the caged forms, the activity of TLR8 is masked but can be restored upon decaging by exposure to UV light. To explain the mechanism clearly, we divide the sites with light responsiveness into three groups. They can separately block the ligands that bind to the pockets of TLR8, change the interaction modes between two TLR8 protomers, and interfere with the interactions between TLR8 cytosolic domains with its downstream adaptor. Specifically, we use this chemical caging strategy to probe and evaluate the function of several tyrosine sites located at the interface of TLR8 homodimers with a previously unknown regulatory mode, which may provide a new strategy for TLR8 modulator development. Effects on downstream signaling pathways are monitored at the transcriptional and translational levels in various cell lines. By photoactivating specific cells within a larger population, this powerful tool can provide novel mechanistic insights, with potential in biotechnological and pharmaceutical applications.

Tetrasubstituted imidazoles as incognito Toll-like receptor 8 a(nta)gonists

Small-molecule modulators of TLR8 have drawn much interests as it plays pivotal roles in the innate immune response to single-stranded RNAs (ssRNAs) derived from viruses. However, their clinical uses are limited because they can invoke an uncontrolled, global inflammatory response. The efforts described herein culminate in the fortuitous discovery of a tetrasubstituted imidazole CU-CPD107 which inhibits R848-induced TLR8 signaling. In stark contrast, CU-CPD107 shows unexpected synergistic agonist activities in the presence of ssRNA, while CU-CPD107 alone is unable to influence TLR8 signaling. CU-CPD107’s unique, dichotomous behavior sheds light on a way to approach TLR agonists. CU-CPD107 offers the opportunity to avoid the undesired, global inflammation side effects that have rendered imidazoquinolines clinically irrelevant, providing an insight for the development of antiviral drugs.

Toll-like receptor 8 (TLR8) plays essential roles in the innate immune response to viral single-stranded RNA (ssRNA), so small molecule modulators of TLR8 are of interest, however adverse effects limit their use. Here, the authors report a tetrasubstituted imidazole CU-CPD107 with dichotomous behaviour, which inhibits R848-induced TLR8 signaling, but shows synergistic activity in the presence of ssRNA, making it a potential antiviral agent.

Structural Evolution and Translational Potential for Agonists and Antagonists of Endosomal Toll-like Receptors

Toll-like receptors (TLRs) are members of a large family of evolutionarily conserved pattern recognition receptors (PRRs), which serve as key components of the innate immune system by playing a pivotal role in sensing "nonself" ligands. Endosomal TLRs (TLR3, TLR7, TLR8, and TLR9) can recognize pathogen-derived nucleic acid and initiate an innate immune response because they react against both self- and non-self-origin nucleic acid molecules. Accordingly, both receptor agonists and antagonists are potentially useful in disparate clinical contexts and thus are globally sought after. Recent research has revealed that agonists and antagonists share an overlapping binding region. This Perspective highlights rational medicinal chemistry approaches to elucidate the structural attributes of small molecules capable of agonism or antagonism or of elegantly switching between the two. The structural evolution of different chemotypes can provide the framework for the future development of endosomal TLR agonists and antagonists.

Small molecule approaches to treat autoimmune and inflammatory diseases (Part II): Nucleic acid sensing antagonists and inhibitors

Nucleic acid sensing pathways play an important role in the innate immune system, protecting hosts against infections. However, a large body of evidence supports a close association between aberrant activation of those pathways and autoimmune and inflammatory diseases. Part II of the digest series on small molecule approaches to autoimmune and inflammatory diseases concentrates on recent advances with respect to small molecule antagonists or inhibitors of the nucleic acid sensing pathways, including endosomal TLRs, NLRP3 inflammasome and cGAS-STING.

Recent trends in the development of Toll-like receptor 7/8-targeting therapeutics

Introduction: Toll-like receptor (TLR) 7 and TLR8 are functionally localized to endosomes and recognize specific RNA sequences. They play crucial roles in initiating innate and adaptive immune responses. TLR7/8 activation protects the host against invading pathogens and enhances immune responses. In contrast, sustained TLR7/8 signaling leads to immune overreaction. Therefore, agonists or antagonists targeting TLR7/8 signaling are favorable drug candidates for the treatment of immune disorders.Areas covered: Basic knowledge about TLR7 and TLR8 and their signaling pathways are briefly reviewed. Various therapeutic agents have been designed to activate or antagonize TLR7/8 signaling pathways, and their safety and efficacy for the treatment of multiple diseases have been investigated in preclinical animal models and clinical trials. TLR7/8 agonists exhibit potent antiviral activity and regulate anti-tumor immune responses. TLR7 agonists have also been used as adjuvants to improve vaccine immunogenicity and generate greater seroprotection. TLR7/8 antagonists are promising candidates for the treatment of autoimmune and inflammatory diseases.Expert opinion: TLR7/8 pathways are favorable targets for immunological therapies. Future research should concentrate on the optimization of drug safety, efficiency, and specificity. Detailed mechanistic studies will contribute to the development of TLR7/8 immunomodulators and novel therapeutic strategies.

Structural and functional understanding of the toll-like receptors

Recognition of invading pathogens by the innate immune system is essential to initiate antimicrobial responses and trigger adaptive immunity. This is largely mediated by an array of pattern-recognition receptor families that are essential for recognizing conserved molecular motifs characteristic of pathogenic microbes. One such family is the Toll-like receptors (TLRs). Activation of TLRs induces production of pro-inflammatory cytokines and type I interferons: the former triggers the synthesis of inflammatory mediators which cause fever, pain and other inflammation, and the latter mediates antiviral responses. Over the past decade, significant progress has been made in structural elucidation of TLRs in higher eukaryotes. The TLR structures with and without agonist and antagonist have been revealed by X-ray crystallography and cryo-electron microscopy studies, demonstrating the activated dimer formation induced by the agonistic ligand and the inhibition mechanism of the antagonistic ligand. Intracellular assembled structures and the TLR-chaperone complex are also reported. As the structural understanding of TLRs becomes better integrated with biochemical and immunological studies, a more comprehensive picture of their architectural and functional properties will emerge. This review summarizes recent advances in structural biological and mechanistic studies on TLRs.

Modulation of the Innate Immune Response by Targeting Toll-like Receptors: A Perspective on Their Agonists and Antagonists

Toll-like receptors (TLRs) are a class of proteins that recognize pathogen-associated molecular patterns (PAMPs) and damaged-associated molecular patterns (DAMPs), and they are involved in the regulation of innate immune system. These transmembrane receptors, localized at the cellular or endosomal membrane, trigger inflammatory processes through either myeloid differentiation primary response 88 (MyD88) or TIR-domain-containing adapter-inducing interferon-β (TRIF) signaling pathways. In the last decades, extensive research has been performed on TLR modulators and their therapeutic implication under several pathological conditions, spanning from infections to cancer, from metabolic disorders to neurodegeneration and autoimmune diseases. This Perspective will highlight the recent discoveries in this field, emphasizing the role of TLRs in different diseases and the therapeutic effect of their natural and synthetic modulators, and it will discuss insights for the future exploitation of TLR modulators in human health.