Introduction: Pattern recognition receptors (PRRs) of the innate immune system mediate and control the activation and progression of adaptive immunity. Toll-like receptors (TLRs) are the most notable of the PRRs: they play crucial roles in protecting the host body against invading pathogens or endogenously released hazardous molecules. Sustained TLR signaling even after the clearance of pathogens or failure to distinguish between ‘self’ and ‘non-self’ molecules can cause inflammatory disorders, autoimmune diseases, and cancer. Areas covered: This review focuses on recently developed therapeutic agents with TLR-antagonistic activities. Expert opinion: In recent years, several research institutes and pharmaceutical companies have achieved fundamental successes in inhibiting or reducing TLR signaling and associated effector mechanisms by using novel inhibitors. These inhibitory molecules include antibodies against TLRs, TLR-derived transmembrane (TM) peptides, bacterial-secreted proteins, and natural or synthetic small molecules, peptides, and proteins. Antagonist developers generally target the TLR ectodomain to block receptor activation. The TM and cytosolic Toll/IL-1 receptor domains also have regions that should be explored for the design of peptide-based and small molecule blocking agents. A number of preclinical and clinical breakthroughs may result in the availability of improved TLR immunomodulatory drugs to address important unmet medical needs.

Toll-like receptors (TLRs) recognize pathogen/damage-associated molecular patterns and initiate inflammatory signaling cascades. Occasionally, overexpression of TLRs leads to the onset of numerous inflammatory diseases, necessitating the development of selective inhibitors to allow a protective yet balanced immune response. Here, we demonstrate that a novel peptide (TIP1) derived from Toll/interleukin-1 receptor (TIR) domain-containing adapter protein inhibited multiple TLR signaling pathways (MyD88-dependent and MyD88-independent) in murine and human cell lines. TIP1 also inhibited NLRP3-mediated IL-1β secretion, as we validated at both the protein and mRNA levels. Biophysical experiments confirmed that TIP1 specifically binds to the BB loop of the TLR4-TIR domain. Animal studies revealed that TIP1 inhibited the secretion of lipopolysaccharide (LPS)-induced proinflammatory cytokines in collagen-induced arthritis (CIA) and kaolin/carrageenan-induced arthritis (K/C) rodent models. TIP1 also rescued animals from sepsis and from LPS-induced kidney/liver damage. Importantly, TIP1 ameliorated the symptoms of rheumatoid arthritis in CIA and K/C rodent models, suggesting that TIP1 has therapeutic potential for the treatment of TLR-mediated autoimmune/inflammatory diseases.

Toll-like receptors (TLRs) are a unique category of pattern recognition receptors that recognize distinct pathogenic components, often utilizing the same set of downstream adaptors. Specific molecular features of extracellular, transmembrane (TM), and cytoplasmic domains of TLRs are crucial for coordinating the complex, innate immune signaling pathway. Here, we constructed a full-length structural model of TLR4—a widely studied member of the interleukin-1 receptor/TLR superfamily—using homology modeling, protein–protein docking, and molecular dynamics simulations to understand the differential domain organization of TLR4 in a membrane-aqueous environment. Results showed that each functional domain of the membrane-bound TLR4 displayed several structural transitions that are biophysically essential for plasma membrane integration. Specifically, the extracellular and cytoplasmic domains were partially immersed in the upper and lower leaflets of the membrane bilayer. Meanwhile, TM domains tilted considerably to overcome the hydrophobic mismatch with the bilayer core. Our analysis indicates an alternate dimerization or a potential oligomerization interface of TLR4-TM. Moreover, the helical properties of an isolated TM dimer partly agree with that of the full-length receptor. Furthermore, membrane-absorbed or solvent-exposed surfaces of the toll/interleukin-1 receptor domain are consistent with previous X-ray crystallography and biochemical studies. Collectively, we provided a complete structural model of membrane-bound TLR4 that strengthens our current understanding of the complex mechanism of receptor activation and adaptor recruitment in the innate immune signaling pathway.