Therapeutic targeting of Toll-like receptors for infectious and inflammatory diseases and cancer

Abstract

Since first being described in the fruit fly Drosophila melanogaster, Toll-like receptors (TLRs) have proven to be of great interest to immunologists and investigators interested in the molecular basis to inflammation. They recognize pathogen-derived factors and also products of inflamed tissue, and trigger signaling pathways that lead to activation of transcription factors such as nuclear factor-kappaB and the interferon regulatory factors. These in turn lead to induction of immune and inflammatory genes, including such important cytokines as tumor necrosis factor-alpha and type I interferon. Much evidence points to a role for TLRs in immune and inflammatory diseases and increasingly in cancer. Examples include clear roles for TLR4 in sepsis, rheumatoid arthritis, ischemia/reperfusion injury, and allergy. TLR2 has been implicated in similar pathologic conditions and also in systemic lupus erythematosus (SLE) and tumor metastasis. TLR7 has also been shown to be important in SLE. TLR5 has been shown to be radioprotective. Recent advances in our understanding of signaling pathways activated by TLRs, structural insights into TLRs bound to their ligands and antagonists, and approaches to inhibit TLRs (including antibodies, peptides, and small molecules) are providing possiblemeans by which to interfere with TLRs clinically. Here we review these recent advances and speculate about whether manipulating TLRs is likely to be successful in fighting off different diseases.

Fig. 1.

TLR signaling pathways. Once activated by their respective ligands, TLRs recruit their specific repertoire of the TIR adapters MyD88, Mal, TRIF, or TRAM, resulting in the recruitment and activation of the IRAKs and TRAF6. This leads to the activation of NF-κB essential modulator (NEMO) and the subsequent phosphorylation and degradation of IκB, the inhibitor of NFκB, rendering NFκB free to translocate from the cytosol to the nucleus and activate κB-dependent genes. IRF7 is also activated downstream of TLRs 7, 8, and 9, leading to its dimerization and translocation into the nucleus and to activation of IFNα and IFN-inducible genes. TLR3 and TLR4 both use TRIF to activate the noncanonical IKKs TBK1 and IKKϵ, resulting in the dimerization and activation of IRF3 and the transcription of IFNβ and IFN-inducible genes.

Fig. 2.

The structure of TLR4/MD-2: molecular basis for ligand binding. A, the structure of human TLR4 (turquoise) bound to MD-2 (yellow) is taken from the crystal structure (Kim et al., 2007). The single nucleotide polymorphisms in TLR4 (D299G and T399I) are shown in green, the cysteine residues in MD-2 critical for LPS binding (Cys95 and Cys105) are shown in red, and the residues in MD-2 (Phe126 and His155) critical for receptor dimerization in response to LPS are shown in pink. B, a model to suggest the structural basis of ligand activation of TLR4/MD-2 (lateral and top views). Using the structural data, a model was made to explain how TLR4/MD-2 might dimerize to form an active complex (Walsh et al., 2008). The two TLR4 molecules are represented in purple and turquoise and the two MD-2 molecules in yellow and green. In this model, there are contacts between the two TLR4 proteins, and each MD-2 touches both TLR4 proteins (see the top view). TLR4 SNP D299G is indicated in red and T399I is indicated in black.

Fig. 3.

Drugs targeting the TLR4/MD-2 signaling pathway. Activation of TLR4 recruits the adapter pairs Mal/MyD88 and TRAM/TRIF. Signaling through Mal/Myd88 recruits IRAK1, IRAK4, Traf6, and TAK1-binding protein (TAB) to activate the NFκB signaling pathway. Signaling through TRAM/TRIF activates NFκB through TRAF6, but also activates signaling through IRF1 and IRF3 through TRAF3. Several drugs now target the TLR4/MD-2 signaling pathway. Eritoran and the AGP compounds bind to the TLR4/MD-2 lipid IA binding site, monoclonal antibodies (for example the neutralizing antibody from NovImmune, Geneva, Switzerland) bind to TLR4. Soluble peptides, such as the BB loop peptides, target the BB loop on the TIR domain, the region of the protein important in receptor dimerization. The small molecular inhibitor TAK-242 targets the signaling domain of the TIR.