Primer: Toll-like receptor signaling pathways--what do rheumatologists need to know?

Toll-like receptors (TLRs) have caught the attention of rheumatologists searching for additional therapeutic targets for diseases such as rheumatoid arthritis and systemic lupus erythematosus. Signaling from these molecules can induce the expression of cytokines such as tumor necrosis factor and interferon alpha. Strategies that target TLRs and their co-receptors (such as MD2 for TLR4 or CD36 for TLR2) might be a more-selective approach than inhibition of global signals such as nuclear factor kappaB or p38 mitogen-activated protein kinase. TLR signaling requires adaptor proteins, including MyD88, Mal, TRIF and TRAM, which are recruited to specific receptors: Mal is used only by TLR2 and TLR4, TRIF is used by TLR3 and TLR4, and TRAM is recruited by TLR4 alone. Mal and TRAM are subject to complex biochemical regulation. Inhibition of Mal or MyD88 blocks the production of inflammatory mediators in synovial tissue. Another possible intracellular target is Unc93b, a protein involved in signaling from TLR3, TLR7 and TLR9. Inhibition of TLR4, TLR7 and TLR9 has produced intriguing results, which indicate that TLRs and their signaling pathways might indeed have great potential as novel targets for the treatment of inflammatory joint disease.