TLR7 and rheumatoid arthritis: Another miRNA also encountered inside synovial fluid exosomes, let-7b, plays a presumably vital role in rheumatoid arthritis because its GU-rich domain is essential for the Toll-like receptor 7 (TLR-7) to bind to an endogenous ligand on the surface of naive synovial fluid macrophages so as to transform them into inflammatory M1 macrophages [99].