Members of the Toll-like receptor (TLR) family, named for the closely related Toll receptor in Drosophila, play a pivotal role in innate immune responses. TLRs recognize conserved motifs found in various pathogens and mediate defense responses. Triggering of the TLR pathway leads to the activation of NF-kappaB and subsequent regulation of immune and inflammatory genes. The TLRs and members of the IL-1 receptor family share a conserved stretch of approximately 200 amino acids known as the Toll/Interleukin-1 receptor (TIR) domain. Upon activation, TLRs associate with a number of cytoplasmic adaptor proteins containing TIR domains, including myeloid differentiation factor 88 (MyD88), MyD88-adaptor-like/TIR-associated protein (MAL/TIRAP), Toll-receptor-associated activator of interferon (TRIF), and Toll-receptor-associated molecule (TRAM). This association leads to the recruitment and activation of IRAK1 and IRAK4, which form a complex with TRAF6 to activate TAK1 and IKK. Activation of IKK leads to the degradation of IkappaB, which normally maintains NF-kappaB in an inactive state by sequestering it in the cytoplasm. TLR9 is highly expressed in macrophages, dendritic cells, and B lymphocytes, and in humans has five isoforms generated by alternative splicing. TLR9 binds to unmethylated CpG motifs present on bacterial DNA and stimulates NF-kappaB via the MyD88 adaptor protein. In contrast to most TLR family members that are localized to the plasma membrane, TLR9 is an intracellular receptor localized to the ER in resting cells. Upon binding to CpG DNA, TLR9 is proteolytically processed and translocates to endo-lysosomal compartments where it binds MyD88, initiating downstream signaling.
