Toll-like receptors (TLRs) are evolutionarily conserved pattern-recognition molecules resembling the toll proteins that mediate antimicrobial responses in Drosophila. These proteins recognize different microbial products during infection and serve as an important link between the innate and adaptive immune responses. TLR9 forms a subfamily along with TLR7 and TLR8 that recognize viral RNA and CpG DNA sequences and are localized in intracellular acidic compartments such as the phagolysosome. Unlike other TLRs which act through adaptor molecules such as TOLLIP, TIRAP, TRIF, and MyD88 to activate various kinases and transcription factors to respond to potential infection, TLR9 is strictly dependent on MyD88.
