TLR9 is localized in the endoplasmic reticulum prior to stimulation.

In mammals, 10 TLRs recognize conserved pathogen-associated molecular patterns, resulting in the induction of inflammatory innate immune responses. One of these, TLR9, is activated intracellularly by bacterial DNA and synthetic oligodeoxynucleotides (ODN), containing unmethylated CpG dinucleotides. Following treatment with CpG ODN, TLR9 is found in lysosome-associated membrane protein type 1-positive lysosomes, and we asked which intracellular compartment contains TLR9 before CpG exposure. Surprisingly, we found by microscopy and supporting biochemical evidence that both transfected and endogenously expressed human TLR9 is retained in the endoplasmic reticulum. By contrast, human TLR4 trafficked to the cell surface, indicating that endoplasmic reticulum retention is not a property common to all TLRs. Because TLR9 is observed in endocytic vesicles following exposure to CpG ODN, our data indicate that a special mechanism must exist for translocating TLR9 to the signaling compartments that contain the CpG DNA.