Toll-like receptor expression in the peripheral nerve

Toll-like receptors comprise a family of evolutionary conserved pattern recognition receptors that act as a first defense line in the innate immune system. Upon stimulation with microbial ligands, they orchestrate the induction of a host defense response by activating different signaling cascades. Interestingly, they appear to detect the presence of endogenous signals of danger as well and as such, neurodegeneration is thought to trigger an immune response through ligation of TLRs. Though recent data report the expression of various TLRs in the central nervous system, TLR expression patterns in the peripheral nervous system have not been determined yet. We observed that Schwann cells express relatively high levels of TLRs, with especially TLR3 and TLR4 being prominent. Sensory and motor neurons hardly express TLRs at all. Through the use of NF-κB signaling as read-out, we could show that all TLRs are functional in Schwann cells and that bacterial lipoprotein, a ligand for TLR1/TLR2 receptors yields the strongest response. In sciatic nerve, basal levels of TLRs closely reflect the expression patterns as determined in Schwann cells. TLR3, TLR4, and TLR7 are majorly expressed, pointing to their possible role in immune surveillance. Upon axotomy, TLR1 becomes strongly induced, while most other TLR expression levels remain unaffected. Altogether, our data suggest that similar to microglia in the brain, Schwann cells might act as sentinel cells in the PNS. Furthermore, acute neurodegeneration induces a shift in TLR expression pattern, most likely illustrating specialized functions of TLRs in basal versus activated conditions of the peripheral nerve.