Expression of complement components in the peripheral nervous system.

We have generated a SAGE (serial analysis of gene expression) library of normal sciatic nerve and found tags encoding for mRNAs of the complement system highly represented. RNA (RT-PCR and northern blot hybridization) and protein (western blot analysis and immunohistochemistry) studies confirmed these findings. High expression of classical pathway components, alternative pathway components and inhibitory components was observed in specific regions of the sciatic nerve. The first components of complement were found in axons, whereas the inhibitory components were detected in the perineurium, thereby protecting the nerve from a complement attack. Immunoreactivity towards activated complement factors was noted in post traumatic neuromas and after acute crush injury, which exemplify nerve regeneration and degeneration. We propose that local production of complement in the peripheral nervous system participates in the protection of healthy nerve and is needed for efficient clearance of myelin after injury: a prerequisite for normal regeneration and remyelination of the peripheral nerve.