A Novel Role of the Perineurium: Evidence for an Essential Function for Macrophage Maintenance and Increase in Isolated Peripheral Nerves.

BACKGROUND AND AIMS: It is an established view that the perineurium surrounding peripheral nerves fulfills pivotal barrier functions to protect nerve fibers from noxious agents of the nerve environment. Here we want to investigate whether the perineurium also influences other endoneurial cells, like macrophages.

METHODS: Peripheral nerve segments-either intact or deprived of their perineurium-were investigated in vitro for up to 9 days. Macrophages were quantified by immunohistochemistry using the pan-marker F4/80. Because elevated macrophage numbers can affect their proliferation and activation, we reduced their abundance in selected experiments by pretreating nerve donor mice with the CSF1-receptor inhibitor PLX5622.

RESULTS: Endoneurial macrophage numbers in isolated nerve segments increased throughout the entire observation period. Removal of the perineurium caused a rapid and robust decline of macrophage numbers, reflecting its role on macrophage maintenance and expansion. Under these conditions, Schwann cells adopted an autophagic phenotype. When macrophages were pharmacologically reduced in number, they still preserved their capacity to expand in the normal nerve segments, with an initial preferential accumulation within the peripheral nerve region near the perineurium. Removal of the perineurium prior to culturing nerves with reduced macrophage numbers resulted again in a substantial loss of macrophages.

INTERPRETATION: We identified a pivotal role of the perineurium in supporting macrophage maintenance and expansion. The perineurium may serve as a source of trophic factors and/or may form a barrier maintaining a critical concentration of trophic factors within the endoneurium. Additionally, macrophages may originate from a cell pool within the perineurium.