Liposome-mediated monocyte depletion during wallerian degeneration defines the role of hematogenous phagocytes in myelin removal.

Newly recruited hematogenous mononuclear cells of the monocyte/macrophage system are suggested to be important effector cells in myelin removal during Wallerian degeneration. Their role has extensively been studied in various in vitro and in vivo models. However, there has been much controversy concerning the role of hematogenous vs. resident cells of the peripheral nervous system in Wallerian degeneration. The present study used a recently established technique to deplete the hematogenous monocyte population by application of dichloromethylene diphosphonate-containing liposomes. Intravenously injected liposomes containing dichloromethylene diphosphonate (Cl2MDP) are ingested by macrophages and monocytes and cause temporary and selective depletion of these cells. The number of LFA-1- and Mac-1- positive macrophages within the nerves was significantly reduced when liposomes were injected shortly after nerve transsection. In these nerves, myelin degradation was significantly less, indicating an essential role of newly recruited phagocytes in this process. Macrophage invasion of degenerating nerves occurred within the first 2 days after transsection. Resident cells of the peripheral nerve participate in myelin removal since macrophage depletion did not completely abolish myelin degradation. These results confirm the important role of hematogenous phagocytes in myelin removal during Wallerian degeneration.