Christine D. Dijkstra

The origin and nature of brain macrophages and microglial cells in the mouse central nervous system (CNS) were investigated. First, the expression and localization of determinants recognized by the different monoclonal antibodies (mAbs) MOMA-1, Mac-1-alpha, and F4/80 (raised against cells of the mononuclear phagocyte system) were immunohistochemically studied in the developing and adult mouse brain. In order to clarify the origin of brain macrophages and microglial cells, we used bacteriophage lambda transgenic mice as donors for bone marrow transplantations in recipient mice of different ages. During ontogeny, numerous MOMA-1-, Mac-1-alpha-, and F4/80-positive blood monocyte-derived brain macrophages (amoeboid microglia) infiltrated the CNS parenchyma. These brain macrophages gradually disappeared from the brain parenchyma at postnatal day 7 (P7). From P17 on, Mac-1-alpha- and F4/80-positive cells were detected within the brain parenchyma with the morphology of resting microglial cells. Transitional forms between brain macrophages and "resting" microglia were not observed in the developing brain. Combined non-radioactive in situ hybridization and immunohistochemistry revealed many MOMA-1-positive bone marrow-derived brain macrophages that were located in the leptomeninges, the ventricles, and occasionally the blood vessel walls. These results show that brain macrophages are of bone marrow origin. Many "resting" microglial cells were detected in the brain, mainly in the white matter. It appeared that about 10% of these cells displayed the transgenic signal. This result indicates that the majority of "resting" microglial cells are of local, presumably neuroectodermal, origin.

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.