ED2-positive perivascular cells act as neuronophages during delayed neuronal loss in the facial nucleus of the rat.

Injection of Fluoro-Gold (FG) into the whisker pad of rats yields a stable retrograde labeling of facial motoneurons. After removal of 10 mm from the facial nerve the microglia phagocytose the FG-prelabeled dead neurons and assume the label. A subsequent brightfield immunostaining of the sections with HRP-DAB as end-product fully quenches the fluorescence of FG from all specifically stained structures (immunoquenching). Combining FG-labeling of neuronophages with immunoquenching, we recently described a population of enigmatic fluorescent cells, found in immediate vicinity to the motoneurons after the general neuronofugal migration of microglia. As the fluorescence of these cells was not quenched after a triple immunostaining with anti neuron-specific enolase, anti-GFAP, and OX-42 (quenching all fluorescence from neurons and glia), they seemed to represent a new, immunologically not identified neuronophage. Now we have further characterized this cell type. Following triple immunostaining, we tested a broad panel of mabs (OX-33, OX-19, OX-18, OX-6, R73, ED1, and ED2) to stain, quench fluorescence, and thus immunotype the unknown phagocytes. Only the mab ED2, the classical marker for perivascular cells, specifically stained the small round neuronophages. This surprising migration of perivascular cells toward decaying neurons was additionally tested and confirmed by intracerebroventricular application of FG prior to resection of the facial nerve Providing evidence for neuronophagia by ED2-positive cells, our results strongly support the hypothesis that the latter are the APC (antigen presenting cells) of the CNS.