ORIGIN, NATURE, AND SOME FUNCTIONAL CONSIDERATIONS OF INTRAVENTRICULAR MACROPHAGES, WITH SPECIAL REFERENCE TO THE EPIPLEXUS CELLS

Intraventricular macrophages encompass the supraependymal, free-floati ng, and epiplexus (Kolmer) cells; the supraependymal cells lie in clos e apposition to the ventricular ependyma, the epiplexus cells are clos ely associated with the choroid plexus epithelium, and the free-floati ng cells are at a variable distance fi om the epithelial surface. Alth ough the three cell types are regarded as one cellular entity, the epi plexus cells preponderate. On scanning electron microscopy, the epiple xus cells display diverse morphological forms, ranging from round to b ipolar to stellate, and bear a variable number of cytoplasmic processe s. Transmission electron microscopy shows the presence of large number s of lysosomes. The phagocytic nature of epiplexus cells is shown by t heir intense staining for nonspecific esterase and active uptake of tr acers, e.g., horseradish peroxidase and rhodamine isothiocynate, admin istered intravenously or intraperitoneally The mode of entry of these tracers in the cerebral ventricles is by way of transepithelial transp ort. In rats, the population of intraventricular macrophages increases steadily after birth until 17 days of age; thereafter, their cell pop ulation remains relatively unchanged. The early upsurge is attributed to proliferation of residential cells and/or influx of circulating mon ocytes/stromal macrophages through the process of ''emperipolesis.'' T he immunophenotypic features of intraventricular macrophages are consi stent with other mononuclear phagocytes being immunoreactive for OX-42 , OX-18, OX-6, and OX-1 and ED1 for the detection of CR3 receptors, MH C class I and II antigens, leucocyte common antigen, and macrophage an tigen, respectively. The expression of these antigens is noticeably en hanced following the injection of lipopolysaccharide (LPS) into postna tal rats. Remarkably, the intraventricular macrophages are induced to express MHC class Il (Ia) antigen after LPS or interferon-gamma inject ions. Furthermore, the expression of transferrin receptors as detected with OX-26 is also upregulated after these treatments. Epiplexus cell s are also elicited to display a de novo expression of nitric oxide sy nthase-like immunoreactivity following intracerebral injection of LPS. They also respond vigorously to a single nonpenetrative blast. Result s of our series of studies suggest that, besides their primary functio n as scavenger cells, the intraventricular macrophages partake in poss ible immunological responses and iron regulation in the ventricular sy stem or the brain as a whole. (C) 1998 Wiley-Liss, Inc.