PHAGOCYTIC-ACTIVITY OF MACROPHAGES AND MICROGLIAL CELLS DURING THE COURSE OF ACUTE AND CHRONIC RELAPSING EXPERIMENTAL AUTOIMMUNE ENCEPHALOMYELITIS

The ED1 monoclonal antibody recognizes an antigen in lysosomal membran es of phagocytes. The expression of this antigen in cells increases du ring phagocytic activity. Here we describe the expression of ED1-immun oreactivity during the various stages of both acute (monophasic) and c hronic relapsing experimental autoimmune encephalomyelitis (EAE) in th e Lewis rat. During the first attack of acute and chronic relapsing EA E, ED1-immunoreactivity was present in macrophages and in cells which displayed morphologic features of activated microglial cells (i.e., ce lls with thick short processes). At the ultrastructural level these ce lls were seen to contain phagocytosed myelin structures in lysosomes. ED1-immunoreactivity in these cells was present in the cytoplasm near lysosomes. During the remission phase of acute EAE and the relapse pha se of chronic relapsing EAE, ED1-positive cells with dendritic morphol ogy not only were present in or nearby lesions, but were also found at sites distant from lesions throughout large parts of the brain. These cells had a morphology comparable to microglial cells in normal brain . A major difference between animals which were in remission and anima ls which on day 25 were suffering from a relapse, was that the latter showed the presence of lesions with darkly stained round ED1-positive macrophages and activated microglial cells. These results indicate tha t during a relapse, newly recruited bloodborne macrophages infiltrate the brain and, together with activated lymphocytes and microglial cell s, recommence a new demyelination process. (C) 1994 Wiley-Liss, Inc.