The rat central nervous system (CNS) during experimental allergic encephalomyelitis (EAE) was analyzed immunohistochemically from the preclinical to recovery stage by using monoclonal antibodies specific for rat T lymphocyte subsets and Ia antigen. Through combination of the avidin-biotin technique and carefully selected fixative, cells with dendritic morphology (DC) and infiltrating mononuclear cells were clearly and intensely demonstrated in the CNS parenchyma during EAE. In normal and complete Freund's adjuvant (CFA)-injected controls, there were no inflammatory foci. Ia (OX3)-positive parenchymal cells were not detected, whereas W3/25 stained DC that were located mainly in the white matter and W3/13 stained axons. At the preclinical stage, 11 days after CNS/CFA sensitization, a few clusters of Ia+ DC were detected in some sections of the spinal cord. The number of Ia+ DC increased as clinical signs developed (P less than 0.001). In rats with a clinical score of 1 or 2, Ia+ DC were mainly located in the perivascular region and closely associated with infiltrating T lymphocytes. However, at moribund state (score 3), Ia+ DC were evenly distributed in gray and white matter on almost all sections of the spinal cord. In recovered rats, the numbers of inflammatory foci and Ia+ DC were less than those in clinical EAE rats (P less than 0.001). Rats without clinical signs throughout the course also contained a few clusters of Ia+ DC. Double immunofluorescent staining with OX3 and anti-glial fibrillary acidic protein (GFAP) antiserum demonstrated that Ia+ DC were negative for GFAP. Their morphology and distribution were similar to those of nucleoside diphosphatase-positive cells, suggesting that Ia+ DC are microglia. In contrast to DC, no astrocytes or endothelial cells express detectable levels of Ia antigen in control and clinical EAE rats. These findings suggest that brain cells other than Ia+ DC may not be involved in the local immune interaction. Ia+ DC may play a significant role in antigen presentation in the CNS with EAE.

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