Ia antigens in the normal rat nervous system and in lesions of experimental allergic encephalomyelitis.

The distribution of the class II major histocompatibility (Ia) antigens has been studied in the normal nervous system and in acute lesions of experimental allergic encephalomyelitis (EAE). EAE was induced in Lewis rats with guinea pig spinal cord in Freund's complete adjuvant. Frozen sections from cord, including the roots and ganglia, were stained for Ia antigens, and some sections were also stained for the hydrolytic enzyme acid phosphatase. In the normal CNS and PNS, there were a few vessel-associated cells or small leukocyte-like cells which expressed Ia antigens. No cells were found which expressed both Ia and acid phosphatase [the phenotype used to describe the activated macrophage group of antigen presenting cells (APCs)]. In EAE, Ia positive cells increased in number prior to the detection of clinical signs. Some of these Ia-positive cells were thought to be astrocytes rather than inflammatory cells. At the height of the disease process large numbers of cells in the EAE lesions were Ia-positive. Among these infiltrating cells were some large acid phosphatase-positive cells which also expressed Ia antigens. These double-positive cells appeared to be APCs in the form of activated macrophages, cells known to be involved in the demyelinating processes of EAE. Our results show that some vascular and vessel-associated cells in the normal nervous system express Ia antigens. We suggest that these and other Ia-positive cells in acute EAE lesions may have a role in antigen presentation.