Identification of CD4- CD8- alpha beta T cells in the subarachnoid space of rats with experimental autoimmune encephalomyelitis. A possible route by which effector cells invade the lesions

Experimental autoimmune encephalomyelitis (EAE) was induced in Lewis rats to elucidate the origin of effector T cells and the route by which they invade lesions. Since mouse studies have suggested that some autoimmune diseases are induced by extrathymic T cells in the liver, we focused our attention on the properties of mononuclear cells (MNC) isolated from the liver and other organs in rats with EAE. A small but significant proportion of LFA-1+ alpha beta T cells was identified in the liver as early as day 7 after immunization with myelin basic protein (MBP). Such LFA-1+ alpha beta T cells were also abundant among MNC attached to the spinal cord (i.e. subarachnoid space), and MNC infiltrated the spinal cord in rats with EAE (day 12). In electron microscopy, MNC attached to the spinal cord were found to be quite unique in terms of their large cell size with well-developed microvilli. More importantly, they were comprised of a considerably large proportion of double-negative CD4- CD8- T cells as well as single-positive CD4+ T cells. However, the cells which infiltrated the spinal cord were mainly CD4+. The present results raise the possibility that the subarachnoid space might be a major site for the expansion of extrathymic T cells in rats with EAE, and that only a limited population of CD4+ T cells invade the spinal cord directly through the outer layer and elicit EAE.