The lesion of acute experimental autoimmune encephalomyelitis. Isolation and membrane phenotypes of perivascular infiltrates from encephalitic rat brain white matter.

Vascular elements were isolated from the brain white matter of rats with acute experimental autoimmune encephalomyelitis following transfer of activated myelin basic protein-specific syngeneic T line lymphocytes. Fractions containing highly purified capillary and small postcapillary vessels and fractions enriched for perivascular infiltrates ("cuffs") were obtained using equilibrium density gradient centrifugation to remove myelin and 1 X g sedimentation in a fetal calf serum gradient to sort the vascular elements. Immunocytochemical analysis revealed that capillary cells were negative for Ia determinants. In small postcapillary vessels with scanty lymphoid cell infiltration, Ia expression was restricted to circumscript areas, often, but not always, around adhering lymphoid cells. Immunocytochemistry combined with cytofluorometry established that the majority of all perivascular lymphocytes expressed the phenotype W3/25, defining either helper T cells or T cells involved in delayed-type hypersensitivity. Less than 3% of the lymphocytes were positive for OX8, which defines suppressor and cytotoxic T subsets. Membrane immunoglobulin expressing B lymphocytes were also less than 10% of the infiltrating cells.