T-cell infiltration and expression of MHC class II antigen by macrophages and microglia in a heterogeneous group in leukoencephalopathy.

We report here on T-cell infiltration and diffuse expression of the major histocompatibility complex (MHC) class II antigen in a heterogeneous group of macrophages and microglia in leukoencephalopathy (LE). Microglia reacting positively for HLA-DR were five times more numerous in LE than those in non-LE cases and were distributed densely in the degenerated white matter but sparsely in the subcortical arcuate fibers. CD4- and CD8-positive lymphocytes were 9 and 15 times more plentiful, respectively, in LE cases; they aggregated in the expanded Virchow-Robin spaces and frequently infiltrated the neural parenchyma. An intimate association of T cells with macrophages, and the expression of leukocyte function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1), accessory molecules in antigen presentation, were observed in each cell in the region of macrophage clusters. These results indicate that expression of MHC antigen is accompanied by cell adhesion molecules and by infiltration of T cells in a heterogeneous group in leukoencephalopathy and suggests their immunocompetence, although it may be secondary to destruction of myelin.