A light and electron microscopic study of HLA-DR positive cells in the human cerebral cortex and subcortical white matter.

Specimens of histologically normal human cerebral cortex and subcortical white matter were obtained during neurosurgical operations and studied by electron microscopy and immunocytochemistry using an antibody against HLA-DR. Greater numbers of labelled cells were present in the white matter than in the overlying cortex. The labelled cells consisted of ramified microglia and perivascular cells. Microglia were often found just outside the walls of small blood vessels, but perivascular cells were actually enclosed by two leaflets of basement membrane in the walls of capillaries, arterioles and venules up to 100 microns in luminal diameter. Labelled microglial processes were often seen enclosing neuronal processes in the cortex and myelinated and unmyelinated axons in the white matter. The enclosed processes appeared healthy, and without features of degenerating neurons. These observations are consistent with a previous suggestion that microglia continually modify the processes of central nervous system neurons by a process of phagocytosis. An intact blood-brain barrier is likely to be of great importance in preventing antigen presentation of the processed neuronal, and perhaps even oligodendrocytic, antigenic peptide fragments to circulating lymphocytes.