Local distribution of microglia in the normal adult human central nervous system differs by up to one order of magnitude

Although microglia are considered to be a sensitive sensor for pathological processes in the central nervous system, there are only a few studies abou t the distribution and density of microglia in the normal human brain. Ther efore, a study of local density of microglial cells was conducted by invest igating 20 normal human brains with no clinical neurological symptoms or di seases and no neuropathological alterations. Microglial cells were visualiz ed by immunolabeling of proteins which are known to be expressed either con stitutively or facultatively, such as CD68, major histocompatibility comple x class II (MHC-II), leukocyte common antigen (LCA), leukocyte chemotactic factor (LCF), macrophage inhibitory factor-related protein (MRP) 8, MRP14, CD4 and allograft-inflammatory factor-1 (AIF-1). CD68, MHC-II and AIF-1 sho wed the highest densities with significant regional differences ranging fro m 0.5% to 16.6% of all cells in the brain parenchyma with significantly mor e microglia in white than in gray matter. LCF and LCA showed a similar patt ern of distribution as the proteins described above, but with lower percent ages of microglial cells. CD4 was not found in the brain parenchyma. We con clude that CD68, MHC-II and AIF-1 define the main microglial cell populatio n, whereas LCF and LCA are expressed by a subpopulation of microglial cells . The brains showed no or a negligible vascular expression of MRP8 and MRP1 4. Information about the local microglia density in the normal human brain can serve as a reference for the evaluation of pathological microglial resp onses.