Origin of CD11b+ macrophage-like cells in the CNS of PLP-overexpressing mice: low influx of haematogenous macrophages and unchanged blood-brain-barrier in the optic nerve.

We have recently reported that overexpression of proteolipid protein in oligodendrocytes leads to a pathologically relevant increase of both CD8+ T-lymphocytes and CD11b+ cells in the CNS. We now focussed on the origin of the CD11b+ cells in the optic nerve, a well established structure for the analysis of the mutant, using bone marrow chimeric mice. Although there is an age-related increase in CD11b+ cells in the myelinated part of the optic nerve of the mutants, the percentage of infiltrating cells was not increased, but enhanced proliferation was detectable. In the non-myelinated optic nerve head, the rate of infiltrating CD11b+ cells and albumin extravasation was high in both genotypes. However, albumin extravasation was also high in the rostral myelinated part, where CD11b+ cell influx was low. Our study demonstrates an intrinsic origin of CD11b+ cells in the presence of an unchanged blood-brain-barrier in a CNS myelin mutant.