Expression of major histocompatibility complex class II antigen on amoeboid microglial cells in early postnatal rat brain following intraperitoneal injections of lipopolysaccharide.

In rats given two single intraperitoneal injections of lipopolysaccharide (LPS) at 1 and 4 days of age and killed at 7 days of age, 11.5--12% of amoeboid microglial cells (AMC) in the supraventricular corpus callosum were induced to express major histocompatibility complex (MHC) class II antigen, as detected with monoclonal antibody OX-6. The MHC class II antigen induced was colocalized with MHC class I antigen and type 3 complement receptors on the same cells. The expression of MHC class II antigen on the plasma membrane of AMC was confirmed in immunoelectron microscopy. Although OX-6-positive AMC often assumed a perivascular position, the majority of them, however, were far removed from the blood vessels. The cytoplasmic processes of the perivascular OX-6-positive AMC appeared to rest directly on the vascular lamina, and in some section profiles they were in contact with a large surface area of the outer wall of small blood vessels. It is concluded from this study that although MHC class II antigen is not constitutively present on AMC, it is, however, inducible under stimulation with LPS. It is, therefore, suggested that the OX-6-positive AMC, especially the perivascular AMC, may have the potentiality to function as antigen-presenting cells in the developing brain when challenged by LPS.