Selective Localization by Neuroglia of Immunoglobulin G in Normal Mice

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Abstract
Plasma proteins including immunoglobulins have been previously localized in neurons with processes extending outside the blood-brain barrier, but not within glia under normal conditions. Immune modulating functions have been proposed for both microglia and astrocytes in several pathological states. Using immunocytochemistry, we have found that large numbers of neuroglial cells contain immunoglobulin G (IgG) in normal mice of the C57 BL/6 strain. Most IgG-positive cells had both the morphology and distribution of microglia, including a higher density in grey matter, and were frequently found in perivascular or perineuronal locations. The accumulation of IgG does not appear to be by nonselective phagocytosis of extracellular fluid, since serum albumin could not be detected within microglia. There was little overlap in the distribution of cellular processes positive for IgG and those which showed astrocytic markers. Neuronal accumulation of plasma proteins was also seen in a distribution described by previous investigators. The function of this selective accumulation of IgG by normal microglia is unknown, but may reflect a role in the immune response within the central nervous system.