We aimed to determine the relative role of quinolinic acid synthesis in purified human microglia, monocyte-derived macrophages and astrocytes in the human brain following immune stimulation. Microglia and macrophages significantly increased quinolinic acid synthesis from tryptophan following activation by either lipopolysaccharide or interferon-gamma. Quinolinic acid synthesis by individual microglia was heterogeneous, and its production by activated macrophages was approximately 32-fold greater than its microglial synthesis. Quinolinic acid synthesis by astrocytes was undetectable. Microglia may, therefore, be the primary endogenous cell type responsible for quinolinic acid synthesis in the brain parenchyma. However, under pathological conditions which precipitate blood-brain barrier compromise and/or leukocytic infiltration, intracerebral quinolinic acid may be derived chiefly from cells of the peripheral immune system such as activated macrophages.

Although a variety of potential sources for reactive oxygen species (ROS) exist in the CNS, brain macrophages, i.e., the microglia, generate large quantities of these reactive species, particularly in response to injury or inflammatory signals. In order to understand how microglia contribute to changes in oxidative status of the CNS and how this might related to disease states, such as Alzheimer disease (AD), we have examined the regulation of superoxide anion and nitric oxide production from rodent and human microglia. Our results indicate that microglia from all species we have studied release superoxide anion, but produce significantly different amounts in response to the same activating agents. Species differences are also found in the ability to generate nitric oxide (NO). In particular, mouse microglia generate large quantities of NO when stimulated, but human and hamster microglia do not produce measurable amounts under the same stimulation conditions. These species differences are important to consider when modeling human disease processes from rodent studies.

A BSTRACT : Microglia are the CNS macrophage and are a primary cellular component of plaques in Alzheimer's disease (AD) that may contribute to the oxidative stress associated with chronic neurodegeneration. We now report that superoxide anion production in microglia or macrophages from 3 different species is increased by long term exposure (24 hours) to A ␤ peptides. Since A ␤ competes for the uptake of opsonized latex beads and for the production of superoxide anion by opsonized zymosan, a likely site of action are membrane receptors associated with the uptake of opsonized particles or fibers. The neurotoxic fibrillar peptides A ␤ (1-42) and human amylin increase radical production whereas a non-toxic, non-fibrillar peptide, rat amylin, does not. We also report that the effect of A ␤ peptides on superoxide anion production is not associated with a concomitant increase in nitric oxide (NO) production in either human monocyte derived macrophages (MDM) or hamster microglia from primary cultures. Since NO is known to protect membrane lipids and scavenge superoxide anion, the lack of A ␤ -mediated induction of NO production in human microglia and macrophages may be as deleterious as the overproduction of superoxide anion induced by chronic exposure to A ␤ peptides.

Abstract:Cytokine-mediated activation of inducible nitric oxide synthase (iNOS) in monocytes or macro-

Previous studies have shown that apolipoprotein E (apoE) plays a role in immune function by modulating tissue redox balance. Using a mouse macrophage cell line (RAW 264.7), we have examined the mechanism by which apoE regulates nitric oxide (NO) production in macrophages. ApoE potentiates NO production in immune activated RAW cells in combination with lipopolysaccharide or polyinosinic:polycytidylic acid (PIC), agents known to induce expression of inducible nitric oxide synthase mRNA and protein. The effect is not observed with apolipoprotein B or heat-inactivated apoE. The combination of PIC plus apoE produced more NO than the level expected from an additive effect of PIC and apoE alone. Furthermore, this increase was observed at submaximal extracellular arginine concentrations, suggesting that apoE altered arginine (substrate) availability. Examination of [(3)H]arginine uptake across the cell membrane demonstrated that arginine uptake was increased by PIC but further increased by PIC plus apoE. Treatment of RAW cells with apoE was associated with an increased apparent V(max) and decreased affinity for arginine as well as a switch in the induction of mRNA for subtypes of cationic amino acid transporters (CAT). Treatment of RAW cells with PIC plus apoE resulted in the loss of detectable CAT1 mRNA and expression of CAT2 mRNA. Regulation of arginine availability is a novel action of apoE on the regulation of macrophage function and the immune response.