Dexamethasone promotes phagocytosis and bacterial killing by human monocytes/macrophages in vitro.

One of the actions of glucocorticoids (GC) in multiple sclerosis (MS) is an inhibitory effect on demyelination. This can be caused by a reduction in the number of infiltrating macrophages and/or by an effect on the phagocytosis of myelin. Here we investigate the effect of GC on the phagocytosis of myelin. Contrary to what was expected, we found that incubation of human monocytes with dexamethasone (DEX) for 48 h augmented (approximately threefold) the phagocytosis of myelin. This enhancement of phagocytosis by human monocytes was not restricted to myelin. Phagocytosis of various particles mediated by different macrophage receptors was increased by DEX. We found that not only the phagocytosis of Staphylococcus aureus bacteria was augmented, but also the killing of these bacteria was at least twice as effective after culture with DEX. Tumor necrosis factor alpha production of human monocyte-derived macrophages induced by lipopolysaccharide and S. aureus was suppressed by DEX. Together our results show that DEX promotes the phagocytosis of particles by human monocytes and thereby may contribute to tissue repair after immune-mediated tissue damage or infection. These data shed a new light on the clinical application of GC.

The role of anti-myelin (auto)-antibodies in the phagocytosis of myelin by macrophages.

Plasma cells secreting antibodies directed to myelin components are present in CNS of MS patients and although the pathogenic role of such antibodies has yet to be established it is apparent from animal studies that anti-myelin antibodies are involved in myelin damage. In this study, we have investigated the effect of disease-promoting anti-myelin mAb on the phagocytosis of myelin by macrophages. Monoclonal antibodies directed to myelin basic protein (MBP)--clones 1, 12, 17, 22, 26, proteolipid protein (PLP), galactocerebroside (GalC) and myelin oligodendrocyte glycoprotein (MOG)--clones Y1, Y4, Y6, Y7, Y9, Y10, Y11 and Z12 were incubated with purified murine myelin labeled with DiI. The degree of phagocytosis of antibody-treated myelin by murine macrophages in vitro was determined using a quantitative flow cytometric assay. In comparison to untreated myelin pretreatment with myelin-specific mAb modified the degree of phagocytosis. The degree of opsonization of myelin was dependent on the isotype of antibody and the epitope recognized in addition to the ability of the mAb to fix complement. The greatest degree of opsonization of myelin was observed with the monoclonal antibody MOG Z12 that has previously been shown to enhance EAE and augment demyelination. These findings suggest a major role for anti-myelin antibodies, in particular antibodies directed to MOG, for the phagocytosis of myelin by macrophages in vitro. This may have relevance to the pathogenesis of myelin damage in vivo and provide a helpful tool for the classification of heterogeneous diseases such as MS.

Reactive oxygen species are required for the phagocytosis of myelin by macrophages.

Reactive oxygen species (ROS) are thought to be involved in the pathogenesis of multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). In this study we showed that the phagocytosis of myelin by macrophages triggers the production of ROS. We also demonstrated that ROS play a crucial role in the myelin phagocytosis. Blocking the ROS production with NADPH oxidase inhibitors (100 microM DPI or 10 mM Apocynin) essentially prevented the phagocytosis of myelin. Furthermore, scavenging of ROS with catalase (H2O2) or mannitol (OH-) decreased the phagocytosis of myelin by macrophages, whereas superoxide dismutase (O2-) did not show this effect. In addition, Lipoic acid (LA), a non-specific scavenger of ROS, also decreased the phagocytosis of myelin by macrophages. In our results, we demonstrate for the first time that ROS appear to play a regulatory role in the phagocytosis of myelin.