Uptake of fibrillar beta-amyloid by microglia isolated from MSR-A (type I and type II) knockout mice.

To characterize the receptors involved in binding fibrillar amyloid A-beta (fA beta), we compared the uptake of fA beta in microglia from wildtype (MSR-A+/+) and MSR-A knockout (MSR-A-/-) mice. On average, there was a 60% reduction in the uptake of Cy3-fA beta in microglia from the MSR-A-/- mice. Cy3-fA beta uptake in the MSR-A-/- mice was still competable by scavenger receptor ligands, including acetylated low-density lipoprotein (Ac-LDL) and fucoidan. This indicates that uptake by MSR-B and/or other MSRs is also involved in the uptake of fA beta by microglia. However, the significant reduction in the uptake of fA beta in the MSR-A-/- microglia suggests that fA beta gets internalized mostly by MSR-As in microglia. Uptake of modified fA beta (ClqA beta) was similar in the MSR-A-/- microglia as in the wildtype indicating that the uptake of the opsonized fA beta is independent of MSR-A.

Effects of incorporation of immunoglobulin G and complement component C1q on uptake and degradation of Alzheimer's disease amyloid fibrils by microglia.

Microglia are macrophage-like immune system cells found in the brain. They are associated with Alzheimer's Disease plaques, which contain fibrillar beta-amyloid (fAbeta) and other components such as complement proteins. We have shown previously that murine microglia bind and internalize fAbeta microaggregates via the type A scavenger receptor, but degradation of internalized fAbeta is significantly slower than normal degradation. In this study, we compared internalization by microglia of fAbeta microaggregates to that of anti-Abeta-antibody-coated fAbeta (IgG-fAbeta) microaggregates and found that the uptake of the latter is increased by about 1.5-fold versus unmodified fAbeta. The endocytic trafficking of IgG-fAbeta is similar to that of fAbeta microaggregates, following an endosomal/lysosomal pathway. We also compared the internalization of fAbeta microaggregates to that of complement protein, C1q-coated fAbeta microaggregates, and found that the levels of uptake are also increased by about 1.5-fold. Rates of degradation of both types of modified fAbeta microaggregates are unchanged compared with unmodified fAbeta microaggregates. We demonstrated by blocking studies that internalization of IgG-fAbeta is mediated by Fc receptors. These data suggest that, in vivo, several different microglial receptors may play a part in internalizing fAbeta, but the involvement of other receptors may not increase the degradation of fAbeta.

Uptake, degradation, and release of fibrillar and soluble forms of Alzheimer's amyloid beta-peptide by microglial cells.

Microglia are phagocytic cells that are the main inflammatory response cells of the central nervous system. In Alzheimer's disease brain, activated microglia are concentrated in regions of compact amyloid deposits that contain the 39-43-amino acid Abeta peptide. We examined the uptake, degradation, and release of small aggregates of fibrillar Abeta (fAbeta) or soluble Abeta (sAbeta) by microglia. We found that although some degradation of fAbeta was observed over 3 days, no further degradation was observed over the next 9 days. Instead, there was a slow release of intact Abeta. The poor degradation was not due to inhibition of lysosomal function, since the rate of alpha2-macroglobulin degradation was not affected by the presence of fAbeta in the late endosomes/lysosomes. In contrast to fAbeta, internalization of sAbeta was not saturable. After internalization, sAbeta was released rapidly from microglia, and very little was degraded. These data show that fAbeta and sAbeta interact differently with microglia but that after internalization a large fraction of both are released without degradation.

Excessive degradation of intracellular protein in macrophages prevents presentation in the context of major histocompatibility complex class II molecules.

The endogenous major histocompatibility complex (MHC) class II presentation pathway allows biosynthesized, intracellular antigens access for presentation to MHC class II-restricted T cells. This pathway has been well documented in B cells and fibroblasts, but may not be universally available in all antigen-presenting cell types. This study compares the ability of different antigen-presenting cells, expressing endogenous C5 protein (fifth component of mouse complement) as a result of transfection, to present their biosynthesized C5 to MHC class II-restricted T cells. B cells and fibroblasts expressing C5 were able to present several epitopes of this protein with MHC class II molecules, whereas macrophages were unable to do so, but readily presented C5 from an extracellular source. However, macrophage presentation of endogenous C5 could be achieved when they were treated with low doses of the lysosomotropic agent ammonium chloride. In the presence of an inhibitor of autophagy, presentation of endogenous C5 was abrogated, indicating that biosynthesized C5 is shuttled into lysosomal compartments for degradation before making contact with MHC class II molecules. Taken together, this suggests that proteolytic activity in lysosomes of macrophages may be excessive, compared with fibroblasts and B cells, and destroys epitopes of the C5 protein before they can gain access to MHC class II molecules. Thus, there are inherent differences in presentation pathways between antigen-presenting cell types; this could reflect their specialized functions within the immune system with macrophages focussing preferentially on internalization, degradation, and presentation of extracellular material.