Impaired lysosomal processing of beta 2-microglobulin by infiltrating macrophages in dialysis amyloidosis

Background. Macrophages may participate in amyloid fibril formation by proc essing the protein precursor. Although this theory seems to apply for amylo idosis, in which proteolytic cleavage is a prerequisite for amyloid fibril formation, it has not been demonstrated for beta 2-microglobulin (beta(2)m) amyloidosis. We aimed to establish the role played by macrophages in beta( 2)m amyloidosis. Methods . We used a double immunogold electron microscopy technique, includ ing mouse antihuman CD68, rabbit antihuman beta(2)m, amyloid P component, a nd lysosome-associated membrane protein (LAMP-1) antibodies. Differential d ensity labeling studies of beta(2)m and amyloid P component were performed extra- and intracellularly to assess protein processing by macrophages. Results. The cells surrounding amyloid fibrils were found to be mostly CD68 positive, suggesting that they were of monocyte-macrophage lineage. Intrac ellular accumulation of amyloid fibrils was also observed; these fibrils we re constantly surrounded by LAMP-1-linked gold particles, demonstrating tha t intracellular beta(2)m was almost exclusively lysosomal. The rough surfac e endoplasmic reticulum was not labeled by beta(2)m antibody, suggesting th at there was no active synthesis of beta(2)m by the cells. As a marker of e ndocytosis, protruded cytoplasmic processes in close relation with the intr acellular accumulations of beta(2)m amyloid fibrils were observed. No diffe rence in density labeling (extracellular vs. intracellular) was observed fo r beta(2)m, whereas intracellular P component labeling was significantly de creased. Conclusions. All of these data are strongly suggestive of phagocytosis and not synthesis of amyloid fibrils by macrophages. Further, they demonstrate an impaired lysosomal processing specific for beta(2)m, as other compounds of the amyloid fibrils (P component) are significantly cleared.