Impaired lysosomal processing of β2-microglobulin by infiltrating macrophages in dialysis amyloidosis

Background. Macrophages may participate in amyloid fibril formation by processing the protein precursor. Although this theory seems to apply for amyloidosis, 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, including mouse antihuman CD68, rabbit antihuman beta(2)m, amyloid P component, and lysosome-associated membrane protein (LAMP-1) antibodies. Differential density 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. Intracellular accumulation of amyloid fibrils was also observed; these fibrils were constantly surrounded by LAMP-1-linked gold particles, demonstrating that intracellular beta(2)m was almost exclusively lysosomal. The rough surface endoplasmic reticulum was not labeled by beta(2)m antibody, suggesting that there was no active synthesis of beta(2)m by the cells. As a marker of endocytosis, protruded cytoplasmic processes in close relation with the intracellular accumulations of beta(2)m amyloid fibrils were observed. No difference in density labeling (extracellular vs. intracellular) was observed for beta(2)m, whereas intracellular P component labeling was significantly decreased. 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.