Intralysosomal accumulation of polyanions. I. Fusion of pinocytic and phagocytic vacuoles with secondary lysosomes

The long-term exposure of macrophages to low concentrations of a number of polyanions leads to their accumulation in high concentration within secondary lysosomes. This was associated with enlargement of the lysosomes, the presence of membranous whorls, and intense toluidine blue staining of the organelles at pH 1.0. After the ingestion of a particulate load by these cells, newly formed phagocytic vacuoles failed to fuse with polyanion-laden lysosomes. The lack of fusion was evident in both fluorescence and electron micrographic studies which followed the transfer of acridine orange or Thorotrast from 2 degrees lysosomes to phagosomes. Agents that inhibited phagosome-lysosome (P-L) fusion included molecules containing high densities of sulfate, sulfonate, or carboxylate residues. Dextran sulfate (DS) in microgram/ml quantities was an excellent inhibitor, whereas nonsulfated dextran (D) was without effect at 1,000-fold higher concentrations. In contrast to their effects on P-L fusion, polyanions failed to influence the fusion of pinocytic vesicles with 2 degrees lysosomes. The uptake, intravacuolar distribution, and intralysosomal digestion of fluid-phase pinocytic markers were unaltered in lysosomes containing either D or DS. Furthermore, subcellular fractionation studies showed that the fluid-phase pinocytic marker HRP was efficiently transferred from pinosomes to large, dense 2 degrees lysosomes containing DS.