It has previously been inferred that the fusion of a macrophage secondary lysosome with a phagosome delivers the entire lysosomal contents uniformly to the phagosome. We found, however, that different fluorescent lysosomal probes can enter phagosomes at remarkably different rates, even though they are initially sequestered together in the same organelles. Thus, sulforhodamine is almost exclusively delivered to yeast-containing phagosomes within 2 h of phagocytosis. But fluoresceinated, high molecular weight dextran accumulates in the same phagosomes only over a period of approximately 24 h. We postulate that the delivery of lysosomal contents may involve an intermittent and incremental process in which individual components can be selectively and sequentially transferred.

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