Technical advance: Caspase-1 activation and IL-1β release correlate with the degree of lysosome damage, as illustrated by a novel imaging method to quantify phagolysosome damage.

In addition to the lysosome's important roles in digestion, antigen processing, and microbial destruction, lysosome damage in macrophages can trigger cell death and release of the inflammatory cytokine IL-1β. To examine the relationship among endocytosis, lysosome damage, and subsequent events, such as caspase-1 activation and IL-1β secretion, we developed a method for measuring lysosome disruption inside individual living cells, which quantifies release of Fdx from lysosomes. Unperturbed, cultured BMM exhibited low levels of lysosome damage, which were not increased by stimulation of macropinocytosis. Lysosome damage following phagocytosis differed with different types of ingested particles, with negligible damage after ingestion of sRBC ghosts, intermediate damage by polystyrene (PS) beads, and high levels of damage by ground silica. Pretreatment with LPS decreased the amount of lysosome damage following phagocytosis of PS beads, silica microspheres, or ground silica. Activation of caspase-1 and subsequent release of IL-1β were proportional to lysosome damage following phagocytosis. The low level of damage following PS bead phagocytosis was insufficient to activate caspase-1 in LPS-activated macrophages. These studies indicate that lysosome damage following phagocytosis is dependent on particle composition and dose and that caspase-1 activation and IL-1β secretion correlate with the extent of lysosome damage.