The role of lysosomal cysteine proteinases as markers of macrophage activation and as non-specific mediators of inflammation

A major portion of the lysosomal proteinases responsible for intracellular degradation of phagocytized proteins are cysteine proteinases, the “acid” cathepsins B, H, L, and S (see [1] for review). As we have shown previously [2, 3], increased cysteine proteinase activity is found in blood plasma of polytraumatized and septic patients as well as in local inflammatory secretions such as bronchoalveolar lavage fluid and peritonitis exudate. Most of this activity is due to cathepsin B, which is relatively stable at neutral pH (half-life about 30 min at pH 7.4) and is protected in the form of reversible complexes with its endogenenous protein inhibitors (stefins, cystatins, and kininogens). Here we report some evidence for the role of cathepsin B as a marker of macrophage activation and of lysosomal cysteine proteinases as potential nonspecific mediators of inflammation.