Synthesis and processing of cathepsin L, an elastase, by human alveolar macrophages

Cathepsin L was partially purified from lysates of freshly isolated macrophages lavaged from lungs of apparently healthy adults and found to be chromatographically and catalytically identical with liver cathepsin L. Western-blotting analysis showed that lung macrophages contain significant levels of a precursor of cathepsin L (43 kDa) in addition to mature enzyme (25 kDa). After culturing for a further 24 h, the precursor disappeared and a new band, corresponding to 34 kDa, appeared, suggesting that the precursor had been processed to an intermediate form of cathepsin L. Biosynthetic labelling of macrophages in vitro with [35S]methionine followed by immunoprecipitation with the cathepsin L antibody confirmed that the cells synthesize cathepsin L as a 43 kDa precursor that is then processed to the mature form (25 kDa) via a 34 kDa intermediate. The precursor, but not the processed forms, was released into the culture medium. During culture in vitro the 34 kDa intermediate accumulated, and little enzyme was processed to the 24 kDa form, consistent with the immunoblot data. Human lung macrophages contain a 1.5 kb transcript of cathepsin L mRNA, whereas none is detectable in human monocytes. These results establish that differentiation of human macrophages within the lung is accompanied by synthesis and expression of an elastinolytic enzyme, cathepsin L. The altered processing of cathepsin L observed during cultivation in vitro suggests caution in the assessment of the elastinolytic potential of human macrophages based on assay in vitro.