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Cathepsin D is a lysosomal acid proteinase that exists as different isoforms. This enzyme is found in most cell types, and macrophages have particularly high levels of this enzyme. Cathepsin D has been implicated in some disease related inflammatory processes that have also been associated with increased macrophage infiltration. The existance of isoforms of cathepsin D suggested the possibility that individual forms of this enzyme may have immunologically unique characteristics. Monoclonal antibodies to cathepsin D could aid in the study of the immunological characteristics of individual isoforms of this enzyme as well as provide a means by which to screen a variety of sample types for cathepsin D levels. In this study we generated a panel of monoclonal anti-cathepsin D antibodies and used these to screen isoelectrically separated cathepsin D samples for unique immunoreactivity patterns. We also used these antibodies to measure changes in the levels of intra- and extracellular cathepsin D that accompany monocyte to macrophage differentiation, and changes that occur in response to the treatment of melanoma cells with tumor necrosis factor (TNF) and interferon. We found that there are unique immunological characteristics associated with individual isoforms of human liver cathepsin D as well as cathepsin D from different cell types. We also observed increases in levels of this enzyme as monocytic cells differentiate to macrophage-like cells. This indicates that cathepsin may be involved in some of the proteolytic processes mediated by macrophages. Different inducers of differentiation resulted in different cathepsin 1D immunoreactivity patterns, suggesting that there may be isoform specificity dependent on the inducer. Cathepsin D levels also increase in neoplastic cells treated with TNF and/or interferon indicating that cathepsin D may play some role in the mechanism of action of the tumor inhibitory effects of these agents. The use of monoclonal antibodies provides assays for cathepsin D which can be used with a variety of sample types and further provides enhanced specificity and sensitivity relative to other assay systems used in the past.

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