The relevance of alpha-naphthyl acetate esterases to various monocyte functions

Human monocytes contain a series of alpha-naphthyl acetate (alpha NA) esterases which are not present in other blood cells and which can be specifically inhibited by bis(4-nitrophenyl)-phosphate (BNPP). This inhibitor is non-toxic at the concentration used and thus enabled studies on the possible significance of this enzyme towards various monocyte functions. BNPP has no noticeable influence on adhesion and spreading of monocytes on glass surfaces, nor does it inhibit the phagocytosis of IgG-coated latex particles. BNPP does, however, diminish the spontaneous cytotoxicity of freshly isolated monocytes towards the erythroleukaemic cell line K562. In the single cell assay in agarose, BNPP treatment of monocytes leads to a decrease in the number of lytic and non-lytic effector-target cell conjugates. In contrast, the antibody-dependent cell-mediated cytotoxicity (ADCC) of monocytes as well as the natural cytotoxicity of lymphocytes towards K562 cells are not influenced by BNPP. The present investigations show that monocyte specific alpha NA esterases are involved in the spontaneous cytotoxicity of monocytes toward tumour cells.