Electron microscopic and immunohistochemical observations of differentiation of human myeloid leukemia line, PL-21.

A new human myeloid leukemia cell line, PL-21, consisting of promyelocytes, was microscopically and immunohistochemically studied for their maturation induced by 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and retinoic acid (RA). More than 80% of PL-21 cells cultured for 4 days with 10 ng/ml of TPA became macrophage-like cells with functional and histochemical properties consistent with monocytes/macrophages. The cells adhered to the plastic flask, developed phagocytic activity and macrophage-specific intracytoplasmic alpha subunit of S-100 protein, and had reduced myeloid-specific cytochemical markers. In contrast, RA-treated PL-21 cells displayed mature neutrophil-like morphology after 7 days of exposure. Most of the cells had reduced nitro blue tetrazolium and acquired phagocytic activity with persistence of myeloid-specific cytochemical markers such as peroxidase, naphthol-AS-D chloroacetate esterase, and Sudan black B. These results indicate that the PL-21 cell line ca be induced to mature into two directions of macrophages and neutrophils by chemical inducers, and will provide a useful tool for studying the differentiation of leukemic cells and searching for other differentiation inducers.