To study the influence of a biologic environment on cultured human leukemia cells, KG-1, KG-1a, and HL-60 cells were inoculated subcutaneously into newborn nude mice. The cells developed myelosarcomas at the site of inoculation and in lungs and kidneys. KG-1 and HL-60 myelosarcomas were successfully passaged through adult nude mice, whereas KG-1a tumors proliferated only after transplantation into newborn hosts. The human nature of the cells forming myelosarcomas in mice was assessed by chromosomal analyses and detection of cross- reactivity with an antibody to the human leukemia cell line K562. We undertook electron microscopic and cytochemical examinations of the cells proliferating in vitro and in the mice. The granules of KG-1 cells in vivo did not react for acid phosphatase, as observed in vitro, and the HL-60 cells proliferating in mice lost the perinuclear myeloperoxidase (MPO) demonstrated in cultured cells. Although the influence of an in vivo selection of cell subpopulations cannot be ruled out, the enzymatic changes are compatible with induced cell differentiation. Conclusive evidence of differentiation in vivo was observed in the KG-1a cell subline. The undifferentiated KG-1a blasts developed cytoplasmic granules and synthesized MPO during proliferation in vivo. These observations indicate that human leukemia cells from established cell lines proliferate in nude mice and may acquire new differentiated properties in response to the in vivo environment.

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