Graded increase in probability of eosinophilic differentiation of HL-60 promyelocytic leukemia cells induced by culture under alkaline conditions

HL-60 cells differentiate primarily to eosinophils instead of neutrophils when cultured with butyric acid if they have previously been cultured under alkaline conditions (pH 7.6). To determine the nature of the process by which this occurs, a group of single-cell derived clones was produced from HL-60 cells after prolonged passage under alkaline conditions. When these clones were induced to mature with butyric acid, each clone demonstrated a characteristic proportion of mature eosinophils and neutrophils. This property was stable for multiple passages. Subclones derived from these clones also demonstrated the same probability of differentiating to an eosinophil as their parent clones. Reversion toward neutrophilic differentiation gradually occurred after several months of culture under conditions of reduced pH. The most highly directed clones demonstrated 90-95% eosinophilic differentiation and continued to differentiate primarily to eosinophils after seven months of culture at the reduced pH. Thus, in HL-60 cells, the tendency to differentiate to an eosinophil is a long-lived, heritable, continuously variable phenotype that is inducible in cells by culture under alkaline conditions. This tendency persists for prolonged periods after the alkaline conditions are removed, but may gradually revert toward neutrophilic differentiation with time.