In vitro culture of Toxoplasma gondii in HL-60 cells cnd cell-mediated immunity against Toxoplasma in dimethylsulfoxide(DMSO)-induced HL-60 cells, i.e., differentiation into granulocytes, were pursued. HL-60 cells were treated with various concentrations of DMSO, and 1.3%(v/v) for 3 day incubation was chosen as the optimal condition for differentiation into granulocytes. The degree of differentiation was assayed in physiological and functional aspects in addition to morphological point. When treated with 1.3% DMSO for 3 days, HL-60 cells did not synthesize DNA materials beyond background level, and showed active chemotactic response to chemotactic peptide, formyl-methionyl-leucyl-phenylalanine(FMLP). Morphologically promyelocytes of high nuclear/cytoplasmic(N/C) ratio changed to granulocytes of relatively low N/C ratio. The relationships between HL-60 cells or DMSO-induced HL-60 cells and Toxoplasma were examined after stain with Giemsa and fluorescent dye (acridine orange). HS-60 cells did not show any sign of toxoplasmacidal activity but showed intracellular proliferation of Toxoplasma to form rosette for 72 hr co-culture. In contrast, DMSO-induced HL-60 cells phagocytosed Toxoplasma within 1 hr, and performed a process of intracellular digestion of Toxoplasma thereafter. With the above results, it is suggested that phagosome-lysosome fusion is one of the critical events for the parasitism by Toxoplasma or for susceptibility of host cells. The in vitro culture system of this study has offered a defined condition to study the protozoan parasite-host cell interactions.
