INTRACELLULAR MULTIPLICATION OF LEGIONELLA-PNEUMOPHILA IN HL-60 CELLSDIFFERENTIATED BY 1,25-DIHYDROXYVITAMIN-D3 AND THE EFFECT OF INTERFERON-GAMMA

We examined leukemic cells, HL-60, an acute promyelocytic leukemia cel l line, after differentiation induced by 1,25-dihydroxyvitamin D3 (D3) and retinoic acid (A) for infection of Legionella pneumophila, the et iologic agent of Legionnaires' disease. We investigated the effect of interferon gamma (IFN-gamma) on the differentiated cells and on the in tracellular growth of the bacteria. An examination of morphological an d antigenic changes in the cells was also included in the study. After 4-day incubation with 10(-6)M D3 or A, the HL-60 cells differentiated into monocyte-like (D3-HL-60) or mature granulocyte-like (A-HL-60) ce lls, respectively. They were then infected with L. pneumophila. Intrac ellular multiplication of the bacteria was evident in D3-HL-60 cells b ut not in HL-60 or A-HL-60 cells. D3-HL-60 cells required a 24-h infec tion time for the intracellular growth of L. pneumophila. D3-HL-60 cel ls activated with human recombinant IFN-gamma for 1-24 h (gamma-IFN-D3 -HL-60 cells) before infection markedly inhibited L. pneumophila multi plication, the effect of IFN-gamma being dose dependent. Surface marke r analysis was carried out in HL-60, D3-HL-60, and gamma-IFN-D3-HL-60 cells. On D3-HL-60 cells, CD11b, CD11c, CD14, and CD35 antigen increas ed, whereas CD71 and HLA-DR antigen decreased. This finding suggested that HL-60 cells differentiated into monocyte-like cells; the acquisit ion of the complement receptors, CD11b(CR3) and CD35(CR1), seemed to b e important for phagocytosis and for the subsequent intracellular mult iplication of L. pneumophila. The gamma-IFN-D3-HL60 cells showed an in crease of CD16, CD36, CD71, and HLA-DR antigen, suggesting that they w ere in an activated state. Our study indicated, first, that D3 can ind uce human leukemic cells to differentiate into functional monocyte-mac rophage-like cells that can support the intracellular multiplication o f L. pneumophila and, second, that these differentiated leukemic cells can be activated by IFN-gamma to markedly inhibit bacterial growth.