NOVEL ASPECT OF AMPHOTERICIN-B ACTION - ACCUMULATION IN HUMAN MONOCYTES POTENTIATES KILLING OF PHAGOCYTOSED CANDIDA-ALBICANS

The influence of low doses of amphotericin B on the capacity of human monocytes to kill Candida albicans was investigated. Killing rates wer e quantified by a novel flow cytometric assay and were found to be 37% +/- 3% (standard error of the mean) after 3 h. Preincubation of monoc ytes for 6 to 20 h with low concentrations of amphotericin B (0.2 mug/ ml) resulted in a markedly augmented fungicidal capacity. Enhancement of killing was 80% +/- 11% (standard error of the mean) over that by t he controls. This effect did not appear to be due to amphotericin B-de pendent monocyte activation; the respiratory burst and expression of h uman leukocyte antigen-DR were unaltered, and no stimulation of interl eukin-1beta release occurred. Cell-associated amphotericin B was extra cted with acetonitrile and was quantified by scanning spectrophotometr y. Amphotericin B appeared to accumulate in the cells, and intracellul ar concentrations attained after overnight incubation in 1 mug of the drug per ml were estimated to be in the range of 50 fg per cell. The f act that intracellular accumulation was responsible for the enhanced f ungicidal capacity of monocytes was supported by the findings that kil ling of Staphylococcus aureus remained normal and enhancement of killi ng of an amphotericin B-resistant C. albicans strain was minimal. Dram atic enhancement of monocyte fungicidal capacity probably extends to o ther amphotericin B-susceptible fungi and could represent a hitherto u nrecognized determinant underlying the curative properties and prophyl actic efficacy of this drug.