Actinobacillus actinomycetemcomitans induces lethal effects on the macrophage-like human cell line U937

We examined the cytotoxicity in culture medium of Actinobacillus actinomycetemcomitans against the human monocyte-macrophage-like cell line U937 using the trypan blue exclusion test and WST-1 test. We found that A. actinomycetemcomitans Y4 showed the highest cytotoxic activity among the three different serotype strains and the cytotoxic effects of both bacterial cells and culture supernatants in A. actinomycetemcomitans Y4 were stronger on phorbol-12-myristate 13-acetate (PMA)-induced U937 cells than uninduced U937 cells. Morphological changes in PMA-induced U937 cells treated with culture supernatants differed from those treated with leukotoxin, and a difference in the susceptibility to 56 degrees C heat treatment was found between culture supernatants and leukotoxin. The cytotoxic activity by WST-1 was determined more rapidly and strongly than that by trypan blue assay. These findings suggested that the cytotoxic effect of A. actinomycetemcomitans was influenced by the differentiation of U937 cells and may be more potent on the respiratory chain than the cell membrane.