SELECTIVE KILLING OF HUMAN MONOCYTES AND CYTOKINE RELEASE PROVOKED BYSPHINGOMYELINASE (BETA-TOXIN) OF STAPHYLOCOCCUS-AUREUS

The best-known activity of Staphylococcus aurens sphingomyelinase C. a lias beta-toxin, is as a hemolysin that provokes hot-cold lysis of ery throcytes which contain substantial amounts of sphingomyelin in the pl asma membrane. Sheep erythrocytes are most susceptible, and me found t hat one hemolytic unit, representing the toxin concentration that elic its 50% hemolysis of 2.5 x 10(8) erythrocytes per ml, corresponds to 0 .05 enzyme units or to approximately 0.25 mu g of sphingomyelinase per ml. The cytotoxic action of beta-toxin on nucleated cells has not bee n described in any detail before, and the present investigation was un dertaken to fill this information gap. We now identify beta-toxin as a remarkably potent monocytocidal agent. At a concentration of 0.001 U/ ml, corresponding to approximately 5 ng/ml, beta-toxin killed over 50% of human monocytes (10(6) cells per ml) within 60 min. By contrast, 1 to 5 mu g of beta-toxin per ml had no cytocidal effects on human gran ulocytes, fibroblasts, lymphocytes, or erythrocytes. A selective monoc ytocidal action was also observed with sphingomyelinase C from Bacillu s cereus and a Streptomyces sp., whereas phospholipase A2 and phosphol ipase D at 100 U/ml were without effect. Monocytes succumbing to the a ction of beta-toxin processed and released interleukin-1 beta, soluble interleukin-6 receptor, and soluble CD14 into the supernatant. Thus, monocyte killing by beta-toxin is associated with cytokine-related eve nts that are important for the initiation and progression of infectiou s disease. These findings uncover a potentially important role for sph ingomyelinase as a determinant of microbial pathogenicity.