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Streptolysin S exists in a cell-bound form and as an extracellular complex between a nonspecific carrier (serum, serum albumin, ribonucleic acid [RNA], Triton, Tween) and a hemolytic moiety (probably a peptide) synthesized by streptococci. Although all the forms of streptolysin S, at 100 hemolytic units, killed mouse leukocyte monolayers, the time needed to kill 100% of the cells varied with the different streptolysin S preparations. Whereas 30 min was sufficient for the cell-bound hemolysin to kill all of the cells, 60 and 180 min were required when RNA streptolysin S and serum streptolysin S, respectively, were employed. Addition of 10% mouse serum to RNA streptolysin S or to cell-bound hemolysin delayed the killing of the leukocytes. The delayed killing observed with serum and albumin hemolysins is probably due to competition for the hemolytic moiety between the carrier molecules and target sites (phospholipids) upon the leukocyte membrane. Serum streptolysin S must be constantly incubated with the cells for 90 min for 100% of the cells to undergo cytopathic changes upon subsequent incubation for an additional 90 min. Streptolysin S inhibitor (trypan blue) added to the system after 30 or 60 min of incubation resulted in the killing of 50 and 100% of the leukocytes, respectively, when the cells were further incubated for 120 min. It is suggested that 30 min of incubation was not sufficient for the transfer of enough streptolysin S molecules upon the cell surface to allow killing of all of the cells. Sublethal amounts of streptolysin S, streptolysin O, and saponin suppressed phagocytosis of streptococci by mouse peritoneal macrophages. This effect was abolished by inhibitors of streptolysin S (trypan blue) and of streptolysin O and saponin (cholesterol). With sublethal amounts of streptolysin S, no inhibition of the reduction of nitro blue tetrazolium by nonphagocytosing cells was observed, but these amounts of streptolysin S caused a 50% inhibition of the reduction of nitro blue tetrazolium by phagocytosing leukocytes. It is suggested that some metabolic systems, which are normally enhanced during phagocytosis, have been affected by sublethal doses of streptolysin S. The results indicate that the in vivo production of small amounts of streptolysins S and O by group A streptococci may inhibit phagocytosis and may thus contribute to the invasiveness and pathogenicity of this microorganism.

Oxygen-Stable Hemolysins of Group A Streptococci VIII. Leukotoxic and Antiphagocytic Effects of Streptolysins S and O