No abstract

The cytotoxic action of the S component of leukocidin from Staphylococcus aureus on rabbit polymorphonuclear leukocytes was supported by the following observations. (i) Leukocytes displayed a large chemotactic response to the S component (10-1o M) as well as to the chemotactic factor N-formylmethionylleucylphenylalanine (10-11 M). (ii) The S component stimulated high levels of phospholipase A2 activity in the cell membranes, with concomitant synthesis and release of prostaglandins. (iii) Uptake of 45Ca into leukocytes exposed to the S component was about double the rate of uptake into untreated cells. The increased 45Ca uptake into the cells was not inhibited by trifluoperazine and ruthenium red. (iv) Indomethacin and alloxazine, which had no effects on the binding of the S component to the cells, attenuated markedly the stimulation of phospholipase A2 activity, the syntheses of prostaglandins, and the increased uptake of 45Ca caused by the S component. The F component of leukocidin, bound to rabbit leukocytes with the aid of the S component, rapidly induced complete release of 86Rb from preloaded leukocytes. This release resulted from stimulation of ouabain-insensitive (Na+ + K+)-adenosine triphosphatase activity and inhibition of cyclic AMP-dependent protein kinase.

The binding of "2'I-labeled S component to rabbit polymorphonuclear leukocytes was found to be concentration dependent and saturable at 37°C. Scatchard analysis of the binding curve gave a straight line, indicating that S component binds to a single population of sites. The dissociation constant, KD, derived from the Scatchard plot was 5.57 x 10-9 M, and the number of binding sites per leukocyte was calculated to be approximately 5,300. Unlabeled S component (10-8 M) or subunit B of cholera toxin (10-7 M) readily competed with 1 I-labeled S component binding, and the labeled S component, preincubated with ganglioside GM1 at equimolar proportions for 5 min, lost the binding capacity to the leukocyte membranes. The binding number of '25I-labeled F component to leukocidinsensitive cells, such as rabbit polymorphonuclear leukocytes and the established human myelocytic leukemia cells, in the absence and in the presence of the unlabeled S component (2.1 nM), was calculated to be 50 and 1,300 molecules per cell, respectively. This increased binding of the labeled F component was time and temperature dependent. The binding number of labeled F component to other cell types comparatively insensitive to leukocidin, such as erythrocytes, adipocytes, intestinal cells, and HeLa cells, was calculated to be less than 50 molecules per cell in spite of the sufficient amount of unlabeled S component bound to their cells. These observations are consistent with the view that in rabbit leukocyte the S component, preferentially bound to the cell surface at 5,300 molecules per cell, contributes to enhance the F component binding up to about 1,300 molecules per cell and may thus play a role of synergistic action of both leukocidin components on the cell membranes in the leukocytolysis.