Staphylococcal scalded skin syndrome: Exfoliative toxin A (ETA) induces serine protease activity when combined with A431 cells

Staphylococcal scalded skin syndrome is the term used for a spectrum of primarily neonatal blistering skin diseases caused by the exfoliative toxins, ETA and ETB, of Staphylococcus aureus. Despite 25 y of research, the toxins' mechanism of action is still poorly understood, although evidence suggests that they may act as serine proteases. In this study, 0.1 mg purified ETA isolated from a baby with pemphigus neonatorum was incubated with A431 cells (a human squamous cell line) at 37 °C for 8, 24 and 48 h and the supernatant tested for protease activity using azocasein as a non-specific substrate. Phosphate-buffered saline was also incubated as negative control. Incubation of ETA with A431 cells for 48 h resulted in a four-fold increase in supernatant azocaseinolytic activity compared with buffer and cells, ETA alone and buffer alone (p < 0.001). Furthermore, this proteolytic activity was inhibited by PMSF (p < 0.001), a specific serine protease inhibitor. These results provide further evidence for the role of the exfoliafive toxins as serine proteases. Furthermore, the A431 cell assay provides a simpler, quicker, cheaper and more acceptable alternative to neonatal mouse epidermis to study the mechanism of action of the exfoliative toxins.