Staphylococcal Superantigen-Like Protein 10 (SSL10) induces necroptosis through TNFR1 activation of RIPK3-dependent signal pathways

Staphylococcal aureus ( S. aureus ) infection can lead to a wide range of diseases such as sepsis and pneumonia. Staphylococcal superantigen like (SSL) proteins, specifically expressed by S. aureus , are shown to be involved in immune evasion during S. aureus infection. Here, we show that SSL10, an SSL family protein, exhibits potent cytotoxicity against human cells (HEK 293T and HUVEC) by inducing necroptosis upon binding to its receptor TNFR1 on the cell membrane. After binding, two distinct signaling pathways are activated downstream of TNFR1 in a RIPK3-dependent manner, i.e. , the RIPK1-RIPK3-MLKL and RIPK3-CaMKII-mitochondrial permeability transition pore (mPTP) pathways. We determined the crystal structure of SSL10 at 1.9 Å resolution and, in conjunction with cytotoxicity analysis of chimeric mutant variants, identified a surface region formed by residues in the N- and C-termini that can potentially serve as the TNFR1 binding site. Knockout of ssl10 in S. aureus profoundly reduced cytotoxicity of culture supernatants, indicating that SSL10 is involved in extracellular cytotoxicity during infection. This study thus provides the first description of cytotoxicity through induction of necroptosis by an SSL protein, and a potential target for clinical treatment of S. aureus -associated diseases.