Group A Streptococcus (GAS, Streptococcus pyogenes) is a Gram-positive, human-associated bacterium that ranges from an asymptomatic colonizer to a significant pathogen. The diseases caused by GAS can range from mild and self-limiting to severe and life-threatening, and post-infectious immune complications can also occur. In all, GAS infections and related sequelae account for more than half a million annual deaths worldwide. GAS is a versatile pathogen that utilizes several virulence factors to contribute to disease, and their effects include lysing host cells, manipulating host immune responses and coagulation, and enabling GAS to adhere to host components. One example is Streptolysin S (SLS), which is especially important for invasive GAS infections. SLS is predominantly associated with lysing erythrocytes, and it also induces inflammation and cytotoxicity in other host cells.
Recent evidence has suggested that SLS targets the membrane-associated ion transporter Band 3 to lyse erythrocytes, challenging the traditional hypothesis that SLS lyses host cells by disrupting cell membranes. However, the possibility that SLS targets other proteins to induce cytolytic effects in non-erythrocytic contexts has not been explored. Here, we use chemical inhibition studies to demonstrate that SLS induces cytotoxicity in human keratinocytes by targeting additional proteins involved in ion transport during GAS infections. These effects could be mitigated by treating keratinocytes with multiple stilbenedisulfonate compounds, which are associated with the inhibition of multiple ion transporters. Subsequent bioinformatics analysis and studies with more specific chemical inhibitors identified the electroneutral sodium-bicarbonate cotransporter NBCn1 as a protein target of SLS during GAS infections of epithelial cells. Targeted disruption of NBCn1 by SLS was associated with intracellular acidification in human keratinocytes, indicating a role for pH regulation in the host response to SLS. Together, these results expand on previously identified host signaling responses to SLS in keratinocytes by identifying a host protein that is targeted by the toxin to cause these signaling events, and support the hypothesis that GAS uses SLS as a multifunctional toxin to increase disease severity in several ways during infection. Further elucidation of how SLS targets host factors to promote disease has implications for the development of therapeutics for invasive GAS infections.