Supplementary Material for: The M1 Protein of <b><i>Streptococcus pyogenes</i></b> Triggers an Innate Uptake Mechanism into Polarized Human Endothelial Cells

Serotype M1 <i>Streptococcus pyogenes</i> is a major human pathogen associated with severe invasive diseases causing high morbidity and mortality. In a substantial number of cases, invasive disease develops in previously healthy individuals with no obvious port of entry. This has led to the hypothesis that the source of streptococci in these cases is a transient bacteraemia. This study focuses on the analysis of interaction of tissue-invasive serotype M1 <i>S. pyogenes</i> with human endothelial cells (EC) of the vascular system. We identify the M1 surface protein of <i>S. pyogenes</i> as the EC invasin which is recognised by polarized human blood EC, thereby triggering rapid, phagocytosis-like uptake of streptococci into polarized EC layers. Upon internalization, the M1 <i>S. pyogenes</i> serotype is incorporated into phagosomes which traffic via the endosomal/lysosomal pathway. However, some of the streptococci successfully evade this innate killing process and hereby mediate their escape into the cytoplasm of the host cell. The results of this study demonstrate that blood EC possess an efficient uptake mechanism for serotype M1 <i>S. pyogenes</i>. Despite efficient phagocytosis, streptococcal survival within EC constitutes one potential mechanism which favours intracellular persistence and thus facilitates continuous infection and dissemination from the primary side of infection into deep tissue.