Molecular mechanisms of platelet activation and immune evasion

All vertebrates are equipped with vital signal transduction mechanisms that establish vascular repair and neutralization of transmitted pathogens. Inappropriate, disproportionate, or deficient responses in hemostasis and immunity frequently lead to thrombotic or inflammatory diseases. Moreover, immune inhibition by external factors, such as components of the S. aureus secretome, is a regular cause of mild or more threatening infection diseases. Modern clinical interference demands for novel therapeutic strategies that act on the level of tight cellular regulation. However, fundamental understanding of the underlying processes linked to these diseases is required prior to the development of new generations of drugs and antibiotics. This thesis describes novel molecular and mechanistic insights into processes of platelet activation, immune activation, and immune inhibition. In chapter 2, we present crystal structures of two GPVI-collagen complexes that provide the molecular details of GPVI-mediated platelet activation. We show that the most N-terminal Ig-like domain (D1-domain) encompasses a conserved collagen-binding site located across the D1 β-sheet formed by strands C, D, F, and F’. Interactions with collagen involve two of the three chains forming a triple helix and require a six-residue GPVI-binding site constituted by the canonical OGPOGP-sequence. Also small amino acids may be tolerated at the first or last position. In chapter 3, we present the crystal structure of the SSL3-TLR2 complex, which reveals the molecular and mechanistic basis for SSL3-mediated inhibition of TLR2-activation. SSL3 binds near the lipopeptide pocket of TLR2 and blocks TLR2-activation via inhibition of lipopeptide-binding and dimerization with TLR6 or TLR1. In chapter 4, we present a novel method to elucidate target-specific and SSL-glycan contributions to binding of S. aureus SSLs and host targets in immunity and hemostasis. Together with the crystal structure presented in chapter 3, we show that TLR2-inhibition by SSL3 is independent of the TLR2-glycans. SSL6-CD47 binding comprises mixed interactions, confirming target-specific binding to CD47. Opposite to that, SSL5-binding to GPVI is mediated by interactions with GPVI-glycans only, and GPVI is therefore not a specific target of SSL5. The results presented in these three chapters are summarized in chapter 5. Also discussed are implications following from our work for current progress in the development anti-thrombotic, anti-inflammatory, and anti-bacterial drugs.