Complement Activation Occurs at the Surface of Platelets Activated by Streptococcal M1 Protein and This Results in Phagocytosis of Platelets

Abstract

Platelets circulate the bloodstream and principally maintain hemostasis. Disturbed hemostasis, a dysregulated inflammatory state, and a decreased platelet count are all hallmarks of severe invasive Streptococcus pyogenes infection, sepsis. We have previously demonstrated that the released M1 protein from S. pyogenes activates platelets, and this activation is dependent on the binding of M1 protein, fibrinogen, and M1-specific IgG to platelets in susceptible donors. In this study, we characterize the M1-associated protein interactions in human plasma and investigate the acquisition of proteins to the surface of activated platelets and the consequences for platelet immune function. Using quantitative mass spectrometry, M1 protein was determined to form a protein complex in plasma with statistically significant enrichment of fibrinogen, IgG3, and complement components, especially C1q. Using flow cytometry, these plasma proteins were also confirmed to be acquired to the platelet surface, resulting in complement activation on M1-activated human platelets. Furthermore, we demonstrated an increased phagocytosis of M1-activated platelets by monocytes, which was not observed with other physiological platelet agonists. This reveals a novel mechanism of complement activation during streptococcal sepsis, which contributes to the platelet consumption that occurs in sepsis.

The necroptotic cell death pathway operates in megakaryocytes, but not in platelet synthesis

“…While we did not implicate LPS-induced thrombocytopenia in the activation of necroptosis in megakaryocytes and platelets, our study provides novel mechanistic insight into a role of activation of extrinsic apoptosis in platelets in LPS-induced thrombocytopenia and sepsis. Pathogenic bacteria isolated from patients with sepsis and M1 protein from Streptococcus pyogenes have been shown to trigger apoptosis of human platelets in vitro 74,75 , suggesting a mechanism by which bacterial pathogens might cause thrombocytopenia in patients with blood stream infections. Future studies are needed to investigate if the mechanistic insight gained from our in vivo study could help the development of targeted therapies combating thrombocytopenia in sepsis patients.…”

The role of platelets in sepsis

“…48 This results in C1q acquisition and complement activation at the platelet surface and increased immune-mediated destruction of these platelets in vitro. 49 Mouse platelets lack the FcγRIIA receptor, which is a significant challenge to investigating these platelet-bacteria interactions in experimental models of sepsis. Importantly, FcγRIIA transgenic mice have been generated, and it is extremely beneficial to study bacterial sepsis and organ dysfunction in this background.…”

“…M protein released from Streptococcus pyogenes forms a complex with plasma fibrinogen and IgG engages the platelet GPIIb/IIIa and FcγRIIA receptors to mediate platelet activation 48 . This results in C1q acquisition and complement activation at the platelet surface and increased immune‐mediated destruction of these platelets in vitro 49 . Mouse platelets lack the FcγRIIA receptor, which is a significant challenge to investigating these platelet‐bacteria interactions in experimental models of sepsis.…”

Playing With Fire: Proinflammatory Virulence Mechanisms of Group A Streptococcus

“…Some M protein alleles may also have weak T cell mitogen activity ( Påhlman et al., 2007 ). Binding of M protein to platelets leads to their activation and thrombosis ( Horváth et al., 2004 ; Shannon et al., 2007 ; Palm et al., 2019 ). Lastly, M protein is a major agonist of TLR2, inducing expression of the numerous proinflammatory molecules regulated by NF-kB ( Pahlman et al., 2006 ), and activates the NLRP3 inflammasome, resulting in proinflammatory cell death by pyroptosis ( Valderrama et al., 2017 ).…”