Regular Article PLATELETS AND THROMBOPOIESIS Bacillus anthracis peptidoglycan activates human platelets through FcgRII and complement

In humans, sepsis is caused by an exaggerated inflammatory response to components of infectious organisms. The inflammation involves platelets which propagate blood coagulation in the vasculature. Thrombocytopenia resulting from in vivo platelet aggregation is common in patients in intensive care units with bacterially driven sepsis and is prognostic for intensive care unit mortality. Thrombocytopenia is associated with changes in vascular permeability, a common feature in sepsis. Furthermore, platelet aggregation contributes to the coagulation dysfunction found in severe sepsis by providing a surface for the propagation of blood coagulation through binding of coagulation factors V and X to promote pathological thrombus formation in the vasculature and organs. The proximal cause of systemic inflammation in gram-negative infections is lipopolysaccharide. However, a corresponding grampositive bacterial product that causes systemic inflammation and can activate platelets is less well defined. The cell walls of gram-positive bacteria are polymers of peptidoglycan (PGN), teichoic acids, and palmitoylated proteins. Many of these polymers are recognized by Toll-like receptors (TLRs) and/or nucleotide-binding oligomerization domain (NOD) sensors expressed by immune cells. PGN, a disaccharide polymer highly crosslinked by a short peptide chain, is the core component of gram-positive cell walls. The minimal essential structures of PGN, isoglutamine-diaminopimelic acid (iE-DAP) and muramyl dipeptide (MDP), are detected by the cytoplasmic NOD1 and 2 receptors. Whether and if mammalian immune cells recognize PGN has been controversial. Earlier studies concluded that PGN stimulated TLR2 but more recent studies showed that TLR2 recognition of PGN was instead due to contaminants in PGN preparations. Our previous work established that extensively purified polymeric PGN stimulates proinflammatory cytokines from innate immune cells by phagocytosis and digestion in lysosomes to the iE-DAP and MDP monomers. We also identified the unusual means by which PGN carries out this response: PGN is opsonized by anti-PGN immunoglobulin G (IgG), ubiquitously present in healthy humans. The IgG-opsonized PGN binds Fcg receptors (FcgR) on monocytes and neutrophils to initiate phagocytosis and lysosomal digestion. Laboratory mice lack anti-PGN IgG and hence murine innate immune cells do not respond to PGN by proinflammatory cytokine production. Thus, the presence of anti-PGN IgG and the expression of a FcgR are necessary for innate immune cells to respond to PGN. Human platelets, but not those of rodents, express FcgRIIa, an activating IgG receptor. Binding of platelet FcgRIIa to IgGopsonized targets induces platelet activation, indicated by platelet aggregation, expression of activated integrin aIIbb3 capable of fibrinogen binding, and exposure of phosphatidylserine (PS).