Key Points: Myeloid cell TF-dependent venous thrombosis is under control of PDI and the complement cascade. C5 deficiency reduces fibrin formation and leukocyte PS exposure with normal platelet deposition in flow-restricted vessels.

The symbiotic gut microbiota plays pivotal roles in host physiology and the development of cardiovascular diseases, but microbiota-triggered pattern recognition signaling mechanisms impacting thrombosis are poorly defined. Here, we show that germ-free and Toll-like receptor (Tlr) 2-deficient mice have reduced thrombus growth following carotid artery injury relative to conventionally raised controls. Germ-free Tlr2^-/- and wild-type mice were indistinguishable, but colonization with microbiota restored a significant difference in thrombus growth between genotypes. We identify reduced plasma levels of von Willebrand Factor (VWF) and reduced VWF synthesis specifically in hepatic endothelial cells as a critical factor that is regulated by gut microbiota and determines thrombus growth in Tlr2^-/- mice. Static platelet aggregate formation on extracellular matrix was similarly reduced in germ-free wild-type, Tlr2^-/-, and heterozygous Vwf^+/- mice that are all characterized by a modest reduction in plasma VWF level. Defective platelet matrix interaction can be restored by exposure to wild-type plasma or to purified VWF dependent on the VWF integrin binding site. Moreover, administration of VWF rescues defective thrombus growth in Tlr2^-/- mice in vivo. These experiments delineate an unexpected pathway, in which microbiota-triggered TLR2 signaling alters the synthesis of pro-adhesive VWF by the liver endothelium and favors platelet integrin-dependent thrombus growth. Key points VWF synthesis in liver sinusoidal endothelial cells is regulated by gut microbiota through TLR2 signaling. Reduced plasma VWF in germ-free and Tlr2^-/- mice cause reduced thrombus formation at the ligation injured carotid artery.

Commensal gut microbiota have recently been implicated in cardiovascular disease (CVD) and cerebrovascular disease. Atherosclerotic plaque formation depends on the colonization status of the host. In addition to host nutrition and the related microbiota‐dependent metabolic changes, activation of innate immune pathways triggers the development of atherosclerosis and supports arterial thrombosis. Gnotobiotic mouse models have uncovered that activation of Toll‐like receptor‐2 by gut microbial ligands supports von Willebrand factor‐integrin mediated platelet deposition to the site of vascular injury. Depending on nutritional factors, the microbiota‐derived choline‐metabolite trimethylamine N‐oxide (TMAO) increases atherosclerotic plaque size, triggers prothrombotic platelet function and promotes arterial thrombus growth. Hence, the composition of the commensal microbiota is an emerging risk factor for CVD. Here, we provide an overview on microbiota‐dependent pathomechanisms that drive the development of CVD and arterial thrombosis.

The essential amino acid tryptophan (Trp) is metabolized by gut commensals, yielding in compounds that affect innate immune cell functions directly, but also acting on the aryl hydrocarbon receptor (AHR), thus regulating the maintenance of group 3 innate lymphoid cells (ILCs), promoting T helper 17 (T_H17) cell differentiation, and interleukin-22 production. In addition, microbiota-derived Trp metabolites have direct effects on the vascular endothelium, thus influencing the development of vascular inflammatory phenotypes. Indoxyl sulfate was demonstrated to promote vascular inflammation, whereas indole-3-propionic acid and indole-3-aldehyde had protective roles. Furthermore, there is increasing evidence for a contributory role of microbiota-derived indole-derivatives in blood pressure regulation and hypertension. Interestingly, there are indications for a role of the kynurenine pathway in atherosclerotic lesion development. Here, we provide an overview on the emerging role of gut commensals in the modulation of Trp metabolism and its influence in cardiovascular disease development.

Interleukin-17A– (IL-17A) and IL-17F–producing CD4⁺ T helper cells (T_H17 cells) are implicated in the development of chronic inflammatory diseases, such as multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE). T_H17 cells also orchestrate leukocyte invasion of the central nervous system (CNS) and subsequent tissue damage. However, the role of IL-17A and IL-17F as effector cytokines is still confused with the encephalitogenic function of the cells that produce these cytokines, namely, T_H17 cells, fueling a long-standing debate in the neuroimmunology field. Here, we demonstrated that mice deficient for IL-17A/F lose their susceptibility to EAE, which correlated with an altered composition of their gut microbiota. However, loss of IL-17A/F in T_H cells did not diminish their encephalitogenic capacity. Reconstitution of a wild-type–like intestinal microbiota or reintroduction of IL-17A specifically into the gut epithelium of IL-17A/F–deficient mice reestablished their susceptibility to EAE. Thus, our data demonstrated that IL-17A and IL-17F are not encephalitogenic mediators but rather modulators of intestinal homeostasis that indirectly alter CNS-directed autoimmunity.

Polyphosphates are linear polymers and ubiquitous metabolites. Bacterial polyphosphates are long chains of hundreds of phosphate units. Here, we report that mouse survival of peritoneal Escherichia coli sepsis is compromised by long-chain polyphosphates, and improves with bacterial polyphosphatekinase deficiency or neutralization using recombinant exopolyphosphatase. Polyphosphate activities are chain-length dependent, impair pathogen clearance, antagonize phagocyte recruitment, diminish phagocytosis and decrease production of iNOS and cytokines. Macrophages bind and internalize polyphosphates, in which their effects are independent of P2Y1 and RAGE receptors. The M1 polarization driven by E. coli derived LPS is misdirected by polyphosphates in favor of an M2 resembling phenotype. Long-chain polyphosphates modulate the expression of more than 1800 LPS/TLR4-regulated genes in macrophages. This interference includes suppression of hundreds of type I interferon-regulated genes due to lower interferon production and responsiveness, blunted STAT1 phosphorylation and reduced MHCII expression. In conclusion, prokaryotic polyphosphates disturb multiple macrophage functions for evading host immunity.