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.

Animal models recapitulating COVID-19 are critical to enhance our understanding of SARS-CoV-2 pathogenesis. Intranasally inoculated transgenic mice expressing human angiotensin-converting enzyme 2 under the cytokeratin 18 promoter (K18-hACE2) represent a lethal model of SARS-CoV-2 infection. We evaluated the clinical and virological dynamics of SARS-CoV-2 using two intranasal doses (10⁴ and 10⁶ PFUs), with a detailed spatiotemporal pathologic analysis of the 10⁶ dose cohort. Despite generally mild-to-moderate pneumonia, clinical decline resulting in euthanasia or death was commonly associated with hypothermia and viral neurodissemination independent of inoculation dose. Neuroinvasion was first observed at 4 days post-infection, initially restricted to the olfactory bulb suggesting axonal transport via the olfactory neuroepithelium as the earliest portal of entry. Absence of viremia suggests neuroinvasion occurs independently of transport across the blood-brain barrier. SARS-CoV-2 tropism was neither restricted to ACE2-expressing cells (e.g., AT1 pneumocytes), nor inclusive of some ACE2-positive cell lineages (e.g., bronchiolar epithelium and brain vasculature). Absence of detectable ACE2 protein expression in neurons but overexpression in neuroepithelium suggest this as the most likely portal of neuroinvasion, with subsequent ACE2 independent lethal neurodissemination. A paucity of epidemiological data and contradicting evidence for neuroinvasion and neurodissemination in humans call into question the translational relevance of this model.

The abundance of evidence suggest that inflammation of immune and non-immune cells may lead to an imbalance of the pro- and anti-coagulant state during viral infections. During systemic infections, the endothelium plays a critical role in regulating hemostasis, and severe imbalances of endothelial function and activation can contribute to organ failure. Viral infections may elevate plasma levels of procoagulant markers such as TAT and D-dimer TF-positive MPs as well as von Willebrand factor (vWF). Although multiple clinical studies are showing the association of viral infection and increased prothrombotic risk, the pathological mechanisms have not been fully identified for most viral infections. Viral infection mediated TLRs activation is both cell type- and species-specific and explains the difficulties in correlating murine model data with the human data. In this review, we briefly discuss the TF-dependent coagulation activation, Toll-like receptors (TLRs) signaling during viral infections, and their contributions to the procoagulant response.

Our results demonstrate a functional role for the commensal microbiota in atherothrombosis. In a ferric chloride injury model of the carotid artery, GF C57BL/6J mice had increased occlusion times compared to colonized controls. Interestingly, in late atherosclerosis, HFD-fed GF Ldlr ^−/− mice had reduced plaque rupture-induced thrombus growth in the carotid artery and diminished ex vivo thrombus formation under arterial flow conditions.