Immunothrombosis versus thrombo-inflammation: platelets in cerebrovascular complications

Abstract

A State-of-the Art lecture titled "Thrombo-Neuroinflammatory Disease" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. First, we would like to advocate for discrimination between immunothrombosis and thrombo-inflammation, as immunothrombosis describes an overshooting inflammatory reaction that results in detrimental thrombotic activity. In contrast, thrombo-inflammation describes the interplay of platelets and coagulation with the immunovascular system, resulting in the recruitment of immune cells and loss of barrier function (hence, hallmarks of inflammation). Both processes can be observed in the brain, with cerebral venous thrombosis being a prime example of immunothrombosis, while infarct progression in response to ischemic stroke is a paradigmatic example of thrombo-inflammation. Here, we review the pathomechanisms underlying cerebral venous thrombosis and ischemic stroke from a platelet-centric perspective and discuss translational implications. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress.

Keywords: blood platelets; cerebral venous thrombosis; immunothrombosis; ischemic stroke; thrombo-inflammation.

Graphical abstract

Figure 1

Essential pathophysiological differences underlying immunothrombosis and thrombo-inflammation in the cerebral vasculature. Both entities involve platelets, the plasmatic coagulation cascade, and immune cells. However, the main difference is the primary outcome: in the case of immunothrombosis, the primary outcome is the occlusion of a blood vessel due to platelet-involved thrombotic activity in response to initial inflammatory stimuli. One example of an immunothrombotic disease is cerebral venous thrombosis. In the context of thrombo-inflammation, the hallmarks are the recruitment of immune cells and the disturbance of barrier functions. This is best exemplified in the context of ischemic stroke, where non-classical functions of platelets are activated during post-ischemic processes. These platelet functions beyond thrombosis then contribute to thrombo-inflammation. EC, endothelial cell; NET, neutrophil extracellular trap; PLT, platelet; RBC, red blood cell.

Figure 2

Cellular and molecular mechanisms regarding cerebral venous thrombosis (CVT) and vaccine-induced thrombotic thrombocytopenia (VITT). (A) One of the severe side effects of adenoviral vector-based SARS-CoV-2 vaccines is VITT. Briefly, the systemic inflammatory status due to vaccination can trigger a release of platelet factor 4 (PF4) from activated platelets. PF4 further forms a complex with vaccine components, which results in the engagement with anti-PF4 autoimmune antibodies (Abs). Once anti-PF4 antibodies are part of an immunocomplex, they are recognized by the FcγRIIA on the surface of platelets and neutrophils, leading to platelet activation and NETosis, ultimately resulting in thrombosis. As a severe complication of VITT, CVT occurs when the thrombi develop right in the brain venous system. The consumption of platelets further leads to secondary thrombocytopenia, which is particularly susceptible to intracerebral hemorrhage upon inflammatory conditions. (B) In an experimental CVT model, Fab-fragments of the anti-C-type lectin-like receptor-2 (CLEC-2) antibody INU1 apparently alter the conformation of CLEC-2. Consequently, CLEC-2 is able to interact with an unidentified ligand, which is enriched in cerebral veins. Then, cooperative signaling of CLEC-2 and α_IIbβ₃ triggers platelet activation, resulting in a rapidly progressing CVT. Blocking CLEC-2 signaling or α_IIbβ₃ (eg, using the anti-α_IIbβ₃ JON/A-F[ab’]₂-fragments) prevents CVT, indicating that this might be a novel therapeutic strategy. EC, endothelial cell.

Figure 3

A simplified model of platelets as orchestrators of thrombo-inflammation (as seen in acute ischemic stroke). The (post-)ischemic endothelium exposes von Willebrand factor (VWF), allowing platelets to interact with VWF via glycoprotein (GP) Ibα, which is critical for infarct progression. Subsequently, platelets become activated via various signaling pathways, including GPVI, PARs, and ADP receptors, among others. Upon activation, platelets shed CD84, thereby recruiting T cells, expose P-selectin (a ligand for PSGL-1 on neutrophils) or phosphatidylserine (PS) (which promotes not only plasmatic coagulation but also supports NETosis), and activate their integrin receptors. On the one hand, plasmatic coagulation results in more thrombin generation, further activating platelets. On the other hand, thrombin cleaves GPV, and soluble GPV dampens fibrin generation. Activated platelets also secrete their granule content, resulting in the release of second-wave mediators such as ADP (from dense granules) as well as protein factors such as high-mobility group box 1 (HMGB1) (from α-granules) that contribute to the recruitment of neutrophils and trigger NETosis. Ultimately, this all converges in the recruitment of immune cells and the disruption of the blood–brain barrier (BBB), demonstrating that ischemic stroke is a thrombo-inflammatory disease. Corresponding strategies of potential intervention and aftermaths are indicated by thumbs up or down (depending on the outcome in experimental models) for future therapeutic developments. ADP, adenosine diphosphate; EC, endothelial cell; NETosis, NET activation and release; PAR, protease-activated receptor; PLD1, phospholipase D1; Syk, spleen tyrosine kinase.