Autoimmune disorders are often associated with low platelet count or thrombocytopenia. In immune-induced thrombocytopenia (IIT), a common mechanism is increased platelet activity, which can have an increased risk of thrombosis. In addition, or alternatively, auto-antibodies suppress platelet formation or augment platelet clearance. Effects of the auto-antibodies are linked to the unique structural and functional characteristics of platelets. Conversely, prior platelet activation may contribute to the innate and adaptive immune responses. Extensive interplay between platelets, coagulation and complement activation processes may aggravate the pathology. Here, we present an overview of the reported molecular causes and consequences of IIT in the most common forms of autoimmune disorders. These include idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus (SLE), antiphospholipid syndrome (APS), drug-induced thrombocytopenia (DITP), heparin-induced thrombocytopenia (HIT), COVID-19 vaccine-induced thrombosis with thrombocytopenia (VITT), thrombotic thrombocytopenia purpura (TTP), and hemolysis, the elevated liver enzymes and low platelet (HELLP) syndrome. We focus on the platelet receptors that bind auto-antibodies, the immune complexes, damage-associated molecular patterns (DAMPs) and complement factors. In addition, we review how circulating platelets serve as a reservoir of immunomodulatory molecules. By this update on the molecular mechanisms and the roles of platelets in the pathogenesis of autoimmune diseases, we highlight platelet-based pathways that can predispose for thrombocytopenia and are linked thrombotic or bleeding events.

Integrin αIIbβ3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbβ3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbβ3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbβ3. Our analysis points to the (autocrine) ADP P2Y₁ and P2Y₁₂ receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

Red blood cells (RBCs) and platelets contribute to the coagulation capacity in bleeding and thrombotic disorders. The thrombin generation (TG) process is considered to reflect the interactions between plasma coagulation and the various blood cells. Using a new high-throughput method capturing the complete TG curve, we were able to compare TG in whole blood and autologous platelet-rich and platelet-poor plasma to redefine the blood cell contributions to the clotting process. We report a faster and initially higher generation of thrombin and shorter coagulation time in whole blood than in platelet-rich plasma upon low concentrations of coagulant triggers, including tissue factor, Russell viper venom factor X, factor Xa, factor XIa, and thrombin. The TG was accelerated with increased hematocrit and delayed after prior treatment of RBC with phosphatidylserine-blocking annexin A5. RBC treatment with ionomycin increased phosphatidylserine exposure, confirmed by flow cytometry, and increased the TG process. In reconstituted blood samples, the prior selective blockage of phosphatidylserine on RBC with annexin A5 enhanced glycoprotein VI–induced platelet procoagulant activity. For patients with anemia or erythrocytosis, cluster analysis revealed high or low whole-blood TG profiles in specific cases of anemia. The TG profiles lowered upon annexin A5 addition in the presence of RBCs and thus were determined by the extent of phosphatidylserine exposure of blood cells. Profiles for patients with polycythemia vera undergoing treatment were similar to that of control subjects. We concluded that RBC and platelets, in a phosphatidylserine-dependent way, contribute to the TG process. Determination of the whole-blood hypo- or hyper-coagulant activity may help to characterize a bleeding or thrombosis risk.

Background Fluorogenic thrombin generation (TG) assays are commonly used to determine global coagulation phenotype in plasma. Whole blood (WB)‐TG assays reach one step closer to physiology by involving the intrinsic blood cells but erythrocytes cause variable quenching of the fluorescence signals, hampering its routine application. Objective To develop a new assay for continuous WB‐TG measurement. Methods In the new WB‐TG assay, the erythrocyte‐caused distortion of signal was solved by continuously mixing the sample during the measurement. The assay was validated by evaluating the reproducibility and comparing with the paper‐based WB‐TG assay (Ninivaggi et al., 2012). Reconstituted human blood and WB from 119 healthy donors was tested to explore the influences of hematocrit and platelet count on TG. Results This novel WB‐TG assay showed good reproducibility, while being less affected by contact activation compared with the previous paper‐based assay. Reconstitution experiments showed that the Lagtime of TG was shortened by the addition of platelets but not erythrocytes. Increasing hematocrit strongly augmented the Peak thrombin, even in the presence of high platelet counts. The Lagtime and Peak of WB‐TG of 119 healthy donors were positively related to erythrocyte count after adjusting for age, sex and oral contraceptive use with multiple linear regression analyses. The reference range and inter‐individual variation of WB‐TG were determined in the healthy cohort. Conclusions A novel WB‐TG assay was developed, which is a straight forward tool to measure the involvement of platelets and erythrocytes in TG and may assist the research of blood cells‐associated coagulation disorders.

The capacity of blood to form thrombin is a critical determinant of coagulability. Plasma thrombin generation (TG), a test that probes the capacity of plasma to form thrombin, has improved our knowledge of the coagulation system and shows promising utility in coagulation management. Although plasma TG gives comprehensive insights into the function of pro- and anticoagulation drivers, it does not measure the role of blood cells in TG. In this literature review, we discuss currently available continuous TG tests that can reflect the involvement of blood cells in coagulation, in particular the fluorogenic assays that allow continuous measurement in platelet-rich plasma and whole blood. We also provide an overview about the influence of blood cells on blood coagulation, with emphasis on the direct influence of blood cells on TG. Platelets accelerate the initiation and velocity of TG by phosphatidylserine exposure, granule content release and surface receptor interaction with coagulation proteins. Erythrocytes are also major providers of phosphatidylserine, and erythrocyte membranes trigger contact activation. Furthermore, leukocytes and cancer cells may be important players in cell-mediated coagulation because, under certain conditions, they express tissue factor, release procoagulant components and can induce platelet activation. We argue that testing TG in the presence of blood cells may be useful to distinguish blood cell–related coagulation disorders. However, it should also be noted that these blood cell–dependent TG assays are not clinically validated. Further standardization and validation studies are needed to explore their clinical usefulness.

The clinical utility of thrombomodulin-modified thrombin generation (TM-TG) in cirrhosis is uncertain. We conducted a prospective study to evaluate the prognostic value of TM-TG in cirrhosis. Patients were recruited during outpatient clinics (compensated and stable decompensated cirrhosis) or if admitted to our inpatient service (acutely decompensated cirrhosis). We performed whole blood (WB) and platelet-poor plasma (PPP) TM-TG at recruitment. All patients were prospectively followed-up for bleeding/thrombosis, hepatic decompensation, and liver-related death. We included 231 patients: 80 with compensated, 70 with stable decompensated, and 81 with acutely decompensated cirrhosis. Median follow-up was 414 days (range: 77-668). Eleven patients, all with acutely decompensated cirrhosis, experienced procedure-related bleeding. Both WB-TG and PPP-TG were more altered in bleeding vs. non-bleeding individuals (lower endogenous thrombin potential [ETP] and peak-height). However, only WB-TG could identify - at the individual patient level - those experiencing major bleeding (all having pre-procedural ETP <350 nmol/L∗min). In acutely decompensated cirrhosis, the AUC of WB-TG ETP for bleeding was 0.854 (95% CI 0.732-0.976), which was higher than that of PPP-TG ETP (0.676; 95% CI 0.524-0.809). Neither WB-TG nor PPP-TG could predict development of thrombosis, mostly portal vein thrombosis (n = 15). In compensated cirrhosis, WB-TG and PPP-TG were comparable between patients who experienced decompensation and those who did not. In decompensated cirrhosis, WB-TG and PPP-TG were more significantly altered in patients experiencing further decompensation/ACLF/liver-related death. A higher WB-TG ETP was linked to a lower risk of progression independently of MELD, Child-Pugh, and C-reactive protein (hazard ratio 0.4, 95% CI 0.21-0.79, p <0.01). In compensated cirrhosis, WB-TG and PPP-TG do not improve risk stratification. In decompensated cirrhosis, WB-TG may be a promising tool for estimating procedure-related bleeding risk. Thrombomodulin-modified thrombin generation (TM-TG) in cirrhosis is a well-established research tool to assess the complex coagulopathy of cirrhosis; however, its clinical utility is uncertain. In acutely decompensated cirrhosis, a TM-modified whole blood (WB)-TG ETP <350 nmol/L∗min predicted major bleeding after invasive procedures, whereas platelet-poor plasma TG indicated a hypo-coagulable state in bleeding patients but could not identify those at risk. Neither WB-TG nor platelet-poor plasma-TG could predict development of portal vein thrombosis, which was predicted by cirrhosis and portal hypertension severity. In decompensated cirrhosis, a better WB-TG capacity was associated with a lower risk of further decompensation, acute-on-chronic liver failure, and liver-related death independently of MELD score/Child-Pugh stage and C-reactive protein.