Cellular immune responses to platelet factor 4 and heparin complexes in patients with heparin-induced thrombocytopenia

The effects of tranexamic acid on platelets in patients undergoing cardiac surgery: a systematic review and meta-analysis

This meta-analysis was designed to evaluate the effects of tranexamic acid (TXA) on platelets in patients undergoing cardiac surgery (CS). Relevant trials were identified by computerized searches of PUBMED, Cochrane Library, EMBASE, OVID, China National Knowledge Infrastructure (CNKI), Wanfang Data and VIP Data till Jun 4th, 2022, were searched using search terms "platelet", "Tranexamic acid", "cardiac surgery", "randomized controlled trial" database search was updated on Jan 1st 2023. Primary outcomes included platelet counts, function and platelet membrane proteins. Secondary outcome included postoperative bleeding. Search yielded 49 eligible trials, which were finally included in the current study. As compared to Control, TXA did not influence post-operative platelet counts in adult patients undergoing on- or off-pump CS, but significantly increased post-operative platelet counts in pediatric patients undergoing on-pump CS [(WMD = 16.72; 95% CI 6.33 to 27.10; P = 0.002)], significantly increased post-operative platelet counts in adults valvular surgery [(WMD = 14.24; 95% CI 1.36 to 27.12; P = 0.03). Additionally, TXA improved ADP-stimulated platelet aggression [(WMD = 1.88; 95% CI 0.93 to 2.83; P = 0.0001)] and improved CD63 expression on platelets [(WMD = 0.72; 95% CI 0.29 to 1.15; P = 0.001)]. The current study demonstrated that TXA administration did not affect post-operative platelet counts in adult patients undergoing either on- or off-pump CABG, but significantly increased post-operative platelet counts in pediatric patients undergoing on-pump CS and adults valvular surgery. Furthermore, TXA improved ADP-stimulated platelet aggression and improved CD63 expression on platelets. To further confirm this, more well designed and adequately powered randomized trials are needed.

Platelet factor 4 induces bone loss by inhibiting the integrin α5-FAK-ERK pathway

BACKGROUND

The effect of platelet factor 4 (PF4) on bone marrow mesenchymal stem cells (BMMSCs) and osteoporosis is poorly understood. Therefore, this study aimed to evaluate the effects of PF4-triggered bone destruction in mice and determine the underlying mechanism.

METHODS

First, in vitro cell proliferation and cell cycle of BMMSCs were assessed using a CCK8 assay and flow cytometry, respectively. Osteogenic differentiation was confirmed using staining and quantification of alkaline phosphatase and Alizarin Red S. Next, an osteoporotic mouse model was established by performing bilateral ovariectomy (OVX). Furthermore, the PF4 concentrations were obtained using enzyme-linked immunosorbent assay. The bone microarchitecture of the femur was evaluated using microCT and histological analyses. Finally, the key regulators of osteogenesis and pathways were investigated using quantitative real-time polymerase chain reaction and Western blotting.

RESULTS

Human PF4 widely and moderately decreased the cell proliferation and osteogenic differentiation ability of BMMSCs. Furthermore, the levels of PF4 in the serum and bone marrow were generally increased, whereas bone microarchitecture deteriorated due to OVX. Moreover, in vivo mouse PF4 supplementation triggered bone deterioration of the femur. In addition, several key regulators of osteogenesis were downregulated, and the integrin α5-focal adhesion kinase-extracellular signal-regulated kinase (ITGA5-FAK-ERK) pathway was inhibited due to PF4 supplementation.

CONCLUSIONS

PF4 may be attributed to OVX-induced bone loss triggered by the suppression of bone formation in vivo and alleviate BMMSC osteogenic differentiation by inhibiting the ITGA5-FAK-ERK pathway.

Pathophysiology and Diagnosis of Drug-Induced Immune Thrombocytopenia

Drug-induced immune thrombocytopenia (DITP) is a life-threatening clinical syndrome that is under-recognized and difficult to diagnose. Many drugs can cause immune-mediated thrombocytopenia, but the most commonly implicated are abciximab, carbamazepine, ceftriaxone, eptifibatide, heparin, ibuprofen, mirtazapine, oxaliplatin, penicillin, quinine, quinidine, rifampicin, suramin, tirofiban, trimethoprim-sulfamethoxazole, and vancomycin. Several different mechanisms have been identified in typical DITP, which is most commonly characterized by severe thrombocytopenia due to clearance and/or destruction of platelets sensitized by a drug-dependent antibody. Patients with typical DITP usually bleed when symptomatic, and biological confirmation of the diagnosis is often difficult because detection of drug-dependent antibodies (DDabs) in the patient’s serum or plasma is frequently not possible. This is in contrast to heparin-induced thrombocytopenia (HIT), which is a particular DITP caused in most cases by heparin-dependent antibodies specific for platelet factor 4, which can strongly activate platelets in vitro and in vivo, explaining why affected patients usually have thrombotic complications but do not bleed. In addition, laboratory tests are readily available to diagnose HIT, unlike the methods used to detect DDabs associated with other DITP that are mostly reserved for laboratories specialized in platelet immunology.

Platelet factor 4 and β-thromboglobulin mRNAs in circulating microparticles of trauma patients as diagnostic markers for deep vein thrombosis

Deep vein thrombosis (DVT) is a common complication after trauma. The development of markers to predict DVT in trauma patients is needed, and circulating microparticles (MPs) and their contents are possible candidates. In this study, we aimed to identify platelet factor 4 (PF4) and β-thromboglobulin (β-TG) mRNAs in circulating MPs as potential markers for DVT diagnosis in trauma patients. Fifteen trauma patients diagnosed with DVT and fifteen matched patients without DVT were included in this study. Fifteen healthy volunteers also were included as controls. Circulating MPs were obtained from the plasma of all study subjects. Annexin V+ MPs and platelet-derived MPs (PMPs) were quantified using flow cytometry. PF4 and β-TG mRNAs in MPs were determined by qPCR, and the common logarithm of relative quantitation (RQ) was calculated using the comparative Ct method. Receiver-operating characteristic (ROC) curves were performed to analyze the diagnostic value of PF4 and β-TG mRNAs. No significant differences were found in Annexin V+ MPs and PMPs levels between trauma patients with and without DVT. However, both PF4 and β-TG mRNAs in MPs from the DVT group were significantly higher than the non-DVT group and healthy controls (P = 0.014 for PF4, P = 0.010 for β-TG). The ROC curve analysis showed that both the PF4 mRNA (area-under curve (AUC) 0.756, P = 0.017) and the β-TG mRNA (AUC 0.751, P = 0.019) had a positive predictive value for DVT. This finding indicates that the PF4 and β-TG mRNAs in MPs may be used as potential biomarkers for DVT diagnosis in trauma patients.

Regulatory T Cells Control PF4/Heparin Antibody Production in Mice

Heparin-induced thrombocytopenia is a relatively common drug-induced immune disorder that can have life-threatening consequences for affected patients. Immune complexes consisting of heparin, platelet factor 4 (PF4), and PF4/heparin-reactive Abs are central to the pathogenesis of heparin-induced thrombocytopenia. Regulatory T (Treg) cells are a subpopulation of CD4 T cells that play a key role in regulating immune responses, but their role in controlling PF4/heparin-specific Ab production is unknown. In the studies described in this article, we found that Foxp3-deficient mice lacking functional Treg cells spontaneously produced PF4/heparin-specific Abs. Following transplantation with bone marrow cells from Foxp3-deficient but not wild-type mice, Rag1-deficient recipients also produced PF4/heparin-specific Abs spontaneously. Adoptively transferred Treg cells prevented spontaneous production of PF4/heparin-specific Abs in Foxp3-deficient mice and inhibited PF4/heparin complex-induced production of PF4/heparin-specific IgGs in wild-type mice. Treg cells suppress immune responses mainly through releasing anti-inflammatory cytokines, such as IL-10. IL-10-deficient mice spontaneously produced PF4/heparin-specific Abs. Moreover, bone marrow chimeric mice with CD4 T cell-specific deletion of IL-10 increased PF4/heparin-specific IgG production upon PF4/heparin complex challenge. Short-term IL-10 administration suppresses PF4/heparin-specific IgG production in wild-type mice. Taken together, these findings demonstrate that Treg cells play an important role in suppressing PF4/heparin-specific Ab production.