Postpartum hemorrhage (PPH) is the leading cause of maternal death worldwide. The World Maternal Antifibrinolytic trial showed that antifibrinolytic tranexamic acid (TXA) reduces PPH deaths. Maternal anemia increases the risk of PPH. The World Maternal Antifibrinolytic-2 trial is now assessing whether TXA can prevent PPH in women with anemia. Low red blood cell (RBC) counts promote fibrinolysis by altering fibrin structure and plasminogen activation. We explored interactions between RBCs and TXA in inhibiting fibrinolysis. We used global fibrinolytic assays (ball sedimentation and viscoelasticity) to monitor the lysis of fibrin containing plasminogen and tissue-type plasminogen activator. We applied a fluorogenic kinetic assay to measure plasmin generation in fibrin clots and scanning electron microscopy to study fibrin structure. According to parallel-line bioassay analysis of the fibrin lysis-time data, the antifibrinolytic potency of 4-128 μM TXA was increased in the presence of 10% to 40% (v/v) RBCs. Global fibrinolysis assays showed that the joint effect of RBCs and TXA was about 15% larger than the sum of their individual effects in the inhibition of fibrinolysis. In plasminogen activation, TXA added the same increment of inhibition to the effect of RBCs at any cell count in the fibrin clot. Regarding fibrin structure, TXA thickened fibrin fibers, which impaired plasminogen activation, whereas RBCs promoted fine fibers that were more resistant to plasmin. The antifibrinolytic potency of TXA is enhanced in fibrin formed in the presence of RBCs through inhibition of plasminogen activation and fibrin lysis, which correlates with modifications of fibrin structures.

Rheumatoid arthritis (RA) is a systemic inflammatory autoimmune disease. The pathomechanism of RA depends on both B and T cells. Regulatory B cells (Breg) have been shown to suppress T-cell immune responses and play a key role in modulating autoimmune processes. We aimed to investigate the possibility of utilizing PD-L1+ Breg cells in downregulating the Th cells’ immune response in healthy individuals and RA patients. We hypothesized that the PD-1/PD-1L interaction plays a key role in this process, which may be defective in autoimmune diseases. We separated T and B cells from the peripheral blood of healthy volunteers and RA patients by magnetic cell sorting, and Th cells and Treg cells were isolated by fluorescence-activated cell sorting. The cytokine production by CD4+ Th cells was detected by intracellular flow cytometry. CpG and CD40L stimulations were applied to induce PD-L1hi expressing Breg cells. We found that the frequency of PD-L1hi cells is significantly lower in all B-cell subsets in RA compared to healthy controls. Functional analysis of induced PD-L1+ Breg cells in coculture with activated autologous Th cells has shown that healthy control samples containing higher levels of PD-L1hi Breg cells significantly inhibit IFN-ү and IL-21 production by Th cells. In contrast, RA patients’ samples with lower levels of PD-L1hi Breg cells failed to do so. Since the expression of PD-L1 on B cells can be modulated in vitro to induce Breg cell suppressive capacity, these data may provide new perspectives for future therapy for RA.

Background: Tranexamic acid (TXA) is a synthetic lysine analog widely used as an antifibrinolytic agent. Large randomized trials have demonstrated life-saving benefits when TXA is administered early in acute hemorrhage, but results regarding prophylactic administration have been conflicting, and several trials have not shown improved clinical outcomes. The mechanisms underlying this discrepancy remain incompletely understood. Objectives: To investigate the molecular and structural mechanisms that determine TXA efficacy in purified fibrin clots under conditions mimicking therapeutic versus prophylactic administration. Methods: We examined fibrinolysis induced by tissue plasminogen activator (tPA) in vitro using confocal microscopy, viscoelastic testing (ClotPro), turbidimetry, and plasmin generation assays at physiologically and therapeutically relevant concentrations of plasminogen and TXA. Scanning electron microscopy (SEM) was employed to assess fibrin structure. Results: When TXA was incorporated into fibrin clots before the addition of tPA, physiological plasminogen concentrations (2.5 µM) reversed the antifibrinolytic effect, resulting in paradoxical acceleration of lysis. By contrast, when clotting and fibrinolysis occurred simultaneously in the presence of TXA and tPA, TXA consistently prolonged lysis time irrespective of plasminogen concentration. SEM demonstrated that TXA, even at concentrations as low as 16 µM, doubled the top-quartile values of the fibrin fiber diameter, altering susceptibility to plasmin-mediated degradation without accelerating plasminogen activation. Conclusions: TXA efficacy is determined not only by dose but also by timing and the plasminogen availability in the clot microenvironment. These findings provide mechanistic insight into the failure of prophylactic TXA administration and highlight the importance of context in optimizing its clinical use.