Le rôle de l'axe CD40L/CD40/NF-κB dans la fonction plaquettaire

CD40 ligand (CD40L) is a thrombo-inflammatory molecule that predicts cardiovascular events. Platelets are the main source of soluble CD40L (sCD40L) in the circulation, with the ability to influence platelet function through its binding to platelet receptors: CD40, αIIbβ3, and α5β1. We have previously demonstrated that stimulation of human platelets with sCD40L induces activation of nuclear factor kappa B (NF-κB), which may play a non-genomic role in priming platelets. Indeed, platelet priming by sCD40L strongly enhances platelet activation and aggregation in response to suboptimal doses of agonists. However, the involvement of the different receptors in platelet priming following activation of NF-κB by sCD40L remains unknown. In addition, the transforming growth factor-B (TGF-B) -activated Kinase (TAK1), a major regulator of inflammation, is activated by sCD40L in nucleated cells, and may be involved in sCD40L/NF-κB signaling. On the other hand, sCD40L is high in the bloodstream of coronary patients, who receive Aspirin (ASA) for secondary prevention. However, antiplatelet therapy with ASA is ineffective in some patients and its efficacy is reduced in patients at high risk for coronary events, increasing their risk of platelet over-activation and developing thrombosis. High blood levels of thrombo-inflammatory mediators, such as sCD40L, may explain such variability. Thus, we undertake this project to elucidate the impact of the sCD40L/NF-κB axis on platelet function and its modulation by ASA. In the first part of our project, we aimed to identify the sCD40L receptors involved in the activation of platelet NF-κB, their downstream signaling and their involvement in platelet aggregation. We have shown that platelets express CD40L receptors: CD40, αIIbβ3 and α5β1 and release sCD40L. sCD40L alone induced activation of platelet NF-κB. This effect was absent in CD40-/- mouse platelets and inhibited by CD40 blockade, but was not affected by αIIbβ3 or α5β1 blockade. The sCD40L/CD40 axis has also activated TAK1 upstream of NF-kB. In functional studies, sCD40L alone had no effect on platelet aggregation but potentiated aggregation in the presence of suboptimal thrombin doses. The inhibitors of CD40, TAK1 and NF-κB abolished this effect. This first study concluded that sCD40L primes platelets via CD40/TAK1/NF-κB signaling pathways, which predisposes platelets to increased activation and aggregation in response to thrombotic stimuli. In the second part of our project, we hypothesized that in the presence of high levels of sCD40L; the effectiveness of ASA may vary. Accordingly, we aimed at determining the effects of ASA on signaling and aggregation of platelets in the presence of sCD40L. The effects of ASA on human platelets treated with sCD40L, in response to suboptimal concentrations of collagen or thrombin, were evaluated on aggregation, thromboxane A2 (TxA2) secretion and phosphorylation p38 MAPK, NF-κB, TAK1 and myosin light chain (MLC). sCD40L significantly increased TxA2 secretion by platelets in response to suboptimal doses of collagen and thrombin; this effect has been reversed by the ASA. ASA did not inhibit the phosphorylation of p38 MAPK, NF-κB, TAK-1, with platelet stimulation by sCD40L either alone or in the presence of agonists. sCD40L potentiated platelet aggregation, an effect completely reversed and partially reduced by ASA in response to a suboptimal dose of collagen and thrombin, respectively. In addition, the effects of ASA on sCD40L-treated platelets with collagen related to inhibition of platelet shape change and phosphorylation of MLC. In summary, ASA does not affect platelet signaling via sCD40L, but prevents its effect on TXA2 secretion and platelet aggregation in response to collagen, via a mechanism involving inhibition of MLC. In conclusion, this project allowed us to determine that platelet priming by sCD40L via activation of NF-κB depends on the CD40 receptor and signaling via TAK1. Thus, the sCD40L/CD40/TAK1/NF-κB axis potentiates activation and aggregation in response to thrombotic stimuli, which may promote occurrence of atherothrombotic events in coronary patients. In addition, since ASA has no effect on platelet signaling via the sCD40L axis, targeting of this axis in platelets may have therapeutic potential in coronary patients with high levels of sCD40L that are none or less responding to ASA.

Keyphrases: CD40L/CD40, NF-κB, Aspirin, Platelets, Thrombosis