Background: In addition to their role in thrombosis and hemostasis, platelets are key mediators of inflammation and altered immunity. Circulating monocyte-platelet aggregates (MPA) represent the crossroads between thrombosis and inflammation and may represent a therapeutic target. While antiplatelet therapy (APT) reduces platelet activity and thrombosis, its effect on MPA is uncertain.

Aims: To analyze the effect of APT on MPA in vitro.

Methods: The effect of different platelet-activating agonists (thromboxane analog U-46619, ADP, PAR4, collagen, and epinephrine) on MPA formation in whole blood (WB) was measured via flow cytometry. Agonist-stimulated WB was incubated in the presence of inhibitors against P-selectin, PSGL-1, PAR1 (ML161), P2Y12 (AZD1283), GPIIb/IIIa (eptifibatide), acetyl salicylic acid (ASA), and dipyridamole and assessed for MPA formation. RNA-Seq data sets of monocytes incubated with healthy platelet releasates (PR) were used to identify platelet-induced upregulation of monocyte transcripts and were validated by RT-qPCR in monocyte-PR co-incubation assays in the presence of APT.

Results: Circulating MPA are increased in prothrombotic and inflammatory diseases including the most recent COVID-19. Monocytes aggregated to platelets have more CD40 and tissue factor expression than monocytes not aggregated to platelets (p<0.05 for each comparison). As expected, targeting P-selectin (85.4% reduction) and PSGL-1 (88.2% reduction) had the greatest attenuation of MPA. Among platelet inhibitors, P2Y12 inhibition was most effective in lowering MPA formation (30.7% reduction) (figure 1).

APT effect on MPA formation in whole bloodFlow cytometry analysis of MPA. 

Incubation of monocytes with platelet releasate induced upregulation of inflammatory mRNA transcripts suppressor of cytokine signaling 3 (SOCS3) and oncostatin m (OSM). Following pretreatment of platelets with APT, both GPIIb/IIIa and P2Y12 inhibition was associated with lower expression of SOCS3 and OSM (figure 2).

Effect of APT on PR-induced inflammatory transcripts in monocytesSOCS3 and OSM in monocytes incubated with APT-treated PR. 

Conclusions: Circulating MPA represent a crossroad of platelet and monocyte activation. We show that APT is associated with both reduced MPA formation and platelet-induced monocyte activation.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/effect-of-antiplatelet-therapy-on-monocyte-platelet-aggregates/