Platelet-neutrophil interactions under thromboinflammatory conditions

Abstract

Platelets primarily mediate hemostasis and thrombosis, whereas leukocytes are responsible for immune responses. Since platelets interact with leukocytes at the site of vascular injury, thrombosis and vascular inflammation are closely intertwined and occur consecutively. Recent studies using real-time imaging technology demonstrated that platelet-neutrophil interactions on the activated endothelium are an important determinant of microvascular occlusion during thromboinflammatory disease in which inflammation is coupled to thrombosis. Although the major receptors and counter receptors have been identified, it remains poorly understood how heterotypic platelet-neutrophil interactions are regulated under disease conditions. This review discusses our current understanding of the regulatory mechanisms of platelet-neutrophil interactions in thromboinflammatory disease.

Fig. 1

Heterotypic cell–cell interactions during vascular inflammation. a During vascular inflammation, neutrophil rolling over and adhesion to the activated ECs are mediated by selectins-their ligands and β2 integrins-ICAM-1, respectively. These adherent and crawling neutrophils allow for platelet adhesion and accumulation. b During arterial thrombosis, platelets adhere to vWF and collagen through GPIb/IX/V complex and GPVI, respectively, thereby inducing platelet aggregation. The adherent platelets support neutrophil rolling and adhesion via the receptor–counter receptor interaction. c The receptor and counter receptors of heterotypic neutrophil–platelet interactions. Heterotypic interactions are mainly mediated by the interactions of P-selectin with PSGL-1 and αMβ2 integrin with GPIbα. Other molecules also contribute to heterotypic interactions, such as platelet JAM-3 binding to neutrophil αMβ2 integrin. Platelet αIIbβ3 integrin can interact with neutrophil αMβ2 integrin through fibrinogen. d Heterotypic EC–neutrophil–platelet interactions can lead to occlusion in microvessels during thromboinflammation. In addition to EC–neutrophil–platelet interactions, RBCs may be trapped and incorporated into cell–cell aggregates

Fig. 2

Receptor-mediated signaling pathways in neutrophils. Under inflammatory conditions, neutrophils can be activated by initial rolling on activated endothelium and by soluble ligands such as TNF-α. TNFR signaling stimulates NF-κB-mediated gene transcription, regulating the expression of numerous pro-inflammatory proteins. GPCR stimulation leads to inhibition of adenylyl cyclase (AC) through Gαi and activation of PI3K and PLCβ through Gβγi. Rac1/2 activity and activated Akt and PKC phosphorylate p47^phox, inducing ROS generation through the NOX2 complex. PLCβ hydrolyzes PIP2 to form IP3 and DAG which in turn mediates calcium release from the ER and DAG-sensitive PKC activation, respectively. Depletion of ER calcium induces STIM1 clustering at the ER-PM junction, interacting with ORAI1, and allowing calcium influx. FcγR signaling leads to ITAM- and Syk-mediated PLCγ2 activation that induces similar downstream effects to those seen from GPCRs. TLR activation leads to NF-κB activation, as seen in TNFR activation. The signaling pathways of GPCRs, FcγR, and TLR enhance MAPK activity and cytosolic calcium levels. During cell activation, exocytosis-mediated granular secretion also induces the membrane translocation of αMβ2 integrin. Neutrophil activation further release histones and form neutrophil extracellular traps (NETs). In addition to DNA and histones, NETs form a scaffold for proteases and trap circulating bacteria and platelets

Fig. 3

Receptor-mediated signaling pathways in platelets. Numerous agonists stimulate platelets through their receptors. Gqα is able to stimulate both Lyn and PLCβ. Lyn then phosphorylates PI3K, leading to PIP3 generation and AKT phosphorylation. Activated AKT stimulates NOS-PKG-MAPK signaling. PLCβ activation allows for generation of DAG and IP₃ from PIP₂. IP3 then binds to the IP3R on the dense tubular system, inducing calcium release into the cytosol. Both DAG and calcium can activate PKC, leading to αIIbβ3 integrin activation. Increases in cytosolic calcium also lead to integrin activation through activation of CalDAG-GEF1. Other GPCRs on the platelet membrane are the P2Y receptors for ADP, the 5-HT receptor for serotonin, and the thromboxane receptor for TXA2. Stimulation of these receptors also induces platelet aggregation through their corresponding G-proteins. Platelet ITAMs, such as FcγR and GPVI, signal through Syk-SLP-76-ADAP and Lyn/Fyn, respectively. Both of these receptors lead to activation of PLCγ2, thus generating IP₃ and DAG and inducing platelet aggregation and granule secretion as described above