First Report of Elevated Monocyte-Platelet Aggregates in Healthy Children

Christina Yip,Vera Ignjatovic,Paul Monagle,Matthew D Linden,Chantal Attard,Christian Schulz

doi:10.1371/journal.pone.0067416

Abstract

Platelets are subcellular fragments which circulate in blood and have well established roles in thrombosis and haemostasis in adults. Upon activation, platelets undergo granule exocytosis and express P-Selectin on the cell membrane which binds a ligand on monocytes, leading to monocyte-platelet aggregation. Elevated circulating monocyte-platelet aggregates in adults are linked to atherothrombosis, but have not been investigated in children where thrombosis is less common. This study aimed to measure monocyte-platelet aggregate formation in children using whole blood flow cytometry. Monocyte-platelet aggregates as well as activation and granule exocytosis of platelets were measured in healthy adults (n = 15, median age 28 years) and healthy children (n = 28, median age 7 years). Monocyte-platelet aggregates in healthy children were elevated compared to healthy adults (37.8±4.4% vs 15.5±1.9% respectively, p<0.01). However, this was not accompanied by any difference in platelet activation (PAC-1 binding 6.8±1.5% vs 6.3±2.0% respectively, p = ns) or granule exocytosis (P-selectin expression 4.4±0.5% vs 3.1±0.5% respectively, p = ns). Despite comparable numbers of platelets bound per monocyte (GPIb MFI 117.3±13.7 vs 130.9±28.6 respectively, p = ns), surface P-selectin expression per platelet-bound monocyte was lower in children compared to adults. We therefore provide the first data of elevated monocyte-platelet aggregates in healthy children.

Highlights

Platelets are small cell fragments of large importance in medicine
When circulating platelets were examined for surface markers of activation with no addition of agonists ex vivo, there was no corresponding increase in activation of the GPIIb/IIIa receptor as measured by PAC-1 binding (6.861.5% vs. 6.362.0% respectively, p = ns) (Figure 1B); or platelet granule exocytosis, as measured by P-selectin expression (4.460.5% vs. 3.160.5% respectively, p = ns) (Figure 1C)
In order to determine whether circulating monocyte-platelet aggregates in children formed as a result of the P-Selectin/P-selectin glycoprotein ligand-1 (PSGL-1) adhesion mechanism known to be responsible for this process in adults, the relative MFI of P-selectin on platelet-bound and – unbound monocyte events was examined

Summary

Introduction

Platelets are small cell fragments of large importance in medicine Their role in haemostasis and the late stage thrombotic complications of cardiovascular disease are well characterised [1]. The platelet P-selectin interacts with P-selectin glycoprotein ligand-1 (PSGL-1), which is constitutively expressed on the surface of circulating monocytes [8] Following this initial tethering, the b2 integrin Mac-1 (CD11b/18), and to a lesser extent LFA-1 on the monocyte stabilise the adhesion [9]. The b2 integrin Mac-1 (CD11b/18), and to a lesser extent LFA-1 on the monocyte stabilise the adhesion [9] These interactions do not develop if PSGL-1 is blocked, or CD62P is not expressed on the platelet [10,11]

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