An imaging flow cytometry method to study platelet-monocyte aggregates using Long COVID as a model
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Background
Long COVID is characterised by persistent systemic inflammation and endothelial dysfunction, with increasing evidence implicating thromboinflammatory mechanisms. Platelet-monocyte aggregates (PMA) represent a sensitive marker of platelet activation and immune-vascular interactions, but their role in Long COVID remains incompletely defined.
Methods
This study quantified circulating PMA in 20 Long COVID patients and 20 healthy controls using a two-colour imaging flow cytometry assay targeting CD14 (a monocyte receptor for pathogen-associated molecular patterns, PAMPs) and CD62P (P-selectin). PMA were expressed as a percentage of total monocytes, and platelet attachment patterns were classified into single versus multiple platelet binding. Statistical analyses included Shapiro-Wilk normality testing, unpaired t-tests, Mann-Whitney U tests or two-way ANOVA as appropriate, and linear regression for correlation analysis.
Results
Circulating PMA were significantly elevated in Long COVID patients compared with controls (29.19 [20.02–37.26] vs 4.59 [2.67–7.16], p < 0.0001). Long COVID samples showed a reduced proportion of monocytes with single platelet attachment and a corresponding increase in multiple platelet binding (p < 0.0001). In controls, %PMA increased with age (p < 0.01), whereas no age association was observed in Long COVID, indicating an elevated baseline independent of age.
Conclusions
Long COVID is associated with markedly increased platelet–monocyte aggregation and altered platelet attachment dynamics, consistent with sustained thromboinflammatory activity. PMA represent a sensitive cellular marker of platelet-driven immune activation and may have utility as an accessible biomarker for stratifying thromboinflammatory burden in Long COVID.
