Phenotypic characterisation of platelets in giant cell arteritis by flow cytometry

Activated platelets have been reported in giant cell arteritis (GCA), an immune-mediated vasculitis of large arteries. However, it is unclear whether platelet surface receptor expression is altered in GCA and how such changes may contribute to vascular inflammation, thromboembolic complications, and response to treatment.

We developed a novel multiparameter flow cytometry panel for platelet phenotyping in whole blood, measuring markers of haemostasis, thrombosis, and thromboinflammation, and combined this with high-dimensional analysis using full annotation using shape-constrained trees (FAUST) to characterise platelet subpopulations. Platelet phenotype and function were examined in GCA patients and compared with controls.

Platelets from GCA patients were functionally responsive to agonist stimulation. CXCR4 expression was reduced at baseline and following activation, with further reductions in active disease. High-dimensional analysis revealed disease-associated shifts in discrete platelet subpopulations, including expansion of a subset with αIIbβ3 activation and CD62P expression but reduced CD63 expression (PAC1+CD62P+CD63-) in GCA versus controls following activation. Conversely, αIIbβ3 activation was reduced in active versus inactive GCA following activation, while the number of CD40L+ve platelets was increased, indicating divergence between thrombotic and inflammatory activation pathways. A subset of platelets expressing CD62P but lacking αIIbβ3 activation or CD63 expression (PAC1-CD62P+CD63-) was decreased in active versus inactive GCA following activation. Prednisolone treatment was associated with receptor-specific modulation of platelet phenotype, including enhanced CD42b shedding upon activation and reduced CD32a expression at basal and following activation. Subpopulation analysis further demonstrated treatment-associated inhibition of α-granule secretion (CD62P) without suppression of αIIbβ3 activation or dense granule release (CD63) in a subset of platelets (PAC1+CD62P-CD63+) following activation.

These findings demonstrate that platelets are functionally altered in GCA and are suggestive of platelet priming and reprogramming driven by the inflammatory milieu and therapeutic interventions. This work advances understanding of platelet involvement in GCA and identifies new avenues for research.