The Role of Mast Cells in Atherosclerotic Plaque Calcification

Background: Calcification is a key feature of atherosclerosis and has been associated with major adverse cardiovascular events. Unstable carotid plaques cause stroke and lesions from those patients have generally more mast cells (MCs) than stable ones. However, recent data from our group showed an upregulation of activated MCs in low calcified, whereas resting MCs were upregulated in high calcified plaques, indicating that MCs may also associate with various aspects of plaque pathology.

Hypothesis and Aim: Our hypothesis is that active vs. resting MC fractions associate with key features of plaque vulnerability, i.e. calcification, intraplaque hemorrhage (IPH) and other immune populations. Our aim is to investigate the association of MC fractions with clinical data from atherosclerosis patients, ultimately aiding to evaluate their therapeutic targeting potential.

Workflow and Results: Deconvolution of transcriptomic data from 127 patients plaques in the Biobank of Karolinska Endarterectomies (BiKE) was performed to enumerate 22 immune cell populations and associate to the clinical features from this cohort. Global gene and protein profiling in BiKE showed that majority of MC markers were upregulated in atherosclerotic plaques compared to control tissues as well as in plaques from symptomatic patients compared to asymptomatic ones.

Analysis of MCs with respect to plaque’s morphological characteristics showed that activated MCs correlated positively with extracellular matrix, calcification and perivascular fat volume. Activated MCs correlated with CD4+ memory cells, and resting MCs with dendritic cells and eosinophils. Histological stainings by plaque tissue microarrays showed the presence of MCs in lesions particularly in regions with IPH and confirmed that MCs generally co-localised with CD3+ lymphocytes.

By stratifying the results according to patient symptoms, activated MCs were elevated with more severe symptoms of plaque instability, however, patients’ medication did not impact MC regulation.

Conclusions: Systematic investigation of MC fractions in human plaques shows that activated MCs associate with increased vulnerability, both when it comes to clinical patient symptoms and morphological plaque features. Further plans include multispectral imaging to define the local environment around MCs in plaques, and detailed in vivo and in vitro studies to investigate the effect of calcification, medication and IPH on MC degranulation.