Einthoven dissertation prizes 2017

The leading cause of death worldwide is cardiovascular disease due to atherosclerosis. In the Netherlands alone, every day 100 people die from atherosclerosis. Therefore, there is a great need to better understand this disease and its progression. In the last 15 years, the role of inflammation and the immune system in atherosclerosis has been increasingly highlighted. The most important immune cells to play a role in atherosclerosis are monocytes. Monocytes are part of the innate immune system, which is rapid, nonspecific and presumably without any memory. The other part of the immune system is the adaptive immune system, which can acquire a memory by producing memory cells. Recently, however, it was discovered that this dichotomy is not true. Monocytes can develop a memory after a first encounter with a microbial stimulus, after which the cell is functionally reprogrammed. Upon a secondary encounter, the monocyte will exhibit a hyperactive response, this is called ‘trained immunity’. Although this can be of great benefit in the context of recurrent infections, it might be detrimental in conditions in which innate immunity itself contributes to tissue injury. In this thesis, we explored the role of inflammation in the development of atherosclerosis in the light of trained immunity. First, we investigated whether different lipoprotein particles can induce trained immunity, similar to micro-organisms. Second, we examined the possibility of a trained immune phenotype in monocytes from patients with atherosclerosis or patients at risk of developing atherosclerosis. Third, we aimed to unravel part of the mechanisms of trained immunity, which is essential for future drug development.

These questions were explored on several levels: in vitro, ex vivo and in vivo studying patients with and without atherosclerosis and on a mechanistic level. In the lab, we studied how monocytes respond to a first encounter with different forms of cholesterol: low-density lipoprotein (LDL), lipoprotein(a) (Lp(a)) and oxidised LDL (oxLDL). How do monocytes respond 6 days after a first encounter with these particles, and can they become trained? Firstly, we demonstrated that both oxLDL and Lp(a), two particles carrying oxidised phospholipids, can induce trained immunity in vitro, i. e. increased production of atherogenic cytokines and chemokines and increased foam cell formation. Translationally, we demonstrated that monocytes from patients with elevated levels of lipoprotein(a) also show this pro-inflammatory phenotype when stimulated ex vivo, indicating a trained immunity phenotype. Secondly, we showed that patients with established atherosclerosis have a different monocyte phenotype in terms of increased cytokine production and changes in the metabolic state compared with healthy controls. Also, patients and controls showed significant differences on an epigenetic level, by decreased histone 3 lysine 27 trimethylation, an inactivating histone mark. Finally, on a mechanistic level, we showed that statins, widely used cholesterol lowering drugs, can prevent the development of trained immunity. We unravelled that the trained immunity phenotype is mediated by accumulation of mevalonate and subsequent stimulation of the insulin-like growth factor (IGF1) receptor. These findings might be used in the future for the development of novel drugs.

This thesis evaluated CAD development and treatment by bioresorbable vascular scaffolds (BVS), while focusing on coronary pathology associated with diabetes mellitus, using intracoronary imaging techniques such as optical coherence tomography (OCT) to obtain highly detailed imaging of the coronary morphology and the vascular response to stent implantation.

First, we used OCT to demonstrate a low incidence of imaging-related complications (0.6%) in an unselected patient population of 1,142 patients with varying indications for imaging. These complications were self-limiting after retrieval of the imaging catheter or easily treatable in the catheterisation laboratory, indicating that OCT is safe to use with a very low event rate. In an experimental setting, we examined the impact of longitudinal catheter displacement on the quantitative assessment of BVS and demonstrated that serial evaluation of matched cross-sections was hampered. This suggested that dedicated analysis methods with per-frame analysis or using software allowing for synchronised evaluation of matched segments, could be more suitable for serial analysis in specific scaffold regions.

Second, OCT and intravascular ultrasound enabled plaque characterisation in vivo in diabetic and non-diabetic swine that were fed a high-fat diet and proved to be complementary for the evaluation of CAD development. Using in vivo OCT, near-infrared spectroscopy (NIRS) and coronary computed tomography angiography and ex vivo vascular function testing and histology, no differences in early atherosclerotic development were observed between diabetic and non-diabetic swine up to 15 months, suggesting that macroscopic atherosclerosis development was not influenced by hyperglycaemia. Additionally, ex vivo vascular function testing demonstrated a shift in balance of different contributors to vascular tone of small coronary arteries during the progression of atherosclerosis, emphasising the importance of longitudinal studies of vascular function in diabetes mellitus and CAD.