Abstract
Atherosclerosis is an inflammatory disease characterized by lipids’ deposition into the arterial wall and its subsequent thickening significantly compromises the lumen and/or eventually leads to plaque rupture and thrombosis conducing to cardiovascular diseases. Lipids-activated endothelium induces the persistent and massive infiltration of blood monocytes in the intima of the arterial wall where they differentiate into macrophages that phagocyte oxidized Low-Density Lipoprotein (oxLDL) inducing their transformation toward foam cells. Foam cells are unable to manage cholesterol efflux and produce cytokines and proteases inducing sustained inflammation and plaque vulnerability. Apoptosis then necrosis of these cells create the pro-thrombotic necrotic lipid core, key component of the vulnerable plaques. Macrophages are therefore characteristic of the plaques but they represent a quite heterogeneous and plastic population gathering different subpopulations presenting, through slight variations of global gene expression, a dual functionality (“heal” or “fight”). The extremities of this continuum are represented by the well-characterized in vitro models, classically activated pro-inflammatory M1 and alternatively activated immunoregulatory M2 macrophages (Mantovani, et al. 2004). Markers of these functional subtypes of macrophages can be found in atherosclerotic lesions (Toutouzas, et al. 2015) and their relative proportion and their specific localization in the plaque could be predictive factors of plaque rupture. If fluorochrome-coupled antibodies directed against M1 or M2 markers do exist, none of such antibodies are validated for in vivo colocalization imaging in humans. Besides, recent immunometabolism data suggested that the functional duality of these cells could be related to their differential utilization of metabolism pathways to produce energy, which may result in different concentrations of intracellular autofluorescent co-enzymes NADH and FAD (Pal & Konkimalla 2016). Therefore a local disequilibrium of the M1/M2 ratio will result in a modification of NADH and FAD autofluorescence (AF) and of the related Optical Redox Ratio (ORR= I(FAD)/I(NADH)). Following an imaging procedure using a linear fluorescence wide-field microscope previously described (Kuznetsov, et al. 1998) and with a table-top two-photon microscope, statistically significant differences in the amounts of NADH were measured between M1 and M2 cells in different cellular models (THP1 cell line and Monocyte-Derived Macrophages) while the relative amount of FAD was approximately constant. To in vitro study more physiopathological models of macrophages, we stimulated cells with acetylated- or oxidized-LDL and human carotids extracts. If expression of M1 or M2 markers in these specific foam cells has already been assessed, their level of NADH and FAD AF has not been addressed so far. Interestingly, these stimulated cells present significant variation of both co-enzymes AF and of the ORR compared to others macrophages highlighting the correlation between the phenotypical status and the ORR. The ultimate goal of the project is to monitor endoscopically without staining the inflammation of the plaque through NADH and FAD two-photon AF imaging of M1 versus M2, together with two other modalities allowing plaque : one dedicated to the detection of lipids (CARS technology) and the other to the detection of the collagenous fibrous cap (SHG).