Association of inflammatory markers and ultrasonographic indicators of endothelial (dys)function in humans
Authors: Klobucar I , Klobucar L , Lechleitner M , Pregartner G , Frank S , Degoricija V 
Published: May 2024

Abstract

Funding Acknowledgements
Type of funding sources: Public grant(s) – National budget only. Main funding source(s): The Austrian Science Fund (FWF)

Introduction
It has been widely known that chronic inflammation is a crucial factor in the process of atherogenesis and atherosclerosis. Also, endothelial dysfunction presents the first and still reversible step of that process.(1) It is a challenge to find a laboratory marker whose levels would be informative about the extent of endothelial dysfunction and atherogenesis in humans.

Purpose
The aim of the present study was to determine associations between markers of chronic, low-grade inflammation and ultrasonographic indicators of endothelial (dys)function in healthy volunteers (H) and in patients with metabolic syndrome (MS).

Methods
Any kind of clinically manifest acute inflammatory or infectious disease, as well as chronic inflammatory, autoimmune or malignant disease was an exclusion criterion for participation. Serum levels of C-reactive protein (CRP) were measured using standard clinical chemistry analyser, and serum levels of interleukin-6 (IL-6) using electro-chemiluminescence immunoassay. Blood vessel wall function was quantified as flow-mediated dilation (FMD) and nitroglycerin-mediated dilation (NMD), which were measured automatically and continuously using the ultrasound device with linear probe (10 MHz) and the software for ultrasound blood vessel diameter analysis,(2) following currently valid guidelines.(3,4,5)

Results
A total of 130 individuals were enrolled in the study (65 H and 65 MS), with a median age of 56 (IQR 50.0, 60.0) years and 47.7% women in each group. CRP and IL-6 levels, although being very low and within the intervals that are considered normal in everyday clinical practice for both groups [CRP - H: 1.2 (0.6, 2.3), MS: 2.4 (1.2, 5.5) μg/mL; IL-6 - H: 2.3 (1.7, 3.0), MS: 4.1 (2.7, 6.8) pg/mL], were significantly higher in the MS compared to H (both p&lt;0.001). The MS group also had significantly lower FMD (p=0.013) and NMD (p=0.033) compared to H. In H, IL-6 levels were significantly negatively correlated with FMD (r=-0.46, p&lt;0.001), but not with NMD; whereas no significant associations of IL-6 levels with FMD or NMD were observed in the MS group. CRP levels were not significantly associated with FMD or NMD in neither the H nor MS group.

Conclusion
Higher IL-6 serum levels in the absence of acute inflammatory or infectious disease are significantly associated with lower FMD values among individuals with low risk for cardiovascular diseases, who are still considered healthy. Direct correlation is attenuated and lost among individuals with developed clinical signs of metabolic syndrome, probably due to its complex pathophysiology, as well as redundant and interdependent mechanisms contributing to the occurrence of endothelial dysfunction in such an environment. Further investigation on larger cohorts is needed to determine whether IL-6 serum levels could serve as a marker of endothelial dysfunction in individuals without clinically developed traditional risk factors and symptoms of cardiovascular disease.