Role of interleukin-17 in endothelial cell activation and vascular function

Endothelial cell (EC) activation is a change of the endothelium from a quiescent state to one that is involved in immune reactions. Activation of ECs is associated with the inception of atherosclerosis. Atherosclerosis is a chronic inflammatory disease that involves adaptive and innate immunity. There are many pro-inflammatory stimuli which activate the endothelium. The pro-inflammatory cytokine interleukin-17 (IL-17) has been shown to activate lung microvascular ECs. Enhanced expression of the IL-17 receptor by synovial ECs is associated with rheumatoid arthritis. These studies suggest that IL-17 plays an important role in EC biology. Nevertheless, the role of IL-17 in EC activation and endothelial dysfunction in the context of hyperlipidemia-induced atherosclerosis has not been studied.;In the current study, we investigated the role of IL-17 in EC activation in vitro with mouse aortic ECs and human aortic ECs. In addition, we used the IL-17/ApoE double knock-out mouse to determine the role of IL-17 in vessel function and atherosclerosis development. First, we found that hyperlipidemia increased the number of IL-17-producing cells in the spleens from wild type mice and ApoE-/- mice that were fed a Western diet when compared to their respective normal chow diet controls. We also found that after treatment with the pro-atherogenic factor, oxidized LDL, there was an increase in the expression of IL-17 receptor by ECs. Using an EC specific array, we found that IL-17 induced significant up-regulation of four genes that are associated with EC activation in mouse aortic ECs. The four genes induced in IL-17-treated mouse aortic ECs were Cxcl1, Cxcl2, Il6, and Csf2. Moreover, we also found that IL-17 induced these four genes in human aortic ECs, and we showed that enhanced monocyte adhesion to ECs was dependent on these four genes.;It was previously observed that a Western diet induced vessel dysfunction in the aortas of ApoE-/- mice. Thus, we sought to determine whether IL-17-deficiency rescues impaired endothelium-dependent relaxation in ApoE-/- mice that were fed a Western diet with the Wire Myograph System. We found that ApoE-/- mice on a 3-week Western diet had impaired endothelium-dependent relaxation when compared to IL-17 -/-ApoE-/- mice. Endothelium-independent relaxation in response to sodium nitroprusside (SNP) and contraction responses induced by potassium chloride (KCl) and phenylephrine (PE) were not different in ApoE -/- mice and IL-17-/-ApoE-/- mice.;Since our in vitro studies and vessel function assay pointed to a pro-atherogenic role for IL-17, we investigated lesion formation in ApoE-/- mice and IL-17-/-ApoE-/- mice. Lesion formation was assessed with Sudan IV staining of the whole aorta and Oil red O staining of aortic sinus cross sections. IL-17 deficiency in ApoE-/- mice did not affect atherosclerotic lesion formation in our study.;Hyperlipidemia is a well-established risk factor for atherosclerosis so we investigated whether the pro-atherogenic role of IL-17 may have been compromised by lipid levels in vivo. The lipid profiles of mice which measured the levels of LDL, HDL, triglyceride, non-esterified free fatty acid, and total cholesterol were determined. The lipid profiles showed that IL-17 deficiency in ApoE-/- mice modulated the levels of the lipids in the plasma.;Taken together, our data suggest that IL-17 is up-regulated in hyperlipidemia and IL-17 induces aortic EC activation. IL-17 also contributes to endothelial dysfunction in ApoE-/- mice induced by a Western diet. Moreover, IL-17 may modify lipid metabolism in mice. The effects of IL-17 on lipid levels may weaken its pro-atherogenic potential and contribute to the lack of an atherosclerotic phenotype in our atherosclerosis study. Our current work has shed light on the role of IL-17 on EC biology and has provided insights into the effects of IL-17 on EC activation, lipid modification, and vascular function. These important findings may serve as the stepping stone to the development of therapeutics that target vascular inflammation and its underlying mechanisms.;The studies in this dissertation were supported by grants from the National Institutes of Health (NIH) and a fellowship from the American Heart Association (AHA).