INVESTIGATING THE PROTECTIVE MECHANISMS CONFERRED BY INTERLEUKIN-15 RECEPTOR ALPHA (Il-15Rα) INHIBITION IN ATHEROSCLEROSIS DEVELOPMENT.

Abstract

Atherosclerosis is a chronic inflammatory disease and a major underlying cause of cardiovascular diseases, which are among the leading causes of death worldwide. Interleukin 15 receptor alpha (IL-15Rα) is a pro-inflammatory cytokine receptor found in both murine and human atherosclerotic plaques. IL-15Rα exists as a membrane- bound receptor as well as a soluble receptor, following proteolytic cleavage at the plasma membrane. However, its role in the pathogenesis of atherosclerosis has not been studied before. This thesis aimed to explore the role of IL-15Rα on atherosclerosis development in vivo, using a murine model system. I investigated the progression of aortic sinus atherosclerosis in Il-15rαKO/KO ApoEKO/KO mice and control ApoEKO/KO mice under a low-fat, cholesterol free normal diet feeding and a high-fat, high-cholesterol diet feeding to expedite atherosclerosis development. I also examined atherosclerosis in the right common carotid arteries of mice, induced by surgically narrowing the arteries to create a hemodynamic stress and feeding the mice with a high fat diet. My findings revealed that Il-15rαKO/KO ApoEKO/KO mice exhibited significantly reduced atherosclerosis compared to control ApoEKO/KO mice across all experimental conditions. I performed reciprocal bone marrow transplantation (BMT) and parabiosis experiments (surgical connection of the circulatory systems of mice) to determine if the protective effect of Il-15rα deletion resulted from the absence of Il-15rα gene expression in hematopoietic cells or in non-bone marrow-derived cells, present locally within atherosclerosis- prone vessels or acting from a distance via a circulating factor. BMT revealed that restoring Il-15Rα expression in bone marrow of Il-15rαKO/KO ApoEKO/KO mice did not increase atherosclerosis development in the recipient mice. Parabiosis between Il-15rαKO/KO ApoEKO/KO mice and ApoEKO/KO mice significantly increased atherosclerosis in the Il-15rαKO/KO ApoEKO/KO mice, while atherosclerosis levels in the ApoEKO/KO mice remained unchanged. These findings suggest that the reduced development of atherosclerosis in the Il-15rαKO/KO ApoEKO/KO mice is attributable to the absence of circulating factor(s) originating from non-bone marrow-derived cells, which could be supplied in trans from the circulation of ApoEKO/KO mice expressing IL-15Rα. To test if IL-15/sIL-15Rα played a role as one of the circulating factors involved in atherosclerosis development, I analyzed atherosclerosis development in mice lacking both Il-15 and Il-15rα. I found that Il-15KO/KO Il-15rαKO/KO ApoEKO/KO mice developed significantly less atherosclerosis when compared to Il-15KO/KO ApoEKO/KO mice, suggesting that the protective effect associated with Il-15rα deletion was independent of IL-15. Lastly, I injected mice with a neutralizing antibody against IL-15Rα to test if pharmacological targeting of the receptor could affect atherosclerosis development. The antibody significantly reduced high- fat diet induced atherosclerosis in the surgically narrowed right common carotid artery of the mice. My studies show, for the very first time, that IL-15Rα plays an active role in atherosclerosis, independently of its only known ligand, IL-15, and that targeting IL-15Rα can reduce atherosclerotic plaque formation. These results may lay the groundwork for development of experimental therapeutics aimed at targeting IL15Rα. Exploring those therapeutics for their effectiveness against human atherosclerotic disease may allow for a multi-tiered approach to treating atherosclerosis, where treatment inhibiting IL-15Rα could be combined with current treatments aimed at cholesterol lowering, to offer enhanced protection against atherosclerosis thus resulting in reduced economic burden associated with atherosclerosis and its subsequent clinical manifestations (myocardial infarction and stroke).