Immune mechanisms in hypertension: defining the role of B cells, macrophage-derived IGF-1 and CXCR2 in vascular and renal inflammation

For several decades it has been known that clinical hypertension is associated with elevated serum levels of antibodies. However, whether antibodies and the cells responsible for their production, B cells, play causative roles in the pathogenesis of hypertension is unknown. There is also evidence that macrophages are activated in hypertension and accumulate in the walls of arteries and the kidneys. However, it is unclear if these cells are activated downstream of antibody production, or whether they contribute to the vascular and renal remodelling and dysfunction associated with hypertension. It is our contention that a better understanding of the role of such immune processes will lead to the development of new anti-hypertensive therapies, and/or the re-purposing of therapies currently reserved for the treatment of auto-immune/auto-inflammatory disorders. In Chapter 3, we demonstrated for the first time that B cells/antibodies play an essential role in the pathogenesis of angiotensin (Ang) II-induced hypertension. First, we showed that hypertension is associated with B cell activation and elevated plasma cell numbers in the spleen, along with increased IgG antibodies in the serum and aortic adventitia. Next, we observed that B cell-activating factor receptor (BAFF-R) knockout mice, which lack mature B cells, were not only resistant to the Ang II-induced elevations in IgG antibodies, but were also protected against aortic macrophage accumulation, fibrosis and stiffening. BAFF-R-/- mice also displayed a blunted pressor response to Ang II, as did mice treated with a B cell- depleting anti-CD20 antibody (similar to the clinically used drug rituximab). Hence, these studies identify a hitherto unappreciated role for B cells in the pathogenesis of hypertension, and provide proof-of-concept that B cell depletion therapy (as used in the treatment of follicular lymphoma and rheumatoid arthritis) may be effective for the treatment of hypertension. In Chapter 4, we explored the role of macrophage-derived IGF-1 in mediating aortic fibrosis and stiffening during hypertension. These studies uncovered a novel paradigm whereby collagen-producing alternatively-activated (AA) macrophages accumulate in the aorta during hypertension and directly promote vascular fibrosis. Knockdown of IGF-1 in macrophages (i.e LysMCre/+xIGF-1fl/fl; MφIGF1KO) or depletion of their circulating precursors, monocytes, prevented the accumulation of collagen-producing AA macrophages, and blunted aortic fibrosis and stiffening. Real-time PCR analysis of aortas from MφIGF1KO mice, and macrophages stimulated ex vivo with IGF-1, suggested that the mechanism by which IGF-1 facilitates fibrosis involves upregulation of CCR5 leading to chemoattraction of AA macrophages into the vessel wall. Thus, these experiments highlight IGF-1 and AA macrophages as important mediators of aortic fibrosis and stiffening and potential therapeutic targets in hypertension. Finally, in Chapter 5, we employed a polymerase chain reaction (PCR) array to identify chemokine ligands and/or receptors that are upregulated in kidneys of mice with hypertension and which could thus represent targets for therapies aimed at preventing leukocyte accumulation and renal inflammation. These studies identified the CXCR2 family ligands CXCL5 and CXCL2 to be the most highly upregulated of all chemokine-related genes following one-kidney/deoxycorticosterone-acetate/salt (1K/DOCA/salt)-induced hypertension. Moreover, increased renal expression of CXCR2 ligands was evident as early as 3 days after the induction of hypertension and correlated temporally with the development of renal fibrosis and elevated blood pressure. Despite these observations, treatment with a CXCR2 antagonist, SB225002, had no effect on 1K/DOCA/salt-induced increases in systolic blood pressure and renal fibrosis and appeared to exacerbate renal inflammation. Thus further studies are required to determine whether the detrimental effects of SB225002 in the kidneys during hypertension are due to its reported off-target effects or reflective of the fact that the CXCR2 ligand/receptor system is not a valid target for therapeutic intervention in this condition. Hypertension is the major risk factor for myocardial ischemia/infarction and stroke, which together constitute the principal causes of death and morbidity worldwide. Currently, up to 20% of hypertensive patients are resistant to current therapies and thus exposed to a heightened risk of cardiovascular events. The studies in this thesis have characterized several immune mechanisms as crucial mediators of hypertension and its associated damage to the blood vessels and kidneys and, as discussed further in Chapter 6, it is hoped that these will be able to be exploited in the future to better manage hypertension in the clinic.