SIHI · Stress-Induced Hypertension and the Role of the Neuroimmune System

Stress-Induced Hypertension and the Role of the Neuroimmune System

Previous studies have found inflammation, in particular T lymphocytes, to be a contributing factor in the genesis of various forms experimental hypertension, including angiotensin II and neurally mediated chronic hypertension. Currently, clinical interventions such as renal denervation (RD) and carotid sinus nerve denervation (CSD) are aimed at targeting the autonomic nervous system to treat hypertension. However, the effects of these blood pressure lowering strategies on the immune system are unknown. We hypothesized that the sympathoinhibitory effect of CSD contributes to reduced T-cell infiltration in the aorta and brainstem, which could improve vascular compliance including the aortic baroreceptor sensitivity and improved baroreflex transmission centrally. Given the sympathetic innervation of immune system organs such as the spleen, thymus and bone marrow, we suggest that reductions in sympathetic activity may be the trigger for the antiinflammatory response, thereby breaking this positive (and pathological) feedback loop. In our study, RD and CSD were performed in the spontaneously hypertensive rat (SHR) and flow cytometry was used to examine tissue infiltration of T lymphocytes (CD3+) in the aorta and brainstem. Bilateral RD was achieved via a retroperitoneal incision to exposure of the renal artery. For CSD, the CS was visualised and branches of the carotid sinus nerve sectioned. Surgical shamoperated SHR (SS) rats underwent the same surgical procedures to expose the kidney and CB but the nerves were left intact. Compared to the SS group, arterial pressure and renal sympathetic nerve activity in the SHR was significantly lowered following both RD and CSD. The percentage of infiltrating CD3+ T lymphocytes in the brainstem (9.6 ± 0.7 vs 6.4 ± 0.8%; t(9) = 2.79, p < 0.05) as well as the aorta (14.5 ± 1.6 vs 10.0 ± 1.1%; t(16) = 2.18, p < 0.05) were significantly reduced following RD (Figure 1-2). Following CBD, there was a trend toward reduced percentage of CD3+ cells in the aorta (t(13) = 1.36, p = 0.19) but unlike RD, there was no change in CD3+ cells in the brainstem (FIGURE 1- 2). These data suggest that there is significant systemic CD3+ cell infiltration in the SHR and that some, but not all, procedures targeting the autonomic nervous system may have benefits in lowering tissue inflammation associated with hypertension. We speculate that the sympathoinhibitory effect of CSD contributes to reduced T-cell infiltration in the aorta and brainstem described herein, which could improve vascular compliance including the aortic baroreceptor sensitivity and improved baroreflex transmission centrally. We next examined the immune cell responses in patients diagnosed with drug resistant hypertension and who were eligible to undergo experimental renal denervation enrolled in the Bristol Heart Institute study. We hypothesized that circulating cells of hypertensive patients exhibit a pro-inflammatory T cell phenotype, as determined by the balance of memory (effector) versus naïve CD3+ cells. Using flow cytometry, subsets of circulating T cells and markers of antigenic memory were analyzed in male hypertensive patients and normotensive subjects. There were no differences in the overall percentage of CD3+ cells in normotensive (63.3 ± 3.7%) (n=5; mean age 34 yrs; mean systolic BP 128.6 mmHg) vs. hypertensive men (62.2 ± 5.5%) (n=6; mean age 53.5 yrs; mean systolic BP 154.7 mmHg) and the percentage of CD4+ and CD8+ T cell subsets was not statistically different between groups. However, T cell subset analysis revealed that the ratio (1.6 ± 0.3 vs 0.7 ± 0.1; P <0.05) of memory (CD45RO+CD3+) vs. naïve (CD45RA+CD3+) T cells as well as the percentage of CD8+CD62L(low) cells (70.4 ± 4.5 vs 52.8 ± 6.3; P <0.05) were increased in hypertension (FIGURE 3). These results demonstrate an imbalance in effector memory / naïve T cells in hypertensive men and provide further evidence that chronic antigen exposure may be involved in the pathogenesis of the inflammatory response hypertension. Moreover, we plan to follow a subset of these patients at 6 and 12 months following RD and CSD to determine if there is a causal link between lowering blood pressure by targeting the autonomic nervous system in humans and a reduction in pro-inflammatory markers, with a particular emphasis on T lymphocytes. Current preliminary data has also been collected and ongoing mouse animal studies using genetically modified models are ongoing to determine whether the response to specific hypertensive stimuli such as the hormone angiotensin II leads to an antigen specific response in the development of hypertension.

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