A novel autoantibody in patients with primary hypertension: antibody against L-type Ca2+ channel:

Immune mechanisms of hypertension

Hypertension affects 30% of adults and is the leading risk factor for heart attack and stroke. Traditionally, hypertension has been regarded as a disorder of two systems that are involved in the regulation of salt-water balance and cardiovascular function: the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS). However, current treatments that aim to limit the influence of the RAAS or SNS on blood pressure fail in ~40% of cases, which suggests that other mechanisms must be involved. This Review summarizes the clinical and experimental evidence supporting a contribution of immune mechanisms to the development of hypertension. In this context, we highlight the immune cell subsets that are postulated to either promote or protect against hypertension through modulation of cardiac output and/or peripheral vascular resistance. We conclude with an appraisal of knowledge gaps still to be addressed before immunomodulatory therapies might be applied to at least a subset of patients with hypertension.

Adaptive Immunity in Hypertension

PURPOSE OF REVIEW

In recent years, a vast body of evidence has accumulated indicating the role of the immune system in the regulation of blood pressure and modulation of hypertensive pathology. Numerous cells of the immune system, both innate and adaptive immunity, have been indicated to play an important role in the development and maintenance of hypertension. The purpose of this review was to summarize the role of adaptive immunity in experimental models of hypertension (genetic, salt-sensitive, and Angiotensin (Ang) II induced) and in human studies. In particular, the role of T and B cells is discussed.

RECENT FINDINGS

In response to hypertensive stimuli such as Ang II and high salt, T cells become pro-inflammatory and they infiltrate the brain, blood vessel adventitia and periadventitial fat, heart, and the kidney. Pro-inflammatory T cell-derived cytokines such as IFN-γ and TNF-α (from CD8+ and CD4+Th1) and IL-17A (from the γδ-T cell and CD4+Th17) exacerbate hypertensive responses mediating both endothelial dysfunction and cardiac, renal, and neurodegenerative injury. The modulation of adaptive immune activation in hypertension has been attributed to target organ oxidative stress that leads to the generation of neoantigens, including isolevuglandin-modified proteins. The role of adaptive immunity is sex-specific with much more pronounced mechanisms in males than that in females. Hypertension is also associated with B cell activation and production of autoantibodies (anti-Hsp70, anti-Hsp65, anti-Hsp60, anti-AT1R, anti-α1AR, and anti-β1AR). The hypertensive responses can be inhibited by T regulatory lymphocytes (Tregs) and their anti-inflammatory IL-10. Adaptive immunity and its interface with innate mechanisms may represent valuable targets in the modulation of blood pressure, as well as hypertension-related residual risk.

Anti-CD3 antibody therapy attenuates the progression of hypertension in female mice with systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a chronic inflammatory autoimmune disorder with prevalent hypertension that significantly contributes to the mortality in this patient population. Pre-clinical and clinical evidence suggests that anti-CD3 antibody therapy may attenuate the development of autoimmune diseases like SLE. However, it is unclear whether this treatment impacts the development of the prevalent hypertension associated with SLE. The present study was designed to determine whether anti-CD3 antibody treatment attenuates the progression of hypertension in female SLE mice with already established renal disease (albuminuria ≥100mg/dL). Female SLE (NZBWF1) and control (NZW) mice were administered either an antibody to CD3ε, a component of the T cell receptor complex expressed on all T cells, or IgG antibody (isotype control) for up to 4 weeks (intranasal; 25μg/week). Spleen weight was lower in SLE mice treated with anti-CD3 antibody than in IgG-treated SLE mice, suggesting that immune system hyperactivity is decreased. Circulating anti-dsDNA autoantibodies were increased in SLE mice compared to controls and were blunted in the anti-CD3-treated SLE mice. The development of hypertension was attenuated in anti-CD3 treated mice with SLE independently of changes in renal injury (assessed by urinary albumin). These data suggest anti-CD3 therapy during autoimmune disease may have added clinical benefit to attenuate cardiovascular risk factors like hypertension.

Immunosuppression With Mycophenolate Mofetil Attenuates Hypertension in an Experimental Model of Autoimmune Disease

Background

Systemic lupus erythematosus (SLE) is a chronic autoimmune disorder that predominantly affects women and is associated with prevalent hypertension, renal injury, and cardiovascular disease. Immune system dysfunction is recognized as an important factor in the pathogenesis of hypertension. We recently showed that preventing autoimmunity prevents the development of hypertension in an experimental model of SLE (female NZBWF1 mice). The present study tests the hypothesis that mycophenolate mofetil (MMF), an immunosuppressive therapy used clinically to treat SLE by depleting proliferating B and T lymphocytes, can improve blood pressure control.

Methods and Results

Female SLE and control (NZW/LacJ) mice were treated daily for 8 weeks with 60 mg/kg MMF. Circulating CD45R⁺ B cells were lower in MMF‐treated SLE mice after 4 weeks of treatment, but neither CD4⁺ nor CD8⁺ T cells were reduced by MMF. Plasma anti–double‐stranded DNA IgG autoantibodies, a marker of SLE disease activity, were higher in SLE mice compared with controls and were lower in SLE mice after 8 weeks of MMF. Mean arterial pressure was elevated in SLE mice compared with controls and lower in SLE mice treated with MMF compared with vehicle‐treated SLE mice. MMF also reduced both renal injury (urinary albumin excretion and glomerulosclerosis) and the infiltration of CD45R⁺ B cells and CD3⁺CD4⁺ T cells in kidneys from mice with SLE.

Conclusions

These data suggest that MMF selectively depleted CD45R⁺ B cells and lowered subsequent autoantibody production, furthering the concept that autoantibodies mechanistically contribute to the pathogenesis of hypertension.

Obligatory Role for B Cells in the Development of Angiotensin II–Dependent Hypertension

Clinical hypertension is associated with raised serum IgG antibodies. However, whether antibodies are causative agents in hypertension remains unknown. We investigated whether hypertension in mice is associated with B-cell activation and IgG production and moreover whether B-cell/IgG deficiency affords protection against hypertension and vascular remodeling. Angiotensin II (Ang II) infusion (0.7 mg/kg per day; 28 days) was associated with (1) a 25% increase in the proportion of splenic B cells expressing the activation marker CD86, (2) an 80% increase in splenic plasma cell numbers, (3) a 500% increase in circulating IgG, and (4) marked IgG accumulation in the aortic adventitia. In B-cell–activating factor receptor–deficient (BAFF-R −/− ) mice, which lack mature B cells, there was no evidence of Ang II–induced increases in serum IgG. Furthermore, the hypertensive response to Ang II was attenuated in BAFF-R −/− (Δ30±4 mm Hg) relative to wild-type (Δ41±5 mm Hg) mice, and this response was rescued by B-cell transfer. BAFF-R −/− mice displayed reduced IgG accumulation in the aorta, which was associated with 80% fewer aortic macrophages and a 70% reduction in transforming growth factor-β expression. BAFF-R −/− mice were also protected from Ang II–induced collagen deposition and aortic stiffening (assessed by pulse wave velocity analysis). Finally, like BAFF-R deficiency, pharmacological depletion of B cells with an anti-CD20 antibody attenuated Ang II–induced hypertension by ≈35%. Hence, these studies demonstrate that B cells/IgGs are crucial for the development of Ang II–induced hypertension and vessel remodeling in mice. Thus, B-cell–targeted therapies—currently used for autoimmune diseases—may hold promise as future treatments for hypertension.

The immunological basis of hypertension

A large number of investigations have demonstrated the participation of the immune system in the pathogenesis of hypertension. Studies focusing on macrophages and Toll-like receptors have documented involvement of the innate immunity. The requirements of antigen presentation and co-stimulation, the critical importance of T cell-driven inflammation, and the demonstration, in specific conditions, of agonistic antibodies directed to angiotensin II type 1 receptors and adrenergic receptors support the role of acquired immunity. Experimental findings support the concept that the balance between T cell-induced inflammation and T cell suppressor responses is critical for the regulation of blood pressure levels. Expression of neoantigens in response to inflammation, as well as surfacing of intracellular immunogenic proteins, such as heat shock proteins, could be responsible for autoimmune reactivity in the kidney, arteries, and central nervous system. Persisting, low-grade inflammation in these target organs may lead to impaired pressure natriuresis, an increase in sympathetic activity, and vascular endothelial dysfunction that may be the cause of chronic elevation of blood pressure in essential hypertension.

Antibodies in the pathogenesis of hypertension

It has long been known that circulating levels of IgG and IgM antibodies are elevated in patients with essential and pregnancy-related hypertension. Recent studies indicate these antibodies target, and in many cases activate, G-protein coupled receptors and ion channels. Prominent among these protein targets are AT₁ receptors, α₁-adrenoceptors, β₁-adrenoceptors, and L-type voltage operated Ca²⁺ channels, all of which are known to play key roles in the regulation of blood pressure through modulation of vascular tone, cardiac output, and/or Na⁺/water reabsorption in the kidneys. This suggests that elevated antibody production may be a causal mechanism in at least some cases of hypertension. In this brief review, we will further describe the protein targets of the antibodies that are elevated in individuals with essential and pregnancy-related hypertension and the likely pathophysiological consequences of antibody binding to these targets. We will speculate on the potential mechanisms that underlie elevated antibody levels in hypertensive individuals and, finally, we will outline the therapeutic opportunities that could arise with a better understanding of how and why antibodies are produced in hypertension.

L-type Ca2+ channel blockers inhibit the window contraction of mouse aorta segments with high affinity

L-type calcium channel blockers (LCCBs) reduce blood pressure more effectively in hypertensive than in normotensive subjects and are more effective in vascular smooth muscle (VSM) than in cardiac muscle. This has been explained by the depolarized resting potential of VSM in comparison with heart muscle cells and during hypertension, because both favor the "high affinity" inactivated state of the L-type calcium channel (LCC). Depolarized resting potentials, however, also increase Ca(2+) influx via window, non-inactivating LCC. The present study investigated whether these channels can be effectively blocked by nifedipine, verapamil or diltiazem, as representatives of different LCCB classes. C57Bl6 mouse aortic segments were depolarized by 50mM K(+) to attain similar degree of inactivation. The depolarization evoked biphasic contractions with the slow force component displaying higher sensitivity to LCCBs than the fast component. Removal of the fast force component increased, whereas stimulation of Ca(2+) influx with the dihydropyridine BAY K8644, a structural analog of nifedipine, decreased the efficacy of the LCCBs. Addition of LCCBs during the contraction caused concentration-dependent relaxation, which was independent of the presence of a fast force component, but still showed lower sensitivity in the presence of BAY K8644. Our data suggest that steady-state contractions by depolarization with 50mM K(+) are completely due to window Ca(2+) influx, which is preferentially inhibited by LCCBs. Furthermore, results point to interactions between the LCCB receptors and Ca(2+) ions or BAY K8644. The high affinity for open, non-inactivating LCC may play a dominant role in the anti-hypertensive effects of LCCBs.