Ellen A. Bernstein

Myeloid Suppressor Cells Accumulate and Regulate Blood Pressure in Hypertension

RATIONALE Chronic inflammation is a major contributor to the progressive pathology of hypertension, and T-cell activation is required for the genesis of hypertension. However, the precise role of myeloid cells in this process is unclear. OBJECTIVE To characterize and understand the role of peripheral myeloid cells in the development of hypertension. METHODS AND RESULTS We examined myeloid cells in the periphery of hypertensive mice and found that increased numbers of CD11b(+)Gr1(+) myeloid cells in blood and the spleen are a characteristic of 3 murine models of experimental hypertension (angiotensin II, L-NG-nitroarginine methyl ester, and high salt). These cells express surface markers and transcription factors associated with immaturity and immunosuppression. Also, they produce hydrogen peroxide to suppress T-cell activation. These are characteristics of myeloid-derived suppressor cells (MDSCs). Depletion of hypertensive MDSCs increased blood pressure and renal inflammation. In contrast, adoptive transfer of wild-type MDSCs to hypertensive mice reduced blood pressure, whereas the transfer of nicotinamide adenine dinucleotide phosphate oxidase 2-deficient MDSCs did not. CONCLUSION The accumulation of MDSCs is a characteristic of experimental models of hypertension. MDSCs limit inflammation and the increase of blood pressure through the production of hydrogen peroxide.

Angiotensin-converting enzyme overexpression in myelomonocytes prevents Alzheimer’s-like cognitive decline

Cognitive decline in patients with Alzheimers disease (AD) is associated with elevated brain levels of amyloid β protein (Aβ), particularly neurotoxic Aβ(1-42). Angiotensin-converting enzyme (ACE) can degrade Aβ(1-42), and ACE overexpression in myelomonocytic cells enhances their immune function. To examine the effect of targeted ACE overexpression on AD, we crossed ACE(10/10) mice, which overexpress ACE in myelomonocytes using the c-fms promoter, with the transgenic APP(SWE)/PS1(ΔE9) mouse model of AD (AD⁺). Evaluation of brain tissue from these AD⁺ACE(10/10) mice at 7 and 13 months revealed that levels of both soluble and insoluble brain Aβ(1-42) were reduced compared with those in AD⁺ mice. Furthermore, both plaque burden and astrogliosis were drastically reduced. Administration of the ACE inhibitor ramipril increased Aβ levels in AD⁺ACE(10/10) mice compared with the levels induced by the ACE-independent vasodilator hydralazine. Overall, AD⁺ACE(10/10) mice had less brain-infiltrating cells, consistent with reduced AD-associated pathology, though ACE-overexpressing macrophages were abundant around and engulfing Aβ plaques. At 11 and 12 months of age, the AD⁺ACE(10/WT) and AD⁺ACE(10/10) mice were virtually equivalent to non-AD mice in cognitive ability, as assessed by maze-based behavioral tests. Our data demonstrate that an enhanced immune response, coupled with increased myelomonocytic expression of catalytically active ACE, prevents cognitive decline in a murine model of AD.

Rediscovering ACE: Novel insights into the many roles of the angiotensin-converting enzyme

Angiotensin-converting enzyme (ACE) is best known for the catalytic conversion of angiotensin I to angiotensin II. However, the use of gene-targeting techniques has led to mouse models highlighting many other biochemical properties and actions of this enzyme. This review discusses recent studies examining the functional significance of ACE tissue-specific expression and the presence in ACE of two independent catalytic sites with distinct substrates and biological effects. It is these features which explain why ACE makes important contributions to many different physiological processes including renal development, blood pressure control, inflammation, and immunity.

Renal Generation of Angiotensin II and the Pathogenesis of Hypertension

The existence of a complete and functional renin-angiotensin system along the nephron is widely recognized. However, its precise role in blood pressure control and, by extension, hypertension is still uncertain. While most investigators agree that overexpressing RAS components along the nephron results in hypertension, two important issues remain: whether the local RAS works as a separate entity or represents an extension of the systemic RAS and whether locally generated angiotensin II has specific renal effects on blood pressure that are distinct from systemic angiotensin II. This review addresses these issues while emphasizing the unique role of local angiotensin II in the response of the kidney to hypertensive stimuli and the induction of hypertension.

Angiotensin-converting Enzyme Overexpression in Mouse Myelomonocytic Cells Augments Resistance to Listeria and Methicillin-resistant Staphylococcus aureus

Gene targeting in ES cells was used to substitute control of angiotensin converting enzyme (ACE) expression from the endogenous promoter to the mouse c-fms promoter. The result is an animal model called ACE 10/10 in which ACE is overexpressed by monocytes, macrophages, and other myelomonocytic lineage cells. To study the immune response of these mice to bacterial infection, we challenged them with Listeria monocytogenes or methicillin-resistant Staphylococcus aureus (MRSA). ACE 10/10 mice have a significantly enhanced immune response to both bacteria in vivo and in vitro. For example, 5 days after Listeria infection, the spleen and liver of ACE 10/10 mice had 8.0- and 5.2-fold less bacteria than wild type mice (WT). In a model of MRSA skin infection, ACE 10/10 mice had 50-fold less bacteria than WT mice. Histologic examination showed a prominent infiltrate of ACE-positive mononuclear cells in the skin lesions from ACE 10/10. Increased bacterial resistance in ACE 10/10 is directly due to overexpression of ACE, as it is eliminated by an ACE inhibitor. Critical to increased immunity in ACE 10/10 is the overexpression of iNOS and reactive nitrogen intermediates, as inhibition of iNOS by the inhibitor 1400W eliminated all in vitro and in vivo differences in innate bacterial resistance between ACE 10/10 and WT mice. Increased resistance to MRSA was transferable by bone marrow transplantation. The overexpression of ACE and iNOS by myelomonocytic cells substantially boosts innate immunity and may represent a new means to address serious bacterial infections.

New insights into the role of angiotensin-converting enzyme obtained from the analysis of genetically modified mice

Angiotensin-converting enzyme (ACE) has been well-recognized for its role in blood pressure regulation. ACE is made by many tissues, though it is most abundantly expressed on the luminal surface of vascular endothelium. ACE knockout mice show a profound phenotype with low blood pressure, but also with hemopoietic and developmental defects, which complicates understanding the biological functions of ACE in individual tissue types. Using a promoter-swapping strategy, several mouse lines with unique ACE tissue expression patterns were studied. These include mice with ACE expression in the liver (ACE 3/3), the heart (ACE 8/8), and macrophages (ACE 10/10). We also investigated mice with a selective inactivation of either the N- or C-terminal ACE catalytic domain. Our studies indicate that ACE plays a role in many other physiologic processes beyond simple blood pressure control.

Nontraditional Roles of Angiotensin-Converting Enzyme

Many articles have described the biochemistry of the renin-angiotensin system (RAS). Simply put, renin and angiotensin-converting enzyme (ACE) produce angiotensin II, which affects the brain (increased thirst), the gut (increased salt absorption), the adrenals (aldosterone production), the kidneys (salt and water retention), the heart (increased cardiac output), and vascular smooth muscle (vasoconstriction). Inhibitors of ACE and the angiotensin II type 1 receptor are now widely used to reduce blood pressure and to treat hypertension, heart failure, and renal disease.1 The effectiveness of these drugs and abundant data from animal models show that the RAS is the bodys central mechanism for regulating blood pressure. Modern studies of the RAS have substantially broadened the known roles of this system by showing that angiotensin II influences cellular proliferation and the inflammatory response. Although this wider interpretation of the RAS encompasses more than control of blood pressure, it still focuses on the effects of angiotensin II in the context of vascular injury. This increased understanding of the RAS inevitably leads to the question of whether, even now, we fully understand all of the physiological roles of the RAS. Here, we argue that the answer is no, that nature uses the components of the RAS and, in particular, ACE for a wide variety of physiological tasks. ### ACE Is Necessary for Renal Development and Normal Male Fertility In understanding the function of the RAS, a powerful tool is the ability to genetically alter mouse genes. Knockout studies of angiotensinogen, renin, ACE, or the angiotensin II type 1 receptor have confirmed the important role of these proteins in regulating blood pressure, but they also underline the wider physiological role for the RAS.2 For example, adult mice lacking ACE show marked abnormalities of renal development characterized by hypertrophy of small arteries, interstitial fibrosis, atrophy of the inner medulla and renal papilla, and dilation of calyces. Renal …

The intrarenal generation of angiotensin II is required for experimental hypertension

Hypertension is a major risk factor for cardiovascular disease. While the cause of hypertension is multifactorial, renal dysregulation of salt and water excretion is a major factor. All components of the renin-angiotensin system are produced locally in the kidney, suggesting that intrarenal generation of angiotensin II plays a key role in blood pressure regulation. Here, we show that two mouse models lacking renal angiotensin converting enzyme (ACE) are protected against angiotensin II and l-NAME induced hypertension. In response to hypertensive stimuli, mice lacking renal ACE do not produce renal angiotensin II. These studies indicate that the intrarenal renin-angiotensin system works as an entity separate from systemic angiotensin II generation. Renal ACE appears necessary for experimental hypertension.

Angiotensin-converting enzyme overexpression in myelocytes enhances the immune response

Abstract Angiotensin-converting enzyme (ACE) plays an important role in blood pressure control. ACE also has effects on renal function, reproduction, hematopoiesis, and several aspects of the immune response. ACE 10/10 mice overexpress ACE in monocytic cells; macrophages from ACE 10/10 mice demonstrate increased polarization toward a proinflammatory phenotype. As a result, ACE 10/10 mice have a highly effective immune response following challenge with melanoma, bacterial infection, or Alzheimer disease. As shown in ACE 10/10 mice, enhanced monocytic function greatly contributes to the ability of the immune response to defend against a wide variety of antigenic and non-antigenic challenges.

Myeloid expression of angiotensin-converting enzyme facilitates myeloid maturation and inhibits the development of myeloid-derived suppressor cells

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous population of immature myeloid cells which accumulate in cancer, infection and chronic inflammation. These cells suppress T-cell function and the immune response. Angiotensin-converting enzyme (ACE) is a peptidase that is now known to regulate aspects of myelopoiesis. Here, we show that ACE expression correlates with myeloid maturation in vitro. Forced ACE overexpression in monocytic cells reduces the generation of MDSCs. In vivo, mice with a genetic change resulting in myeloid cell ACE overexpression have reduced numbers of blood and splenic MDSCs in a tumor model and in a model of chronic inflammation induced by complete Freund’s adjuvant. In contrast, ACE-null mice produce large numbers of MDSCs during chronic inflammation. Macrophages from mice with myeloid ACE overexpressing are more pro-inflammatory and have more tumor-killing activity than cells from wild-type mice. Thus, manipulating myeloid ACE activity can interfere with MDSC development and the maturation of myeloid cells.