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.

Angiotensin-converting enzyme (ACE) plays a central role in blood pressure regulation by producing the vasoconstrictor angiotensin II. When ACE knockout mice were studied, they presented with a complicated phenotype, including cardiovascular, reproductive, hematologic and developmental defects. The complexity of an ACE knockout mouse emphasizes the advantages and disadvantages of the classic knockout strategy. An animal lacking all ACE is very different from a wild type animal, and can be modeled as representing an extreme phenotype. To understand the role of ACE in a tissue and organ specific fashion, our group used targeted homologous recombination to create mouse models in which a promoter swapping strategy results in very restricted tissue patterns of ACE expression. Mice with ACE expression only in the heart, termed ACE 8/8 mice, present with atria enlargement and electrical conduction defects, but normal ventricular function. Mice with ACE expression only in monocytes and macrophages, termed ACE 10/10 mice, have a marked resistance to the growth of melanoma due to an enhanced immune response characterized by increased tumor specific CD8+ T cells and increased proinflammatory cytokines. These mice may define a new means of augmenting the immune response, potentially useful in human clinical situations. The promoter swapping strategy permits scientific investigation of questions unapproachable by other experimental approaches.

Background: It has been proposed that elements of the renin angiotensin system expressed in the arterial wall are critical for the development of atherosclerosis. Angiotensin converting enzyme (ACE) is highly expressed by the endothelium and is responsible for a critical enzymatic step in the generation of angiotensin II. However, the functional contribution of ACE expression in the vascular wall in atherogenesis is unknown. Therefore, we made use of unique genetic models in which mice without expression of ACE in the vascular wall were crossed with apoE-/- mice in order to determine the contribution of tissue ACE expression to atherosclerotic lesion formation. Methods and Results: Mice expressing either a soluble form of ACE (ACE 2/2) or mice with somatic ACE expression restricted to the liver and kidney (ACE 3/3) on an ApoE-/- background were placed on a standard chow or Western diet for 6 months. Atherosclerotic lesion area in the ACE 2/2 mice was significantly lower than that seen in the ACE 3/3 mice. However, these animals also had significantly lower blood pressure and reduced plasma ACE activity which precluded establishing a specific causal relationship between absent tissue ACE activity and decreased atherosclerotic lesion extent. Therefore, we studied the ACE 3/3 mice which are normotensive and lack vascular ACE expression. In the ACE 3/3 animals, atherosclerotic lesion area was no different from wild type controls despite reduced plasma ACE activity. Conclusions: We concluded that under these experimental conditions, expression of ACE in the arterial wall is not required for atherosclerotic lesion formation.

Background: Macrophages are ubiquitous in all stages of atherosclerosis, exerting tremendous impact on lesion progression and plaque stability. Because macrophages in atherosclerotic plaques express angiotensin-converting enzyme (ACE), current dogma posits that local myeloid-mediated effects worsen the disease. In contrast, we previously reported that myeloid ACE overexpression augments macrophage resistance to various immune challenges, including tumors, bacterial infection and Alzheimer's plaque deposition. Here, we sought to assess the impact of myeloid ACE on atherosclerosis. Methods: A mouse model in which ACE is overexpressed in myelomonocytic lineage cells, called ACE10, was generated and sequentially crossed with ApoE-deficient mice to create ACE10/10ApoE−/− (ACE10/ApoE). Control mice were ACEWT/WTApoE−/− (WT/ApoE). Atherosclerosis was induced using an atherogenic diet alone, or in combination with unilateral nephrectomy plus deoxycorticosterone acetate (DOCA) salt for eight weeks. Results: With an atherogenic diet alone or in combination with DOCA, the ACE10/ApoE mice showed significantly less atherosclerotic plaques compared to their WT/ApoE counterparts (p < 0.01). When recipient ApoE−/− mice were reconstituted with ACE10/10 bone marrow, these mice showed significantly reduced lesion areas compared to recipients reconstituted with wild type bone marrow. Furthermore, transfer of ACE-deficient bone marrow had no impact on lesion area. Conclusion: Our data indicate that while myeloid ACE may not be required for atherosclerosis, enhanced ACE expression paradoxically reduced disease progression.