Angiotensin-converting enzyme enhances the oxidative response and bactericidal activity of neutrophils

Neutrophils are an essential component of the innate immune response.¹  As first responders, blood neutrophils engulf and kill invading pathogens. Not surprisingly, neutrophil number and functional maturation are tightly regulated by several factors, including cytokines and bioactive peptides.^1-3 

Activation of neutrophil reduced NAD phosphate (NADPH) oxidase (NOX2) and the subsequent generation of superoxide, hydrogen peroxide, and other reactive oxygen species (ROS) play a central role in the antibacterial activities of neutrophils.^2-4  ROS not only kill pathogens in phagosomes, but also activate other important antimicrobial mechanisms, including the release of proteins and fibrils to entrap and kill bacteria, a process called neutrophil extracellular trap (NET) formation.^2,3,5,6 

Angiotensin-converting enzyme (ACE) is a key component of the renin-angiotensin system, and is responsible for converting angiotensin I to the vasoactive peptide angiotensin II (Ang II).^7,8  Millions of patients take ACE inhibitors to treat hypertension and cardiovascular disease. Unlike renin, which is very limited in tissue expression and enzymatic specificity, ACE is expressed in many tissues and is enzymatically promiscuous; in addition to angiotensin I, ACE can cleave many other peptides such as bradykinin, substance P, enkephalins, and several other peptides.⁹  Because of this, ACE affects diverse biological processes, including renal development, male reproduction, and several aspects of the immune response.^8-13  For example, ACE affects the functional maturation of both myeloid and erythroid lineage cells.¹⁴ 

Our laboratory has investigated the role of ACE in monocytic function. We found that overexpression of ACE in monocytes and macrophages upregulates the immune responses of these cells.^13-18  Neutrophils and monocytes are derived from a common precursor and share many biological features. This led us to investigate the natural role of ACE in neutrophil function and whether ACE overexpression would enhance neutrophil function. Studying both ACE knockout (KO) mice and a new line of transgenic mice called Neu^ACE, we found that ACE plays an important physiologic role in the antimicrobial activities of neutrophils. Further, the upregulation of ACE in neutrophils strongly enhances antibacterial immunity in mice. This appears to be the result of a marked increase of NOX2 activity and ROS generation associated with cell activation. Clinically, neutrophil transfusion is used in immunosuppressed patients. The overexpression of ACE in neutrophils endows these cells with a substantially higher capacity to transfer immune resistance to bacterial infection. In contrast, ACE inhibitors appear deleterious to neutrophil function. Given the importance of the innate immune response, these findings raise the possibility of a very novel means of increasing immune resistance.

All animal experiment protocols were approved by the Cedars-Sinai Institutional Animal Care and Usage Committee. ACE KO mice were described previously.¹⁹  These mice were back-crossed to C57BL/6 mice for 10 or more generations.

To generate Neu^ACE mice, mouse ACE complementary DNA was modified to contain the mouse c-fms promoter before the transcription start site (see supplemental Figure 3A, available on the Blood Web site).^15,20  C57BL/6 mice were then made transgenic for this construct using standard approaches. Screening of several different founder lines identified the Neu^ACE mouse family, which was unique in overexpressing only ACE in neutrophils. Neu^ACE mice are distinct from the previously reported ACE 10/10 mice.¹⁵  The Neu^ACE mice were bred to homozygosity for the transgenic construct. All mice were 8 to 12 weeks old. Both male and female mice were used and no phenotypic differences were noted.

Methicillin-resistant Staphylococcus aureus (MRSA; strain LAC, US300), green fluorescent protein (GFP)-expressing. S aureus (GFP-Staph., RN4220), Klebsiella pneumoniae (ATCC 10031), and Pseudomonas aeruginosa (ATCC 35032) have been previously described.^21,22  They were grown as described in the supplemental Methods.

Bacteria were washed twice and resuspended in sterile phosphate buffered saline (PBS). Bacteria were adjusted to the desired concentrations by absorbance assuming an optical density of 600 nm of 0.3 was equivalent to 1 × 10⁸ colony-forming units per milliliter (CFUs/mL). CFU numbers used in each experiment were confirmed by plating. For skin infection, mice were shaved the day before subcutaneous injection of 100 μL PBS per mouse flank at the bacterial doses described in the figures. The lesion size was quantified over the course of 3 days postinfection as described.²³  If not indicated otherwise, the mice were euthanized and skin lesions were excised and homogenized in 1 mL of PBS for determination of CFU by serial dilutions. Afterward, the homogenate was centrifuged for 5 minutes and the supernatant was used for cytokine measurement.

For systemic infection, ∼1 × 10⁷ CFU of MRSA was injected into mice through the retro-orbital vein. After 48 hours, blood was drawn via the retro-orbital vein, and, in some experiments, mice were euthanized and splenic single-cell suspensions were prepared for analysis of ACE, myeloperoxidase (MPO), and cell subpopulations.

All other methods, such as bone marrow (BM) transplantation, ACE inhibitors, neutrophil isolation and depletion, in vitro blood killing, superoxide, ROS, and MPO measurements, or NET formation are previously published protocols described in detail in supplemental Methods.

We first evaluated ACE expression in neutrophils following MRSA infection because activated neutrophils markedly change the profile of expressed proteins. Wild-type (WT) mice were challenged with MRSA (IV) and, after 2 days, ACE expression was determined using flow cytometry (FCM). In response to acute MRSA stimulation, both blood and splenic neutrophils increased surface ACE expression (Figure 1A).

We next challenged ACE KO and WT mice with a subcutaneous injection of MRSA (10⁷ CFU/mouse flank). Three days postinfection, the mice were euthanized, and the skin lesional area and lesional bacterial burden were measured. ACE KO mice have significantly larger skin lesions (fourfold) and more bacteria per lesion (3.3-fold) compared with WT mice (Figure 1B).

ACE KO mice have systemic differences from WT mice, including low blood pressure. Because of this, we investigated the role of ACE in innate immunity using BM transplantation. WT mice were transplanted with BM from either WT mice or ACE KO mice. Western blot verified successful transplantation (supplemental Figure 1). Eight weeks after transplantation, mice were challenged with subcutaneous MRSA. WT mice transplanted with ACE KO BM exhibited substantially increased skin lesional area (4.9-fold) and lesional bacterial counts (3.5-fold) compared with the transplantation of WT BM (Figure 1C). Thus, these studies indicate an important role of ACE in the innate immune response to MRSA.

To examine the ability of phagocytic cells to kill MRSA, we used an MRSA blood killing assay in which peripheral blood from WT and ACE KO mice was mixed with MRSA ex vivo. Viable bacteria were determined at 2 and 5 hours. Bacterial clearance was significantly lower in the blood of ACE KO mice compared with that of WT mice (Figure 1D). To directly assess the ability of ACE KO neutrophils to kill MRSA, an intracellular killing assay was performed using purified neutrophils from BM. MRSA was added at a multiplicity of infection (MOI) of 10, and phagocytosis continued for 20 minutes. Extracellular bacteria were then killed and the number of viable bacteria as CFU was determined after cell lysis. Intracellular bacterial killing by neutrophils from WT mice was substantially better than neutrophils from ACE KO mice (Figure 1E). The peripheral blood of ACE KO animals has equivalent numbers of peripheral neutrophils and other immune cells as compared with WT mice (supplemental Figure 2).

Because the lack of ACE expression in neutrophils and other hematopoietic cells was associated with a diminished innate immune response, we also investigated the phenotype of animals overexpressing ACE in neutrophils. C57Bl/6 mice were made transgenic for a construct in which ACE complementary DNA is under the control of the c-fms promoter (supplemental Figure 3). This approach does not alter the natural ACE gene in the transgenic mice. Founder mice were screened for high neutrophil ACE expression. Eventually 1 family, termed Neu^ACE, was bred to homozygosity for the transgenic construct. By western blot, FCM, and ACE activity measurement, Neu^ACE neutrophils express approximately 12- to 18-fold higher levels of ACE compared with equivalent WT cells (Figure 2). As an example, Neu^ACE neutrophils make 17.6-fold more ACE than equivalent WT cells by ACE activity assay. Other leukocytes (monocytes, macrophages, dendritic cells, B cells, and T cells) and organs (lung, kidney, liver, and spleen) of Neu^ACE mice were not significantly different in ACE expression from WT (supplemental Figures 4-8). The percentages of blood and BM neutrophils were not statistically different between Neu^ACE mice and WT (supplemental Figure 9).

Experiments tested the response of Neu^ACE mice to subcutaneous MRSA injection (2 × 10⁸ CFU/mouse flank). Three days after infection, Neu^ACE mice had significantly smaller skin lesions (24 ± 1 mm²) and fewer bacteria (7.8 × 10⁷ ± 7E6 CFU/mL) compared with WT mice (lesion area, 107.6 ± 16 mm²; bacterial count, 3.4 × 10⁸ ± 6E7 CFU/mL) (Figure 3A, P < .0005). To confirm that this difference was due to neutrophils, both Neu^ACE and WT mice were infected with MRSA following anti-polymorphonuclear neutrophil antibody-induced neutropenia. In the absence of neutrophils, the response to MRSA was not different between Neu^ACE and WT mice. We also studied MRSA infection in Neu^ACE and WT mice pretreated for 7 days with the ACE inhibitor ramipril. The inhibitor was continued during the 3-day experiment. ACE blockade significantly increased the lesional area and the bacterial burden in both WT and Neu^ACE mice. It also eliminated any differences between the 2 strains (Figure 3B). These data suggest that ACE enzymatic activity is important for neutrophil antibacterial activity in both Neu^ACE and WT mice.

Next, we examined the ability of peripheral blood from Neu^ACE mice to kill MRSA in vitro (Figure 3C). At both 2 and 5 hours after addition of MRSA, bacterial titers were significantly lower in blood from Neu^ACE mice compared with blood from WT mice. However, when this experiment was performed using blood from mice pretreated for 7 days with ramipril, blood from Neu^ACE mice was less effective in clearing bacteria and there was now no difference between blood from Neu^ACE and WT mice.

To investigate whether ACE itself directly participates in bacterial killing, in vitro blood killing was assessed using blood pretreated for only 30 minutes with ramipril. Here, there was no pretreatment of the mice. When blood from WT or Neu^ACE was then challenged with MRSA, there was no difference in killing between the samples with or without the acute addition of ramipril (supplemental Figure 10). Additionally, we found no direct effects of pure mouse ACE (affinity purified from mice kidney and lung) on the growth of MRSA (supplemental Figure 11). Thus, ACE activity itself does not participate in the killing of bacteria.

Finally, we investigated the effectiveness of Neu^ACE and WT blood when challenged with either P aeruginosa or K pneumoniae. Similar to the results with MRSA, substantially better in vitro killing of these pathogens was observed by Neu^ACE blood compared with WT (supplemental Figure 12A-B). Together, data in Figures 1-3 indicate that ACE catalytic activity affects the physiologic function of neutrophils. Further, ACE overexpression enhances neutrophil antibacterial response against several bacterial species.

To further compare bacterial killing in Neu^ACE and WT neutrophils, we measured phagocytosis. BM neutrophils were mixed with S aureus producing GFP (MOI ∼20). Bacterial internalization over time was determined by FCM. We found no difference in bacterial internalization between Neu^ACE and WT neutrophils at 0 to 20 minutes (Figure 3D). At 30 to 60 minutes, a slightly lower number of bacteria was measured in Neu^ACE neutrophils compared with WT neutrophils, but these differences were not significant.

To investigate phagocytic killing, neutrophils were allowed to phagocytize bacteria (MOI ∼10) for 30 minutes. Extracellular bacteria were then killed with gentamicin and the loss of GFP signal was followed by FCM. The loss of GFP, which parallels the killing of ingested S aureus,²⁴  was significantly more rapid in Neu^ACE than in WT neutrophils at both 2 and 5 hours (Figure 3E). To more precisely quantitate intracellular killing of MRSA, we counted intracellular CFU at 2 and 5 hours. This also showed a significantly higher rate of bacterial killing in Neu^ACE neutrophils compared with WT neutrophils (Figure 3F). When these experiments were repeated in the presence of DPI (an inhibitor of ROS production by flavoenzymes), we now saw no difference in bacterial killing efficiency between Neu^ACE and WT neutrophils.

The production of ROS is very important for neutrophil antimicrobial activity.^2,25  To directly examine whether the ROS response in neutrophils changes with ACE status, we used an assay in which neutrophil oxidation of the intracellular indicator DHR 123 was determined by FCM under normal and activated conditions. To activate cells, we challenged mice IV with MRSA for 2 days. There was significantly increased oxidation of DHR 123 in Neu^ACE neutrophils compared with WT, indicating higher production of ROS in the Neu^ACE cells (Figure 4A).

To directly compare the in vivo importance of ROS production to phagocytic killing, Neu^ACE and WT mice were challenged with subcutaneous MRSA, but animals were now treated with DPI. Under these conditions, both strains showed equivalent skin lesion area and bacterial burden (Figure 4B). Thus, both in vitro and in vivo results show that increased ROS production by Neu^ACE neutrophils contributes to their better killing of bacteria.

In neutrophils, NOX2 oxidase and MPO plays an important role in ROS-mediated bacterial killing.²⁶  We examined NOX2 activity by measuring superoxide production using a cytochrome c reduction assay. Compared with WT, there was a highly significantly increase of superoxide generation (> twofold, P < .0005) in Neu^ACE neutrophils following MRSA infection (Figure 4C), suggesting ACE expression directly affects NOX2 activity. We next examined the level of NOX2 subunits in the neutrophil membrane fraction since upon activation, cytosolic subunits of NOX2 assemble with membrane gp91-phox.⁴  When neutrophils were exposed to MRSA for 15 minutes, we found a significantly higher level of NADPH oxidase subunits p47-phox and p67-phox (two- to threefold, P < .05) in Neu^ACE neutrophil membranes compared with WT, whereas no significant difference was observed in abundance of gp91-phox (Figure 4D). When we compared total cellular expression of NADPH oxidase subunits, we found no significant difference between Neu^ACE and WT neutrophils (Figure 4E), which indicates that ACE most probably affects NOX2 activation rather than protein expression. Analysis of superoxide production by neutrophils from ACE KO mice showed the opposite effect; these cells made significantly less superoxide than WT cells in response to phorbol 12-myristate 13-acetate (PMA) stimulation (supplemental Figure 13).

Because phosphorylation of p47-phox is critical for activation of NOX2, we measured membrane p47-phox phosphorylation (pSer345) upon activation with MRSA. Compared with WT neutrophils, p47-phox phosphorylation was significantly higher in Neu^ACE neutrophils at 15 minutes after MRSA-mediated activation (Figure 4Fiii, P < 002). In contrast, at an earlier time point (5 minutes) and in control groups (0 minutes) with no MRSA infection, we found no significant difference in p47-phox phosphorylation. ACE inhibition with ramipril eliminated the difference in membrane phospho-p47-phox levels between Neu^ACE and WT cells (Figure 4Fiv).

We assessed specific MPO activity and total peroxidase activity in MRSA-activated neutrophils. No difference was observed between WT and Neu^ACE neutrophils (supplemental Figure 14). To directly study the role of NOX2 in enhanced antibacterial activity of Neu^ACE neutrophils, we measured MRSA killing efficiency of blood killing in the presence of the highly specific NOX2 blocker, gp91 ds-tat.²⁷  NOX2 inhibition with this peptide totally reverted Neu^ACE neutrophils to a WT phenotype as measured by blood killing of MRSA (Figure 4G).

We previously published a mouse model called ACE 10/10 in which ACE is overexpressed in monocytes, macrophages, and other myelomonocytic cells.¹⁵  Evaluation of ACE 10/10 macrophages showed that these cells also overexpress superoxide in response to PMA (supplemental Figure 5viii). Thus, increased ACE activity leads to enhanced superoxide production by neutrophils and macrophages.

Other than phagocytic killing, neutrophils kill pathogens by releasing NET.^28,29  An important stimulus for NET formation is granulocytic ROS production.^5,6,29  To study NET formation in Neu^ACE mice, BM neutrophils were purified and then treated with PMA. NET formation was analyzed by fluorescent microscopy following DNA staining. Neu^ACE neutrophils showed a higher rate of NET formation compared with WT (Figure 5A). Further, MRSA-induced NET formation was quantitated using an assay that specifically measured NET-associated neutrophil elastase. The amount of NET-associated elastase paralleled the release of double-stranded DNA, a major component of NETs.^30,31  Neu^ACE neutrophils release significantly more NET-associated elastase than equivalent WT cells (Figure 5B). The inhibition of ROS by DPI reduced NET-associated elastase release by Neu^ACE neutrophils, eliminating the difference between these cells and WT neutrophils (Figure 5B). ACE inhibition by ramipril also abrogated the higher rate of NET formation in Neu^ACE mice (Figure 5C).

Neutrophils also attack pathogens using cytokines that not only enhance neutrophil recruitment but also act to recruit and activate other immune cells.^21,32,33  We examined production of interleukin 1β (IL-1β) as it is a critical cytokine for antibacterial activity.^21,33  Three days after subcutaneous MRSA inoculation, IL-1β levels in skin lesions were measured by enzyme-linked immunosorbent assay (ELISA). Lesions from Neu^ACE mice contained significantly more IL-1β than equivalent lesions from WT mice (Figure 5D). When the same experiment was performed in mice treated with DPI, the difference between Neu^ACE and WT mice disappeared. This same experiment was also performed with mice depleted of neutrophils, or with mice treated for 1 week with ramipril. Now, there was no difference in IL-1β production between Neu^ACE and WT mice (supplemental Figure 15).

To directly investigate granulocyte production of IL-1β, neutrophils were highly purified from the BM of Neu^ACE and WT mice (>98% by FCM). These cells were exposed to MRSA in vitro. After 8 hours, cells were lysed and IL-1β within the lysate (cells plus supernatant) was assessed by ELISA. Again, this experiment showed that neutrophils from Neu^ACE mice expressed significantly more IL-1β than WT when exposed to MRSA (Figure 5E).

Ang II is an important product of ACE that exerts most effects following binding to cell surface AT1 receptors.^34,35  The Ang II/AT1 receptor pathway is known to induce ROS in the cardiovascular system and in circulating leukocytes.^36,37  To investigate whether this pathway influences ROS production in Neu^ACE neutrophils, we analyzed superoxide production in these and WT neutrophils in the presence of the AT1 receptor antagonist losartan. Experiments were also performed in the presence or absence of the ACE inhibitor ramipril. Losartan did not block superoxide generation in Neu^ACE cells (Figure 6A). This was in contrast to ACE inhibition by ramipril, which reduced superoxide production by Neu^ACE cells and abolished the difference between Neu^ACE and WT cells (Figure 6B). Similarly, increased peroxide production in response to MRSA was completely eliminated by ramipril, but was not affected by losartan (supplemental Figure 16). In addition, we did not find any difference in the blood levels of Ang II between WT and Neu^ACE mice (supplemental Figure 17). These data indicate that increased ROS production in Neu^ACE neutrophils exposed to MRSA is not due to Ang II–mediated binding to the AT1 receptor, but is due to ACE activity.

To study the role of the Ang II/AT1 axis in vivo, groups of Neu^ACE and WT mice were treated for 1 week with losartan or with the renin inhibitor aliskiren. Both drugs were effective as measured by a reduction of blood pressure (supplemental Figure 18). The mice were then challenged with subcutaneous MRSA. Neither losartan nor aliskiren affected the lesion size or bacterial counts of Neu^ACE or WT mice compared with equivalent mice not treated with the drugs (Figure 6C-D). Similarly, the treatment of WT mice with Ang II for several days also showed no effects on MRSA skin lesion size, lesional bacteria counts, or blood killing of MRSA (supplemental Figure 19). Thus, these data indicate that Ang II is not responsible for the neutrophil antibacterial activity in Neu^ACE mice. Further, blocking other known ACE-mediated peptide pathways, such as bradykinin/B2R, substance p/NK1R, and Ac-SDKP, had no effects on bacterial killing in mice (supplemental Figure 20).

We explored using Neu^ACE neutrophil therapy for controlling MRSA infection in neutropenic mice. These studies were designed to mimic patients who were neutropenic from chemotherapy and receive neutrophil transfusions to combat infection. Mice were administered a single intraperitoneal dose of cyclophosphamide (230 mg/kg) that was sufficient to render them neutropenic for 4 days (supplemental Figure 21). One day after cyclophosphamide, the mice were infected subcutaneously with MRSA, and 1 day after this, the mice received an IV transfusion of neutrophils purified from the BM of either Neu^ACE or WT mice. Mice without neutrophil transfer served as controls. At 3 days postinfection, there was no substantial difference in lesion size between mice treated with WT neutrophils or Neu^ACE neutrophils (Figure 7A). In contrast, the bacterial burden was affected (Figure 7B). Specifically, although the transfer of WT neutrophils into neutropenic mice reduced the bacterial burden, the transfer of Neu^ACE neutrophils was far more effective and reduced the average lesional bacterial counts to only 1/50th that observed in the absence of neutrophil transfer (Figure 7B).

We describe a new line of mice in which ACE is selectively overexpressed in granulocytes. Granulocytes are the first line of defense against acute bacterial infection.^1,2  We now present both in vitro and in vivo evidence that the innate response to acute MRSA infection is increased in mice overexpressing ACE in neutrophils. In contrast, mice lacking ACE activity due to genetic means or because of ACE inhibitors showed the oppose effect: reduced superoxide production and increased susceptibility to infection. Indeed, the comparison of 3 groups of neutrophils (cells lacking ACE expression, cells having WT levels of ACE, and cells overexpressing ACE) showed a direct relationship in which increasing ACE expression is increasingly advantageous in vivo and in in vitro killing by blood, a common assay used to assess neutrophil function. Not only is blood killing of MRSA augmented by ACE overexpression, but a similar analysis of P aeruginosa and K pneumoniae also showed increased killing.

A major question is how does ACE enhance the immune response? Experiments in which an ACE inhibitor was acutely added directly to the blood killing assays showed no effects. In contrast, 7 days’ exposure of mice to an ACE inhibitor, a time sufficient to affect newly formed neutrophils, was associated with significantly diminished bacterial killing. These experiments suggest that it is ACE catalytic activity that is important for the effect of this protein. However, ACE does not directly participate in bacterial killing but probably modifies the phenotype of neutrophils to enhance their immune effectiveness. Granulocytes overexpressing ACE produce more ROS upon activation. This plays a very important role in the increased effectiveness of these neutrophils, as indicated by the effects of the ROS inhibitor DPI. The ACE-overexpressing neutrophils also produce a better NET response and produce increased levels of the pro-inflammatory cytokine IL-1β. These may be downstream effects of increased ROS expression, as indicated by the elimination of the Neu^ACE advantage following administration of DPI.

In neutrophils, NOX2 is the major source of oxidation. Activation and consequent superoxide production by NOX2 requires assembly of cytosolic subunits with NOX2 in the cell membrane.^38,39  Our results showed increased accumulation of cytosolic subunits p47-phox and p67-phox in the membrane of Neu^ACE neutrophils compared with WT following activation with MRSA. Importantly, we found no difference in the total expression of these subunits in Neu^ACE and WT neutrophils, indicating ACE enhances activation rather than increases expression of these subunits. It is known that p47-phox activates through phosphorylation and that it not only self-recruits, but also recruits other cytosolic subunits to the membrane, including p67-phox and p40-phox.³⁸  As expected, a higher level of membrane p47-phox in Neu^ACE neutrophils was also associated with higher levels of phosphorylation. ACE inhibition by ramipril eliminated the difference between Neu^ACE and WT neutrophils in membrane-associated phospho-p47-phox (pSer345). How ACE influences p47-phox phosphorylation needs to be investigated further. For example, we have not yet determined that status of other p47-phox phosphorylation sites or the kinases responsible for increased phosphorylation. Further, we have not characterized natural scavengers of superoxide, such as superoxide dismutase (SOD) activity, in Neu^ACE mice.

In the cardiovascular system, Ang II is intimately associated with blood pressure regulation.^9,34,35  In mice, ROS plays an important role in Ang II–mediated hypertension. Specifically, Ang II is no longer able to induce hypertension if ROS production is pharmacologically or genetically blocked.^36,40,41  What is particularly interesting in our studies is that analysis of the role of ACE vs the AT1 receptor (Ang II) indicates that the enhanced response to infection is independent of AT1 effects but fully dependent on ACE catalytic activity. Indeed, data using the renin inhibitor aliskiren argue that no angiotensin peptide is mediating the Neu^ACE phenotype. We also found that blocking 3 other well-studied ACE-mediated peptide pathways (bradykinin/B2R, substance p/NK1R, and Ac-SDKP) showed no effects on the control of MRSA infection in mice. At present, we do not know the peptides that stimulate increased ROS production following immune activation.

Enhanced susceptibility to infection is observed in patients undergoing chemotherapy.^42,43  Such patients become neutropenic with reduced resistance to infection. This can be treated by the transfusion of exogenous neutrophils.⁴⁴  We have modeled immunosuppression with MRSA infection in mice. These experiments showed the advantage of exogenous neutrophil transfusion. They also showed that granulocytes expressing high levels of ACE (Neu^ACE) appear far more effective in reducing the bacterial counts within MRSA skin lesions.

ACE inhibitors are used by millions of patients to treat hypertension, diabetic nephropathy, heart failure, and other cardiovascular diseases. Many of these patients are elderly. Although clinical studies have documented the effectiveness of renin-angiotensin system blockade, it is now possible to achieve such blockade with either an ACE inhibitor or an Ang II AT1 receptor antagonist. ACE inhibitors are generally considered safe, but some studies have noted an association between ACE inhibition and increased risk of infection. For example, studies demonstrated a higher incidence of urinary tract infections in individuals taking ACE inhibitors.^45,46  A clinical study of sepsis comparing ACE inhibitors and AT1 antagonists showed that the risk of bacterial infection was increased with ACE inhibition but not with an AT1 receptor antagonist.⁴⁷  These results have been challenged by other publications.^48,49  Although ACE affects many physiologic systems, any reduction of neutrophilic function might be expected to contribute to an increased risk of infection; therefore, our findings have clinical implications and suggest that the link between the use of ACE inhibitors and infection should be further explored.

The authors thank Makoto Katsumata (Mouse Genetics Core, Cedars Sinai Medical Center, Los Angeles, CA) for help in generating the Neu^ACE mice and Stacey Kolar (Division of Pediatric Infectious Diseases and Immunology, Cedars Sinai Medical Center, Los Angeles, CA) for her suggestions concerning the in vitro blood killing experiments.

This study was supported by grants from the National Institutes of Health (NIH), National Heart, Lung, and Blood Institute (P01HL129941 and R01HL110353) (K.E.B.), NIH, National Institute of Allergy and Infectious Diseases (R21AI114965) (K.E.B.), and NIH, National Institute of Diabetes and Digestive and Kidney Diseases (R03DK101592) (R.A.G.-V).

Contribution: Z.K. was responsible for study conception and experimental design, and experimental input with assistance from X.Z.S. (intellectual advice, bone marrow transplantation, and neutrophil oxidative burst assay), E.A.B. (animal husbandry and transgenic mice genotyping), S.F. (vector construction), J.F.G., M.E., and T.V.Z. (blood collection and ACE activity measurement); R.A.G.-V. contributed intellectual advice and support; G.Y.L. contributed intellectual advice, helped formulate the hypothesis, and edited the manuscript; K.E.B. contributed intellectual advice and project coordination; and Z.K. and K.E.B. analyzed data and prepared the manuscript.

Conflict-of-interest disclosure: R.A.G.-V. is an employee and a stockholder of Pfizer, Inc. The remaining authors declare no competing financial interests.

Correspondence: Kenneth E. Bernstein, Department of Pathology, Cedars-Sinai Medical Center, 8700 Beverly Blvd, Los Angeles, CA 90048; kenneth.bernstein@cshs.org.