Differential expression of CD14, CD36 and the LDL receptor on human monocyte-derived macrophages. A novel cell culture system to study macrophage differentiation and heterogeneity

Advances in Continuous Microfluidics-Based Technologies for the Study of HIV Infection

HIV-1 is the causative agent of acquired immunodeficiency syndrome (AIDS). It affects millions of people worldwide and the pandemic persists despite the implementation of highly active antiretroviral therapy. A wide spectrum of techniques has been implemented in order to diagnose and monitor AIDS progression over the years. Besides the conventional approaches, microfluidics has provided useful methods for monitoring HIV-1 infection. In this review, we introduce continuous microfluidics as well as the fabrication and handling of microfluidic chips. We provide a review of the different applications of continuous microfluidics in AIDS diagnosis and progression and in the basic study of the HIV-1 life cycle.

Dietary 23-hydroxy ursolic acid protects against atherosclerosis and obesity by preventing dyslipidemia-induced monocyte priming and dysfunction

BACKGROUND AND AIMS

We demonstrated that dietary ursolic acid (UA) reduces atherosclerotic lesion size and improves kidney function in diabetic mice. Based on structure-function analyses of naturally occurring UA analogs, we synthesized 23-hydroxy ursolic acid (23-OHUA), a compound with structural features predicted to enhance its bioavailability and anti-atherogenic properties compared to UA. The goal of this study was to determine the anti-obesogenic and atheroprotective properties of 23-OHUA and its mechanism of action.

METHODS

We performed chemotaxis assays to determine IC₅₀ of phytochemicals on primed THP-1 monocytes. We fed 12-week old female LDLR^-/- mice a high-fat diet (HFD) or a HFD supplemented with either 0.05% UA or 0.05% 23-OHUA, and measured monocyte priming, weight gain and atherosclerotic lesion size after 6 and 20 weeks.

RESULTS

Both dietary UA and 23-OHUA prevented dyslipidemia-induced loss of MKP-1 activity, and hyper-chemotactic activity, hallmarks of blood monocytes priming and dysfunction, but they did not affect plasma lipids or blood glucose levels nor WBC and monocyte counts. After 20 weeks, mice fed 23-OHUA showed 11% less weight gain compared to HFD-fed control mice and a 40% reduction in atherosclerotic plaque size, whereas UA reduced lesion size by only 19% and did not reduce weight gain.

CONCLUSIONS

Dietary 23-OHUA reduces weight gain and attenuates atherogenesis in mice by protecting monocytes against metabolic stress-induced priming and dysfunction. Based on its mechanism of action, 23-OHUA may represent a novel therapeutic approach for the prevention and treatment of obesity and atherosclerosis.

Quantification of sterol-specific response in human macrophages using automated imaged-based analysis

BACKGROUND

The transformation of normal macrophage cells into lipid-laden foam cells is an important step in the progression of atherosclerosis. One major contributor to foam cell formation in vivo is the intracellular accumulation of cholesterol.

METHODS

Here, we report the effects of various combinations of low-density lipoprotein, sterols, lipids and other factors on human macrophages, using an automated image analysis program to quantitatively compare single cell properties, such as cell size and lipid content, in different conditions.

RESULTS

We observed that the addition of cholesterol caused an increase in average cell lipid content across a range of conditions. All of the sterol-lipid mixtures examined were capable of inducing increases in average cell lipid content, with variations in the distribution of the response, in cytotoxicity and in how the sterol-lipid combination interacted with other activating factors. For example, cholesterol and lipopolysaccharide acted synergistically to increase cell lipid content while also increasing cell survival compared with the addition of lipopolysaccharide alone. Additionally, ergosterol and cholesteryl hemisuccinate caused similar increases in lipid content but also exhibited considerably greater cytotoxicity than cholesterol.

CONCLUSIONS

The use of automated image analysis enables us to assess not only changes in average cell size and content, but also to rapidly and automatically compare population distributions based on simple fluorescence images. Our observations add to increasing understanding of the complex and multifactorial nature of foam-cell formation and provide a novel approach to assessing the heterogeneity of macrophage response to a variety of factors.

Dyslipidemic Diet-Induced Monocyte "Priming" and Dysfunction in Non-Human Primates Is Triggered by Elevated Plasma Cholesterol and Accompanied by Altered Histone Acetylation

Monocytes and the recruitment of monocyte-derived macrophages into sites of inflammation play a key role in atherogenesis and other chronic inflammatory diseases linked to cardiometabolic syndrome and obesity. Previous studies from our group have shown that metabolic stress promotes monocyte priming, i.e., enhanced adhesion and accelerated chemotaxis of monocytes in response to chemokines, both in vitro and in dyslipidemic LDLR^-/- mice. We also showed that metabolic stress-induced monocyte dysfunction is, at least to a large extent caused by the S-glutathionylation, inactivation, and subsequent degradation of mitogen-activated protein kinase phosphatase 1. Here, we analyzed the effects of a Western-style, dyslipidemic diet (DD), which was composed of high levels of saturated fat, cholesterol, and simple sugars, on monocyte (dys)function in non-human primates (NHPs). We found that similar to mice, a DD enhances monocyte chemotaxis in NHP within 4 weeks, occurring concordantly with the onset of hypercholesterolemia but prior to changes in triglycerides, blood glucose, monocytosis, or changes in monocyte subset composition. In addition, we identified transitory decreases in the acetylation of histone H3 at the lysine residues 18 and 23 in metabolically primed monocytes, and we found that monocyte priming was correlated with the acetylation of histone H3 at lysine 27 after an 8-week DD regimen. Our data show that metabolic stress promotes monocyte priming and hyper-chemotactic responses in NHP. The histone modifications accompanying monocyte priming in primates suggest a reprogramming of the epigenetic landscape, which may lead to dysregulated responses and functionalities in macrophages derived from primed monocytes that are recruited to sites of inflammation.

Macrophage TGF-β1 and the Proapoptotic Extracellular Matrix Protein BIGH3 Induce Renal Cell Apoptosis in Prediabetic and Diabetic Conditions

Metabolically stressed kidney is in part characterized by infiltrating macrophages and macrophage-derived TGF-β1 that promote the synthesis of various ECM molecules. TGF-β1 strongly enhances the expression of the gene TGFBI that encodes a cell-adhesion class, proapoptotic ECM protein called BIGH3. We hypothesized that in a diabetic environment a relationship between infiltrating macrophages, macrophage-derived TGF-β1, and BIGH3 protein promotes renal cell death. To investigate this hypothesis, we used our mouse model of diabetic complications. Mice on a high-fat diet developed hypercholesterolemia, and exposure to streptozotocin rendered hypercholesterolemic mice diabetic. Immunohistochemical images show increased macrophage infiltration and BIGH3 protein in the kidney cortices of hypercholesterolemic and diabetic mice. Macrophages induced a two-fold increase in BIGH3 expression and an 86% increase in renal proximal tubule epithelial cell apoptosis. TGF-β1 antibody and TGF-β1 receptor chemical antagonist blocked macrophage-induced apoptosis. BIGH3 antibody completely blocked apoptosis that was induced by TGF-β1, and blocked apoptosis induced by exogenous recombinant BIGH3. These results uncover a distinctive interplay of macrophage-derived TGF-β1, BIGH3 protein, and apoptosis, and indicate that BIGH3 is central in a novel pathway that promotes diabetic nephropathy. Macrophage TGF-β1 and BIGH3 are identified as prediabetic biomarkers, and potential therapeutic targets for intervention in prediabetic and diabetic individuals.

BIGH3 protein and macrophages in retinal endothelial cell apoptosis

Diabetes is a pandemic disease with a higher occurrence in minority populations. The molecular mechanism to initiate diabetes-associated retinal angiogenesis remains largely unknown. We propose an inflammatory pathway of diabetic retinopathy in which macrophages in the diabetic eye provide TGFβ to retinal endothelial cells (REC) in the retinal microvasculature. In response to TGFβ, REC synthesize and secrete a pro-apoptotic BIGH3 (TGFβ-Induced Gene Human Clone 3) protein, which acts in an autocrine loop to induce REC apoptosis. Rhesus monkey retinal endothelial cells (RhREC) were treated with dMCM (cell media of macrophages treated with high glucose and LDL) and assayed for apoptosis (TUNEL), BIGH3 mRNA (qPCR), and protein (Western blots) expressions. Cells were also treated with ΤGFβ1 and 2 for BIGH3 mRNA and protein expression. Inhibition assays were carried out using antibodies for TGFβ1 and for BIGH3 to block apoptosis and mRNA expression. BIGH3 in cultured RhREC cells were identified by immunohistochemistry (IHC). Distribution of BIGH3 and macrophages in the diabetic mouse retina was examined with IHC. RhRECs treated with dMCM or TGFβ showed a significant increase in apoptosis and BIGH3 protein expression. Recombinant BIGH3 added to RhREC culture medium led to a dose-dependent increase in apoptosis. Antibodies (Ab) directed against BIGH3 and TGFβ, as well as TGFβ receptor blocker resulted in a significant reduction in apoptosis induced by either dMCM, TGFβ or BIGH3. IHC showed that cultured RhREC constitutively expressed BIGH3. Macrophage and BIGH3 protein were co-localized to the inner retina of the diabetic mouse eye. Our results support a novel inflammatory pathway for diabetic retinopathy. This pathway is initiated by TGFβ released from macrophages, which promotes synthesis and release of BIGH3 protein by REC and REC apoptosis.

Ursolic acid protects monocytes against metabolic stress-induced priming and dysfunction by preventing the induction of Nox4

AIMS

Dietary supplementation with ursolic acid (UA) prevents monocyte dysfunction in diabetic mice and protects mice against atherosclerosis and loss of renal function. The goal of this study was to determine the molecular mechanism by which UA prevents monocyte dysfunction induced by metabolic stress.

METHODS AND RESULTS

Metabolic stress sensitizes or "primes" human THP-1 monocytes and murine peritoneal macrophages to the chemoattractant MCP-1, converting these cells into a hyper-chemotactic phenotype. UA protected THP-1 monocytes and peritoneal macrophages against metabolic priming and prevented their hyper-reactivity to MCP-1. UA blocked the metabolic stress-induced increase in global protein-S-glutathionylation, a measure of cellular thiol oxidative stress, and normalized actin-S-glutathionylation. UA also restored MAPK phosphatase-1 (MKP1) protein expression and phosphatase activity, decreased by metabolic priming, and normalized p38 MAPK activation. Neither metabolic stress nor UA supplementation altered mRNA or protein levels of glutaredoxin-1, the principal enzyme responsible for the reduction of mixed disulfides between glutathione and protein thiols in these cells. However, the induction of Nox4 by metabolic stress, required for metabolic priming, was inhibited by UA in both THP-1 monocytes and peritoneal macrophages.

CONCLUSION

UA protects THP-1 monocytes against dysfunction by suppressing metabolic stress-induced Nox4 expression, thereby preventing the Nox4-dependent dysregulation of redox-sensitive processes, including actin turnover and MAPK-signaling, two key processes that control monocyte migration and adhesion. This study provides a novel mechanism for the anti-inflammatory and athero- and renoprotective properties of UA and suggests that dysfunctional blood monocytes may be primary targets of UA and related compounds.

Regulation of Monocyte Adhesion and Migration by Nox4

We showed that metabolic disorders promote thiol oxidative stress in monocytes, priming monocytes for accelerated chemokine-induced recruitment, and accumulation at sites of vascular injury and the progression of atherosclerosis. The aim of this study was to identify both the source of reactive oxygen species (ROS) responsible for thiol oxidation in primed and dysfunctional monocytes and the molecular mechanisms through which ROS accelerate the migration and recruitment of monocyte-derived macrophages. We found that Nox4, a recently identified NADPH oxidase in monocytes and macrophages, localized to focal adhesions and the actin cytoskeleton, and associated with phospho-FAK, paxillin, and actin, implicating Nox4 in the regulation of monocyte adhesion and migration. We also identified Nox4 as a new, metabolic stress-inducible source of ROS that controls actin S-glutathionylation and turnover in monocytes and macrophages, providing a novel mechanistic link between Nox4-derived H₂O₂ and monocyte adhesion and migration. Actin associated with Nox4 was S-glutathionylated, and Nox4 association with actin was enhanced in metabolically-stressed monocytes. Metabolic stress induced Nox4 and accelerated monocyte adhesion and chemotaxis in a Nox4-dependent mechanism. In conclusion, our data suggest that monocytic Nox4 is a central regulator of actin dynamics, and induction of Nox4 is the rate-limiting step in metabolic stress-induced monocyte priming and dysfunction associated with accelerated atherosclerosis and the progression of atherosclerotic plaques.

Redox regulation of MAPK phosphatase 1 controls monocyte migration and macrophage recruitment

Monocytic adhesion and chemotaxis are regulated by MAPK pathways, which in turn are controlled by redox-sensitive MAPK phosphatases (MKPs). We recently reported that metabolic disorders prime monocytes for enhanced recruitment into vascular lesions by increasing monocytes' responsiveness to chemoattractants. However, the molecular details of this proatherogenic mechanism were not known. Here we show that monocyte priming results in the S-glutathionylation and subsequent inactivation and degradation of MKP-1. Chronic exposure of human THP-1 monocytes to diabetic conditions resulted in the loss of MKP-1 protein levels, the hyperactivation of ERK and p38 in response to monocyte chemoattractant protein-1 (MCP-1), and increased monocyte adhesion and chemotaxis. Knockdown of MKP-1 mimicked the priming effects of metabolic stress, whereas MKP-1 overexpression blunted both MAPK activation and monocyte adhesion and migration induced by MCP-1. Metabolic stress promoted the S-glutathionylation of MKP-1, targeting MKP-1 for proteasomal degradation. Preventing MKP-1 S-glutathionylation in metabolically stressed monocytes by overexpressing glutaredoxin 1 protected MKP-1 from degradation and normalized monocyte adhesion and chemotaxis in response to MCP-1. Blood monocytes isolated from diabetic mice showed a 55% reduction in MKP-1 activity compared with nondiabetic mice. Hematopoietic MKP-1 deficiency in atherosclerosis-prone mice mimicked monocyte priming and dysfunction associated with metabolic disorders, increased monocyte chemotaxis in vivo, and accelerated atherosclerotic lesion formation. In conclusion, we identified MKP-1 as a central redox-sensitive regulator of monocyte adhesion and migration and showed that the loss of MKP-1 activity is a critical step in monocyte priming and the metabolic stress-induced conversion of blood monocytes into a proatherogenic phenotype.

Nox4 is a novel inducible source of reactive oxygen species in monocytes and macrophages and mediates oxidized low density lipoprotein-induced macrophage death

RATIONALE

The enhanced formation of intracellular reactive oxygen species (ROS) induced by oxidized low-density lipoprotein (OxLDL) promotes macrophage death, a process likely to contribute to the formation of necrotic cores and the progression of atherosclerotic lesions. Yet macrophage deficiency of phagocytic NADPH oxidase (Nox2), the primary source of ROS in macrophages, does not reduce atherosclerotic lesion development in mice. This suggests an as yet unidentified NADPH oxidase may be present in macrophages and responsible for the intracellular ROS formation induced by OxLDL.

OBJECTIVE

The aim of this study was to identify the source of intracellular ROS involved in macrophage death.

METHODS AND RESULTS

Nox4 was expressed in human monocytes and mature macrophages, and was localized to the endoplasmic reticulum and to defined foci within the nucleus. Nox4 colocalized with p22(phox), and both proteins were upregulated in response to OxLDL stimulation, whereas Nox2/gp91(phox) levels remained unchanged. Induction of Nox4 expression, intracellular ROS formation and macrophage cytotoxicity induced by OxLDL were blocked by MEK1/2 inhibition, but not by inhibitors of p38-MAPK (mitogen-activated protein kinase), JNK (Jun N-terminal kinase), or JAK2 (Janus kinase 2). Small interfering RNA knockdown of Nox4 inhibited both intracellular ROS production and macrophage cytotoxicity induced by OxLDL, whereas Nox4 overexpression enhanced both OxLDL-stimulated ROS formation and macrophage death.

CONCLUSIONS

Nox4 is a novel source of intracellular ROS in human monocytes and macrophages. Induction of Nox4 by OxLDL is mediated by the MEK1/ERK pathway and required for OxLDL cytotoxicity in human macrophages, implicating monocytic Nox4 in atherogenesis.

Macrophage heterogeneity in atherosclerotic plaques

PURPOSE OF REVIEW

The varied behaviour of macrophages and foam cells during atherosclerosis and its clinical sequelae prompt the question whether all these activities can be the property of a single cell population.

RECENT FINDINGS

Subsets of monocytes with distinct patterns of surface markers and behaviours during inflammation have recently been characterized and shown to have complementary roles during progression of atherosclerosis. A variety of macrophage phenotypes derived from these monocyte subsets in response to mediators of innate and acquired immunity have also been found in plaques. Based on functional properties and genomic signatures, they may have different impacts on facets of plaque development, including fibrous cap and lipid core formation.

SUMMARY

Monocyte and macrophage phenotypic diversity is important in atherogenesis. More work is needed to define consistent marker sets for the different foam cell phenotypes in experimental animals and humans. Cell tracking studies are needed to establish their relationship with monocyte subtypes. In addition, genetic and pharmacological manipulation of phenotypes will be useful to define their functions and exploit the resulting therapeutic potential.

Enhancing the performance of a point-of-care CD4+ T-cell counting microchip through monocyte depletion for HIV/AIDS diagnostics

CD4+ T cell counts are important tests used to stage HIV-positive patients, enabling clinicians to make informed antiretroviral treatment decisions and to monitor the therapeutic outcomes. However, state-of-the-art CD4 counting methods based on flow cytometry are not applicable in resource-limited settings, due to their high cost and technical requirements. In previous work, we reported the development of a cell isolation microchip that can be used at the point of care for CD4 counts. In that microfluidic chip, CD4+ T cells were separated from 10 microL of whole blood, and enumerated via either light microscopy or impedance sensing. The microchip counts matched flow cytometry results in the intermediate CD4 count range, between 200-800 cells/microL, but displayed a positive bias at absolute CD4 counts below 200 cells/microL, due largely to monocyte contamination. To enhance the performance in the low CD4 count range, we report here an improved design of a two-stage microfluidic device to deplete monocytes from whole blood, followed by CD4+ T cell capture. Using the double-stage device combined with a high viscosity rinsing solution, we obtained microchip CD4 counts comparable to flow cytometry results in the full clinically relevant range. In addition to CD4 counting, the strategy of contaminant depletion prior to target cell isolation can be easily adapted to immunoaffinity capture of other cell types that lack a unique surface marker from a complex biological fluid.

Antigen-induced immunomodulation in the pathogenesis of atherosclerosis

Atherosclerosis is a chronic inflammatory disorder characterised by the accumulation of monocytes/macrophages, smooth muscle cells, and lymphocytes within the arterial wall in response to the release of proinflammatory molecules. Such accumulation results in the formation of the atherosclerotic plaque, which would eventually evolve to complications such as total artery occlusion, rupture, calcification, or aneurysm. Although the molecular mechanism responsible for the development of atherosclerosis is not completely understood, it is clear that the immune system plays a key role in the development of the atherosclerotic plaque and in its complications. There are multiple antigenic stimuli that have been associated with the pathogenesis of atherosclerosis. Most of these stimuli come from modified self-molecules such as oxidised low-density lipoproteins (oxLDLs), beta2glycoprotein1 (β2GP1), lipoprotein a (LP(a)), heat shock proteins (HSPs), and protein components of the extracellular matrix such as collagen and fibrinogen in the form of advanced glycation-end (AGE) products. In addition, several foreign antigens including bacteria such as Porphyromonas gingivalis and Chlamydia pneumoniae and viruses such as enterovirus and cytomegalovirus have been associated with atherosclerosis as potentially causative or bystander participants, adding another level of complexity to the analysis of the pathophysiology of atherosclerosis. The present review summarises the most important scientific findings published within the last two decades on the importance of antigens, antigen stimulation, and adaptive immune responses in the development of atherosclerotic plaques.

Heterogeneity of human macrophages in culture and in atherosclerotic plaques

Research suggests that monocytes differentiate into unique lineage-determined macrophage subpopulations in response to the local cytokine environment. The present study evaluated the atherogenic potential of two divergent lineage-determined human monocyte-derived macrophage subpopulations. Monocytes were differentiated for 7 days in the presence of alternative macrophage development cytokines: granulocyte-macrophage colony-stimulating factor to produce granulocyte-macrophage-CSF macrophages (GM-Mac), or macrophage colony-stimulating factor (M-CSF) to produce M-Mac. Gene chip analyses of three monocyte donors demonstrated differential expression of inflammatory and cholesterol homeostasis genes in the macrophage subpopulations. Quantitative PCR confirmed a fivefold elevation in the expression of genes that promote reverse cholesterol transport (PPAR-gamma, LXR-alpha, and ABCG1) and macrophage emigration from lesions (CCR7) in GM-Mac compared to that in M-Mac. Immunocytochemistry confirmed enhanced expression of the proinflammatory marker CD14 in M-Mac relative to GM-Mac. M-Mac spontaneously accumulated cholesterol when incubated with unmodified low-density lipoprotein whereas GM-Mac only accumulated similar levels of cholesterol after protein kinase C activation. Immunostained human coronary arteries showed that macrophages with similar antigen expression to that of M-Mac (CD68(+)/CD14(+)) were predominant within atherosclerotic lesions whereas macrophages with antigen expression similar to GM-Mac (CD68(+)/CD14(-)) were predominant in areas devoid of disease. The identification of macrophage subpopulations with different gene expression patterns and, thus, different potentials for promoting atherosclerosis has important experimental and clinical implications and could prove to be a valuable finding in developing therapeutic interventions in diseases dependent on macrophage function.

Newly recruited human monocytes have a preserved responsiveness towards bacterial peptides in terms of CD11b up-regulation and intracellular hydrogen peroxide production

The transmigration of peripheral human monocytes to the interstitium is a fundamental step in the host-defence mechanism against infections. Little is known about the state of function of in vivo transmigrated interstitial monocytes prior to differentiation into macrophages and dendritic cells. We hypothesized that newly recruited interstitial monocytes have a preserved responsiveness against bacterial-related peptides, giving them a specific role in the immediate defence against invading pathogens. In order to test this hypothesis, we explored the responsiveness of in vivo transmigrated as well as peripheral monocytes, in terms of CD11b expression and H(2)O(2) production towards the bacterial-related peptide formylmethionylleucylphenylalanine (fMLP) by the use of a skin chamber technique. In addition, we analysed the concentration of interleukin (IL)-8, monocyte chemotactic protein-1 (MCP-1) and tumour necrosis factor (TNF)-alpha in the skin blister exudates and in the circulation. We demonstrate that in vivo-transmigrated monocytes had a fivefold higher CD11b expression compared to monocytes obtained from the peripheral circulation. fMLP exposure induced a significantly higher CD11b expression on transmigrated cells compared to peripheral monocytes. In addition, newly recruited monocytes had a preserved H(2)O(2) production. The interstitial concentration of IL-8, MCP-1 and TNF-alpha was significantly higher in blister exudates compared to that in the peripheral circulation. Thus, in vivo transmigrated human monocytes preserve their capacity to respond towards bacterial peptides in terms of CD11b up-regulation and H(2)O(2) generation. These data strengthen a role for newly recruited interstitial human monocytes in the immediate defence against invading pathogens.

Molecular mechanism of glutathione-mediated protection from oxidized low-density lipoprotein-induced cell injury in human macrophages: role of glutathione reductase and glutaredoxin

Macrophage death is a hallmark of advanced atherosclerotic plaque, and oxidized low-density lipoprotein (OxLDL) found in these lesions is believed to contribute to macrophage injury. However, the underlying mechanisms of this phenomenon are only poorly understood. Here we show that in human monocyte-derived macrophages, OxLDL depleted intracellular glutathione (GSH) and inhibited glutathione reductase, resulting in a marked diminution of the glutathione/glutathione disulfide ratio. In the absence of OxLDL, an 80% depletion of intracellular GSH levels did not affect cell viability, but glutathione depletion dramatically increased OxLDL-induced cell death. Conversely, supplementation of intracellular GSH stores with glutathione diethyl ester substantially diminished OxLDL toxicity. OxLDL also promoted protein-S-glutathionylation, which was increased in macrophages pretreated with the glutathione reductase inhibitor BCNU. Knockdown experiments with siRNA directed against glutathione reductase and glutaredoxin showed that both enzymes are essential for the protection of macrophages against OxLDL. Finally, the peroxyl-radical scavenger Trolox did not prevent GSH depletion but completely blocked OxLDL-induced protein-S-glutathionylation and cell death. These data suggest that OxLDL promotes ROS formation and protein-S-glutathionylation by a mechanism independent from its effect on GSH depletion. Neither mechanism was sufficient to induce macrophage injury, but when stimulated concurrently, these pathways promoted the accumulation of protein-glutathione mixed disulfides and cell death.

Consequences and Therapeutic Implications of Macrophage Apoptosis in Atherosclerosis

Macrophage apoptosis occurs throughout all stages of atherosclerosis, yet new findings in vivo suggest that the consequences of this event may be very different in early versus late atherosclerotic lesions. In early lesions, where phagocytic clearance of apoptotic cells appears to be efficient, macrophage apoptosis is associated with diminished lesion cellularity and decreased lesion progression. In late lesions, however, a number of factors may contribute to defective phagocytic clearance of apoptotic macrophages, leading to secondary necrosis of these cells and a proinflammatory response. The cumulative effect of these late lesional events is generation of the necrotic core, which, in concert with proatherogenic effects of residual surviving macrophages, promotes further inflammation, plaque instability, and thrombosis. Thus, the ability or lack thereof of lesional phagocytes to safely clear apoptotic macrophages may be an important determinant of acute atherothrombotic clinical events. Further understanding of the mechanisms involved in macrophage apoptosis and phagocytic clearance might lead to novel therapeutic strategies directed against the progression of advanced plaques.

A novel thiol oxidation-based mechanism for adriamycin-induced cell injury in human macrophages

Adriamycin is a widely used antitumor antibiotic, but its use has been limited by its cytotoxicity in both cardiomyocytes and non-cardiac tissues. While adriamycin's ability to redox cycle via one-electron transfer reactions and generate ROS is thought to promote cardiotoxicity, the mechanisms involved in non-cardiac tissue injury are not clear. Here we show that prolonged exposure (48 h) of human monocyte-derived macrophages to adriamycin at concentrations as low as 1 microM promotes caspase-independent cell death. Treatment of cells with scavengers of superoxide and peroxyl radicals blocked adriamycin-induced oxidation of dichlorodihydrofluorescein (DCFH) but did not prevent macrophage injury. Macrophages treated with either adriamycin or the thiol oxidant diamide showed elevated levels of glutathione disulfide and increased protein-S-glutathionylation prior to cell injury, indicating that thiol oxidation is involved in adriamycin-induced macrophage death. Furthermore, inhibition of glutathione reductase (GR) with 1,3-bis[2-chloroethyl]-1-nitrosourea or transfection of macrophages with small inhibitory RNA (siRNA) directed against GR or glutaredoxin (Grx) potentiated adriamycin-induced macrophage injury. Thus, both GR and Grx appear to play a crucial role in protecting macrophages from adriamycin-induced cell injury. These findings suggest a new mechanism for adriamycin-induced tissue injury whereby thiol oxidation, rather than one-electron redox cycling and ROS generation, mediates adriamycin-induced cell damage.

Lipoprotein aggregation protects human monocyte-derived macrophages from OxLDL-induced cytotoxicity

Oxidative modifications render low density lipoprotein cytotoxic and enhance its propensity to aggregate and fuse into particles similar to those found in atherosclerotic lesions. We showed previously that aggregation of oxidized LDL (OxLDL) promotes the transformation of human macrophages into lipid-laden foam cells (Asmis, R., and J. Jelk. 2000. Large variations in human foam cell formation in individuals. A fully autologous in vitro assay based on the quantitative analysis of cellular neutral lipids. Atherosclerosis. 148: 243-253). Here, we tested the hypothesis that aggregation of OxLDL enhances its clearance by human macrophages and thus may protect macrophages from OxLDL-induced cytotoxicity. We found that increased aggregation of OxLDL correlated with decreased macrophage injury. Using 3H-labeled and Alexa546-labeled OxLDL, we found that aggregation enhanced OxLDL uptake and increased cholesteryl ester accumulation but did not alter free cholesterol levels in macrophages. Acetylated LDL was a potent competitor of aggregated oxidized LDL (AggOxLDL) uptake, suggesting that scavenger receptor A plays an important role in the clearance of AggOxLDL. Inhibitors of actin polymerization, cytochalasin B, cytochalasin D, and latrunculin A, also prevented AggOxLDL uptake and restored OxLDL-induced cytotoxicity. This suggests that OxLDL-induced macrophage injury does not require OxLDL uptake and may occur on the cell surface. Our data demonstrate that aggregation of cytotoxic OxLDL enhances its clearance by macrophages without damage to the cells, thus allowing macrophages to avoid OxLDL-induced cell injury.

Heterogeneous expression of apolipoprotein-E by human macrophages

Apolipoprotein-E (apoE) is expressed at high levels by macrophages. In addition to its role in lipid transport, macrophage-derived apoE plays an important role in immunoregulation. Previous studies have identified macrophage subpopulations that differ substantially in their ability to synthesize specific cytokines and enzymes, however, potential heterogeneous macrophage apoE expression has not been studied. Here we examined apoE expression in human THP-1 macrophages and monocyte-derived macrophages (MDM). Using immunocytochemistry and flow cytometry methods we reveal a striking heterogeneity in macrophage apoE expression in both cell types. In phorbol-ester-differentiated THP-1 macrophages, 5% of the cells over-expressed apoE at levels more than 50-fold higher than the rest of the population. ApoE over-expressing THP-1 macrophages contained condensed/fragmented nuclei and increased levels of activated caspase-3 indicating induction of apoptosis. In MDM, 3-5% of the cells also highly over-expressed apoE, up to 50-fold higher than the rest of the population; however, this was not associated with obvious nuclear alterations. The apoE over-expressing MDM were larger, more granular, and more autofluorescent than the majority of cells and they contained numerous vesicle-like structures that appeared to be coated by apoE. Flow cytometry experiments indicated that the apoE over-expressing subpopulation of MDM were positive for CD14, CD11b/Mac-1 and CD68. These observations suggest that specific macrophage subpopulations may be important for apoE-mediated immunoregulation and clearly indicate that subpopulation heterogeneity should be taken into account when investigating macrophage apoE expression.

Immunophenotype and functional properties of feline dendritic cells derived from blood and bone marrow

Dendritic cells (DCs) are a heterogeneous population of cells of fundamental importance in initiating innate as well as specific immune responses. The identity and function of DCs in the cat are unknown, although they are likely pivotal in the response to infection. In this study, feline DCs were derived by 3-10-day culture of adherent blood mononuclear cells (PBMCs) and bone marrow mononuclear cells (BMMCs) in the presence of IL 4 and GM-CSF. BMMC consistently yielded a greater number of DCs than PBMC, and there were fewer macrophages than DC from both compartments. DCs expressed a distinct constellation of surface molecules, which included CD1a, CD1b, and CD1c, CD11b, CD14, and 2-3-fold higher levels of MHC class I and II molecules than co-cultured macrophages or fresh blood monocytes. DCs displayed typical cytoplasmic processes, limited non-specific esterase activity, and acquired antigen by phagocytosis, pinocytosis, and binding to specific receptors. Cytokine-exposed cells induced proliferation of allogeneic lymphocytes. Thus, the cells derived by these culture conditions had markers and functions analogous to immature myeloid DCs. Availability of feline DCs will enable investigation of their role in infectious disease and their potential therapeutic application.

Role of monocytes in atherogenesis

This review focuses on the role of monocytes in the early phase of atherogenesis, before foam cell formation. An emerging consensus underscores the importance of the cellular inflammatory system in atherogenesis. Initiation of the process apparently hinges on accumulating low-density lipoproteins (LDL) undergoing oxidation and glycation, providing stimuli for the release of monocyte attracting chemokines and for the upregulation of endothelial adhesive molecules. These conditions favor monocyte transmigration to the intima, where chemically modified, aggregated, or proteoglycan- or antibody-complexed LDL may be endocytotically internalized via scavenger receptors present on the emergent macrophage surface. The differentiating monocytes in concert with T lymphocytes exert a modulating effect on lipoproteins. These events propagate a series of reactions entailing generation of lipid peroxides and expression of chemokines, adhesion molecules, cytokines, and growth factors, thereby sustaining an ongoing inflammatory process leading ultimately to lesion formation. New data emerging from studies using transgenic animals, notably mice, have provided novel insights into many of the cellular interactions and signaling mechanisms involving monocytes/macrophages in the atherogenic processes. A number of these studies, focusing on mechanisms for monocyte activation and the roles of adhesive molecules, chemokines, cytokines and growth factors, are addressed in this review.

Imatinib inhibits the in vitro development of the monocyte/macrophage lineage from normal human bone marrow progenitors

The antileukaemic tyrosine kinase inhibitor, imatinib, has been reported to inhibit specifically the growth of bcr-abl expressing CML progenitors at levels of 0.1-5.0 microM, by blocking the ATP-binding site of the kinase domain of bcr-abl. Inhibition of the c-abl, platelet-derived growth factor receptor and stem cell factor receptor (c-kit) tyrosine kinases by imatinib has also been reported. Here, we demonstrate that imatinib significantly inhibits in vitro monocyte/macrophage development from normal bone marrow progenitors, while neutrophil and eosinophil development was less affected. Monocyte/macrophage inhibition was observed in semisolid agar and liquid cultures at concentrations of imatinib as low as 0.3 microM. The maturation of monocytes into macrophages was also found to be impaired following treatment of cultures with 1.0 microM imatinib. Imatinib blocked monocyte/macrophage development in cultures stimulated with and without M-CSF, suggesting that inhibition of the M-CSF receptor, c-fms, by imatinib was unlikely to be responsible. Imatinib may therefore have an inhibitory activity for other kinase(s) that play a role in monocyte/macrophage differentiation. This inhibition of normal monocyte/macrophage development was observed at concentrations of imatinib achievable pharmacologically, suggesting that imatinib or closely related derivatives may have potential for the treatment of diseases where monocytes/macrophages contribute to pathogenesis.

Oxidized LDL promotes peroxide-mediated mitochondrial dysfunction and cell death in human macrophages: a caspase-3-independent pathway

Several studies suggest that macrophage death and subsequent lysis contribute to the development of advanced atherosclerotic lesions. Although oxidized LDL (OxLDL) is thought to contribute to lesion formation and induces macrophage apoptosis, the mechanisms underlying macrophage lysis have not been well defined. To determine if induction of apoptosis in human macrophages also promotes cell lysis, we studied caspase-3 activation by OxLDL and activating anti-Fas antibodies. We found that Fas-induced activation of caspase-3 does not promote macrophage lysis and caspase-3 activation is not required for OxLDL-induced macrophage lysis. OxLDL induces the formation of peroxides, but not superoxide, and decreases mitochondrial membrane potential. Scavengers of peroxyl radicals restore mitochondrial membrane potential and prevent macrophage lysis, implicating peroxyl radicals in both mitochondrial dysfunction and macrophage lysis induced by OxLDL. We conclude that macrophage death induced by OxLDL results in cell lysis, but it does not require activation of Fas or caspase-3. The full text of this article is available at http://www.circresaha.org.

C-terminal fragment of alpha1-antitrypsin activates human monocytes to a pro-inflammatory state through interactions with the CD36 scavenger receptor and LDL receptor

Monocyte scavenger receptor, CD36 has been implicated in the pathogenesis of atherosclerosis as a major oxidised LDL receptor mediating lipid accumulation and foam cell formation. Previously, we found that treatment of monocyte cultures with the carboxyl terminal fragment of alpha1-antitrypsin (C-36) increases lipid binding and uptake, induces LDL receptor mRNA and CD36 receptor protein expression, and also significantly increases production of pro-inflammatory molecules. To assess the role of the CD36 receptor in proatherogenic monocyte activation by the C-36 fragment, we tested whether specific anti-CD36 receptor antibodies would block the effects of C-36 on monocyte activation. We find that pre-incubation of cells with anti-LDL and anti-CD36 receptor antibodies (10 microg/ml) blocks binding of 125I-C-36 by about 50%. Similarly, cells pre-incubated with oxidised LDL or native LDL at concentrations from 2.5 to 10 microg/ml showed a loss of 125I-C-36 binding (up to 49 and 57%) and uptake (up to 47 and 59.8%), respectively. In parallel experiments, monocytes were first incubated for 1 or 6 h with anti-CD36 antibodies (10 microg/ml) prior to adding C-36 peptide. Anti-CD36 antibodies suppressed C-36-induced production of gelatinase B, monocyte chemoattractant protein-1, interleukin-6 and cellular oxygen consumption to control levels, whereas levels of TNFalpha were unaffected. In contrast, saturation of LDL receptors with excess of anti-LDL (20 microg/ml) significantly inhibited C-36 induced TNFalpha levels. Results indicate that the C-36 peptide binds to both LDL and CD36 scavenger receptors which involves selective upregulation of pro-inflammatory molecules and activation of the respiratory burst in human monocytes. This also supports important roles for CD36 and LDL receptors in atherogenesis and suggests that blockade of CD36 receptor can be protective in pro-inflammatory activation of human monocytes.

Apoptosis induced by oxidized low density lipoprotein in human monocyte-derived macrophages involves CD36 and activation of caspase-3

Macrophage death may play a crucial role in the progression of atherosclerotic lesions. Here we present evidence that CD36 is involved in oxidized LDL (OxLDL)-induced apoptosis in human monocyte-derived macrophages. Anti-CD36 mAb SMO and OKM-5 reduced the number of apoptotic cells in OxLDL-treated macrophages by more than 94%, but they did not block ceramide-triggered apoptosis. Thrombospondin inhibited the induction of apoptosis by OxLDL in a dose-dependent manner with an IC50 of 10-30 microM. OxLDL did not induce apoptosis in CD36-negative macrophages, demonstrating the essential role of this scavenger receptor in OxLDL-triggered programmed cell death. Neither anti-CD36 Ig nor thrombospondin triggered programmed cell death suggesting that binding to CD36 alone is not sufficient to initiate apoptosis. However, inhibitors of OxLDL-induced apoptosis did not block the uptake of 3H-labeled OxLDL. In contrast, acetylated LDL and polyinosinic acid, ligands of scavenger receptor A (SRA), inhibited uptake of 3H-labeled OxLDL by 65 and 49%, respectively, but did not block OxLDL-induced apoptosis, indicating that SRA is not involved in this process. OxLDL also stimulated caspase-3 activity in human macrophages. Activation of caspase-3 was blocked by anti-CD36 Ig and the caspase-3 inhibitor Z-DEVD-FMK. These results suggest that binding of OxLDL to CD36 initiates a yet unknown OxLDL-specific signaling event, which leads to the rapid activation of caspase-3 resulting in apoptosis of human macrophages. Our data demonstrate a novel role for CD36 in macrophage biology with likely consequences for the development of atherosclerotic lesions.

Vitamin E supplementation of human macrophages prevents neither foam cell formation nor increased susceptibility of foam cells to lysis by oxidized LDL

Several studies in macrophage cell lines, rodent macrophages, and animal models of atherosclerosis suggest that vitamin E may prevent the formation of foam cells. We tested this hypothesis in a recently developed, fully autologous in vitro model of human foam cell formation. During maturation, macrophages continuously increased their alpha-tocopherol/total cholesterol ratio, demonstrating that these cells accumulate alpha-tocopherol at an even higher rate than cholesterol. In the presence of unsupplemented serum, we observed no correlation between serum vitamin E levels and the increase in the cellular alpha-tocopherol/total cholesterol ratio. In contrast, under supplemented conditions, a 3.1-fold increase in the mean serum alpha-tocopherol/total cholesterol ratio resulted in a corresponding mean 3.5-fold increase in the cellular alpha-tocopherol/total cholesterol ratio. Vitamin E loading had no effect on the lipid composition of macrophages and did not affect their growth. Foam cell formation was stimulated in mature unsupplemented and vitamin E-loaded macrophages for 1 week with 50 microg autologous aggregated low density lipoprotein (LDL) in the presence of unsupplemented and vitamin E-loaded serum, respectively. We observed no effect of vitamin E supplementation on the formation of foam cells. However, foam cell formation resulted in a 36% and 44% reduction in the cellular alpha-tocopherol/total cholesterol ratio in unsupplemented and vitamin E-supplemented foam cells, respectively. The loss of vitamin E was accelerated with increasing concentrations of aggregated LDL and was accompanied by an increase in the susceptibility of these foam cells to succumb to the cell lytic effects of oxidized LDL (OxLDL). However, vitamin E supplementation did not protect macrophages or foam cells from OxLDL-mediated cell lysis, suggesting that vitamin E loss in foam cells is not the cause of their increased susceptibility to cell lysis. Our results suggest that the beneficial effects of vitamin E on cardiovascular disease observed in humans are due neither to a reduction in the propensity of macrophages to form foam cells nor to an increased resistance of these cells to cytolytic OxLDL.

Large variations in human foam cell formation in individuals: a fully autologous in vitro assay based on the quantitative analysis of cellular neutral lipids

The transformation of monocyte-derived macrophages into lipid-laden foam cells constitutes a characteristic and crucial event in the development of the earliest atherosclerotic lesions. We investigated whether the propensity to form foam cells varies among individuals. We developed a fully autologous foam cell assay based on a recently developed novel culture technique for human monocyte-derived macrophages (Wintergerst ES, Jelk J, Asmis, R. Differential expression of CD14, CD36 and the LDL receptor on human monocyte-derived macrophages. A novel cell culture system to study macrophage differentiation and heterogeneity, Histochem. Cell Biol. 1998;110:231-241). Thin layer chromatography and laser densitometry were used to determine cholesterol, triglyceride and cholesteryl ester levels in human macrophages. Aggregated LDL obtained by vortexing was found to be a reproducible stimulus of foam cell formation in human macrophages. In our hands, Cu(2+)-oxidized LDL also induced cholesteryl ester accumulation, but only when vortexed. We found that foam cell formation in an individual varied by less than 25% over a 10-month period. In contrast, we observed a sevenfold difference in foam cell formation among eight male volunteers. The transfer of foam cells into culture medium with freshly thawed autologous serum resulted in a 75% regression within 1 week, independent of the amount of cellular cholesteryl esters accumulated. Foam cell formation correlated neither to serum nor to cellular cholesterol and triglyceride levels. The propensity to form foam cells could therefore represent a novel indicator of individual risk of atherogenesis.