TLR/MyD88 and liver X receptor alpha signaling pathways reciprocally control Chlamydia pneumoniae-induced acceleration of atherosclerosis

The role of immune system in atherosclerosis: Molecular mechanisms, controversies, and future possibilities

Numerous cardiovascular disorders have atherosclerosis as their pathological underpinning. Numerous studies have demonstrated that, with the aid of pattern recognition receptors, cytokines, and immunoglobulins, innate immunity, represented by monocytes/macrophages, and adaptive immunity, primarily T/B cells, play a critical role in controlling inflammation and abnormal lipid metabolism in atherosclerosis. Additionally, the finding of numerous complement components in atherosclerotic plaques suggests yet again how heavily the immune system controls atherosclerosis. Therefore, it is essential to have a thorough grasp of how the immune system contributes to atherosclerosis. The specific molecular mechanisms involved in the activation of immune cells and immune molecules in atherosclerosis, the controversy surrounding some immune cells in atherosclerosis, and the limitations of extrapolating from relevant animal models to humans were all carefully reviewed in this review from the three perspectives of innate immunity, adaptive immunity, and complement system. This could provide fresh possibilities for atherosclerosis research and treatment in the future.

A review of the signaling pathways of aerobic and anaerobic exercise on atherosclerosis

Atherosclerosis (AS), a chronic inflammatory vascular disease with lipid metabolism abnormalities, is one of the major pathological bases of coronary heart disease. As people's lifestyles and diets change, the incidence of AS increases yearly. Physical activity and exercise training have recently been identified as effective strategies for lowering cardiovascular disease (CVD) risk. However, the best exercise mode to ameliorate the risk factors related to AS is not clear. The effect of exercise on AS is affected by the type of exercise, intensity, and duration. In particular, aerobic and anaerobic exercise are the two most widely discussed types of exercise. During exercise, the cardiovascular system undergoes physiological changes via various signaling pathways. The review aims to summarize signaling pathways related to AS in two different exercise types and provide new ideas for the prevention and treatment of AS in clinical practice.

Protein-enriched extracts from housefly (Musca domestica) maggots alleviates atherosclerosis in apolipoprotein E-deficient mice by promoting bile acid production and consequent cholesterol consumption

Currently, atherosclerosis control is important to prevent future heart attacks or strokes. Protein-enriched extract (PE) from housefly maggots (Musca domestica) can inhibit the development of atherosclerosis partially through its antioxidant effects. Whether PE exerts other anti-atherosclerosis functions remains unclear. Here, PE was found to simultaneously promote cholesterol metabolism effects in apolipoprotein E knockout (ApoE^-/- ) mice. Bile acid synthesis plays a key role in regulating cholesterol homeostasis in atherosclerosis. Whether PE alleviates atherosclerosis by promoting bile acid production and consequent cholesterol consumption was further explored. First, 8-week-old male ApoE^-/- mice were recruited and fed on a cholesterol-enriched diet. After 8 weeks, these mice were divided into three groups and received gavage administration of PE, simvastatin, and saline for another 8 weeks. Atherosclerosis severity was then assessed. Real-time quantitative polymerase chain reaction and western blot were employed to determine the expression of hepatic ATP-binding cassette transporter A1 (ABCA1), liver X receptor α (LXRα), and peroxisome proliferator-activated receptor-γ (PPAR-γ). Serum levels of high-density lipoprotein-cholesterol (HDL), low-density lipoprotein-cholesterol (LDL), and total cholesterol (TC) were determined by enzyme-linked immunoassay. Results revealed that PE reversed the formation of atherosclerotic lesion; increased the expression of PPAR-γ, LXRα, and ABCA1; increased the amount of bile flow and total bile acid; reduced the serum level of LDL and TC; and increased the level of HDL. In conclusion, enhancement on bile acid production and consequent cholesterol consumption may partially contribute to the anti-atherosclerotic effects of PE. The reversal of PPARγ-LXRα-ABCA1 signaling pathway may be involved in this process.

Anti-PCSK9 monoclonal antibody attenuates high-fat diet and zymosan-induced vascular inflammation in C57BL/6 mice by modulating TLR2/NF-ƙB signaling pathway

Objectives

Excess intake of a high-fatty diet (HFD) together with zymosan administration mediates vasculitis response which leads to impaired serum lipid levels and causes arterial stiffness. In the development of new cholesterol-lowering medications, PCSK9 inhibitor (proprotein convertase subtilisin/kexin type 9) is an emerging therapeutic. The goal of the present study was to see whether anti-PCSK9 mAb1 might prevent vasculitis in C57BL/6 mice by blocking TLR2/NF-B activation in HFD and Zymosan-induced vasculitis.

Materials and Methods

Protein-protein molecular docking was performed to validate the binding affinity of anti-PCSK9 mAb1 against TLR2. Under the experimental study, mice were randomly allocated to the following groups: Group I: standard mice diet (30 days) + Zymosan vehicle (sterile PBS solution of 5mg/ml on 8^th day); Group II: HFD (30 days) + Zymosan ( single IP dose 80 mg/kg on day 8^th); Group III: HFD+Zymosan + anti-PCSK9 mAb1 (6 mg/kg, s.c. on 10^th and 20^th days); Group IV: HFD+Zymosan+anti-PCSK9 mAb1 (10 mg/kg, s.c. on 10^th and 20^th days).

Results

In comparison with the low dose of anti-PCSK9 mAb1 (6 mg/kg), the high dose of anti-PCSK9 mAb1 (10 mg/kg) together with HFD and Zymosan inhibited vasculitis more effectively by decreasing aortic TLR2 and NF-B levels, reducing serum TNF- and IL-6, and up-regulating liver LDLR levels, which down-regulated serum LDL-C and improved serum lipids levels. Histopathological studies showed that anti-PCSK9 mAb1 treatment reduced plaque accumulation in the aorta of mice.

Conclusion

These findings indicate that anti-PCSK9 mAb1 has therapeutic potential in reducing HFD and Zymosan-induced vascular inflammation.

Chlamydia pneumoniae Infection Induces Vascular Smooth Muscle Cell Migration and Atherosclerosis Through Mitochondrial Reactive Oxygen Species-Mediated JunB-Fra-1 Activation

Infection is closely related to atherosclerosis, which is a major pathological basis for cardiovascular diseases. Vascular smooth muscle cell (VSMC) migration is an important trigger in development of atherosclerosis that is associated with Chlamydia pneumoniae (C. pneumoniae) infection. However, the mechanism of VSMC migration remains unclear, and whether antioxidant could be a therapeutic target for C. pneumoniae infection-induced atherosclerosis also remains unknown. The results showed that C. pneumoniae infection mainly impaired mitochondrial function and increased the level of mitochondrial reactive oxygen species (mtROS). The expressions of protein JunB, Fra-1 and Matrix metalloproteinase 2 (MMP) evidently increased after C. pneumoniae infection, and the interaction between JunB and Fra-1 was also enhanced. After scavenging mtROS by antioxidant Mito-TEMPO, the increasing expressions of JunB, Fra-1, MMP2 and the capacity of VSMC migration induced by C. pneumoniae infection were all inhibited. In comparison with infected ApoE^-/- mice, the level of ROS in atherosclerotic lesion in ApoE^-/-TLR2^-/- mice with C. pneumoniae infection decreased. Knocking out TLR2 suppressed the expressions of JunB, Fra-1 and MMP2 in VSMCs and the formation of atherosclerotic lesion after C. pneumoniae infection. Furthermore, after using small interfering RNA to inhibit the expression of TLR2, the level of mtROS and the expressions of JunB, Fra-1 and MMP2 apparently decreased. Taken together, C. pneumoniae infection may promote VSMC migration and atherosclerosis development by increasing the level of mtROS through TLR2 to activate the JunB-Fra-1/MMP2 signaling pathway. The data provide the first evidence that antioxidant could reduce C. pneumoniae infection-induced VSMC migration and atherosclerosis.

Modulatory role of atorvastatin against high-fat diet and zymosan-induced activation of TLR2/NF-ƙB signaling pathway in C57BL/6 mice

OBJECTIVES

Accumulated evidence provides a strong connection between the immune system and vascular inflammation. The innate immune system's main sensors are toll-like receptors (TLRs). Zymosan (Zym), a fungal product, induces an inflammatory response via activating TLR2 of the immune system. Atorvastatin, a potent statin, possesses pleiotropic effects including immunomodulatory, lipid-lowering, and anti-inflammatory. Therefore, the current study aimed to evaluate the protective role of atorvastatin against a high-fat diet (HFD) and Zym-induced vascular inflammation in C57BL/6 mice via modulation of TLR2/NF-ƙB signaling pathway.

MATERIALS AND METHODS

In silico study was conducted to confirm the binding affinity of atorvastatin against TLR2. Under in vivo study, mice were divided into four groups: Normal control: normal standard chow-diet fed for 30 days + Zym vehicle (sterile PBS, 5 mg/ml on 8th day); HFD (30 days) + Zym (80 mg/kg, IP, on 8th day); HFD/Zym + atorvastatin vehicle (0.5% CMC, p.o., from 10th to 30th day); HFD/Zym + atorvastatin (3.6 mg/kg, p.o., from 10th to 30th day).

RESULTS

Atorvastatin treatment along with HFD and Zym inhibited vascular inflammation by suppressing the levels of aortic TLR2, cardiac NF-ƙB and decrease in serum TNF-α and IL-6. Further, there was an increase in hepatic LDLR levels, resulting in a decrease in serum LDL-C and an increase in HDL-C levels. Histopathological examination of the aorta showed a reduction in plaque accumulation with the atorvastatin-treated group as compared with HFD and Zym-treated group.

CONCLUSION

Atorvastatin attenuates vascular inflammation mediated by HFD and Zym through suppression of TLR2, NF-ƙB, TNF-α, IL-6, and upregulation of LDLR levels.

An overview of the innate and adaptive immune system in atherosclerosis

Cardiovascular disease is the leading cause of death globally. Coronary artery disease (CAD) is a chronic inflammatory disease usually caused by atherosclerosis, in which the coronary arteries become narrowed by atheromatous plaque. Plaques in atherosclerosis are formed through the accumulation of lipids and various immune cells. Both adaptive and innate immune systems are involved in the pathogenesis of atherosclerosis and facilitate plaque formation and disease progression. Almost all immune system cells, including neutrophils, B cells, T cells monocytes, macrophages, foam cells, and dendritic cells (DCs), play a vital role in atherosclerotic plaque. Atherogenesis, the normal function of the endothelium, is initially disrupted and, then, cells of the immune system are recruited to the endothelium following increased expression of cell adhesion molecules. Accumulation of immune cells and lipids leads to the formation of a necrotic nucleus. As the disease progresses, smooth muscle cells form fibrous layers, whose rupture results in exposing the necrotic nucleus and thrombosis. Accordingly, the present review was conducted to determine the role of different cells in innate and adaptive immune systems in inhibition and progression of atherosclerosis.

Porphyromonas gingivalis disrupts vascular endothelial homeostasis in a TLR-NF-κB axis dependent manner

Cardiovascular disease is still the leading cause of mortality worldwide. Vascular endothelial dysfunction is viewed as the initial step of most cardiovascular diseases. Many studies have indicated that periodontal pathogens, especially Porphyromonas gingivalis, are closely correlated with vascular endothelial homeostasis, but the function of P. gingivalis and the underlying mechanisms are still elusive. To illuminate the effects and elucidate the mechanisms of P. gingivalis on endothelial structural integrity, we developed P. gingivalis infection models in vivo and in vitro. Endothelial cell proliferation, differentiation and apoptosis were detected. Here, we showed that P. gingivalis can impair endothelial integrity by inhibiting cell proliferation and inducing endothelial mesenchymal transformation and apoptosis of endothelial cells, which reduce the cell levels and cause the endothelium to lose its ability to repair itself. A mechanistic analysis showed that TLR antagonist or NF-κB signalling inhibitor can largely rescue the damaged integrity of the endothelium caused by P. gingivalis, suggesting that TLR-NF-κB signalling plays a vital role in vascular endothelial homeostasis destroyed by P. gingivalis. These results suggest a potential intervention method for the prevention and treatment of cardiovascular disease.

Infection and atherosclerosis: TLR-dependent pathways

Atherosclerotic vascular disease (ASVD) is a chronic process, with a progressive course over many years, but it can cause acute clinical events, including acute coronary syndromes (ACS), myocardial infarction (MI) and stroke. In addition to a series of typical risk factors for atherosclerosis, like hyperlipidemia, hypertension, smoking and obesity, emerging evidence suggests that atherosclerosis is a chronic inflammatory disease, suggesting that chronic infection plays an important role in the development of atherosclerosis. Toll-like receptors (TLRs) are the most characteristic members of pattern recognition receptors (PRRs), which play an important role in innate immune mechanism. TLRs play different roles in different stages of infection of atherosclerosis-related pathogens such as Chlamydia pneumoniae (C. pneumoniae), periodontal pathogens including Porphyromonas gingivalis (P. gingivalis), Helicobacter pylori (H. pylori) and human immunodeficiency virus (HIV). Overall, activation of TLR2 and 4 seems to have a profound impact on infection-related atherosclerosis. This article reviews the role of TLRs in the process of atherosclerosis after C. pneumoniae and other infections and the current status of treatment, with a view to providing a new direction and potential therapeutic targets for the study of ASVD.

TLR2/CXCR4 coassociation facilitatesChlamydia pneumoniaeinfection-induced atherosclerosis

Toll-like receptor 2 (TLR2) and C-X-C motif chemokine receptor 4 (CXCR4) have both been shown to be involved in atherosclerosis. We demonstrate for the first time the presence of TLR2/CXCR4 coassociation during C. pneumoniae infection-induced atherosclerosis. Amazingly, blocking of both TLR2 and CXCR4 significantly retards and even almost reverses this infection-induced atherosclerosis. Our work reveals new mechanisms about C. pneumoniae infection-induced atherosclerosis and identifies potential new therapeutic targets for the prevention and treatment of atherosclerosis.

Chlamydia and Lipids Engage a Common Signaling Pathway That Promotes Atherogenesis

BACKGROUND

Recent studies indicate that Toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88) signaling promote the development of high fat diet-induced atherosclerosis in hypercholesterolemic mice.

OBJECTIVES

The authors investigated the role of TLR4/MyD88 signaling in hematopoietic and stromal cells in the development and infection-mediated acceleration of atherosclerosis.

METHODS

The authors generated bone marrow chimeras between wild-type and Tlr4^-/- mice, as well as wild-type and Myd88^-/- mice. All mice were on the Apoe^-/- background and fed high fat diet. The authors infected the chimeric mice with C. pneumoniae (CP) and fed them high fat diet.

RESULTS

Aortic sinus plaques and lipid content were significantly reduced in Apoe^-/- mice that received Tlr4^-/-or Myd88^-/- bone marrow compared with control animals despite similar cholesterol levels. Similarly, Tlr4 or Myd88 deficiency in stromal cells also led to a reduction in the lesion area and lipid in aortic sinus plaques. MyD88 expression only in CD11c+ dendritic cells (myeloid cells) in cells was sufficient in otherwise MyD88-deficient mice to induce CP infection-mediated acceleration of atherosclerosis, underlining the key role of MyD88 in CD11c+ dendritic cells (myeloid cells). Whereas CP infection markedly accelerated atherosclerosis in TLR4- or MyD88-positive chimeras, CP infection had a minimal effect on atherosclerosis in TLR4- or MyD88-deficient mice (either in the hematopoietic or stromal cell compartments).

CONCLUSIONS

The authors show that both CP infection and metabolic stress associated with dyslipidemia use the same innate immune response pathway, utilizing TLR4/MyD88 signaling, with similar relative contributions in bone marrow-derived hematopoietic cells and in stromal cells. Further studies are required to understand this intricate and complex cross talk among innate and adaptive immune systems in various conditions to more effectively design dendritic cell-mediated atheroprotective vaccines and other therapeutic strategies.

Chlamydia pneumoniae Hijacks a Host Autoregulatory IL-1β Loop to Drive Foam Cell Formation and Accelerate Atherosclerosis

Pathogen burden accelerates atherosclerosis, but the mechanisms remain unresolved. Activation of the NLRP3 inflammasome is linked to atherogenesis. Here we investigated whether Chlamydia pneumoniae (C.pn) infection engages NLRP3 in promoting atherosclerosis. C.pn potentiated hyperlipidemia-induced inflammasome activity in cultured macrophages and in foam cells in atherosclerotic lesions of Ldlr^-/- mice. C.pn-induced acceleration of atherosclerosis was significantly dependent on NLRP3 and caspase-1. We discovered that C.pn-induced extracellular IL-1β triggers a negative feedback loop to inhibit GPR109a and ABCA1 expression and cholesterol efflux, leading to accumulation of intracellular cholesterol and foam cell formation. Gpr109a and Abca1 were both upregulated in plaque lesions in Nlrp3^-/- mice in both hyperlipidemic and C.pn infection models. Mature IL-1β and cholesterol may compete for access to the ABCA1 transporter to be exported from macrophages. C.pn exploits this metabolic-immune crosstalk, which can be modulated by NLRP3 inhibitors to alleviate atherosclerosis.

Chlamydia pneumoniae Infection Exacerbates Atherosclerosis in ApoB100only/LDLR-/- Mouse Strain

Aims

Hyperlipidaemia model animals have been used to elucidate the role of Chlamydia pneumoniae (Cpn) infection in atherosclerosis. The aims of this study were to investigate the proatherogenic effect of multiple Cpn infections in ApoB100only/LDLR^−/− mice which based on lipid profile can be regarded as the most suitable mouse model of human hypercholesterolemia and to compare the lesion development to that in a major atherosclerosis model ApoE^−/− mice.

Methods and Results

Aorta samples of ApoB100only/LDLR^−/− mice infected three times with Cpn were subjected to morphometric analyses. Morphometric evaluation disclosed that Cpn infections exacerbated atherosclerosis development in the aortic root and descending aorta of the mice fed with normal diet. Viable Cpn was detected in the ascending aorta by RT-PCR. Chlamydial 16SrRNA expression showed the presence of viable Cpn in the aorta of infected animals. A similar rate of acceleration of atherosclerosis was observed when the infection protocol was applied in ApoB100only/LDLR^−/− and in ApoE^−/− mice.

Conclusion

Similar to ApoE^−/− mice, ApoB100only/LDLR^−/− mice with more human-relevant serum lipoprotein composition develop increased atherosclerosis after Cpn infections; thus this mouse strain can be used as a model of infection-related atherosclerosis enhancement and can provide further evidence for the proatherogenic influence of Cpn in mice.

Hepatic Tm6sf2 overexpression affects cellular ApoB-trafficking, plasma lipid levels, hepatic steatosis and atherosclerosis

The human transmembrane 6 superfamily member 2 (TM6SF2) gene has been implicated in plasma lipoprotein metabolism, alcoholic and non-alcoholic fatty liver disease and myocardial infarction in multiple genome-wide association studies. To investigate the role of Tm6sf2 in metabolic homeostasis, we generated mice with elevated expression using adeno-associated virus (AAV)-mediated gene delivery. Hepatic overexpression of mouse Tm6sf2 resulted in phenotypes previously observed in Tm6sf2-deficient mice including reduced plasma lipid levels, diminished hepatic triglycerides secretion and increased hepatosteatosis. Furthermore, increased hepatic Tm6sf2 expression protected against the development of atherosclerosis in LDL-receptor/ApoB48-deficient mice. In cultured human hepatocytes, Tm6sf2 overexpression reduced apolipoprotein B secretion and resulted in its accumulation within the endoplasmic reticulum (ER) suggesting impaired ER-to-Golgi trafficking of pre-very low-density lipoprotein (VLDL) particles. Analysis of two metabolic trait-associated coding polymorphisms in the human TM6SF2 gene (rs58542926 and rs187429064) revealed that both variants impact TM6SF2 expression by affecting the rate of protein turnover. These data demonstrate that rs58542926 (E167K) and rs187429064 (L156P) are functional variants and suggest that they influence metabolic traits through altered TM6SF2 protein stability. Taken together, our results indicate that cellular Tm6sf2 level is an important determinant of VLDL metabolism and further implicate TM6SF2 as a causative gene underlying metabolic disease and trait associations at the 19p13.11 locus.

Activation of LXRα improves cardiac remodeling induced by pulmonary artery hypertension in rats

Inflammatory factors regulated by NF-κB play a significant role in PAH and myocardial hypertrophy. LXR activation may inhibit myocardial hypertrophy via suppressing inflammatory pathways; it is unknown whether LXR is also involved in PAH-induced myocardial hypertrophy or remodeling. To further explore the protective effect of LXR in PAH-induced cardiac hypertrophy and remodeling, a PAH model was developed, and T0901317, an agonist of LXR, was used to examine the effect of LXR activation. PAH rats demonstrated obvious cardiac hypertrophy and remodeling in the right ventricle, but significant improvement of cardiac hypertrophy and remodeling was observed in PAH rats treated with T0901317. Through RT-PCR, Western blot and ELISA examination, NF-κB, IL-6, TNF-α, and iNOS were found to be significantly reduced in PAH rats treated with T0901317 compared to PAH rats treated with DMSO. Apoptosis was also significantly reduced in PAH rats treated with T0901317. Thus, LXR activation may inhibit PAH-induced cardiac hypertrophy and remodeling by inhibiting NF-κB-mediated inflammatory pathways.

An Unsettled Debate About the Potential Role of Infection in Pathogenesis of Atherosclerosis

Association of infection with atherosclerosis is by no means new. Several sero-epidemiological and pathologic studies as well as animal models have shown a link between infection and atherosclerosis. Exciting discoveries in recent times related to role of inter-individual genetic variation in modulating inflammatory response to infection have reignited the enthusiasm in proving a causal link between infection and atherosclerosis. The purpose of this article was to review and analyze the available evidence linking infection with atherosclerosis.

NLRP4 is an essential negative regulator of fructose-induced cardiac injury in vitro and in vivo

High fructose consumption leads to metabolic syndrome and enhances cardiovascular disease risk. However, our knowledge of the molecular mechanism underlying the cardiac disease caused by fructose feeding is still poor. Nod-like receptors (NLRs) are intracellular sensors, responding to a variety of intracellular danger signals to induce injuries. NLRP4 is a negative regulator of nuclear factor-κB (NF-κB) signaling pathway through interactions with kinase IκB kinase (IKK). Here, we illustrated that NLRP4 attenuates pro-inflammatory cytokines releasing, including Transforming growth factor (TGF-β1), Tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interleukin-18 (IL-18) and interleukin-6 (IL-6), in fructose-treated cardiac cells by means of RT-qPCR, and western blotting analysis. In addition, NLRP4 could reduce the expression of TANK-binding kinase 1/interferon regulatory factor 3 (TBK1/IRF3), reducing inflammation response and achieving its anti-hypertrophic action. TBK1 plays critical roles in the IRF3 signaling pathway, modulating inflammation response. The inhibition of IKK/NF-κB signaling pathway by NLRP4 is confirmed by NLRP4 over-expression and knockdown. In vivo, high fructose feeding induced cardiac injury, accompanied with reduced expression of NLRP4 in heart tissue samples, indicating the possible role of NLRP4 in ameliorating heart injury. In conclusion, the findings above indicated that NLRP4 is an important mediator of cardiac remodeling in vitro and in vivo through negatively regulating TBK1/IRF3 and IKK/NF-κB signaling pathways, indicating that NLRP4 might be a promising therapeutic target against cardiac inflammation.

Manipulation of Host Cholesterol by Obligate Intracellular Bacteria

Cholesterol is a multifunctional lipid that plays important metabolic and structural roles in the eukaryotic cell. Despite having diverse lifestyles, the obligate intracellular bacterial pathogens Chlamydia, Coxiella, Anaplasma, Ehrlichia, and Rickettsia all target cholesterol during host cell colonization as a potential source of membrane, as well as a means to manipulate host cell signaling and trafficking. To promote host cell entry, these pathogens utilize cholesterol-rich microdomains known as lipid rafts, which serve as organizational and functional platforms for host signaling pathways involved in phagocytosis. Once a pathogen gains entrance to the intracellular space, it can manipulate host cholesterol trafficking pathways to access nutrient-rich vesicles or acquire membrane components for the bacteria or bacteria-containing vacuole. To acquire cholesterol, these pathogens specifically target host cholesterol metabolism, uptake, efflux, and storage. In this review, we examine the strategies obligate intracellular bacterial pathogens employ to manipulate cholesterol during host cell colonization. Understanding how obligate intracellular pathogens target and use host cholesterol provides critical insight into the host-pathogen relationship.

Microbiota, Immune Subversion, and Chronic Inflammation

Several host-adapted pathogens and commensals have evolved mechanisms to evade the host innate immune system inducing a state of low-grade inflammation. Epidemiological studies have also documented the association of a subset of these microorganisms with chronic inflammatory disorders. In this review, we summarize recent studies demonstrating the role of the microbiota in chronic inflammatory diseases and discuss how specific microorganisms subvert or inhibit protective signaling normally induced by toll-like receptors (TLRs). We highlight our work on the oral pathogen Porphyromonas gingivalis and discuss the role of microbial modulation of lipid A structures in evasion of TLR4 signaling and resulting systemic immunopathology associated with atherosclerosis. P. gingivalis intrinsically expresses underacylated lipid A moieties and can modify the phosphorylation of lipid A, leading to altered TLR4 signaling. Using P. gingivalis mutant strains expressing distinct lipid A moieties, we demonstrated that expression of antagonist lipid A was associated with P. gingivalis-mediated systemic inflammation and immunopathology, whereas strains expressing agonist lipid A exhibited modest systemic inflammation. Likewise, mice deficient in TLR4 were more susceptible to vascular inflammation after oral infection with P. gingivalis wild-type strain compared to mice possessing functional TLR4. Collectively, our studies support a role for P. gingivalis-mediated dysregulation of innate and adaptive responses resulting in immunopathology and systemic inflammation. We propose that anti-TLR4 interventions must be designed with caution, given the balance between the protective and destructive roles of TLR signaling in response to microbiota and associated immunopathologies.

Signaling events in pathogen-induced macrophage foam cell formation

Macrophage foam cell formation is a key event in atherosclerosis. Several triggers induce low-density lipoprotein (LDL) uptake by macrophages to create foam cells, including infections with Porphyromonas gingivalis and Chlamydia pneumoniae, two pathogens that have been linked to atherosclerosis. While gene regulation during foam cell formation has been examined, comparative investigations to identify shared and specific pathogen-elicited molecular events relevant to foam cell formation are not well documented. We infected mouse bone marrow-derived macrophages with P. gingivalis or C. pneumoniae in the presence of LDL to induce foam cell formation, and examined gene expression using an atherosclerosis pathway targeted plate array. We found over 30 genes were significantly induced in response to both pathogens, including PPAR family members that are broadly important in atherosclerosis and matrix remodeling genes that may play a role in plaque development and stability. Six genes mainly involved in lipid transport were significantly downregulated. The response overall was remarkably similar and few genes were regulated in a pathogen-specific manner. Despite very divergent lifestyles, P. gingivalis and C. pneumoniae activate similar gene expression profiles during foam cell formation that may ultimately serve as targets for modulating infection-elicited foam cell burden, and progression of atherosclerosis.

Immune dysregulation mediated by the oral microbiome: potential link to chronic inflammation and atherosclerosis

Cardiovascular disease is an inflammatory disorder characterized by the progressive formation of plaque in coronary arteries, termed atherosclerosis. It is a multifactorial disease that is one of the leading causes of death worldwide. Although a number of risk factors have been associated with disease progression, the underlying inflammatory mechanisms contributing to atherosclerosis remain to be fully delineated. Within the last decade, the potential role for infection in inflammatory plaque progression has received considerable interest. Microbial pathogens associated with periodontal disease have been of particular interest due to the high levels of bacteremia that are observed after routine dental procedures and every day oral activities, such as tooth brushing. Here, we explore the potential mechanisms that may explain how periodontal pathogens either directly or indirectly elicit immune dysregulation and consequently progressive inflammation manifested as atherosclerosis. Periodontal pathogens have been shown to contribute directly to atherosclerosis by disrupting endothelial cell function, one of the earliest indicators of cardiovascular disease. Oral infection is thought to indirectly induce elevated production of inflammatory mediators in the systemic circulation. Recently, a number of studies have been conducted focusing on how disruption of the gut microbiome influences the systemic production of proinflammatory cytokines and consequently exacerbation of inflammatory diseases such as atherosclerosis. It is clear that the immune mechanisms leading to atherosclerotic plaque progression, by oral infection, are complex. Understanding the immune pathways leading to disease progression is essential for the future development of anti-inflammatory therapies for this chronic disease.

Chlamydia pneumoniae Infection and Inflammatory Diseases

Chlamydia pneumoniae, an obligate intracellular bacterial pathogen, has long been investigated as a potential developmental or exacerbating factor in various pathologies. Its unique lifestyle and ability to disseminate throughout the host while persisting in relative safety from the immune response has placed this obligate intracellular pathogen in the crosshairs as a potentially mitigating factor in chronic inflammatory diseases. Many animal model and human correlative studies have been performed to confirm or deny a role for C. pneumoniae infection in these disorders. In some cases, antibiotic clinical trials were conducted to prove a link between bacterial infections and atherosclerosis. In this review, we detail the latest information regarding the potential role that C. pneumoniae infection may have in chronic inflammatory diseases.

Anti-atherosclerotic effect of housefly (Musca domestica) maggot-derived protein-enriched extracts by dampened oxidative stress in apolipoprotein E-deficient mice

Despitemany therapeutic advances, atherosclerosis remains the leading cause of morbidity and mortality in developed countries.

Specific Inflammatory Stimuli Lead to Distinct Platelet Responses in Mice and Humans

Introduction

Diverse and multi-factorial processes contribute to the progression of cardiovascular disease. These processes affect cells involved in the development of this disease in varying ways, ultimately leading to atherothrombosis. The goal of our study was to compare the differential effects of specific stimuli – two bacterial infections and a Western diet – on platelet responses in ApoE^-/- mice, specifically examining inflammatory function and gene expression. Results from murine studies were verified using platelets from participants of the Framingham Heart Study (FHS; n = 1819 participants).

Methods

Blood and spleen samples were collected at weeks 1 and 9 from ApoE^-/- mice infected with Porphyromonas gingivalis or Chlamydia pneumoniae and from mice fed a Western diet for 9 weeks. Transcripts based on data from a Western diet in ApoE^-/- mice were measured in platelet samples from FHS using high throughput qRT-PCR.

Results

At week 1, both bacterial infections increased circulating platelet-neutrophil aggregates. At week 9, these cells individually localized to the spleen, while Western diet resulted in increased platelet-neutrophil aggregates in the spleen only. Microarray analysis of platelet RNA from infected or Western diet-fed mice at week 1 and 9 showed differential profiles. Genes, such as Serpina1a, Ttr, Fgg, Rpl21, and Alb, were uniquely affected by infection and diet. Results were reinforced in platelets obtained from participants of the FHS.

Conclusion

Using both human studies and animal models, results demonstrate that variable sources of inflammatory stimuli have the ability to influence the platelet phenotype in distinct ways, indicative of the diverse function of platelets in thrombosis, hemostasis, and immunity.

Infection and Atherosclerosis Development

Atherosclerosis is a chronic disease hallmarked by chronic inflammation, endothelial dysfunction and lipid accumulation in the vasculature. Although lipid modification and deposition are thought to be a major source of the continuous inflammatory stimulus, a large body of evidence suggests that infectious agents may contribute to atherosclerotic processes. This could occur by either direct effects through infection of vascular cells and/or through indirect effects by induction of cytokine and acute phase reactant proteins by infection at other sites. Multiple bacterial and viral pathogens have been associated with atherosclerosis by seroepidemiological studies, identification of the infectious agent in human atherosclerotic tissue, and experimental studies demonstrating an acceleration of atherosclerosis following infection in animal models of atherosclerosis. This review will focus on those infectious agents for which biological plausibility has been demonstrated in animal models and on the challenges of proving a role of infection in human atherosclerotic disease.

Chlamydia pneumoniae acute liver infection affects hepatic cholesterol and triglyceride metabolism in mice

OBJECTIVE

Chlamydia pneumoniae has been linked to atherosclerosis, strictly associated with hyperlipidemia. The liver plays a central role in the regulation of lipid metabolism. Since in animal models C. pneumoniae can be found at hepatic level, this study aims to elucidate whether C. pneumoniae infection accelerates atherosclerosis by affecting lipid metabolism.

METHODS

Thirty Balb/c mice were challenged intra-peritoneally with C. pneumoniae elementary bodies and thirty with Chlamydia trachomatis, serovar D. Thirty mice were injected with sucrose-phosphate-glutamate buffer, as negative controls. Seven days after infection, liver samples were examined both for presence of chlamydia and expression of genes involved in inflammation and lipid metabolism.

RESULTS

C. pneumoniae was isolated from 26 liver homogenates, whereas C. trachomatis was never re-cultivated (P < 0.001). C. pneumoniae infected mice showed significantly increased serum cholesterol and triglycerides levels compared both with negative controls (P < 0.001 and P = 0.0197, respectively) and C. trachomatis infected mice (P < 0.001). Liver bile acids were significantly reduced in C. pneumoniae compared to controls and C. trachomatis infected mice. In C. pneumoniae infected livers, cholesterol 7α-hydroxylase (Cyp7a1) and low-density lipoprotein receptor (Ldlr) mRNA levels were reduced, while inducible degrader of the low-density lipoprotein receptor (Idol) expression was increased. Hypertriglyceridemia was associated to reduced expression of hepatic carnitine palmitoyltransferase-1a (Cpt1a) and medium chain acyl-Coenzyme A dehydrogenase (Acadm). Pro-inflammatory cytokines gene expression was increased compared to negative controls. Conversely, in C. trachomatis infected animals, normal serum lipid levels were associated with elevated pro-inflammatory cytokines gene expression, linked to only a mild disturbance of lipid regulatory genes.

CONCLUSION

Our results indicate that C. pneumoniae mouse liver infection induces dyslipidemic effects with significant modifications of genes involved in lipid metabolism.

Danger signaling in atherosclerosis

All aspects of the pathogenesis of atherosclerosis are critically influenced by the inflammatory response in vascular plaques. Research in the field of innate immunity from the past 2 decades has uncovered many novel mechanisms elucidating how immune cells sense microbes, tissue damage, and metabolic derangements. Here, we summarize which triggers of innate immunity appear during atherogenesis and by which pathways they can contribute to inflammation in atherosclerotic plaques. The increased understanding gained from studies assessing how immune activation is associated with the pathogenesis of atherosclerosis has provided many novel targets for potential therapeutic intervention. Excitingly, the concept that inflammation may be the core of cardiovascular disease is currently being clinically evaluated and will probably encourage further studies in this area.

Inflammation-induced foam cell formation in chronic inflammatory disease

Atherosclerosis is the leading cause of cardiovascular disease and is both a metabolic and inflammatory disease. Two models describe early events initiating atherosclerotic plaque formation, whereby foam cells form in response to hyperlipidaemia or inflammation-associated stimuli. Although these models are inextricably linked and not mutually exclusive, identifying the unique contribution of each in different disease settings remains an important question. Circulating monocytes are key mediators of atherogenesis in both models as precursors to lipid-laden foam cells formed in response to either excess lipid deposition in arteries, signalling via pattern-associated molecular patterns or a combination of the two. In this review, we assess the role of monocytes in each model and discuss how key steps in atherogenesis may be targeted to enhance clinical outcomes in patients with chronic inflammatory disease.

Mast cells play an important role in chlamydia pneumoniae lung infection by facilitating immune cell recruitment into the airway

Mast cells are known as central players in allergy and anaphylaxis, and they play a pivotal role in host defense against certain pathogens. Chlamydia pneumoniae is an important human pathogen, but it is unclear what role mast cells play during C. pneumoniae infection. We infected C57BL/6 (wild-type [WT]) and mast cell-deficient mice (Kit(W-sh/W-sh) [Wsh]) with C. pneumoniae. Wsh mice showed improved survival compared with WT mice, with fewer cells in Wsh bronchoalveolar lavage fluid (BALF), despite similar levels of cytokines and chemokines. We also found a more rapid clearance of bacteria from the lungs of Wsh mice compared with WT mice. Cromolyn, a mast cell stabilizer, reduced BALF cells and bacterial burden similar to the levels seen in Wsh mice; conversely, Compound 48/80, a mast cell degranulator, increased the number of BALF cells and bacterial burden. Histology showed that WT lungs had diffuse inflammation, whereas Wsh mice had patchy accumulations of neutrophils and perivascular accumulations of lymphocytes. Infected Wsh mice had reduced amounts of matrix metalloprotease-9 in BALF and were resistant to epithelial integral membrane protein degradation, suggesting that barrier integrity remains intact in Wsh mice. Mast cell reconstitution in Wsh mice led to enhanced bacterial growth and normal epithelial integral membrane protein degradation, highlighting the specific role of mast cells in this model. These data suggest that mast cells play a detrimental role during C. pneumoniae infection by facilitating immune cell infiltration into the airspace and providing a more favorable replicative environment for C. pneumoniae.

A single infection with Chlamydia pneumoniae is sufficient to exacerbate atherosclerosis in ApoE deficient mice

Several studies have demonstrated a strong link between Chlamydia pneumoniae (Cp) infection and atherosclerosis progression/exacerbation. Here, we try to understand whether a single administration of Cp could exacerbate atherosclerosis. Apoe(-/-) mice were intranasally infected with Cp followed by a high fat diet. Mice were sacrificed at different time points after Cp infection to monitor the development of the atheroma. Cp infection increased lipid content in the aortic sinus of Apoe(-/-) mice starting from 8 weeks. This was associated with increased numbers of active myeloid dendritic cells and plasmacytoid DCs which were co-localized with T-cells in the atherosclerotic plaque. The serum levels of IFN-γ showed a Th1-like environment typical of atherosclerosis. In conclusion, we demonstrate that one dose of Cp could exacerbate atherosclerotic lesion development, triggering innate immune cell accumulation early on that allowed the involvement of Th1-like cells in the exacerbation of the atherosclerotic plaque at later time points.

Distinct gene signatures in aortic tissue from ApoE-/- mice exposed to pathogens or Western diet

Background

Atherosclerosis is a progressive disease characterized by inflammation and accumulation of lipids in vascular tissue. Porphyromonas gingivalis (Pg) and Chlamydia pneumoniae (Cp) are associated with inflammatory atherosclerosis in humans. Similar to endogenous mediators arising from excessive dietary lipids, these Gram-negative pathogens are pro-atherogenic in animal models, although the specific inflammatory/atherogenic pathways induced by these stimuli are not well defined. In this study, we identified gene expression profiles that characterize P. gingivalis, C. pneumoniae, and Western diet (WD) at acute and chronic time points in aortas of Apolipoprotein E (ApoE^-/-) mice.

Results

At the chronic time point, we observed that P. gingivalis was associated with a high number of unique differentially expressed genes compared to C. pneumoniae or WD. For the top 500 differentially expressed genes unique to each group, we observed a high percentage (76%) that exhibited decreased expression in P. gingivalis-treated mice in contrast to a high percentage (96%) that exhibited increased expression in WD mice. C. pneumoniae treatment resulted in approximately equal numbers of genes that exhibited increased and decreased expression. Gene Set Enrichment Analysis (GSEA) revealed distinct stimuli-associated phenotypes, including decreased expression of mitochondrion, glucose metabolism, and PPAR pathways in response to P. gingivalis but increased expression of mitochondrion, lipid metabolism, carbohydrate and amino acid metabolism, and PPAR pathways in response to C. pneumoniae; WD was associated with increased expression of immune and inflammatory pathways. DAVID analysis of gene clusters identified by two-way ANOVA at acute and chronic time points revealed a set of core genes that exhibited altered expression during the natural progression of atherosclerosis in ApoE^-/- mice; these changes were enhanced in P. gingivalis-treated mice but attenuated in C. pneumoniae-treated mice. Notable differences in the expression of genes associated with unstable plaques were also observed among the three pro-atherogenic stimuli.

Conclusions

Despite the common outcome of P. gingivalis, C. pneumoniae, and WD on the induction of vascular inflammation and atherosclerosis, distinct gene signatures and pathways unique to each pro-atherogenic stimulus were identified. Our results suggest that pathogen exposure results in dysregulated cellular responses that may impact plaque progression and regression pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1176) contains supplementary material, which is available to authorized users.

Activation of an innate immune receptor, Nod1, accelerates atherogenesis in Apoe-/- mice

Atherosclerosis is essentially a vascular inflammatory process in the presence of an excess amount of lipid. We have recently reported that oral administration of a nucleotide-binding oligomerization domain (Nod)-1 ligand, FK565, induced vascular inflammation in vivo. No studies, however, have proven the association between Nod1 and atherosclerosis in vivo. To investigate a potential role of NOD1 in atherogenesis, we orally administered FK565 to apolipoprotein E knockout (Apoe(-/-)) mice for 4 wk intermittently and performed quantification of atherosclerotic lesions in aortic roots and aortas, immunohistochemical analyses, and microarray-based gene expression profiling of aortic roots. FK565 administration accelerated the development of atherosclerosis in Apoe(-/-) mice, and the effect was dependent on Nod1 in non-bone marrow origin cells by bone marrow transplantation experiments. Immunohistochemical studies revealed the increases in the accumulation of macrophages and CD3 T cells within the plaques in aortic roots. Gene expression analyses of aortic roots demonstrated a marked upregulation of the Ccl5 gene during early stage of atherogenesis, and the treatment with Ccl5 antagonist significantly inhibited the acceleration of atherosclerosis in FK565-administered Apoe(-/-) mice. Additionally, as compared with Apoe(-/-) mice, Apoe and Nod1 double-knockout mice showed reduced development of atherosclerotic lesions from the early stage as well as their delayed progression and a significant reduction in Ccl5 mRNA levels at 9 wk of age. Data in the present study show that the Nod1 signaling pathway in non-bone marrow-derived cells contributes to the development of atherosclerosis.

Chlamydia pneumoniae harness host NLRP3 inflammasome-mediated caspase-1 activation for optimal intracellular growth in murine macrophages

Chlamydia pneumoniae is an obligate intracellular pathogen that replicates within a vacuole and acquires host cell nutrients. We show that C. pneumoniae utilizes host innate immune signaling NLRP3/ASC/caspase-1 inflammasome for intracellular growth. Bone marrow-derived macrophages (BMMs) secreted mature interleukin-1β upon infection with C. pneumoniae depending on the NLRP3 inflammasome activation. Intracellular growth of C. pneumoniae was severely impaired in BMMs from Nlrp3(-/-), Asc(-/-), and Casp1(-/-) mice but not wild type or Nlrc4(-/-) mice. Furthermore defective NLRP3 inflammasome components led to accumulation of lipid droplets inside the infected BMMs, suggesting that uptake and/or utilization of lipids is disturbed in the absence of NLRP3 inflammasome activation. These results suggest C. pneumoniae has evolved to harness both host innate immune response and NLRP3 inflammasome activation, for the acquisition of essential nutrients necessary for intracellular growth. This unique property of C. pneumoniae may shed a new light on how C. pneumoniae increase the risk of atherosclerosis and metabolic syndrome.

Distinct lipid a moieties contribute to pathogen-induced site-specific vascular inflammation

Several successful pathogens have evolved mechanisms to evade host defense, resulting in the establishment of persistent and chronic infections. One such pathogen, Porphyromonas gingivalis, induces chronic low-grade inflammation associated with local inflammatory bone loss and systemic inflammation manifested as atherosclerosis. P. gingivalis expresses an atypical lipopolysaccharide (LPS) structure containing heterogeneous lipid A species, that exhibit Toll-like receptor-4 (TLR4) agonist or antagonist activity, or are non-activating at TLR4. In this study, we utilized a series of P. gingivalis lipid A mutants to demonstrate that antagonistic lipid A structures enable the pathogen to evade TLR4-mediated bactericidal activity in macrophages resulting in systemic inflammation. Production of antagonistic lipid A was associated with the induction of low levels of TLR4-dependent proinflammatory mediators, failed activation of the inflammasome and increased bacterial survival in macrophages. Oral infection of ApoE^−/− mice with the P. gingivalis strain expressing antagonistic lipid A resulted in vascular inflammation, macrophage accumulation and atherosclerosis progression. In contrast, a P. gingivalis strain producing exclusively agonistic lipid A augmented levels of proinflammatory mediators and activated the inflammasome in a caspase-11-dependent manner, resulting in host cell lysis and decreased bacterial survival. ApoE^−/− mice infected with this strain exhibited diminished vascular inflammation, macrophage accumulation, and atherosclerosis progression. Notably, the ability of P. gingivalis to induce local inflammatory bone loss was independent of lipid A expression, indicative of distinct mechanisms for induction of local versus systemic inflammation by this pathogen. Collectively, our results point to a pivotal role for activation of the non-canonical inflammasome in P. gingivalis infection and demonstrate that P. gingivalis evades immune detection at TLR4 facilitating chronic inflammation in the vasculature. These studies support the emerging concept that pathogen-mediated chronic inflammatory disorders result from specific pathogen-mediated evasion strategies resulting in low-grade chronic inflammation.

Several human pathogens express structurally divergent forms of lipid A, the endotoxic portion of lipopolysaccharide (LPS), as a strategy to evade host innate immune detection and establish persistent infection. Expression of modified lipid A species promotes pathogen evasion of host recognition by Toll-like receptor-4 (TLR4) and the non-canonical inflammasome. The Gram-negative oral anaerobe, Porphyromonas gingivalis, expresses lipid A structures that function as TLR4 agonists or antagonists, or are immunologically inert. It is currently unclear how modulation of P. gingivalis lipid A expression contributes to innate immune recognition, survival, and the ability of the pathogen to induce local and systemic inflammation. In this study, we demonstrate that P. gingivalis expression of antagonist lipid A species results in attenuated production of proinflammatory mediators and evasion of non-canonical inflammasome activation, facilitating bacterial survival in the macrophage. Infection of atherosclerosis-prone ApoE^−/− mice with this strain resulted in progression of chronic inflammation in the vasculature. Notably, the ability of P. gingivalis to induce local inflammatory bone loss was independent of lipid A modifications, supporting distinct mechanisms for induction of local versus systemic inflammation. Our work demonstrates that evasion of immune detection at TLR4 contributes to pathogen persistence and facilitates low-grade chronic inflammation.

Semisynthetic bile acid FXR and TGR5 agonists: physicochemical properties, pharmacokinetics, and metabolism in the rat

We report on the relationship between the structure-pharmacokinetics, metabolism, and therapeutic activity of semisynthetic bile acid analogs, including 6α-ethyl-3α,7α-dihydroxy-5β-cholan-24-oic acid (a selective farnesoid X receptor [FXR] receptor agonist), 6α-ethyl-23(S)-methyl-3α,7α,12α-trihydroxy-5β-cholan-24-oic acid (a specific Takeda G protein-coupled receptor 5 [TGR5] receptor agonist), and 6α-ethyl-3α,7α-dihydroxy-24-nor-5β-cholan-23-sulfate (a dual FXR/TGR5 agonist). We measured the main physicochemical properties of these molecules, including ionization constants, water solubility, lipophilicity, detergency, and protein binding. Biliary secretion and metabolism and plasma and hepatic concentrations were evaluated by high-pressure liquid chromatography-electrospray-mass spectrometry/mass spectrometry in bile fistula rat and compared with natural analogs chenodeoxycholic, cholic acid, and taurochenodexycholic acid and intestinal bacteria metabolism was evaluated in terms of 7α-dehydroxylase substrate-specificity in anaerobic human stool culture. The semisynthetic derivatives detergency, measured in terms of their critical micellar concentration, was quite similar to the natural analogs. They were slightly more lipophilic than the corresponding natural analogs, evaluated by their 1-octanol water partition coefficient (log P), because of the ethyl group in 6 position, which makes these molecules very stable toward bacterial 7-dehydroxylation. The hepatic metabolism and biliary secretion were different: 6α-ethyl-3α,7α-dihydroxy-5β-cholan-24-oic acid, as chenodeoxycholic acid, was efficiently conjugated with taurine in the liver and, only in this form, promptly and efficiently secreted in bile. 6α-Ethyl-23(S)-methyl-3α,7α,12α-trihydroxy-5β-cholan-24-oic acid was poorly conjugated with taurine because of the steric hindrance of the methyl at C23(S) position metabolized to the C23(R) isomer and partly conjugated with taurine. Conversely, 6α-ethyl-3α,7α-dihydroxy-24-nor-5β-cholan-23-sulfate was secreted in bile unmodified and as 3-glucuronide. Therefore, minor structural modifications profoundly influence the metabolism and biodistribution in the target organs where these analogs exert therapeutic effects by interacting with FXR and/or TGR5 receptors.

Deficiency of CCAAT/enhancer binding protein-epsilon reduces atherosclerotic lesions in LDLR-/- mice

The CCAAT/enhancer binding proteins (C/EBPs) are transcription factors involved in hematopoietic cell development and induction of several inflammatory mediators. C/EBPε is expressed only in myeloid cells including monocytes/macrophages. Atherosclerosis is an inflammatory disorder of the vascular wall and circulating immune cells such as monocytes/macrophages. Mice deficient in the low density lipoprotein (LDL) receptor (Ldlr−/−) fed on a high cholesterol diet (HCD) show elevated blood cholesterol levels and are widely used as models to study human atherosclerosis. In this study, we generated Ldlr and Cebpe double-knockout (llee) mice and compared their atherogenic phenotypes to Ldlr single deficient (llEE) mice after HCD. Macrophages from llee mice have reduced lipid uptake by foam cells and impaired phagokinetic motility in vitro compared to macrophages from llEE mice. Also, compared to llEE mice, llee mice have alterations of lipid metabolism, and reduced atheroma and obesity, particularly the males. Peritoneal macrophages of llee male mice have reduced mRNA expression of FABP4, a fatty acid binding protein implicated in atherosclerosis. Overall, our study suggests that the myeloid specific factor C/EBPε is involved in systemic lipid metabolism and that silencing of C/EBPε could decrease the development of atherosclerosis.

Tannerella forsythia BspA increases the risk factors for atherosclerosis in ApoE(-/-) mice

OBJECTIVE

The aim of this study was to evaluate the effects of Tannerella forsythia and its major surface virulence factor, BspA, on the progression of atherosclerosis in ApoE(-/-) mice and the expression of lipid metabolism-related genes.

METHODS

PMA-differentiated THP-1 cells were treated with BspA to detect foam cell formation. The proximal aortas of ApoE(-/-) mice injected with T. forsythia or BspA were stained with oil red O to examine lipid deposition. The serum levels of CRP, HDL, and LDL were detected by ELISA. The liver tissue of T. forsythia- or BspA-injected ApoE(-/-) mice was examined for mRNA expression of lipid metabolism-related genes, such as liver X receptors (LXRα and LXRβ) and ATP-binding cassette transporter A1 (ABCA1).

RESULTS

Tannerella forsythia and BspA induced foam cell formation in THP-1 cells and accelerated the progression of atherosclerotic lesions in ApoE(-/-) mice. Mouse serum levels of CRP and LDL were increased, and HDL was decreased by T. forsythia and BspA. The expression levels of LXRα and LXRβ, and ABCA1 in liver tissue were decreased by T. forsythia and BspA.

CONCLUSIONS

Tannerella forsythia and BspA augmented atherosclerotic lesion progression in ApoE(-/-) mice. This process may be associated with downregulation of lipid metabolism-related gene expression.

Chlamydia pneumoniae infection in mice induces chronic lung inflammation, iBALT formation, and fibrosis

Chlamydia pneumoniae (CP) lung infection can induce chronic lung inflammation and is associated with not only acute asthma but also COPD exacerbations. However, in mouse models of CP infection, most studies have investigated specifically the acute phase of the infection and not the longer-term chronic changes in the lungs. We infected C57BL/6 mice with 5×10⁵ CP intratracheally and monitored inflammation, cellular infiltrates and cytokine levels over time to investigate the chronic inflammatory lung changes. While bacteria numbers declined by day 28, macrophage numbers remained high through day 35. Immune cell clusters were detected as early as day 14 and persisted through day 35, and stained positive for B, T, and follicular dendritic cells, indicating these clusters were inducible bronchus associated lymphoid tissues (iBALTs). Classically activated inflammatory M1 macrophages were the predominant subtype early on while alternatively activated M2 macrophages increased later during infection. Adoptive transfer of M1 but not M2 macrophages intratracheally 1 week after infection resulted in greater lung inflammation, severe fibrosis, and increased numbers of iBALTS 35 days after infection. In summary, we show that CP lung infection in mice induces chronic inflammatory changes including iBALT formations as well as fibrosis. These observations suggest that the M1 macrophages, which are part of the normal response to clear acute C. pneumoniae lung infection, result in an enhanced acute response when present in excess numbers, with greater inflammation, tissue injury, and severe fibrosis.

The role of the vascular dendritic cell network in atherosclerosis

A complex role has been described for dendritic cells (DCs) in the potentiation and control of vascular inflammation and atherosclerosis. Resident vascular DCs are found in the intima of atherosclerosis-prone vascular regions exposed to disturbed blood flow patterns. Several phenotypically and functionally distinct vascular DC subsets have been described. The functional heterogeneity of these cells and their contributions to vascular homeostasis, inflammation, and atherosclerosis are only recently beginning to emerge. Here, we review the available literature, characterizing the origin and function of known vascular DC subsets and their important role contributing to the balance of immune activation and immune tolerance governing vascular homeostasis under healthy conditions. We then discuss how homeostatic DC functions are disrupted during atherogenesis, leading to atherosclerosis. The effectiveness of DC-based "atherosclerosis vaccine" therapies in the treatment of atherosclerosis is also reviewed. We further provide suggestions for distinguishing DCs from macrophages and discuss important future directions for the field.

Mouse models of atherosclerosis: explaining critical roles of lipid metabolism and inflammation

Atherosclerosis is the most common cause of death globally. It is a complex disease involving morphological and cellular changes in vascular walls. Studying molecular mechanism of the disease is hindered by disease complexity and lack of robust noninvasive diagnostics in human. Mouse models are the most popular animal models that allow researchers to study the mechanism of disease progression. In this review we discuss the advantage and development of mouse as a model for atherosclerotic research. Along with commonly used models, this review discusses strains that are used to study the role of two critical processes associated with the disease-lipid metabolism and inflammation.

Inflammation and atherosclerosis: direct versus indirect mechanisms

It is now widely accepted that the development of atherosclerotic lesions involves a chronic inflammatory response that includes both innate and adaptive immune mechanisms. However, it is still unclear precisely what induces the inflammatory response. Furthermore, inflammation within the blood vessel can be divided into direct mechanisms where the primary inflammatory events occur within the intima of the blood vessel and contribute to both the initiation and progression of the plaques and indirect mechanisms where inflammation at nonvascular sites can contribute to the progression of the lesions. The direct mechanisms include lipid deposition and modification, influx of lipoprotein associated factors and microparticles derived from many different cell types, and possibly bacterial and viral infection of vascular cells. Indirect mechanisms derive from inflammation related to autoimmune diseases, smoking, respiratory infection, and pollution exposure, and possibly periodontal disease and gastric infection. The mechanisms include secretion of cytokines and other inflammatory factors into the circulation with subsequent uptake into the plaques, egress and recruitment of activated inflammatory cells, formation of dysfunctional HDL and crossreactive autoantibodies.

Plasmacytoid dendritic cells play a role for effective innate immune responses during Chlamydia pneumoniae infection in mice

Plasmacytoid dendritic cells (pDCs) are known for their robust antiviral response and their pro-tolerance effects towards allergic diseases and tissue engraftments. However, little is known about the role pDCs may play during a bacterial infection, including pulmonary Chlamydia pneumoniae (CP). In this study, we investigated the role of pDCs during pulmonary CP infection. Our results revealed that depletion of pDCs during acute CP infection in mice results in delayed and reduced lung inflammation, with an early delay in cellular recruitment and significant reduction in early cytokine production in the lungs. This was followed by impaired and delayed bacterial clearance from the lungs which then resulted in a severe and prolonged chronic inflammation and iBALT like structures containing large numbers of B and T cells in these animals. We also observed that increasing the pDC numbers in the lung by FLT3L treatment experimentally results in greater lung inflammation during acute CP infection. In contrast to these results, restimulation of T-cells in the draining lymph nodes of pDC-depleted mice induced greater amounts of proinflammatory cytokines than we observed in control mice. These results suggest that pDCs in the lung may provide critical proinflammatory innate immune responses in response to CP infection, but are suppressive towards adaptive immune responses in the lymph node. Thus pDCs in the lung and the draining lymph node appear to have different roles and phenotypes during acute CP infection and may play a role in host immune responses.

Coordinate stimulation of macrophages by microparticles and TLR ligands induces foam cell formation

Aberrant activation of macrophages in arterial walls by oxidized lipoproteins can lead to atherosclerosis. Oxidized lipoproteins convert macrophages to foam cells through lipid uptake and TLR signaling. To investigate the relative contributions of lipid uptake and TLR signaling in foam cell formation, we established an in vitro assay using liposomes of defined lipid compositions. We found that TLRs signaling through Toll/IL-1R domain-containing adapter inducing IFN-β promoted foam cell formation by inducing both NF-κB signaling and type I IFN production, whereas TLRs that do not induce IFN, like TLR2, did not enhance foam cell formation. Addition of IFN-α to TLR2 activator promoted robust foam cell formation. TLR signaling further required peroxisome proliferator-activated receptor α, as inhibition of peroxisome proliferator-activated receptor α blocked foam cell formation. We then investigated the ability of endogenous microparticles (MP) to contribute to foam cell formation. We found that lipid-containing MP promoted foam cell formation, which was enhanced by TLR stimulation or IFN-α. These MP also stimulated foam cell formation in a human skin model. However, these MP suppressed TNF-α production and T cell activation, showing that foam cell formation can occur by immunosuppressive MP. Taken together, the data reveal novel signaling requirements for foam cell formation and suggest that uptake of distinct types of MP in the context of activation of multiple distinct TLR can induce foam cell formation.

Innate immune responses to Chlamydia pneumoniae infection: role of TLRs, NLRs, and the inflammasome

Chlamydiae are important human pathogens that are responsible for a wide rage of diseases with a significant impact on public health. In this review article we highlight how recent studies have increased our knowledge of Chlamydia pneumoniae pathogenesis and mechanisms of innate immunity directed host defense against C. pneumoniae infection.

Protective role for TLR4 signaling in atherosclerosis progression as revealed by infection with a common oral pathogen

Clinical and epidemiological studies have implicated chronic infections in the development of atherosclerosis. It has been proposed that common mechanisms of signaling via TLRs link stimulation by multiple pathogens to atherosclerosis. However, how pathogen-specific stimulation of TLR4 contributes to atherosclerosis progression remains poorly understood. In this study, atherosclerosis-prone apolipoprotein-E null (ApoE(-/-)) and TLR4-deficient (ApoE(-/-)TLR4(-/-)) mice were orally infected with the periodontal pathogen Porphyromonas gingivalis. ApoE(-/-)TLR4(-/-) mice were markedly more susceptible to atherosclerosis after oral infection with P. gingivalis. Using live animal imaging, we demonstrate that enhanced lesion progression occurs progressively and was increasingly evident with advancing age. Immunohistochemical analysis of lesions from ApoE(-/-)TLR4(-/-) mice revealed an increased inflammatory cell infiltrate composed primarily of macrophages and IL-17 effector T cells (Th17), a subset linked with chronic inflammation. Furthermore, enhanced atherosclerosis in TLR4-deficient mice was associated with impaired development of Th1 immunity and regulatory T cell infiltration. In vitro studies suggest that the mechanism of TLR4-mediated protective immunity may be orchestrated by dendritic cell IL-12 and IL-10, which are prototypic Th1 and regulatory T cell polarizing cytokines. We demonstrate an atheroprotective role for TLR4 in response to infection with the oral pathogen P. gingivalis. Our results point to a role for pathogen-specific TLR signaling in chronic inflammation and atherosclerosis.