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Promotion of atherosclerosis by Helicobacter cinaedi infection that involves macrophage-driven proinflammatory responses

Abstract

Helicobacter cinaedi is the most common enterohepatic Helicobacter species that causes bacteremia in humans, but its pathogenicity is unclear. Here, we investigated the possible association of H. cinaedi with atherosclerosis in vivo and in vitro. We found that H. cinaedi infection significantly enhanced atherosclerosis in hyperlipidaemic mice. Aortic root lesions in infected mice showed increased accumulation of neutrophils and F4/80+ foam cells, which was due, at least partly, to bacteria-mediated increased expression of proinflammatory genes. Although infection was asymptomatic, detection of cytolethal distending toxin RNA of H. cinaedi indicated aorta infection. H. cinaedi infection altered expression of cholesterol receptors and transporters in cultured macrophages and caused foam cell formation. Also, infection induced differentiation of THP-1 monocytes. These data provide the first evidence of a pathogenic role of H. cinaedi in atherosclerosis in experimental models, thereby justifying additional investigations of the possible role of enterohepatic Helicobacter spp. in atherosclerosis and cardiovascular disease.

Introduction

Atherosclerosis is the major cause of many cardiovascular diseases (CVDs)1. A widely accepted key player in the initiation and progression of atherosclerosis is the chronic inflammatory response1,2, but the initiating factor in this chronic inflammation in atherosclerotic tissues is still unclear3. The generally accepted paradigm implicates a proinflammatory mechanism involving lipid deposition and chemical modification as the initiator of inflammation in atherosclerotic tissues1,3,4. An alternative hypothesis that focuses on microbial factors has emerged during the past decades; it suggests that specific pathogens, such as Chlamydiapneumoniae5, cytomegalovirus6, herpes simplex virus7, influenza viruses8, Porphyromonasgingivalis9, Aggregatibacter actinomycetemcomitans10, Toxoplasma gondii11 and a few other reported pathogens, may be sources of inflammation. However, a number of studies claimed a lack of association of the above-mentioned microbes with the pathogenesis of atherosclerosis12,13,14,15. Several clinical trials also failed to confirm antibiotic treatment (for C. pneumoniae and Helicobacter pylori) or prevention through vaccination (for influenza virus) as effective interventions for CVD, which resulted in limited enthusiasm for the belief that microbes are aetiological agents in the pathogenesis of atherosclerosis3. Although failure of antibiotic trials suggests that the specific pathogens did not contribute to the disease process, failure may also indicate a lack of antibiotic susceptibility and/or the presence of multiple pathogens in plaque3,16. Therefore, the possible role of microbes that are closely related to human habitation, including those yet unidentified in atherosclerosis patients, cannot be ruled out.

Helicobacter cinaedi is the most frequently reported nongastric Helicobacter species isolated from both immunocompromised and immunocompetent patients17,18. Accumulating evidence suggests that H. cinaedi infection may be more common than previously thought because detecting H. cinaedi by conventional culture methods is difficult18,19,20,21,22. More important, even after successful diagnosis, treatment in many cases seems troublesome because of the growing resistance of H. cinaedi to several antibiotics. This resistance often results in failure to eradicate bacteria from infected patients after prolonged therapy21. Asymptomatic humans and animals with latent persistent infections have also been reported23,24,25,26. In fact, a recent study indicated that apparently asymptomatic human carriers may serve as a substantial source of bacterial spread through the faecal-oral route27.

Many cases of H. cinaedi bacteremia were reported in past decades, but clinical manifestations and epidemiology of diseases caused by this bacterium are still unclear17,18,20. Evidence suggests that H. cinaedi may have particular vascular tropism so that it can cause bacteremia and endovascular infections28, which are pathological manifestations that do not occur in H. pylori-infected patients. In support of such a vascular tropism, Lewis et al.29 reported clinical isolation of H. cinaedi from blood of a patient with myopericarditis and more recently we reported the apparent localization of H. cinaedi antigens in human atherosclerotic tissues25. Taken together, these clinical observations, if one assumes the possibility of an overlooked laboratory diagnosis, suggest that H. cinaedi is a clinically relevant bacterium that may contribute to the development of atherosclerotic CVD. Whether H. cinaedi has any pathogenic role in the pathogenesis of atherosclerosis remains unclear, however.

In the present study, we utilized in vivo and in vitro experimental models to investigate the proatherogenic potential of H. cinaedi infection. Pathological and morphometric analyses of aortic tissues showed significantly enhanced development of atherosclerotic lesions in orally infected hyperlipidaemic mice. H. cinaedi infection also induced extensive foam cell formation in cultured macrophages and differentiation of THP-1 monocytes into macrophages. These observations warrant additional research to elucidate the aetiological role of H. cinaedi and other enterohepatic Helicobacter spp. in CVD patients.

Results

Atherosclerosis in the aortic root of H. cinaedi-infected B6.Apoeshl mice

B6.Apoeshl mice are spontaneously hyperlipidaemic mice with an apolipoprotein E deficiency and have been utilized to study the development of atherosclerosis30. To investigate the involvement of H. cinaedi in the pathogenesis of atherosclerosis, we orally infected B6.Apoeshl mice with H. cinaedi. All mice were fed a high-cholesterol (1.25%) diet for 8 weeks and cross-sections of the aortic sinus were stained with Oil Red O for quantification of lesion areas. H. cinaedi infection substantially increased the size of atherosclerotic lesions in the aortic sinus compared with that in uninfected control mice (Fig. 1a). To observe and quantify the lipid-rich atherosclerotic plaques in similarly infected mice, aortas from another cohort pair of experimentally infected and control mice were opened longitudinally and stained with Sudan IV, which revealed that H. cinaedi infection increased both the number and the size of the atherosclerotic plaques in the aortas, particularly in aortic arch regions (Fig. 1b–g). To evaluate the pathogenic potential of H. cinaedi in mouse models that are less susceptible to atherosclerosis development, we similarly infected wild-type (WT) and endothelial nitric oxide synthase deficient (eNOS−/−) mice in a separate experiment. However, no atherosclerotic lesions developed in WT and eNOS−/− mice in either infected or control groups during the study period of 8 weeks (Fig. 1c).

Figure 1
figure1

Oral infection with H. cinaedi enhanced atherosclerosis in B6.Apoeshl mice.

Mice were killed at 8 weeks after oral infection. (a) Lesions in the aortic sinus stained with Oil Red O (red) and the stained areas were quantified and expressed as % plaque in the total aortic sinus area. Sections were counterstained with haematoxylin. Scale bars, 200 μm. Data are means ± s.d. *P < 0.05, n = 5 in each group. (b) Representative photographs of aortic arch regions with white plaques (yellow arrows) in B6.Apoeshl mice. Images are representative of an independent experiment separate from (a) with 5 mice per group. (c) Representative images of Sudan IV-stained (red) aortas from mice with the indicated genotypes. B6.Apoeshl aortas are from mice described in (b). (d–g) Percentages of Sudan IV-stained atherosclerotic plaques in the total aortic areas of B6.Apoeshl mice from (c). *P < 0.05.

Histopathological and immunohistochemical analyses of the aortic sinus

In B6.Apoeshl mice, we found two stages of atherosclerotic lesion development, as determined according to previously reported criteria9. The primary stage featured Oil Red O-stained lesions that resembled foam cell lesions. Both infected and control groups had lesions close to the aortic valve attachment site. The infected group had more abundant lesions in the free aortic wall compared with the control group (Fig. 2a). The advanced stage of atherosclerotic lesion development had a mixture of Oil Red O-stained cells, acellular zones and an inflammatory infiltrate. The infected group had higher number of advanced lesions in both the free aortic walls and the aortic valve attachment region compared with the control group (Fig. 2a). Both infected and control groups demonstrated F4/80 immunostaining of macrophages, but the infected group showed considerably pronounced immunostaining compared with the control group (Fig. 2b). Lesions in infected mice also manifested increased recruitment of neutrophils (Fig. 2c). Enhanced F4/80 immunostaining and neutrophil accumulation in lesions of the infected group indicated enhanced inflammatory changes in the vascular walls of these mice.

Figure 2
figure2

H. cinaedi infection induced advanced atherosclerotic lesion development and enhanced macrophage and neutrophil accumulation in atherosclerotic lesions of B6.Apoeshl mice.

Mice were killed at 8 weeks after oral H. cinaedi infection. (a) Representative Oil Red O-stained cross-sections of aortic root lesions from H. cinaedi-infected mice or PBS-treated mice fed a high-cholesterol diet (HCD) (n = 5 in each group). Most lesions in the vehicle-treated mice were adjacent to the valve attachment and were mainly first-stage foam cell lesions. Lesions in H. cinaedi-infected mice, however, were more abundant throughout the free aortic wall, had acellular cores (arrow) and represented the advanced stage of atherosclerosis. Proximal aortas of eNOS−/− and WT mice appeared normal after 8 weeks of infection. L, aortic lumen; I, intima; M, media; and A, adventitia. Scale bars in right panels, 20 μm. (b) Serial cross-sections of the aortic sinus of B6.Apoeshl mice showing F4/80 immunostaining and Oil Red O staining of advanced lesions of infected and vehicle-treated control mice (n = 5 in each group). Percentages of F4/80-immunostained areas in aortic sinus lesions are also shown. Data are means ice (Scale bars, 30 μm. *P < 0.05. (c) Representative cross-sections of the aortic sinus of B6.Apoeshl mice showing immunostaining of neutrophils (n = 4 in each group). The percentage of neutrophil-positive area in the total aortic sinus lesion is shown. Data are means. S. Scale bar, 100 μm. *P < 0.05. DIC, differential interference contrast.

Inflammatory gene expression in aortic tissues during H. cinaedi infection

To clarify the molecular basis of H. cinaedi-induced atherogenic changes in the vascular wall, a separate group of B6.Apoeshl mice was orally infected and their aortic tissues were analysed after 4 weeks of infection. To exclude inflammatory effects of a high-fat diet, these mice were fed a normal chow diet throughout the study period. Gene expression analysis by real-time reverse transcriptase PCR revealed significantly increased expression of inducible NOS (iNOS), interleukin-1β (IL-1β) and Toll-like receptor (TLR) 4 in aortic tissues of infected mice compared with that in control mice (P < 0.05) (Fig. 3). C-C motif chemokine 2 (CCL2) and intercellular adhesion molecule-1 (ICAM-1), two prominent mediators responsible for leukocyte adhesion and recruitment in the vascular wall31,32, were also significantly upregulated in aortas of infected mice (P < 0.05) (Fig. 3).

Figure 3
figure3

Relative gene expression levels in aortas of control and infected B6.Apoeshl mice.

Mice were orally infected with H. cinaedi or sham-treated with PBS (control) and were killed after 4 weeks of feeding a normal chow diet (n = 3 in each group). Total RNA from aortic tissues was extracted and mRNAs were quantified via real-time reverse transcriptase PCR. The relative quantities of experimental mRNA were normalized to the quantity of β-actin mRNA. Data are means ± s.d. *P < 0.05 versus control.

Detection of H. cinaedi mRNA and DNA in B6.Apoeshl mice after infection

During the experimental period, no mice had any symptoms of H. cinaedi infection such as diarrhea or cellulitis, which are seen in infected patients17,18,33. To confirm that H. cinaedi infection did indeed occur and to investigate potential sites of bacterial persistence, we attempted to detect H. cinaedi mRNA or DNA in various tissues and faecal samples from the same mice that were utilized in the study of inflammatory gene quantification. Blood samples were cultured first in a growth medium to increase the quantity of bacterial DNA for efficient amplification by our recently described nested PCR method27. Nested PCR amplified H. cinaedi DNA from two (of three) blood culture samples (Fig. 4a). Also, kidney (all three) and faecal (two of three) samples from infected mice contained bacterial DNA (Fig. 4a). No H. cinaedi DNA was identified in uninfected control mice (Fig. 4). In addition, immunohistochemical analyses showed the presence of H. cinaedi in caecal tissues (see Supplementary Fig. S1 online). This finding indicated successful colonization of the intestinal tract, which may be required for establishment of subsequent persistent infection.

Figure 4
figure4

Nested PCR detection of bacterial DNA and RNA in specimens of H. cinaedi-infected and vehicle-treated B6.Apoeshl mice.

The nested PCR assay used two pairs of primers that amplify the cdt gene of H. cinaedi into a 359-bp DNA fragment. Amplification product of mouse glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA was used as a loading control. Each lane indicates an individual mouse. P, PCR-positive control; N, PCR-negative control. (a) The 359-bp region of bacterial cdt gene was detected in DNA samples extracted from mouse-specimens at 4 weeks after H. cinaedi infection. Vehicle-treated mice had no bacterial cdt. All samples had amplifiable host DNA as shown by the representative image for GAPDH PCR. (b) Total RNA from aortic tissue was subjected to reverse transcriptase PCR followed by nested PCR detection of cdt. Uncropped gel images are shown in Supplementary Fig. S7.

Because bacterial mRNAs are known to have a half-life in seconds to minutes and therefore detection of bacterial mRNA as well as bacterial DNA has been applied in many studies to identify the live, viable and even metabolically active (i.e., infectious) bacteria in tissue samples34. In the present study, analysis of mRNA in aortic tissues by reverse transcription and nested PCR showed amplification of the transcript of the cytolethal distending toxin gene cdt of H. cinaedi in the infected mice. Because this toxin is known to be a major pathogenic factor of H. cinaedi, its mRNA detection strongly indicates the presence of viable and potentially infectious (proliferative) forms of H. cinaedi in aortic tissues (Fig. 4b).

Although H. cinaedi DNA could not be detected in liver and spleen tissues (Fig. 4a), infected mice had a higher ratio of spleen weight to body weight than did control mice (see Supplementary Fig. S2 online). Caecal tissues in infected mice showed mild infiltration of F4/80+ macrophage cells and neutrophils (see Supplementary Fig. S1 online). Shen et al. reported similar trend of moderate inflammation responses, e.g., mild typhlocolitis in the cecocolic junction with increased iNOS expression, in B6 and IL-10-defecient mice infected with H. cinaedi35. The present immunohistochemical analyses also revealed not only strong iNOS expression but also remarkable formation of 8-nitro-cGMP and S-guanylated proteins in caecal tissues in infected mice (cf. Supplementary Fig. S1 online). We identified recently 8-nitro-cGMP as well as protein S-guanylation as prominent biomarkers and mediators for inflammatory responses associate with excessive production of NO (from iNOS in this case) and reactive oxygen species36,37,38,39. Therefore, though not extensive, appreciable levels of inflammation may occur in caecal tissues, which is considered as a characteristic for H. cinaedi infected or the colonized B6 and B6.Apoeshl mice.

Macrophage-derived foam cell formation during H. cinaedi infection

We then used a cell culture system to clarify the role of H. cinaedi in macrophage-derived foam cell formation (Fig. 5). Peritoneal macrophages from WT mice and human monocyte-derived macrophages (HMDMs) were infected with H. cinaedi in the presence or absence of human low-density lipoprotein (LDL). Foam cell formation was assessed by using Oil Red O staining. Only H. cinaedi-infected cells accumulated lipid droplets, the hallmark of foam cell formation; uninfected and H. pylori-infected cells did not (Fig. 5a). Analysis of cellular lipid contents revealed that H. cinaedi-infected macrophages accumulated an increasing number of lipid droplets rich in cholesteryl esters in an infection dose-dependent manner (Fig. 5b and Supplementary Fig. S3a online). Free cholesterol is toxic to cells, which therefore normally convert it to cholesteryl ester for cellular storage in lipid droplets. In fact, as shown in Fig. 5b, the level of free cholesterol did not increase significantly during infection with H. cinaedi at an MOI up to 10, whereas the level of cholesteryl ester was elevated even at an infection dose of MOI 1.0. Moreover, lipid droplets found to be accumulated in H. cinaedi-infected HMDMs without human LDL added to culture media (Fig. 5c). A similar trend was observed for peritoneal macrophages infected with H. cinaedi (data not shown). This result is in clear contrast to the cellular response seen for C. pneumoniae-induced foam cells, when infected cells accumulated lipid droplets only in the presence of exogenous LDL in culture media; infected cells without exogenous LDL did not transform into foam cells40.

Figure 5
figure5

H. cinaedi induced macrophage foam cell formation in vitro.

(a) Oil Red O staining of peritoneal macrophages from WT mice infected with H. cinaedi or H. pylori at an MOI of 0.1 for 24 h and the number of Oil Red O-positive lipid droplets per cell. Macrophages treated with acetylated LDL (AcLDL) (50 μg ml−1) were used as a positive control for foam cell formation. Nuclei were stained with DAPI. Scale bars, 20 μm. (b) Cholesterol content of peritoneal macrophages infected with different doses of H. cinaedi. (c) Oil Red O staining of HMDMs infected for 24 h with H. cinaedi (0.1 MOI) in the presence or absence of human LDL and the percentage of foam cells. Cells were counterstained with haematoxylin. Scale bars, 10 μm. In all experiments cells were cultured in the medium containing 10% fetal bovine serum. Data are means ± s.d. from a representative experiment (n = 3). *P < 0.05, **P < 0.01 versus uninfected cells.

We further examined the possibility that cholesterol derived by bacteria might have contributed to the foam cell formation in absence of exogenous LDL, utilizing lipoprotein-deficient serum (LPDS) and analyzed foam cell formation by H. cinaedi-infected RAW cells. Supplementary Fig. S3b shows that the infected cells did not accumulate lipid droplets when fetal bovine serum (FBS) in the culture media was replaced with LPDS. However, the infected cells started to induce foam cell formation, when LDL was added to the cell culture with LPDS. Standard FBS contains approximately 300 μg/ml cholesterol, consisting partly of LDL cholesterol, according to manufacture's data. Therefore, our observation indicated that H. cinaedi-infected macrophages utilized the LDL cholesterol available from the 10% serum in the cell culture media, rather than H. cinaedi-derived cholesterol, to accumulate lipid droplets.

Levels of ABCG1 in H. cinaedi-infected macrophages

Evidence has suggested critical roles for ATP-binding cassette (ABC) transporters A1 and G1 in macrophage foam cell formation and atherosclerosis development41,42. To evaluate the effect of H. cinaedi in ABCG1 expression in macrophages/monocytes, therefore, we analysed lysates of infected and uninfected peritoneal macrophages and THP-1 monocyte-derived macrophages by using Western blot analysis. As Figure 6a and b illustrates, H. cinaedi-infected mouse peritoneal macrophages and THP-1 monocyte-derived macrophages had significantly reduced levels of ABCG1 (P < 0.05). Also, THP-1 monocytes had reduced ABCG1 levels as seen by immunocytochemistry and the infected cells manifested a morphological change (Fig. 6c). H. pylori-infected THP-1 monocyte-derived macrophages, however, had an increase in ABCG1 protein. ABCA1 expression in THP-1 monocyte-derived macrophages was not affected by infection with H. cinaedi or H. pylori (Fig. 6b). Lipopolysaccharide (LPS) treatment of THP-1 monocyte-derived macrophages produced no apparent change in ABCG1 and ABCA1 expression (Fig. 6b).

Figure 6
figure6

ABCG1 was downregulated and LDLR was upregulated in H. cinaedi-infected macrophages.

Western blot analysis of whole-cell lysates of (a) WT peritoneal macrophages and (b) THP-1 monocyte-derived macrophages infected with the indicated bacteria at an MOI of 0.1 or treated with Escherichia coli LPS (100 ng ml−1) for 24 h. Data are means ± s.d. from a representative experiment (n = 3). *P < 0.05, **P < 0.01 versus the PBS-treated control. (c) THP-1 monocytes were infected with H. cinaedi at an MOI of 0.1 for 5 days or were uninfected and ABCG1 was detected by immunohistochemistry. Scale bar, 5 μm. (d) Western blot analysis of whole-cell lysate protein from HMDMs infected with the indicated bacteria (0.1 MOI) or treated as indicated for 24 h. (e) Quantification of the densitometric measurement of relative band intensity of mature LDLR from the Western blot. (f) Representative fluorescent confocal images showing expression of LDLR and TLR2 in HMDMs infected with H. cinaedi (0.1 MOI) or treated with vehicle for 24 h. Scale bars, 10 μm. In all experiments (a to f) cells were cultured in the medium containing 10% fetal bovine serum. (g) Schematic representation of H. cinaedi-induced foam cell formation mechanism. Uncropped blots (a, b and d) are shown in Supplementary Fig. S8.

Expression of LDL receptor (LDLR) in H. cinaedi-infected macrophages

The LDLR on macrophages reportedly played a role in atherosclerotic lesion development in C57BL/6 mice43. We found that H. cinaedi infection induced significantly increased expression of LDLR protein in HMDMs (P < 0.05), with or without the addition of human LDL to the culture medium (Fig. 6d–f). While LDLR expression was likely decreased by exogenous addition of LDL (though not statistically significant), it was significantly attenuated by the treatment with acetylated LDL (AcLDL) in cultured HMDMs. However, H. cinaedi-infection obviously upregulated the expression of LDLR (Fig. 6d,e), despite accumulation of cholesteryl ester (Fig. 5b). This finding thus indicates that cholesteryl ester derived from bacteria, if any, did not affect the LDLR expression. On the contrary, infection with a similar dose of H. pylori did not increase LDLR expression markedly compared with expression in uninfected control cells (Fig. 6d,e). The class A scavenger receptor SRA1, which binds and takes up AcLDL/oxidized LDL (OxLDL)44, was downregulated in both H. cinaedi- and H. pylori-infected HMDMs. The class B scavenger receptor SRB1, which mediates uptake of OxLDL and high-density lipoprotein45, was unaltered by H. cinaedi infection but was markedly upregulated in H. pylori-infected cells (Fig. 6d). However, plasma levels of total cholesterol, LDL cholesterol and triglycerides were unchanged in H. cinaedi-infected B6.Apoeshl mice (see Supplementary Fig. S4 online).

All cell cultures were employed at infection doses of H. cinaedi below MOI 10, because, at MOI of less than 10, free cholesterol was not increased (Fig. 5b) as well as the cell viability did not decreased, as shown and discussed below. Specifically, the MOI of 0.1 was chosen mostly in the present analyses revealing the notable data shown in Fig. 6. Therefore, we now interpret that the finding of increased expression of LDL receptors coupled with reduced expression of ABCG1 is physiologically relevant. More importantly, the finding presented in Supplementary Fig. S3b, as mentioned above, may strongly support our interpretation that upregulation of LDLR and downregulation of ABCG1 of the culture macrophages induced by H. cinaedi infection may be promoting the cellular uptake of LDL cholesterol originated from FBS. In other words, upregulation of LDLR and downregulation of ABCG1 in H. cinaedi-infected macrophages indicated that foam cell formation was mediated by altered LDL metabolism (Fig. 6g).

H. cinaedi infection-induced differentiation of THP-1 monocytes into macrophages

As mentioned earlier, the infected mice in this study developed bacteremia (Fig. 4a), which is often seen in H. cinaedi-infected patients18. During bacteremia, circulating monocytes are exposed to invading bacteria or bacterial components. Available evidence indicates that the circulating monocytes adhere to activated endothelial cells and migrate into sites that are vulnerable to atherosclerotic lesion formation46. Therefore, to assess the effect of H. cinaedi infection on the differentiation of monocytes, we used THP-1 monocytes and analysed them for macrophage-like morphological changes such as diffused or amoeboid morphology of cells with cytoplasmic vacuoles47. Figure 7a shows representative Giemsa-stained and differential interference contrast (DIC) microscopic images of THP-1 cells at 5 days after infection with H. cinaedi. PBS-treated control cells maintained a round shape and apparently had a high nucleus-to-cytoplasm ratio. In contrast, cells infected with viable H. cinaedi as well as positive control cells stimulated with phorbol myristate acetate (PMA) showed an obvious change to diffused and enlarged morphology with cytoplasmic vacuoles. Cells treated with heat-killed H. cinaedi had only a minimal morphological change, as did cells treated with LPS (Fig. 7b). However, treatment with heat-killed H. cinaedi bacteria increased the attachment phenomenon of THP-1 cells compared with that in vehicle-treated cells (see Supplementary Fig. S5a online). Infection with live H. cinaedi led to reduction in total number of THP-1 cells (cf. Supplementary Fig. S5b online), which may support our findings related to infection-induced morphological differentiation. Regarding the potentially toxic effect of free cholesterol, the cells were likely suffered from its cytotoxicity only at MOI of 50, where their viability was attenuated (Supplemental Fig. S5c) and the free cholesterol level was elevated (Fig. 5b). However, decrease in cell number at MOI 10 in Supplemental Fig. S5b probably was not due to cytotoxicity of free cholesterol, but rather because of growth inhibition associated with advanced differentiation of THP-1 cells to foam cells. In fact, at MOI of less that 10, free cholesterol was not increased (Fig. 5b) as well as the cell viability did not decreased (Supplemental Fig. S5c). Treatment of cells with Pam3CSK4, a TLR2 ligand48, resulted in an attachment phenomenon and macrophage-like morphology close to those of the heat-killed H. cinaedi-treated cells (Fig. 7a,b). Pretreatment of THP-1 cells with TLR2 or TLR4 neutralizing antibodies significantly inhibited H. cinaedi-induced morphological differentiation (P < 0.05) (Fig. 7c), which indicated the involvement of these two TLRs. Finally, assessment of macrophage marker expression by Western blot analysis and immunocytochemistry revealed that H. cinaedi-infected THP-1 cells had an increased CD14 level compared with that in uninfected cells (see Supplementary Fig. S6 online). Expression of ICAM-1 also increased in THP-1 cells in an H. cinaedi infection dose-dependent manner (see Supplementary Fig. S6 online).

Figure 7
figure7

H. cinaedi induced morphological differentiation of THP-1 monocytes.

Suspended THP-1 cells were seeded in eight-well glass chamber slides and were infected with viable or heat-killed H. cinaedi at an MOI of 0.1 or were treated with E. coli LPS (100 ng ml−1), Pam3CSK4 (100 ng ml−1), or PMA (100 nM) for up to 7 days. (a) Representative bright-field images of Giemsa-stained cells and DIC images are shown at day 5 after infection or treatment. Scale bars in the Giemsa panel, 20 μm; scale bars in the DIC panel, 5 μm. (b) Percentages of cells with macrophage-like morphology, i.e., diffused or amoeboid shapes and cytoplasmic vacuoles, as shown by Giemsa staining, at the indicated time points. (c) Cells were treated with control isotype IgG or anti-TLR2 or anti-TLR4 IgG antibodies for 3 h and were then infected with live H. cinaedi (0.1 MOI) for 3 days. Percentages of morphologically changed cells were determined after Giemsa staining. For (b) and (c), at least 100 Giemsa-stained cells were counted manually at each time point for each treatment. In all experiments (a to c) cells were cultured in the medium containing 10% fetal bovine serum. Experiments were performed in triplicate. Data are means ± s.d. from a representative experiment (n = 3). *P < 0.05, **P < 0.01 versus PBS-treated cells.

Discussion

Our current study reports that H. cinaedi, a difficult-to-detect and emerging enterohepatic pathogen, enhanced the development of atherosclerosis in hyperlipidaemic mice. This increased atherosclerosis in H. cinaedi-infected mice was associated with activation of proatherogenic pathways, as evidenced by increased accumulation of macrophage foam cells and neutrophils in atherosclerotic lesions. Successful identification of H. cinaedi mRNA in aortic tissues of infected mice indicated the presence of bacteria, although infection was apparently asymptomatic. Our cell culture studies with peritoneal macrophages, THP-1 cells and HMDMs indicated that H. cinaedi infection altered the expression of cholesterol receptors or transporters and caused foam cell formation, which is a hallmark of atherosclerosis progression. These observations, combined with our previous report on immunohistochemical detection of H. cinaedi in human atherosclerotic tissues25, suggested a close association of H. cinaedi with atherosclerosis in infected individuals. Further investigations will be required to determine whether H. cinaedi plays a causative role in development of atherosclerosis in infected patients.

H. cinaedi has been primarily isolated from immunocompromised patients with various clinical manifestations, such as meningitis, bacteremia, cellulitis, enteritis and was less commonly isolated from patients with no recognized defect in host defense17,18,20. Our recent study27 however, revealed that H. cinaedi is likely colonized in the intestinal tract among healthy individuals, who may be then affected asymptomatically with its persistent infection. Therefore, we sought to clarify any pathological and clinical consequences of a possible persistent infection of H. cinaedi focusing on its potential to develop atherosclerosis.

For the present study, we administered H. cinaedi orally, as a natural route of infection, because our recent research identified the DNA of H. cinaedi in faeces from healthy volunteers and H. cinaedi-infected patients, which suggested a faecal-oral route of transmission27. For H. cinaedi identification in infected mice, we detected mRNA and DNA derived from cdt gene of H. cinaedi by using nested PCR assay we recently established27 (Fig. 4), because H. cinaedi is difficult to be isolated effectively via conventional bacterial culture especially with small amounts of tissue specimens (e.g., mouse aorta) due to the fastidious and slowly growing nature of this microaerobic bacteria as we reported earlier18,19,27. Although the present mouse model of H. cinaedi infection was apparently asymptomatic in view of severe clinical manifestations due to typical enteritis (e.g., diarrhea) and septicemia, the finding of H. cinaedi mRNA as well as its DNA identified unequivocally in either aortic or other tissues of infected mice is considered to be indicative of bacterial live cell activities indeed present in vivo in mice infected with H. cinaedi (Fig. 4). This interpretation was further supported by clear evidence of inflammatory responses in systemic as well as vascular lesions (Figs. 3 and S1). In fact, detection of H. cinaedi mRNA and DNA in aorta and blood culture samples of infected mice in this study (Fig. 4), together with an earlier report of recurrent bacteremia in patients18, may reflect the vascular tropism of H. cinaedi and its ability to cause systemic infection. The presence of bacterial mRNA and DNA in samples collected at 4 weeks after infection also indicated that H. cinaedi caused a chronic infection in these experimental mice. Therefore, chronic systemic dissemination of bacteria or bacterial components and active infection of aortic tissues may have directly caused proinflammatory gene expression in vascular walls (Fig. 3) and promoted the progression of atherosclerosis (Figs. 1 and 2).

The results of this study do not necessarily prove direct causality, however, because the B6.Apoeshl mice that we used have a preexisting genetic risk factor for atherosclerosis. Also, our current finding of no atherosclerotic lesions in WT and eNOS−/− mice fed a high-cholesterol diet at 8 weeks after H. cinaedi infection (Fig. 1c) emphasizes the importance of genetic susceptibility. Earlier studies reported that WT and eNOS−/− mice rarely developed atherosclerotic lesions during the short period of study49,50. In addition to genetic factors, another risk factor that was recently described is phosphatidylcholine metabolites that are generated by the metabolic activity of gut flora51. To maintain the natural intestinal habitat, we did not eliminate (e.g., with antibiotic treatment) the existing normal gut flora of animals in this study. Determination of changes in intestinal flora, if any, that occur because of H. cinaedi infection is an important subject that warrants future investigations. Regarding the association of bacterial infection with atherosclerosis, Koren et al. performed some molecular epidemiological analysis with microbiota of oral cavity, gut and atherosclerotic plaques from patients with atherosclerosis by using the pyrosequencing of the bacterial 16S rRNA gene52. In their study, any particular 16S rRNA gene-associated PCR amplicon was not listed for Helicobacter species. However, earlier reports suggest that comparison of the near-complete 16S rRNA gene sequences may have also led to incorrect identification of H. cinaedi, until a full genome sequencing that we and others completed recently27,53. Therefore, it may be possible that the H. cinaedi gene might have been overlooked and remains mostly uncertain. In any events, a notable atherosclerosis promoting potential of H. cinaedi is clarified herein, which further support the atherosclerosis pathogenesis involving polymicrobial mechanism to cause chronic inflammatory response in human vasculatures. In fact, additional epidemiological studies of atherosclerosis patients are needed to determine the cause-and-effect relationship in humans. Nonetheless, the results of this study demonstrate a potential of H. cinaedi for enhancement of atherosclerosis and provide mechanistic insights into H. cinaedi-related atherosclerosis (Fig. 8).

Figure 8
figure8

Schematic representation of the proposed mechanism of H. cinaedi-related atherosclerosis.

SMC, smooth muscle cell.

Given the well-known functions of macrophage foam cells in the initiation and progression of atherosclerosis2, the enhanced foam cell accumulation found here (Fig. 2b) can be considered a key event in the enhancement of atherosclerotic plaque development in infected B6.Apoeshl mice (Fig. 1). Our current cell culture studies also confirmed a direct role of H. cinaedi in macrophage foam cell formation (Fig. 5 and Supplementary Fig. S3 online). In our earlier study25, we clearly demonstrated the presence of H. cinaedi antigens in CD68+ foam cells in human atherosclerotic tissues. These observations together suggest a causative role of H. cinaedi in macrophage foam cell formation during development of atherosclerosis in humans. However, additional studies are required to prove this hypothesis.

In our study here, H. cinaedi-infection markedly reduced the level of the cholesterol efflux-related protein ABCG1 in mouse peritoneal macrophages and THP-1 monocyte-derived macrophages (Fig. 6a–c). ABCG1 serves a critical function in the homeostasis of cholesterol metabolism, with an ultimate impact on monocyte-endothelial cell interaction54, plasma high-density lipoprotein level55, macrophage apoptosis56 and atherosclerosis development57. Therefore, the ability of H. cinaedi to downregulate ABCG1 in macrophages may be a pathophysiologically relevant mechanism for H. cinaedi-induced foam cell formation.

H. cinaedi infection substantially upregulated the expression of LDLR protein in HMDMs (Fig. 6d–f), although plasma levels of total cholesterol, LDL cholesterol and triglycerides were unchanged in H. cinaedi-infected B6.Apoeshl mice (see Supplementary Fig. S4 online). H. cinaedi infection had no significant effect on the expression of SRA1 and SRB1 (Fig. 6d), two scavenger receptors for modified LDL4. These results suggested that H. cinaedi-induced foam cell formation involves modulation of unmodified LDL metabolism. Li et al. reported a similar finding of increased macrophage LDLR expression in response to acute inflammation58. Infection of HMDMs with H. pylori, however, did not significantly alter the expression of LDLR along with SRA1, whereas it caused significantly increased expression of SRB1 in our study (P < 0.05) (Fig. 6d). Expression of ABCG1 and ABCA1 also markedly increased in H. pylori-infected THP-1 monocyte-derived macrophages (Fig. 6b). These contrasting effects of H. pylori and H. cinaedi on cholesterol receptors and transporters explain in part why macrophages infected with H. pylori did not transform into foam cells (Fig. 5a). Despite the same genus of these two bacterial species, their effects on cellular lipid metabolism appear to be distinct because of pathogenic characteristics that are yet to be elucidated.

H. cinaedi infection also led to differentiation of THP-1 monocytes, which depended mainly on TLR2/TLR4 signalling (Fig. 7c). TLR signalling reportedly played important roles in both murine and human atherosclerosis48,59. In our study, H. cinaedi-infected mice showed increased mRNA expression of both TLR2 and TLR4 in aortic tissues (Fig. 3), which suggests the involvement of these two pattern recognition receptors in H. cinaedi-related atherosclerosis. However, detailed investigations, perhaps utilizing gene knockout macrophages and mouse models, are required to confirm the role of individual TLRs in H. cinaedi-induced monocyte differentiation, macrophage foam cell formation and atherosclerosis enhancement in mice.

H. cinaedi possesses several virulence factors that may contribute to its pathogenic role in atherosclerosis. According to our recent report53, the complete genome sequence of H. cinaedi indicates the presence of several virulence factors such as cytolethal distending toxin (CDT), alkyl hydroperoxide reductase, neutrophil activation protein, invasion antigen B, a potential type VI secretion system (T6SS), clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) systems. Shen et al. showed that CDT was an important factor for H. cinaedi-induced typhlocolitis in interleukin-10-deficient mice35. This toxin from other bacteria such as Campylobacter jejuni and Helicobacter hepaticus was also suggested to operate in immune modulation and persistent bacterial colonization in mice60,61. Therefore, CDT of H. cinaedi may contribute to bacterial persistence in hosts, which is often found in H. cinaedi-infected patients21,22. However, further investigations are necessary to explore the exact pathogenic role of virulence factors of H. cinaedi in the context of atherosclerosis.

In summary, H. cinaedi infection significantly enhanced the development of atherosclerosis in spontaneously hyperlipidaemic mice. Increased expression of proinflammatory genes, increased accumulation of neutrophils and induction of macrophage-derived foam cell formation underlie the pathogenic mechanism of enhanced atherosclerosis, which may be mediated directly by H. cinaedi bacteria invading the vasculature. Even though our study does not present evidence of direct causal effects, our findings provide the first data demonstrating the association of H. cinaedi with the enhancement of atherosclerosis. Additional investigations to determine the aetiological/contributing role of H. cinaedi and other enterohepatic Helicobacter spp. in atherosclerotic CVD development are thus warranted.

Methods

Bacterial culture

A clinical isolate of H. cinaedi (strain PAGU 0616)18,19 was used throughout these studies. H. cinaedi and H. pylori were grown as described in Supplementary Information online. Heat-killed H. cinaedi bacteria were prepared by incubating a bacterial suspension in PBS at 70°C for 30 min. For infection studies, late log-phase bacterial inoculum doses were adjusted by measuring the optical density of the bacterial suspension at 620 nm.

Animals and oral challenge with H. cinaedi

All protocols for animal experiments using mice were approved by the Animal Care and Use Committee, Kumamoto University. H. cinaedi infection was produced in mice in compliance with the institutional guidelines and regulations, with an effort to minimize the number of animals used and their suffering. WT C57BL/6 mice were purchased from CLEA Japan, Inc. (Tokyo, Japan). eNOS−/− (C57BL/6-Nos3tm1unc) mice were purchased from Jackson Laboratories (West Grove, PA, USA). Spontaneous hyperlipidaemic mice, C57BL/6.KOR-Apoeshl (B6.Apoeshl) mice30, were purchased from Japan SLC (Shizuoka, Japan). All mice were maintained on a standard chow diet and were housed in the Center for Animal Resources and Development, Kumamoto University. Mice of each genotype were divided into two groups: infection and control, with each group comprising five 6- to 8-week-old male mice. Animals were infected with H. cinaedi according to the method by Shen Z. et al35 with a slight modification. They established this protocol for infection of B6 and IL10−/− mice with H. cinaedi and followed up the infected mice until 12 and 36 weeks postinfection, respectively and showed via PCR method that colonization peaks at 12 weeks postinfection in B6 mice35. In our experiment, briefly, each mouse in the infection groups received 109 CFU of live H. cinaedi in 200 μl of PBS at pH 7.4 in each oral dose. The infection groups received three doses of H. cinaedi in the first week and a fourth dose in the fourth week of the study period. Mice in control groups were mock-treated with PBS. Throughout the study period, mice were fed a high-fat diet with a high cholesterol level (1.25%). All mice were killed 8 weeks after the initial oral challenge. Before their death, mice were fasted for 16 h. Blood and tissue samples were collected for analysis. The in vivo experiments for H. cinaedi infection were repeated at least five times with different cohort designs containing more than fifty B6.Apoeshl mice (with or without a high-cholesterol diet) subjected to the various pathological analyses at each time point after infection. This repetitional experimental design was employed to ensure precision and reproducibility of our current in vivo study, as described in detail in Supplementary Materials and Methods.

Pathological assessment of atherosclerotic lesions

The heart and proximal section of the aorta were removed from the mice and cleaned of fat and adventitial tissue. The top halves of the hearts were fixed in 4% paraformaldehyde at 4°C and were embedded in Tissue-Tek optimal cutting temperature (OCT) compound (Miles, Elkhart, IN, USA). Atherosclerotic plaque formation was analysed according to the method previously described62,63,64,65 with modifications. Briefly, the aortic root area was identified by the proximal presence of aortic valve leaflets. For preparing the cryosections of aortic sinus, an experienced pathologist (Y.F., coauthor of this paper) blinded to the identity of samples decided the start and end points of aortic sinus based on the morphology of sections under a light microscope. Consecutive sections (7 μm thick) in an entire aortic root area were obtained and mounted serially on ten different slides each numbered from 1 to 10. By the end of sampling/mounting, each slide had ten sections each separated by 70 μm tissue distance. The same numbered slide from each mouse was used for Oil red O staining. To evaluate the lesions, eight sections — four at the end of the aortic sinus and four at the junction between the aortic sinus and the ascending aorta and each section mounted on the same slide was separated by 70 μm (7 μm/section × 10 slides) in the original tissue — were used for the analysis according to the protocol published by Yagyu et al62. The sections were stained with Oil Red O and counterstained with haematoxylin. Whole aortas were collected and stained with Sudan IV. ImageJ software (NIH) was used to measure total lumen area including the intimal layer of the aortic sinus, toal area of whole aorta and the areas of atherosclerotic plaques. The plaque area was then divided with the area of aortic lumen or the whole aorta and the resultant value was expressed as percentage of atherosclerotic plaque area. GraphPad Prism 6.0 (GraphPad Software Inc, San Diego, CA) was used to generate the graphs showing atherosclerotic lesion percentages between control and infection groups of mice.

Nested PCR for detection of H. cinaedi DNA from faecal, tissue and blood specimens

To detect bacterial DNA via a nested PCR assay and to investigate the gene expression profile in aortic tissues, we performed a separate experiment with three control and five infected B6.Apoeshl mice that were fed a normal chow diet for 4 weeks. Mice were infected with H. cinaedi as described above, except they did not receive the fourth dose at week 4. All mice were fasted overnight and then were killed after faecal samples were collected. At the time of their death, blood samples were collected aseptically by means of orbital lobe puncture and the remaining blood was washed away by injecting 10 ml of PBS into the left ventricle to flush out the systemic circulation. Specimens of liver, kidney, spleen, aorta and caecal tissues were sequentially collected while maintaining the strictest possible aseptic conditions to avoid DNA contamination among specimens.

DNA from these specimens was extracted by using commercially available kits (Qiagen, Valencia, CA, USA). The extracted DNA was then used for a nested PCR assay, as previously described27, to detect the cdt gene of H. cinaedi. Because the sensitivity of detection by nested PCR was at least 102 CFU ml−1 of biological sample27, we initially cultured freshly collected blood for 3 days in BHI media according to the protocol for bacterial culture as described above. We then centrifuged the cultured blood specimens at 5,000 × g for 5 min and the resultant pellets were subjected to DNA extraction and nested PCR.

RNA isolation from aortas and amplification by real-time reverse transcriptase PCR

Total RNA from aortic tissues was extracted with TRIzol reagent (Invitrogen, Carlsbad, CA, USA) followed by DNase I treatment. The extracted RNA was then reverse-transcribed into cDNA by using random primers (Takara Bio, Otsu, Japan) and was subjected to PCR amplification. Levels of the following inflammatory markers were analysed by using PCR: iNOS, TLR2, TLR4, VCAM-1, ICAM-1, CCL2 and IL-1β. All primers were designed via Primer3plus66. Primer sequences (see Supplementary Table S1 online) and other details of the real-time PCR method are described in the Supplementary Information online.

Cell treatments and H. cinaedi infection

HMDMs, peritoneal macrophages and THP-1 monocyte-derived macrophages were cultured in RPMI 1640 with 10% fetal bovine serum all through the experiments. For foam cell-related studies, H. cinaedi or H. pylori bacteria suspended in PBS were added to these cells at the desired MOI. Cells were then incubated for 24 h with or without human LDL (50 μg ml−1) (Intracel, Frederick, MD, USA). After 24 h of infection or treatments, cells were washed twice with PBS and used for analysis. Preparation of macrophages and other details of cell treatments appear in the Supplementary Information online.

Monocyte differentiation studies

THP-1 cells (1 × 105 cells per well) were seeded in eight-well glass chamber slides and incubated overnight before being used in experiments. H. cinaedi bacteria suspended in PBS were added at the desired MOI, or the cells were treated with TLR ligands as described above followed by incubation at 37°C with 5% CO2 for 1, 3, 5 and 7 days. To observe the attachment phenomenon, suspended and attached cells were collected separately, followed by counting with an automated cell counter (Invitrogen). For morphological analysis, cells were fixed with paraformaldehyde, gently washed twice with PBS, stained with 2% Giemsa (Merck, Darmstadt, Germany) and viewed under a microscope at × 400 magnification. At least 100 cells were counted in five separate fields to determine the percentage of morphologically changed cells. Morphological alterations included a change in cell shape from round to amoeboid, an apparent change in the nucleus-to-cytoplasm ratio and the presence of cytoplasmic vacuoles. In some experiments, cells were treated for 3 h with neutralizing antibodies against TLR2 and TLR4 (BioLegend, San Diego, CA) at a concentration of 10 mg ml−1 and were then infected or treated as described.

Lipid body staining and foam cell analysis

Cells were fixed in 4% buffered formalin for 10 min, washed in distilled water, rinsed in 60% isopropanol and stained with 0.2% Oil Red O for 15 min. Stained cells were then cleaned in 60% isopropanol and were counterstained with haematoxylin. The aqueous mounting agent Aquatex (Merck, Summit, NJ, USA) was used to mount cells for light microscopic observations. For fluorescence analysis, cells were mounted in 10% glycerol in 20 mM Tris-HCl (pH 7.4) with 4′,6-diamidino-2-phenylindole (DAPI). Foam cells were observed via light or fluorescence microscopy (excitation wavelength 510–560; emission wavelength 575–590) at × 400 magnification with a Nikon Eclipse E1000 microscope (Nikon, Tokyo, Japan). Cells with more than 10 lipid droplets were defined as foam cells and the percentage of foam cells formed in each condition was determined according to a previously described method67. The lipid content of macrophages was also determined by means of a colorimetric method. Details of the procedure can be found in the Supplementary Information online.

Confocal microscopy

For all microscopic studies, cells were grown in glass chamber slides and were stimulated or infected as described. Caecal and aortic sinus tissues from the mice were fixed and embedded in OCT according to the procedure described earlier in the section on pathological assessment of atherosclerotic lesions. For determining infiltration of inflammatory cells, sections were stained with either rat anti-mouse anti-neutrophil antibody, clone 7/4, (ABD Serotec, Oxford, UK), or with rat anti-mouse F4/80 antibody (ABD Serotec, Oxford, UK) for macrophage staining. For confirming colonization of H. cinaedi in the intestinal tract, cecal tissues from infected and uninfected mice were similarly fixed and stained with a rabbit polyclonal antibody raised against a 30 kDa major antigenic protein of H. cinaedi19,25. Additional details of the immunostaining procedure are given in the Supplementary Information online. Stained cells and tissues were then examined with a Nikon Eclipse TE2000-E (and Nikon EZ-C1 software) laser scanning confocal microscope (Nikon) with a × 100 oil-immersion objective. Images were edited via ImageJ software and Adobe Photoshop CS4 version 11 (Adobe Systems, Waltham, MA, USA).

Western blot analysis

Cells were lysed in an ice-cold lysis buffer containing 20 mM Tris-HCl (pH 8.0), 140 mM NaCl, 10% glycerol, 1% NP-40, 100 mM NaF, 1 mM Na3OV4 and protease inhibitor cocktail (cOmplete ULTRA Tablets, Mini, Roche, Manheim, Germany). Cell lysates (10 μg) were subjected to immunoblot analysis as described in the Supplementary Information online.

Statistical analysis

Statistical significance was defined as P < 0.05 in a two-tailed Student's t-test. Analyses were performed by means of PASW Statistics, Version 18 (SPSS, Chicago, IL, USA).

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Acknowledgements

We thank Judith B. Gandy for her excellent editing of the manuscript. This work was supported in part by a Grant-in-Aid for Challenging Exploratory Research (grant number 21659109) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. S.K. was supported by the Japanese Government Scholarship (Monbukagakusho, MEXT) for doctoral studies.

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S.K., H.N.A.R., T.O. and T.A. designed the experiments, performed the analyses and wrote the paper. Y.F. and M.T. helped with cell biology, gene expression analyses and mouse studies. T.M., K.Ono and K.Oyama performed nested PCR analyses. J.Y., K.A.A., M.M.R. and H.N.A.R. provided technical help with immunochemical and immunoblot analyses. T.S., T.I., S.F. and Y.K. performed data analyses and edited the paper. T.A. supervised the study.

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Khan, S., Rahman, H., Okamoto, T. et al. Promotion of atherosclerosis by Helicobacter cinaedi infection that involves macrophage-driven proinflammatory responses. Sci Rep 4, 4680 (2014). https://doi.org/10.1038/srep04680

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