Genital Chlamydia infection in hyperlipidemic mouse models exacerbates atherosclerosis

Abstract

Background and aims.

Atherosclerosis is a chronic inflammatory disease, and recent studies have shown that infection at remote sites can contribute to the progression of atherosclerosis in hyperlipidemic mouse models. In this report, we tested the hypothesis that genital Chlamydia infection could accelerate the onset and progression of atherosclerosis.

Methods.

Apolipoprotein E (Apoe^−/−) and LDL receptor knockout (Ldlr^−/−) mice on a high-fat diet were infected intra-vaginally with Chlamydia muridarum. Atherosclerotic lesions on the aortic sinuses and in the descending aorta were assessed at 8-weeks post-infection. Systemic, macrophage, and vascular site inflammatory responses were assessed and quantified.

Results.

Compared to the uninfected groups, infected Apoe^−/− and Ldlr^−/− mice developed significantly more atherosclerotic lesions in the aortic sinus and in the descending aorta. Increased lesions were associated with higher circulating levels of serum amyloid A-1, IL-1β, TNF-α, and increased VCAM-1 expression in the aortic sinus, suggesting an association with inflammatory responses observed during C. muridarum infection. Genital infection courses were similar in Apoe^−/−, Ldlr^−/−, and wild type mice. Further, Apoe^−/− mice developed severe uterine pathology with increased dilatations. Apoe-deficiency also augmented cytokine/chemokine response in C. muridarum infected macrophages, suggesting that the difference in macrophage response could have contributed to the genital pathology in Apoe^−/− mice.

Conclusions.

Overall, these studies demonstrate that genital Chlamydia infection exacerbates atherosclerotic lesions in hyperlipidemic mouse and suggest a novel role for Apoe in full recovery of uterine anatomy after chlamydial infection.

Graphical Abstract

Introduction

Atherosclerosis is a chronic inflammatory disease initiated by endothelial dysfunction, macrophage activation, and lipid accumulation in the vasculature¹. Several studies provide evidence that infectious agents may accelerate atherosclerotic processes. For example, Porphyromonas gingivalis, Helicobacter pylori, HIV, and Chlamydia pneumoniae infection have been linked with atherosclerosis in humans^2–4. Respiratory C. pneumoniae infection and Chlamydia heat shock protein have been associated with atherosclerosis in multiple human serologic studies, providing some of the first evidence of an association between infection and atherosclerosis^5–7. C. pneumoniae has been detected in human atherosclerotic arteries in multiple studies^5–8. Mouse models of Chlamydia lung infection support the clinical observations from C. pneumoniae association with atherosclerosis. C. pneumoniae respiratory infection accelerates hyperlipidemia-induced atherosclerotic lesion development in C57BL/6J (wild type, WT)^{9, 10}, apolipoprotein-deficient (Apoe^−/−)¹¹, and low-density lipoprotein receptor-deficient (Ldlr^−/−) mice^{12, 13}.

Genital Chlamydia trachomatis infection is the most prevalent sexually transmitted bacterial infection and is a common cause of pelvic inflammatory disease (PID)¹⁴. Chlamydia infection followed by inflammation of the uterine lining (termed endometritis) and/or fallopian tubes (termed salpingitis), can lead to infertility in some women¹⁵. A link between genital chlamydial infection and atherosclerosis merits investigation because of the high prevalence of Chlamydia infection in youth, high prevalence of cardiovascular disease in adulthood, and the socioeconomic association of both infection and cardiovascular disease^{16, 17}. However, there have been no studies investigating whether or not genital Chlamydia trachomatis infection primes the host for accelerated atherosclerosis. We hypothesize that the inflammatory response to genital C. trachomatis infection could enhance the progression of atherosclerosis. This hypothesis was tested using the mouse model for genital Chlamydia infection, which has served as a powerful tool in the identification of key innate and adaptive inflammatory pathways^{18, 19}. Using two independent hyperlipidemic mouse models Apoe^−/− and Ldlr^−/− mice, we show that C. muridarum genital infection exacerbates atherosclerotic lesions. Interestingly, Apoe^−/− mice uteri also developed severe dilatation post infection, suggesting a Apoe-deficiency promotes uterine pathology after chlamydial infection.

Materials and methods

Chlamydial stocks, mice, and diet:

Chlamydia muridarum, “Nigg” strain (UAMS, Little Rock, AR) was propagated as described in an earlier study¹⁸. Wild type mice (WT, C57BL/6J 000664), mice deficient in Apoe (Apoe^−/−, 002052) or Ldlr (Ldlr^−/−, 002207) in C57BL/6J background were purchased from the Jackson Laboratory. Institutional Animal Care and Use Committees at the University of Arkansas for Medical Sciences, University of Pittsburgh, and UNC at Chapel Hill approved all animal experiments. Apoe^−/− mice were fed a high-fat diet (TD88137, 42% fat and 0.5% cholesterol), and Ldlr^−/− mice were fed a high-fat (15.8% fat)/high-cholesterol (1.25%) diet (TD94059)^{20, 21}.

Genital infections of mice:

Seven days prior to infection, all mice (8-wks) received 2.5 mg of Depo-Provera subcutaneously in 100 μl of PBS. A week later, one set of mice were anesthetized with nembutal (240–250 μl of 5 mg/ml stock) and infected by administering 3 × 10⁵ infectious forming units (IFU) of C. muridarum in 10 ml SPG buffer (250 mM sucrose, 10 mM sodium phosphate, 5 mM L-glutamate) into the vaginal vault. Uninfected mice did not receive SPG-buffer into the vaginal vault, since previous studies from our laboratory have observed no inflammation or no uterine pathology from SPG buffer. Beginning one week later, uninfected and infected mice were fed a high-fat (Apoe^−/−) or a high-fat/high cholesterol diet (Ldlr^−/−) for 7 weeks (Fig 1A). Mice were euthanized at 8 weeks (56-days) post-infection. Chlamydial shedding was determined by swabbing the cervix and vaginal vault of infected mice with a calcium alginate swab (Spectrum Medical Instruments, Los Angeles, CA) at various times post-infection, and IFUs in the swabs were determined as described in an earlier study¹⁹.

Fig 1.

Aortic lesions were increased in Apoe^−/− mice with intra vaginal infection of C. muridarum. Apoe^−/− mice (N=10) were infected with 3 × 10⁵ IFU vaginally and were fed high fat diet starting one week after infection. Mice were sacrificed 8-weeks post infection (A). Aortic sinus (N=6) and descending aorta (N=10) were stained using oil red O to visualize plaques and average lesion area plotted. Aortic sinus from a representative mouse from each group is shown and (B) lesion area in aortic sinus (C) for all mice determined. Descending aorta from a representative mouse from each group is shown (D) and lesion area in descending aorta (E) for all mice determined. Significance determined by ‘t’ test. Figure represents one of two independent experiments.

Genital histopathology:

Genital tract tissues were extracted en bloc from mice and tissues were fixed in 10% buffered formalin and embedded in paraffin. Longitudinal sections (4 mm) were stained with hematoxylin and eosin (H&E) and evaluated by a pathologist to whom experimental design was not disclosed. Scoring was done as described in an earlier study¹⁹. Luminal distention of the uterine horns and dilatation of the oviducts was graded from 1 to 4, with a grade 4 representing peak severity or frequency of the parameter.

Atherosclerotic lesions:

The heart and descending aorta were excised and fixed in PBS/4% formalin/30% sucrose overnight before mounted in optimal cutting temperature medium and frozen at −70°C. Aortic sinus cryosections (8 μm) were stained with Oil Red O²⁰. Atherosclerotic lesions were quantified as previously described²⁰, by determining the lesion area in each of the five sections for each mouse. En face analysis of the descending aorta was performed after staining descending aorta with Oil Red O²⁰.

Plasma lipids, SAA, and cytokines:

Concentrations of plasma total cholesterol was determined using kits from Biovision (Milpitas, CA) as previously described²⁰. Plasma TNF-α and serum amyloid A-1 (SAA-1) levels were determined by sandwich ELISA using kits specific for mouse TNF-α (RND Systems) and SAA-1 (Invitrogen). A HEK-IL-1 receptor reporter cell line (HEK-IL-1Ra, Invivogen) was used to estimate the functionally active mature form of mouse IL-1β in the plasma. Plasma samples from uninfected and infected Apoe^−/− and Ldlr^−/− mice were used in this bioassay along with recombinant mouse IL-1β as a standard. A HEK-IL-1Ra cell line expressing IL-1Ra linked to secretory alkaline phosphatase (SEAP) as a reporter was treated for 16 h with plasma from chlamydia infected Apoe^−/− and Ldlr^−/− mice. SEAP in the supernatant collected from the HEK-IL-1Ra cell line was quantified using Quantiblue substrate (Invivogen).

Immunohistochemical analysis:

Serial aortic sinus cryosections (8 μm) were stained with goat anti-mouse vascular cell adhesion molecule-1 (VCAM-1) IgG (10 μg/ml, RND systems) using Vectastain ABC reagent (Vector Laboratories Inc.). The sections were developed with DAB (3,3’-diaminobenzidine) and counterstained with Mayer’s hematoxylin. Images were captured using an Olympus microscope. Sections stained with goat IgG were used as a non-specific IgG control. The percentage of VCAM-1+ staining area was determined by measuring the total aortic sinus area. Macrophages at the lesion site were characterized by staining cryosections with the MOMA-2 antibody²².

Chlamydia at the lesion site:

DNA was prepared from the aorta, including the aortic arches, of Chlamydia-infected Apoe^−/− mice. Detection of chlamydial DNA was determined by quantitative PCR using C. muridarum-specific primers for gene coding for 16s rRNA as previously described²³. Purified chlamydial DNA was used as a positive control.

Bone marrow-derived macrophages:

Bone marrow-derived macrophages (BMDM) were generated from WT, Apoe^−/− and Ldlr^−/− mice, as previously described²⁰. After 6–8 days in culture, BMDM (0.8 × 10⁵/well) were infected with C. muridarum (1 MOI). Total RNA was isolated from uninfected and infected BMDM, and expression of cytokines and chemokines were determined by quantitative RT-PCR (qRT-PCR), using primers described in earlier studies^{20, 24, 25}, and in Supplemental Table 1. To confirm infection by observation of chlamydial inclusions, BMDM (0.8 × 10⁵/well) were seeded onto coverslips, infected in parallel, and fixed with methanol for 10 min at room temperature at 24-hour post infection. Coverslips were stained for chlamydial inclusion using mouse anti-chlamydial immune sera, followed by anti-mouse alexaFluor-488 secondary antibody (Invitrogen) and mounted with ProLong Antifade (Invitrogen)¹⁹.

Statistical analyses:

Columns and error bars represent mean ± SD. Differences between two groups were considered significant when p < 0.05 using the two-tailed Student t test. Infection course was analyzed by 2-way ANOVA and significance of uterine pathological scores was determined using the Mann-Whitney U test. All data were analyzed using InStat version 3.1a for Macintosh (GraphPad, San Diego, CA).

Results

C. muridarum genital infection results in increased aortic lesions in both Apoe^−/− and Ldlr^−/− mice:

Apoe^−/− and Ldlr^−/− mice, fed a hyperlipidemic diet, were used to determine whether genital chlamydial infection accelerates the progression of atherosclerosis. Infection course was assessed by chlamydial IFU measurements in genital swabs from Apoe^−/− and Ldlr^−/− mice and found to be comparable to WT mice (Supplemental Fig 1A and B). Both knockout mouse strains cleared infection by day 25 and IFUs were not detected in the swabs, thereafter.

To determine whether atherosclerosis was exacerbated in mice infected with C. muridarum, atherosclerotic lesions were compared between infected and uninfected mice at 8 weeks post-infection that were given hyperlipidemic diet starting a week after infection (Fig. 1A). Larger lesion areas were observed in the aortic sinuses of infected Apoe^−/− mice (Fig. 1B), which was significantly different between uninfected and infected mice (Fig. 1C). En face analysis of the descending aorta, including aortic arch, also showed significantly increased lesions in mice that were infected with C. muridarum (Fig. 1D and E). Ldlr^−/− mice were used as another model to independently test and confirm the finding from Apoe^−/− mice. The aortic sinus atherosclerotic lesion area in infected Ldlr^−/− mice was significantly larger (50% increase) than in the uninfected Ldlr^−/− mice (Fig. 2AB). En face analysis showed a similar increased in descending aorta of infected mice (Fig 2C). At this early time point (7 weeks on a high fat diet), Chlamydia-infected wild type mice (fed a high fat diet) did not show detectable atherosclerotic lesions (data not shown). These observations demonstrate that genital chlamydial infection exacerbates atherosclerosis in Apoe^−/− and Ldlr^−/− mice, two well-characterized hyperlipidemic mouse models.

Fig 2. Aortic lesions were increased in Ldlr^−/− mice infected with C. muridarum.

Ldlr^−/− mice (N=10) were infected with 3 × 10⁵ IFU vaginally and were fed high fat/high cholesterol diet starting one week after infection (as shown in Figure 1A). Mice were sacrificed 8-weeks post infection. Aortic sinus (N=5) was stained using oil red O to visualize plaques. Aortic sinus (A) from a representative mouse from each group is shown and average lesion area (B) plotted. En face analysis of descending aorta from a representative mouse from each group is shown (C). Uninfected Ldlr^−/− mice fed a high-fat/high cholesterol diet was used as control. Significance determined by ‘t’ test.

Chlamydia infection induces systemic and arterial inflammation.

Cholesterol levels were similar in uninfected and infected Apoe^−/− mice, and similar results were observed in Ldlr^−/− mice (Supplemental Fig. 2A and 2B). Therefore, increased atherosclerotic lesions in infected mice could not be attributed to plasma cholesterol levels. To evaluate if systemic inflammatory responses²⁶ were different in uninfected and infected mice, SAA-1, TNF-α, and IL-1β levels were measured in the plasma at 8-weeks post infection. SAA-1, TNF-α, and IL-1β levels were elevated in the plasma of Chlamydia-infected Apoe^−/− mice (Fig. 3A–C). Plasma levels of bioactive IL-1β were also significantly higher in Chlamydia-infected Ldlr^−/− mice compared to the corresponding uninfected mice at same time post infection (Fig. 3D). To determine if inflammatory responses were different in the vascular endothelium between uninfected and infected mice, we analyzed the expression of an inflammation marker; VCAM-1 at the lesion site in the aortic sections. VCAM-1 served as a candidate inflammatory marker as VCAM-1-dependent monocyte adhesion and transmigration promote formation of fatty streak lesions²⁷. Significantly more VCAM-1+ staining was observed in the aortic lesions of infected Apoe^−/− mice compared with uninfected Apoe^−/− mice (Fig. 4A and B). A similar trend was also observed in infected Ldlr^−/− mice, compared with uninfected Ldlr^−/− mice (Fig. 4C and D). Immunohistochemical analyses of the aortic sinus using MOMA2 antibody showed more macrophages in aortic lesions of infected Apoe^−/− mice, compared to uninfected mice (Fig. 4E). These findings suggest that chlamydial infection-induced systemic inflammatory response could induce endothelial cell activation, resulting in increased adherence, and transmigration of monocytes at the lesion site in infected mice.

Figure 3. Increased levels of SAA, TNF-α, and IL-1β in the plasma of infected hyperlipidemic mice.

Apoe^−/− mice (N=10) or Ldlr^−/− were infected with 3 × 10⁵ IFU genitally, and one week after infection mice were fed hyperlipidemic diet. Mice were sacrificed 8-weeks post infection (7 weeks on a high fat diet). Plasma from uninfected mice were used as controls. Plasma levels of SAA-1 (A), TNF-α (B) in Apoe^−/− mice, and IL-1β in Apoe^−/− (C), and Ldlr^−/− (D) were determined as described in methods. Significance determined by ‘t’ test.

Fig 4.

Increased VCAM-1 expression and macrophages in Chlamydia-infected mice. Representative aortic sinus sections from uninfected or Chlamydia-infected Apoe^−/− (A, B) and or Ldlr^−/− (C, D) were stained with anti-mouse VCAM-1 IgG to detect VCAM-1 protein expression at the lesion site. Arrows indicate VCAM-1 positive staining. (E), Macrophages in aortic sections from uninfected and Chlamydia-infected or Apoe^−/− mice was determined as described the Methods section. Under similar conditions, aortic sections incubated with an isotype-matched rat IgG control were minimal. Representative of five aortic sinus sections from uninfected and chlamydia-infected Apoe^−/− or Ldlr^−/− mice are presented (100 × magnification).

Clinical studies have shown that C. pneumoniae lung infections result in dissemination of C. pneumoniae at the vascular site^{28, 29}. Therefore, we investigated whether increased incidence of lesions observed in hyperlipidemic mice is caused by dissemination of C. muridarum to the vascular site. Quantitative PCR analyses of DNA from the aortic arch at day 8 weeks post-infection did not show chlamydial DNA (Supplemental Fig. 3A and B). These findings suggest that there is no persistent chlamydial infection in the lesion-prone vascular site. However, this does not preclude dissemination of Chlamydia to the vascular sites in early stages of infection.

Increased uterine pathology was observed in C. muridarum-infected Apoe^−/− mice.

Chlamydia infection leads to significant pathology of the oviduct, characterized by fluid filled hydrosalpinx. Additionally, some mice also develop uterine pathology with dilatation. Excision of the genital tract at 8 weeks post-infection showed no significant differences in hydrosalpinx incidence between WT and Ldlr^−/− mice and between WT and Apoe^−/− mice, that were fed high fat diet (data not shown). However, infected Apoe^−/− mice developed severe uterine dilatation relative to infected WT mice (Fig. 5A). Histopathological observation of H&E stained section showed dilated uteruses in infected Apoe^−/− mice with severe thinning of uterine wall (Fig. 5B). To determine if feeding high fat diet exacerbated uterine pathology, in an independent study, Apoe^−/− mice were infected and fed a high fat or regular chow diets (non-hyperlipidemic diet). Histopathological scoring of genital tract tissues showed more uterine horns with a score of 4 in high fat diet fed Apoe^−/− mice. However, the differences in uterine dilatation between mice fed high fat or regular chow diet were not statistically different, although both were significantly increased compared to uninfected mice (Fig 5C). Histological scoring of inflammatory cells at this time revealed no significant differences in acute (neutrophils), chronic (mononuclear), or plasma cells in mice fed high fat (Fig. 5D) or regular diets (data not shown). Further, the severe uterine dilatation observed in infected Apoe^−/− mice, was not observed in infected Ldlr^−/− mice (data not shown). These data suggest that uterine pathology observed in infected Apoe^−/− mice is an outcome of absence of Apoe and not due to hyperlipidemia.

Fig 5. Severe uterine dilatation in Apoe^−/− mice in comparison with WT mice.

(A), Representative photograph of genital tracts from infected WT and Apoe^−/− mice (N=10). (B) H&E staining of longitudinal sections of genital tracts WT and Apoe^−/− mice (N=5), and (C) histopathological scoring for immune cells and dilatation (N=5). (D) Dilatation scores from an independent experiment where 5 mice from each group were fed high fat diet or fed a regular chow diet. Statistics were determined by non-parametric two-tailed Mann Whitney test.

Inflammatory cytokine response to Chlamydia infection is increased in Apoe^−/− macrophages.

To determine if increased genital pathology in Apoe^−/− mice is a result of increased inflammatory response of immune cells to genital Chlamydia infection, we generated BMDM from WT and Apoe^−/− mice and their inflammatory gene expression in response to Chlamydia infection determined. Chlamydia-infected WT macrophages showed increased Ifnb, Il1b, Il6, and Tnfa expression (Supplemental Fig. 4A) compared to uninfected macrophages. Moreover, Apoe-deficient macrophages infected with Chlamydia showed higher pro-inflammatory cytokine response (Supplemental Fig. 4A) compared to infected WT macrophages. We then determined whether macrophages from two hyperlipidemic mouse models differ in cytokine response following Chlamydial infection. BMDM from Apoe^−/− and Ldlr^−/− mice were infected with Chlamydia, and their cytokine response to infection determined by qRT-PCR. Interestingly, some cytokine and chemokine expression of Apoe^−/− BMDM were significantly higher than those of Ldlr^−/− BMDM (Fig. 6). To rule out if the increased cytokine response was a result of increased infection, infected macrophages were stained for chlamydial inclusion. No differences in inclusion size or numbers were observed between Apoe^−/− and Ldlr^−/− mice BMDM macrophages (Supplemental Fig. 4B). These findings suggest that Apoe-deficiency augments chlamydial infection-induced pro-inflammatory cytokine response, which could contribute to genital pathology.

Fig 6.

Augmented inflammatory responses in Chlamydia infected Apoe^−/− macrophages. BMDM from Apoe^−/− and Ldlr^−/− were infected with C. muridarum (1 MOI) for 8 h, and expression of inflammatory cytokine and chemokine responses was determined by qRTPCR. Uninfected macrophages were used as controls. Values are mean ± SD of triplicates. Representative two independent experiments are presented. Cytokine expression in Chlamydia infected Apoe^−/− BMDM was compared to infected Ldlr^−/− BMDM. Significance determined by ‘t’ test.

Discussion

In this study we addressed the hypothesis that genital chlamydial infection can accelerate atherosclerosis under hyperlipidemic conditions. Using two hyperlipidemic mouse models (Apoe^−/− and Ldlr^−/−), we observed increased atherosclerotic lesions following Chlamydia genital infection. The infected mice had enhanced systemic and local inflammation, indicated by increased SAA, TNF-α, and IL-1β in plasma, and upregulation of VCAM-1 at the lesion site. In addition, we also discovered a novel role for Apoe in the maintenance of uterine architecture after chlamydial infection is resolved in the genital tract.

There is significant but conflicting literature regarding C. pneumoniae lung infection promoting atherosclerosis in the mouse model. Earlier studies have reported increased atherosclerotic lesions in Apoe^−/− or Ldlr^−/− mice inoculated intra-nasally at multiple times with C. pneumonia^{11, 12, 30}. However, other studies showed no differences in total lesion area^{31, 32}. Different C. pneumoniae strains, age of the mice, mouse strains used; the frequency and route of infections could be contributing to the conflicting reports. Unlike previous studies^{33, 34}, we have used genital C. muridarum infection model which requires a single inoculation to establish genital infection to examine genital C. muridarum infection contributing to atherosclerotic lesions in mice. Atherosclerotic lesions were analyzed as early as 8 weeks post-infection, including 7 weeks on a hyperlipidemic diet. Unlike other studies where lesions are measured at later times (> 12 weeks on a high-fat diet)^{31, 35}, we chose to collect samples at this early time when lesions are not fully developed in Apoe^−/− mice, to detect increases in lesion area post infection, before excessive lesions appeared in uninfected hyperlipidemic mice.

Bacterial infection or bacterial ligands regulate cholesterol homeostasis by regulating reverse cholesterol transporter expression³⁶. However, we found no differences in plasma cholesterol levels between infected and uninfected mice, indicating that the increased lesions in the infected groups are independent of plasma lipid levels. Infectious microbial agent-induced atherosclerosis could occur by direct effect through infection of vascular cells and/or indirectly by induction of inflammatory cytokines by infection at other sites, such as the aortic vascular endothelium. C. pneumoniae was detected within atherosclerotic plaques, suggesting an association between atherosclerosis and C. pneumoniae infection³⁷. A recent report has reported that intranasal administration of C. muridarum resulted in the infection of blood monocytes, which subsequently could transport C. muridarum to the vascular site³⁸. In our study, PCR analyses did not show chlamydial 16s ribosomal RNA in the aortic arch at 8 weeks post-infection. However, this does not rule out the possibility of early dissemination of C. muridarum to the mouse aorta, as we have observed early dissemination followed by rapid clearance of C. muridarum in the lungs and spleens of WT mice³⁹. Further, C. muridarum infection has been shown to persist in the gut well after it has been cleared from the genital tract^{40, 41}. Whether or not this persistence in the gut could have affected atherosclerotic lesions needs further investigation. Dissemination of C. muridarum is likely due to its ability to grow in macrophages⁴². However, with the exception of the less prevalent human Chlamydia strain like Lymphogranuloma venereum, genital strains of C. trachomatis grow poorly in human macrophages. Nevertheless, immunodeficiency or co-infection with other sexually transmitted infectious pathogens can compromise host immunity and allow C. trachomatis to survive in macrophages and disseminate to remote sites.

In mice, systemic inflammation induced by either intraperitoneal LPS injection or by cecal ligation puncture accelerated atherosclerosis by increased VCAM-1 expression and monocyte adhesion to the aorta^{43, 44}. Further, we have shown that C. muridarum infection in Apoe^−/− and Ldlr^−/− mice increased expression of VCAM-1 and macrophage accumulation at the lesion site. We have also shown that chlamydial genital infection results in augmented systemic inflammatory response as evidenced by elevated levels of TNF-α and IL-1β. Mechanistic studies have indicated that TNF-α severely accelerates lesion development by upregulating vascular cell adhesion molecules and CD36, one of the scavenger receptors implicated in foam cell formation and atherosclerosis^{45, 46}. These findings suggest that C. muridarum-induced production of TNF-α and IL1b could increase endothelial cell VCAM-1 expression resulting in increased monocyte adhesion, and subsequent accelerated atherosclerosis in infected mice^{27, 47}.

Genital Chlamydia infection in the mouse model leads to the development of fluid-filled oviduct pathology, described as hydrosalpinx. Occasionally, C57BL/6 mice also develop uterine cysts or swelling post-infection. However, in most animals, the uterine horns recover fully and resume an intact epithelial layer. A surprise finding in this study was that infected Apoe^−/− mice developed severe uterine pathology in addition to oviduct pathology. The uterine swelling led to complete loss of the epithelial architecture. We have reported similar uterine horn pathology in Irf3^−/− mice and suggested increased cellular proliferation as an associated phenotype⁴⁸. However, the exact mechanism for this phenotype is not clear. The uterine pathology observed in Apoe^−/− mice was not associated with a high fat diet, and not observed in Ldlr^−/− mice, suggesting an alternate function specifically for Apoe in uterus. Apoe is expressed in macrophages, and is essential for efficient cholesterol efflux⁴⁹. However, several new functions of Apoe have emerged. An anti-inflammatory role has been attributed to Apoe as it aids in clearance of apoptotic bodies by macrophages⁵⁰. In this report, we showed that Chlamydia infection augmented cytokine expression in Apoe-deficient macrophages compared to WT and Ldlr-deficient macrophages. These findings suggest that lack of anti-inflammatory function of Apoe may contribute to the augmented inflammatory cytokine response following Chlamydia infection. Apoe^−/− mice display impaired immunity to Listeria monocytogene⁵¹ and Klebsiella pneumoniae⁵² infection. Apoe has also been shown to inhibit cell proliferation and migration⁵³, suggesting that in the absence of Apoe, excessive proliferation of uterine stromal cells could lead to the observed dilatation.

In conclusion, our findings show that inflammatory response during a genital Chlamydia infection may be sufficient to promote the formation of atherosclerotic lesions under hyperlipidemic conditions in mouse models. To our knowledge, this is the first study to suggest a link between genital Chlamydia infection-induced PID and subclinical atherosclerosis, suggesting a causal link between genital infection and atherosclerosis. Further clinical studies are required to establish this association in humans. Additionally, the contribution of Apoe in uterine health during infection warrants further investigation.

Highlights.

Genital chlamydia infection exacerbated atherosclerosis in Apoe−/− and Ldlr−/−, two independent hyperlipidemic mouse models of atherosclerosis.

Chlamydia infection increased circulating levels of TNF-α and IL-1β.

Chlamydia infection also increased VCAM-1 expression and macrophages in atherosclerotic lesions.

Chlamydia infection increased pro-inflammatory cytokines and chemokines mRNA expression.

Interestingly, Chlamydia infection showed uterine pathology only in apoE-deficient mice.