PMCCPMCCPMCC

Search tips
Search criteria 

Advanced

 
Logo of najmedsciHomeCurrent issueInstructionsSubmit article
 
N Am J Med Sci. 2013 March; 5(3): 169–181.
PMCID: PMC3632020

Chlamydophila Pneumoniae Infection and Cardiovascular Disease

Abstract

Atherosclerosis is a multifactorial vascular inflammatory process; however, the inciting cause for inflammation remains unclear. Two decades ago, Chlamydophila pneumoniae (formerly Chlamydia pneumoniae) infection was proposed as a putative etiologic agent. We performed a PubMed search using the keywords Chlamydia and atherosclerosis in a Boolean query to identify published studies on C. pneumoniae and its role in atherogenesis, and to understand research interest in this topic. We found 1,652 published articles on this topic between 1991 and 2011. We analyzed relevant published studies and found various serological, molecular, and animal modeling studies in the early period. Encouraged by positive results from these studies, more than a dozen antibiotic clinical-trials were subsequently conducted, which did not find clinical benefits of anti-Chlamydophila drug therapy. While many researchers believe that the organism is still important, negative clinical trials had a similar impact on overall research interest. With many novel mechanisms identified for atherogenesis, there is a need for newer paradigms in Chlamydophila-atherosclerosis research.

Keywords: Atherosclerosis, Chlamydophila pneumonia, Cardiovascular disease, Infection

Introduction

Atherosclerosis is a leading cause of global mortality and morbidity, among noncommunicable disorders.[1] Atherosclerosis leads to coronary artery disease (CAD), cerebrovascular disease (CVD), and peripheral vascular disease (PVD), which have a wide clinical spectrum including stable or unstable angina (UA), acute myocardial infarction (AMI) or sudden cardiac death (SCD), transient ischemic attacks (TIA), ischemic strokes, vascular dementias, intermittent claudication, and gangrene. Further, atherosclerosis is responsible for a large burden of chronic kidney disease (CKD) and hypertensive heart diseases. At the population level, nine risk factors [smoking, history of hypertension, diabetes, waist/hip ratio, dietary patterns, physical activity, consumption of alcohol, blood apolipoproteins (Apo), and psychosocial factors] account for more than 90% of the population-attributable risk.[2] Despite these strong associations, interactions between these risk factors and an underlying inciting cause remain inadequately explained.[3]

A number of studies have found that inflammation of the vessel wall is an important mechanism responsible for initiation, progression, sclerosis, erosion, and rupture of atherosclerotic plaques.[4] A low-grade infection either by a virus (cytomegalovirus or human herpes viruses) or a bacterium, [Helicobacter pylori or Chlamydophila pneumoniae (formerly Chlamydia pneumoniae)] has been suggested as a possible etiology for this inflammatory activity,[4] For the last two decades, C. pneumoniae has been the strongest candidate organism. C. pneumoniae is an ubiquitous, obligate intracellular Gram-negative bacterium, and is a common respiratory pathogen.[5] It has been shown that C. pneumoniae infects human mononuclear cells, and this has also been demonstrated after respiratory challenge in animal studies.[6] These infected mononuclear cells transmigrate into circulation [peripheral blood mononuclear cells (PBMC)] and secondarily infect endothelial cells by cell-to-cell transmission of C. pneumoniae. This event could trigger a series of immunological phenomenon leading to atherosclerosis[6] [Figure 1]. Further, it has been demonstrated in animal studies that C. pneumonia infection reduces high-density lipoprotein (HDL) levels and intra-plaque hemorrhages.[7] Another mechanism for development of atherosclerosis is mediated through expression of heat shock proteins (HSP) on the endothelium. HSPs are normally located in the endothelium, and have a protective function. Anti-HSP60 antibodies induce endothelial damage, and smooth muscle proliferation. C. pneumonia infection leads to production of antibodies as bacterial HSP has a sequence homology with human HSP. Thus, molecular mimicry between human HSP60 and bacterial 60 k-Da HSP contributes to atherosclerosis.[8]

Figure 1
Postulated inflammatory pathways and role of Chlamydophila pneumoniae in causation of atherosclerosis

Studies in animal models have isolated C. pneumoniae from coronary,[911] carotid,[12,13] and peripheral arteries.[14] These isolation studies triggered a debate if association of C. pneumoniae infection and atherosclerosis is causal, or if the infectious agent is merely an innocent bystander.[1517] In a landmark study, Hu and coworkers[18] demonstrated that Chlamydophila infection could induce atherogenesis in low-density lipoprotein (LDL)-knockout mice, only in the presence of a high-cholesterol diet. This experiment further lent credence to the hypothesis that infection and hypercholesterolemia are essential causal components leading to atherogenesis. The current article reviews Chlamydophila atherosclerosis literature with reference to causal significance of this association and traces investigator interest in this hypothesis from the last two decades.

Materials and Methods

We performed a PubMed search to identify studies about the role of C. pneumoniae in atherosclerosis. We used a Boolean query [(C. pneumoniae) and (atherosclerosis OR coronary OR stroke OR peripheral vascular disease OR cerebral OR hypertension OR diabetes)] and identified a total of 1,668 studies. We classified published literature by year of publication and described three distinct study designs: (A) Seroepidemiological studies; (B) Molecular studies; and (C) Clinical trials. We excluded animal and cell culture studies and have elaborated human studies alone. We included serology studies, which had used either a case-control study design at a defined cross section in time, or studies nested in well-defined community cohorts. Molecular studies included those that demonstrated C. pneumoniae DNA in circulating mononuclear cells or in vascular tissue using electron microscopy, molecular diagnostics, immunochemistry, or cell line culture techniques. Clinical trials included in the description were randomized control trials, which aimed at evaluating reduction in mortality or cardiovascular events with antibiotic therapy in patients with early atherosclerotic disease.

Results

The chronologic distribution of studies evaluating Chlamydophila-atherosclerosis association shows a peak in the year 2000, and a gradual decline in number of publications since the year 2003 [Figure 2]. Published literature has three overlapping periods, with most serology studies being published in the first decade (1991-2001) followed by accumulation of molecular evidence. Between 2003 and 2006, negative results in large clinical trials heralded a gradual decline in research publications pursuing this hypothesis.[19] Subsequently, molecular studies have continued to demonstrate presence of Chlamydophila antigens in atheromatous tissue, and researchers have argued that failed antibiotic trials do not mean that the hypothesis is refuted. The seroepidemiological, molecular, and clinical trial evidence for and against this hypothesis is detailed below.

Figure 2
Number of PubMed indexed articles (n=1668) by year of publication on Chlamydophila pneumoniae infection and atherosclerosis

Seroepidemiological studies

Various seroepidemiological studies tested the hypothesis of association between exposure to C. pneumoniae and cardiovascular outcomes. Two broad study designs have tested this hypothesis. First, case-control studies with a cross-sectional design are hospital- or community-based studies, where exposure is determined in cases, after the outcome had already taken place. Second, case-control studies nested in well-defined community cohorts or prospective case-control studies, where exposure is determined from serum samples, which had been stored prior to occurrence of the outcome. These later studies characterize temporal relationship between exposure and outcome.

Case-control studies: Cross-sectional design

Of a total of 47 studies identified, 44 were included in two systematic reviews[20,21] [Table 1], and a majority of them (23 out of 44) reported an unequivocal positive association. Another 16 studies reported a non-significant positive outcome [odds ratio (OR) for C. pneumoniae seropositivity was greater than 1.0, but the 95% confidence interval (CI) crossed 1, while remaining five studies reported negative point estimates (with OR of less than 1.0)]. The range of point estimates was wide (0.7-17.0), representing a considerable heterogeneity in the results. The results of 29 studies included in the review by Bloemankamp et al.[20] were pooled using the random effects model, and the weighted pooled OR was 2.0 (95% CI 1.5-2.6).

Table 1
Case-control studies that evaluated the relationship between exposure to Chlamydophila pneumoniae seropositivity and cardiovascular risk in a cross-sectional manner

Most of the earlier studies had evaluated immunoglobulin (Ig) G antibodies, which signify past infection. Subsequently, IgA antibodies were shown to be better associated with the presence of C. pneumoniae in the atheromatous tissue, signifying a persistent infection. Different studies showed a higher prevalence of IgA antibodies among patients with established athermanous disease as compared to healthy controls.[2224] The point estimates risk ratios in these studies were higher and confidence intervals significant even after adjusting for confounders.[2529]

Case-control studies: Prospective design

These cohorts from which cases and controls were sampled had been followed up for 3-15 years [Table 2]. Eighteen studies with this design were summarized in the two systematic reviews,[20,21] and only two of them reported a positive association of C. pneumoniae seropositivity with cardiovascular events [Osserwade et al. (OR 2.8 95% CI 1.3-5.8)[30] and Roivainen et al. (1.7 95% CI 1.2-2.5)[31]]. The point estimates were equivocal in nine studies, and negative in seven others. The weighted pooled odds of 15 of these studies reviewed by Bloemankamp et al.[20] was 1.1 (95% CI 0.8-1.4). In a meta-regression of these studies, the regression coefficient was estimated to be -0.04 (-0.08, -0.01) for every additional year of follow-up of the cohort, implying that for every additional one year of follow-up, the point estimate in these studies was lowered by 0.04. The accuracy of serology for detection of Chlamydophila has also been variable across studies, and at different cut-offs [Table 3].

Table 2
Prospective case-control studies that evaluated the relationship between Chlamydophila pneumoniae seropositivity and cardiovascular risk
Table 3
Accuracy of serology with presence of Chlamydophila pneumoniae infection

Molecular studies

Circulating C. pneumoniae DNA

It has been hypothesized that PBMC positive for C. pneumoniae DNA detected by polymerase chain reaction (PCR) techniques[32] could be considered as a surrogate marker for chronic C. pneumoniae infection. Case only studies (which have reported the prevalence of C. pneumoniae–positive DNA in different population subgroups) and case-control studies (where cases include patients with atherosclerosis-associated cardiovascular conditions and healthy controls) have been used to determine risk estimates. Smeija et al.[32] published a systematic review of these studies. A few[13,3337] additional studies have appeared since the publication of this review. The prevalence of C. pneumoniae DNA in PBMC ranges from 2.5% to 46.2% in healthy blood donors and other healthy individuals who were studied in prevalence studies (in one study, all medical student controls were negative). The pooled odds of presence of C. pneumoniae DNA in PBMC as a risk factor for cardiovascular events was 2.03 (95% CI 1.34-3.08).[32]

Identification of C. pneumoniae in vascular tissue

More than 40 studies have used various techniques (immunocytochemistry techniques were used to detect the presence of specific antibodies or antigens in tissue aspirates, PCR, and in situ hybridization to demonstrate the presence of C. pneumoniae DNA; electron microscopy was used to demonstrate the intact bacterium and cell culture to isolate viable Chlamydophila organisms[4]) to determine the presence of C. pneumoniae in vascular tissues. The results of these studies have been compiled by Boman et al.,[4] and in the 2679 specimens analyzed by all authors by different techniques, immunocytochemistry (676 specimens), electron microscopy (97 specimens), and PCR (2294 specimens) showed positive results in 49.7%, 39.1%, and 24.3% cases, respectively. There was a wide heterogeneity in the results across studies [Table 4], and only 7.3% of the 451 specimens were culture positive.

Table 4
Detection of Chlamydophila pneumonia in atheromatous tissue specimens

Mechanism of C. pneumoniae–induced atherosclerosis

It has been hypothesized that respiratory infections by C. pneumoniae may result in hematogenous spread through PBMC. The organism has effects on endothelium and vascular smooth muscle cells, mediated through cytokines [interleukin (IL)-1, IL6, IL8, tumor necrosis factor (TNF)-α, interferon (IFN)-γ, platelet-derived growth factor (PDGF), HSP60, etc) resulting in upregulation of inflammation,[38] endothelial apoptosis,[39] and vascular smooth muscle proliferation. These events are likely precursors of atheroma formation. In the last five years, additional signaling mechanisms [mediated through Interferon regulatory factors 3 and 7, toll-like receptor (TLR)-2/4, IL-8, Intercellular Adhesion Molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1, extracellular signal-regulated kinase (ERK)-1/2, nuclear factor-kappa B (NF-kB), IL-23, IL-6, IL-1beta, transforming growth factor (TGF)-beta, and chemokine (C-C motif) ligand (CCL-20)][4042] have been postulated to have a role in the initiation and progression of atheromatous lesions. C. pneumoniae also has a role in lipid accumulation in the vessel wall by upregulating lecithin-like oxidized LDL receptors (LOX-1) in both endothelial and vascular smooth muscle cells.[43,44] In the same period, four studies[4548] did not find convincing evidence of presence of Chlamydophila genome in the atheromatous tissue, and argued in favor of alternative mechanisms. However, it is likely that C. pneumoniae infection may only be providing an initial trigger and is transient rather than persistent. The mechanism mediated through molecular mimicry between human and bacterial HSP60 and production of anti-HSP antibodies is attractive, as it strengthens ‘hit-and-run’ hypothesis for the organism.

Clinical trials

Various Clinical trials evaluated whether eradication of C. pneumoniae is beneficial in the secondary prevention of cardiovascular events. The evidence that such a therapy could be useful had come from animal studies, where administration of high-dose azithromycin for 10 weeks was associated with a reduction in intimal thickening.

Initial studies reported between 1997 and 2001 (Gupta et al., ACADEMIC, ROXIS, CLARIFY, and Leowattana et al.) had fewer subjects, shorter duration of antibiotic therapy, and shorter durations of follow-up. Subsequently, larger trials were launched (WIZARD, ACES, ANTIBIO, AZACS, and PROVE-IT) where antibiotics were administered for 3 months to 2 years, and duration of follow-up was 1-4 years [Table 5]. These later trials had greater statistical power to detect smaller differences between the intervention and non-intervention arms. In a meta-analysis summarizing the evidence from 19,217 patients in 11 randomized controlled trials, the point estimates for mortality reduction and reduction in secondary cardiovascular event were 1.02 (95% CI 0.89-1.16) and 0.92 (95% CI 0.81-1.04), respectively.[49]

Table 5
Randomized control trials that studied the impact of antibiotic therapy on subsequent cardiovascular events

Discussion

There is an ongoing debate whether the association between C. pneumoniae infection and cardiovascular outcomes is causal or the organism is merely an innocent bystander. The main arguments supporting a causal role are: biological plausibility and a consistent finding that atherosclerosis associated with vascular inflammation, is inducible by C. pneumoniae in laboratory experiments. The arguments against causality are poor association between seropositivity and cardiovascular outcomes, lack of consistency in demonstration of C. pneumoniae in vascular tissue, and failed attempts to show benefits of eradication of the organism. It, however, may be argued that we may be constrained by traditional Koch's postulates in an attempt to prove causality. In case of C. pneumonia, it is likely that the organism is not a singular causal factor in atherogenesis or its progression, its presence transient after an initial trigger, and eradication a failed aim because the persistence of the organism in atheromatous tissue and penetration of drugs are both questionable.

Seroepidemiological studies are useful in framing initial hypothesis, but these also have a major limitation. An important assumption in seroepidemiological studies is that the presence of anti-C. pneumoniae antibodies is a surrogate measure of chronic C. pneumoniae infection. This assumption may not hold true as there is a poor correlation between serology and detection of C. pneumoniae in vascular tissues [Table 3]. At low titers, serology had a poor specificity, and the number of false-positive results was high. At high titers, serology had a poor sensitivity but a high specificity.[4] Anti-Chlamydophila antibodies are measured using microimmunofluoroscence (MIF) technique, which needs an experienced microscopist to interpret the results.[4] Different studies have used in-house tests, using a variable cut-off to define positivity, and this approach is prone for misclassification error.[50] Owing to these limitations, there is a need for a better serological test to define chronic Chlamydophila infection.[15,51]

Infection due to Chlamydophila species is common, has seasonal variations, and different species exhibit antigenic cross-reactivity.[51] This leads to a high background prevalence of seropositivity as well as presence of C. pneumoniae DNA in PBMC. This diminishes the strength of association between evidence of past infection and atherosclerotic diseases.[52] Further, age, smoking status, and socioeconomic status are potential confounders in the relationship between exposure to C. pneumoniae and atherosclerosis. Systematic reviews[20,21] have revealed that studies that had adequately adjusted for confounders had a lower point estimate as compared to studies where adjustment was inadequate [1.1 (0.8-1.7) vs. 1.9 (1.2-3.0)]. In a meta-regression,[20] the regression coefficient for the degree of adjustment was -0.10 (-0.23, 0.02), implying that for every additional degree of adjustment, the point estimate is lowered by 0.1.

Despite reasonable molecular and serologic evidence, antibiotic trials failed to improve clinical outcomes. While this is a strong evidence against the Chlamydophila-atherosclerosis hypothesis, and may indicate an absence of the organism from either circulation or atheromatous plaques. The lack of protective effect in these antibiotic trials has been a major setback for the C. pneumoniae-atherosclerosis hypothesis, and calls for a reappraisal of its pathological mechanisms. Four main arguments are proposed to counter these negative results. First, the organism is difficult to eradicate and refractory to current anti-Chlamydophila antibiotics. Second, most patients in antibiotic trials had advanced atheromatous lesions that had already reached an irreversible stage. Third, the bacterium might be acting by a hit-and-run mechanism, in which case secondary prevention strategies are unlikely to be beneficial. Finally, the presence of other causal factors, such as hypercholesterolemia may be essential for the organism to induce atherosclerosis, and hence this mechanism may operate in specific subpopulations. Researchers argue that negative antibiotic trials should not put a premature end to C. pneumoniae–atherosclerosis hypothesis,[7,19] rather these should stimulate research into newer treatment strategies targeting Chlamydophila-specific proteins and machinery directly involved in their survival, replication, and maintenance.[53]

Infectious disease etiology for atherosclerosis is an attractive hypothesis, as it can cause a paradigm shift in preventive strategies. Current etiologies have led to primary and secondary prevention strategies targeting multiple risk factors, and reinforcing positive and negative behaviors is an intensive life-long process.[107,108] On the contrary, a single infectious etiology can stimulate research into vaccine development, and has the potential for better prevention and cure. In recent years, same paradigm shift has been adopted for cervical cancers. However, with multiple-candidate organisms, the most promising of these failing trials, there has never been a setback. Newer approaches to understand its pathogenesis and identifying a successful clinical application must continue.

Footnotes

Source of Support: Nil.

Conflict of Interest: None declared.

References

1. Murray JL, Lopez AD. Global mortality, disability, and contribution of risk factors: Global burden of disease study. Lancet. 1997;349:1436–42. [PubMed]
2. Yusuf S, Hawken S, Ounpuu S, Dans T, Avezum A, Lanas F, et al. Effect of potentially modifiable risk factors associated with myocardial infarction in 52 countries (the INTERHEART study): Case-control study. Lancet. 2004;364:937–52. [PubMed]
3. Broxmeyer L. Heart disease: The greatest ‘risk’ factor of them all. Med Hypotheses. 2004;62:773–9. [PubMed]
4. Boman J, Hammerschlag MR. Chlamydia pneumoniae and atherosclerosis: Critical assessment of diagnostic methods and relevance to treatment studies. Clin Microbiol Rev. 2002;15:1–20. [PMC free article] [PubMed]
5. Grayston JT. Background and current knowledge of Chlamydia pneumoniae and atherosclerosis. J Infect Dis. 2000;181(Suppl 3):S402–10. [PubMed]
6. Mahony JB, Coombes BK. Chlamydia pneumoniae and atherosclerosis: Does the evidence support a causal or contributory role? FEMS Microbiol Lett. 2001;197:1–9. [PubMed]
7. Rosenfeld ME, Campbell LA. Pathogens and atherosclerosis: update on the potential contribution of multiple infectious organisms to the pathogenesis of atherosclerosis. Thromb Haemost. 2011;106:858–67. [PubMed]
8. Erkkila L, Laitinen K, Haasio K, Tiirola T, Jauhiainen M, Lehr HA, et al. Heat shock protein 60 autoimmunity and early lipid lesions in cholesterol-fed C57BL/6JBom mice during Chlamydia pneumoniae infection. Atherosclerosis. 2004;177:321–8. [PubMed]
9. Ramirez JA. Isolation of Chlamydia pneumoniae from the coronary artery of a patient with coronary atherosclerosis. The Chlamydia pneumoniae/Atherosclerosis Study Group. Ann Intern Med. 1996;125:979–82. [PubMed]
10. Apfalter P, Loidl M, Nadrchal R, Makristathis A, Rotter M, Bergmann M, et al. Isolation and continuous growth of Chlamydia pneumoniae from arterectomy specimens. Eur J Clin Microbiol Infect Dis. 2000;19:305–8. [PubMed]
11. Jackson LA, Campbell LA, Kuo CC, Rodriguez DI, Lee A, Grayston JT. Isolation of Chlamydia pneumoniae from a carotid endarterectomy specimen. J Infect Dis. 1997;176:292–5. [PubMed]
12. Kuo CC, Gown AM, Benditt EP, Grayston JT. Detection of Chlamydia pneumoniae in aortic lesions of atherosclerosis by immunocytochemical stain. Arterioscler Thromb. 1993;13:1501–4. [PubMed]
13. Sessa R, Di Pietro M, Schiavoni G, Galdiero M, Cipriani P, Romano S, et al. Chlamydia pneumoniae in asymptomatic carotid atherosclerosis. Int J Immunopathol Pharmacol. 2006;19:111–8. [PubMed]
14. Kuo CC, Coulson AS, Campbell LA, Cappuccio AL, Lawrence RD, Wang SP, et al. Detection of Chlamydia pneumoniae in atherosclerotic plaques in the walls of arteries of lower extremities from patients undergoing bypass operation for arterial obstruction. J Vasc Surg. 1997;26:29–31. [PubMed]
15. Ieven MM, Hoymans VY. Involvement of Chlamydia pneumoniae in atherosclerosis: More evidence for lack of evidence. J Clin Microbiol. 2005;43:19–24. [PMC free article] [PubMed]
16. Liu C, Waters DD. Chlamydia pneumoniae and atherosclerosis: from Koch postulates to clinical trials. Prog Cardiovasc Dis. 2005;47:230–9. [PubMed]
17. Shor A, Phillips JI. Chlamydia pneumoniae and atherosclerosis. JAMA. 1999;282:2071–3. [PubMed]
18. Hu H, Pierce GN, Zhong G. The atherogenic effects of chlamydia are dependent on serum cholesterol and specific to Chlamydia pneumoniae. J Clin Invest. 1999;103:747–53. [PMC free article] [PubMed]
19. Epstein SE, Zhu J, Najafi AH, Burnett MH. Insights Into the Role of Infection in Atherogenesis and in Plaque Rupture. Circulation. 2009;119:3133–41. [PubMed]
20. Bloemenkamp DG, Mali WP, Visseren FL, van der Graaf Y. Meta-analysis of sero-epidemiologic studies of the relation between Chlamydia pneumoniae and atherosclerosis: Does study design influence results? Am Heart J. 2003;145:409–17. [PubMed]
21. Danesh J, Collins R, Peto R. Chronic infections and coronary heart disease: is there a link? Lancet. 1997;350:430. [PubMed]
22. Toss H, Gnarpe J, Gnarpe H, Siegbahn A, Lindahl B, Wallentin L. Increased fibrinogen levels are associated with persistent Chlamydia pneumoniae infection in unstable coronary artery disease. Eur Heart J. 1998;19:570–7. [PubMed]
23. Korner I, Blatz R, Wittig I, Pfeiffer D, Ruhlmann C. Serological evidence of Chlamydia pneumoniae lipopolysaccharide antibodies in atherosclerosis of various vascular regions. Vasa. 1999;28:259–63. [PubMed]
24. Gnarpe J, Sparr A, Naas J, Lundback A. Serological analysis of specific IgA to Chlamydia pneumoniae: Increased sensitivity of IgA antibody detection using prolonged incubation and high antigen concentration. APMIS. 2000;108:357–62. [PubMed]
25. Strachan DP, Carrington D, Mendall MA, Ballam L, Morris J, Butland BK, et al. Relation of Chlamydia pneumoniae serology to mortality and incidence of ischaemic heart disease over 13 years in the caerphilly prospective heart disease study. BMJ. 1999;318:1035–9. [PMC free article] [PubMed]
26. Shimada K, Daida H, Mokuno H, Watanabe Y, Sawano M, Iwama Y, et al. Association of seropositivity for antibody to Chlamydia-specific lipopolysaccharide and coronary artery disease in Japanese men. Jpn Circ J. 2001;65:182–7. [PubMed]
27. Kinjo K, Sato H, Ohnishi Y, Hishida E, Nakatani D, Mizuno H, et al. Joint effects of Chlamydia pneumoniae infection and classic coronary risk factors on risk of acute myocardial infarction. Am Heart J. 2003;146:324–30. [PubMed]
28. Voorend M, Faber CG, van der Ven AJ, Kessels F, Bruggeman CA, Lodder J. Chlamydia pneumoniae is a likely risk factor for ischemic stroke in young patients. J Stroke Cerebrovasc Dis. 2004;13:85–91. [PubMed]
29. Piechowski-Jozwiak B, Mickielewicz A, Gaciong Z, Berent H, Kwiecinski H. Elevated levels of anti-Chlamydia pneumoniae IgA and IgG antibodies in young adults with ischemic stroke. Acta Neurol Scand. 2007;116:144–9. [PubMed]
30. Ossewaarde JM, Feskens EJ, De Vries A, Vallinga CE, Kromhout D. Chlamydia pneumoniae is a risk factor for coronary heart disease in symptom-free elderly men, but Helicobacter pylori and cytomegalovirus are not. Epidemiol Infect. 1998;120:93–9. [PubMed]
31. Roivainen M, Viik-Kajander M, Palosuo T, Toivaen P, Leionen M, Saikku P, et al. Infections, inflammation and the risk of Coronary heart disease. Circulation. 2000;101:252–7. [PubMed]
32. Smieja M, Mahony J, Petrich A, Boman J, Chernesky M. Association of circulating Chamydia pneumonia DNA with cardiovascular disease A systematic review. BMC Infect Dis. 2003;2:21. [PMC free article] [PubMed]
33. Podsiadly E, Przyluski J, Kwiatkowski A, Kruk M, Wszola M, Nosek R, et al. Presence of Chlamydia pneumoniae in patients with and without atherosclerosis. Eur J Clin Microbiol Infect Dis. 2005;24:507–13. [PubMed]
34. Mitusch R, Luedemann J, Wood WG, Berger K, Schminke U, Suter M, et al. Asymptomatic carotid atherosclerosis is associated with circulating chlamydia pneumoniae DNA in younger normotensive subjects in a general population survey. Arterioscler Thromb Vasc Biol. 2005;25:386–91. [PubMed]
35. Tsirpanlis G, Chatzipanagiotou S, Ioannidis A, Moutafis S, Poulopoulou C, Nicolaou C. Detection of Chlamydia pneumoniae in peripheral blood mononuclear cells: Correlation with inflammation and atherosclerosis in haemodialysis patients. Nephrol Dial Transplant. 2003;18:918–23. [PubMed]
36. Wang SS, Tondella ML, Bajpai A, Mathew AG, Mehranpour P, Li W, et al. Circulating Chlamydia pneumoniae DNA and advanced coronary artery disease. Int J Cardiol. 2007;118:215–9. [PubMed]
37. Yamaguchi H, Yamada M, Uruma T, Kanamori M, Goto H, Yamamoto Y, et al. Prevalence of viable Chlamydia pneumoniae in peripheral blood mononuclear cells of healthy blood donors. Transfusion. 2004;44:1072–8. [PubMed]
38. Atik B, Johnston SC, Dean D. Association of carotid plaque Lp-PLA (2) with macrophages and Chlamydia pneumoniae infection among patients at risk for stroke. PLoS One. 2010;5:e11026. [PMC free article] [PubMed]
39. Lysenko AI, Solov’eva NA. The implication of Chlamydia pneumoniae in damage to human aortic endotheliocytes in atherosclerosis. Arkh Patol. 2010;72:21–5. [PubMed]
40. Benagiano M, Munari F, Ciervo A, Amedei A, Paccani SR, Mancini F, et al. Chlamydophila pneumoniae phospholipase D (CpPLD) drives Th17 inflammation in human atherosclerosis. Proc Natl Acad Sci U S A. 2012;109:1222–7. [PubMed]
41. Jha HC, Srivastava P, Prasad J, Mittal A. Chlamydia pneumoniae heat shock protein 60 enhances expression of ERK, TLR-4 and IL-8 in atheromatous plaques of coronary artery disease patients. Immunol Invest. 2011;40:206–22. [PubMed]
42. Buss C, Opitz B, Hocke AC, Lippmann J, van Laak V, Hippenstiel S, et al. Essential role of mitochondrial antiviral signaling, IFN regulatory factor (IRF) 3, and IRF7 in Chlamydophila pneumoniae-mediated IFN-beta response and control of bacterial replication in human endothelial cells. J Immunol. 2010;184:3072–8. [PubMed]
43. Prochnau D, Rodel J, Prager K, Kuersten D, Heller R, Straube E, et al. Induced expression of lectin-like oxidized ldl receptor-1 in vascular smooth muscle cells following Chlamydia pneumoniae infection and its down-regulation by fluvastatin. Acta Microbiol Immunol Hung. 2010;57:147–55. [PubMed]
44. Yoshida T, Koide N, Mori I, Ito H, Yokochi T. Chlamydia pneumoniae infection enhances lectin-like oxidized low-density lipoprotein receptor (LOX-1) expression on human endothelial cells. FEMS Microbiol Lett. 2006;260:17–22. [PubMed]
45. Bennermo M, Nordin M, Lundman P, Boqvist S, Held C, Samnegard A, et al. Genetic and environmental influences on the plasma interleukin-6 concentration in patients with a recent myocardial infarction: A case-control study. J Interferon Cytokine Res. 2011;31:259–64. [PubMed]
46. Brykczynski M, Zych A, Goracy I, Maczynska I, Wojciechowska-Koszko I, Mokrzycki K, et al. Evaluation of the level of antibodies against Chlamydophila (Chlamydia) pneumoniae in post-surgery heart ischaemia patients and their clinical conditions-a six-year study. Arch Med Sci. 2010;6(2):214–20. [PMC free article] [PubMed]
47. Reverter JL, Tassies D, Alonso N, Pellitero S, Sanmarti A, Reverter JC. Non-detectable Chlamydophila pneumoniae DNA in peripheral leukocytes in type 2 diabetes mellitus patients with and without carotid atherosclerosis. Med Clin (Barc) 2012;138:11–4. [PubMed]
48. Tremolada S, Delbue S, Ferraresso M, Carloni C, Elia F, Larocca S, et al. Search for genomic sequences of microbial agents in atherosclerotic plaques. Int J Immunopathol Pharmacol. 2011;24:243–6. [PubMed]
49. Andraws R, Berger JS, Brown DL. Effects of antibiotic therapy on outcomes of patients with coronary artery disease: a meta-analysis of randomized controlled trials. JAMA. 2005;293:2641–7. [PubMed]
50. Dowell SF, Peeling RW, Boman J, Carlone GM, Fields BS, Guarner MR, et al. Standardizing Chlamydia pneumoniae assays: reccomendations from the Centers for Disease Control and prevention (USA) and Laboratory center for disease control (Canada) Clin Infect Dis. 2001;33:492–503. [PubMed]
51. Apfalter P. Chlamydia pneumoniae, stroke, and serological associations: Anything learned from the atherosclerosis-cardiovascular literature or do we have to start over again? Stroke. 2006;37:756–8. [PubMed]
52. Rothman KJ, Poole C. A strengthening program for weak associations. Int J Epidemiol. 1988;17:955. [PubMed]
53. Deniset JF, Pierce GN. Possibilities for therapeutic interventions in disrupting Chlamydophila pneumoniae involvement in atherosclerosis. Fundam Clin Pharmacol. 2010;24:607–17. [PubMed]
54. Blasi F, Cosentini R, Raccanelli R, Massari FM, Arosio C, Tarsia P, et al. A possible association of Chlamydia pneumoniae infection and acute myocardial infarction in patients younger than 65 years of age. Chest. 1997;112:309–12. [PubMed]
55. Thomas GN, Scheel O, Koehler AP, Bassett DC, Cheng AF. Respiratory Chlamydial infections in a Hong Kong teaching hospital and association with coronary heart disease. Scand J Infect Dis Suppl. 1997;104:30–3. [PubMed]
56. Gabriel AS, Gnarpe H, Gnarpe J, Hallander H, Nyquist O, Martinsson A. The prevalence of chronic Chlamydia pneumoniae infection as detected by polymerase chain reaction in pharyngeal samples from patients with ischaemic heart disease. Eur Heart J. 1998;19:1321–7. [PubMed]
57. Boman J. High prevalence of Chlamydia Pneumoniae DNA in peripheral blood mononuclear cells in patients with cardiovascular disease and in patients with middle aged blood donors. J Infect Dis. 1998;178:274–7. [PubMed]
58. Mazzoli S, Tofani N, Fantini A, Semplici F, Bandini F, Salvi A, et al. Chlamydia pneumoniae antibody response in patients with acute myocardial infarction and their follow-up. Am Heart J. 1998;135:15–20. [PubMed]
59. Cook PJ, Honeybourne D, Lip GY, Beevers DG, Wise R, Davies P. Chlamydia pneumoniae antibody titers are significantly associated with acute stroke and transient cerebral ischemia: The West Birmingham Stroke Project. Stroke. 1998;29:404–10. [PubMed]
60. Anderson JL, Carlquist JF, Muhlestein JB, Horne BD, Elmer SP. Evaluation of C-reactive protein, an inflammatory marker, and infectious serology as risk factors for coronary artery disease and myocardial infarction. J Am Coll Cardiol. 1998;32:35–41. [PubMed]
61. Miyashita N, Toyota E, Sawayama T, Matsumoto A, Mikami Y, Kawai N, et al. Association of chronic infection of Chlamydia pneumoniae and coronary heart disease in the Japanese. Intern Med. 1998;37:913–6. [PubMed]
62. Cellesi C, Sansoni A, Casini S, Migliorini L, Zacchini F, Gasparini R, et al. Chlamydia pneumoniae antibodies and angiographically demonstrated coronary artery disease in a sample population from Italy. Atherosclerosis. 1999;145:81–5. [PubMed]
63. Shimada K, Mokuno H, Watanabe Y, Sawano M, Sato H, Kurata T, et al. Association between chlamydial infection and coronary artery disease. J Cardiol. 1999;34:259–65. [PubMed]
64. Sessa R, Di Pietro M, Santino I, del Piano M, Varveri A, Dagianti A, et al. Chlamydia pneumoniae infection and atherosclerotic coronary disease. Am Heart J. 1999;137:1116–9. [PubMed]
65. Abdelmouttaleb I, Danchin N, Ilardo C, Aimone-Gastin I, Angioi M, Lozniewski A, et al. C-Reactive protein and coronary artery disease: Additional evidence of the implication of an inflammatory process in acute coronary syndromes. Am Heart J. 1999;137:346–51. [PubMed]
66. Altman R, Rouvier J, Scazziota A, Absi RS, Gonzalez C. Lack of association between prior infection with Chlamydia pneumoniae and acute or chronic coronary artery disease. Clin Cardiol. 1999;22:85–90. [PubMed]
67. Leowattana W, Mahanonda N, Bhuripunyo K, Leelarasamee A, Pokum S, Suwimol B. The prevalence of Chlamydia pneumoniae antibodies in Thai patients with coronary artery disease. J Med Assoc Thai. 1999;82:792–7. [PubMed]
68. Kaykov E, Abbou B, Friedstrom S, Hermoni D, Roguin N. Chlamydia preumoniae in ischemic heart disease. Isr Med Assoc J. 1999;1:225–7. [PubMed]
69. Nobel M, De Torrente A, Peter O, Genne D. No serological evidence of association between chlamydia pneumonia infection and acute coronary heart disease. Scand J Infect Dis. 1999;31:261–4. [PubMed]
70. Kontula K, Vuorio A, Turtola H, Saikku P. Association of seropositivity for Chlamydia pneumoniae and coronary artery disease in heterozygous familial hypercholesterolaemia. Lancet. 1999;354:46–7. [PubMed]
71. Elkind MS, Lin IF, Grayston JT, Sacco RL. Chlamydia pneumoniae and the risk of first ischemic stroke: The Northern Manhattan Stroke Study. Stroke. 2000;31:1521–5. [PubMed]
72. Romeo F, Martuscelli E, Chirieolo G, Cerabino LM, Ericson K, Saldeen TG, et al. Seropositivity against Chlamydia pneumoniae in patients with coronary atherosclerosis. Clin Cardiol. 2000;23:327–30. [PubMed]
73. Glader CA, Boman J, Saikku P, Stenlund H, Weinehall L, Hallmanns G, et al. The proatherogenic properties of lipoprotein (a) may be enhanced through the formation of circulating immune complexes containing Chlamydia pneumoniae-specific IgG antibodies. Eur Heart J. 2000;21:639–46. [PubMed]
74. Hoffmeister A, Rothenbacher D, Wanner P, Bode G, Persson K, Brenner H, et al. Seropositivity to chlamydial lipopolysaccharide and Chlamydia pneumoniae, systemic inflammation and stable coronary artery disease: Negative results of a case-control study. J Am Coll Cardiol. 2000;35:112–8. [PubMed]
75. Ammann P, Marschall S, Kraus M, Schmid L, Angehrn W, Krapf R, et al. Characteristics and prognosis of myocardial infarction in patients with normal coronary arteries. Chest. 2000;117:333–8. [PubMed]
76. Jantos CA, Krombach C, Wuppermann FN, Gardemann A, Bepler S, Asslan H, et al. Antibody response to the 60-kDa heat-shock protein of Chlamydia pneumoniae in patients with coronary artery disease. J Infect Dis. 2000;181:1700–5. [PubMed]
77. Mendis S, Arseculeratne YM, Withana N, Samitha S. Chlamydia pneumoniae infection and its association with coronary heart disease and cardiovascular risk factors in a sample South Asian population. Int J Cardiol. 2001;79:191–6. [PubMed]
78. Bloemenkamp DG, Mali WP, Tanis BC, Rosendaal FR, van den Bosch MA, Kemmeren JM, et al. Chlamydia pneumoniae, Helicobacter pylori and cytomegalovirus infections and the risk of peripheral arterial disease in young women. Atherosclerosis. 2002;163:149–56. [PubMed]
79. Hasan ZN. Association of Chlamydia pneumoniae serology and ischemic stroke. South Med J. 2011;104:319–21. [PubMed]
80. Rai NK, Choudhary R, Bhatia R, Singh MB, Tripathi M, Prasad K, et al. Chlamydia pneumoniae seropositivity in adults with acute ischemic stroke: A case-control study. Ann Indian Acad Neurol. 2011;14:93–7. [PMC free article] [PubMed]
81. Miettinen H, Lehto S, Saikku P, Haffner SM, Ronnemaa T, Pyorala K, et al. Association of Chlamydia pneumoniae and acute coronary heart disease events in non-insulin dependent diabetic and non-diabetic subjects in Finland. Eur Heart J. 1996;17:682–8. [PubMed]
82. Glader CA, Stegmayr B, Boman J, Stenlund H, Weinehall L, Hallmans G, et al. Chlamydia pneumoniae antibodies and high lipoprotein (a) levels do not predict ischemic cerebral infarctions. Results from a nested case-control study in Northern Sweden. Stroke. 1999;30:2013–8. [PubMed]
83. Nieto FJ, Folsom AR, Sorlie PD, Grayston JT, Wang SP, Chambless LE. Chlamydia pneumoniae infection and incident coronary heart disease: the Atherosclerosis Risk in Communities Study. Am J Epidemiol. 1999;150:149–56. [PubMed]
84. Fagerberg B, Gnarpe J, Gnarpe H, Agewall S, Wikstrand J. Chlamydia pneumoniae but not cytomegalovirus antibodies are associated with future risk of stroke and cardiovascular disease: A prospective study in middle-aged to elderly men with treated hypertension. Stroke. 1999;30:299–305. [PubMed]
85. Danesh J, Whincup P, Walker M, Lennon L, Thomson A, Appleby P, et al. Chlamydia pneumoniae IgG titres and coronary heart disease: prospective study and meta-analysis. BMJ. 2000;321:208–13. [PMC free article] [PubMed]
86. Wald NJ, Law MR, Morris JK, Zhou X, Wong Y, Ward ME. Chlamydia pneumoniae infection and mortality from ischaemic heart disease: large prospective study. BMJ. 2000;321:204–7. [PMC free article] [PubMed]
87. Siscovick DS, Schwartz SM, Corey L, Grayston JT, Ashley R, Wang SP, et al. Chlamydia pneumoniae, herpes simplex virus type 1, and cytomegalovirus and incident myocardial infarction and coronary heart disease death in older adults: the Cardiovascular Health Study. Circulation. 2000;102:2335–40. [PubMed]
88. Roivainen M, Viik-Kajander M, Palosuo T, Toivanen P, Leinonen M, Saikku P, et al. Infections, inflammation, and the risk of coronary heart disease. Circulation. 2000;101:252–7. [PubMed]
89. Johnsen SP, Overvad K, Ostergaard L, Tjonneland A, Husted SE, Sorensen HT. Chlamydia pneumoniae seropositivity and risk of ischemic stroke: A nested case-control study. Eur J Epidemiol. 2005;20:59–65. [PubMed]
90. Volanen I, Jarvisalo MJ, Vainionpaa R, Arffman M, Kallio K, Angle S, et al. Increased aortic intima-media thickness in 11-year-old healthy children with persistent Chlamydia pneumoniae seropositivity. Arterioscler Thromb Vasc Biol. 2006;26:649–55. [PubMed]
91. Sakurai-Komada N, Koike KA, Kaku Y, Hiraki M, Cui R, Sankai T, et al. Chlamydia pneumoniae infection was associated with risk of mortality from coronary heart disease in Japanese women but not men: The JACC Study. J Atheroscler Thromb. 2010;17:510–6. [PubMed]
92. Maass M, Bartels C, Kruger S, Krause E, Engel PM, Dalhoff K. Endovascular presence of Chlamydia pneumoniae DNA is a generalized phenomenon in atherosclerotic vascular disease. Atherosclerosis. 1998;140(Suppl 1):S25–30. [PubMed]
93. Kaul R, Uphoff J, Wiedeman J, Yadlapalli S, Wenman WM. Detection of Chlamydia pneumoniae DNA in CD3+lymphocytes from healthy blood donors and patients with coronary artery disease. Circulation. 2000;102:2341–6. [PubMed]
94. Berger M, Schroder B, Daeschlein G, Schneider W, Busjahn A, Buchwalow I, et al. Chlamydia pneumoniae DNA in non-coronary atherosclerotic plaques and circulating leukocytes. J Lab Clin Med. 2000;136:194–200. [PubMed]
95. Gupta S, Camm AJ. Chronic infection in the etiology of atherosclerosis--the case for Chlamydia pneumoniae. Clin Cardiol. 1997;20:829–36. [PubMed]
96. Muhlestein JB, Anderson JL, Carlquist JF, Salunkhe K, Horne BD, Pearson RR, et al. Randomized secondary prevention trial of azithromycin in patients with coronary artery disease: primary clinical results of the ACADEMIC study. Circulation. 2000;102:1755–60. [PubMed]
97. Gurfinkel E, Bozovich G, Beck E, Testa E, Livellara B, Mautner B. Treatment with the antibiotic roxithromycin in patients with acute non-Q-wave coronary syndromes.The final report of the ROXIS Study. Eur Heart J. 1999;20:121–7. [PubMed]
98. Neumann F, Kastrati A, Miethke T, Pogatsa-Murray G, Mehilli J, Valina C, et al. Treatment of Chlamydia pneumoniae infection with roxithromycin and effect on neointima proliferation after coronary stent placement (ISAR-3): a randomised, double-blind, placebo-controlled trial. Lancet. 2001;357:2085–9. [PubMed]
99. Leowattana W, Bhuripanyo K, Singhaviranon L, Akaniroj S, Mahanonda N, Samranthin M, et al. Roxithromycin in prevention of acute coronary syndrome associated with Chlamydia pneumoniae infection: a randomized placebo controlled trial. J Med Assoc Thai. 2001;84(Suppl 3):S669–75. [PubMed]
100. Stone AF, Mendall MA, Kaski JC, Edger TM, Risley P, Poloniecki J, et al. Effect of treatment for Chlamydia pneumoniae and Helicobacter pylori on markers of inflammation and cardiac events in patients with acute coronary syndromes: South Thames Trial of Antibiotics in Myocardial Infarction and Unstable Angina (STAMINA) Circulation. 2002;106:1219–23. [PubMed]
101. Zahn R, Schneider S, Frilling B, Seidl K, Tebbe U, Weber M, et al. Antibiotic therapy after acute myocardial infarction: a prospective randomized study. Circulation. 2003;107:1253–9. [PubMed]
102. O’Connor CM, Dunne MW, Pfeffer MA, Muhlestein JB, Yao L, Gupta S, et al. Azithromycin for the secondary prevention of coronary heart disease events: the WIZARD study: A randomized controlled trial. JAMA. 2003;290:1459–66. [PubMed]
103. Cercek B, Shah PK, Noc M, Zahger D, Zeymer U, Matetzky S, et al. Effect of short-term treatment with azithromycin on recurrent ischaemic events in patients with acute coronary syndrome in the Azithromycin in Acute Coronary Syndrome (AZACS) trial: A randomised controlled trial. Lancet. 2003;361:809–13. [PubMed]
104. Grayston JT, Kronmal RA, Jackson LA, Parisi AF, Muhlestein JB, Cohen JD, et al. Azithromycin for the secondary prevention of coronary events. N Engl J Med. 2005;352:1637–45. [PubMed]
105. Cannon CP, Braunwald E, McCabe CH, Grayston JT, Muhlestein B, Giugliano RP, et al. Antibiotic treatment of Chlamydia pneumoniae after acute coronary syndrome. N Engl J Med. 2005;352:1646–54. [PubMed]
106. Jespersen CM, Als-Nielsen B, Damgaard M, Hansen JF, Hansen S, Helo OH, et al. Randomised placebo controlled multicentre trial to assess short term clarithromycin for patients with stable coronary heart disease: CLARICOR trial. BMJ. 2006;332:22–7. [PMC free article] [PubMed]
107. Litinov D, Mahini H, Garilnabi M. Anti-oxidant and anti-inflammatory role of Paraoxanase I: implication in arteriosclerosis diseases. N Am J Med Sci. 2012;4:523. [PMC free article] [PubMed]
108. Paul J, Dasgupta S, Ghosh MK. Carotid artery intima media thickness as a surrogate marker of atherosclerosis in patients with Chronic Renal Failure on Hemodialysis. N Am J Med Sci. 2012;4:77. [PMC free article] [PubMed]

Articles from North American Journal of Medical Sciences are provided here courtesy of Medknow Publications