Role-of-Infections-in-Atherogenesis

Sandeep T Laroia, Apar Kishor Ganti, Anil Potti

Department of Medicine, University of North Dakota
School of Medicine and Health Sciences, USA

In 1921, Ophuls1 proposed that infections could lead to atherosclerosis. This hypothesis was based on pathologic specimens of blood vessels, which showed macrophage infiltrates and foam cells. Five decades later, in 1978, Fabricant et al.2 once again showed arterial lesions similar to atherosclerosis in chicken infected with the avian herpesvirus. Recent advances in diagnostic techniques have facilitated the re-emergence of this hypothesis. It is now well accepted that atherosclerosis is an inflammatory process, and a natural corollary of this concept is that microorganisms could be the prime initiators of this process.3 Data from several studies indicate an increased prevalence of chronic infections in atherogenesis. The agents that have been implicated in this process are Chlamydia pneumoniae, Helicobacter pylori, herpes simplex virus, and cytomegalovirus (CMV).4–6 In this article, we evaluate the evidence available both for and against the different etiological agents, and their  current status in the pathogenesis of atherosclerosis.

Basic Scheme of Atherogenesis

Simplified, atherogenesis is the passage of low-density lipoprotein (LDL)-cholesterol through dysfunctional endothelium at points of low shear stress. In addition, mechanical stress can lead to the aggravation of this phenomenon.7 LDL-cholesterol penetrates the dysfunctional endothelium and undergoes oxidation. The oxidized LDL causes further endothelial dysfunction. Monocytes penetrate the endothelium, differentiate into macrophages, attract more macrophages and the resultant foam cells, which are lipid-laden macrophages, accumulate in the region. These cells may ultimately rupture, causing the release of toxic inflammatory mediators that trigger a fibroproliferative response from smooth muscles.8, 9

Potential Role-of-Infections-in-Atherogenesis

Endothelial dysfunction, the trigger for atherogenesis, can be induced by systemic or local infection. Multiple mechanisms have been proposed which include the following:

  1. Bacterial endotoxins and tumor necrosis factor (TNFalpha) can inhibit vasodilator nitric oxide generated by endothelial-dependent processes;10

  2. Endothelial stunning, a mechanism which hypothesizes that periods of endothelial inactivity can be induced by a brief exposure to endotoxin;11

  3. Direct infection of the endothelium by infectious agents, especially the herpesviruses including CMV;12

  4. Altered expression of growth-controlling proteins by vascular smooth muscles after infection with certain viruses, which leads to these cells obtaining a growth advantage and thus may contribute to atherosclerosis and restenosis.13

Furthermore, C-reactive protein (CRP) and fibrinogen (acute phase reactants), which are strong independent predictors of subsequent cardiovascular events, are found to be elevated in infectious states as well, and elevated levels of cytokines are found in both infections and acute coronary syndromes. These circulating cytokines may cause abnormal endothelial function, increased thrombosis, and toxic free radical generation, leading to accelerated atherogenesis.14,15

Evidence for the Role of Infections in Atherosclerosis

Herpesviridae, especially CMV, H. pylori, and C. pneumoniae, have been extensively studied for their effect on atherosclerosis. Tables 1, 2, and 3 list the various studies that showed a positive correlation between atherogenesis and infection by herpesviruses,16–30 H. pylori,31–41 and C. pneumoniae,42–63 respectively. These studies included both human and experimental research. The experimental works listed include those involving both experimental animals and tissue studies.

Evidence Against the Role of Infections in Atherosclerosis

Though there are a number of reports favoring the role of infections in atherosclerosis, the number of reports arguing against such a role is equally impressive. In the discussion that follows, a few reports which showed no effect of infection in the pathogenesis of atherogenesis have been listed

Evidence against herpesviridae: Siscovick et al.19 did not find any correlation between the presence of IgG antibodies to CMV and the risk of acute myocardial infarction (AMI) or coronary artery disease (CAD) in elderly patients. Choussat et al.63 studied systemic markers of inflammation in patients with unstable angina or non-Q wave MI, and the relationship between these markers, seropositivity to chronic infections (CMV, H. pylori, and C. pneumoniae), and prognosis. They found no association between the levels of each inflammatory marker and the serologic status. Furthermore, levels of inflammatory proteins in patients seronegative to all 3 agents were comparable to those of patients seropositive to 2 or 3 infectious agents. The composite end-points of death, MI, recurrent angina, or revascularization at 1-year follow-up did not differ according to the serologic status.63 To determine if CMV infection is a risk factor for primary CAD and the association between CMV infection and CAD (>50% blockage in any coronary artery), Adler et al.64 investigated nearly 900 successive nontransplant patients undergoing coronary angiography. By the use of logistic regression, they found that CMV seropositivity (p=0.462), the level of IgG antibodies to CMV whole-cell antigen (p=0.98), or the levels of IgG antibodies to CMV glycoprotein B (p=0.67) were not significantly associated with CAD. These data suggest that CMV infection is not a major risk factor for the development of primary CAD in adults.64

 

Evidence against Helicobacter pylori: A Finnish group failed to show a statistically significant relationship between patients infected with H. pylori and CAD; interestingly, this study showed higher levels of serum triglycerides in patients seropositive for H. pylori.65 In their study, Tsai and Huang66 showed that H. pylori seropositivity was not associated with several coronary risk factors in either cases or controls. The proportion of H. pylori-positive patients was higher among cases with triple-vessel disease (77.5%) than in those with double-vessel (67.3%) and single-vessel (65.7%) disease; however, the differences were not statistically significant. In this study, no increase was found in H. pylori seropositivity in subjects with CAD. In a recent review, Menge et al.67 found that the present data were inconclusive regarding the association between H. pylori infection and CAD. They concluded that proposed links between H. pylori infection and coronary heart disease (CHD), such as hyperhomocysteinemia or autoimmune mechanisms due to cross-reacting antibodies to H. pylori heat-shock protein with human endothelium-derived heat-shock protein, need further confirmation. Quinn et al.68 studied the relationship between angiographically defined CAD and serologic evidence of H. pylori infection in 488 patients undergoing elective coronary angiography. There was no association between H. pylori infection and CAD.68 Basili et al.69 retrospectively analyzed 149 subjects who underwent an esophagogastroduodenoscopy, in whom the search for H. pylori was histologically performed, and found that the prevalence of CAD was not significantly different from that observed in H. pylori-free patients (26% v. 21%; p=0.527). Lastly, a meta-analysis of 18 epidemiological studies involving over 10 000 patients failed to demonstrate any significant association between H. pylori infection and CAD.70 Khurshid et al.71 prospectively studied 179 patients undergoing coronary angiography for suspected CAD and found that H. pylori infection rates were similar in patients with normal and abnormal coronary arteries, and infection with H. pylori was not an independent risk factor for CAD. In patients with CAD, H. pylori infection was not a risk factor for more severe disease.71

Evidence against Chlamydia pneumoniae

Patient data: The Physicians Health Study72 prospectively measured IgG antibodies against C. pneumoniae in 343 participants with first MI. A similar number of age- and smoking-matched controls were also followed up for a period of 12 years. The prevalence of seropositivity was the same in both the groups.72 Markus et al.73 obtained ultrasonic images of the carotid artery to determine the intima–media thickness (IMT) and the thickness of any atheroma plaques, and found no evidence that serological evidence of C. pneumoniae infection is associated with early atherosclerosis. They also found no evidence that C. pneumoniae results in a chronic systemic inflammatory state.73 Hoffmeister et al.74 investigated the association between seropositivity to chlamydial lipopolysaccharide (cLPS) or C. pneumoniae and angiographically documented CAD. They also examined the relationship between serostatus and markers of systemic inflammation. Their results indicated no strong association between C. pneumoniae and CAD, and they concluded that the increased systemic inflammation in patients with CAD did not seem to be due to seropositivity to cLPS or C. pneumoniae.74 In their meta-analysis, Danesh et al.75 reviewed 15 studies to examine the association between CAD and serum markers of chronic C. pneumoniae infection. They did not find any strong association between C. pneumoniae IgG titers and incident CAD. Romeo et al.76 tried to correlate the severity of CAD with seropositivity to C. pneumoniae prospectively. They found no significant difference in IgG and IgA seropositivity among patients with stable CAD, unstable CAD, and controls. They concluded that only a small percentage of patients with CAD demonstrate seropositivity against C. pneumoniae. A recent case–control study investigated the relationship between the presence of C. pneumoniae IgG and IgA and angiographically diagnosed CAD. When cases were compared with controls whose angiographic results were normal, after adjusting for established risk factors (cholesterol, smoking, hypertension, diabetes, age, gender, and family history), the estimated risk of CAD was 0.79 for the presence of IgG and 0.94 for IgA. These results do not support an association between C. pneumoniae infection and CAD.77 In a recent review, Wong et al.78 concluded that more evidence is required before C. pneumoniae can be accepted as playing a role in atherosclerosis.

Animal data: Aalto-Setala et al.79 infected apolipoprotein E (apo E)-deficient mice with C. pneumoniae and placed them on either a high- or low-fat diet. They found that C. pneumoniae infection did not influence the lesion size in either mouse strain. They also could not demonstrate C. pneumoniae by polymerase chain reaction in any of the atherosclerotic lesions of the infected animals. They did not find any inflammatory signs in the myocardium of C. pneumoniae-infected mice. They concluded that C. pneumoniae infection did not accelerate atherogenic changes in the aortic root of apo E-deficient mice.79 Blessing et al.80 inoculated C57BL/6J mice with C. pneumoniae. They observed inflammatory changes in the heart or aorta in a small number of chronically infected mice but no evidence of atherosclerotic lesions in any of them. Their findings suggested that chronic C. pneumoniae infection could induce inflammatory changes in the heart and aorta of C57BL/6J mice but did not initiate definitive atherosclerosis.80 In a recent review, Haberbosch and Jantos81 concluded from the present data that chronic infection with the pathogen is not an independent risk factor for atherosclerosis.

Intervention Studies

The ACADEMIC trial82 reported 302 patients with seropositivity to C. pneumoniae. Subjects received azithromycin and placebo for 3 months. At 6 months, the azithromycin-treated group showed a reduced global index of inflammation, which comprised CRP, TNF-alpha; and interleukins 1 and 6 compared with a placebo. However, cardiovascular events were similar in the two groups at 6 months. At 2-year follow-up, there was a 20%–30% risk reduction.82 Parchure et al.83 carried out a randomized, prospective, double-blind, placebo-controlled trial in 40 male patients with documented CAD and positive C. pneumoniae IgG antibody titers. They showed that patients who received azithromycin had a significant improvement in flow-mediated dilatation of the brachial artery. They then concluded that treatment with azithromycin had a favorable effect on endothelial function in patients with documented CAD and evidence of C. pneumoniae infection, irrespective of antibody titer levels.83

A pilot study of 60 survivors of AMI with persistent elevated anti-chlamydial antibody titers was designed so that subjects were randomized to receive placebo or azithromycin. Azithromycin-treated patients showed an apparent reduction in cardiovascular events from 28% to 8%. There was no significant difference between a singleand double-dose course of azithromycin.84 The ROXIS trial85 randomized 202 patients with unstable angina or non-Q wave MI to roxithromycin or a placebo. At the end of the treatment period, the rates of recurrent ischemia were 1% v. 5.4%, MI 0% v. 2.2%, and ischemic events 0% v. 2.2% in the roxithromycin v. placebo group, respectively.85  At 6 months, the individual and composite event rates remained lower in the roxithromycin group, but the difference was not statistically significant.86 In the AZACS trial,87 patients with acute coronary syndromes (unstable angina or MI) were randomized in a double-blind, placebocontrolled fashion to either azithromycin 500 mg/day followed by 250 mg/day for 4 days or a matching placebo.  They found that in patients with AMI or unstable angina, short-term treatment with azithromycin did not have an effect on the recurrence of ischemic events during a 6-month follow-up period. There was no difference between patients who tested positive for the presence of C. pneumoniae antibodies and those who did not.87 Similarly, in the WIZARD trials,88 which sought to assess the use of antibiotics to prevent recurrent CHD, the antibiotic regimen comprising weekly azithromycin in adult patients >6 weeks post-MI with elevated C. pneumoniae IgG titers, achieved a 7% nonsignificant reduction in the incidence of recurrent cardiovascular disease at 11 weeks. The baseline titer of IgG antibodies against C. pneumoniae had no effect on the outcome.88

Conclusions

Atherosclerosis is an inflammatory process. Hence, microorganisms could be the prime initiators of this process. Endothelial dysfunction, the trigger for atherogenesis, can be induced by systemic or local infection. Circulating cytokines, released by any stimulus, may cause abnormal endothelial function, increased thrombosis and toxic free-radical generation, leading to accelerated atherogenesis. As in leprosy, Whipple’s disease, syphilis and ehrlichiosis, Koch’s postulates cannot be used to determine the role of infection in atherosclerotic disease. Regarding the role of infection in CAD, important indirect evidence comes from prevention of the disease by means of specific interventions.

The data for and against the role of infection in atherosclerotic vascular disease are equally impressive. However, the few prospective clinical trials evaluating the role of antibiotics in the secondary prevention of CAD have not shown a significant decrease in clinically major cardiovascular events. Larger studies with a longer duration of follow-up may be more useful in assessing the exact pathogenetic mechanism of infection in atherosclerotic heart disease, and also the role of antibiotic therapy in the treatment of CAD.

Based on the current data, however, it is not possible to be certain one way or the other about the role of infection in the pathogenesis of atherogenesis, and the subsequent complications of atherosclerosis. Further studies are required to solve this complex problem.

Correspondence:

Dr Sandeep T Laroia,
Department of Internal Medicine,
University of North Dakota
School of Medicine,
1919 Elm Street N, Fargo,
ND 58102, USA.
e-mail: sandeeplaroia@meritcare.com

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