CLINICAL STUDIES
Circulating chlamydia pneumoniae DNA as a predictor of coronary artery disease
Yuk-ki Wong, BSc, (Hons), MRCP*  ,
Keith D. Dawkins, FRCP, FACC and
Michael E. Ward, PhD*
* Molecular Microbiology Department, Southampton University Medical School, Southampton, United Kingdom
Wessex Cardiothoracic Unit, Southampton General Hospital, Southampton, United Kingdom
Manuscript received October 30, 1998;
revised manuscript received March 10, 1999,
accepted July 6, 1999.
Reprint requests and correspondence: Dr. Yuk-ki Wong, Wessex Cardiothoracic Unit, Southampton General Hospital, Tremona Road, Southampton, Hampshire, United Kingdom S016 6YD
YW2{at}soton.ac.uk
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Abstract
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OBJECTIVE
To determine whether current vascular Chlamydia pneumoniae (CPn) infection as diagnosed by circulating CPn DNA is more common in subjects with coronary artery disease (CAD).
BACKGROUND
Serological, pathological and animal studies have associated CPn with CAD and preliminary trials suggest antibiotics may prevent adverse coronary events. C. pneumoniae is thought to disseminate systemically within macrophages. We therefore detected CPn DNA in blood to determine whether its presence was a predictor of CAD.
METHODS
One thousand, two hundred and five subjects attending for diagnostic and interventional coronary arteriography were recruited. The mononuclear cell layer and platelets were separated from collected blood and the polymerase chain reaction (PCR) was used to detect CPn DNA.
RESULTS
Circulating CPn DNA was found in 8.8% of 669 men with CAD compared with 2.9% of 135 men with normal coronary arteries (odds ratio [OR] 3.2, 95% confidence interval [CI] 1.1–8.9). In men with CAD, those with CPn DNA had higher mean platelet counts than those without CPn DNA. Monocyte counts and indirect fibrinogen levels were also raised but not significantly so. By contrast, no association of circulating CPn DNA and CAD was seen in women.
CONCLUSIONS
Circulating CPn DNA is a predictor of CAD in men. Unlike serology, it is a specific indicator of current infection and is a means of identifying subjects who may potentially benefit from antichlamydial therapy.
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Abbreviations and Acronyms
| | CAD | = coronary artery disease | | CI | = confidence interval | | CPn | = Chlamydia pneumoniae | | OR | = odds ratio | | PBS | = phosphate buffered saline | | PCR | = polymerase chain reaction |
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The serological association between antibody to the obligate intracellular bacterium Chlamydia pneumoniae (CPn) and coronary artery disease (CAD) was first reported in 1988 (1) and has subsequently been confirmed by numerous serological studies (2). Furthermore, CPn antigen and DNA are commonly found in atherosclerotic plaque (3,4). Intranasal inoculation of the organism in rabbits has also been shown to induce aortic inflammatory changes consistent with early
atherosclerosis (5) and to exacerbate disease progression (6). Such changes could be prevented by antichlamydial treatment with azithromycin (6). However, not all patients with CAD will have been infected with CPn, and antibiotics are only indicated for those who have infection. A simple method for diagnosing current vascular infection with CPn is required. Previous studies have used the presence of IgG and IgA antibody to CPn as a proxy for chronic infection, but the validity of this approach is uncertain (7). C. pneumoniae infection generally starts in the respiratory tract and probably disseminates systemically in the blood stream within alveolar macrophages (8). We therefore assessed the prevalence of CPn DNA in the blood mononuclear cells of 1,205 patients referred for coronary arteriography and correlated the results with clinical and other parameters.
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Methods
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This study was approved by the local regional ethics committee. Subjects were recruited consecutively from individuals referred for diagnostic and interventional coronary arteriography to the Wessex Cardiothoracic Unit, a tertiary referral center, from the end of August 1997 to the beginning of March 1998. Because they were likely to form a small group, subjects under 30 were excluded. A few subjects who did not have blood taken during their current admission were also excluded. Data available included a full blood count and, in most cases, indirect fibrinogen levels and fasting lipids. From each subject, 4 to 5 mls of blood collected into a tube containing ethylenediaminetetraacetic acid (EDTA), were available for serology and for the detection of CPn DNA by the polymerase chain reaction (PCR). Cardiac risk factors were recorded, and hypercholesterolemia was diagnosed if the fasting total cholesterol was greater or had ever been greater than 5.2 mmol/l. A family history of CAD in a sibling, parent, grandparent or sibling of a parent was recorded. The Carstairs deprivation score standardized against national data was calculated from 1991 UK census data (Manchester Information, Datasets and Associated Services) using the residential postcode and census enumeration district. Coronary arteriograms were interpreted by a consultant cardiologist and classified as "normal," "minor irregularities" or "significant disease."
Blood mononuclear cell PCR.
EDTA blood was collected from patients around the time of cardiac catheterization. Samples were stored overnight at 4°C for processing the next day. The plasma was clarified by centrifugation and the supernatant stored at –72°C for serology. Blood cells were mixed with an equal volume of phosphate buffered saline (PBS) and layered onto 3 ml of Lymphoprep (Nycomed, Denmark). The mononuclear cell layer, which also contained platelets, was then collected after centrifugation at 500g for 15 min, washed twice in PBS and stored at –72°C. DNA was prepared from the blood mononuclear cells by conventional methods involving digestion with proteinase K, extraction with phenolchloroform, precipitation with sodium acetate-ethanol and the addition of 50 µl of distilled water. To check for PCR inhibitors, 3 µl of extracted DNA was spiked with 5 pg of lambda phage DNA followed by PCR with lambda specific primers (9). C. pneumoniae PCR (10) was performed using 3 µl DNA at the original concentration and where inhibition was detected, at a 10-fold dilution. Amplification products were visualized by electrophoresis on a 2% agarose gel and staining with ethidium bromide. Stringent precautions were taken against DNA contamination. Negative controls (distilled water) were used during DNA extraction and interspersed every five to seven samples during PCR. To control sensitivity, a positive control was used consisting of CPn DNA equivalent to 1–10 elementary bodies in 3 µl water.
Serology.
A time-resolved fluoroscopic immunoassay was used (11). Antigen was whole, purified CPn organisms (VR1310) and the labelling agent was Europium-labelled streptavidin (Wallac Oy, Finland). Antibody levels were measured as fluorescence counts using a Wallac 1234 fluorometer.
Data analysis.
Conditional multiple logistic regression (SPSS version 8) was used to explore possible associations of the presence of CPn DNA and conventional coronary risk factors with CAD. Of those subjects who did have CAD, possible associations of the presence of CPn DNA with indirect fibrinogen and various hematological variables were investigated after adjusting for age, smoking and month of recruitment. The two-tailed t test was used to compare continuous variables with normal distributions and the chi-square test used for categorical variables. The distribution of Carstairs score was skewed and subjects were categorized into three groups of equal size.
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Results
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Subjects recruited.
One thousand two hundred five subjects were recruited (804 men, 401 women). Significant CAD was found by coronary arteriography in 669 (83.2%) men and 244 (60.8%) women while 135 men and 157 women had normal coronary arteries or coronary arteries with mild irregularities only. Of these latter subjects, 73 (54.1%) men and 81 (51.6%) women had been investigated for chest pain; the majority of the remainder had valvular heart disease. As expected, the prevalence of conventional coronary risk factors, apart from deprivation as measured by the Carstairs score, was greater in those with CAD (Table 1). The Carstairs score takes into account unemployment, social class, overcrowding and car ownership and 62.5% of all subjects had a Carstairs score that was better than the national average while scores were unavailable for 5.1% of subjects.
C. pneumoniae DNA in blood mononuclear cells.
One hundred subjects (8.3%) had CPn DNA in their blood mononuclear cells. Men with CAD were significantly more likely to have circulating CPn DNA than men with normal coronaries (Table 1), but this difference was not seen in women. Among men with CAD, those who were positive for CPn had higher mean platelet counts (Table 2). Mean monocyte counts and indirect fibrinogen levels were also higher, but not significantly so. Similar comparisons were not made among women because the number of subjects was smaller.
Serology.
Plasma samples were assayed in duplicate, with intra- and interassay coefficients of variation of 4% and 20%, respectively. Antibody to CPn showed a bimodal distribution. For analysis, the cut-off point for seropositivity was predefined as the value midway between the two peaks. Men were more likely to be seropositive than women, but no association of serology with CAD was observed for either sex (Table 1). Subjects who were seropositive were more likely to have CPn DNA in their blood mononuclear cells, but this was not statistically significant (76/834 vs. 23/346, odds ratio [OR] = 1.4, 95% confidence interval [CI] = 0.9 to 2.3).
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Discussion
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Detection of current vascular infection by CPn.
We have shown in a large study that CPn DNA can be detected by PCR in the blood mononuclear cells of patients referred for coronary arteriography. This is the first time that the clinical and hematological correlates of vascular chlamydial infection have been assessed in patients clearly shown to be infected because of the presence of circulating CPn DNA. In both laboratory animals (12) and human subjects (3), the persistence of vascular CPn DNA has been associated with the presence of viable organism. Other studies have attempted to equate the persistence of specific IgG and IgA antibody to CPn to the presence of chronic infection with the organism. However, data on the serological response associated with culture confirmed chronic CPn infection are sparse, and no generally agreed standards for chlamydial serology exist. Culture confirmed chronic respiratory infection with CPn is not always associated with a specific antibody response (13), and it is often not possible to predict subjects with CPn in their blood vessels on the basis of their serology (14,15). A recent autopsy study of young Alaskan natives (mean age 34.1) did find that subjects who had CPn in their blood vessels were more likely to have had a specific IgG titre of  128 an average 8.7 years before death (16). However, that serology is a blunt predictive tool was demonstrated by the fact that 15 of 32 subjects with a prior titre of 128 were positive for CPn compared with three of seven subjects without measurable antibody. Culture of CPn is an exacting and tedious procedure and is not a feasible option for the routine detection of vascular infection with this organism. Our results suggest that for CPn vascular infection, as for C. trachomatis genital tract infection, PCR or other nucleic acid-based amplification procedures are the most appropriate method for detecting current infection (17).
One limitation of blood mononuclear cell PCR is that infection in the blood does not necessarily imply infection of blood vessels. Also, as infection of blood vessels by CPn in living subjects cannot generally be diagnosed, the sensitivity and specificity of blood mononuclear cell PCR for diagnosing such infection is unknown. To distinguish between acute and chronic infection, future studies will need to test subjects on more than one occasion to document the persistence of circulating CPn DNA. Such studies will also help determine the reproducibility of this test.
Association of CPn infection and CAD.
Our study population was typical in that it showed the expected correlations with known risk factors of coronary disease. In males, interestingly, circulating CPn DNA was a stronger predictor of CAD than any of the known cardiac risk factors, with an adjusted odds ratio of 3.4 although the prevalence of vascular CPn infection was low. By contrast, in women, the presence of CPn DNA was not a predictor of CAD. The reason for this is unknown, but aortic stenosis has been associated with CPn infection (18), and approximately half the subjects with normal coronary arteries had valvular heart disease. Nevertheless, exclusion of these subjects did not significantly alter the results of the analysis. Among men with CAD, those with CPn DNA had higher platelet counts, a feature of some infectious diseases although not previously reported for CPn. Previous studies have reported that CPn infection is associated with raised fibrinogen levels and that this may explain the increased cardiovascular risk (19). However, in this study, indirect fibrinogen levels were not significantly raised. Men were more likely than women to be seropositive for CPn as has been widely reported, but in neither sex was there an association of specific antibody with CAD, in agreement with several recent studies (20,21) including two that were prospective (22,23).
Population differences in the prevalence of CPn infection.
During the course of this study, a similar but smaller study was published with dramatically different results (24). In Ume , Northern Sweden, a remarkable 59.4% of 101 patients attending for coronary arteriography (mean age 64) were found to have CPn DNA as were 46% of 52 blood donors (mean age 49). Also, 11 of 14 (78.6%) patients found to have normal coronary arteries were positive for CPn compared with 49 of 86 (57%) patients with CAD. The much higher prevalence of CPn infection reported from Ume might reflect population or methodological differences. Epidemics of CPn respiratory disease have been described throughout Scandinavia and an unusual 94% of patients and 90% of blood donors were seropositive for CPn, possibly the highest seroprevalence rates ever reported. To exclude the possibility that methodological differences may account for our differing results, we retested specimens from our all male subjects with normal coronary arteries using the touch-down PCR method of the Swedish study, but found only one more positive result. It is likely, therefore, that there are genuine population differences, but further studies are necessary to clarify these points.
Antibiotic intervention trials.
An association between circulating CPn DNA and CAD does not necessarily imply causation. We recently found that patients undergoing redo coronary artery bypass graft surgery were just as likely to have CPn in their new internal mammary artery grafts as in their failed grafts (9). Also, the Alaskan study above found no difference in disease severity between coronary artery segments that had evidence for CPn and those that did not. However, a small secondary prevention trial has reported that azithromycin reduces the incidence of further adverse coronary events following myocardial infarction (25). A second study also reported significant benefits after thirty days follow-up but not after six months (26), while preliminary reports from a third study also found no benefit (27). Further trials are under way, with one trial giving antibiotics for periods of up to one year without evidence of prior infection (28). Antimicrobials are only likely to benefit those who are currently infected and CPn can be considered neither universal nor the only organism that might be important in CAD. For reasons of appropriate prescribing alone, antimicrobials should only be given to subjects in whom there is good reason to believe there is current CPn infection. In our population many CAD patients did not have circulating CPn DNA. Although further research is required, the demonstration of circulating CPn DNA in blood may be a useful, possibly prognostic test for selecting patients for antibiotic trials designed to clarify the role of CPn in CAD.
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Acknowledgments
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We thank the British Heart Foundation and the Wessex Cardiac Trust for financial support and our colleagues in the Wessex Cardiothoracic Center for facilitating access to their patients.
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Footnotes
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This study was supported by a grant from the British Heart Foundation. Dr. Yuk-ki Wong was supported by a grant from Wessex Cardiac Trust.
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Abstract
Methods