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From the Department of Epidemiology, The Johns Hopkins University School of Hygiene and Public Health, Baltimore, Md (F.J.N., G.W.C., M.S.), National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md (P.S.), Division of Hematology, University of Texas Medical School, Houston, Tex (K.W.), and the Division of Molecular Virology, Baylor College of Medicine, Houston, Tex (E.A., J.L.M.).
| Abstract |
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Key Words: atherosclerosis cytomegalovirus fibrinogen hemostasis lipoprotein(a)
| Introduction |
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Experimental evidence suggests that CMV infection is associated with a procoagulant state,6 7 resulting in changes that could reflect endothelial dysfunction, possibly leading to atherosclerosis. To our knowledge, no epidemiologic study has examined these issues. The main objective of the present report is to describe the association between hemostatic parameters and serum CMV antibodies measured in two sets of samples from the ARIC study previously used for case-control studies of viral infection and atherosclerosis.4 5 The association between CMV antibodies and Lp(a) levels was also examined because of recent suggestions of a possible interaction between CMV infection, Lp(a) levels, and impaired fibrinolysis.8
| Methods |
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Longitudinal Sample
Of the 4020 participants from Washington County, Md (one of the
ARIC sites), 1410 had also participated in a health survey conducted in
the county in 1974 that included a collection of about 15 cc of
nonfasting blood.10 Of these 1410 persons, 949 both had
serum collected in 1974 that was available for assay and had completed
the first and second ARIC examinations. These individuals form the
source population for the selection of 150 cases of carotid
atherosclerosis and 150 frequency-matched controls that
were included in a nested case-control study reporting on the
longitudinal association between viral antibodies and
atherosclerosis.5 All but two of these 300
participants were white.
As described in detail in a previous report,5 case-control status was based on measurements of the IMT obtained by B-mode ultrasound at three carotid artery sites (common, bifurcation, and internal carotid).11 Measurements by trained sonographers were obtained on both sides of the neck. Carotid atherosclerosis was defined based on the average IMT for all measurements taken at the ARIC baseline (first) and follow-up (second) visits. The 150 participants with the highest overall mean IMT were considered cases (mean IMT, 1.04 mm; range, 0.88 mm to 1.59 mm). Controls were the participants with the lowest mean IMT within 10-year age and gender strata, frequency matched to the number of cases in each stratum (mean IMT, 0.62 mm; range, 0.43 mm to 0.74 mm).
Cross-sectional Sample
In addition to the Washington County set of cases and controls,
we attempted confirmatory analyses of the longitudinal results
on another set of ARIC participants who were included in a previously
reported cross-sectional study of herpes virus infections and carotid
IMT.4 This study included 340 matched case-control pairs
selected from among participants from all four ARIC centers and found a
modest association between carotid IMT and CMV antibodies measured in
serum concurrently collected at the time of ARIC baseline examination.
The IMT case-control definition in this study used different cut-point
criteria than that used on the longitudinal sample.4 In
brief, cases had at least two measurements of carotid IMT >2.5 mm
or bilateral thickening corresponding to a maximum IMT
1.7 mm in
the internal carotid, and/or
1.8 mm in the carotid bifurcation,
and/or
1.6 mm in the common carotids. Controls were selected
among participants with all IMT measurements below the 75th percentile
of the cohort distribution. These additional data also allowed us to
examine the cross-sectional association of CMV antibodies and plasma
hemostatic factors.
Covariate Information
Information on cigarette smoking, history of diabetes, and
educational level was obtained at the baseline ARIC examination (1987
through 1989) by trained interviewers following a standardized
protocol.9 Medication use was assessed by asking the
participants to bring to the clinic all medications taken in the
previous 2 weeks. Participants taking lipid-lowering medications were
classified as being treated for
hypercholesterolemia. Antihypertensive
medication use was assessed by asking the participant whether they had
taken medication for high blood pressure during the past 2 weeks. Blood
pressure measurements and venipuncture were carried out
after participants had been fasting for at least 12 hours. Sitting
blood pressures were measured with a standardized Hawksley random-zero
sphygmomanometer after 5 minutes' rest. Systolic and
diastolic blood pressures were defined as the first phase
and the fifth phase of Korotkoff sounds. The average of the second and
third of three measurements (with 30 seconds' rest between them) is
used in this paper.
Laboratory Processing
The serum samples collected in 1974 were frozen at -73°C. In
August 1994, the samples were thawed, aliquoted, and shipped with dry
ice to the laboratory at the Division of Molecular Virology, Baylor
College of Medicine, Houston, Tex. The serum samples collected at the
ARIC examination (1987 through 1989) were collected for clinical
chemistry analysis and had been thawed once and refrozen at
-70°C. They were thawed again for separation into aliquots sent for
CMV antibodies analysis to the same laboratory in Houston. CMV
antibodies were measured as previously described.2 5 12 13
Briefly, solid-phase radioimmunoassay was used for the detection of
antibodies using the whole antigen of CMV strain AD169 as capture
antigen. Positive-negative values represent the ratio of the
average counts of the test serum with CMV antigen to the average counts
of the test serum with control antigens from uninfected cells, which is
always <2. Titration of serum showed that the positive-negative values
were directly related to antibody concentrations.
Lp(a) was measured as the total protein component, which is approximately 30% of the total lipoprotein mass, by using a double-antibody enzyme-linked immunosorbent assay technique for apoprotein(a) detection.14 Hemostatic variables in plasma collected as part of the baseline (1987 through 1989) examination of the ARIC Study were measured following a protocol previously described.15 16 17 In brief, fibrinogen was measured by the thrombin time titration method. Factor VII and factor VIII activities were assessed by determining the ability of the testing sample to correct the clotting time of human factor VII or factor VIII defective plasma. VWF and protein C antigens were determined by enzyme-linked immunosorbent assays. Antithrombin III activity was measured by a chromogenic substrate method. aPTT was measured on an automated coagulometer. White blood cell counts were determined in local hospitals at each of the field centers within 24 hours after venipuncture using automated counters.18 Previous publications have documented the reliability of these measurements as well as their distribution in the ARIC population.18 19 20 21 22
Statistical Analysis
The comparison of the distribution of study variables in
cases and controls has been the subject of previous
publications.4 5 The present analysis focuses
on the relation between CMV antibody levels and hemostatic
parameters. Mean levels of hemostatic
parameters according to CMV antibody level categories were
obtained by analysis of covariance, while adjusting for
case-control status, age, gender, and other potential confounding
variables. For the longitudinal analyses, CMV antibody
levels were categorized using the same three levels previously found
related to case-control status in a graded fashion: positive-negative
ratio <4; 4-19.9; and
20.5 As a result of the lower
levels of CMV antibodies in the cross-sectional samples,4
the categories for CMV antibodies in the corresponding analyses
were established as positive-negative ratio <2; 2-7.9; and
8, which
resulted in similar proportional distribution of the study population
as that of the longitudinal analyses. Linear trend tests were
carried out by testing the statistical significance of the linear
regression coefficient for a three-level ordinal variable
corresponding to these categories, while adjusting for possible
confounders. With the exception of aPTT and protein C, all hemostatic
variables were slightly skewed. Thus, all linear regression
analyses for hemostatic variables were repeated using
log-transformed values with results practically identical to those
reported here (not shown). Stratified conditional and unconditional
logistic regression analyses were used to investigate whether
Lp(a) and hemostatic variables could be effect modifiers of the
association between CMV antibodies and atherosclerosis,
which we have previously documented.4 5 All statistical
analyses were conducted using SAS version 6.10 (SAS Institute,
Cary, NC).
| Results |
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As previously reported,5 CMV antibody levels in serum
samples collected in 1974 were markedly higher than in serum samples
collected in 1987 through 1989. Among control subjects from Washington
County in the two sets of samples, for example, the median CMV
positive-negative values were 8.3 and 3.8, respectively. A total of 36
participants had their CMV antibodies measured in both surveys. With
the exception of individuals with low antibody levels in 1974, most of
these individuals had substantially lower CMV antibody levels in their
second measurement (Fig 1
). All quintiles above the median were higher
in 1974 than in 1987 through 1989 samples, whereas the quintiles below
the median were similar (not shown). The corresponding Pearson
correlation was r=.66.
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In the longitudinal analyses, after adjustment for age,
gender, and case-control status, CMV positive-negative values in 1974
sera were negatively associated with aPTT in plasma collected in 1987
through 1989 (Table 2
). However, antibody
levels were positively associated with plasma levels of vWF, factor
VIII, and protein C. After additional adjustment for variables that
have been described as correlates both of viral antibodies and of
hemostatic factors (serum cholesterol levels,
triglyceride levels, cigarette smoking, and educational
level),5 23 the estimates in Table 2
remained
virtually unchanged (not shown). Other hemo-static
parameters such as fibrinogen levels and white blood cell
count were not related to CMV antibody levels in the longitudinal
analysis (Table 2
).
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When the cross-sectional relationship between CMV titers and
hemostatic parameters was explored, different patterns of
association were found (Table 2
). The CMV positive-negative categories
shown on the right side of Table 2
were constructed so that the
proportion of participants in each category was comparable to that in
the longitudinal analyses (left side of Table 2
). In these
analyses, a trend for factor VIII was still present but
much weaker, whereas the trends for aPTT, vWF, and protein C were
entirely absent. However, these cross-sectional analyses show a
direct association between CMV antibodies and antithrombin III and
fibrinogen levels. Different categorizations of these cross-sectional
CMV positive-negative ratios, as well as further adjustment for
potential confounders (serum cholesterol levels,
triglyceride levels, cigarette smoking, and educational
level), showed essentially the same trends (not shown). There were no
statistically significant interactions between any of the
parameters shown in Table 2
and case-control status; and
when analyses were restricted to control subjects, the
associations seen in Table 2
were still evident, although the results
were no longer statistically significant due to the smaller sample size
(not shown).
The association between CMV antibodies and Lp(a) was also investigated.
Although no clear trends on the geometric mean Lp(a) protein levels
according to the levels of CMV antibodies shown in Table 2
were
observed (results not shown), serum samples that were positive for CMV
antibodies in 1987 through 1989 were more likely to have high levels of
Lp(a) protein than negative CMV sera. Among CMV positive samples (CMV
positive-negative ratio >2), 25.1% had Lp(a) protein levels in the
upper quartile for the entire ARIC population [Lp(a) protein
148
µg/mL], as compared with only 14.1% among CMV negative sera
(odds ratio, 1.9; 95% confidence intervals, 1.2 to 3.2).
To assess whether Lp(a) and the investigated hemostatic
parameters modify the association between CMV antibodies
and atherosclerosis described in previous
publications,4 5 stratified analyses were
conducted (Table 3
). For both the
longitudinal analysis (based on the 1974 CMV antibodies) and
the cross-sectional analysis (based on the 1987 through 1989
values), a positive association between CMV antibodies and
atherosclerosis was restricted to or was stronger among
those with values of Lp(a) protein and fibrinogen above the median
levels for the ARIC Study population. In contrast, only weak or no
consistent effect modification was observed for factor VII
(Table 3
) and for the other hemostatic parameters
considered in this article (not shown).
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| Discussion |
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Several mechanisms have been proposed to explain these associations.3 5 Herpesvirus might be involved in atherogenesis by altering the lipid metabolism of involved cells.1 33 34 Viral infection might induce endothelial injury, triggering the development of the atherosclerotic plaque.35 In fact, CMV infection of the endothelium increases the adherence of polymorphonuclear leukocytes, one of the earliest steps in the atherogenesis process.36 Furthermore, the presence of CMV in smooth muscle cells from atherectomy specimens has been shown to be associated with enhanced p53 accumulation in patients developing coronary restenosis after angioplasty.37 Replicating CMV destroys proliferating smooth muscle cells, possibly impairing vessel repair of atherosclerotic lesions.38
Another possible mechanism whereby CMV infection is related to
atherosclerotic disease or its manifestations is by inducing hemostatic
dysfunction and thrombosis.8 In the present study, CMV
antibody titers in 1974 were associated with 1987 through 1989 plasma
levels of certain hemostatic parameters including factor
VIII, vWF, and protein C (Table 2
). In addition, there was an inverse
association with aPTT, an overall screening test for the intrinsic
coagulation pathway. The association of CMV titer with vWF may stem
from an increased synthesis due to endothelial
dysfunction. The association with factor VIII may reflect that the
latter factor exists in plasma as a noncovalent complex with
vWF.39 In turn, the apparently paradoxical increased
levels of protein C associated with high CMV antibody levels could stem
from a compensatory mechanism to counteract the increase in
procoagulant factors.
In contrast to the longitudinal results, the cross-sectional
analyses showed that the associations above were much less
evident or had even disappeared, such as for aPTT, vWF, and protein C.
The reasons for this apparent inconsistency are unclear.
One possible reason is that a particularly virulent CMV strain affected
the Washington County population during the early or mid-1970s and
resulted in long-term endothelial damage leading to the
observed longitudinal associations. This could also explain the results
presented in Fig 1
, ie, the
remarkably higher CMV titers observed in the 1974 samples as compared
with the titers in the 1987 through 1989 samples collected in the
individuals who were included in both the longitudinal and the
cross-sectional studies. Alternatively, these results could stem from a
"saturation effect." Provided that the prevalence of CMV infection
increases with age, reaching very high proportions in late
adulthood,40 it is possible that in 1974 when this cohort
was relatively young, there was still some kind of range (real
negatives versus positives). This may not have been the case (or less
so) later, at the time of the ARIC examination, when the apparent
negatives might have been "false-negatives." The concurrent
associations of CMV antibodies with fibrinogen and antithrombin III in
1987 through 1989 samples may reflect a more acute reaction to a more
recent viral reactivation.
Positive CMV antibodies were associated with Lp(a) protein levels in the present study, only when both variables were categorically defined as binary variables. This result was surprising, given the relatively weak dependence of Lp(a) levels in environmental factors vis-a-vis genetic determinants.41 Although it has been suggested that Lp(a) may be an acute phase reactant,42 the pathophysiologic interpretation of transient increases of Lp(a) following acute myocardial infarction and other disease states remain controversial.43 We failed to observe a significant dose-response trend, however, and thus our finding needs to be replicated in other populations.
Of the hemostatic parameters associated with anti-CMV titers in the studies described in this manuscript, fibrinogen, factor VIII, and vWF have been associated with prevalent coronary disease in the ARIC Study.23 Other studies have documented an association of incident coronary disease and previous levels of fibrinogen and factor VIII,42 43 vWF,43 44 45 and protein C.43 44 45 46 Of these, only fibrinogen was related to carotid atherosclerosis in the ARIC Study.23 Lp(a) protein levels also have been found to be associated with carotid atherosclerosis in this population.22
Overall, these results suggest that CMV infection may be associated with chronic hypercoagulability and are consistent with previous in vitro studies documenting that CMV infection produces a depletion of vWF of cultured endothelial cells.6 Other studies have documented the procoagulant properties of CMV and other herpesvirus infections47 48 49 and that this phenomenon depends on plasma coagulation factors.7 The altered hemostatic parameters could stem from mechanical damage or inflammatory activation of the endothelial lining of the vascular wall due to the viral infection,7 changes that could eventually lead to atherosclerosis.50 Furthermore, the finding that the association between CMV and atherosclerosis seen in previous reports from our group4 5 are practically restricted to individuals with high levels of Lp(a) protein and fibrinogen supports the hypothesis that the prothrombotic properties of CMV may at least partially explain its possible atherogenic effect.8 A previous study found that another infectious agent (Chlamydia pneumoniae) was more strongly associated with angiographically-defined coronary artery disease when in combination with high Lp(a) levels.53 The interplay between raised prothrombotic activity, reduced fibrinolytic activity, increased Lp(a) levels, and atherosclerosis has been emphasized recently.51 The striking homology between Lp(a) and plasminogen may result in Lp(a) binding to fibrin and may block fibrinolysis by preventing activation of plasminogen to plasmin.8 41 51 The combination of the procoagulant effects of CMV infection and decreased fibrinolysis may be promoting the development of atherosclerosis.
To our knowledge, this is the first epidemiologic study in apparently healthy adult individuals showing an association of CMV infection and levels of hemostatic parameters and Lp(a), as well as their possible interaction regarding atherogenesis. These findings suggest that hemostatic variables may be a link in the relationship between CMV infection and atherosclerosis, a role that deserves future investigation.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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We thank Linda Schramm and Sandra C. Hoffman, MPH, at the Johns Hopkins Training Center for Public Health Research, Hagerstown, Md, for their help in managing and handling the serum samples. The Washington County clinical center, central laboratories, and the ultrasound reading center of the ARIC Cooperative Group, their institutions, coinvestigators, and principal staff who contributed to this report are as follows: The Johns Hopkins University, Baltimore, Md: Carol Shearer, Rita Timmons, Joyce B. Chabot, and Carol Christman; University of Texas Medical School, Houston: Valerie Stinson, Pam Pfile, Hoang Pham, and Teri Trevino; The Methodist Hospital, Atherosclerosis Clinical Laboratory, Houston: Doris J. Epps, Charles E. Rhodes, and Selma M. Soyal; Bowman-Gray School of Medicine, Ultrasound Reading Center, Winston-Salem, NC: Christy Jones, Kathy Joyce, Mary Louise Lauffer, and Suzanne Pillsbury; University of North Carolina, Chapel Hill, Coordinating Center: Ken Kaufman, PhD, Ho Kim, Charmaine M. Marquis, and Alison Meyer.
| Footnotes |
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Received November 21, 1996; accepted February 24, 1997.
| References |
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