Data from this study support the hypothesis that monocyte and macrophage activation in HIV disease may contribute to the formation of vulnerable atherosclerotic plaque. Specifically, we demonstrate that the monocyte/macrophage-specific marker, sCD163, is uniquely associated with increased noncalcified plaque among young, asymptomatic, HIV-infected men with low or undetectable levels of viremia and a long duration of HIV disease. sCD163 was significantly associated with increased noncalcified plaque burden, independent of traditional risk factors. These data lend insight into the dynamic process by which cardiovascular disease develops among HIV-infected patients with low or undetectable plasma virus and suggest that macrophage activation, independent of traditional risk factors, contributes to a unique phenotype of inflammatory noncalcified coronary plaque among patients with long-term HIV infection.
In this study, we show that traditional risk factors, including age, Framingham risk score, and cholesterol and low-density lipoprotein levels, were significantly associated with increased calcium score, a measure of calcified plaque burden. In contrast, sCD163 and duration of known HIV infection were positively associated with noncalcified plaque. In addition, sCD14, another marker of monocyte activation, was associated with sCD163 and with noncalcified plaque in univariate analysis. However, sCD163, but not sCD14, remained significantly associated with plaque in the multivariate regression analysis that also controlled for traditional risk factors. This was true in a combined analysis of all patients, controlling for HIV infection, and in the HIV-infected subgroup alone. In contrast, the relationship between sCD163 and noncalcified plaque was not significant in multivariate regression among HIV-uninfected patients.
In contrast to our data on sCD163, markers of generalized inflammation, such as C-reactive protein level, did not show a relationship with plaque in our study, suggesting that, among the large number of patients with controlled viremia, monocyte/macrophage activation—and not generalized inflammation—contributes to increased plaque.
Taken together, these data are significant in that they suggest that specific types of atherosclerotic disease have different etiologies in the HIV-infected population. Those patients with traditional risk factors resulting from dyslipidemia and insulin resistance, hypertension, and other traditional risk factors will be expected to have more calcified lesions. However, we now show that even those with few traditional risk factors can have noncalcified disease and that this may be related to monocyte/macrophage activation, a point not previously appreciated in the HIV-infected population. Plaques with necrotic cores are known to be more prone to rupture, and histologically, increased macrophage infiltration is seen in these necrotic cores [3
]. Taken together, our data suggest a potential mechanism by which HIV infection may produce a phenotype of lipid-rich inflamed atherosclerotic plaques potentially via up-regulated monocyte activation that may be more vulnerable to rupture.
Inflammatory and immunologic factors may play important roles contributing to increased coronary artery disease in the HIV population, as suggested by studies like SMART [14
]. Triant et al  and Lichtenstein et al [17
] recently showed, among large cohorts, that acute myocardial infarction rates are related to low CD4 cell count, after controlling for traditional risk factors and ART . In addition, Kaplan et al [18
] recently showed that HIV-associated T cell activation is related to subclinical carotid atherosclerosis. The current data suggest—to our knowledge, for the first time—that monocyte/macrophage activation may also contribute to subclinical atherosclerosis.
The current data extend the data from prior studies, which showed endothelial dysfunction by brachial artery flow mediated vasodilation among those with long-standing low viral loads [19
]. We now demonstrate that, among patients receiving stable ART who have undetectable viremia, monocyte activation is significant and relates most to noncalcified coronary plaque, the level of which is increased among HIV-infected patients. Atherosclerosis is an inflammatory process in which monocytes and T lymphocytes play important roles [20
]. We and others have previously shown that CMV-induced immune responses may also be contributory to atherosclerosis in HIV-infected patients [1
]. In the current study, we found that sCD163 related not only to HIV RNA levels but also with CMV IgG titers, and this may be another factor contributing to monocyte activation in our study population. Indeed, animal data suggest acute CMV infection may mediate trafficking of inflammatory monocyte/macrophages from the bone marrow via CCR2 [25
]. In contrast, sCD163 did not relate to EBV viral capsid IgG titers in our study. Taken together, these data suggest that CMV titers may be uniquely related to monocyte activation, as measured by sCD163, rather than a reflection of a nonspecific B cell activation.
sCD163 has been previously described as a plasma marker of atherosclerosis in HIV-uninfected patients [8
] Activated macrophages expressing CD163 have been found in atherosclerotic plaque in HIV-uninfected patients [27
]. These activated macrophages are thought to participate in the atherosclerotic process and contribute to the pathogenesis of cardiovascular disease. However, the specific role played by CD163 itself in the development of atherosclerosis is unknown [28
]. In this study, the first among HIV-infected patients to investigate the relationship between sCD163 and atherosclerotic plaque, we find a significant association of sCD163 with the pro-inflammatory CD14+
monocyte subsets as well as with sCD14. In addition, we demonstrate that levels of LPS in plasma, a measure of circulating microbial products that have crossed the gastrointestinal tract, are also elevated in HIV-infected patients in association with sCD163. LPS and bacterial products have been found in humans with chronic HIV infection and SIV-infected monkeys and correlate with development of AIDS [30
]. LPS is a potent stimulant with other macrophage activation products of CD163 expression and secretion by monocyte/macrophages [32
]. However, LPS was not itself associated with plaque. In addition, chronicity of HIV infection related to degree of monocyte activation as indicated by sCD163 level. Together, these data suggest that HIV-infected individuals have an increased risk of atherosclerosis due to macrophage-mediated mechanisms, likely resulting from chronic immune activation despite low or undetectable plasma virus.
In contrast to the data from the SMART study, our data in carefully phenotyped patients assessed by coronary angiography do not suggest a relationship between D
-dimer and subclinical atherosclerotic disease. The lack of an association between D
-dimer and coronary atherosclerosis in our study may be due to the fact that we investigated early asymptomatic cardiovascular disease before an acute coronary event, in contrast to data from the SMART study [34
The current study has limitations but a number of strengths. We cannot fully determine the contribution of prior inflammation or risk factors to current plaque burden from an assessment at a single point in time in this cross-sectional study. However, nadir CD4 cell count, a marker of prior disease severity, was not related to plaque in the HIV-infected patients. We were unable to study women, and our data cannot be generalized to that group, and future studies will be critical in that population. Nonetheless, our study prospectively enrolled a large number of patients with longstanding HIV infection, largely well controlled, with minimal traditional risks and without known coronary disease and a well-matched control group of HIV-uninfected patients. We used detailed phenotyping of plaque morphology and measurements of monocyte/macrophage activation markers to find a novel relationship with monocyte activation and noncalcified plaque in HIV-infected patients.
Taken together, these data demonstrate monocyte activation in association with increased plaque comprise predominantly noncalcified plaque in this population. In particular, our data demonstrate that sCD163, a marker of monocyte activation, is strongly and uniquely associated with non-calcified coronary lesions in HIV-infected patients. Development of non-calcified lesions related to persistent monocyte activation may predispose HIV-infected patients to plaque rupture and premature CAD. Our data do not negate the impact and need to modify traditional risk factors, which do appear to be associated with increased calcified lesions. The data suggest the need for additional studies to assess how monocyte activation may contribute to atherosclerosis and ways in which this can be minimized among patients with longstanding HIV disease.