Current models of atherosclerosis pathogenesis suggest that endothelial damage stems from a variety of insults and results in a positive feedback mechanism, in which the production of EPCs in the bone marrow is increased, leading to partial reconstitution of normal endothelium.
We observed higher EPC frequencies in HIV-positive as compared to HIV-negative subjects, a finding that supports our previous observations indicating an increased endothelial stress in HIV-infected subjects [19
]. In contrast to other studies showing lower levels of EPCs and CFU-ECs together with similar frequency of ECFCs in untreated HIV-positive subjects as compared to HIV-negative subjects [20
], we observed higher EPC frequencies in HIV-positive as compared to HIV-negative subjects irrespective of the presence of ART, or subsequent change in c-IMT. With regards to the impact of ART on EPCs, our lack of a difference between ART-treated and untreated groups is limited by the smaller number of subjects on ART, as our study was focused on a priori
selected subjects based on prospective c-IMT change rather than balanced for ART versus non-ART subjects at baseline.
EPCs can serve as predictors of early subclinical atherosclerosis [22
] or cardiovascular events [23
] in HIV-negative subjects, while low CD4+
T-cell count has been reported as a robust risk factor for increased c-IMT in HIV-positive subjects [24
]. In this study, despite higher EPC frequencies in HIV-positive as compared to HIV-negative subjects, c-IMT was not associated with EPC frequencies or CD4+
T-cell count. Interestingly though, there was a positive association between EPC frequencies and CD4+
T-cell count. One explanation for this finding could be that although failure to produce EPCs may not be a major cause of HIV-associated atherosclerosis, increased EPC frequencies in HIV infection may represent the enrichment of dysfunctional EPCs; therefore they are not associated with direct c-IMT changes. It is also possible as suggested by Chironi et al.
] that c-IMT, measured in the common carotid segment free from atherosclerosis, is not a specific marker of atherosclerosis and may represent medial hypertrophy that is a non-atherosclerotic process [25
]. A potential for future EPC functional analysis could include matched flow-mediated dilation measures of the brachial artery, widely used as a measure of endothelial function [26
], as this was not performed in this cohort.
Our study has several limitations. First, our sample size was too small to rule out the presence of a clinically meaningful effect of EPCs on outcome. A larger study including a group of atherosclerotic HIV-negative subjects may be necessary to distinguish between the effects induced by vascular damage and those induced by HIV infection. Second, cryopreservation resulted in 50% decrease in EPC frequency, and as a result although cryopreserved PBMC were as informative as fresh PBMC from the same donors in documenting an EPC frequency rise in HIV-infected subjects when compared to HIV-negative subjects, they may be limiting to evidence weak associations with c-IMT, if present. Third, as c-IMT reflects a cumulative exposure to risk factors while circulating EPCs may reflect acute ongoing endothelial stress, additional functional studies and long-term follow-up of subjects with increased EPC frequencies will be needed to address if EPCs are functional, if they are associated with cardiovascular events independently of c-IMT changes and if a cardiac clinical event in HIV-infected subjects is preceded by a decrease of the otherwise increased EPC frequencies.
In summary, our data showing increased levels of steady-state circulating EPCs independently of c-IMT change support the hypothesis that HIV infection does not impede EPCs from emerging in circulation as an indication of chronic vascular stress irrespective of ART-mediated suppression.