Introduction.

HIV infection has been associated with increased cardiovascular disease risk [1] but the mediators of the increased risk have not been specifically identified. Endothelial dysfunction is a critical initial step of atherogenesis which subsequently contributes to the progression and clinical manifestations of atherosclerosis [2], [3]. Long-term use of protease inhibitors (PIs) has been associated with endothelial dysfunction [4]. Factors other than PI use that may contribute to endothelial dysfunction in HIV-infected patients include untreated HIV infection itself [5] treatment-associated lipid changes [4], [6] and the lipodystrophy syndrome [7].

Mediators and markers of endothelial dysfunction have been sought, such as lipids and lipoproteins and circulating markers of inflammation and vascular activation, but the majority of these factors have not been significantly associated with endothelial function as measured by brachial flow-mediated dilation (FMD) [5], [8]. Lack of a consistent association between FMD and CD4 cell count suggests that immune status is not directly related to endothelial dysfunction [5], [8], [9], . In a large cohort study, there was also no association between CD4 cell count and risk of myocardial infarction [12]. Despite careful study, the specific mediators of HIV-associated endothelial dysfunction have not been identified

CD4 T cell depletion in gut-associated lymphoid tissue (GALT) occurs within 4–6 weeks of primary HIV infection [13].Simian immunodeficiency virus-infected macaque models have demonstrated that gut bacteria are a source of circulating bacterial lipopolysaccharide (LPS) [14]. CD4 T-cell depletion in GALT coincides with increased expression of genes associated with inflammation and decreased expression of genes regulating epithelial barrier and digestive functions, mucosal repair and regeneration which suggests disruption of the gut microenvironment [13]. Increased levels of LPS that occur in patients with chronic progressive HIV infection decreased after 48 weeks of effective anti-retroviral therapy (ART) but did not normalize [15]. These increased levels of LPS are associated with persistent immune activation [15], so by extension persistent immune activation itself is another potential mechanism of HIV-associated endothelial dysfunction.

Gut microbial translocation leads to increased circulating levels of LPS and other microbial products. LPS activates endothelial cells via a distinct signaling pathway and this may directly influence cardiovascular disease pathogenesis [16], [17]. Soluble CD14 (sCD14) is a circulating glycoprotein that binds LPS, subsequently allowing the interaction of LPS with another signaling receptor. Cell associated CD14 is a multifunctional receptor, a glycoprotein expressed on the surface of monocytes, macrophages and neutrophils with a specificity for LPS and other bacterial wall derived components [16]. The presence of sCD14 is required for LPS-induced injury and activation of endothelial cells.

Our aim was to establish a relationship between microbial translocation measures and endothelial dysfunction in HIV-infected subjects in two well-characterized study populations [5], [8] which used FMD as a marker of endothelial function. As has been shown in recent studies, impaired FMD has been associated with worse long-term cardiovascular disease prognosis [2], [18]. A wide variety of inflammatory and vascular biomarkers were previously performed in both parent studies including high sensitivity C-reactive protein (hsCRP), lipids, lipoproteins, interleukin-6, adiponectin and vascular cell adhesion molecule-1 (VCAM-1) but none correlated with FMD.

Abstract
Background

Circulating levels of microbial products are increased in HIV infection, and provoke endothelial dysfunction in other disease settings.

Methodology/Principal Findings

We examined data from a cross-sectional single site study at Indiana University (Indiana, N = 85) and a 24- week multicenter prospective study of antiretroviral therapy (ART) initiation (ACTG 5152s, N = 75). Brachial artery flow-mediated dilation (FMD) was measured by ultrasound. Plasma lipopolysaccharide (LPS) and soluble CD14 (sCD14) levels were measured from stored specimens and correlated with FMD values using Pearson correlations. The Indiana subjects were 63% male with a mean age of 39 years and a median CD4 count of 406 cells/mm3 (388 not on ART, 464 on ART). The 5152s subjects were 92% were male with a mean age of 35 years and a median CD4 count of 251 cells/mm3 at entry which increased to 396 cells/mm3 on ART. When analyzing the two cohorts individually or in combination neither sCD14 nor LPS correlated significantly with FMD. In a pre-specified subgroup analysis of the Indiana subjects receiving ART (N = 46, mean ART duration 40 months) LPS was inversely correlated with FMD (r = −0.33, p = 0.02), but not sCD14 (r = −0.01, p = 0.9). Multivariate analysis confirmed LPS as an independent predictor of FMD in this subgroup (p = 0.02).

Conclusions/Significance

In HIV- infected individuals on prolonged ART, higher LPS levels are associated with worse endothelial function but not in untreated subjects or at 24 weeks after ART initiation. Persistent microbial translocation may contribute to arterial dysfunction and the increased cardiovascular disease risk observed in individuals on long-term ART.