Our study shows that biomarkers associated with different pathways of atherosclerosis - inflammation and coagulation and endothelial dysfunction - were higher in HIV-infected children compared HEU children. In the HIV-infected children, elevations of these biomarkers were independent of metabolic factors (hyperlipidemia or insulin resistance), with the exception of the selectins (both P and E), where differences appeared to be influenced strongly by lipid levels. Biomarkers of endothelial dysfunction, sICAM and sVCAM, and biomarkers of inflammation, CRP and MCP-1, were associated with higher HIV viral loads.
Atherosclerosis is considered an inflammatory process.[29
] Triggers that can initiate vascular injury include lipids, lipoproteins, angiotensin II, cytokines, glycosylation products, oxidative stress, and infectious agents.[11
] This injury results in the activation of nuclear factor-κB (NF-κB) with several pro-inflammatory cytokines released including molecules that increase leukocyte rolling and adherence to the endothelium, leukocyte migration through the endothelium, and recruitment of more inflammatory cells. Activated macrophages secrete several cytokines and growth factors that promote maturation of the atheromatous lesion.
Biomarkers such as hsCRP are independent predictors of future CVD in adults and there is emerging evidence of their utility in children.[18
] Other biomarkers that reflect leukocyte adherence, migration, and chemotaxis have also been associated with increased CVD risk in HIV-uninfected populations.[19
] We found that CRP and MCP-1, biomarkers associated with inflammation, were associated with increased viral load. In the Strategic Management of Antiretroviral Therapy (SMART) study, hsCRP and IL-6 levels were associated with viral load and CVD all-cause mortality risk in HIV-infected adults.[31
] Even in patients with viral suppression, the levels of these biomarkers were about 40–60% higher than in an HIV-uninfected population.[32
] However, not all studies have shown that hsCRP levels are associated with adverse CVD events.[33
HDL-cholesterol and higher triglycerides were associated with biomarkers of inflammation although the HDL effect was diminished in the HIV model when viral load was considered. HDL-cholesterol, thought to be critical in the “reverse transport” of cholesterol from arterial plaques, may also have direct anti-inflammatory effects[34
] by decreasing E-selectin[35
] (associated with leukocyte tethering and rolling) and limiting expression of vascular adhesion molecules such as VCAM and ICAM.[36
] Other studies show that postprandial triglycerides or triglyceride-rich lipoproteins are associated with activation of NF-κB[37
] or that very-low-density-lipoproteins (VLDL) can increase expression of leukocyte adhesion factors.[38
] We found that triglycerides were associated with higher MCP-1 and E-selectin. The putative role of selectins is to facilitate the tethering and rolling of leukocytes along the endothelium; hyperlipidemia may induce endothelial injury and activate this process. Both P- and E-selectin levels were associated with hyperlipidemia, even after adjusting for HIV status.
Although a hypercoagulable state is associated with CVD risk, few studies have evaluated biomarkers associated with thrombosis in HIV-infected patients. Fibrinogen is positively associated with mortality in HIV,[31
] but whether this translates to increased CVD risk is unclear. PI therapy is associated with increased fibrinogen levels in the Fat Redistribution and Metabolic Change Study (FRAM).[39
] We found fibrinogen was positively correlated with LDL-cholesterol levels in HIV-infected children. Fibrinogen may represent coagulation risk, but may also reflect inflammation.
Several studies in adults report associations between endothelial dysfunction markers with HIV disease severity.[40
] We found that MCP-1, sICAM, and sVCAM levels were higher in the HIV-infected children compared to HEU and that higher levels were associated with viral load, independent of metabolic status. These findings suggest that HIV itself may cause immune activation and resulting endothelial injury.[41
] These biomarkers are associated with all-cause mortality in non-HIV populations[42
] and sVCAM levels are associated with increased carotid intima media thickness (cIMT) in HIV-infected adults.[43
] The HIV tat
proteins induce VCAM-1, ICAM-1, and MCP-1. ICAM was elevated in HIV-infected children compared to controls and elevations were inversely related to CD4 counts.[44
] In addition MCP-1 is proposed to activate viral infection.[45
] Treatment interruptions are associated with increased levels of sVCAM, ICAM, and P-selectin,[46
] suggesting the influence of viral activity on expression of these biomarkers.
We did not find a strong effect of ARVs on the biomarkers we studied due possibly to the collinearity of ARVs on metabolic outcomes. PI therapy was associated with higher fibrinogen and NNRTI was associated with higher CRP. In cell culture, ARVs can alter endothelial cell mitochondrial DNA thereby increasing production of reactive oxygen species,[47
] endothelial cell permeability,[49
] and leukocyte adhesion[50
]. Thus, ARV therapy could directly or indirectly (through changes in the metabolic profile) increase levels of biomarkers.
Studies on vascular inflammation and structural/functional vascular dysfunction (i.e.
, vessel compliance, distensibility, and structure) in HIV-infected children have been limited.[51
] We have recently shown that similar biomarkers are also associated with central adiposity and decreased immune function (lower CD4 counts), although we had limited ability to evaluate the effect of lipids on these biomarkers.[22
] Some studies show vascular stiffness (through flow-mediated dilation studies) and cIMT were greater in HIV-infected children than controls[53
] with differences independent of known CVD risk factors and ARV therapy,[53
] while others show an association with ARVs, especially PIs.[52
] Longitudinal follow-up in one study revealed cIMT was similar in HIV and controls.[51
] Interestingly, some pre-HAART studies in children showed increased coronary artery calcifications,[57
] suggesting the contribution of baseline immune activation to CVD risk.
We did find some unexpected results. For instance, BMI was inversely related to sVCAM. In non-HIV cohorts, adiposity is associated with higher levels of these biomarkers.[58
] However, in our cohort, we suspect that higher viral loads (sicker children) were associated with lower BMI and, in turn, higher viral loads were associated with many of our biomarkers including sVCAM. We also selected our groups by whether the child met a hyperlipidemia definition and our groups may not represent HIV as a whole. However, the rates of hyperlipidemia in the PHACS cohort as a whole were similar to our study sample. Lastly, we did find that children with hyperlipidemia were more likely to have a family member with diabetes. This is an unusual association and may be a result of reporting bias.
In conclusion, we show that biomarkers associated with vascular inflammatory pathways are increased in HIV-infected children when compared to appropriately matched HEU children. Higher levels of these biomarkers were independently associated with HIV viral load (MCP-1, sICAM and sVCAM) and/or hyperlipidemia (MCP-1, fibrinogen, P-selectin and E-selectin). Our findings provide further evidence that HIV-infected children are at risk for CVD. Since many of these biomarkers were associated with modifiable risk factors, such as hyperlipidemia, interventions to modify these risks should be considered in future studies.