HIV-infected patients are at an increased risk of developing atherosclerosis and cardiovascular disease than age-matched HIV-seronegative adults, yet the mechanisms accounting for this effect remain poorly defined. To clarify the role of several factors putatively associated with atherosclerosis in HIV patients, we used high resolution ultrasound to quantify subclinical atherosclerosis in a large cohort of HIV-infected individuals. Nested within this cohort was a sizeable group of HIV-seropositive individuals who are able to maintain an undetectable HIV viral load in the absence of anti-retroviral therapy (elite controllers). Compared to uninfected controls, carotid IMT was higher among all groups of HIV subjects, irrespective of antiretroviral treatment or the level of viremia. HIV controllers were of particular interest as they lacked the previously suggested key risk factors needed for the development of HIV-associated atherosclerosis namely, exposure to anti-retroviral therapy, high level viremia, and/or advanced immunodeficiency. The unexpected finding of a high IMT in HIV controllers suggests that other factors contribute to the pathogenesis of HIV-associated atherosclerosis. We have previously reported that “elite” control of HIV is typically associated with high levels of T cell activation (as compared to HIV-seronegatives),[9
] and extend these observations to include C-reactive protein, a marker of systemic inflammation. Collectively, these data argue for a possible role of persistent HIV-associated inflammation as a potential cause for accelerated atherosclerosis in HIV disease.
One potential explanation for our findings was that we inadvertently selected a group of HIV-seronegative controls who lacked risk factors and/or had unusually low levels of IMT. This is unlikely as our seronegative controls had carotid IMT values between the 50th
percentile of the Multi-Ethnic Study of Atherosclerosis Study cohort aged 45 to 54 years while our HIV controllers had IMT values far in excess of 75th
percentile of this cohort.[21
] In fact, the IMT of our HIV-infected subjects, including that of the HIV controllers, was similar to that of patients with heterozygous familiar hypercholesterolemia before the era of intensive statin use,[22
] a condition almost invariably associated with premature atherosclerosis and coronary heart disease. Our findings suggest that HIV may be associated with a similarly accelerated form of atherosclerosis.
Although “elite” controllers lack readily detectable HIV RNA using conventional assays, they clearly harbor replication competent virus and almost invariably have measurable plasma HIV RNA using very sensitive assays.[23
] Determining the degree to which this low level HIV replication among HIV controllers is responsible for increased cardiovascular risk has implications for antiretroviral treated patients, since they too often have evidence of persistent low-level viremia (and perhaps low level viral replication) [25
Among HIV controllers, C-reactive protein levels, a marker of systemic inflammation, are elevated and indistinguishable from that of the other HIV-seropositive groups. In uninfected patients, atherosclerosis is a disease with a strong pro-inflammatory component.[7
] Activated T cells, macrophages, and mast cells act in concert to release factors leading to atherosclerosis initiation, progression, plaque instability, and intraluminal thrombus formation.[7
] Within the context of HIV infection, heightened antigen-specific T cells responses and more recently levels of interleukin-6, a key stimulus to the hepatic production of C-reactive protein, have been associated with accelerated atherosclerotic disease.[9
] Here, we used measurements of hsCRP to confirm that HIV disease is a pro-inflammatory state, even among the HIV controllers. In uninfected individuals, hsCRP independently predicts cardiovascular disease risk[28
], while in HIV-seropositive patients, hsCRP clearly predicts mortality.[30
] Although the presence of relatively high hsCRP levels in our controllers confirms that long-term host-mediated control of HIV is a pro-inflammatory state, the fact that CRP only explained a small proportion of the differences in IMT between the HIV controllers and HIV-seronegative persons suggests that other inflammatory mediators are more likely causally associated with premature atherosclerosis in HIV disease.
Several factors may contribute to systemic inflammation in HIV-infected controllers. First, HIV infection is associated with an early and rapid destruction of CD4+ T cell population within the intestinal lymphoid tissue.[31
] This loss of anti-microbial defense mechanism may lead to the chronic entry of bacterial pro-inflammatory products including endotoxin (lipopolysaccharide) into the blood stream (bacterial translocation).[32
] We have demonstrated that compared to controls, HIV controllers (as well as HIV non-controllers) have much higher circulating levels of lipopolysaccharide[14
] a product associated with endothelial dysfunction[33
] and early atherogenesis.[35
]. Increases in lipopolysaccharide also activate the innate and adaptive immune systems.[32
] Accordingly, higher levels of bacterial lipopolysaccharide potentially result in endothelial dysfunction, immune activation, and chronic inflammation leading to accelerated atherosclerosis among individuals with HIV, including HIV controllers. Second, other viruses in patients with HIV may be involved in immune activation and inflammation and hence atherosclerosis. Most HIV patients, including controllers, are seropositive for cytomegalovirus (CMV). Among all HIV patients, we have shown that increases in CMV-specific T-cell responsiveness correlate directly with the extent of atherosclerosis.[9
] In preliminary data from our group, HIV controllers appear to have higher CMV-specific T cell responses compared to uninfected controls (Hunt, PH unpublished data), suggesting that inflammatory responses related to CMV infection may play a role in HIV-associated atherosclerosis. Lastly, strong HIV-specific T cell responses – the very responses that are thought to help controllers maintain control of HIV replication –[13
] may also be contributing to generalized inflammation even in the absence of clinically detectable viremia.[42
This study has several limitations that deserve comment. This was a cross-sectional study, where the rate of progression in carotid IMT cannot be measured. This study design makes it particularly challenging to compare treated and untreated disease, as those on and off therapy almost certainly have very different prior disease histories that are difficult to measure. The comparisons between the HIV-seronegative subjects and the HIV controllers is less problematic, as the two groups likely differ primarily in the exposure variable of interest. Also, the magnitude of the differences between the HIV-seronegatives and the controllers is so great that unmeasured confounders are unlikely to solely account for our observed differences. Another potential limitation is the absence of studies correlating carotid IMT with cardiovascular event rates in HIV population. However, extensive data[21
] suggest that carotid IMT is a powerful predictor of cardiovascular outcomes in patients without HIV. Extrapolation to HIV patients is reasonable, although remains to be proven. Finally, it is important to stress that the role of inflammation in causing the relatively high levels of IMT in our controllers is based largely on inferences from our prior studies in which we demonstrate that long-term host-mediated control of HIV replication is associated with high levels of T cell activation. We plan in future studies to investigate in more detail the relationship between inflammation and IMT progression among a larger number of HIV controllers. Larger studies will also allow us to control for other potential factors that might be related to persistent inflammation in HIV-infected controllers, including injection drug abuse and/or the presence of certain other co-infections (e.g., CMV, HBV, HCV).
In summary, HIV infection is associated with premature atherosclerosis, as measured by IMT. This occurs even in the absence of detectable viremia, overt immunodefiency and exposure to antiretroviral therapy, and appears to be independent of traditional cardiac risk factors. While antiretroviral therapy and advanced immunodeficiency likely contribute independently to atherosclerosis in HIV patients, our results with the controllers demonstrate that these elements are not a prerequisite for higher levels of subclinical atherosclerosis. Whether the premature atherosclerosis observed in our controllers is due directly to an inflammatory process remains to be established. From a clinical perspective, our observations suggest that all HIV-infected individuals even those doing apparently well with or without antiretroviral treatment may benefit from aggressive cardiovascular risk management and perhaps the use of anti-inflammatory agents such as statins [43
] although this remains unproven treatment in this setting. Finally our observations have implications for individuals treated adequately with antiretroviral therapy by current standards, since it remains possible that persistent low level viral replication which can occur despite otherwise effective therapy may still prove to be harmful. In the future, achieving and maintaining even lower viral loads than current therapies permit in individuals with HIV will need to be investigated both in terms of HIV disease and cardiovascular risk.