After adjustment for clinical and HIV-related predictors in a multivariate model, age, DBP, antihypertensive medication treatment, smoking, and nadir CD4+ T-cell count remained significant predictors of arterial stiffness (Table 2). Specifically, a nadir CD4+ T-cell count below 350 cells/μl was independently associated with a 7.3% increase in AIx@75 (95% CI 2.6–11.9, P = 0.003) compared with a nadir CD4+ T-cell count of at least 350 cells/μl. When analyzed as a continuous variable, nadir CD4+ T-cell count was a significant predictor of AIx@75 in unadjusted analyses, and remained significant predictor after adjustment for other clinical and HIV-related parameters. Each 1-SD decrease in nadir CD4+ T-cell count was associated with a 2.3% increase in AIx@75 (95% CI 0.4–4.2, P = 0.02).
HIV-infected persons have a higher risk of developing cardiovascular disease compared with age-matched uninfected persons [1,2,24]. The mechanism for this increase is unknown, but is almost certainly multifactorial. Prior work from our group and others suggest that uncontrolled viral replication (and its effect on biomarkers of inflammation) appear to be causally associated with this increased risk [3,24,25]. Among treated patients, certain drugs such as the protease inhibitor class and perhaps abacavir are also associated with increased risk of disease [1,6,7]. The relative degree of immunodeficiency – as defined by nadir and recent CD4+ T-cell counts – may also be associated with cardiovascular risk. Here, we performed a detailed assessment of arterial stiffness by pulse wave analysis and tonometry. Arterial stiffness has been associated with all-cause mortality, cardiovascular mortality , coronary artery disease , and stroke  in HIV-uninfected persons. As expected, age, BP, and antihypertensive medication use were associated with increased arterial stiffness in our population. We also found that the peripheral nadir CD4+ T-cell count was a strong and consistent predictor of both PWV and arterial stiffness. This association appeared to be independent of other important clinical factors that are known to influence measures of arterial stiffness such as age, BP, and diabetes mellitus. More importantly, the relationship between nadir CD4+ T-cell count and arterial stiffness was independent of other HIV-associated characteristics, including HIV duration, the use of protease inhibitors, and current CD4+ T-cell count.
Among long-term treated patients, there is a growing body of evidence that suggests the degree of prior or residual immunodeficiency while on therapy is associated with the short-term risk of cardiovascular disease. In our earlier study , we found that a nadir CD4+ T-cell count of 200 cells/μl or less was associated with carotid intima–media thickness (IMT), whereas other studies [28,29] have failed to detect an effect. The discrepancies between study results may be due to differences in methods of assessment of carotid artery IMT . Studies [31–33] have also suggested that the on-therapy CD4+ T-cell count is independently associated with an increased risk of cardiovascular disease. These observations suggest that for unclear reasons, persistent immunodeficiency during HAART has negative cardiovascular consequences. Given the manner in which treatment has been historically administered, the vast majority of individuals in these cohorts had chronic infection at the time they started HAART, and most had a CD4 nadir below 350 cells/μl. These studies were hence unable to determine whether earlier initiation of HAART is associated with better cardiovascular outcomes than delayed initiation of HAART. Defining with more precision, the role of prior or current immunodeficiency in driving heart disease could prove to be informative for the ongoing debate as to when to start combination ART . Our data show that advanced immunodeficiency as represented by a nadir CD4+ T-cell count below 350 cells/μl is independently associated with increased arterial stiffness. Although it is not possible to conclude from our cross-sectional study that earlier initiation of ART may help reduce cardiovascular risk, our study results provide important initial evidence that might support the further pursuit of prospective studies addressing this question.
The pathophysiologic mechanism by which immunodeficiency may mediate arterial stiffness and cardiovascular risk remains unclear. In the SMART study, elevations in interleukin-6 and D-dimer were strongly associated with all-cause mortality, suggesting that interruption of HAART may result in higher levels of HIV-associated inflammation . In our study, there was no correlation with hs-CRP and the degree of arterial stiffness; however, this may have been due to insufficient power to detect differences between participants.
Chronic activation of the immune system in HIV infection may be due to microbial translocation in the gastrointestinal tract, leading to elevated levels of circulating microbial products such as lipopolysaccharide, which may activate immune and inflammatory pathways . It is known that residual microbial translocation during suppressive HAART is associated with the degree of immune reconstitution, as reflected by CD4+ T-cell count recovery . It has also been shown that initiation of HAART at CD4+ T-cell nadir of 350 cells/μl or less is associated with incomplete reconstitution of T-cell subsets and T-cell activation . It is thus possible that the relationship between nadir CD4+ T-cell counts and arterial stiffness may be mediated via microbial translocation, which, in turn, may activate inflammatory pathways, leading to premature atherosclerosis.
Our study has many limitations common to cross-sectional studies. Although our study demonstrates a strong association between nadir CD4+ T-cell count and measures of arterial stiffness, it was a cross-sectional observational study, and is, therefore, subject to potential selection biases and limitations in establishing cause–effect relationships. Our data argue for early use of ART; however, we were unable to analyze whether treatment initiation during the acute versus chronic phases of HIV infection (and potential associated differences in lifestyle or behavioral factors) affected cardiovascular risk, independent of nadir CD4+ T-cell count, due to limited number of individuals who started HAART during acute infection.
In addition, although a nadir CD4 T-cell count above 350 cells/μl appeared to be associated with improved cardiovascular risk in our study, it is unclear from our data whether this relationship extends beyond a CD4+ T-cell count of 500 cells/μl, as few participants met these criteria. Thus, whether earlier initiation of HAART at a CD4+ T-cell count of 500 cells/μl rather than 350 cells/μl may impact cardiovascular risk is unclear. It is also unclear from our study whether it is exposure to HIV replication or low CD4+ T-cell counts or both that were driving vascular dysfunction. However, because nadir but not current CD4+ T-cell count in persons with undetectable viral load predicted arterial stiffness suggests that duration of untreated HIV or exposure to viremia may be more important than persistent depressed CD4+ T-cell counts. Finally, we failed to detect consistent associations between diabetes mellitus, smoking, and our two vascular outcome measures. This may be due to the limited number of individuals with clinical diabetes mellitus. Also, a recent systematic review  examining cardiovascular risk factors associated with PWV showed that only six of 44 studies reported an association between smoking and PWV, thus the lack of association between these parameters in our study is not surprising. Finally, given that arterial stiffness is a surrogate and not direct measure of atherosclerosis, the ultimate clinical significance of our findings is unclear. Prior studies have shown a 5 m/s increase in PWV to be associated with two-fold greater odds of all-cause mortality in hypertensive patients , and each quartile increase in AIx was associated with two-fold greater odds of coronary artery disease . Whether or not our findings of increased AIx@75 or increased PWV in association with nadir CD4+ T-cell counts below 350 cells/μl translate into clinically significant outcomes is unclear and will require further study.
Despite these limitations, the strengths of this study are that participants were recruited from two unique ongoing longitudinal cohorts, which included individuals who were treated with HAART both early and late in the course of their HIV infection, and were thus extremely well characterized with respect to their clinical and HIV-related parameters. Our study is the largest study examining arterial stiffness to date, and may provide important initial evidence to prompt longitudinal studies addressing whether early initiation of HAART may have any impact on cardiovascular risk.
Among treated HIV-infected individuals, increased arterial stiffness was independently associated with both traditional cardiovascular risk factors, including age and diabetes, as well as a low nadir CD4 cell count. Although inferences drawn from our findings must be interpreted with caution due to the cross-sectional nature of our study, our data may provide initial evidence that earlier initiation of ART before low CD4 cell counts occur may be a means of reducing cardiovascular risk among individuals with HIV infection. Prospective studies are needed to evaluate potential beneficial effects of HAART initiation at higher CD4 T-cell counts on cardiovascular risk.
This research was supported by grants from the National Institutes of Health (NIH)/University of California, San Francisco (UCSF)–Gladstone Institute of Virology and Immunology Center for AIDS Research, P30-AI027763 (J.E.H); from the NIH grants #5R01-HL095130 and 5K23-AI066885 (P.Y.H.); from the National Institute of Allergy and Infectious Diseases grant #K24-AI069994 (S.G.D.), the UCSF/Gladstone Center for AIDS Research grants #P30-AI27763 and P30-MH59037; and the UCSF Clinical and Translational Science Institute grant #UL1 RR024131–01. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIH.
P.Y.H. has received honoraria from Gilead, and grant support from Pfizer. S.G.D. has received grant support from Pfizer, Merck, Bristol-Myers Squibb, Roche, and Gilead, and honoraria from GlaxoSmithKline.
J.E.H., S.G.D., F.M.H., and P.Y.H. were responsible for the study's conception and design. J.E.H., Y.X., and A.S. were involved in the study performance. J.E.H. did the statistical analyses and wrote the manuscript, and all authors participated in critical review and substantial input to the final manuscript.
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