In a large, community-based sample of middle-aged to older men and women, we investigated the relations of circulating angiogenic cell phenotypes with a comprehensive set of vascular function measures. We observed modest associations of higher CD34+ and CD34+/KDR+ concentrations with lower digital microvessel dilation but with higher peripheral arterial stiffness in unadjusted models. In addition, higher CD34+ concentrations were associated with lower central arterial stiffness in unadjusted models. Only the modest relation of higher CD34+ and lower microvessel dilation measured by PAT persisted in models adjusting for standard cardiovascular risk factors. In all models, we did not observe associations of CFU with any of the vascular measures. Overall, our findings suggest that these circulating angiogenic cell phenotypes are largely not associated with conventional measures of vasodilator function or arterial stiffness after accounting for standard cardiovascular risk factors.
Our findings stand in contrast to prior work indicating that depletion of progenitor cell phenotypes relates to vascular dysfunction. Prior human studies that related angiogenic cell phenotypes to vasodilator function and arterial stiffness have been restricted to small and selected samples, and the results from these studies have not been consistent. Lower CFU has been associated with impaired brachial artery flow mediated dilation in men and with lower arterial elasticity in older individuals.8,15
In healthy young men17
and patients with Type 1 diabetes mellitus,13
higher levels of CD34+/KDR+ cells have been associated with higher flow-mediated dilation. In patients with coronary artery disease, one study found a relation of CD34+/KDR+ cells with coronary endothelial dysfunction25
whereas another did not.26
There are several potential explanations for our findings. The present study included a large, unselected sample with comprehensive vascular testing and risk factor evaluation. Thus, the observed lack of association may reflect lower vascular disease burden. In addition, the assessment of associated cardiovascular risk factors in a large sample facilitated adjustment in multivariable models to reduce the potential for confounding. Therefore, it remains possible that variation in circulating quantities of angiogenic cells are associated with vascular dysfunction through pathways that also involve the development of certain traditional risk factors, such as diastolic blood pressure and body mass index. In addition, available techniques for identifying progenitor cell-related phenotypes in humans remain non-specific,1
even though many of these same cell-based phenotypes have been demonstrated to exhibit angiogenic capacity in other settings, most notably in the setting of acute or chronic ischemia.27–29
Furthermore, the angiogenic cell phenotypes, as measured, may also vary in individuals in response to exogenous or endogenous stressors. Thus, current methods for detecting variation in angiogenic cell capacity may lack adequate specificity, particularly in an ambulatory cohort.
Relatively little is known about the behavior of circulating angiogenic cell phenotypes in relation to the progression of chronic vascular disease. Although we did not observe effect modification by CVD status, it remains possible that variability in both measured and unmeasured cell-based phenotypes are important but not as easily detectable in individuals who are relatively healthy or have subclinical disease, compared to individuals with clinically manifest acute or chronic illness.27,28
In addition, mild abnormalities in vascular function occurring in association with depletion of angiogenic cells may, in turn, promote their increased production and/or mobilization into the circulation in the effort to maintain relative vascular homeostasis.30
This could account for the unexpected inverse association observed between CD34+ cells and PAT ratio.
The possibility that production and mobilization might compensate for depletion of any particular cell type to a greater extent in milder rather than more severe vascular disease states could explain the more consistent associations observed in studies relating CFU and/or CD34+ cells with measures of atherosclerosis.31–33
Indeed, we observed in this same Framingham cohort that variation in CFU was significantly associated with measures of coronary artery and abdominal aortic calcification.33
Compared to the measures of vascular function in the present study, measures of arterial calcification are likely to remain more stable in each individual. In addition, large artery calcification represents more advanced vascular disease reflecting cumulative alterations in the arterial wall that appear to relate more directly to angiogenic cell populations. On the other hand, subclinical alterations in vascular function, existing in the absence of marked anatomic disease, constitute early and potentially reversible vascular disease.34,35
Interpreting the present findings in this context, we speculate that measurable variation in circulating angiogenic cells may associate more closely with advanced structural rather than early and dynamic functional vascular abnormalities. Vascular dysfunction that precedes the development of anatomic atherosclerosis may yet be associated with alterations in angiogenic cell turnover; however, current techniques for accurately quantifying supply versus mobilization of these cellular phenotypes remain limited.36
Since our analyses were cross-sectional, our findings do not exclude the possibility that variation in angiogenic cell quantity is related to later development of vascular dysfunction in a time-dependent manner. This limitation applies particularly to analyses of flow-mediated dilation measurements, which were measured at the prior examination cycle to the angiogenic cell populations. There is limited data regarding the stability of flow-mediated dilation (or angiogenic cell phenotypes) over a similar period of time in community-based cohorts. In contrast to flow-mediated dilation, arterial tonometry and digital vascular function measures were performed concurrently with the cell assays. With respect to cell phenotyping, the assays used in the present analysis differed somewhat from those used in prior studies. For CFU phenotyping, our analyses were performed on buffy coat as opposed to whole blood specimens;8
however, this difference is not expected to alter variation in colony formation significantly. For the CD34+ and CD34+/KDR+ assays, we isolated non-adherent rather than adherent peripheral blood mononuclear cells;7,10
however, prior studies have demonstrated consistent findings when using these two approaches.1
The lack of significant associations observed in our analyses could relate, in part, to overfitting of models that included multiple cardiovascular risk factors as covariates. Our sample included middle-aged to older adults of predominantly European ancestry; thus, the generalizability of our findings to other age groups and racial/ethnic populations remains untested.17
The main results of our analysis in a large community-based sample indicate lack of an association between angiogenic cell phenotypes, assessed using established assays, and vascular functional phenotypes after adjustment for traditional cardiovascular risk factors. These findings suggest that our understanding of the relationship between circulating angiogenic cells and vascular function remains limited.