Abstract Body (Do not enter title and authors here): Introduction
Endothelial Progenitor Cells (EPCs) are a marker of endothelial regenerative capacity and promote vascular healing. EPCs are phenotyped by CD34-expressing mononuclear cells that can differentiate into hematopoietic, endothelial, and non-hematopoietic lineages, and dual expression with CD133 identifies an early EPC-enriched subpopulation. Flow-mediated dilation (FMD) is a non-invasive ultrasound technique that assesses brachial artery endothelial function and reflects endothelial nitric oxide bioavailability. Previous studies have shown an association between lower levels of EPCs and impaired vascular function in younger populations. Younger individuals with risk factors also have higher levels of EPCs, an association not seen in elderly populations. There has been no study demonstrating an association between specific subgroups of EPCs and FMD, especially in older populations.

Hypothesis
We hypothesize that lower FMD will be associated with reductions in circulating EPCs in an older population.

Methods
In 125 patients (Mean age 62.8, 38% male, 41.4, % black) with or without mild cognitive impairment, EPCs were enumerated by flow cytometry as CD45^med mononuclear cells expressing CD34 and its subsets expressing CD133. Linear regression was conducted testing the association between circulating EPCs and FMD, comparing the highest to lowest quartile of EPC expression, with adjustments made for age, sex, race, body mass index, history of heart failure, and chronic kidney disease.

Results
After a multivariate adjustment, the highest quartile of CD34+ cells was associated with higher FMD (b=2.16, p=0.045) as well as CD34+ cells co-expressing CD133 (b=1.89, p=0.048).

Conclusions
Lower FMD was associated with lower levels of circulating EPCs in an elderly population. This suggests that elderly individuals that preserve higher levels of circulating EPCs as they age may have improved vascular functioning.