© 2007 American Thoracic Society
Circulating Endothelial Cells in Obstructive Sleep Apnea: An Important Methodological LessonTo the Editor:We read with interest the recent article by El Solh and colleagues claiming a link between circulating endothelial cells (CECs) and obstructive sleep apnea (OSA) (1). However, two aspects of the methodology used by El Solh and coworkers warrant further discussion. CECs can be quantified by a number of methods, the commonest being immunomagnetic beads (IB) and flow cytometry. In both of these techniques, anti-CD146 (an endothelial-associated antigen) is frequently used (2). To ensure specificity for CECs, further markers or morphological features are identified. In the case of IB, secondary staining with lectin ulex europeaus is required. Similarly, with flow cytometry (FC), a multiparametric approach using multiple markers is essential. El Solh and coworkers (1) rely solely on CD146 to discriminate CECs by FC. This technique is not supported by much of the recent literature, as CD146 is also now recognized to be present on other cells. This includes not only certain tumor lines (as highlighted by El Solh and coworkers in their article) but also a subpopulation of normal activated lymphocytes (3–5). Therefore, the concurrent use of a leukocyte discriminating marker (e.g., CD45 or a combination of CD33, CD19, CD3, CD14, CD21) is recommended. Thus, El Solh and coworkers may have inadvertently measured other leukocytes. In addition, the fluorochrome conjugated to annexin V appears to have been an inappropriate choice. Importantly, the authors have used both anti-CD146 and annexin V conjugated to phycoerythrin (PE), together. This unfortunate error will undoubtedly have resulted in the failure to distinguish between the three possible groups of cells (CD146+, annexin V+, and CD146+/annexin V+ cells) during FC. Given the intimate relationship between endothelial damage and cardiovascular disease, it seems logical that this process may also occur as a consequence of OSA alone. However, further exploration of this relationship must be based on contemporary methodological processes.
City Hospital, Birmingham, United Kingdom FOOTNOTES Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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