Detection and quantification of mature circulating endothelial cells using flow cytometry and immunomagnetic beads: A methodological comparison

Circulating endothelial cells and circulating progenitor cells in breast cancer: relationship to endothelial damage/dysfunction/apoptosis, clinicopathologic factors, and the Nottingham Prognostic Index

BACKGROUND AND METHODS

Abnormal circulating endothelial cell (CEC) and circulating progenitor cell (CPC) numbers are present in cancer, but their relationship with angiogenesis, apoptosis, vascular biology, and prognosis is unclear. We prospectively studied 160 patients with breast cancer and 63 age-matched controls free of breast cancer, measuring CECs (CD45(-)/CD146(+)/CD34(+)) and CPCs (CD45(-)/CD133(+)/CD34(+)) by flow cytometry and plasma markers of endothelial damage/dysfunction (von Willebrand factor), apoptosis (Fas/Fas-L) and angiogenesis (vascular endothelial growth factor [VEGF], angiogenin) by ELISA. These were compared with clinicopathophysiologic features and the Nottingham Prognostic Index (NPI). An additional blood sample was taken 6 to 8 weeks after surgery from 15 women to test the effect of tumor removal.

RESULTS

CECs were significantly higher in the NPI poor prognostic group compared with moderate and good prognostic groups, and the cancer-free controls, whereas CPCs were lower in the poor prognosis group (both P < .05). Levels of von Willebrand factor, VEGF, angiogenin, and Fas-L (but not soluble Fas) were abnormal in breast cancer compared with controls (P < .05), with no relationship to prognosis groups. VEGF (P = .04) and angiogenin (P = .001) were markedly different after surgery. In multivariate analysis, vascular invasion (P < .05) and tumor size (P < .001) were independently associated with CECs. CPCs did not significantly associate with NPI in a linear regression model; age (P < .05) was a negative predictor, whereas Her-2 status (P < .05) positively predicted CPCs. After adjustment, no variable independently predicted CPC levels.

CONCLUSIONS

CECs and CPCs demonstrate a strong relationship with NPI groups, but only CECs positively predict higher NPI scores and correlate with tumor invasiveness and size, possibly reflecting total tumor vascular volume.

Endothelial progenitor cells: identity defined?

In the past decade, researchers have gained important insights on the role of bone marrow (BM)-derived cells in adult neovascularization. A subset of BM-derived cells, called endothelial progenitor cells (EPCs), has been of particular interest, as these cells were suggested to home to sites of neovascularization and neoendothelialization and differentiate into endothelial cells (ECs) in situ, a process referred to as postnatal vasculogenesis. Therefore, EPCs were proposed as a potential regenerative tool for treating human vascular disease and a possible target to restrict vessel growth in tumour pathology. However, conflicting results have been reported in the field, and the identification, characterization, and exact role of EPCs in vascular biology is still a subject of much discussion. The focus of this review is on the controversial issues in the field of EPCs which are related to the lack of a unique EPC marker, identification challenges related to the paucity of EPCs in the circulation, and the important phenotypical and functional overlap between EPCs, haematopoietic cells and mature ECs. We also discuss our recent findings on the origin of endothelial outgrowth cells (EOCs), showing that this in vitro defined EC population does not originate from circulating CD133⁺ cells or CD45⁺ haematopoietic cells.

A six-colour flow cytometric method for simultaneous detection of cell phenotype and apoptosis of circulating endothelial cells

Blood circulating endothelial cells (CECs), with their resting and activated subsets, (rCECs and aCECs) and circulating progenitors cells (CEPs) are two extremely rare cell populations that are important in tissue vascularization. Their number and function are modulated in diseases involving vascular injury, such as human tumours. Although a consensus on the phenotypic definition of endothelial cells, as well as on the optimal enumeration technique, is still lacking, the number of clinical studies based on assessment of these cells is rapidly expanding, as well as the analytical methods employed. The present study aimed to develop a rapid and sensitive flow cytometric method of quantifying and characterizing CECs (with both their subsets and the apoptotic fraction) and CEPs. We analysed peripheral blood samples from 21 subjects with a six-colour flow cytometric approach allowing detection of the cell phenotype of CECs and CEPs using a monoclonal antibodies panel and a dedicated gating strategy. Apoptotic CECs were detected with Annexin V and dead cells with 7-amino-actinomycin D staining. The described technique proved to be a new, reliable, tool increasing our knowledge of the biology of CECs and CEPs and can readily be applied in the study of many pathological conditions characterized by endothelial damage.

Flow cytometric assessment of monocyte activation markers and circulating endothelial cells in patients with localized or metastatic breast cancer

BACKGROUND

Monocyte activation in cancer patients may be reflective of anticancer activity. However, studies indicate that recruitment of macrophages can actually promote tumor growth and angiogenesis. Assessment of other microenvironmental cells such as circulating endothelial cells (CECs) may provide additional information regarding disease progression. The objective of this study was to assess monocyte activation and CECs in breast cancer patients and determine the potential clinical relevance during disease progression.

METHODS

Patients (n = 41) with localized or metastatic breast cancer who were not currently receiving treatment were eligible for study inclusion. Peripheral blood was collected and analyzed by flow cytometry for monocyte activation (Leuko64 assay kit), and for CECs (CD146(+)CD45(-) phenotype).

RESULTS

Metastatic breast cancer patients demonstrated a higher monocyte CD64 index relative to normal donors and localized breast cancer patients (P < 0.05). Furthermore, breast cancer patients had a lower monocyte CD163 index relative to normal donors (P = 0.008). Localized breast cancer patients demonstrated higher levels of CD146(+)CD45(-) cells CECs relative to metastatic breast cancer patients and normal donors. Within the localized breast cancer population, levels of CD146(+)CD45(-) cells increased with disease stage (P < 0.05).

CONCLUSIONS

These results suggest that monocyte activation and CECs may play a role in breast cancer progression. We speculate that monocyte activation may reflect a reaction to metastatic cells and/or response to tissue damage caused by metastatic growth in distant organs. Furthermore, the observation that CECs increase with disease stage in localized breast cancer suggests that CECs could be a useful surrogate marker for disease progression in this patient population.

The relationship of circulating endothelial cells to plasma indices of endothelial damage/dysfunction and apoptosis in acute coronary syndromes: implications for prognosis

BACKGROUND

While coronary artery disease has been linked to both endothelial damage and cellular apoptosis, their inter-relationships and impact on cardiovascular (CV) outcomes has been barely explored.

AIMS

First, we investigated the inter-relationships between circulating endothelial cells (CECs, and index of endothelial damage) and circulating plasma markers of endothelial damage/dysfunction [von Willebrand factor (VWF), soluble E selectin (sEsel)] and apoptosis [soluble Fas (sFas), Fas ligand (sFasL) and their ratio, sFas/sFasL] in patients presenting with acute coronary syndrome (ACS). Second, we assessed their prognostic values for major adverse CV events (MACE) in ACS.

METHODS

We studied 211 patients with ACS, who were compared with 60 healthy controls (HC) and 45 'disease controls' (patients with stable coronary artery disease, CAD). Simultaneous blood samples for CECs (immunobead method), VWF, sESel, sFas and sFasL (ELISA) were taken within 24 h of presentation of ACS and at 48 h post admission.

RESULTS

CEC, sEsel and VWF levels were significantly higher among the ACS groups compared with the CAD and HC (P < 0.05) groups. sFas was higher (P = 0.016) and sFasL lower (P = 0.021) in ACS compared with controls (HC and CAD). There was a significant increase in sFas/sFasL ratio with increasing disease severity (P = 0.0004). There were significant correlations between CECs and both VWF and sEsel (both P < 0.01) but no correlations between CECs and either sFas or sFas ligand. On univariate survival analysis, CECs were associated with an increased risk of both MACE [hazard ratio (HR) 2.4 (95% CI 1.2-4.1); P = 0.009] and cardiovascular death [HR 2.95 (95% CI 1.01-8.81); P = 0.047]. On multivariate Cox regression analysis, only VWF (and not CECs) remained as an independent predictor of MACE [HR 1.02 (95% CI 1.005-1.040); P = 0.009].

CONCLUSION

CECs were associated with abnormal plasma indices of endothelial damage/dysfunction and not apoptosis, despite abnormalities of all these markers being associated with ACS. VWF remained as an independent predictor of MACE, on multivariate analysis.

Quantification of circulating endothelial and progenitor cells: comparison of quantitative PCR and four-channel flow cytometry

Background

Circulating endothelial cells (CEC) and endothelial precursor cells (CEP) have been suggested as markers for angiogenesis in cancer. However, CEC/CEP represent a tiny and heterogeneous cell population, rendering a standardized monitoring in peripheral blood difficult. Thus, we investigated whether a PCR-based detection method of CEC/CEP might overcome the limitations of rare-event flow cytometry.

Findings

To test the sensitivity of both assays endothelial colony forming cell clones (ECFC) and cord blood derived CD45^- CD34⁺ progenitor cells were spiked into peripheral blood mononuclear cells (PBMNC) of healthy volunteers. Samples were analyzed for the expression of CD45, CD31, CD34, KDR or CD133 by 4-color flow cytometry and for the expression of CD34, CD133, KDR and CD144 by qPCR. Applying flow cytometry, spiked ECFC and progenitor cells were detectable at frequencies ≥ 0.01%, whereas by qPCR a detection limit of 0.001% was achievable. Furthermore, PBMNC from healthy controls (n = 30), patients with locally advanced rectal cancer (n = 20) and metastatic non-small cell lung cancer (NSCLC, n = 25) were analyzed. No increase of CEC/CEP was detectable by flow cytometry. Furthermore, only CD34 and KDR gene expression was significantly elevated in patients with metastatic NSCLC. However, both markers are not specific for endothelial cells.

Conclusion

QPCR is more sensitive, but less specific than 4-channel flow cytometry for the detection of CEC/CEP cell types. However, both methods failed to reliably detect an increase of CEC/CEP in tumor patients. Thus, more specific CEC/CEP markers are needed to validate and improve the detection of these rare cell types by PCR-based assays.

Quantification of circulating mature endothelial cells using a whole blood four-color flow cytometric assay

Circulating endothelial cells (CEC) are currently proposed as a potential biomarker for measuring the impact of anti-angiogenic treatments in cancer. However, the lack of consensus on the appropriate method of CEC measurement has led to conflicting data in cancer patients. A validated assay adapted for evaluating the clinical utility of CEC in large cohorts of patients undergoing anti-angiogenic treatments is needed. We developed a four-color flow cytometric assay to measure CEC as CD31(+), CD146(+), CD45(-), 7-amino-actinomycin-D (7AAD)(-) events in whole blood. The distinctive features of the assay are: (1) staining of 1 ml whole blood, (2) use of a whole blood IgPE control to measure accurately background noise, (3) accumulation of a large number of events (almost 5 10(6)) to ensure statistical analysis, and (4) use of 10 microm fluorescent microbeads to evaluate the event size. Assay reproducibility was determined in duplicate aliquots of samples drawn from 20 metastatic cancer patients. Assay linearity was tested by spiking whole blood with low numbers of HUVEC. Five-color flow cytometric experiments with CD144 were performed to confirm the endothelial origin of the cells. CEC were measured in 20 healthy individuals and 125 patients with metastatic cancer. Reproducibility was good between duplicate aliquots (r(2)=0.948, mean difference between duplicates of 0.86 CEC/ml). Detected HUVEC correlated with spiked HUVEC (r(2)=0.916, mean recovery of 100.3%). Co-staining of CD31, CD146 and CD144 confirmed the endothelial nature of cells identified as CEC. Median CEC levels were 6.5/ml (range, 0-15) in healthy individuals and 15.0/ml (range, 0-179) in patients with metastatic carcinoma (p<0.001). The assay proposed here allows reproducible and sensitive measurement of CEC by flow cytometry and could help evaluate CEC as biomarkers of anti-angiogenic therapies in large cohorts of patients.

Measuring angiogenic cytokines, circulating endothelial cells, and endothelial progenitor cells in peripheral blood and cord blood: VEGF and CXCL12 correlate with the number of circulating endothelial progenitor cells in peripheral blood

Circulating endothelial cells (CECs) and endothelial progenitor cells (EPCs) are thought to play an important role in the vascularization of damaged tissues and cancers. These cells are also required for tissue-engineered blood vessels and to help skin substitutes revascularize more efficiently. A standard approach to the phenotyping and enumeration of CEC and EPC is key to the development of new therapies, and the identification of biomarkers within the blood that regulate their levels may be important for the treatment of cancer. We have devised an improved multiparameter flow cytometric assay for CEC and circulating EPC enumeration. This assay uses antibodies recognizing CD133 and CD34 to identify EPC and CEC, respectively, and incorporates specific markers CD144 and vascular endothelial growth factor receptor 2 (VEGFR-2) for both CEC and EPC cells. In peripheral blood (PB), mean CEC numbers were 55 +/- 95 mL(-1) and mean EPC numbers were 44 +/- 58 mL(-1) (n = 60). We also found a significant correlation of both plasma VEGF (r = 0.90, p < 0.001) and CXCL12 (r = 0.84, p < 0.001) with EPCs, but not CECs. The cytokines also correlated with each other (r = 0.85, p < 0.001). In umbilical cord blood (UCB) we found on average 13 times more CEC (719 +/- 338 mL(-1)) and 7 times more EPC (299 +/- 245 mL(-1)) than in PB. However, serum VEGF and CXCL12 levels in UCB did not correlate with either EPC or CEC numbers. These results suggest a major role for VEGF and CXCL12 in the control of marrow-derived EPCs in adult PB and provide normal data for comparison with patient populations.

Quantitative detection of circulating endothelial cells in vasculitis: comparison of flow cytometry and immunomagnetic bead extraction

BACKGROUND

Circulating endothelial cells (CECs) are biomarkers for endothelial cell (EC) injury and are quantified using immunomagnetic bead extraction (IBE), or flow cytometry (FC). Reports suggest that there is good agreement between these methods for CEC quantification.

OBJECTIVES

We examined levels of agreement between these techniques in children with systemic vasculitis.

METHODS

We added HUVEC or human pulmonary artery EC to whole blood to optimize FC gating strategies for EC. EC-optimized FC was then compared with IBE for CEC enumeration in 25 children with vasculitis and 20 healthy controls.

RESULTS

Using Bland-Altman analysis, agreement between IBE and EC-optimized FC was poor in children with vasculitis (n = 25) and healthy controls (n = 20): IBE consistently detected higher values than the EC-optimized FC method: the mean difference between the two techniques was 60 CECs mL(-1), 95% CI +/-374 CECs mL(-1) (paired analyses of 45 individuals). Agreement was poorest for vasculitis patients: mean difference (IBE - EC-optimized FC) 120 CECs mL(-1), 95% CI +/-460 CECs mL(-1) (P = 0.018). We identified three reasons for this discrepancy: (i) sub-optimal FC gating parameters previously used for detecting CECs; (ii) inherent lack of sensitivity of FC compared with IBE for CEC rare event detection; and (iii) use of lysis buffers required for FC causing CEC lysis.

CONCLUSIONS

There was poor agreement between EC-optimized FC and IBE for the quantification of CECs from children with active vasculitis and controls. We emphasize that in this clinical setting the two techniques are not directly comparable when comparing results obtained using these different methodologies.

Circulating endothelial cells in oncology: pitfalls and promises

Adequate blood supply is a prerequisite in the pathogenesis of solid malignancies. As a result, depriving a tumour from its oxygen and nutrients, either by preventing the formation of new vessels, or by disrupting vessels already present in the tumour, appears to be an effective treatment modality in oncology. Given the mechanism by which these agents exert their anti-tumour activity together with the crucial role of tumour vasculature in the pathogenesis of tumours, there is a great need for markers properly reflecting its impact. Circulating endothelial cells (CEC), which are thought to derive from damaged vasculature, may be such a marker. Appropriate enumeration of these cells appears to be a technical challenge. Nevertheless, first studies using validated CEC assays have shown that CEC numbers in patients with advanced malignancies are elevated compared to healthy controls making CEC a potential tool for among other establishing prognosis and therapy-induced effects. In this review, we will address the possible clinical applications of CEC detection in oncology, as well as the pitfalls encountered in this process.

CD146-based immunomagnetic enrichment followed by multiparameter flow cytometry: a new approach to counting circulating endothelial cells

BACKGROUND

Circulating endothelial cells (CECs) have emerged as non-invasive biomarkers of vascular dysfunction. The most widely used method for their detection is CD146-based immunomagnetic separation (IMS). Although this approach has provided consensus values in both normal and pathologic situations, it remains tedious and requires a trained operator.

OBJECTIVES

Our objective was to evaluate a new hybrid assay for CEC measurement using a combination of pre-enrichment of CD146+ circulating cells and multiparametric flow cytometry measurement (FCM).

PATIENTS AND METHODS

CECs were determined in peripheral blood from 20 healthy volunteers, 12 patients undergoing coronary angioplasty, and 30 renal transplant recipients, and blood spiked with cultured endothelial cells. CD146+ cells were isolated using CD146-coated magnetic nanoparticles and labeled using CD45-fluorescein isothiocyanate and CD146-PE or isotype control antibody and propidium iodide before FCM. The same samples were also processed using CD146-based immunomagnetic separation as the reference method.

RESULTS

The hybrid assay detected CECs, identified as CD45(dim)/CD146(bright)/propidium iodide(+), with high size-related scatter characteristics, and clearly discriminated these from CD45(bright)/CD146(dim) activated T lymphocytes. The method demonstrated both high recovery efficiency and good reproducibility. Both IMS and the hybrid assay similarly identified increased CEC levels in patients undergoing coronary angioplasty and renal transplantation, when compared to healthy controls. In patients, CEC values from these two methods were of the same order of magnitude and highly correlated. Bland-Altman analysis revealed poor statistical agreement between methods, flerrofluid-FCM providing higher values than IMS.

CONCLUSION

This new hybrid FCM assay constitutes an accurate alternative to visual counting of CECs following CD146-based IMS.

A maximal exercise bout increases the number of circulating CD34+/KDR+ endothelial progenitor cells in healthy subjects. Relation with lipid profile

Mobilization of bone marrow-derived endothelial progenitor cells (EPC) might explain exercise-induced improvement of endothelial function. We assessed whether a maximal exercise bout could alter the number of circulating EPC in healthy subjects and whether this effect is related to their cardiovascular risk profile. Additionally, we investigated possible mediators of this effect, namely nitric oxide (NO) bioavailability and vascular endothelial growth factor (VEGF) release. Healthy subjects (group 1, n = 11; group 2, n = 14) performed a symptom-limited cardiopulmonary exercise test on a bicycle ergometer. Numbers of CD34+/kinase insert domain receptor (KDR)+ cells were determined by flow-cytometric analysis, either after magnetic separation of CD34+ cells (group 1) or starting from whole blood (group 2). Serum concentrations of VEGF and NO metabolites were measured by using ELISA. Following exercise, EPC increased by 76% (15.4 +/- 10.7 cells/ml vs. 27.2 +/- 13.7 cells/ml; P = 0.01) in group 1 and by 69% in group 2 (30.9 +/- 14.6 cells/ml vs. 52.5 +/- 42.6 cells/ml; P = 0.03). The increase in EPC correlated positively with LDL and total cholesterol/HDL ratio and negatively with peak oxygen consumption and oxygen consumption at anaerobic threshold. VEGF levels increased with exercise, with a strong trend toward significance (P = 0.055). NO levels remained unchanged. The present study demonstrates that a maximal bout of exercise induces a significant shift in CD34+ cells toward CD34+/KDR+ cells. This response was larger in subjects with a less favorable lipid profile.