There is accumulating evidence that circulating endothelial cells (CECs) play an important role in angiogenesis. An increased neovascularisation has been documented in bone marrow and lymph nodes of patients with chronic lymphocytic leukemia (CLL). The aim of our study was to compare the number of CECs in peripheral blood of CLL patients and healthy donors and to correlate these numbers with bone marrow microvessel density (MVD). In addition we checked if the number of CECs correlates with the response to treatment. We analyzed the total (CECs), activated endothelial cells (aCECs), apoptotic endothelial cells (apoCECs) and endothelial precursors (CPECs) count by the four colour flow cytometry in the peripheral blood of 80 untreated CLL patients and 29 healthy donors. We also evaluated the MVD in the bone marrow biopsy in 21 CLL patients and 11 controls, using the hot spot analysis of vessels density (x200 HPS). In 34 patients treated with purine analogs (Cladribine or Fludarabine) as first line treatment, we analyzed the influence of the initial number of CECs on treatment results. We found significantly higher numbers of CECs, aCECs, apoCECs and CPECs in CLL patients than in healthy controls (p=0.0001, p=0.001, p=0.0001 and p=0.001 respectively). The total number of CECs was significantly higher in the group of patients with advanced disease (Rai stage 3–4) as compared to patients in Rai stage 0–2 (p=0.02). A correlation was found between CECs and apoCECs number and the β2-microglobulin level (r=0.4, p=0.01 and r=0.3, p=0.01 respectively). The MVD was significantly higher in the bone marrow of CLL patients (median 60, range 27–147) as compared to controls (median 20, range 10–60) (p=0.01). In addition, the number of aCECs correlated with the MVD in CLL patients (r= 0.5, p= 0.02). Moreover, the patients with CECs numbers above median had significantly lower probability to achieve response to treatment with purine analogs (p=0.048). In conclusion, the number of CECs is increased in CLL patients and correlates with vessel density and tumor stage. These findings indicate that analysis of CECs may be a suitable, noninvasive marker of angiogenesis in CLL patients. Evaluation of CECs could be useful to identify CLL patients with low probability of response to purine analogs that may benefit from alternative therapy including anti-angiogenic agents.

Author notes

Disclosure: No relevant conflicts of interest to declare.