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The angiogenic status of CLL tissues is influenced by cells present in the microenvironment, including CLL
cells. Indeed, CLL cells are known to establish reciprocal interactions with stromal cellular components [28-30] .
These interactions affect cellular functions and modify the CLL cell gene expression pattern, resulting in the
so-called “angiogenic switch” [31-33] . This review describes the angiogenic factors produced by CLL cells, their
regulation by internal or external factors, and their function in angiogenesis and other cellular processes.
We also summarize the anti-angiogenic therapies that have been evaluated in CLL and the results obtained.

CLL CELLS PRODUCE PRO- AND ANTIANGIOGENIC FACTORS AND THEIR RECEPTORS
Angiogenic factors produced by CLL cells
Previous excellent reviews provide a detailed characterization of the angiogenic factors present in CLL and
their possible role in the disease [20,21,34,35] . The present review expands and updates the reported studies. CLL
cells spontaneously synthesize and secrete pro- and antiangiogenic molecules, including basic fibroblast
growth factor (bFGF) [21,34-36] , vascular endothelial growth factor (VEGF) [36-38] , platelet-derived growth factor
(PDGF) , thrombospondin-1 (TSP-1) , angiopoietin-2 (Ang-2) , and matrix metalloproteinase-9
[39]
[36]
[36]
(MMP-9) [40-42] [Figure 1]. The secreted factors are found in the conditioned media of cultured CLL cells as
well as in the plasma/serum and urine of CLL patients, and their levels vary during the course of the disease.
Several groups have analyzed samples from CLL patients by enzyme-like immunoassays and have
demonstrated that elevated levels of bFGF in lymphocytes and plasma correlate with advanced stages of the
disease (Rai stage III/IV) [43-45] . In another study, bFGF levels in urine were also higher in CLL patients than
[22]
in controls, but they did not significantly correlate with the clinical stage .

VEGF is clearly the most studied angiogenic factor in CLL. The human VEGF family comprises five
members: VEGF-A, VEGF-B, VEGF-C, VEGF-D, and placental growth factor [46-48] . VEGF-A is the best
characterized and comprises five different isoforms (VEGF , VEGF , VEGF , VEGF , and VEGF ) that
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arise by alternative splicing of the VEGF gene [46-49] . Using ELISA, Western blot, and RT-PCR analyses of
concentrated CLL cell culture media, Chen et al. demonstrated the presence of VEGF and VEGF , with
[38]
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molecular weights of 28 and 42 kDa, respectively. VEGF is the predominant form in CLL and the best
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studied in terms of expression, regulation and signaling; we refer to this isoform as VEGF throughout this
review.
A correlation was found between elevated levels of serum VEGF in early CLL stages (Rai I/II) and the risk of
CLL progression/progression-free survival, supporting the role of VEGF as a prognostic marker in this
disease [26,50,51] . The amount of plasma PDGF was also higher in CLL patients than in controls and strongly
correlated with the levels of VEGF and with advanced stages (Rai II-IV) and poor prognosis markers (ZAP-
[52]
70 or CD38 positive) . High concentrations of Ang-2 mRNA and plasma protein were also found in
several cohorts of CLL patients, with a significant correlation with an aggressive phenotype (unmutated
IGHV, CD38 positive), advanced Binet stages (B-C), and shorter survival [53-55] . Similarly, the intracellular
and serum concentrations of MMP-9 were higher in CLL than in normal lymphocytes [40-42] . These elevated
MMP-9 levels were detectable at early CLL stages and correlated with the risk of disease progression [56,57] .
[40]
In contrast to the above-mentioned factors, the levels of the antiangiogenic molecule TSP-1 , both mRNA
[58]
and protein, were higher in low-risk CLL patients (Rai I/II) than in high-risk patients (Rai stage > II) . The
[36]
same pattern was observed when TSP-1 was quantitated in the conditioned medium of CLL cells cultured
for 24 h . These studies indicate that expression and secretion of pro- and antiangiogenic molecules is an
[36]
active process in CLL, with a clear proangiogenic switch as disease progresses (see below). Quantitation of
these factors, however, is not done routinely in the clinic when diagnosing and staging CLL, and their
amounts are usually determined as complementary indicators or for specific studies. Accordingly, the levels
of angiogenic factors are not commonly included among the criteria used to decide the initiation of