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VEGF receptor 1 (VEGFR1) and VEGF receptor 2 (VEGFR2). Expression of all three tyrosine kinase VEGF
receptors (VEGFR1, VEGFR2, and VEGFR3) at the CLL cell surface was also demonstrated by flow
[62]
cytometry (13 patient samples) and immunocytochemical (27 patient samples) analyses . The non-
enzymatic VEGF receptor neuropilin-1 was also shown to be present in CLL cells, at both mRNA and
protein levels, and with a higher expression than in control B-cells [63,64] . VEGF binds to VEGFR1 and
VEGFR2 but not to VEGFR3, whose ligands are VEGF-C and -D [46-49] . Although the binding affinity of
VEGF to VEGFR1 and VEGFR2 is high, VEGFR2 is the main receptor responsible for delivering
intracellular signaling upon interaction with VEGF [20,61] . Therefore, most studies on the CLL system have
focused on the functional consequences of the VEGF/VEGFR2 interaction.

[61]
The levels of expression of VEGFR2 in CLL cells are variable. Ferrajoli et al. showed that elevated levels of
VEGFR2 protein, measured in cell extracts, correlated with shorter survival. An association between high
VEGFR2 expression and advanced Rai stages (III/IV) was also observed in that study, although it was not
statistically significant. Conversely, we analyzed the constitutive VEGFR2 expression on 38 CLL samples by
flow cytometry and did not find a correlation with Rai/Binet stages . Despite this, high expression of
[65]
VEGFR2 appears to be a characteristic of CLL cells, suggesting an active role of this receptor in the
pathology of the disease.

Ang-2 and TSP-1 receptors
The receptor for Ang-2 is Tie-2, one of the two members (Tie-1 and Tie-2) of the Tie receptor tyrosine
kinase family . While expression of Tie-1 in CLL cells was early recognized [36,67,68] , the expression of Tie-2
[66]
on these cells has been controversial. Analyses by PCR and flow cytometry on nine plasma CLL samples
failed to show Tie-2 expression, while Tie-2 was present on endothelial cells used as control . Treatment of
[69]
these CLL cells with various stimuli (CD40L, hypoxia, and stromal/endothelial cells) did not induce Tie-2
expression. In the same study, bone marrow-derived CLL cells expressed Tie-2, detected by PCR and flow
[69]
cytometry, suggesting a role for the microenvironment in Tie-2 expression on CLL .
Aguirre Palma et al. did not find a constitutive expression of Tie-2 by PCR analyses of peripheral blood
[68]
CLL cells. However, they observed a transient expression of Tie-2 upon stimulation of these cells with
Ang-2. Another study found that a small percentage of peripheral blood CLL cells expressed Tie-2 at the cell
[71]
[70]
surface . Because Tie-2 undergoes rapid internalization upon activation these authors performed
intracellular staining and found a moderate positive expression of Tie-2 receptor. CLL cells infiltrating
lymph nodes also expressed Tie-2. The presence of Tie-2 was further confirmed by Western blotting and
[70]
PCR analyses . It therefore appears that expression of the Tie-2 receptor in CLL cells is subjected to
modulation by several external factors.

Two main TSP-1 receptors, CD36 and CD47, are expressed by CLL cells. CD36 expression was initially
studied by Rutella et al. on 24 CLL samples. Using flow cytometry and immunofluorescence analyses,
[72]
they found variable expression of CD36 in most CLL cases. Elevated CD36 expression was associated with
[72]
advanced disease (Rai stage III/IV, organ infiltration) . CD47 was also shown to be present in CLL cells
and to mediate several functions upon binding to TSP-1, including cell death and cytoskeleton
reorganization [73,74] .

MMP-9 receptors
Although MMP-9 is mainly a secreted protease found in soluble form in serum and cell conditioned media,
we and others have consistently found MMP-9 at the CLL cell surface [40-42,75] . Using immunoprecipitation
with anti-MMP-9 antibodies, function blocking antibodies, siRNA transfection, and immunofluorescence