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Novel Insights into the Biology of CLL
Mark C. Lanasa1
1Division of Medical Oncology, Duke University Medical Center, Durham NC
Significant advancements in the care of patients with chronic lymphocytic leukemia (CLL) have occurred over the pastdecade. Nonetheless, CLL remains incurable outside of allogeneic transplantation. CLL is the most common leukemia
in the United States and Europe, and new treatments and therapeutic strategies are clearly needed. To address this
need, the pathogenesis of CLL has been an area of intense ongoing investigation. These international efforts illuminate
a complex biology that is reliant on the interplay of inherited, environmental, and host factors. This broad review will
discuss the recent advances in our understanding of CLL biology including the elucidation of inherited and acquired
genetic changes; the role of the B-cell receptor and B-cell receptor signaling; CLL cell kinetics; and the interactions in
the microenvironment between CLL cells, other immune cells, and stromal elements. This improved understanding of
disease pathogenesis is facilitating the development of novel therapeutic treatment strategies.
Chronic lymphocytic leukemia (CLL) remains an enigmatic dis-
ease. Although the first clinical description of chronic lymphoidleukemia was published over 150 years ago, the etiology of CLL is
unknown, the cell of origin of CLL is unknown, and CLL remains
incurable outside of allogeneic transplantation. CLL is the most
common leukemia in the United States, with approximately 15,500
new diagnoses per year. Because of the relatively long survival of
patients with CLL, it is by far the most prevalent leukemia, with an
estimated 95,000 Americans living with CLL.1 CLL lymphocytes
have a characteristic immunophenotype: CD5, CD19, CD20dim,
CD23, and surface immunoglobulindim. Although the majority of
patients are asymptomatic at diagnosis, the relentless accumulation
of CLL lymphocytes leads to symptomatic disease, need for
CLL-directed therapy, disease-related complications, and approxi-
mately 4500 CLL attributable deaths per year in the United States.
Several characteristics of CLL facilitate basic and translational
research: (i) the high population prevalence; (ii) the malignant cells
are easily obtained through venous phlebotomy; (iii) most patients
have an asymptomatic phase that allows for longitudinal evaluation;
and (iv) CLL is has a relatively long disease-specific survival.
Therefore, CLL has become a model system for the investigation of
B-cell lymphoproliferative disorders. In the 5 years since the
biology of CLL was last broadly reviewed in the American Society
of Hematology education session, tremendous progress has been
made in the understanding of CLL disease biology, and this review
will focus on these discoveries. Specifically, important advances
have been made in identifying inherited and acquired genetic
mutations, the role of B-cell receptor (BCR) signaling, and the
interplay between the malignant B cells and the tumor microenviron-ment. These advances reveal CLL to be a disease that is dependent
on on the interplay of inherited, environmental, and host factors.
Inherited Genetic FactorsThe most important risk factor for the development of CLL is a
family history of CLL. Among patients with newly diagnosed CLL,
8% to 10% have a family history of CLL. CLL has a heritability that
is twice that of common solid tumors with known low-prevalence,
high penetrance causative genes, such as breast and colon cancer. 2,3
Pedigree analysis using data from the Swedish Family Cancer
Database showed the relative risk of CLL among first-degree
relatives of persons with CLL to range between 7.0 and 8.5. This
equivalent risk across all first-degree relatives suggests an inherited,rather than environmental, basis for familial CLL. 4 Though families
with four or more cases of CLL have been reported, they are
extremely rare. Some CLL kindreds suggest a dominant inheritance,
therefore, genome-wide linkage studies in familial CLL have been
performed. The largest and most recent reported study added 101
new cases of familial CLL to a previously reported cohort of 105
families.5 This study was the first statistically significant genome-
wide linkage scan and identified chromosome 2q21.2 as associated
with inheritance of CLL. However, no causative genes have been
identified at this locus. Overall, linkage studies in CLL have been
limited by the low number of affected individuals per family, late
age of onset, and presumed genetic heterogeneity.
Complete sequencing of the human genome revealed large numbers
of common single nucleotide polymorphisms (SNPs). This discov-
ery led to the hypothesis that the inheritance of complex diseases
may be due to the coinheritance of these common variants. Di
Bernardo et al6 performed a multistage genome-wide SNP associa-
tion (GWA) study to evaluate the contribution of common variants
to the inheritance of CLL. This study identified six novel loci
associated with the development of CLL, thus providing the first
evidence for the contribution of common (population prevalence
5%) genetic variants to the development of CLL. Five of these SNPs
have been validated in an independent GWA.7 Subsequently, four
additional susceptibility loci have been identified through follow-up
analyses from the initial Di Bernardo study.8 Identification of these
genes informs our understanding of CLL biology. For example, theGWA linked interferon regulatory factor 4 (IRF4), a key regulator
of lymphocyte maturation and proliferation, with risk of developing
CLL. The 10 loci identified to date individually confer a small risk
of disease, with relative risks ranging from 1.2 to 1.6. As a group,
these risk alleles account for 10% of the total heritability of CLL.
Because GWA can detect only those alleles with a population
prevalence of 5%, it is possible that much of the inherited risk of
CLL is due to uncommon inherited variants (prevalence 5%).
Massively parallel sequencing technologies may enable the discov-
ery of these rare variants and illuminate the hidden heritability
in CLL.
CHRONIC LYMPHOCYTIC LEUKEMIA
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Acquired Genetic FactorsIn a landmark report by the German CLL Study Group, Dohner et
al9 showed that acquired chromosomal abnormalities involving
chromosomes 11, 12, 13, and 17 are common in CLL, and that these
abnormalities predict both time to first treatment and CLL-specific
survival. The genes principally responsible for the adverse progno-
sis associated with del 17p13 and del 11q22 were recognized to be
TP53 and ATM, respectively. The genes that contribute to CLL
pathogenesis in trisomy 12 remain unknown. There is ongoing
debate regarding whether trisomy 12 confers an increased risk of
disease progression; unlike the Dohner study, our institutional
experience is that patients with trisomy 12 do follow a somewhat
more aggressive disease course than those patients with normal
FISH (fluorescence in situ hybridization).10 Deletion of 13q14 is the
most common and most favorable cytogenetic abnormality in CLL.
The responsible genes at this locus were initially unclear. In 2002,
Calin et al11 showed that the microRNA 15/16 (miR 15/16) cluster
were the critical deleted genes in this region. MicroRNAs are small
(approximately 20 nucleotides) nonprotein coding RNAs that
modulate the level of specific proteins by binding sequence-
complimentary mRNAs. miR-15 and miR-16 were subsequently
shown to be negative regulators of BCL2.12 This discovery is
notable in that it was the first association of clinical disease with
microRNAs.
The Dohner study9 established interphase cytogenetics (FISH) as
the clinical standard of care for evaluation of chromosomal aberra-
tions in CLL because the low proliferative capacity of most CLL
limits the clinical utility of metaphase cytogenetics. However,
emerging data shows that the evaluation of four (or five) chromo-
somal loci by FISH obscures the significant heterogeneity and
complexity of acquired chromosomal defects in CLL. The applica-
tion of high-density SNP arrays illuminates some of this complex-
ity. For example, copy number neutral loss of heterozygosity is
observed in a significant number of CLL cases, and this finding
cannot be detected with either FISH or standard karyotyping. Using
SNP arrays, Ouilette et al13 showed significant heterogeneity among
13q14 deletions, with some deletions extending centromeric to
include the retinoblastoma gene. A follow-up study showed that
13q14 deletions that include retinoblastoma are associated with
genomic complexity,14 a finding associated with aggressive disease
in prior clinical reports. Although data currently available are
inadequate to support the widespread application of SNP arrays to
routine clinical care, this technology may ultimately be a useful
adjunct to routine clinical FISH.
At the chromosomal level, del 17p13 is more homogenous than
other acquired genomic defects. High-density SNP arrays showed
that the deletion breakpoint typically falls within chromosomal band
17p11.2 and extends telomeric to include most of the p arm.15
Deletions of 17p13 are almost always monoallelic, and it was
initially unclear why monoallelic loss of TP53 would cause such a
dramatic change in CLL proliferative capacity, chemosensitivity,
and prognosis. It is now understood that 80% of cases with del
17p13 have single nucleotide somatic mutations of the TP53 allele
on the other chromosome 17 (the allele in trans), thereby causing
near complete loss of TP53 function. Although uncommon, TP53
inactivating mutations can occur in the absence of del 17p13, and
these mutations apparently confer an equivalent adverse risk as del
17p13.16 TP53 is a multimeric protein with four identical subunits,
and these inactivating mutations may act in a dominant negative
manner, thus abrogating TP53 function even in the presence of a
wild-type allele in trans. Although only 5% of CLL cases show
deletion or mutation of 17p13 at diagnosis, somatic mutations occur
in approximately one-third of relapsed or chemotherapy refractory
patients. Whether this represents clonal selection or clonal evolution
or both is an area of active investigation.
Epigenetic changes are also relevant to the pathogenesis of CLL.
Epigenetic changes are noninherited chromosomal modifications
that affect gene transcription, such as methylation of gene promoters
and acetylation of histone-bound DNA. An example of epigenetic
change, global hypomethylation of DNA, has been described in
CLL. Between 2% and 8% of CpG islands (gene promoter
elements) are aberrantly methylated in CLL when compared with
normal B cells, a finding that suggests DNA methylation broadly
affects the transcriptional profile of CLL. A germline mutation in
death-associated protein kinase 1 (DAPK1) that segregated with
risk of CLL was identified by Raval et al18 in a family with multiple
CLL cases. The authors showed that DAPK1 expression is silenced
through promoter methylation in the majority of CLL cases,
suggesting a central role for both epigenetic modification and
DAPK1 in CLL leukemogenesis.18 Using high-density methylation
microarrays evaluating over 27,000 CpG sites, Kanduri et al19
identified differential patterns of methylation that were dependent
on the mutation status of the BCR. Poor-risk CLL showed
methylation profiles that facilitated signaling through proliferative
cellular pathways, including MAPK (mitogen-activated protein
kinase) and NF-B (nuclear factor- light-chain enhancer of
activated B cells), a finding that directly relates the clinical
phenotype of poor-risk CLL to methylation of BCR-mediated
signaling pathways.
The Role of BCR in CLLInvestigation of the immunoglobulin gene has led to findings that
are central to the current understanding of CLL (Table 1). During
B-cell development, variable-diversity-joining (VDJ) recombina-
tion generates an immunoglobulin heavy chain (IGH), and an
immunoglobulin light chain (IGL) is created through isotype-
specific VJ recombination. There is tremendous combinatorial
potential in this process, with perhaps 1010 potential IGH-IGL
sequence combinations. However, patients with CLL use a highly
restricted set of BCRs.20 Approximately 14% of all CLL cases
express the heavy chain VH169, and an additional 18% express
VH434. This finding of marked restriction of the IGH repertoire
led to the hypothesis that tonic stimulation by specific auto or
alloantigens drives expansion of the malignant clone.21,22 Intra-
clonal diversification of some CLL further supports the antigen-
drive hypothesis.23
During the germinal center reaction, somatic mutations are gener-
ated in the BCR to increase affinity for target antigen. Approxi-
mately half of CLL cases show IGH mutations, and the other half
Table 1. The mutation status of the immunoglobulin heavy chain (IGVH)
as a central determinant of CLL biology
IGVH Unmutated IGVH Mutated
Cell of origin Mature (antigen experienced) B
cell
Mature (antigen experienced) B
cell
Postulated mechanism of
maturation
Germinal center, T-help
dependent
T-help independent, germinal
center independent (?)
Chromosomal abnormalities del 11q22, del 17p13 del 13q14 (sole)
Frequency of BCR stereotypy 40% 10%
Antigen recognition Polyreactive oligoreactive
Response to BCR cross-linking Activation, proliferation Anergy or no response
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show an unmutated IGH, typically defined as 2% deviation
from germline sequence. Patients with mutated IGH typically
follow a more indolent disease course than those with unmutated
IGH24. This initially led to the hypothesis that CLL may be two
distinct entities, one deriving from naive (IGH unmutated) B cells
and the other from postgerminal center memory (IGH mutated) B
cells. However, gene expression profiling studies in CLL show both
IGH mutated and unmutated CLL to be derived from memory B
cells.25,26 The mechanism by which some CLL escape IGH mutation
is unclear. One hypothesis proposes that certain antigens favor ahelper T-cell independent or germinal center independent matura-
tion that does not involve IGH mutation. Alternatively, the IGH
unmutated CLL cells may be autoreactive and targeted for apopto-
sis, but avoid cell death through either tonic BCR stimulation or
transforming genetic events.
Recent experiments using phage display libraries have attempted to
better define the types of antigens and epitopes recognized by IGH
mutated and unmutated CLL. Monoclonal antibodies (MAbs)
generated from the BCR of IGH-mutated CLL bound epitopes with
structurally related amino acid motifs, whereas unmutated CLL
recognized multiple different epitopes.27 This suggests that in vivo
multiple, potentially structurally divergent antigens can bind and
stimulate CLL cells through the BCR. This conclusion is provoca-
tive because six of eight of the CLL MAbs investigated had
stereotyped BCRs. Stereotypy is the observation of near complete
sequence homology of the complimentarity-determining region 3
(CDR3) of the IGH28; the CDR3 largely defines the antigen
specificity of the BCR. For example, approximately 1% of all CLL
express VH1 69 that are virtually identical within the CDR3.29 The
presence of a stereotyped BCR is associated with adverse risk,
independent of IGH mutation status, although stereotypy is more
commonly observed among unmutated (40%) than mutated CLL
(10%). The observation of stereotypy has been forwarded as
evidence of selective antigenic pressure in CLL; however, MAbs
derived from these BCRs may be oligo- or polyreactive in vivo.
MAbs derived from stereotyped BCRs have also been shown to
react with conserved epitopes exposed on apoptotic cells.30 The
selection of stereotyped BCRs may be related to a combination of
antigenic pressure and CDR3 sequence facilitated BCR signaling.
Given the clear importance of the BCR in the clinical behavior of
CLL, the role of BCR-mediated signaling is another area of
significant interest. The surface density of the BCR is lower in CLL
than in most B-cell subsets, suggesting CLL may have attenuated
signaling responses to antigen binding. Experimental evidence
shows that, in vitro, approximately half of all CLL retain BCR-
mediated signaling, and that the IGH mutation status and other
molecular characteristics used for clinical prognostication (eg,
CD38 and ZAP70 [zeta-associated protein 70]) are important
determinants of BCR responsiveness. After cross-linking the BCRs
with isotype-specific antibodies, Lanham et al31 showed global
increases in tyrosine phosphorylation, a marker of induction of
intracellular signaling. The responsiveness to cross-linking was
strongly associated with IGH mutation status (P .0005), as well as
expression of CD38 (P .05).31 Using gene expression profiling,
Guarini et al32 showed that BCR cross-linking with anti-IgM led to
the transcription of gene pathways regulating cell-cycle regulation,
cytoskeletal organization, and proliferation; but, these changes were
only observed among IGH unmutated cases. BCR ligation leads to
enhanced intracellular signaling among ZAP70 expressing CLL,
and the introduction of ZAP70 into CLL not otherwise expressing
ZAP70 augments BCR-mediated signaling, suggesting a direct role
for ZAP70 in BCR-mediated signaling.33 Muzio et al34 showed that
CLL B cells that do not respond to BCR ligation (typically
IGH-mutated cases) show activation of cellular pathways that
suggest anergy. BCR signaling in vivo is likely even more
complicated: assuming that there are multiple antigens capable of
binding the BCR, the binding affinity for a specific antigen likely
determines whether the response is activating or anergic. Although
this complexity in vivo remains incompletely understood, in
general, BCR ligation in IGH unmutated CLL leads to predomi-
nantly activating and proliferative responses, whereas BCR signal-ing in IGH-mutated CLL favors anergic and antiapoptotic re-
sponses. Additionally, the adverse risk associated with ZAP70 and
CD38 expression may be directly related to their effects on cell
signaling and activation.
CLL Cell KineticsThe conventional view of CLL had been that it is primarily a disease
of failed apoptosis and passive accumulation. This view is sup-
ported by the observation that the great majority ( 98%) of
peripherally circulating CLL cells are arrested in G0 or the early G1
phase and have overexpression of antiapoptotic proteins, such as
BCL2. Other observations suggest that CLL may have significant
proliferative capacity. As discussed in the prior section, IGH
unmutated CLL has the capacity for proliferative responses to BCR
ligation. Many CLL express markers of cellular activation (eg,
CD38, CD49d, and CD69), providing a link between CLL cell
biology and prognostics. Clonal evolution and Richters transforma-
tion are observed in some cases. In a series of elegant experiments,
Messmer et al 35 evaluated the in vivo kinetics of CLL by having
patients consume fixed doses of deuterated heavy water (2H2O)
for 84 days. The deuterium was incorporated into newly synthesized
DNA during the S phase. Using mass spectrometry, the birth and
death rates of CLL cells were then determined. Although there
was significant variability in birth rates (0.11%1.76%/day), all
patients had a rate of new cell formation of at least 10 9 new CLL
cells per day. Patients with higher birth rates (exceeding 0.35%/day)
were more likely to have symptomatic disease. Significant variabil-
ity was also observed in the cellular death rate, and the balance
between birth and death likely determines both white blood cell
trend and risk of progression.35
The deuterated water experiments also provided novel insights into
CLL intraclonal heterogeneity and CLL cell trafficking. By flow
sorting the CD38 and CD38 fractions from CLL patients receiv-
ing deuterated water, Calissano et al36 showed that the CD38
fraction proliferated at a greater rate than the CD38 fraction. It has
previously been shown that, regardless of the proportion of CD38
cells, these cells show significantly increased expression of prolifera-
tion factors ZAP70, Ki67, and telomerase when compared with the
CD38 fraction.37 In a more recent report, Calissano et al provided
preliminary evidence that new CLL cells have a distinct immuno-
phenotype: CD5hiCXCR4low, whereas resting CLL cells are
CD5lowCXCR4hi. Gene expression array analysis of flow sorted
new and resting cell fractions that showed that new cells
differentially express genes related to proliferation, cellular activa-
tion, and cell signaling, whereas resting cells predominantly
express genes involved in apoptosis, cell death, and migration. 38
These studies have led to the hypothesis that CLL cells in the blood
compartment eventually enter a pathway of cellular senescence.
Upregulation of CXCR4 (a CXC chemokine receptor) promotes
reentry into the tumor microenvironment, where the cells receive
prosurvival signals. Rather than undergoing apoptosis, a subset of
resting CLL cells are activated, proliferate, downregulate CXCR4,
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and are then released back into the peripheral blood. These studies
shed light on the phenotypic continuum of CLL cells and better
define the life cycle of a CLL B cell, as well as the central role of the
tumor microenvironment in maintaining the malignant clone.
The Tumor Microenvironment in CLLOver the past decade, the essential role of the tumor microenviron-
ment in the survival and progression of CLL has become increas-
ingly clear. The tumor microenvironment describes an admixture of
malignant cells with host immune cells, stromal elements, and
vascular cells that create a niche wherein signals can be transmitted
through antigen presentation, cellcell interactions, and paracrine
signaling. The microenvironments differ in the bone marrow and
secondary lymphoid organs, where the former contains mesenchy-
mal stromal cells (MSC) within vascular niches and in the latter
nurse-like cells (NLCs), T cells and follicular dendritic cells are
present.
NLCs are large CD14 mononuclear cells that are abundant in the
secondary lymphoid organs of patients with CLL.39 NLCs and
MSCs constitutively express the chemokines CXCL12 (stromal-
derived factor-1, Sdf-1) and CXCL13. CLL cells express CXCR4
(CD184), the receptor for Sdf-1, and in vitro studies have shown
that engagement of Sdf-1 with CXCR4 promotes cell survival
through activation of the signal transducer and activator of transcrip-
tion 3 (STAT3) and MAPK pathways. Blockade of this interaction
with a small molecule inhibitor leads to decreased CLL viability,
suggesting this interaction is a potential therapeutic target.40 Addi-
tional studies have shown that CLL cell viability can be significantly
enhanced in vitro with Sdf-1, although viability is further enhanced
with NLCs over Sdf-1, suggesting that NLCs also deliver additional
prosurvival signals. It is now understood that, in addition to Sdf-1,
NLCs also secrete B-cell activating factor (BAFF) and a proliferation-
inducing ligand (APRIL). Engagement of these ligands with CLL
cells induces intracellular NF-B1 and MCL-1, whereas sdf-1
induces ERK1/2 and AKT. As such, NLCs can deliver multiple
prosurvival signals that utilize distinct signaling pathways.41 Engage-
ment of CD40, expressed on the surface of CLL cells, with CD154
(CD40L), expressed on CD4 T cells found in CLL pseudofollicles,
also induces the NF-B pathway. These diverse interactions have
important implications for CLL therapy, particularly as mechanisms
of chemotherapy resistance. A recent report showed that in vitro
coculture of CLL cells with CD154 expressing fibroblasts induced
1000-fold resistance to ABT-737, a small molecular inhibitor of
BCL2 and BCL-XL.42 Similarly, coculture of CLL cells with MSCs
induces resistance to fludarabine, cyclophosphamide, and dexameth-
asone through maintenance of MCL-1.43 Taken together, these in
vitro data strongly suggest that the tumor microenvironment en-
hances CLL cell resistance to both apoptosis and chemotherapy.
Investigation of novel mechanisms of interaction between the tumor
microenvironment and CLL cells is an active area of research. CLL
cells have the capacity to affect local angiogenesis within the
vascular niche. Microvessel density is increased in bone marrow of
CLL patients, with highest densities found at the periphery of
lymphoid aggregates. CLL cells have the capacity to secrete a
variety of angiogenic cytokines, including vascular endothelial
growth factor (VEGF), basic fibroblast growth factor (bFGF), and
thrombospondin-1 (TSP-1).44 More recently, Ghosh et al45 de-
scribed the contribution of CLL cell-derived microvesicles as a
mechanism by which CLL cells can module the local microenviron-
ment. Microvesicles are cell membrane-derived particles that can
deliver cell surface receptors, activated signaling proteins, or nucleic
acids to target cells. The authors found that CLL cell-derived mi-
crovesicles were increased in patients with advanced-stage CLL, and
that microvesicles could activate AKT/mTOR signaling in MSCs.45
Thus, microvesicles represent a novel mechanism of CLL signaling
within the microenvironment. Finally, toll-like receptors (TLRs) are
cell surface receptors that are part of the innate immune system that
bind structurally conserved microbial antigens and activate immune
responses. Recent data shows that TLRs are expressed and func-
tional in CLL; binding of TLRs with cognate antigen-activated
NF-B expression and induced surface expression of activationmarkers CD25 and CD80.46 Given the architectural complexity,
number of different cell types present, and clinical importance of
these interactions, investigation of the tumor microenvironment will
continue to be an active and fruitful area of research in CLL.
T-Cell Abnormalities in CLLMost malignancies are associated with decreased numbers of
circulating T cells, but in CLL they are elevated 2.5 to 4 times
normal, at least pretherapy.47 Although T cells are increased in
number, the T-cell repertoire is significantly contracted in CLL,
with oligoclonal and monoclonal subsets.48,49 CLL cells secrete
immunomodulatory cytokines, such as interleukin-6, interleukin-
10, and tumor growth factor- (TGF-), which shift the helper
T-cell response from a Th1 response to an anergic Th2 response. 50,51
Suppressive regulatory T cells (Treg) are also increased in patients
with CLL.52 A mechanism for this increase may be direct CLLT-
cell contact through CD27 and CD70; this interaction also induces
BCL2 within the Treg, making the T reg cell population in CLL
patients more resistant to apoptosis than in normal individuals.53
These qualitative and quantitative T-cell abnormalities likely allow
the CLL lymphocytes to avoid cell-mediated immune responses,
despite the fact that CLL cells express tumor-specific antigens that
can be presented by major histocompatibility complex molecules.
To better understand the mechanism underlying impaired T-cell
responses in CLL, Gorgun et al54 performed gene expression array
analyses of purified CD4 and CD8 T cells. Among CD4 T cells,
genes related to cell differentiation were differentially expressed
between T cells derived from previously untreated CLL patients and
healthy controls. Gene pathways controlling cytoskeleton forma-
tion, vesicle trafficking, and cytotoxicity were differentially ex-
pressed in CD8 T cells. When CLL cells were cocultured with
CD4 or CD8 T cells obtained from healthy controls, the abnormal
patterns of gene expression were induced, suggesting that these
T-cell defects are due to direct cellcell contact. 54 In light of the
observed dysregulation of cytoskeleton assembly genes, a subse-
quent study by Ramsay et al55 evaluated the capacity of T cells to
form a normal immunological synapse. This study showed that T
cells derived from CLL patients could not form normal cellcell
contact and that recruitment of T-cell signaling proteins to the
synapse was blocked. Similarly, coculture of CLL cells with donor
T cells blocked the formation of an immunologic synapse.55 The
inability of T cells derived from CLL patients to form a normal
immunologic synapse likely impairs both antigen recognition and
cellular cytotoxicity. Taken together, this global impairment in
T-cell function may be an important cause of tumor progression,
increased susceptibility to infection, and secondary malignancies in
patients with CLL.
Using Novel Biologic Insights to Develop an
Integrated Model of Disease BiologyDetailed laboratory-based and translational investigation has yielded
tremendous progress in our understanding of CLL. One of the
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significant challenges at this time is to assemble this body of
knowledge into a unified model of disease pathogenesis. Epidemio-
logic data implies the importance of ethnicity (likely a marker of
genetic risk) and age (a marker of immune senescence). A proposed
model follows: in a genetically susceptible host, tonic stimulation
by a stereotyped antigenlikely a low-affinity autoantigen
initiates expansion of a premalignant clone. Through some combina-
tion of tonic antigenic stimulation, acquisition of somatic genetic
mutations, and timely support from the tumor microenvironment,
the potentially autoreactive clone escapes immune surveillance andbegins to expand. In the tumor microenvironment, cells receive
proliferative signals from CD4 T cells and antiapoptotic signals
from NLCs and MSCs. Initially supported by the tumor microenvi-
ronment, the increasing clone may begin to further modulate the
microenvironment and immune response to favor survival and
progression of the clone. Among IGH unmutated clones, BCR-
transmitted signals provide predominantly proliferative signals,
whereas IGH-mutated clones express gene pathways related to
cellular anergy. The CLL cells continue to receive antigenic
stimulation, and as the clone continues to proliferate, additional
somatic mutations are acquired, and this may diminish the reliance
of the clone on both antigenic stimulation and microenvironment
support.
Some central questions to be investigated include the molecular
basis of the 2:1 male:female gender bias in CLL, the mechanism by
which some CLL acquire somatic mutations and others do not (ie,
the molecular determinants of IGH mutation status), the mechanism
by which preemergent CLL clones escape immune surveillance and
deletion, and whether clonal evolution can be avoided through
suppression of the proliferative compartment of CLL.
Finally, this scientific progress has created numerous targets of
novel therapeutic interventions. Lineage-restricted cell surface
molecules, such as ROR1 and CD37, are targets of novel MAbs.
Lenalidomide has been shown to reverse the abnormal immunologic
synapse formation in CLL, and also modulates the microenviron-
ment through monocyte and NK cell activation. Novel small
molecule inhibitors of BCR-mediated signaling are currently being
tested in clinical trials. CAL101 selectively blocks a phosphatidyl-
inositol 3-kinase isoform related to downstream signaling from
CXCR4. Additional trials studying small molecular inhibitors of
antiapoptotic proteins, such as BCL2, are underway. Elucidation of
the mechanisms of chemotherapy resistance related to loss of p53
function has led to strategies to circumvent p53-dependent path-
ways. The advances in our fundamental understanding of the
mechanisms of disease in CLL will undoubtedly lead to improved
therapies for our patients.
DisclosuresConflict-of-interest disclosure: The author has consulted and been
affiliated with the Speakers Bureau for GlaxoSmithKline and
Genentech, and has received research funding from Celgene and
Eleos.
Off-label drug use: None disclosed.
CorrespondenceMark C. Lanasa, MD, PhD, Assistant Professor of Medicine,
Division of Medical Oncology, Duke University Medical Center,
DUMC Box 3872, 1 Trent Dr., Morris Building Room 25153,
Durham, NC 27710; Phone: (919) 684-8964; Fax: (919) 684-5325;
e-mail: [email protected]
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