*Thomas Enzler,1-3*Arnon P. Kater,1,4*Weizhou Zhang,2George F. Widhopf II,1Han-Yu Chuang,1Jason Lee,1EstherAvery,1
Carlo M. Croce,5Michael Karin,2and Thomas J. Kipps1
1Moores Cancer Center and2Laboratory of Gene Regulation and Signal Transduction, Departments of Medicine, Pharmacology and Pathology, School of
Medicine, University of California San Diego, La Jolla;3Department of Medicine, Stanford University School of Medicine, CA;4Department of Hematology,
Academic Medical Center,Amsterdam, The Netherlands; and5The Ohio State University Cancer Center, Columbus
Results of heavy-water labeling studies
have challenged the notion that chronic
lymphocytic leukemia (CLL) represents
an accumulation of noncycling B cells.
We examined leukemia cell turnover in
E?-TCL1 transgenic (TCL1-Tg) mice,
which develop a CLL-like disease at 8 to
12 months of age. We found that leukemia
cells in these mice not only had higher
proportions of proliferating cells but also
apoptotic cells than did nonleukemic lym-
phocytes. We crossed TCL1-Tg with
BAFF-Tg mice, which express high levels
of CD257. TCL1?BAFF-Tg mice devel-
oped CLL-like disease at a significantly
younger age and had more rapid disease
progression and shorter survival than
TCL1?BAFF-Tg mice had similar propor-
tions of proliferating cells, but fewer pro-
portions of dying cells, than did the CLL
cells of TCL1-Tg mice. Moreover, leuke-
mia cells from either TCL1?BAFF-Tg or
TCL1-Tg mice produced more aggressive
disease when transferred into BAFF-Tg
mice than into wild-type (WT) mice. Neu-
tralization of CD257 resulted in rapid re-
duction in circulating leukemia cells.
These results indicate that the leukemia
cells of TCL1-Tg mice undergo high lev-
els of spontaneous apoptosis that is off-
set by relatively high rates of leukemia
cell proliferation, which might allow for
acquisition of mutations that contribute
to disease evolution. (Blood. 2009;114:
Chronic lymphocytic leukemia (CLL), the most common adult
leukemia in Western countries, is a disease of neoplastic, monoclo-
nal CD5?B cells, which accumulate in the blood, marrow, and
secondary lymphoid tissues. For years this disease has been viewed
as a prime example of a tumor with very low levels of cell turnover
in which intrinsic resistance to apoptosis was responsible for a
gradual accumulation of neoplastic CD5?B cells.1Consistent with
this notion, CLL cells express high levels of antiapoptotic Bcl-2
family members as well as inhibitors of apoptosis proteins.2-5
However, recent studies have challenged this view. CLL cells
were found to undergo spontaneous apoptosis in vitro under
conditions that could support growth of established B-cell lines.6
Spontaneous apoptosis of CLL cells could be inhibited by acces-
derived marrow stromal cells,7follicular dendritic cells,8or nurse-
like cells (NLCs),6,9-11suggesting that CLL cells do not have an
intrinsic resistance to apoptosis. In particular, NLCs produce
factors that can enhance leukemia cell survival in vitro.6,12Among
these factors is a member of the tumor necrosis factor family, most
notably CD257 (B cell–activating factor of tumor necrosis factor
family [BAFF], also called tumor necrosis factor and apoptosis
lator).10CLL B cells express 3 receptors for CD257, namely
CD269 (formerly called B-cell maturation antigen), CD267 (for-
merly called transmembrane activator and CAML interactor), and
CD268 (formerly called BAFF receptor, or BR3).13,14Interaction of
CD257 with these receptors can enhance leukemia cell survival in
vitro, a mechanism that potentially could contribute to disease
progression in vivo.14The existence of cycling cells in CLL
patients is suggested by recent studies using heavy water to label
leukemia cells in vivo.15,16These studies demonstrated that some
patients with apparently indolent disease might generate up to 1%
of their entire leukemia cell population each day, implying that in
CLL there might be high rates of cell turnover that counterbalance
such high rates of leukemia cell proliferation. As such, we
hypothesize that B-cell survival factors such as CD257 could
enhance disease progression merely by inhibiting leukemia cell
turnover, thereby allowing for enhanced accumulation of nascent
leukemia cells in vivo.
We examined for this in an animal model for CLL, namely mice
made transgenic (Tg) for the human T-cell leukemia 1 (TCL1) gene
controlled by the immunoglobulin ? heavy-chain promoter/
enhancer, known as E?-TCL1-Tg mice (here noted as TCL1-Tg
mice). Such animals have constitutive high-level expression of
Tcl1 in mature B cells. At 8 to 12 months of age, these mice
develop a CLL-like disease that shares many features with human
CLL, such as excessive accumulation of monoclonal CD5?B cells
in blood and lymphoid tissues.17Originally identified in T-cell
leukemia, Tcl1 enhances Akt kinase activity, mediates its nuclear
translocation, and influences cell activation and survival.18More
recently it was shown that Tcl1 is expressed by human CLL cells
and also interacts with c-Jun, JunB, and c-Fos to inhibit activator
Submitted June 29, 2009; accepted August 17, 2009. Prepublished online as
Blood First Edition paper, September 15, 2009; DOI 10.1182/blood-2009-06-
*T.E.,A.P.K., and W.Z. shared first authorship.
The online version of this article contains a data supplement.
The publication costs of this article were defrayed in part by page charge
payment. Therefore, and solely to indicate this fact, this article is hereby
marked ‘‘advertisement’’ in accordance with 18 USC section 1734.
© 2009 by TheAmerican Society of Hematology
4469BLOOD, 12 NOVEMBER 2009?VOLUME 114, NUMBER 20
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