B Cell Receptor and BAFF Receptor Signaling Regulation
of B Cell Homeostasis1
Wasif N. Khan2
B lymphocyte homeostasis depends on tonic and in-
duced BCR signaling and receptors sensitive to tro-
phic factors, such as B cell-activating factor receptor
(BAFF-R or BR3) during development and mainte-
nance. This review will discuss growing evidence sug-
gesting that the signaling mechanisms that maintain
B cell survival and metabolic fitness during selection
at transitional stages and survival after maturation rely
on cross-talk between BCR and BR3 signaling. Recent
anisms underlying this crosstalk. In this review I also
propose a model for regulating the amplitude of BCR
signaling by a signal amplification loop downstream of
the BCR involving Btk and NF-?B that may facilitate
BCR-dependent B cell survival as well as its functional
coupling to BR3 for the growth and survival of B
lymphocytes. The Journal of Immunology, 2009, 183:
checkpoints at early and late transitional stages in the spleen (1).
Tonic as well as BCR-induced signals, together with B cell-acti-
vating factor (BAFF)3receptor (BR3) signals, facilitate the pro-
duction and maintenance of immunocompetent pools of mature
follicular (Fo)BI and FoBII cells and marginal zone (MZ) B cells
while remaining self-tolerant (2). Although the molecular mecha-
nisms that implement developmental checkpoints during periph-
eral B cell development and survival remain poorly defined, new
insights have emerged from the distinct ways B cells at different
developmental stages respond to BCR and BR3 engagement. Re-
cent evidence also suggests that Bruton’s tyrosine kinase (Btk) and
the transcription factor NF-?B, particularly c-Rel, are central in the
regulation of B cell survival through a BCR/Btk signaling axis that
constitutes a positive autoregulatory loop to increase signal
strength with B cell maturation (3, 4). This signaling axis also
mediates crosstalk between BCR and BR3, which is emerging as
cell receptor signaling guides the selection of immature
B cells in the bone marrow. After exit from the bone
marrow, these immature B cells go through additional
a fundamentally important mechanism to regulate B cell survival.
Although critical components and events downstream of the BCR
are well known, the emerging mechanisms of BCR signaling
through a positive feedback signaling loop and its role in the re-
enforcement of BR3 function are subjects of intense research and
will be discussed in this review.
BCR signaling in transitional and mature B cell populations
sitional (T2) and mature B cells is hampered by the presence of
self-reactive BCRs that were not selected against in the bone
marrow. Clonotypes bearing these BCRs must either be si-
lenced or eliminated. Thus, negative selection of self-reactive
multiple mechanisms, including deletion, anergy, or receptor
editing upon Ag encounter (5–7). In vivo, strong BCR signals
are proposed to drive these processes, whereas transition into a
T2 B cell may occur through weak BCR engagements or in a
ligand-independent fashion through tonic BCR signaling (8–
11). Analysis of the peripheral B cell repertoire, both in mice
and humans, supports the model in which the formation of a
mature B cell repertoire is also regulated by a positive selection
checkpoint that likely occurs at later stages of transitional B cell
pool is regulated by developmental stage-specific quantitative
and qualitative alterations in BCR signaling. Thus, alterations
in BCR signaling during T1 to T2 transition play a critical role
do not undergo apoptosis due to a strong BCR signal increase
basal or tonic BCR signaling by a developmentally regulated
default pathway. These T1 cells gradually increase the expres-
not shown). This increases BCR signaling competence and
BAFF-dependent survival potential of T2 relative to T1 B cells
as was recently suggested (4). Regardless of the mechanism of
selection, a failure to implement transitional B cell checkpoints
Department of Microbiology and Immunology, Miller School of Medicine, University of
Miami, Miami, FL 33136
Received for publication December 4, 2008. Accepted for publication June 25, 2009.
This article must therefore be hereby marked advertisement in accordance with 18 U.S.C.
Section 1734 solely to indicate this fact.
1This work was supported in part by National Institutes of Health AI060729 (to
2Address correspondence and reprint requests to Dr. Wasif N. Khan, Department of Mi-
crobiology and Immunology, Miller School of Medicine, 1600 Northwest Tenth Avenue,
3147A Rosenstiel Medical Science Building, University of Miami, Miami, FL 33136.
E-mail address: firstname.lastname@example.org
3Abbreviations used in this paper: BAFF, B cell-activating factor; BR3, BAFF receptor;
Btk, Bruton’s tyrosine kinase; DAG, diacylglycerol; Fo, follicular; int, intermediate; IKK,
I?B kinase; IP3, inositol-1,4,5-triphosphate; LSM, lipid second messenger; MZ, marginal
zone; PKC, protein kinase C; SLE, systemic lupus erythematosus; T1, early transitional
type 1 (B cell); T2, late transitional type 2 (B cell); TACI, transmembrane activator and
calcium-modulating/cyclophilin ligand-interacting protein.
Copyright © 2009 by The American Association of Immunologists, Inc. 0022-1767/09/$2.00
is associated with human autoimmune diseases such as SLE, as
reflected by a disproportionate increase in T1 cells in SLE pa-
tients (16, 17).
The cellular and molecular basis by which checkpoints during
transitional to mature B cell development is implemented is the
tion, there are at least three subpopulations of transitional B cells;
T1 (AA4?IgMhighIgD?CD23?), T2 (AA4?IgMhighIgDhigh
CD23?CD21int; where “int” is intermediate), and T2-
preMZ (AA4?IgMhighIgD?CD23?CD21high) (1, 13, 15,
18–21) before their selection into mature Fo or MZ B cells
(Fig. 1). According to this scheme, T1 B cells develop into
T2 B cells, which in turn are thought to serve as the precur-
sor to either subsequent transitional (T2-preMZ) or mature
follicular (FoBI: IgMintIgDhighCD23?CD21int; FoBII:
IgMhighIgDhighCD23?CD21int) B cells (4, 19, 20). The T2-
preMZ cells likely give rise to MZ B cells (4, 19, 20).
The distinct biological consequences in T1 and T2 B cells to
BCR engagement do suggest differences in the nature of intra-
cellular signaling, the downstream targets, or both in the two
investigations have shed some light on potential differences in
the intracellular signaling responses within transitional B cell
populations to BCR engagement. Furthermore, analyses of the
B cells in genetic models of defective BCR signaling suggest
stage-specific functions for some of the signaling components.
However, the biochemical evidence of a stage-specific function
for BCR signaling is largely lacking. Our proteomics analysis of
biochemically enriched lipid rafts from T1 and T2 cells after
BCR cross-linking did not reveal any major qualitative differ-
ences among the known signaling components (J. Llanes and
W. N. Khan, unpublished results). Thus, quantitative differ-
ences in BCR signaling may determine distinct biological out-
comes within the transitional B cell subpopulations. Concep-
tually, quantitative differences in the signaling programs can
implement distinct thresholds or strengths of BCR signaling,
which is considered to be the major mechanism of B cell fate
decisions. Consistent with this, potential positive regulatory
through T2 and mature B cell stages (discussed below).
Because of the significance of phosphoinositide metabolism
ers and we have investigated BCR induction of inositol-1,4,5-
triphosphate (IP3) and diacylglycerol (DAG) lipid second mes-
sengers (LSMs) in immature or transitional B cell subsets (15,
25–27). We found that BCR stimulation in vitro induced the
not in T1 or Btk-deficient B cells (15, 25, 26). However, the
reduced accumulation of LSM metabolism in T1 relative to T2
B cells does not seem to affect the NF-?B signaling pathway, at
least at early time points after BCR stimulation (28–30).
Consistent with this hypothesis T1, T2, and mature B cell
populations can activate I?B kinase (IKK) and induce deg-
radation of I?B?, processes that require the activation of
(BCR), the immature B cells leave the bone marrow and emigrate to the spleen as T1 B cells. In the spleen, T1 B cells develop into T2 B cells, which in turn are
thought to serve as the precursor to either subsequent transitional (T2-preMZ) or mature FoBI and FoBII B cells. The T2-preMZ cells likely give rise to MZ B cells.
An analogous pathway of B cell maturation has been recently described to exist in the bone marrow (19, 20).
3562 BRIEF REVIEWS: BCR/BR3 REGULATION OF B CELL BIOLOGY
protein kinase C (PKC) ? by DAG and Ca?2. In addition,
NF-?B DNA binding is also comparable among T1, T2, and
mature B cell populations at early time points (Refs. 28 and
31 and unpublished data). Despite NF-?B activation, genes
encoding antiapoptotic A1 and Bcl-xLproteins are not in-
duced in T1 but are induced in T2 and mature B cells, sug-
gesting the existence of a T1 stage-specific gene suppression
mechanism that selectively affects gene expression in re-
sponse to BCR but not to TLR stimulation (28, 31).
The comparable activation of NF-?B in transitional and ma-
ture B cell subsets may be explained by the complex regulation
of LSMs after their formation, which is differentially regulated
in T1 vs more mature B cells. For example, despite reduced
BCR-induced levels of IP3, T1 cells can however increase intra-
cellular calcium concentration ([Ca2?]i) in response to BCR
engagement (15, 32). This conflict could possibly be explained
by the more rapid breakdown of DAG and the formation of
mature B cells by the rapid action of DAG kinases and IP3ki-
are responsible for activating NF-?B in response to BCR re-
mains to be determined.
We recently described how, in addition to the classical
NF-?B activation discussed above, a second phase of NF-?B
sis (28). This long-term c-Rel induction coincides with in-
creased levels of antiapoptotic genes as well as up-regulation of
BR3 and its substrate, p100 (NF-?B2), in T2 and mature B
cells. Thus, in T2 and mature B cells two distinct NF-?B-de-
pendent mechanisms control B cell survival; the initial activa-
tion of the classical NF-?B pathway followed by the long-term
induction of c-Rel. The mechanisms that control c-Rel nuclear
activity independently of the classical NF-?B pathway remain
unknown. Together, these two phases of gene regulation pro-
vide sufficient levels of antiapoptotic proteins for extended pe-
factor. Consistently, the promoter regions of antiapoptotic
genes associated with B cell survival, A1 and Bcl-xL, are direct
targets of the c-Rel subunit of NF-?B. We propose that sus-
tained induction of c-Rel in T2 and mature B cells is a critical
regulator of differential survival of T1 and T2 B cells (28).
Btk and NF-?B positive autoregulatory loop downstream of BCR
Btk and the transcription factor NF-?B are central in the reg-
ulation of B cell apoptosis (3, 4, 30, 36–38). Recent evidence
suggests that Btk levels change with BCR stimulation, which
may regulate the strength and extent of BCR signaling. Intra-
cellular levels of Btk may play an important role in a quantita-
tive increase in BCR signaling with B cell maturation. Nisitani
et al. showed that Btk protein levels in splenic B cells are max-
imally increased (7- to 10-fold) within 4 h of BCR stimulation
by a posttranscriptional mechanism involving Btk and PI3K
(39). This up-regulation also occurs in vivo after T cell-depen-
dent or T cell-independent immunization. The in vivo studies
suggested that, in addition to BCR signaling, Btk induction
also involves other receptors in B cells. Furthermore, NF-?B
loop (40). Because genetic evidence indicates that B cell depen-
dence on NF-?B increases with maturation, it stands to reason
that BCR signaling via Btk would lead to an increase in intra-
cellular levels of Btk and increased activation of its signaling
ing apparatus and a quantitative increase in BCR signaling po-
in mediating BCR function, and Btk levels are rate limiting in
the transmission of certain BCR signal transduction pathways,
including c-Rel induction (28, 41). Indeed, the ability to acti-
vate NF-?B in a sustained manner, particularly the c-Rel
NF-?B subunit, increases with B cell maturation (28). In con-
trast to T1 cells, BCR-induced c-Rel signaling in T2 and ma-
ture B cells regulates the expression and function of BR3, en-
hancing and reinforcing prosurvival signaling under BR3
control (28, 42, 43).
Unique contributions of sustained c-Rel activation to B cell survival
It is notable that NF-?B-responsive antiapoptotic genes, in-
cluding Bcl-xLand A1, which are induced in T2 and mature Fo
B cells are directly controlled by the c-Rel subunit of NF-?B
lines (47). Thus, we investigated the possibility that T1 B cells
may be unable to activate c-Rel to the extent and/or duration
necessary to induce an antiapoptotic program and, thus, render
them sensitive to BCR-induced apoptosis. This model would
also predict that apoptosis-resistant T2 and mature B cells are
capable of long-term c-Rel response. Consistent with this
model, BCR signaling in mature B cells induced long-term nu-
clear expression of c-Rel, which coincided with the stable ex-
pression of antiapoptotic genes. A similar c-Rel response was
also observed in T2 and mature Fo B cells in response to BCR
stimulation, whereas T1 cells failed to do so (28). This long-
dependent manner. Like Btk-deficient B cells, T1 cells also
failed to up-regulate sustained c-Rel expression, whereas sus-
tained c-Rel expression was evident in T2 cells (28). A require-
ment for Btk, and by inference DAG and IP3, in BCR-induced
NF-?B in c-Rel gene transcription. Binding sites for both tran-
scription factors have been identified in the c-Rel promoter;
however, it is controversial as to which sites contribute to con-
stitutive vs Ag receptor-induced c-Rel gene expression (48, 49).
Because basal c-Rel expression is regulated by PI3K, the BCR/
PI3K pathway likely forms a BCR signaling module that regu-
BCR/Btk signaling module would be more relevant in BCR-
induced B cell survival, which may include positive selection of
T2 cells and activation of mature B cells. We propose that the
ability to sustain nuclear c-Rel by the two BCR signaling mod-
ules is of paramount importance in the apoptotic vs survival re-
sponse of B cells at the resting state and to BCR engagement.
Functional coupling of BCR and BR3
It is clear from the above discussion that BCR signaling differ-
entially controls survival in T1 and T2 cells and thus regulates
3563The Journal of Immunology
the entrance of select B cell clones into the mature B cell com-
ations in the strength of BCR signaling (5, 10, 13, 14). Evi-
dence suggests that trophic or environmental signals influence
to the development of autoimmune diseases (51–53). BAFF,
also known as TNFSF13B, BLys, TALL-1, THANK, or
zTNF4, is the most critical of these trophic factors affecting the
regulation of B cell maturation and the subsequent mainte-
nance of mature B cells (2, 54). Discovery of the BAFF/BR3
system has reshaped our thinking of how the size of the B lym-
phocyte compartment is managed. BAFF-transgenic mice have
greatly increased numbers of peripheral B cells as well as serum
57). Consistent with this, overproduction of BAFF has been
cell malignancies. Emerging evidence suggests that BR3 is as
critical for B cell survival as the BCR, and that BCR and BR3
are functionally linked in the regulation of B cell survival (28,
Three TNF receptors are known to bind BAFF: the B cell
maturation Ag (BCMA), a transmembrane activator and
calcium-modulating/cyclophilin ligand-interacting protein
(TACI), and BR3 (54). BR3 and TACI are expressed on all
B cell subsets (lowest level on T1 B cells, highest levels on T2
and MZ B cells), whereas BCMA is expressed primarily on
plasma cells and, hence, is dispensable for the development of
primary B cell repertoire (58). Targeted gene deletion of BR3
profoundly reduces the numbers of transitional as well as ma-
ture B cells in the spleen; however, all subpopulations are de-
tected (59, 60). A naturally occurring mutation (A/WySnJ) in
the BR3 cytoplasmic tail results in a similar B cell deficiency
with BAFF does not increase mature splenic B cells (58). In
contrast, mice deficient for TACI have increased splenic B cells
and serum Igs, which was suggested to mean a potential nega-
circulating BAFF to become available, which can bind to BR3
and increase B cell numbers.
Recent reports demonstrate that BCR regulates the expres-
sion of BR3 (42). Thus, BCR may also control B cell survival
and sensitivity to environmental cues indirectly via this prosur-
vival receptor (28, 43, 54). Others and we have examined how
BCR signaling influences BR3 expression and function. We
found that BCR-induced expression of BR3 requires Btk and
portant in the regulation of B cell survival directly by inducing
the expression of antiapoptotic genes but also indirectly via in-
creasing the expression and function of BR3 (28, 43). It is in-
teresting to note that Btk-deficient mice (Fig. 2A) have a spe-
the positive selection, whereas MZ B cells are present. One in-
terpretation is that, in the absence of Btk, BCR generates
weaker signals that favor the development of MZ B cells (19,
20). The other but not mutually exclusive possibility is that cir-
culating BAFF is increased due to reduced Fo BI cells in Btk-
deficient mice. The increased BAFF may support MZ B cell
survival in this relatively B lymphopenic environment. Consis-
tent with this idea, preferential expansion of MZ B cells has
been observed in other lymphopenic mice.
However, unlike BCR, BR3 has been shown to activate both the
classical and alternative NF-?B pathways (Fig. 2B). Among the
three BAFF receptors, only BR3 activates the alternative NF-?B
pathway. Also, the alternative pathway is more robustly activated
than the classical NF-?B pathway by BR3 signaling (64, 66, 67).
The activation of the alternative NF-?B pathway involves NF-
?B-inducing kinase and activation of IKK?, which phosphor-
ylates p100 and leads to its processing into p52 (64, 65). The
play defective in vitro survival in the presence of soluble BAFF
NF-?B pathway via Btk-dependent mechanisms. BCR signaling increases the levels of Btk protein via posttranscriptional as well as NF-?B-mediated tran-
scriptional up-regulation of the gene encoding Btk (40). In this model, successive increases of Btk and the NF-?B positive autoregulatory loop would lead
signaling produces long-term nuclear expression of c-Rel and its target antiapoptotic genes (e.g., A1 and Bcl-xL) promote B cell survival. The sustained c-Rel
response also results in expression of BR3 and its substrate, p100, enhancing and re-enforcing BR3 survival signaling through activation of the alternative
NF-?B pathway. B, BAFF engagement with BR3 also activates the classical NF-?B pathway via a Btk-dependent mechanism, resulting in an increase in BR3
and p100 (67). This affords BR3 with self-sufficiency in an autoregulatory feedback mechanism for low-level sustained activation of both NF-?B pathways,
further strengthening BR3-mediated B cell survival.
Schematic of BCR signal amplification and BCR reinforcement of BR3 survival signaling. A, BCR signaling leads to activation of the classical
3564 BRIEF REVIEWS: BCR/BR3 REGULATION OF B CELL BIOLOGY
suggests that NF-?B is at least partially responsible for BAFF-
mediated B cell survival and that both the classical and alterna-
tive pathways are involved in this process (64, 66–68). Re-
cently a novel mode of NF-?B-dependent gene expression by
BR3 was discovered. In this unexpected mechanism, BR3 asso-
ciates with IKK?/c-Rel and histone H3 in the nucleus where
What is the significance of activating multiple NF-?B path-
ways? A hypothesis would be that the quantity and quality of
the specific NF-?B DNA binding subunits that translocate to
the nucleus and activate genes may be critically important in
determining the distinct biological outcomes in different tran-
sitional and mature B cell populations. Because BCR signaling
is at the center of all B cell selection processes, the regulation
and extent of BCR signaling to NF-?B (and other signaling
pathways) in B cells undergoing negative selection, positive se-
lection, or maintenance must be clearly different and may in-
fluence NF-?B subunit composition and the extent of DNA
binding to distinct target genes, including BR3. This delicate
regulation combined with BR3-mediated activation of primar-
ily the alternative NF-?B pathway may produce specific re-
sponses in distinct B cell populations.
Yet another mechanism that promotes the intersection of
BCR with BR3 signaling and strengthens their functional cou-
pling is BCR regulation of the BR3 substrate, p100 (Fig. 2).
BAFF interaction with BR3 activates the alternative NF-?B
pathway, which involves IKK?-mediated phosphorylation and
the subsequent proteolytic processing of p100 (NF-?B2) to
p52, which preferentially dimerizes with RelB, and p52/RelB
translocate to the nucleus (64, 65). Because proteolysis is an ir-
reversible process, conversion to p52 results in the elimination
the alternative pathway must require continuous production of
p100. Consistent with this model, recent studies show that
p100 (28, 43). p100 is regulated at the transcriptional level via
Btk- and c-Rel-dependent mechanisms (28). These results sug-
gest that one mechanism for providing a p100 supply for the
sustained activation of the alternative NF-?B pathway involves
BCR signaling through the classical NF-?B pathway and c-Rel
(28). Indeed, loss of p50 results in a decrease in BCR-induced
expression of BR3 as well as p100 (67).
signaling in B cell development, independent contribution of
these receptors is evident by genetic studies with mice partially
manuscript in preparation). The functional link between BCR
and BR3 also has ramifications for the development of B cell
pathologies, including autoimmunity and the survival of B cell
lymphomas. This suggests that when BCR signaling is per-
tion and, thus, inappropriate B cell survival. Because TLRs are
potent activators of the classical NF-?B pathway, they also in-
(G. Carlesso, K. L. Hoek, and W. N. Khan, unpublished data),
BR3. This regulation of BR3 expression and function raises the
possibility that TLR engagement in B cells in an inappropriate
context may lead to the development of autoimmune condi-
tions, breakdown in B cell tolerance, or development of B cell
In addition to receiving re-enforcement from the BCR-in-
duced expression of p100, BR3 is also self-sufficient in main-
taining the signaling loop between classical and alternative
NF-?B pathways (66, 67). Several observations are consistent
with this possibility. First, BR3 is also capable of activating the
classical NF-?B pathway in a Btk- and phospholipase C-?2-
dependent manner and proceeds via the phosphorylation of
IKK? and degradation of I?B? (67, 70). Furthermore, BR3
engagement with BAFF up-regulated transcriptional activation
of the genes encoding BR3, as well as p100 and RelB. Unlike
BCR, however, BR3 required p50/RelA heterodimers but did
not require c-Rel for these processes. Although the survival of
resting mature B cells is dependent on BCR signaling, these
findings suggest that BR3 has the potential to maintain long-
term activity of the alternative NF-?B pathway through activa-
tion of the classical NF-?B pathway. This model is consistent
with the findings that Btk- or p50-deficient B cells survive
ing BAFF (66, 67).
Although this review is focused on the NF-?B aspect of BR3
signaling, several other mechanisms of BR3-mediated B cell
survival have been described that are independent of NF-?B
tion of BR3 with BAFF leads to activation of the PI3K/Akt, ERK, and Pim-2
signaling pathways via IKK?, apparently via mechanisms independent of tran-
scriptional activity of the NF-?B pathway. Phosphoinositide-dependent ki-
nase-1 (PDK1) and PKC? are also implicated in Akt activation in response to
mic sequestration of the Forkhead transcription factor FOXO3a, which, when
in the nucleus, targets the expression of the proapoptotic gene Bim. ERK ac-
tivity further reduces cellular levels of the Bim protein by phosphorylation and
degradation. Concomitantly, Akt induces transcriptional induction of the an-
tiapoptotic gene Mcl-1 and blocks the inhibition of protein translation by 4E-
BP1, including that of the Mcl-1 protein. Akt/mammalian target of rapamycin
(mTOR) and Pim-2 signaling implements this inhibition of 4E-BP1 in re-
sponse to BAFF. In addition to skewing the balance toward prevalence of an-
tiapoptotic proteins, Akt plays a critical role in cellular growth and anabolism
via the rapamycin-sensitive mTOR pathway. Another mechanism through
which BAFF interaction with BR3 prevents cell death is by blocking the entry
of proapoptotic PKC? to the nucleus. Thus, BAFF regulates B cell survival and
growth by preventing apoptosis at multiple levels and by mobilizing a major
cellular metabolic pathway involving mTOR under Akt control (71).
Schematic of BAFF receptor (BR3) survival signaling. Interac-
3565 The Journal of Immunology
transcriptional activity (Fig. 3; 71). These include down-regu-
lation of the proapoptotic molecule Bim and up-regulation of
Mcl-1 via the ERK and Akt pathways, which are activated by a
novel IKK? function (72–74). BAFF-induced activation of
Akt/mTOR and Pim-2 signaling pathways are essential for the
regulation of B cell growth and overall metabolic fitness (75).
Akt is activated by PKC? and phosphoinositide-dependent ki-
nase-1 targeting of Ser473and Thr308downstream of PI3K in
response to BAFF (74, 75). The implications of the levels of
BR3 discussed in this review should be applicable to the NF-
?B-independent mechanisms as well.
The rules that govern B cell responses to BCR and BR3 signal-
Multiple outcomes, including apoptosis, survival, growth, dif-
ferentiation, and proliferation in B cells, take place following
engagement of these receptors. Although mechanisms regulat-
ing signal transduction pathways downstream of BCR have
the focus of intensive research. The current understanding is
that T1 cells in the spleen die upon BCR engagement due to a
failure to activate transcription factors or transcription of anti-
apoptotic genes. Potential positive selection of T2 cells is regu-
lated by gain of resistance to BCR-induced apoptosis. This
change from T1 to T2 cells is accompanied by an ability to in-
duce sustained activation of c-Rel and the stable expression of
antiapoptotic genes. This mode of c-Rel activation also en-
dows T2 cells with the ability to grow and survive in response
to BAFF by regulating the expression of BR3 and its sub-
strate p100 and hence, re-enforcing the long-term activation
of the alternative NF-?B pathway. These survival character-
istics are retained after maturation in Fo B cells. However, in
the resting state PI3K-mediated c-Rel expression may regu-
late B cell survival. Thus, therapeutic intervention of the
positive signal amplification loop in BCR-induced Btk ex-
pression, its positive consequences on c-Rel activation, and
BCR re-enforcement of the growth and survival function of
BR3 may provide potential therapeutic targets in the treat-
ment of autoimmune diseases and B cell lymphomas.
I thank Dr. Emily S. Clark for careful reading of the manuscript and helpful
discussions and Jacqueline A. Wright for helpful discussions.
The authors have no financial conflict of interest.
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3567The Journal of Immunology