By
From the Molecular and Cell Biology Department, University of California, Berkeley, Berkeley, California 94720-3200
Within peripheral lymphoid organs, complex interactions occur between lymphocytes and antigen-presenting cells during an immune response to antigen. Although there is considerable recognition of the fact that
activation of innate immune signaling pathways is necessary for productive adaptive immune responses to occur, a
great deal is still unknown about how this coordination is
achieved at a molecular level. NF- NF- NF- In this issue of The Journal of Experimental Medicine, Franzoso et al. report on the phenotype of knockout mice lacking p52. These mice have impaired splenic marginal zone
architecture and defective germinal center formation after
immunization with T-dependent antigens. Intriguingly, in
contrast to knockout mice of the most closely related p50
family member, defective T-dependent responses in p52
knockout mice appear to result from a defect in accessory cell function, rather than lymphocyte function.
Some clues as to the function of different NF- Consistent with these localization studies, p52 and RelB
appear to play important and distinct roles in antigen-presenting cell function. Based on staining with markers for
specific macrophage populations, Franzoso et al. report that
p52 knockout mice have altered splenic marginal zone architecture (16). The marginal zone surrounding lymphoid
follicles is thought to be an important site for regulation of
cell migration during an immune response (17). Metallophilic macrophages, detected with the MOMA-1 antibody, are absent from the inner marginal zones of spleens from
p52 knockout mice. This defect appears relatively specific
as marginal zone macrophages, detected with the ERTR-9
antibody, are present in the outer marginal zones of these
mice. Lymphoid follicles of p52 knockout mice are also
abnormal, with depleted and absent B cell follicular areas,
and the presence of BM8-staining macrophages that are
normally excluded from the white pulp of wild-type mice. Immunization of p52 knockout mice with T-dependent antigens results in an impaired antibody response that is characterized by an inability to form germinal centers and follicular dendritic cell networks. Interestingly, this inability to
generate germinal centers is not a lymphocyte-intrinsic property as adoptively transferred p52-deficient lymphocytes were
able to form germinal centers.
Several studies implicate RelB as a critical regulator of
the differentiation of dendritic cells. Dendritic cells are potent antigen-presenting cells that enter resting tissues as precursors and, after antigenic exposure, differentiate and migrate to draining lymph nodes. In RelB knockout mice,
numbers of dendritic cells are severely reduced (18). In
studies examining RelB activity in immature and differentiated human dendritic cells, immunohistochemical staining
demonstrated RelB within differentiated lymph node interdigitating and follicular dendritic cells, but not undifferentiated dendritic cells in normal skin (20). Active nuclear
RelB was detected by supershift assay only in differentiated
dendritic cells and in activated B cells.
The distinct phenotypes observed in antibody responses
of other knockout mice suggest that individual NF- Recent work has begun to clarify a role for NF- As with
T lymphocytes, analyses of different knockout B cells reveal that NF- It has been recognized for some time that general B cell
activators, in addition to activating B cell differentiation,
also participate in regulation of isotype switching. Recent
studies from several groups have begun to pinpoint NF- The involvement of NF- The importance of
NF- Results from transgenic and knockout
mice are beginning to reveal how NF- The recognition that NF-
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B/Rel transcription
factors have been the focus of considerable interest over the
past few years, in part because they seem ideally positioned
to integrate information from both innate and adaptive immune signaling pathways. Recent develoments with gene-targeted knockout mice indicate that NF
B/Rel transcription factors are critical regulators of immune responses at
the level of both antigen-presenting cells and lymphocytes.
B/Rel transcription factors function as dimers held
latently in the the cytoplasm of cells by a family of inhibitor
I
B proteins (for reviews see references 1). There are
five known mammalian NF-
B/Rel proteins: Rel (c-Rel),
p65 (RelA), RelB, p50 (NFKB1), and p52 (NFKB2). Both
the p105 precursor of p50, and the p100 precursor of p52,
possess domains that function as I
Bs, and there exist at
least five distinct I
B proteins: I
B
, I
B
, I
B
, I
B
,
and bcl-3.
B/Rel transcription factors are activated by a surprising variety of different signaling pathways involved in
immune function and development. Signaling pathways involved in innate immune responses that activate these factors include a newly identified human homologue of
Drosphila Toll (4), the cytokines TNF-
and IL-1
, the
chemotactic peptide fMet-Leu-Phe (5), as well a variety of
different bacterial and viral products (1). Signaling pathways involved in adaptive immune responses that activate
these factors include key lymphocyte receptor signaling
pathways such as antigen receptors on B and T cells, CD28
on T cells, and CD40 on B cells (1). These signaling
pathways converge on phosphorylation and degradation of
I
Bs, which unmask a nuclear localization signal that leads
to translocation of NF-
B/Rel dimers into the nucleus. Recently, several of the kinases involved in phosphorylation of I
Bs have been identified, and studies of these long
sought after I
B kinases should provide significant insight
into the regulation of activation of NF-
B/Rel transcription factors (6).
B/Rel family members in immune function have come from analyses
of gene and protein expression in lymphoid organs.
Whereas p50, Rel, and RelA are found highly expressed in
all hematopoietic-derived cells, including lymphocytes, the
expression pattern of p52 and RelB appears to be more restricted (13). In situ analysis of protein expression in human
tonsil and lymph nodes reveals that physiologically high
levels of nuclear p52 is restricted to accessory cells of the
immune system including follicular dendritic cells, dendritic cells, and macrophages in the T cell zone (14). In adult lymphoid tissues, high expression of RelB is limited to dendritic cells found in the periarterial lymphatic sheaths of the
spleen, the deep cortex of the lymph nodes, and thymic
medulla (15).
B/
Rel members likely regulate different cellular events that
take place during antibody responses. Rel knockout mice,
for example, exhibit defective antibody responses to both
T-independent and T-dependent antigens (21). In contrast,
p50 knockout mice have defective responses to only T
cell-dependent antigens but germinal center formation is
normal (22), whereas p52 and bcl-3 knockout mice have
defective T-dependent responses as well as defective germinal center formation (23, 24). In addition, defective antibody responses in p52 knockout mice are observed only in
the absence of immunization with adjuvant, whereas defective antibody responses in Rel and p50 knockout mice are
observed in mice immunized with adjuvant. Additional
adoptive transfer experiments should help sort out whether
the cellular basis for these defects occurs in lymphocytes,
antigen-presenting cells, or both.
B/Rel transcription factors in
the regulation of T cell activation. In the absence of exogenous IL-2, activation and subsequent proliferation of T
cells is critically dependent on the c-Rel transcription factor. T cells from c-Rel knockout mice are unresponsive to
most mitogenic stimuli including concanavalin A, and cross-linking of antigen receptor and CD28 (21). Exogenous IL-2,
however, can restore the ability of these c-Rel knockout T
cells to proliferate, suggesting a possible role for c-Rel in
the regulation of IL-2 transcription. Several groups have
recently defined such a role for c-Rel, by demonstrating
that stimulation of CD28 on T cells induces the association
of c-Rel to a CD28 response element (CD28RE) in the
IL-2 promoter (25). This CD28RE is a variant NF-
B
binding site that was previously defined as critical for transcriptional upregulation of IL-2 by CD28 receptor activation in T cells.
B/Rel factors regulate B cell activation and proliferation by a variety of general B cell activators, including lipopolysacharride, CD40 ligand, and antigen-receptor cross-linking (21, 22, 28, 29). Interestingly,
emerging studies also reveal that NF-
B/Rel activation by
these same general B cell activators regulates immunoglobulin heavy chain class switching to specific isotypes.
B/
Rel transcription factors as important signaling intermediates in the regulation of isotype switching by general B cell
activators. Snapper's group has recently demonstrated through
in vitro studies that B cells from Rel and p50 knockout
mice have distinct intrinsic defects in their ability to class
switch to IgE, IgA, and IgG1 (30). Stavnezer's group
has identified
B binding sites within the germline CH promoters regulating class switching to IgG1 and IgE, and has demonstrated distinct regulation of these
B binding sites
by different NF-
B/Rel members (33). Birshtein's group
has also identified
B binding sites within the 3
IgH enhancers that have been implicated in isotype switch regulation (36). Taken together, these studies suggest that distinct NF-
B/Rel dimers may regulate class switching to
specific isotypes.
B/Rel factors in isotype switching is intriguing because it suggests several cell-intrinsic
mechanisms by which B cells could use the complex regulation of NF-
B/Rel factors to further regulate cell-extrinsic signals. Studies with B cell lines and primary B cells indicate that the composition of NF-
B/Rel dimers found in
B cells is variable and reflects both the activation and differentiation state of a B cell, presumably due in part to transcriptional cross-regulation of different family member genes
(38). Thus, activation of the same extracellular signaling
pathways may lead to the translocation of different NF-
B/
Rel dimers depending upon the state of the B cell when activated. Since the pool of NF-
B/Rel dimers available for
translocation likely reflects information from multiple signaling pathways, use of NF-
B/Rel transcription factors to
regulate isotype switching may represent one mechanism
by which information from innate and adaptive signaling
pathways can be integrated in the response of B cells.
B/Rel transcription factors in immune regulation is
highlighted by defects observed in immune homeostasis in
knockout mice. Recent studies from Bravo's group suggest
a similar regulatory role for both the p105 precursors of
p50, and the p100 precursor of p52 (41). In mice lacking
both p105 and p50 proteins, no obvious developmental
immune defects were observed (22). In contrast, Bravo, R.,
D. Dambach, E. Claudio, C. Ryan, and H. Ishikawa (personal communication) report that mice lacking the p105
precursor, but still expressing p50, exhibit chronic inflammation. This result suggests an important role in maintaining immune homeostasis for the p105 precursor. Similarly,
mice lacking both p100 and p52 proteins reported in this
issue by Franzoso et al. do not exhibit an inflammatory
phenotype, whereas mice lacking only the p100 precursor
exhibit gastric hyperplasia, enlarged lymph nodes, and enhanced cytokine production by activated T cells (41). These
studies suggest an important role in maintaining immune
homeostasis for both p105 and p100 precursors. A complex inflammatory phenotype characterized by myeloid hyperplasia and splenomegaly due to extramedullary hemopoiesis
has also been reported for RelB knockout mice (19).
B/Rel transcription factors function as critical mediators of immune responses. The complex regulation of this transcription factor
family by multiple innate and adaptive signaling pathways
within multiple cell lineages, however, represent formidable obstacles to developing a clearer understanding of how
these transcription factors function in immune regulation.
Although reconstitution and transfer experiments using different knockout models will continue to be crucial in sorting out the individual contributions of different cell lineages to defective responses, it will likely remain a difficult
issue to define which NF-
B/Rel dimers regulate specific
responses due to the disruption of multiple dimer complexes in individual knockout mice.
B/Rel transcription factors
are critical regulators of immune responses at the level of
both antigen-presenting cells and lymphocytes poses several
intriguing issues for future study: (a) Are the defects in germinal center formation and antigen-presenting cells reported
for p52 knockout mice related to the defects observed in
TNF ligand and receptor knockout mice (16)? (b) When
and where are NF-
B/Rel transcription factors activated
by innate and adaptive immune signaling pathways during
an immune response? (c) Can information from the activation of multiple signaling pathways be integrated within individual cells through NF-
B/Rel transcription factors?
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Footnotes |
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Address correspondence to William C. Sha, Molecular and Cell Biology Department, University of California, Berkeley, 441 LSA, Berkeley, CA 94720-3200. Phone: 510-643-2783; Fax: 510-643-2784; E-mail: bsha{at}uclink4.berkeley.edu
Received for publication 25 November 1997.
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