By
From the * Institute of Medical Microbiology, Immunology and Hygiene, Technical University of
Munich, D-81675 Munich, Germany; the Institute for Genetics, University of Cologne,
D-50931 Cologne, Germany; § Serono Pharmaceutical Research Institute, Plan-les-Ouates,
CH-1228, Switzerland; and the
Engelhardt Institute of Molecular Biology and Belozersky Institute
of Physico-Chemical Biology, 117984 Moscow, Russia
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Abstract |
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The formation of germinal centers (GCs) represents a crucial step in the humoral immune response. Recent studies using gene-targeted mice have revealed that the cytokines tumor necrosis factor (TNF), lymphotoxin (LT) , and LT
, as well as their receptors TNF receptor p55
(TNFRp55) and LT
R play essential roles in the development of GCs. To establish in which
cell types expression of LT
R, LT
, and TNF is required for GC formation, LT
R
/
,
LT
/
, TNF
/
, B cell-deficient (BCR
/
), and wild-type mice were used to generate reciprocal or mixed bone marrow (BM) chimeric mice. GCs, herein defined as peanut agglutinin-binding (PNA+) clusters of centroblasts/centrocytes in association with follicular dendritic
cell (FDC) networks, were not detectable in LT
R
/
hosts after transfer of wild-type BM. In
contrast, the GC reaction was restored in LT
/
hosts reconstituted with either wild-type or
LT
R
/
BM. In BCR
/
recipients reconstituted with compound LT
/
/BCR
/
or
TNF
/
/BCR
/
BM grafts, PNA+ cell clusters formed in splenic follicles, but associated
FDC networks were strongly reduced or absent. Thus, development of splenic FDC networks
depends on expression of LT
and TNF by B lymphocytes and LT
R by radioresistant stromal cells.
![]() |
Introduction |
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In the course of a humoral immune response, germinal centers (GCs)1 represent the microenvironment where antigen-specific B cells efficiently undergo clonal expansion, diversification of their Ig genes, selection for clones bearing B cell receptors (BCRs) of high affinity to the antigen, and differentiation into memory or plasma cells (for reviews, see references 1). To support the GC reaction, B cells, T cells, and follicular dendritic cells (FDCs), the three major cellular constituents of GCs, have to interact in an intricate way. In brief, FDCs trap and retain native antigen complexed with antibody and complement, and also provide contact-dependent antigen-nonspecific costimulatory signals that facilitate chemotaxis and proliferation of B cells (4, 5). It is believed that the trapped antigen can be best endocytosed by B cells with increased affinity of their BCRs to the antigen (4, 5). These B cells then present the processed antigen to CD4+ T cells, which on their part, together with FDCs, support further proliferation and differentiation of the selected B cells towards preplasma cells or memory B cells (4, 5).
Over the past few years, several members of the TNF/
lymphotoxin (LT) receptor and ligand families have been
shown to play essential roles in the induction of GCs. Prominent members mediating this function are the TNFRp55,
the LTR, and their ligands TNF, LT
, and LT
. Both
LT
and TNF form homotrimers (6, 7) which signal via
TNFRp55 and TNFRp75 (8, 9). LT
3 is only secreted by
activated lymphocytes and NK cells (7), whereas TNF3 exists both in a membrane-bound and soluble form (10) and
can be produced by many different cell types, including
macrophages, T, and B cells (11). LT
can also associate with LT
, a membrane-bound type II protein (14, 15).
The LT
1
2 heterotrimer engages the LT
R, which is expressed on macrophages and in lymphoid and visceral tissues, but not on T or B lymphocytes (16). Mice deficient in TNFRp55, LT
R, TNF, or LT
all lack GCs, as
defined by the absence of both peanut agglutinin-binding
(PNA+) clusters of centroblasts/centrocytes and associated
FDC networks in B cell areas (19). LT
/
mice differ
in their phenotype in that some small PNA+ clusters are
detectable in the B cell areas, but associated FDC networks
are absent or strongly reduced (22). Expression of LT
and TNFRp55 by B cells and radioresistant stromal cells,
respectively, was shown to be required for GC formation
(25). However, no data concerning the cell lineages required to express LT
R, LT
, and TNF for GC development are available.
This study demonstrates that engagement of the LTR
on radioresistant stromal cells is mandatory for creating an
intact splenic microarchitecture, allowing T-B cell segregation and establishment of GCs. Moreover, for full development of splenic FDC networks, B lymphocytes are required to produce both LT
and TNF.
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Materials and Methods |
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Mice.
LTBone Marrow Transfer.
Bone marrow (BM) cells were harvested by flushing femurs and tibias of donor mice with cold RPMI 1640 medium (Seromed) supplemented with 10% heat-inactivated FCS (Seromed), 2 mM L-glutamine (Seromed), 50 µM 2-ME (GIBCO BRL), 50 µg/ml streptomycin (Seromed), and 100 U/ml penicillin. Cells were washed and depleted of mature T cells using magnetic beads coated with anti-Thy1.2 mAb (Dynal) according to the manufacturer's protocol. After the depletion, cells were counted, washed, and resuspended in PBS. Recipient mice were irradiated with 9.5 Gy using a 137Cs irradiator (Buchler) and injected intravenously with 2 × 106 BM cells in 0.2 ml PBS. For mixed BM transfer, 1.5 × 106 cells of each genotype were injected in a total volume of 0.2 ml PBS.Immunization.
6-8 wk after the BM transfer, the mice received an intraperitoneal injection of 5 µg of alum-precipitated (4-hydroxy-3-nitrophenyl-acetyl)-chicken gamma globulin (molar ratio 19:1; NP19-CG) in 0.2 ml PBS.Evaluation of Chimerism.
10 d after immunization, mixed BM chimeric mice were killed, and BM, spleens, lymph nodes, and sera were collected. One half of each spleen was frozen for immunohistology, and the other half and all lymph nodes were used to purify B lymphocytes. Single cell suspensions were prepared using nylon cell strainers (Becton Dickinson). Cells were washed and incubated in Tris-buffered ammonium chloride solution to lyse erythrocytes. Cells were then washed in RPMI 1640 medium and in PBS containing 0.5% BSA. At this stage, chimerism of some mice was determined by flow cytometry on a FACScan®, using mAbs directed against the Ly5.1 (CD45.1) and Ly5.2 (CD45.2) isoforms conjugated to FITC or PE (PharMingen). B cells were purified by magnetic cell sorting using mouse CD45R (B220) microbeads, MACS® VS+ separation columns and a MACS® magnet (Miltenyi Biotec) according to the instructions provided by the manufacturer. The selected fractions were additionally depleted of remaining T cells by magnetic cell sorting using anti-Thy1.2-coated Dynabeads (Dynal). BM cells were depleted of T cells and of IgM+/IgG+ cells using a cocktail of Dynabeads coated with anti-Thy1.2 or anti-IgM/IgG antibodies (Dynal). The purity of cell populations was confirmed by flow cytometry. The percentage of LTImmunohistochemistry.
Tissue samples were embedded in tissue-freezing medium (Leica) and snap-frozen in 2-methylbutane (Merck) prechilled by liquid nitrogen. Cryostat sections (7 µm) were fixed for 8 min in acetone (Merck), dried, and preincubated for 30 min with PBS containing 5% (vol/vol) goat serum, 1% (wt/vol) BSA, and 0.15% (vol/vol) hydrogen peroxide (Sigma). Blocking of endogenous biotin was performed using an avidin-biotin blocking kit (Vector) according to the manufacturer's protocol. For double labeling, sections were incubated for 30 min with (a) biotinylated PNA diluted 1:500 (Vector) and rat anti- mouse CR1 (CD35) diluted 1:100 (clone 8C12; PharMingen); (b) biotinylated PNA diluted 1:500 (Vector) and rat anti-mouse B220 diluted 1:100 (clone RA3-6B2; PharMingen); and (c) biotinylated mouse anti-mouse IgD diluted 1:50 (clone 1.3-5), rat anti-mouse CD4 diluted 1:100 (GK1.5; PharMingen), and rat anti-mouse CD8 used as a 1:2 diluted hybridoma supernatant (clone 53.6-72; American Type Culture Collection). Rat IgG2a and IgG2b (PharMingen) were used as isotype controls. Single labeling was performed with FDC-M1 mAb (clone 4C11). After washing, alkaline phosphatase (AP)-conjugated streptavidin (Sigma) and/or horseradish peroxidase-coupled mouse anti-rat IgG (Dianova) were added. After 30 min incubation and washing, color development for bound AP and horseradish peroxidase was consecutively performed with an AP reaction kit (Vector) according to the manufacturer's instructions and with 3-aminoethyl-carbazole (Sigma) as described (32). In addition, fluorescent microscopy was used for analysis of sections labeled with PNA and FDC-M2 mAb (clone 209) as described previously (25).Measurement of Antigen-specific IgG.
10 d after immunization, NP-specific IgG antibodies were detected using sandwich ELISAs with NP12-BSA- or NP5-BSA-conjugated ELISA plates (10 µg/ml in carbonate buffer [pH 9.5]). Murine NP-specific IgG antibodies were detected with an AP-conjugated goat anti- mouse IgG antiserum (Dianova). The substrate used was p-nitrophenyl phosphate (Sigma). For calculation of arbitrary binding units of NP-specific IgG antibodies, the standard NP-reactive mAb, N1G9 (33), was included on each ELISA plate. ![]() |
Results |
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To address which cell
types have to express LT or LT
R to initiate and maintain GC reactions, a reciprocal BM transplantation approach was used. BM from C57BL/6 wild-type donors was
transferred into myeloablatively irradiated LT
R
/
recipients (B6
LT
R
/
) and vice versa (LT
R
/
B6).
In the same manner, LT
R
/
LT
/
, B6
LT
/
,
and LT
/
B6 chimeric mice were generated. As a
control, BM from C57BL/6 wild-type donors was taken to
repopulate irradiated C57BL/6 wild-type recipients (B6
B6). After 8 wk, mice were immunized intraperitoneally with 5 µg NP19-CG adsorbed to alum, and 10 d later
spleens and sera were taken for analysis. For detection of
GCs, a double labeling of spleen sections was performed
using the plant lectin PNA and the mAb 8C12. PNA binds
to centroblasts/centrocytes, whereas the mAb 8C12 is directed against the murine complement receptor CR1, which is highly expressed on FDCs and at lower levels by
B lymphocytes (34). As shown in Fig. 1, E-H, in the spleens
of B6
LT
R
/
animals only a few, small PNA+ cell
aggregates were observed, and FDC networks were completely absent. A similar phenotype was found in immunized LT
R
/
mice (21). An aliquot of the wild-type
BM used to reconstitute the LT
R
/
recipients was also
transferred into LT
/
recipients. In these mice, the wild-type BM-derived cells were capable of restoring GC formation, proving the intrinsic competence of the donor BM
cells (data not shown). Morphologically intact GCs, i.e.,
PNA+ cell clusters in association with FDC networks,
were also detectable in spleens from LT
R
/
LT
/
(Fig. 1, I and J), LT
R
/
B6 (data not shown), and
B6
B6 (Fig. 1, A and B) chimeras. In contrast, wild-type
mice reconstituted with LT
/
BM had fewer and smaller
PNA+ cell clusters with almost undetectable FDC networks (Fig. 1, M and N). Presence or absence of FDC networks in association with PNA+ cell clusters was confirmed by double labeling with PNA and FDC-M2 mAbs
or by single labeling with FDC-M1 mAbs (data not
shown).
|
The anatomical localization of GCs was determined by
labeling splenic sections with either PNA and anti-B220
mAbs or anti-IgD, anti-CD4, and anti-CD8 mAbs. Downregulation of IgD on most GC B cells and relatively low
numbers of T cells scattered in GCs compared with compact T cell areas in periarteriolar lymphoid sheaths served as
criteria for the identification of GCs. In LTR
/
LT
/
(Fig. 1, K and L), LT
R
/
B6 (data not
shown), and B6
B6 (Fig. 1, C and D) chimeric mice, all
GCs were correctly localized in the B cell areas. Conversely, in spleens of LT
R
/
mice reconstituted with
wild-type BM, PNA+ cell aggregates were found around
central arterioles (Fig. 1, G and H). T and B cells segregated
normally in LT
R
/
LT
/
(Fig. 1 L), LT
R
/
B6 (data not shown), and B6
B6 (Fig. 1 D) chimeric
mice, forming distinct periarteriolar lymphoid sheaths and B
cell follicles. In contrast, in B6
LT
R
/
mice, T and B
cells were mixed despite the presence of hematopoietically derived LT
R+/+ donor cells (Fig. 1 H). Taken together,
the failure of LT
R+/+ BM-derived cells to restore GCs
and an intact splenic architecture in LT
R
/
recipients
provides evidence that LT
R on radioresistant stromal cells
is required for these functions. However, LT
R
/
BM-derived cells were capable of establishing GCs in LT
/
recipients. This shows that for GC development in adult
mice the presence of LT
R on radiosensitive BM-derived
cells is dispensable, whereas the presence of LT
on these
cells is necessary. The latter conclusion is further supported
by the finding that transfer of LT
/
BM into wild-type
C57BL/6 recipients severely impaired GC formation.
Expression of LT and TNF by
hematopoietic cell lineages is required for GC formation
(results above, and reference 35). Yet it is unclear whether
a single hematopoietic lineage is necessary and, perhaps,
sufficient for the production of LT
and/or TNF, or
whether different cellular sources can redundantly provide these ligands in GC reactions. In particular for TNF, the
latter appears to be possible, since a great variety of cell
types (e.g., macrophages, granulocytes, T cells, B cells,
dendritic cells) can produce both soluble and membrane-bound TNF. Similarly, LT
can be synthesized by three
distinct cell types, namely T, B, and NK cells (15, 16, 36).
Thus, to address the question of which cell type is required to express LT
and TNF for GC establishment, compound BM chimeric mice were made. BM cells from
BCR-deficient donors were mixed in a 1:1 ratio with BM
cells from TNF
/
or LT
/
donors and transferred into
myeloablatively irradiated BCR
/
recipients (LT
/
+ BCR
/
BCR
/
; TNF
/
+ BCR
/
BCR
/
).
Control groups were established by reconstituting myeloablatively irradiated BCR
/
recipients with mixed BM
from Ly5.1+ C57BL/6 wild-type donors and LT
/
or
TNF
/
donors (LT
/
+ B6
BCR
/
; TNF
/
+ B6
BCR
/
). Since BCR
/
BM cannot give rise to mature B cells, peripheral B cells in the LT
/
+ BCR
/
BCR
/
and TNF
/
+ BCR
/
BCR
/
chimeras
were genetically deficient in LT
and TNF, respectively. All other radiosensitive BM-derived cell populations consisted of a mixture of wild-type and gene-targeted cells. In
control groups (LT
/
+ B6
BCR
/
; TNF
/
+ B6
BCR
/
), all BM-derived cell populations
including
B cells
were composed of wild-type and genetically deficient cells. After 6-7 wk, mice were immunized intraperitoneally with 5 µg of alum-precipitated NP19-CG, and 10 d
later BM, spleen, lymph nodes, and serum were taken for
analysis. BM chimerism and B cell chimerism were determined by Southern blotting and/or flow cytometric analysis of the CD45 isoforms Ly5.1 and Ly5.2 (see Materials
and Methods, and Table I). BM chimerism was found to be
comparable between experimental and control groups (Table I). Immunohistochemical analysis of splenic sections
from LT
/
+ BCR
/
BCR
/
chimeras revealed
the presence of few PNA+ cell clusters. Associated FDC
networks were absent or significantly reduced in size (Fig.
2, E and F). Most of the PNA+ cell clusters were localized
within B cell areas (Fig. 2, E-H). Albeit segregated from the
T cell areas, the B cell areas did not represent well-defined
B cell follicles. Spleens of LT
/
+ B6
BCR
/
control mice showed numerous PNA+ cell clusters in association with large FDC networks, most of them correctly localized within B cell follicles (Fig. 2, A-D). With regard to
FDC network formation, the absence of TNF production by B cells in TNF
/
+ BCR
/
BCR
/
chimeras resulted in a phenotype similar to the one found in LT
/
+ BCR
/
BCR
/
chimeras: few PNA+ cell clusters
contained considerably underdeveloped FDC networks, and most clusters lacked immunohistochemically detectable
networks altogether (Fig. 2, M and N). However, in these
mice, approximately two thirds of the PNA+ cell clusters
were located around central arterioles in T cell areas (Fig.
2, O and P) and only one third was found in B cell areas
(not shown). B cells segregated from T cells and some B cell follicles were observed (not shown). In TNF
/
+ B6
BCR
/
control chimeras, most PNA+ cell clusters
associated with FDC networks were readily detectable in
distinct B cell follicles (Fig. 2, I-L). All results regarding splenic FDC network formation in compound BM chimeras were confirmed by double labeling with PNA and
FDC-M2 mAbs (data not shown). Taken together, the results demonstrate that expression of both LT
and TNF by
B cells is required for the development of mature splenic
FDC networks, but not for the formation of PNA+ cell
clusters.
|
|
To investigate whether
the observed abnormal GC reactions correlated with impaired specific IgG responses, NP-specific IgG titers were
quantified in the sera of animals 10 d after immunization. Densely or sparsely haptenated BSA (NP12-BSA or NP5-BSA) was used for coating of ELISA plates, allowing detection of both low and high affinity NP-specific IgG. B6 B6 and LT
R
/
B6 chimeras responded to immunization with a comparable production of specific IgG, whereas
B6
LT
R
/
animals did not mount a significant primary IgG response (Fig. 3, A and B). Moreover, significant
defects were not observed in B6
LT
/
, LT
/
B6, or LT
R
/
LT
/
chimeras (data not shown). In
B6
LT
R
/
chimeras, the impaired primary IgG response correlated with multiple defects in the organization
of peripheral lymphoid tissues such as lack of lymph nodes
and Peyer's patches, disruption of T-B cell segregation, and
aberrant PNA+ cell clusters without FDC networks in the
spleen (macroscopic examination, and Fig. 1, E-H). In contrast, LT
/
+ BCR
/
BCR
/
and TNF
/
+ BCR
/
BCR
/
mice contained lymph nodes, PNA+
cell clusters, and distinct T and B cell areas, yet differed from their control groups regarding splenic FDC network
formation (Fig. 2). Here, differences in NP-specific IgG titers between experimental and control groups were not statistically significant (Fig. 3, C and D), implying that a lack
or strong reduction of splenic FDC networks does not necessarily lead to impaired specific primary IgG responses.
|
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Discussion |
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Mice deficient for the TNFRp55 or the LTR lack FDC
networks and correctly localized PNA+ cell clusters, demonstrating the requirement of signals from these receptors for
GC formation (20, 21). Development of PNA+ cell clusters
and FDC networks in TNFRp55
/
mice is not rescued
by transplantation of wild-type BM (25, 26), whereas
TNFRp55
/
BM is as efficient as wild-type BM in reconstituting these structures in LT
/
mice (26). These data
indicate that for establishment of GCs, TNFRp55 is required
on radioresistant stromal cells and not on radiosensitive BM-derived cells (25, 26). In the present study, LT
R
/
LT
/
, LT
R
/
B6, and B6
LT
R
/
BM chimeric mice were used for the analysis of GC development, and evidence is provided that for formation of FDC networks expression of LT
R, like TNFRp55, is required on
radioresistant cells, but not on BM-derived radiosensitive
cells. Since FDCs are known to withstand high doses of irradiation (37), it is likely that putative FDC precursors are radioresistant cells that depend on signals from the TNFRp55
(25, 26) and the LT
R (this study) for differentiation to mature FDCs (Fig. 4). Alternatively, it is conceivable that radioresistant stromal cells different from FDC precursors exist
that provide molecules needed for FDC maturation and depend on signals from the TNFRp55 and/or the LT
R in
order to fulfill this function.
|
Comparable to LTR
/
mice, mice with a disrupted
LT
gene are devoid of marginal zones, proper T-B cell
segregation, correctly localized PNA+ cell clusters, and
FDC networks (20, 38, 39). Recently, experiments by Fu
et al. (28) and Gonzalez et al. (29) led to the identification
of the cellular source of LT
needed for the establishment
of FDC networks and correctly localized PNA+ cell clusters in the spleen. The first group used BM from LT
/
mice mixed together with BM from TCR
/
or BCR
/
mice to reconstitute LT
/
mice. In chimeric mice, all T
or B lymphocytes were deficient in LT
, whereas the
other cells of hematopoietic origin consisted of a mixture
of LT
/
and LT
+/+ genotypes. The formation of
PNA+ cell clusters and FDC networks was precluded only
in BM chimeras in which B cells were deficient for LT
,
indicating that LT
-producing B cells are essential for the
establishment of GCs (28). Gonzalez et al. (29) adoptively
transferred purified lymphocytes into SCID mice and also
showed the dependence of the FDC network on LT
-producing B lymphocytes. Since LT
together with LT
can
form ligands specific for LT
R (16), it was suggested
that LT
1
2 heterotrimers on B cells are required for FDC
development. In line with this hypothesis, wild-type B cells
transferred into SCID mice that were simultaneously
treated with LT
R-Fc fusion protein did not induce FDC
networks (29). In our study, a mixture of BM cells from
BCR
/
and LT
/
mice were transferred into irradiated
BCR
/
recipients. In the absence of LT
-producing B
cells, but not of LT
-producing T cells (Endres, R., M.B.
Alimzhanov, and K. Pfeffer, unpublished results), FDC
networks were strongly reduced or absent. However,
PNA+ cell clusters in B cell areas were observed. These results are in contrast to the findings of Fu and colleagues,
who did not detect any correctly localized PNA+ cell clusters and FDC-specific labeling in BCR
/
+ LT
/
LT
/
compound BM chimeras (28). This discrepancy
may be explained by different experimental conditions, i.e.,
Fu et al. reconstituted LT
/
mice, which show a severely disorganized splenic architecture (20, 38), whereas
here B cell-deficient mice are reconstituted, which apart
from missing B cells and FDC networks appear to have a
conserved splenic architecture (Endres, R., M.B. Alimzhanov, and K. Pfeffer, unpublished results). Alternatively, it is
conceivable that LT
/
B cells still produce LT
homotrimers which engage the TNFRp55 and thereby induce
GC formation, although much less efficiently than wild-type B cells. However, since signaling via the LT
R appears indispensable for FDC development (this study, and
reference 21), in order to explain the appearance of few
underdeveloped FDC clusters in LT
/
+ BCR
/
BCR
/
chimeras and in LT
/
mice (this study, and
references 22, 24), one has to assume that ligands other
than LT
1
2 can engage the LT
R on radioresistant cells
at least to some extent. A recently identified member of the
TNF ligand family, LIGHT, may serve this function, since
it was shown to bind to the LT
R (40).
Besides LT3 homotrimers, soluble and membrane-bound forms of TNF3 signal via the TNFRp55. TNF
/
and TNFRp55
/
mice have comparable phenotypes in that
they lack B cell follicles, FDC networks, and correctly localized PNA+ cell clusters (19, 20). Since TNF is known to be
produced by many cell types (11), we asked whether B
cell-derived TNF is required for the induction of splenic
FDC networks by generating compound TNF
/
+ BCR
/
BCR
/
BM chimeras. Surprisingly, these
chimeras did not contain mature splenic FDC networks,
showing a phenotype similar to the one observed in chimeras devoid of LT
-producing B cells. Thus, TNF is yet another mediator in B cell-pre-FDC interactions which
lead to FDC network development. In contrast to TNF
/
mice (19), TNF
/
+ BCR
/
BCR
/
chimeras have
few underdeveloped FDC networks associated with PNA+
cell clusters. This indicates that, in the absence of B cell- derived TNF, TNF from cells other than B cells can provide signals for FDC network development albeit to a
limited extent. It is noteworthy that Alexopoulou et al. observed a reduced production of TNF in LT
/
mice after
LPS treatment (24). The authors suggested that in the
LT
/
mouse strain (38), TNF gene expression is altered
by the neor cassette used to inactivate the LT
gene (24).
The mutation introduced in the LT
gene (22) presumably
does not interfere with the expression of the neighboring
TNF and LT
genes. Therefore, differences in GC formation between chimeras devoid of LT
-producing B cells
versus LT
-producing B cells could result from different amounts of TNF produced by LT
/
and LT
/
B
cells, respectively. LT
/
B cells, but not LT
/
B cells,
might fail to provide local TNF concentrations high enough to allow formation of few underdeveloped GCs.
One simple model of splenic FDC network maturation,
which accommodates data from several groups (25, 26, 28, 29, and this study), is depicted in Fig. 4. Radioresistant
stromal cells have to receive at least two different signals,
one via the LTR (this study) and the other via the
TNFRp55 (25, 26), in order to give rise to a local FDC
network. Most likely, these radioresistant cells represent
hitherto unidentified FDC precursors, either residing at the
site of B cell accumulation or attracted to this location by
the B cells. Alternatively, the radioresistant cells are not FDC precursors, but serve as inducible sources of unknown
factors required for differentiation of FDC precursors to
mature local FDC networks. The cytokines required for
FDC network development, TNF3, LT
3 and LT
1
2, are
provided by B lymphocytes (28, 29, and this study). To
date, the temporal and spatial interrelations of TNFRp55
and LT
R transduced signals remain unclear. It is possible that these two signals act during distinct stages of FDC differentiation.
![]() |
Footnotes |
---|
Address correspondence to Klaus Pfeffer, Institute for Medical Microbiology, Immunology and Hygiene, Technical University of Munich, Trogerstr. 9, D-81675 Munich, Germany. Phone: 49-89-4140-4132; Fax: 49-89-4140-4183; E-mail: klaus.pfeffer{at}lrz.tu-muenchen.de
Received for publication 8 September 1998 and in revised form 13 October 1998.
The continuous and generous support of H. Wagner is greatly appreciated. The authors thank E. Schaller, U. Huffstadt, S. Weiss, and K. Mink for expert technical assistance. The authors are also grateful to H.R. Rodewald and G. Kollias for providing Ly5.1+ C57BL/6 and TNFThis study was supported by the Deutsche Forschungsgemeinschaft (grant Pf 259/2-4) and the Sonderforschungsbereich 391 and 243, Klinische Forschergruppe Postoperative Immunparalyse und Sepsis (grant Si208/5-1). M.B. Alimzhanov was supported by a fellowship from the European Molecular Biology Organization. S.A. Nedospasov is an international research scholar of the Howard Hughes Medical Institute.
Abbreviations used in this paper AP, alkaline phosphatase; FDC, follicular dendritic cell; GC, germinal center; LT, lymphotoxin; BCR, B cell receptor; PNA, peanut agglutinin.
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References |
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