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-Deficient Mice and Alymphoplasia (aly) Mice Defined by the Chimeric Analysis1





*
First Department of Internal Medicine, School of Medicine, Ehime University, Ehime, Japan;
Division of Informative Cytology, Institute for Enzyme Research, University of Tokushima, Tokushima, Japan;
Department of Immunology and Inflammation, Biogen Inc., Cambridge, MA 02142;
Reproductive Engineering Section, Mitsubishi Kasei Institute of Life Sciences, Tokyo, Japan; and
¶
Genome Information Research Center, Osaka University, Osaka, Japan
| Abstract |
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(LT
)-deficient mice and alymphoplasia
(aly) mice, a natural mutant strain, manifest a quite
similar phenotype: lack of lymph nodes (LN) and Peyers patches (PP),
with disturbed spleen architecture. The mechanisms underlying the
defective lymphoid organogenesis in these mice were investigated by
generating aggregation chimeras; ex vivo fused morulae were implanted
into pseudo-pregnant host females and allowed to develop to term.
Chimeric mice between LT
-deficient mice and wild-type mice restored
LN and PP almost completely, suggesting that LT
expressed by
circulating bone marrow-derived cells is essential for lymphoid
organogenesis as well as for organization of spleen architecture. By
contrast, chimeric mice between aly mice and wild-type
mice showed only limited restoration of LN and PP. This suggests that
the putative aly gene product does not act as a
circulating ligand for lymphoid organogenesis, like LT
. Rather,
abnormal development of lymphoid organs in aly mice
seems most likely due to the defective development of the incipient
stromal cells of the LN and PP. Supporting this hypothesis,
up-regulation of VCAM-1 on aly mouse embryonic
fibroblasts by signals through LT
R, which is exclusively expressed
by nonlymphoid cells, was disturbed. These studies demonstrate that
LT
and the putative aly gene product together control
lymphoid organogenesis with a close mechanistic relationship in their
biochemical pathways through governing the distinct cellular
compartments, the former acting as a circulating ligand and the latter
as a LT
R-signaling molecule expressed by the stroma of the lymphoid
organs. | Introduction |
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+
cells enters the LN using
4
7, a ligand for MAdCAM-1 (4, 5), mechanisms about how the incipient lymphoid organogenesis
takes place still remain enigmatic.
Recent studies with gene-targeted mice manifesting abnormal development
of the lymphoid organs have provided a new insight into these undefined
processes. Lymphotoxin-
(LT
), originally discovered as a
proinflammatory molecule (6), has turned out to be an
essential factor that controls the genesis of the secondary lymphoid
organs as well as for the organized spleen architecture
(7, 8, 9). Lymphotoxin exists as two distinct forms: LT
3
as a soluble form and LT
/
heteromer (mainly as LT
1LT
2) as a
membrane-associated form, and each engages distinct receptor(s), the
former with TNFR-I, TNFR-II, and herpesvirus entry mediator, and the
latter with LT
R, respectively (10, 11, 12, 13, 14). Mice deficient
for LT
were born with systemic absence of LN and PP (15, 16). Subsequent analyses of mice deficient for other
LT
-related molecules have provided more detailed information on the
action of LT for the lymphoid organogenesis. Although mice deficient
for LT
also lacked PP and most LN, they did possess mesenteric LN
(17, 18). These results suggested that LT
1LT
2 plays
major roles of LT in the development of LN and PP, and that there also
exists an LT
/
heteromer-independent pathway required for the
development of mesenteric LN. This hypothesis was later proven by the
in vivo administration of LT
R-Ig and TNFR-I-Ig fusion protein, which
blocks the signals through LT
R and TNFR-I, respectively. Normal mice
treated in utero with LT
R-Ig alone lacked systemic LN except for
mesenteric LN, whereas concomitant administration of LT
R-Ig and
TNFR-I-Ig or anti-TNF blocked the development of all LN, including
mesenteric LN, indicating that TNF-TNFR-I axis also contributes to LN
genesis (19). Consistent with this model, mice deficient
for both LT
and TNFR-I lacked all LN, including mesenteric LN
(20). Conflicting data, however, also exist. It was
demonstrated that mice deficient for LT
R lack all LN, including
mesenteric LN, suggesting that LT
R is a primary receptor responsible
for the development of all LN (21). Phenotypic difference
between LT
-deficient mice and LT
R-deficient mice also suggested
that LT
R binds not only to LT
/
heteromer, but also to other
ligand(s), such as LIGHT (14), which may mediate signals
required for the development of mesenteric LN. Thus, the integration of
the detailed phenotypic analyses of these gene-targeted mice with
current perspectives of TNF/LT biology may illuminate many aspects of
the lymphoid organogenesis.
Studies with knockout mice of TNF/LT-related molecules have also
unraveled essential actions of LT and TNF for the organization of
lymphoid structure. In mice deficient for either LT
(22, 23), LT
(17, 18), or TNF (24, 25),
organized clusters of follicular dendritic cell (FDC) and germinal
centers (GC) are absent from the spleen. In
LT
-/- mice, formation of FDC clusters and GC
was restored by transplantation of normal bone marrow (BM), indicating
that the LT
-expressing cells required to establish these lymphoid
structures are derived from BM (22). Subsequent analyses
have pointed out B cells as essential source of LT required for this
action (26, 27). In contrast to
LT
-/- mice, when TNFR-I-deficient mice
(28, 29) or LT
R-deficient mice (30) were
reconstituted with wild-type BM cells, they showed no detectable FDC
clusters or GC formation, suggesting that both TNFR-I and LT
R
expression on non-BM-derived cells are necessary for the establishment
of these structures. Thus, both BM-derived and non-BM-derived cells
governed by the TNF/TNFR family members contribute to the organization
of lymphoid structure.
In addition to these studies with gene-targeted mice, alymphoplasia
(aly) mice, an autosomal recessive natural mutant strain,
have provided a novel and unique model for the abnormal development of
lymphoid organs (31). Like
LT
-/- mice, aly mice lack all LN
and PP, and spleen architecture such as development of GC and FDC
clusters as well as marginal zone formation is disturbed
(31, 32, 33). aly mice manifest additional
immunodeficiencies, including disorganized thymic architecture, low
serum Ig level, and impaired allogenic skin rejection, which are not
observed in LT
-/- mice (15, 16, 31). Although a gene responsible for this mutant strain has been
mapped to chromosome 11 by linkage analysis (34), little
is known about how the putative aly gene product contributes
to the lymphoid organogenesis. Because LT
-/-
mice and aly mice manifest a quite similar phenotype (i.e.,
lack of systemic LN and PP, and disorganized spleen architecture),
comparative studies on the mechanisms underlying the abnormal lymphoid
organ development in these mice may provide clearer vision for the
lymphoid organogenesis.
Although dominant roles of LT
R as well as supportive roles of TNFR-I
in the development of secondary lymphoid organs have been demonstrated
as described above, exact mechanisms about how LT control the
development of lymphoid organs through these receptors are still
largely unknown. Furthermore, we have no clues for the identity of the
putative aly gene product that, like LT, is indispensable
for lymphoid organ development. Our studies were undertaken to clarify
the mechanisms underlying the defective lymphoid organogenesis in
LT
-/- mice and aly mice. For this
purpose, we have generated aggregation chimeras; ex vivo fused morulae
were implanted into pseudo-pregnant host females and allowed to develop
to term. Chimeric analyses demonstrate that LT
and the putative
aly gene product control lymphoid organogenesis by governing
distinct cellular compartments. LT
, expressed by the BM-derived
cells, is essential not only for the organization of spleen
architecture, but also for lymphoid organogenesis. By contrast, the
putative aly gene product from BM-derived cells, if
expressed, has no major roles in the development of secondary lymphoid
organs. Rather, lack of LN and PP in aly mice may be caused
by a defect of non-BM-derived cells, possibly through the defective
development of the incipient stromal cells of the LN and PP. Finally,
we demonstrate that signaling through LT
R, but not through TNFR-I,
is impaired in embryonic fibroblasts (EF) isolated from aly
mice. Because LT
R is exclusively expressed by nonlymphoid cells
(11, 13), defective LT
R signaling in aly
mice is consistent with the idea that lack of lymphoid organogenesis in
this strain is caused by the defect of non-BM-derived cells.
| Materials and Methods |
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LT
-/- mice were kindly provided by Dr. Chaplin
(15). Transgenic mice expressing green fluorescence
protein (GFP) under the control of a chicken
-actin promoter and CMV
enhancer (hereafter GFP-Tg) were generated as previously described
(35). aly mice (31) and
C57BL/6J mice were purchased from CLEA Japan (Osaka, Japan).
LT
-/- mice have agouti coat color, whereas
aly mice and GFP-Tg are black coat color. Mice were
maintained and bred under pathogen-free conditions. All animals used
were handled in accordance with the Guide for Animal Experimentation of
Ehime University, School of Medicine (Ehime, Japan), and experiments
were initiated with 8- to 12-wk-old mice.
Generation of chimeric mice
To generate chimeras, the experimental morulae either from
LT
-/- mouse matings or aly mouse
matings were fused ex vivo with morulae from GFP-Tg matings, implanted
together into pseudo-pregnant host females, and allowed to develop to
term, as previously described (36). Chimeric mice between
LT
-/- mice and aly mice were
generated in the same way.
Flow-cytometric analysis
Suspensions of spleen and thymus were prepared by teasing the
tissues between two frosted microscope slides. Spleen cell suspensions
were depleted of RBC by osmotic lysis. For the assessment of
GFP-expressing cells, cells were then subjected to the analysis with a
FACScaliber flow cytometer (Becton Dickinson, San Jose, CA) with
CELLQuest soft ware, as previously described (37). For the
assessment of membrane-bound LT expression, spleen cells were
stimulated with immobilized anti-CD3-
mAb (clone 145-2C11;
PharMingen, San Diego, CA) and Con A (5 µg/ml) for 18 h. After
washing twice with PBS, cells were incubated with LT
R human IgG1
fusion protein (LT
R-Ig) (38) and then with
PE-conjugated anti-human IgG (Calbiochem, La Jolla, CA). Cells were
analyzed with a FACScaliber flow cytometer.
Immunohistochemistry
Ten days after i.p. injection of 100 µl of a 10% SRBC suspension in PBS, spleens were harvested and frozen sections were stained with anti-CD45R/B220 mAb, anti-Thy-1 mAb, anti-CR1 mAb (8C12) (PharMingen), and peanut agglutinin (Vector Laboratories, Burlingame, CA), as previously described (22, 23).
Assessment of signaling through LT
R and TNFR-I using EF
EF were established by the standard procedure (39).
EF from both aly mice and C57BL/6J wild-type mice were
cultured in RPMI 1640 medium (Life Technologies, Grand Island, NY)
supplemented with 10% heat-inactivated FBS (Life Technologies), 2 mM
L-glutamine, 100 U/ml penicillin, and 100 µg/ml
streptomycin at a density of 2 x 105
cells/well in a 6-well culture plate. Eighteen hours after incubation
with either control mAb Ha4/8 (1 µg/ml), agonistic anti-LT
R
mAb AC.H6 (1 µg/ml) (19), or human rTNF (100 U/ml)
(Genzyme, Cambridge, MA), EF were harvested from the plate using 10 mM
EDTA, and subjected to the flow-cytometric analysis using
FITC-conjugated anti-VCAM-1 mAb (PharMingen), as described above.
Expression of VCAM-1 on EF was assessed by the EPICS ELITE (Coulter,
Hialeah, FL), according to the manufacturers instruction.
| Results |
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Although there is some phenotypic difference between
LT
-/- mice and aly mice, these
two strains possess an extremely similar phenotype: lack of LN and PP,
and disorganized spleen architecture. Genetic analysis has demonstrated
that LT
itself is not a gene responsible for the aly
mutation; with the use of linkage analysis, aly gene has
been mapped to mouse chromosome 11 in which neither LT genes nor
LT-receptor genes are located. It is still possible, however, that the
homozygous aly gene mutation may perturb the expression of
membrane-associated LT to cause the LT
-deficient phenotype in
aly mice. To test this, we first examined the expression of
LT mRNA from aly spleen cells with Northern blot analysis.
Both LT
expression after Con A stimulation and constitutive LT
expression from aly homozygous mice
(aly/aly) were indistinguishable from those
seen in aly heterozygous mice (aly/+) (data not
shown). Membrane-associated LT expression in aly mice was
more directly tested by flow cytometry using LT
R-Ig, which binds to
membrane-associated LT
1LT
2. There was no binding of LT
R-Ig to
unstimulated aly/+ spleen cells. After anti-CD3-
plus
Con A stimulation, aly/+ spleen cells expressed
membrane-associated LT detected by this fusion protein (Fig. 1
A). Control polyclonal human
IgG did not show any binding to aly/+ spleen cells under
these conditions (data not shown). As expected, spleen cells from
LT
-/- mice showed no binding to this fusion
protein even after stimulation (Fig. 1
B). Although LT
R-Ig
may also bind to LIGHT on activated lymphocytes, another ligand for
LT
R, this did not occur under this experimental setting. LT
R-Ig
bound to aly/aly spleen cells after anti-CD3-
plus
Con A stimulation (Fig. 1
C), demonstrating that
membrane-associated LT expression is retained in aly mice
with this in vitro system.
|
-/- mice
If membrane-associated LT is present and functional per se in
aly mice, as suggested above, cells from aly/aly
should reverse the phenotype of LT
-/- mice.
This was tested by generating a chimeric mouse between
LT
-/- mice and aly mice, in which
cells from both strains coexist and interact with each other throughout
development. LT
-/- mouse morulae were fused
ex vivo with morulae obtained from aly/aly matings. One
chimeric mouse was born, in which the contribution from both strains
was verified by the coat color; both LT
-/-
mouse-derived agouti hairs and aly/aly-derived black hairs
were observed in this animal (data not shown). Upon detailed
inspection, mesenteric LN (Fig. 2
A) and one lumbar LN (Fig. 2
B) were observed in this chimera, demonstrating that lack
of LN in LT
-/- mice was restored by the
cells from aly mice. Alternatively, the restoration of LN
may represent the compensation for aly phenotype by the
putative aly gene product-sufficient cells from
LT
-/- mice. In either case,
membrane-associated LT from aly mouse cells needs to be
functional in order for this complementation to take place. Taken
together, these results clearly demonstrate that lack of LN genesis
in aly mice cannot be attributed to the lack of functional
membrane-associated LT.
|
Defective lymphoid organ development in LT
-/- mice
is almost completely restored by the generation of chimeras with normal
animals
We have demonstrated previously that lack of FDC clusters as well
as the defective GC formation in the spleen from
LT
-/- mice were restored by the transfer of
wild-type BM cells, indicating that LT
-expressing cells required to
establish these lymphoid structures are derived from BM (22, 28). In contrast to the spleen architecture, development of LN
and PP was not restored in the same animals (37). Assuming
that LT
-expressing cells required to generate LN and PP are also BM
derived, we speculated that lack of LN and PP in
LT
-/- mice is developmentally fixed. These
hypotheses, however, have not been formally proven, because BM cells
were transferred only into the adult mice in these experiments. We have
approached this issue by generating aggregation chimeras between
LT
-/- mouse morulae and
LT
+/+ mouse morulae (summarized in Table I
); both LT
-deficient BM-derived cells
and LT
-sufficient BM-derived cells are expected to circulate in the
body and to interact with the incipient stromal cells of the lymphoid
organs throughout the development. Because we used GFP-Tg as
LT
+/+ mice, chimeric contribution from each
strain can be readily monitored by the detection of GFP. In particular,
detection of GFP from the thymocytes and/or splenocytes by flow
cytometry enabled us to focus on the chimerism of BM-derived cells,
which are the major source of membrane-associated LT.
|
-/-/GFP-7) of 11 chimeras
generated showed no evidence of chimeric contribution from GFP-Tg; coat
color was agouti from LT
-/- mice, and no GFP
expression from the body as well as from the thymocytes and splenocytes
was detected. Upon detailed inspection, no LN and PP were observed in
this mouse. Histological analysis demonstrated disturbed T cell/B cell
organization without GC and FDC formation in the spleen (data not
shown), suggesting that this mouse was apparently derived only from
LT
-/- mice. Except for this mouse, the other
10 mice showed contribution from both LT
-/-
mice and GFP-Tg. Coat color and GFP expression from the body (i.e.,
muscles, fat tissues, and peritoneum) as well as from the thymocytes
and/or splenocytes showed wide variety of contribution from the donors
in each chimera. Upon detailed inspection, all chimeric mice showed
lymphoid organ development indistinguishable from that seen in
wild-type mice, as exemplified in Fig. 2
-/-/GFP-6 showed only one PP with
normal LN genesis. Percentages of the LT
-expressing cells evaluated
by the detection of GFP from thymocytes varied among the chimeras,
ranging from 8 to 66%, and this variation did not apparently affect
the extent of restoration of lymphoid organogenesis in this range.
Development of LN and PP occurred irrespective of the coat color,
status of GFP expression from the body as long as LT
-expressing
cells exist in the spleen and thymus. As expected, spleen architecture
was also restored in these mice. Organized T cell/B cell segregation
and FDC cluster formation were present in the spleens from all chimeric
mice (Fig. 3
-expressing BM-derived
cells, if present throughout the development, can restore the lymphoid
organogenesis in LT
-/- mice, supporting the
idea that LT
-dependent interactions must occur during development in
order for LN and PP to develop.
|
To characterize the putative aly gene product, chimeric mice between aly mice and GFP-Tg were generated and evaluated for the restoration of lymphoid organ development. Because both aly and GFP-Tg have black coat color, chimerism cannot be assessed by the coat color. Chimerism could be evaluated, however, by the detection of GFP in tissues as well as from the thymocytes and/or splenocytes. Of 12 mice generated, one mouse (aly/GFP-6) did not show any evidence for the contribution from GFP-Tg: complete absence of GFP expression in tissues and from thymocytes and splenocytes, no LN and PP, and disturbed T cell/B cell segregation without GC and FDC formation in the spleen.
Of the other 11 chimeras, two mice (aly/GFP-10 and
aly/GFP-11) showed no inguinal LN (Fig. 2
D), and
one mouse (aly/GFP-5) had only right inguinal LN. Although
four mice had more than four PP, seven mice had either less than three
(aly/GFP-5, aly/GFP-7, and aly/GFP-12)
or no PP at all (aly/GFP-8, aly/GFP-9,
aly/GFP-10, and aly/GFP-11). Furthermore, one
mouse that lacked inguinal LN and PP (aly/GFP-11) had only
one small mesenteric LN (Table I
). Development of other LN such as
mandibular, iliac, lumbar, popliteal, and axillary LN was
indistinguishable from those seen in wild-type mice. It is particularly
important to note that there are many spleen cells derived from GFP-Tg
even in the chimeric animals that had defective development of LN
and/or PP; mice that lacked both inguinal LN and PP
(aly/GFP-10 and aly/GFP-11) showed many
GFP-expressing cells in the spleens (Fig. 4
, C and D).
Because BM-derived cells from GFP-Tg should have the normal putative
aly gene product, if expressed, this suggests that lack of
LN and/or PP in these chimeras was caused independent of the presence
of putative aly gene product on the BM-derived cells. This
limited restoration of lymphoid organ development sharply contrasts to
that seen in the chimeras between LT
-/- mice
and GFP-Tg in which relatively small numbers of the LT
-expressing
BM-derived cells were sufficient to restore the lymphoid
organogenesis.
|
Possible involvement of LT
R signaling in the abnormal lymphoid
organ development in aly mice
The similar phenotypes of the LT
-deficient and aly
mouse strains suggested that there might be a close mechanistic
relationship in their affected biochemical pathways. The data described
above showed no detectable alteration in the expression of the
membrane-associated LT ligand in aly mice. We next examined
downstream elements of the pathway at the level of LT
R signaling
using EF isolated either from aly mice or from C57BL/6J
wild-type mice. In the wild-type EF, VCAM-1 expression was up-regulated
upon incubation with human TNF, which binds and signals exclusively
through mouse TNFR-I (Fig. 5
A). Either agonistic
anti-LT
R mAb AC.H6 (Fig. 5
C) or rLT
1LT
2 (data
not shown) also induced up-regulation of VCAM-1 from wild-type EF,
although to a lesser extent compared with TNF stimulation. By contrast,
up-regulation of VCAM-1 after stimulation either with agonistic
anti-LT
R mAb (Fig. 5
D) or with rLT
1LT
2 (data
not shown) was absent from aly EF, although human TNF
induced up-regulation of VCAM-1 on aly EF (Fig. 5
B). These results suggest that signaling through LT
R,
but not through TNFR-I, is impaired in aly mice.
|
| Discussion |
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-/- mice and aly mice, a
natural mutant strain, were investigated by a novel approach. The
results demonstrate that the phenotype of aly mice is not
due to the lack of functional membrane-bound LT, but that defective
signaling through LT
R seems likely. Studies using chimeric mice
suggest that LT required for the genesis of LN and PP is provided by
BM-derived cells, as shown for the organization of spleen architecture
(22). By contrast, BM-derived cells do not account for the
abnormal development of secondary lymphoid organs in aly
mice. Thus, LT
and the putative aly gene product together
control lymphoid organogenesis by governing the distinct cellular
compartments with a close mechanistic relationship in their biochemical
pathways.
Identification of the aly gene and/or aly gene
product promises to facilitate our understanding of the mechanisms for
lymphoid organogenesis as well as for immunodeficiency. The only
information on aly gene to date, however, is its chromosomal
location (34). Because LT
-/-
mice and aly mice manifest a quite similar phenotype, we
first investigated whether aly mice have membrane-associated
LT. The results indicated that activated splenocytes from
aly mice expressed membrane-associated LT whose function was
proven by the complementation experiment; mesenteric LN and lumbar LN
were restored in a chimera between LT
-/-
mice and aly mice.
To clarify the roles of LT
and the putative aly gene
product in lymphoid organogenesis, we have tested how the phenotype of
mice deficient for those molecules can be rescued by generating
chimeric mice with normal animals. In each chimeric mouse, cellular
contribution from the donor strain is expected to be random. In fact,
percentages of GFP-expressing BM-derived cells, coat color, and GFP
expression in tissues varied among the mice. In the case of chimeric
mice between LT
-/- mice and GFP-Tg, this
variation did not apparently affect the extent of restoration of
lymphoid organogenesis. Gene dosage effect of LT, however, has been
reported. Mice heterozygous for both lt
and
lt
(LT
+/-LT
+/- mice)
showed complete lack of PP, and some of those mice also lacked
inguinal LN (20). Although there was one chimera
(LT
-/-/GFP-6) that had only one PP, the
percentage of GFP-expressing (i.e., LT
-expressing) thymocytes in
this mouse was not low (66%). The reason for the poor PP genesis
despite sufficient numbers of LT
-expressing cells in this particular
animal remains unknown.
Although transgenic expression of LT
under the control of rat
insulin promoter in LT
-/- mice restored some
LN such as mesenteric LN and cervical LN, development of inguinal LN,
popliteal LN, and PP did not take place (40). Furthermore,
reconstituted LN did not contain FDC clusters, and T cell/B cell
segregation in the spleen remained disturbed in these mice. In contrast
to these transgenic mice, chimeric mice between
LT
-/- mice and LT
-sufficient mice,
GFP-Tg, restored not only the development of all LN and PP, but also
spleen architecture completely. We speculate that the difference
between transgenic studies and the chimeric analyses in the present
study is due to the differential location of the cells expressing
LT
, the former mainly at ectopic sites such as pancreatic
cells,
kidney, and skin, and the latter at more physiological sites,
respectively.
Restoration of lymphoid organogenesis in chimeric mice between
aly mice and GFP-Tg took place only partially. This limited
restoration of lymphoid organ development sharply contrasts to that
seen in the chimeras between LT
-/- mice and
GFP-Tg; there were many aly/GFP-Tg chimeric mice that lacked
LN and/or PP with many GFP-expressing BM-derived cells. This suggests
that the putative aly gene product from BM-derived cells, if
expressed, has no major role in the genesis of secondary lymphoid
organs. Rather, lack of LN and PP in aly mice may be caused
by the defect of non-BM-derived cells, possibly through the defective
development of the incipient stromal cells of the lymphoid organs. We
speculate that stroma of such missing LN and PP in chimeric mice were
destined to derive and develop from cells contributed by aly
mice. In this scenario, BM-derived cells from aly mice have
no major responsibility for the lack of LN and PP in this strain, which
is suggested by the complementation experiment with
LT
-/- mice, as described above.
It may be informative to determine the stromal origin of the restored LN in these chimeras. Our efforts, however, to detect the GFP from the LN and spleen stroma under the fluorescent microscope were not successful; although we could detect strong GFP signals not only from BM-derived cells but also from some nonlymphoid tissues such as muscles and fat tissues, GFP expression from the lymphoid stroma did not give us a clear signal sufficient to determine the origin of the donor. Introduction of a detection marker whose expression is more ubiquitous may be required to solve this issue.
It is now clear that all LN are not generated equally. Differential
requirement for TNF/LT receptor family members has been demonstrated in
detail for the development of mesenteric LN (15, 16, 17, 18). Mice
deficient in BLR1, a chemokine receptor expressed mainly on mature B
cells, have defective development of inguinal LN and PP
(41). Furthermore, LN at different anatomic sites are
generated at a particular developmental stage in an ordered fashion
(42). It may merit attention that inguinal LN and PP,
which develop at a later gestational stage, were susceptible to the
failure of restoration in our aly/GFP-Tg chimeric mice. This
is also true for LT
-/- mice, in which LT
gene was introduced under the control of rat insulin promoter
(40), and for LT
-/- mice in
which agonistic anti-LT
R mAb was injected in utero to restore LN
genesis (19). The reason that inguinal LN and PP become
preferential targets for the developmental loss in these mice remains
unknown.
To date, a number of classes of genes have been implicated for the
generation of secondary lymphoid organs. Cytokine-related genes other
than LT have been recognized to be involved in the lymphoid
organogenesis; mice deficient in either IL-2R
chain
(43), IL-7R
, or Jak3 (44) have defective
PP development. Chemokine receptor BLR1 has also been demonstrated to
be essential for the development of inguinal LN and PP, as well as for
the formation of B cell follicles in the spleen (41).
Recently, it was also demonstrated that mice deficient in
osteoprotegerin ligand have defective LN genesis (45). In
addition, targeted deletion of transcription factors has caused
abnormal development of lymphoid organs and/or lymphoid cells;
Hox11-deficient mice lack spleen (46), Id2-deficient mice
lack LN and PP (47), and Ikaros-deficient mice lack cells
of all lymphoid lineages, LN, and PP (48). Mice deficient
for both NF-
B1 (p50) and NF-
B2 (p52) also lack LN with
disorganized spleen architecture (49). Thus, there are
broader spectrum of factors than had been anticipated that influence
the lymphoid organogenesis. Then, what kind of gene might
aly be? Up-regulation of VCAM-1 after stimulation with
agonistic anti-LT
R mAb was absent from aly EF,
suggesting that abnormal LT
R signaling may account for the abnormal
lymphoid organ development in aly mice. Consistent with this
result, injection of agonistic anti-LT
R mAb into pregnant
aly female, which could induce lymphoid organogenesis in
LT
-/- mice (19), did not
restore the development of LN in the progeny (our unpublished data).
Furthermore, these results well reconcile the fact that LT
R is
exclusively expressed by nonlymphoid cells, with the results
demonstrating that lack of LN and PP genesis in aly mice is
caused by the defect of non-BM-derived cells. Because LT
R gene
itself is on mouse chromosome 6 (38), not on chromosome
11, and that LT
R mRNA expression from EF was indistinguishable
between aly/+ mice and aly/aly mice by RT-PCR
(unpublished data), the putative aly gene product should
work downstream of the LT
R. In light of the fact that aly
mice manifest some additional immunodeficient phenotypes to that seen
in mice deficient for LT
R (21), it is reasonable to
speculate that the putative aly gene product may be involved
in the signaling more than through LT
R. Molecular cloning of
aly gene is required to solve this issue.
aly mice also manifest some additional phenotypes of
immunodeficiency to that seen in LT
-/- mice.
LT
-/- mice, however, manifest more profound
defects than aly mice in some aspects; aly mouse
spleens contain a relatively well-formed T cell area, whereas
LT
-/- mouse spleens lack organized T cell
area completely (Fig. 3
, A and B ). Recently,
Alexopoulou et al. have demonstrated that spleen architecture, but not
LN and PP genesis, in LT
-/- mice can be
greatly improved by the transgenic expression of TNF, suggesting that
defective signaling through TNFR-I in addition to the loss of LT
R
signaling can be attributed to the disturbed spleen architecture in
LT
-/- mice (50). In
aly mice, signaling through TNFR-I was not apparently
affected, as demonstrated by the up-regulation of VCAM-1 on EF as well
as by the normal NF-
B induction from thymocytes after stimulation
with human TNF (unpublished data). Thus, preserved TNFR-I signaling in
aly mice may explain the less disturbed organized T cell
area in aly mice than that from
LT
-/- mice, although the putative
aly gene product should be involved in other receptor
signaling beyond LT
R.
The histological abnormality of the spleen in aly mice was
not fully corrected by the BM transfer from wild-type mice (31, and our
unpublished data). Based on this result, involvement of the stromal
element in the abnormal lymphoid structure in aly mice was
postulated. It was also demonstrated that no histogenesis of the LN or
PP took place in adult aly mice that had received either
BALB/c BM cells (31) or aly/+ BM cells
(unpublished data). It is not clear, however, whether these results
indicate the involvement of the stromal element in the abnormal
lymphoid organogenesis in aly mice; failure of LN and PP
genesis by wild-type BM cells could indicate that defective lymphoid
organogenesis in aly mice is developmentally fixed. In fact,
by manipulating LT-LT
R axis of wild-type mice or
LT
-/- mice in utero, it was demonstrated
that LN genesis is largely completed by day 17 of gestation (19, 42). It is our intention that we have introduced chimeric
analyses that overcome the developmental barrier. Using chimeric
analyses, we have demonstrated that lack of LN and PP genesis in
aly mice seems most likely due to the defective stromal
development of the lymphoid organs. It is important, however, to
emphasize that aly mice do have some defect in the
BM-derived cells as well. Abnormal function of the BM-derived cells in
aly mice is exemplified by the fact that serum IgM and IgG
in aly mice increased to normal levels after BM
transplantation from wild-type mice (31). We speculate
that the putative aly gene product plays important roles
both in non-BM-derived cells and in BM-derived cells; abnormal
development of LN and PP in aly mice is due to the defect in
non-BM-derived cells, as demonstrated in the present study, and
some of the impaired immune function may be caused by the lack of
the putative aly gene product in the BM-derived cells, as
suggested by the BM transfer experiments. The role of the putative
aly gene product in immune regulation is currently under
investigation.
There are several types of knockout mice that manifest abnormal GC
and/or FDC development without gross defective lymphoid organogenesis
(51). Conversely, mice that exhibit abnormal lymphoid
organ development with intact GC and FDC development also exist; mice
heterozygous for both lt
and lt
had the
ability to develop GC and FDC in the spleen in the absence of PP and/or
inguinal LN (20). LT
-/- mice
expressing transgenic TNF lacked all LN and PP, but retained a
suboptimal capacity to develop primary B cell follicles that contained
FDC clusters in the spleen, and these structures could support the
formation of GC (50). Furthermore, Id2-deficient mice
retain normal spleen architecture without LN and PP (47).
In this study, we demonstrated that some of the aly-derived
chimeras lacked some LN and/or PP with intact GC and FDC development in
the spleen. Taken together, organization of the lymphoid structure
takes place independently of lymphoid organogenesis, and vice versa.
Thus, signals required for the development of lymphoid organs may be
distinct from those for the organization of lymphoid architecture.
| Note added in proof. |
|---|
|
|
|---|
| Acknowledgments |
|---|
R-Ig, agonistic anti-LT
R mAb, and rLT
1LT
2. We
also thank Masayuki Miyasaka and Hiroyasu Nakano for stimulating the
discussion. | Footnotes |
|---|
2 Address correspondence and reprint requests to Dr. Mitsuru Matsumoto, Division of Informative Cytology, Institute for Enzyme Research, University of Tokushima, 3-18-15 Kuramoto, Tokushima 770-8503, Japan. E-mail address: ![]()
3 Abbreviations used in this paper: LN, lymph node; BM, bone marrow; EF, embryonic fibroblast; FDC, follicular dendritic cell; GC, germinal center; GFP, green fluorescence protein; HEV, high endothelial venule; LT, lymphotoxin; PP, Peyers patch; Tg, transgenic mice. ![]()
Received for publication March 9, 1999. Accepted for publication May 12, 1999.
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