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Cutting Edge |









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Emory Transplant Center and Department of Surgery, Emory University School of Medicine, Atlanta, GA 30322; and the Departments of
Surgery,
Pathology, and
Medicine, University of Chicago, Chicago, IL 60637
| Abstract |
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(Mig) and secondary lymphoid chemokine (SLC) gene expression
within allografts and spleens respectively. Blocking membrane
lymphotoxin did not inhibit rejection mediated by CD4+ T
cells. Combining disruption of membrane lymphotoxin and treatment with
CTLA4-Ig inhibited rejection in wild-type mice. These data demonstrate
that membrane lymphotoxin is an important regulatory molecule for
CD8+ T cells mediating rejection and suggest a strategy to
avoid costimulation blockade-resistant
rejection. | Introduction |
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Lymphotoxin-related proteins, which belong to the TNF/TNFR
superfamilies, are also important regulators of immune responses and as
such are promising targets for modulating recipient responses to
transplanted organs. Members of the lymphotoxin-related family of
proteins expressed by T cells include herpes virus entry mediator
(HVEM)5 and membrane lymphotoxin (mLT). HVEM
binds a TNF-like molecule homologous to lymphotoxins, showing
inducible expression, competing with HSV glycoprotein D for HVEM, a
receptor expressed by T lymphocytes (LIGHT), which is expressed by
immature dendritic cells (DC), resulting in enhanced T cell
proliferation and cytokine production, suggesting that HVEM/LIGHT
interactions provide a costimulatory signal for T cells
(12). The binding of mlT expressed by T cells and
activated B cells to lymphotoxin
receptor (LT
R) expressed on
stromal cells and cells of monocytic origin has been shown to be
critical for the development of lymphoid organs (13) and
for the regulation of chemokine production (14).
Disruption of these molecular interactions has been reported to impair
the immune response to viruses and tumors (15, 16, 17, 18, 19).
The purpose of the current study was to determine whether manipulation of these lymphotoxin-related molecules could be used to inhibit costimulation blockade-resistant, CD8+ T cell-mediated allograft rejection. To test this hypothesis we used a heterotopic murine intestinal transplant model. This model offers the advantage that, unlike the murine cardiac transplant model, either CD4+ or CD8+ T cells can mediate rejection (20). However, because this model lacks physical characteristics amenable to serial, noninvasive monitoring, and because recipient survival is not dependent upon the survival of the heterotopic graft, the assessment of transplanted intestines was based upon their histologic appearance.
| Materials and Methods |
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C57BL/6 (H-2b), C57BL/6 x C3H/HeJ
(B6C3F1/J, H-2bxk), and
C57BL/6-Cd4tmlMak
(CD4-/-, H-2b) mice were
purchased from The Jackson Laboratory (Bar Harbor, ME) and the National
Cancer Institute (Frederick, MD). LT
-/-
mice, back-crossed six generations onto the C57BL/6 background, have
been previously described (21).
Intestinal transplantation and histologic graft assessment
Intestinal transplantation was performed as described (20). Intestine grafts were revascularized by anastomosing the portal vein to the recipient inferior vena cava and the superior mesenteric artery to the recipient infrarenal aorta. The jejunum was exteriorized as a stoma and the ileum was anastomosed to the side of the recipient jejunum. Specimens for histologic assessment were fixed in 10% buffered formalin and embedded in paraffin. H&E-stained 3-µm sections were evaluated by a pathologist in a "blinded" fashion. Rejection was graded according to the following definitions: 0, no rejection; 1, scattered apoptotic crypt cells; 2, focal crypt destruction; and 3, mucosal ulceration with or without transmural necrosis.
Study design
To avoid graft-vs-host disease (GVHD), a F1 into parent transplant model was used. Allografts were procured from B6C3F1/J mice (H-2bxk). The genetic background of all recipient mice was C57BL/6J (H-2b). Technical failures, defined as mice that died within the first 3 days, were excluded from analysis.
Fusion proteins and mAb
LT
R-Ig is comprised of the LT
R extracellular domain
attached to the CH2 and CH3
domains of human IgG1 (13). Recipient mice were treated
with 100 µg administered i.p. on days 0 and 7. Mice in control groups
were treated with human IgG. The fusion protein mCTLA4-Ig was provided
by M. Collins (Genetics Institute, Cambridge, MA). Recipient mice were
treated with 50 µg of mCTLA4-Ig administered i.p. every other day for
14 days beginning on the day of transplantation. BBF6-BF2, a mAb that
is specific for the
-chain of mlT, was provided by J. Browning
(Biogen, Cambridge, MA). This mAb was administered i.p. at a dose of
100 µg on days 0, 3, and 7.
RT-PCR
Total RNA was isolated from intestinal grafts frozen by liquid nitrogen using a RNeasy Mini Kit (Qiagen, Hilden, Germany). Samples were treated with RNase-free DNase (Amersham Pharmacia Biotech, Piscataway, NJ). Total RNA (35 µg) was reverse transcribed using the First Strand cDNA Synthesis kit (Amersham Pharmacia Biotech). The mRNA encoding GAPDH, Mig, and SLC were detected by real-time RT-PCR using the ABI Prism 7700-sequence detection system (Applied Biosystems, Foster City, CA) as described (11). The primer and probe sequences were designed using the software program Primer Express Version 1.0 (Applied Biosystems). Sequences used to detect GAPDH, Mig, and SLC are as follows: GAPDH forward primer, 5'-TTCACCACCATGGAGAAGGC-3'; reverse primer, 5'-GGCATGGACTGTGGTCATGA-3'; probe, 5'-TGCATCCTGCACCACCAACTGCTTAG-3'; Mig forward primer, 5'-GCCATGAAGTCCGCTGTTCT-3'; reverse primer, 5'-GGTTCCTCGAACTCCACACTG-3'; probe, 5'-TTCCTTTTGGGCATCATCTTCCTGGA-3'; SLC forward primer, 5'-AGACTCAGGAGCCCAAAGCA-3'; reverse primer, 5'-GTTGAAGCAGGGCAAGGG T-3'; and probe, 5'-CCACCTCATGCTGGCCTCCGTC-3'.
Reactions were performed using 20-µl reaction volumes and the TaqMan Universal PCR master mixture (Applied Biosystems). PCR conditions: 50°C for 2 min, 95°C for 10 min. Each amplification was 15 s at 95°C and 1 min at 60°C for 40 cycles. PCR results were analyzed using the relative standard curve method.
Statistical analysis
Rejection grades were compared using the Kruskal-Wallis test for samples from multiple groups and the Mann-Whitney U test for samples from two groups. Continuous variables were compared using the unpaired t test with Welch correction. Calculations were performed using InStat version 2 (GraphPad, San Diego, CA). Values of p < 0.05 were considered significant.
| Results and Discussion |
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-/-
recipients. Although there was a trend toward a decrease in the
severity of rejection in LT
-/- recipients
(p = 0.07 vs wild-type controls), all
recipients did develop rejection (Fig. 1
-/- mice was mediated by
CD4+ T cells. We have previously shown that
CD4+ T cell-mediated rejection can be blocked by
CTLA4-Ig (7). Therefore, to test the hypothesis,
LT
-/- recipient mice were treated with
CTLA4-Ig. Rejection was completely inhibited when
LT
-/- mice were treated with CTLA4-Ig,
whereas CTLA4-Ig had no effect on rejection in wild-type recipients
(Fig. 1
-related molecules contribute
significantly to the process of allograft rejection and that this
effect is predominantly on CD8+ T cells. Several
mechanisms could explain this effect. Secreted lymphotoxin
LT
3, which is absent in
LT
-/- mice, may contribute to intestinal
allograft rejection. Similarly, mlT, which is comprised of LT
and
LT
, is also absent in LT
-/- mice and may
play a role in allograft rejection. Alternatively, the disruption of
splenic architecture and the paucity of lymph nodes associated with the
LT
-/- phenotype (22) may be
responsible for the impaired immune response to intestinal allografts.
This is consistent with the observation that splenectomized
aly/aly mutant mice, which lack lymph nodes and splenic
tissue, fail to reject heart allografts (23).
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R-Ig that binds mlT and LIGHT but not
LT
3. We have previously shown that in
CD4-/- mice rejection is dependent upon
CD8+ T cells (20). Treatment with
LT
R-Ig significantly inhibited CD8+ T
cell-mediated rejection at day 14 (Fig. 2
-/- mice is at least in part due to the
disruption of mlT. The failure of LT
R-Ig to promote long-term
allograft survival in this model could be due to incomplete blockade of
mlT and LIGHT or the ability of other effector mechanisms to mediate
rejection independent of mlT and LIGHT.
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-/- mice, the results obtained using
LT
R-Ig cannot be attributed to an alteration in the structure of
secondary lymphoid organs or loss of expression of
LT
3. However, in addition to binding mlT,
LT
R-Ig also binds to LIGHT (24). The interaction of
LIGHT with HVEM has been shown to deliver a signal that costimulates T
cells (12, 19). Inhibition of the HVEM/LIGHT interaction
by LT
R-Ig has been reported to inhibit tumor growth, prevent the
development of GVHD, and impair the development of alloreactive CTL
(18, 19). Thus, the inhibition of allograft rejection
associated with LT
R-Ig could either be a consequence of its ability
to bind mlT or its ability to bind LIGHT, thereby blocking the
HVEM/LIGHT costimulatory pathway.
To distinguish between the effects of blocking mlT and LIGHT on
the rejection of intestinal allografts by CD8+ T
cells, we treated CD4-/- recipient mice with
BBF6-BF2, a mAb specific for the
-chain of the mlT complex. Blockade
of mlT using this anti-LT
mAb significantly inhibited the
rejection of intestinal allografts by CD4-/-
mice (Fig. 2
, p < 0.01). These data confirm those
obtained using LT
R-Ig and directly demonstrate that mlT contributes
to CD8+ T cell-mediated allograft rejection.
Interestingly, the anti-LT
mAb had little effect on intestinal
allograft rejection mediated by CD4+ T cells in
CD8-/- recipients (MRG 2.0 ± 0,
n = 3). These data describe an important new role for
mlT in allograft rejection and identify a novel strategy to inhibit
costimulation blockade-resistant rejection. However, it should be noted
that these data do not exclude a potential contribution of LIGHT to
this process.
Mechanisms by which biological agents inhibit rejection can
be grouped into those that deplete alloreactive T cells, those
that prevent complete T cell activation and induce anergy by
blocking costimulatory signals, those that alter T cell
differentiation, those that impair T cell migration, and those that
induce the development of regulatory cells. Of these possible
mechanisms, the engagement of HVEM by LIGHT costimulates T cells
(12) and the engagement of mlT by LT
R augments
chemokine production (14), which in turn regulates cell
migration. We postulated that one or both of these mechanisms
contributed to the protective effect of disrupting lymphotoxin-related
molecules on intestinal allograft rejection and undertook studies to
test this hypothesis. Characteristically, agents that block T cell
costimulatory molecules prevent complete T cell activation and
consequently inhibit Ag-driven T cell proliferation. Unlike CLTA4-Ig,
LT
R-Ig did not inhibit the proliferation of naive T cells to
alloantigens in vitro over a broad range of concentrations nor did it
inhibit the proliferation of CD4+ or
CD8+ T cells in vivo in a GVHD model (data not
shown). These data suggest that LT
R-Ig may inhibit rejection by
mechanisms other than the prevention of T cell costimulation and
activation.
Not having observed an effect of LT
R-Ig on T cell
costimulation, its effect on chemokine production was examined. Given
the demonstrated role of Mig in allograft rejection (25),
the effect of LT
R-Ig and the anti-LT
mAb on Mig gene
expression was determined. Treatment of either wild-type or
CD4-/- recipient mice with LT
R-Ig
significantly reduced Mig gene expression within intestinal allografts
(Fig. 3
, A and B).
The anti-LT
mAb also significantly reduced Mig gene expression
in CD4-/- recipients (Fig. 3
B).
These data, together with the knowledge that migration of immune cells
is regulated by concentration gradients of chemoattractant molecules
such as Mig, suggest that LT
R-Ig and anti-LT
mAb-induced
alterations in chemokine production within intestinal allografts may
inhibit rejection by impairing the migration of leukocytes to the
allograft. Although our data do not directly test this hypothesis, the
importance of this potential mechanism warrants future
investigation.
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R-Ig or an anti-LT
mAb dramatically reduced
SLC gene expression within the spleens of
CD4-/- or wild-type intestinal allograft
recipients (Fig. 4
mAb as compared with CD4-/-
recipients treated with a control mAb (data not shown). This is also
compatible with our previous data that demonstrated a decrease in the
number of splenic DC in wild-type, untransplanted mice treated with
LT
R-Ig (13, 22). It has previously been demonstrated
that following abdominal transplantation of allogeneic hearts DC
migrate to the spleen (27) and that the spleen is an
important site for T cell priming (23). In light of these
observations, our data suggest that disrupting mlT inhibits chemokine
production within the spleen, thereby impairing DC migration and T cell
priming.
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R-Ig failed to inhibit rejection of intestinal allografts in
wild-type mice (MRG 2.4 ± 0.5, n = 5 vs 2.4
± 1.0 for wild-type control mice, n = 10). Given our
previous observation that blocking mlT had little effect on
CD4+ T cell-mediated rejection, we treated
wild-type recipients with a combination of CTLA4-Ig to inhibit
CD4+ T cell-mediated rejection and LT
R-Ig to
inhibit CD8+ T cell-mediated rejection. Combined
treatment significantly inhibited intestinal allograft rejection in
wild-type recipients (MRG 1.1 ± 1.1, n = 10 vs
CTLA4-Ig MRG 2.6 ± 0.7, n = 8 or LT
R-Ig MRG
2.4 ± 0.5, n = 5; p = 0.01 and
0.04 respectively). In models where rejection is more dependent upon
CD4+ T cells, short courses of treatment with
some agents have been shown to promote long-term allograft acceptance
through the induction of regulatory T cells (9). Our data
demonstrate that controlling both CD4+ and
CD8+ T cells significantly inhibited rejection in
the intestinal transplant model and suggest that this approach may be
useful clinically. However, although significantly less severe, the
observation that seven of the 10 recipients did develop rejection
suggests that this strategy may not promote tolerance. In summary, these data provide the first demonstration that mlT regulates the recipient immune response to transplanted organs. Equally important, blockade of this pathway inhibits CD8+ T cell-mediated allograft rejection, suggesting an approach to costimulation blockade-resistant rejection. Our data, together with published data, suggest that treatment-induced alterations in chemokine production may contribute to this effect, perhaps by impairing the migration of T cells and DC to the allograft and spleen. Lastly, these data demonstrate that combinations of agents that bind mlT and costimulatory molecules may represent a clinically applicable immunosuppressive strategy when both CD4+ and CD8+ T cells contribute to allograft rejection.
| Acknowledgments |
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| Footnotes |
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2 X.G. and J.W. contributed equally to this work. ![]()
3 Y.-X.F. and K.A.N. should be considered co-senior authors. ![]()
4 Address correspondence and reprint requests to Dr. Kenneth A. Newell, Department of Surgery, Emory University, WMB 5105, 1639 Pierce Drive, Atlanta, GA 30322. E-mail address: kenneth.newell{at}emory.org ![]()
5 Abbreviations used in this paper: HVEM, herpes virus entry mediator; LT
R, lymphotoxin
receptor; mLT, membrane lymphotoxin; LIGHT, a TNF-like molecule homologous to lymphotoxins, showing inducible expression, competing with HSV glycoprotein D for HVEM, a receptor expressed by T lymphocytes; DC, dendritic cell; SLC, secondary lymphoid chemokine; MRG, mean rejection grade; Mig, monokine induced by IFN-
; GVHD, graft-vs-host disease. ![]()
Received for publication July 31, 2001. Accepted for publication September 10, 2001.
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-induced chemokine Mig. J. Immunol. 163:4878.This article has been cited by other articles:
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