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Short Communications |
§
From the Cardiovascular Biology Laboratory,*
Harvard School of Public Health, Brigham and Women's
Hospital,
and Harvard Medical
School,§
Boston, Massachusetts; and Merck
Research Laboratories,
Rahway, New Jersey
| Abstract |
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| Introduction |
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In recent years, the cytokine-inducible isoform of nitric oxide synthase (NOS2) has attracted interest as a potential regulator of both acute and chronic rejection.2,3 However, recent reports have shown that various measures to reduce NOS activity produce contrasting effects in various models of acute and chronic rejection used. For example, in acute rejection studies involving rat cardiac and lung transplant models, pharmacological synthase inhibitors or NO precursors have been shown to attenuate the course of rejection.4-8 In contrast, in a mouse chronic rejection model where NOS2 is reduced by targeted gene disruption there is accelerated development of graft vasculopathy, the hallmark of chronic rejection.9 At this point, it remains unclear whether these contrasting results should be attributed to different rejection models or different measures to alter NOS activity or whether they reflect opposing biological effects of NOS2 during acute and chronic rejection.
To study independently the effects of NOS2 on acute and chronic allograft rejection within the same genetic strain combination, we placed MHC class I/II mismatched allografts into recipients with and without T-cell depleting immunosuppressive therapy.10,11 To assess the effect of NOS2 in these models, we induced a NOS2-deficient state by using recipient mice with targeted deletion of the NOS2 gene.12 We compared the impact of recipient NOS2-deficiency on ventricular contractility, parenchymal rejection, and intragraft NOS expression in cardiac allografts undergoing acute or chronic rejection placed into NOS2-deficient and wild-type recipients.
| Materials and Methods |
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For this study, 63 heterotopic cardiac transplantations were studied. This included analysis of 27 chronically rejecting transplants completed as part of another body of work examining arteriosclerotic parameters.9 Male CBA/CaJ (H-2k) mice (allografts) or C57BL/6J (H-2b) mice (isografts) aged 6 to 8 weeks were used as heart donors. As organ recipients male mice deficient in inducible nitric oxide synthase (NOS2-/-) on a C57BL/6J x 129SvEv (H-2b) background12 were compared with wild-type recipients. Two different wild-type strains were used as comparative control groups. The first control group consisted of mosaic B6J129/Sv (H-2b) wild-type recipients (hereafter referred to as B6/129). In addition, we used pure C57BL/6J (H-2b) recipients (hereafter referred to as B6) as a second reference group as this transplantation model had been originally established and characterized with the pure B6 strain combination.11 NOS2-deficient mice were generously supplied by Dr. Carl Nathan (Cornell University, New York, NY), who produced them in collaboration with J.S. Mudgett. CBA/CaJ (stock number 000654), B6 (stock number 000664) and B6/129 mice (stock number 101045) were obtained from Jackson Laboratory (Bar Harbor, ME).
Heterotopic Cardiac Transplantation
Vascularized heterotopic abdominal cardiac transplantation was
performed as described by Corry et al10
and
transplants were harvested as described
previously.11
Grafts from NOS2-/- recipients
were compared with those from wild-type recipients. Acute rejection was
studied in allografts placed into nonimmunosuppressed recipients
(NOS2-/- n = 7, wild-type B6 n = 10,
and wild-type B6/129 n = 8). CBA grafts placed into B6
or B6/129 recipients typically fail by day 8. Hence, grafts were
harvested 7 days after transplantation, when the characteristic
histological features of acute rejection are maximally developed but
before final graft failure, to ensure adequate in vivo
perfusion of the graft before tissue harvest and fixation. Chronic
rejection was studied in allografts placed into immunosuppressed
recipients (NOS2-/- n = 12, wild-type B6
n = 8 and wild-type B6/129 n = 7) that
were harvested 55 days after transplantation. At this point, grafts in
wild-type recipients developed histological manifestations of chronic
rejection with preserved ventricular function (palpation scores
2).
Immunosuppressive therapy to delay onset of rejection and produce grafts undergoing chronic rejection included anti-CD4 and anti-CD8 antibodies (GK1.5 and 2.43; 2.0 mg i.p., days 14 and then weekly to day 28) as described previously.11 As demonstrated by FACS analysis of splenocytes, this program reduces CD4+ and CD8+ cells by >94% during the treatment.11 At day 55 after transplantation, splenic CD4+ cells were 48% of the control level and CD8+ were 15% of the control level, indicating ongoing low-level immunosuppression. Native hearts from transplanted recipients (NOS2-/- and wild-type) exposed to the same circulation were used as one control group. Isografts placed into untreated recipients (NOS2-/- and wild-type) and exposed to the same surgical procedure were used as a second control group. Graft function was evaluated daily by measuring the force of palpable heart beat and assigning a score ranging from 0 (no palpable heart beat) to 4 (maximal strength heart beat).
Histological Analysis of Rejection
After perfusion with phosphate-buffered saline (PBS), cardiac allografts were harvested at 7 days (acute rejection) or 55 days (chronic rejection) post-transplantation. Transverse heart sections were fixed in Methyl Carnoy's solution and embedded in paraffin. Sections (4 µm) were stained with hematoxylin and eosin and Verhoeff's elastin for histological evaluation. Slides were examined by light microscopy and allografts were graded for severity of rejection using a modified International Society for Heart and Lung Transplantation grading system on a scale of 0 (no rejection) to 4 (severe rejection).13 Grading was performed by two independent observers in a blinded fashion. Scores are reported as mean value for all grafts in each recipient group.
Reverse Transcriptase-Polymerase Chain Reaction
Relative gene transcript levels for NOS2 were measured using
RT-PCR as published previously.11
This method
allows triplicate analysis of 5 to 11 representative grafts from each
group simultaneously. The cDNA panel was prepared from the following
hearts: native hearts (wild-type B6 n = 8, NOS2-/-
n = 11), isografts (wild-type B6 n = 6,
NOS2-/- n = 5), acutely rejecting allografts
(wild-type B6 n = 8, NOS2-/- n = 7)
and chronically rejecting allografts (wild-type B6 n =
7, NOS2-/- n = 11). Primer sequences, sequence
accession numbers, annealing temperatures, and cycle numbers
were as follows:
![]() | (1) |
Triplicate samples were amplified using the following thermal cycling parameters: denaturation at 94°C for 30 seconds, annealing at a primer-optimized temperature for 20 seconds, and extension at 72°C for 60 seconds (increased by 2 seconds/cycle) followed by a final extension of 7 minutes at the end of all cycles. 32P-dCTP (150,000 cpm/reaction) was included for quantitative PCR studies. The amount of incorporated 32P-dCTP in amplified product bands from dried agarose gels was measured by volume integration (Molecular Dynamics, Sunnyvale, CA). The corrected level of the specific product was derived by dividing the amplified product value by the mean value for the control gene G3PDH in the respective sample.
Immunohistological Analysis
To localize NOS2 expression within the rejecting hearts, immunostaining for NOS2 was performed in paraffin sections (4 µm) from acutely and chronically rejecting allografts transplanted into wild-type recipients. Polyclonal rabbit anti-NOS2 (1:250, 60 minutes at room temperature) was prepared by Jeffrey R. Weidner and Richard A. Mumford (Merck Research Laboratories, Rahway, NJ) and kindly provided to us by Carl Nathan.14 Negative controls included omission of primary or secondary antibody and staining of native heart sections from NOS2-/- mice.
Statistical Analysis
For comparison of two groups, an unpaired t-test was used. A probability value < 0.05 was considered significant. For comparison of more than two groups, analysis of variance (ANOVA) was used. If the ANOVA was significant, the Bonferroni/Dunn procedure was used as a post hoc test. Group data are expressed as mean ± SEM.
| Results |
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As shown in Figure 1A
, acute
rejection was associated with significant induction of NOS2 gene
transcript levels in allografts from wild-type recipients (0.66 ±
0.08 relative units) compared with native hearts (0.01 ± 0.01
relative units, P < 0.0001) and isografts (0.02
± 0.01 relative units, P < 0.0001). In contrast, in
acutely rejecting allografts placed into NOS2-/- recipients NOS2
transcript levels (0.04 ± 0.01 relative units) did not show a
significant increase when compared to the baseline levels in native
hearts (0.02 ± 0.01 relative units, P = 0.24) or
isografts (0.04 ± 0.01 relative units, P = 0.87).
During chronic rejection, grafts placed into wild-type recipients
showed significantly increased NOS2 transcript levels (0.40 ±
0.08 relative units) when compared to baseline levels in native
(P < 0.0001) or isograft
(P < 0.0002) controls. However, this increase
was significantly smaller than in acutely rejecting hearts
(P = 0.003). Interestingly, chronically
rejecting grafts placed into NOS2-/- recipients (0.17 ± 0.02
relative units) also showed a significant increase in NOS2 transcript
levels when compared to native hearts (P <
0.0001), isografts (P < 0.0001) as well as
acutely rejecting allografts (P < 0.0001).
However, this induction during chronic rejection was significantly
lower than in wild-type recipients (P = 0.0034).
Thus, disruption of recipient sources of NOS2 resulted in completely
abolished intragraft transcript levels of NOS2 during acute rejection,
whereas during chronic rejection, grafts in NOS2-deficient recipients
still show partially conserved induction of NOS2 expression.
|
NOS2 Promotes Acute Rejection
As shown in Figure 2A
, at day 7
after transplantation, allografts placed into untreated wild-type
recipients (B6 n = 10, B6/129 n = 8)
had multifocal inflammatory infiltrates consisting of lymphocytes and
macrophages. The allograft parenchyma showed patches of myocyte damage
and necrosis (Figure 2A)
. There were no differences between grafts
placed into B6 and B6/129 controls. In contrast, allografts placed into
untreated NOS2-/- recipients (n = 7) showed
only sporadic foci with fewer infiltrating mononuclear cells. The
parenchymal architecture was preserved showing only infrequent myocyte
damage (Figure 2C)
. This resulted in significantly lower mean
histological grading scores (Figure 2B)
in allografts placed into
NOS2-deficient recipients (1.6 ± 0.4) compared to those in either
wild-type control group (wild-type B6 2.9 ± 0.2,
P = 0.002; wild-type B6/129 2.7 ± 0.3,
P = 0.012). Scores for the pure and mosaic wild-type
recipient groups were not significantly different. Hence, in our acute
rejection model the reduction in rejection scores in grafts from
nonimmunosuppressed NOS2-/- recipients indicates that NOS2 promotes
parenchymal destruction in the acutely rejecting heart.
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Immunosuppressed wild-type recipients produced allografts with
sparse interstitial fibrosis, inflammatory infiltrates, and rare
myocyte damage (Figure 2D)
. In contrast, allografts placed into
immunosuppressed NOS2-/- recipients showed more diffuse inflammatory
infiltrates, frequent patches of myocyte necrosis, advanced
interstitial fibrosis, and scattered interstitial edema and hemorrhage
(Figure 2F)
. As shown in Figure 2E
, mean histological grading scores
were significantly higher in allografts from NOS2-deficient recipients
(3.8 ± 0.1 [NOS2-/-] compared with those in allografts from
wild-type recipients 1.8 ± 0.2 [wild-type B6; P
< 0.0001] or 1.5 ± 0.4 [wild-type B6/129; P <
0.0001]). Of note, all grafts in each group showed diffusely diseased
vessels. However, as previously established in another set of
transplants,9
NOS2-deficiency of the recipient
resulted in increased severity of transplant arteriosclerosis.
NOS2 Protects Long-Term Graft Function
The impact of NOS2-deficiency on myocardial function was estimated
by scoring the force of the palpable heartbeat in chronically rejecting
grafts (55 days). As shown in Figure 3
,
mean palpation scores in chronically rejecting grafts placed into
NOS2-/- recipients (n = 12) were significantly
lower (0.3 ± 0.1) than scores for both wild-type control groups
(B6 [n = 8] 2.3 ± 0.4, P < 0.0001; B6/129
[n = 7] 2.3 ± 0.2, P <
0.0001). Similar palpation (chronic rejection) and rejection (acute and
chronic rejection) scores in allografts from both wild-type control
groups (B6 and B6/129) suggest a comparable alloimmune reponse in these
inbred strains.
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| Discussion |
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NO, NOS2, and Acute Rejection
The observed reduction in allograft rejection in NOS2-deficient recipients extends previous findings in other acute rejection models correlating pharmacologic inhibition of NOS with prolonged allograft survival and reduced histological grades of rejection.4-6,8 Immune cell-derived NOS2 mediates destructive effects on the allograft parenchyma. NO may be directly inducing morphological damage to cardiac myocytes. Increased NO synthesis has been associated with an increase in myocyte death. This effect could be prevented by addition of a nitric oxide synthase inhibitor.15 Programmed cell death (apoptosis) has been recognized as one of the causes of NO-mediated cardiac myocyte loss in the transplanted heart.16 In situ detection of apoptotic myocytes paralleled NOS2 expression in a rat model of cardiac rejection.16 In vivo gene transfection of endothelial NOS into rat cardiac myocytes has been shown to induce apoptotic cell death, an effect which could be abolished by pretreatment with a NOS inhibitor.17
NO, NOS2, and Chronic Rejection
With regard to chronic rejection, this study represents the first in vivo assessment of NOS2-mediated actions on parenchymal parameters. NOS2-deficiency resulted in lower palpation scores, more graft destruction with higher rejection scores, and more vascular thickening. This demonstrates that NOS2 has a protective role against ventricular failure in chronically rejecting allografts. The mechanisms for this protective effect deserve further study. Our leading hypothesis is that antiproliferative effects of NO produced by up-regulation of NOS2 in the donor vasculature18 mediate the protective effects in the transplanted myocardium. In the future, the role of donor-derived NOS2 sources could be further addressed by using NOS2-knockout mice as donors after an appropriate immunosuppressive regimen for the immunogenetically different reverse strain combination (H-2b into H-2k) is identified.
Recently, we showed that NOS deficiency was associated with an increase in the severity of transplant arteriosclerosis.9 Comparison of arteriosclerotic lesion development in grafts placed into NOS2-/- and wild-type recipients showed a twofold increase in severity of luminal occlusion in association with NOS2-deficiency. Myocyte necrosis, interstitial edema, and mononuclear infiltration characteristic of parenchymal rejection are hard to distinguish from ischemic injury. Attenuation of transplant-associated lesion development in the chronically rejecting allograft would reduce ischemic myocardial damage. We showed that NOS2 deficiency correlates with increases in rejection scores and arteriosclerotic severity. It is believed that the fibrotic parenchymal changes in chronically rejecting hearts result, at least in part, from ischemic insults from diffuse, obliterative vascular thickening.19 Therefore, it would be reasonable to hypothesize that by maintaining vessel patency, NOS2-mediated anti-arteriosclerotic effects would result in less ischemic damage.
| Conclusion |
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| Footnotes |
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Supported by a grant of the Milton Foundation and the Abraham Fund. JK was supported by a fellowship grant from the Deutsche Forschungsgemeinschaft, Bonn, Germany.
Accepted for publication August 5, 1998.
| References |
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