N omega -monomethyl-L-arginine inhibits nitric oxide production in murine cardiac allografts but does not affect graft rejection

Nitrosylhemoglobin in photodynamically stressed human tumors growing in nude mice

The role of nitric oxide in human tumor biology and therapy has been the subject of extensive studies. However, there is only limited knowledge about the mechanisms of NO production and its metabolism, and about the role NO can play in modern therapeutic procedures, such as photodynamic therapy. Here, for the first time, we report the presence of nitrosylhemoglobin, a stable complex of NO, in human lung adenocarcinoma A549 tumors growing in situ in nude mice. Using electron paramagnetic resonance spectroscopy we show that the level of nitrosylhemoglobin increases in the course of photodynamic therapy and that the phenomenon is local. Even the destruction of strongly vascularized normal liver tissue did not induce the paramagnetic signal, despite bringing about tissue necrosis. We conclude that photodynamic stress substantiates NO production and blood extravasation in situ, both processes on-going even in non-treated tumors, although at a lower intensity.

The complex role of iNOS in acutely rejecting cardiac transplants

This review summarizes the evidence for a detrimental role of nitric oxide (NO) derived from inducible NO synthase (iNOS) and/or reactive nitrogen species such as peroxynitrite in acutely rejecting cardiac transplants. In chronic cardiac transplant rejection, iNOS may have an opposing beneficial component. The purpose of this review is primarily to address issues related to acute rejection, which is a recognized risk factor for chronic rejection. The evidence for a detrimental role is based upon strategies involving nonselective NOS inhibitors, NO neutralizers, selective iNOS inhibitors, and iNOS gene deletion in rodent models of cardiac rejection. The review is presented in the context of the impact on various components, including graft survival, histological rejection, and cardiac function, which may contribute to the process of graft rejection in toto. Possible limitations of each strategy are discussed in order to understand better the variance in published findings, including issues related to the potential importance of cell localization of iNOS expression. Finally, the concept of a dual role for NO and its downstream product, peroxynitrite, in rejection vs immune regulation is discussed.

Tetrahydrobiopterin compounds prolong allograft survival independently of their effect on nitric oxide synthase activity

BACKGROUND

In previous work, the four-amino analogue of tetrahydrobiopterin, a novel, selective inhibitor of inducible nitric oxide synthase, has been shown to prolong survival of murine cardiac allografts.

METHODS

To further elucidate the underlying molecular immunosuppressive mechanism, we compared the effect of four-amino tetrahydrobiopterin with that of the unsubstituted parent compound tetrahydrobiopterin and of N-(iminoethyl)-L-lysine (L-NIL), a nonpterin inhibitor of inducible nitric oxide synthase using a murine cardiac transplant model. We analyzed allograft survival, intragraft gene expression in grafts by microarray and real-time polymerase chain reaction, graft nitrotyrosine staining by immunohistochemistry and plasma nitrite plus nitrate levels by high-performance liquid chromatography.

RESULTS

Allograft survival was significantly prolonged by tetrahydrobiopterin and cyclosporin A, but not by L-NIL although decreased plasma nitrite plus nitrate levels confirmed nitric oxide synthase inhibition in vivo. As compared to allogeneic untreated controls, intragraft peroxynitrite formation and hence nitrotyrosine staining was lowered in all groups except in cyclosporine A-treated animals. Gene expression profiles obtained by microarray analysis demonstrated that cyclosporine A was able to counteract the expression changes of more than half of the genes differently expressed in syngeneic grafts versus allografts, whereas tetrahydrobiopterin compounds and L-NIL showed only smaller influences on gene expression profiles.

CONCLUSIONS

These results demonstrate that the four-amino substitution, which is essential for inhibition of nitric oxide synthase, is not required for the immunosuppressive effect of tetrahydrobiopterin compounds. We describe a novel immunosuppressive role of pharmacologically applied tetrahydrobiopterin.

Variable efficacy ofN6-(1-iminoethyl)-L-lysine in acute cardiac transplant rejection

We examined the efficacy and mechanism of action of N6-(1-iminoethyl)-l-lysine (l-NIL), a highly selective inhibitor of inducible nitric oxide (NO) synthase (iNOS), on acute cardiac transplant rejection. l-NIL produced a concentration-dependent attenuation of plasma NO by-products and a decrease in nitrosylation of heme protein without altering protein levels of iNOS. At postoperative day 4, l-NIL did not alter the increased binding activities for transcription factors nuclear factor-κB and activator protein-1. Whereas l-NIL decreased inflammatory cell infiltration, graft survival was only prolonged at the dose of 1.0 μg/ml that incompletely blocked NO production. Higher l-NIL concentrations (30 and 60 μg/ml) ablated the increased NO production but failed to improve graft survival and even potentiated NF-κB binding activity examined at day 6. Alloimmune activation indicated by increased cytokine gene expression for interferon-γ, interleukin-6, and interleukin-10 was inhibited in grafts only by treatment with 1.0 μg/ml l-NIL. These findings suggest a complex role of NO in acute cardiac allograft rejection. Partial inhibition of iNOS is beneficial to graft survival, whereas total ablation may oppose any benefits to graft survival. These studies have important implications in understanding the dual role of NO in acute rejection and help to reconcile discrepancies in the literature.

Mechanisms of the protective action of diethyldithiocarbamate-iron complex on acute cardiac allograft rejection

In this study, we examined the actions of diethyldithiocarbamate-iron (DETC-Fe) complex in acute graft rejection heterotopically transplanted rat hearts. Chronic treatment with DETC-Fe inhibited the increase in plasma nitric oxide (NO) metabolites and nitrosylation of myocardial heme protein as determined by electron paramagnetic resonance (EPR) spectroscopy. Pulse injection with DETC-Fe normalized NO metabolites. We verified intragraft trapping of NO in vivo by pulse injection with DETC-Fe by the detection within allografts of an anisotropic triplet EPR signal for DETC-Fe-NO adduct with resonance positions (g tensor factors for perpendicular and parallel components, respectively g( perpendicular ) = 2.038 and g( parallel ) = 2.02; hyperfine coupling of 12.5 G). DETC-Fe prolonged graft survival and decreased histological rejection scores. DNA binding activity for nuclear factor (NF)-kappaB and activator protein-1 was increased in allografts and prevented by DETC-Fe. Abrogation of the activation of NF-kappaB by DETC-Fe was associated with increased IkappaBalpha inhibitory protein. Western blotting and RT-PCR analysis revealed that DETC-Fe inhibited inducible NO synthase protein and gene expression. Gene expression for the proinflammatory cytokine interferon-gamma was also decreased by DETC-Fe. Thus DETC-Fe limits NF-kappaB-dependent gene expression and possesses significant immunosuppressive properties.

Non-heme iron protein: a potential target of nitric oxide in acute cardiac allograft rejection

We examined iron nitrosylation of non-heme protein and enzymatic activity of the Fe-S cluster protein, aconitase, in acute cardiac allograft rejection. Heterotopic transplantation of donor hearts was performed in histocompatibility matched (isografts: Lewis --> Lewis) and mismatched (allografts: Wistar-Furth --> Lewis) rats. On postoperative days (POD) 4-6, Western blot analysis and immunohistochemistry revealed inducible nitric-oxide synthase (iNOS) protein in allografts but not isografts. EPR spectroscopy revealed background signals at g = 2.003 (for semiquinone) and g = 2.02 and g = 1.94 (for Fe-S cluster protein) in isografts and normal hearts. In contrast, in allografts on POD4, a new axial signal at g = 2.04 and g = 2.02 appeared that was attributed to the dinitrosyl-iron complex formed by nitrosylation of non-heme protein. Appearance of this signal occurred at or before significant nitrosylation of heme protein. Iron nitrosylation of non-heme protein was coincidental with decreases in the nonnitrosylated Fe-S cluster signal at g = 1.94. Aconitase enzyme activity was decreased to approximately 50% of that observed in isograft controls by POD4. Treatment with cyclosporine blocked the (i) elevation of plasma nitrate + nitrite, (ii) up-regulation of iNOS protein, (iii) decrease in Fe-S cluster EPR signal, (iv) formation of dinitrosyl-iron complexes, and (v) loss of aconitase enzyme activity. Formation of dinitrosyl-iron complexes and loss of aconitase activity within allografts also was inhibited by treatment of recipients with a selective iNOS inhibitor, l-N(6)-(1-iminoethyl)lysine. This report shows targeting of an important non-heme Fe-S cluster protein in acute solid organ transplant rejection.

Prolonging organ allograft survival: potential role of nitric oxide scavengers

A growing number of studies suggest a key role of nitric oxide (NO) derived from the inducible NO synthase (iNOS) isoform as a signalling molecule leading to acute organ transplant rejection. Current theory suggests that NO targets certain tissue proteins for nitrosylation or nitration leading to inhibition of enzyme/protein function and to cell death via apoptosis. Gene expression of iNOS and formation of nitrotyrosine residues have been confirmed in biopsies of rejecting grafts in humans. Experimental attempts to delay graft rejection by treatment with iNOS enzyme inhibitors have yielded conflicting results. An alternative strategy to alter rejection mediated by NO is to scavenge and/or neutralise the actions of excess NO, thereby by-passing the inhibition of iNOS enzyme activity. This review summarises recent laboratory evidence that new experimental NO scavengers/neutralisers have potential value to prolong graft survival. To date, various metal-based NO scavenging/neutralising compounds have been shown to enhance cardiac allograft survival in the absence of immunosuppression. When used in combination with low-dose cyclosporin, these agents produce a synergistic action to enhance graft survival or even to produce "permanent graft survival" under certain prolonged drug regimens. A portion of this benefit may be accounted for by the property of some of these compounds to display immunosuppressant and anti-inflammatory activity in vivo. These properties are based on findings including the following: (i) attenuating cell infiltration into the graft; (ii) attenuating activation of NFkappaB (a transcription factor important for upregulation of various inflammatory genes); (iii) attenuating cyclin D3 gene expression (a marker of cell proliferation; (iv) antagonising autoimmune activation (as determined by attenuated cytokine gene expression in splenocytes isolated from treated animals but stimulated for several days ex vivo in mixed lymphocyte cultures).

A ruthenium (III) polyaminocarboxylate complex, a novel nitric oxide scavenger, enhances graft survival and decreases nitrosylated heme protein in models of acute and delayed cardiac transplant rejection

Nitric oxide (NO) derived from the up-regulation of inducible NO synthase (iNOS) is believed to play an important role in organ rejection. In experimental models of acute cardiac transplant rejection (i.e., without immunosuppression), treatment using NOS inhibitors to prevent acute rejection have yielded conflicting results. This is most likely due to potential inhibition of constitutive NOS. Accordingly, agents that trap NO directly may have some advantage. In the current study, we evaluated the actions of a ruthenium-based NO scavenger, AMD6221, to inhibit the nitrosylation of myocardial protein and to prolong cardiac allograft survival in a model of acute cardiac transplant rejection (without immunosuppression). In addition, we evaluated the efficacy of AMD6221 used in combination with low-dose cyclosporine (CsA) (i.e., a model of delayed graft rejection). Heterotopic abdominal cardiac transplantation was performed using rat strains with disparities at major and minor histocompatibility loci. Grafts were harvested on postoperative day 6 for histologic examination or analysis of myocardial protein nitrosylation using electron paramagnetic resonance (EPR) spectroscopy. Other animals were monitored twice daily to determine rejection times. Plasma was also taken at postoperative day 6 for determining the concentration of NO by-products (nitrate plus nitrite). Treatment with AMD6221 either prolonged graft survival and/or caused a marked decrease in myocardial nitrosylprotein formation as determined by EPR spectroscopy. In vivo scavenging of NO by AMD6221 was verified by high-performance liquid chromatography analysis of nitrosylated-drug in plasma samples. Low-dose CsA given alone or in combination with AMD6221 completely blocked formation of myocardial nitrosylprotein complexes. Whereas low-dose CsA alone prolonged graft survival, combination therapy with CsA plus AMD6221 produced a synergistic effect on graft survival. These studies indicate that treatment with a ruthenium-based NO scavenger, such as AMD6221, may be an effective regimen used alone or in combination with CsA to protect myocardial proteins from posttranscriptional modification and to prolong cardiac graft survival.

Myocyte function and [Ca 2+ ]i homeostasis during early allogenic heart transplant rejection

BACKGROUND

Recent studies from our laboratory have demonstrated that in vivo contractile function of rejecting mouse heterotopic abdominal heart allografts 5 days after transplantation is depressed to 40% of that of syngenic controls, and that this depression of function is prevented by the nitric oxide synthase (NOS) inhibitor NG-monomethyl-l-arginine. However, the mechanisms of altered myocyte function caused by nitric oxide production in this setting are not established.

METHODS

We measured intracellular calcium concentration ([Ca2+]i) transients (fluo-3, confocal microscopy), fractional shortening (video motion), and L-type Ca2+ currents (whole-cell patch clamp) 5 days after transplantation in ventricular myocytes freshly isolated from syngenic (Balb/C into Balb/C) and allogenic (Balb/C into C3H) transplants.

RESULTS

L-type Ca2+ currents, [Ca2+]i transient amplitudes, and fractional shortening did not differ between nonrejecting, syngenic and rejecting, allogenic transplants. Catecholamine responsiveness as analyzed by the change in the peak [Ca2+]i transient induced by 100 nM isoproterenol was also similar. Superfusion with l-arginine, an NOS substrate, caused decreased shortening with no change in [Ca2+]i transients in allogenic myocytes, but had no effect in syngenic myocytes.

CONCLUSIONS

Depressed contractile function of rejecting allogenic heart transplants in vivo appears to be caused in part by an NOS-dependent decrease in myofilament Ca2+ sensitivity.

Protective role of the L-arginine-nitric oxide synthase pathway on preservation injury after rat liver transplantation

BACKGROUND

A major problem complicating liver transplantation is the preservation injury that results from cold storage and subsequent ischemia/reperfusion injury after organ revascularization. The L-arginine-nitric oxide (NO) pathway has been recognized to play critical roles during infection, inflammation, organ injury, and transplant rejection. Recent data indicates that NO synthesis has beneficial effects in several models of liver injury. The purpose of this study is to examine the role of the L-arginine-NO pathway on preservation injury in an experimental model of rat liver transplantation.

METHODS

Orthotopic liver transplantation was performed in syngeneic (LEW to LEW) rats. Liver preservation injury was determined by measuring serum liver function tests 6 to 48 hours after transplantation. In some experiments, rats received L-arginine supplementation 0 to 24 hours after transplantation. In other experiments, NO synthase inhibitors (L-NAME or L-NIL) were injected at the time of isograft revascularization.

RESULTS

L-Arginine supplementation decreased hepatic transaminase levels at all time points examined (6-48 hours). L-Arginine produced a significant improvement in liver preservation injury by 12 hours after reperfusion. The NO synthase inhibitor L-NAME caused a significant increase in liver injury 24 hours after injection. The inducible NO synthase (iNOS)-specific inhibitor L-NIL had no significant effect on liver injury.

CONCLUSIONS

The results show that L-arginine supplementation and NO synthesis improve hepatic injury and have a protective role in the transplanted liver graft. The protective effect may be mediated by low-level cNOS-derived NO.

Endobronchial transfection of naked viral interleukin-10 gene in rat lung allotransplantation

BACKGROUND

Recent studies suggest that viral interleukin-10 (vIL-10) suppresses alloimmune response in transplantation. Tissue mRNA expression of inducible nitric oxide synthase (iNOS) and exhaled nitric oxide (NO) levels have been observed to increase in lung allograft rejection. The aims of this study were to examine the feasibility of vIL-10 gene transfer into rat lung allografts and to investigate its effect on subsequent allograft rejection.

METHODS

Male Lewis rats (RT1l) underwent left lung transplantation with allografts from Brown Norway rats (RT1n). The donor rats were endobronchially transfected 2 minutes before harvest with 400 microg (group I, n = 5), 600 microg (group II, n = 5), or 800 microg (group III, n = 5) of naked pCMVievIL-10. Group IV (n = 5) animals, serving as control, received 400 microg of naked pCF1-CAT. All recipients were sacrificed on postoperative day 5. Transgene expression of vIL-10 was assessed by both reverse transcriptase-polymerase chain reaction and immunohistochemistry. Allograft gas exchange, exhaled NO level, histologic rejection score, and mRNA expression of graft cyokines were also assessed.

RESULTS

Transgene expression of lung graft vIL-10 was detected by both reverse transcriptase-polymerase chain reaction and immunohistochemistry. The iNOS mRNA expression in groups I, II, and III was significantly lower than that of group IV (p < 0.05, analysis of variance). Exhaled NO levels in groups I, II, and III were significantly lower than in group IV (p < 0.01, analysis of variance). There was no significant difference between groups with respect to gas exchange, peak airway pressure, or histologic rejection score.

CONCLUSIONS

It appears that endobronchial transfection of naked vIL-10 plasmid in a rat lung allotransplant model is feasible and suppresses lung iNOS mRNA expression and exhaled NO levels. An association between iNOS upregulation and high exhaled NO levels in lung allograft resection was also noted.

NOX 100, a nitric oxide scavenger, enhances cardiac allograft survival and promotes long-term graft acceptance

BACKGROUND

We examined the role of nitrosative stress in allograft destruction.

METHODS

Rats undergoing cardiac transplants received NOX-100, a water-soluble nitric oxide (NO) scavenger with antioxidant properties, with or without low-dose cyclosporine (CsA). Graft survival, NO production, and nuclear factor kappa B (NF-kappaB) activity were studied.

RESULT

Using NOX-100 daily until rejection prolonged graft survival (11.6+/-0.6 vs. 7.4+/-0.2 days; P<0.05). Daily low-dose CsA (2.5 mg/kg im) for 7 days or until rejection also prolonged survival (12.6+/-0.5 and 21.6+/-1.6 days, respectively; P<0.01 vs. Controls). Low-dose CsA for 7 days and NOX-100 for 30 days prolonged graft survival (45.0+/-4.7 days; P<0.01 vs. all groups.). NOX-100 had no effect on whole blood CsA levels. Combination therapy until Day 100 resulted in 1 graft loss at Day 116 and indefinite survival in 3 animals (>300 days), which accepted a second WF strain heart without further immunosuppressive therapy but promptly rejected a third party (ACI) cardiac allograft. NOX-100 and CsA reduced nitrate and nitrite, and combination therapy completely normalized NO through to Day 30. Electron paramagnetic resonance spectroscopic analysis demonstrated reduction of signals for nitrosylmyoglobin and nitrosyl-heme with NOX-100 and elimination of signals with CsA alone or combination therapy. Activity of myocardial NF-kappaB decreased with monotherapy vs. untreated allografts. Combination therapy resulted in further inhibition of NF-kappaB up to Day 30. The extent of graft survival correlated with the extent of NO scavenging and NF-kappaB inhibition. Short-term combination therapy had no effect on graft lymphocytic infiltrate on Days 15, 20, and 30.

CONCLUSION

These data support a role for both oxidative and nitrosative stress in rejection and the immunoregulatory potential of antioxidant therapy after transplantation.

Reduction of myocardial nitrosyl complex formation by a nitric oxide scavenger prolongs cardiac allograft survival

Nitric oxide synthase (NOS) inhibitors have been shown to reduce NO but yield conflicting results on cardiac allograft survival. In this study, we provide an alternative approach specifically to examine the efficacy of a NO scavenger on nitrosyl complex formation and graft survival in a model of heterotopic cardiac transplantation. Efficacy was examined under both acute and chronic conditions (i.e., without or with immunosuppression, respectively). Electron paramagnetic resonance (EPR) spectroscopy of frozen myocardial tissue from untreated allografts showed progressive increases in nitrosylheme and nitrosomyoglobin before graft failure. These signals were not seen in either isografts or native hearts of allograft recipients. Both plasma nitrate plus nitrite and myocardial nitrosyl complex formation in cardiac allografts were significantly decreased in recipient animals treated with the NO scavenger, NOX-100, or by low-dose cyclosporine (CsA). Both interventions were nearly equivalent in significantly prolonging graft survival. The short-term combination treatment of both NOX-100 plus CsA completely eliminated myocardial nitrosyl complex formation and synergistically prolonged graft survival. Long-term combination drug treatment (days 0-100) followed by cessation of therapy resulted in permanent graft acceptance with no evidence for nitrosyl complex formation. These studies support a role of NO in cardiac allograft rejection. Furthermore, these studies indicate a potential therapeutic value of NO scavengers in preventing organ rejection.

Inducible nitric oxide synthase promotes cytokine expression in cardiac allografts but is not required for efficient rejection

BACKGROUND

Inducible nitric oxide synthase (iNOS) is enhanced during acute rejection. Pharmacologic inhibition of nitric oxide synthase (NOS) activity has had variable effects on graft survival in a number of animal models. To further characterize the requirement and effects of iNOS during acute allograft rejection, we examined rejection responses of mice completely deficient of iNOS.

METHODS

Heterotopic cardiac allografts were performed using wild-type and iNOS deficient mice (iNOS[-/-]) as recipients. Graft survival was determined by abdominal palpation. At days 3 and 7 following transplantation, grafts were harvested and analyzed histologically. Cytokine messenger RNA (mRNA) expression was measured by ribonuclease protection assay.

RESULTS

Mean survival time of cardiac allografts did not differ between wild-type (18 +/- 3 days) and iNOS(-/-) recipients (16 +/- 2 days). At 3 days, findings of moderate acute rejection were seen in both recipients groups, although modestly reduced in iNOS(-/ -) mice. By 7 days, allografts in both groups demonstrated severe rejection. Within grafts at day 3, there was a 3-fold reduction in IL-1beta expression and a 4-fold reduction in IL-1RA in iNOS(-/-) recipients (p = 0.03 andp = 0.04, respectively) compared to wild-type recipients. Expression of other proinflammatory cytokines was detected in the grafts from both recipients, but was not significantly different. Finally, rejection responses to iNOS(-/-) cardiac allografts were nearly identical to wild-type allografts.

CONCLUSIONS

Rejection of cardiac allografts by iNOS(-/-) mice occurs in a similar fashion to wild-type recipients, with extensive inflammation and proinflammatory cytokine production. While iNOS may play a role in cytokine induction by macrophages, these studies suggest that iNOS is not required for efficient cardiac graft rejection.

Relative importance of cytotoxic T lymphocytes and nitric oxide-dependent cytotoxicity in contractile dysfunction of rejecting murine cardiac allografts

BACKGROUND

Previous in vitro studies have suggested that both cytotoxic T lymphocyte (CTL)-mediated and non-CTL-mediated myocyte lysis occur during murine cardiac heterotopic allograft rejection, but the relative importance of these injury mechanisms on myocardial function is not established. We therefore compared the in vivo effects of depletion of CTL and inhibition of nitric oxide synthase (NOS) on contractility of the rejecting heart.

METHODS

Syngeneic (BALB/c into BALB/c) and allogeneic (BALB/c into C57/B16) heterotopic abdominal cardiac transplants were performed. In some of the allogeneic transplants, CD8+ lymphocytes were depleted by intraperitoneal injection of anti-CD8 monoclonal antibody. NOS inhibition was accomplished by continuous infusion of NG-monomethyl-L-arginine via a subcutaneous osmotic pump. Five days after transplantation, the abdominal cavity was opened and the transplanted heart exposed. Base to apex developed force was measured during spontaneous beating at a diastolic stretch of 4 g by placing a suture through the apex of the heart and attaching it to a strain gauge. Effects of interventions on graft survival were determined by recording the days required for loss of palpable graft contractions.

RESULTS

Allogeneic hearts showed a significant reduction in systolic force compared to non-rejecting syngeneic hearts. Depletion of CD8+ cells improved contractility significantly relative to non-depleted allogeneic hearts, but contractility remained significantly reduced relative to syngeneic hearts. Developed force in allogeneic hearts was also improved by NOS inhibition (P<0.01), and NG-monomethyl-L-arginine infusion slightly prolonged graft survival.

CONCLUSION

Both CTL-mediated and NOS-dependent (possibly macrophage-mediated) mechanisms contribute to contractile dysfunction during early cardiac allograft rejection in this model. However, NOS inhibition combined with CTL depletion only slightly prolongs graft survival in this model.

Electron spin resonance analysis of heme-nitrosyl and reduced iron-sulfur centered complexes in allogeneic, heterotopic cardiac transplants: effects of treatment with pyrrolidine dithiocarbamate

Inhibition of inducible nitric oxide synthase (iNOS) prolongs allograft survival suggesting a role for nitric oxide (.NO) in allograft rejection. Induction of iNOS is regulated by the oxidant-sensitive, nuclear factor kappa B (NF-kappaB) in many cell types. In the present study using electron spin resonance (ESR) spectroscopy, we evaluated whether pyrrolidine dithiocarbamate (PDTC), a metal chelator and antioxidant, might limit .NO production during the development of rejection in cardiac allografts. We performed either isogeneic (Lewis to Lewis) or allogeneic (Wistar-Furth to Lewis) heterotopic abdominal cardiac transplantation. Allograft recipients received daily injections of PDTC or aminoguanidine (a known inhibitor of iNOS). At postoperative days 4 or 6, grafted and native hearts of transplant recipients were flushed with cardioplegic solution to remove blood contamination. ESR data of allografts revealed a triplet nitrogen signal (aN=17.5 G) and centered at g=2.012 and an additional broad signal at g=2.08. This signal was not seen in either isografts or native hearts of either isograft or allograft recipients. Based upon these parameters, these signals are attributed to nitrosomyoglobin. This signal was inhibited by treatment with aminoguanidine or PDTC. Under these conditions, PDTC also prolonged graft survival from 6.6+/-0.2 to 11.7+/-0.3 days. Thus, it is conceivable that nitrosylmyoglobin formation precedes rejection in cardiac allografts and inhibition of nitrosomyoglobin with agents such as PDTC contribute to improved graft survival.

Nitric oxide protects cardiomyocytes against tert-butyl hydroperoxide-induced formation of alkoxyl and peroxyl radicals and peroxidation of phosphatidylserine

We studied protective effects of nitric oxide against tert-butyl hydroperoxide-induced oxidative damage to cardiac myocytes. Two distinct free radicals species--alkoxyl radicals associated with non-heme iron catalytic sites and myoglobin protein-centered peroxyl radicals--were found in low-temperature EPR spectra of cardiac myocytes exposed to t-BuOOH. The t-BuOOH-induced radical formation was accompanied by site-specific oxidative stress in membrane phospholipids (peroxidation of phosphatidylserine) assayed by fluorescence HPLC after metabolic labeling of cell phospholipids with oxidation-sensitive cis-parinaric acid. An NO-donor, (Z)-1-[N-(3-ammonio-propyl)-N-(n-propyl) amino]-diazen-1-ium-1,2-diolate], protected cardiac myocytes against tert-butyl hydroperoxide-induced: (i) formation of non-protein- and protein-centered free radical species and (ii) concomitant peroxidation of phosphatidylserine. Thus nitric oxide can act as an effective antioxidant in live cardiomyocytes.

Skin allograft rejection in mice lacking inducible nitric oxide synthase

BACKGROUND

During allograft rejection, up-regulation of cytokine-inducible nitric oxide synthase (iNOS) leads to the production of large amounts of nitric oxide (NO). The net effect of NO on the alloimmune response is, however, difficult to predict because of its diverse biological effects, which include potentially opposing roles as an effector and immunoregulatory molecule.

METHODS

In this study, the role of iNOS on the in vitro and in vivo alloimmune response was defined using mutant mice that lack a functional iNOS gene. The ability of spleen cells obtained from iNOS-deficient mutants to proliferate and to produce cytokines in response to irradiated BALB/c stimulator cells was determined, and the rate at which iNOS-deficient mice were able to reject BALB/c skin allografts was observed.

RESULTS

Spleen cells from homozygous iNOS-deficient (129xMF1)F1 mice, when compared with cells from heterozygous control mice, showed an increased in vitro proliferative response and produced substantially higher levels of interferon-gamma, and also more interleukin-2 and interleukin-12, in response to allogeneic stimulation. The kinetics of BALB/c skin graft rejection were comparable in heterozygous control animals and iNOS-deficient mice. Moreover, no net effect of iNOS on skin allograft rejection was apparent in mice treated with depleting monoclonal antibodies (mAb) to CD4 or CD8 T cells, either alone or in combination, or in mice treated with both anti-CD8 mAb and a neutralizing anti-tumor necrosis factor mAb.

CONCLUSIONS

These results show that iNOS has an immunomodulatory effect on the in vitro alloimmune response but lack of iNOS has no net influence on the kinetics of murine skin allograft rejection in either unmodified recipients or recipients in which the early contribution of T-cell subsets and tumor necrosis factor-alpha to graft rejection has been abrogated.

Nitric oxide prevents oxidative damage produced by tert-butyl hydroperoxide in erythroleukemia cells via nitrosylation of heme and non-heme iron. Electron paramagnetic resonance evidence

We studied protective effects of NO against tert-butylhydroperoxide (t-BuOOH)-induced oxidations in a subline of human erythroleukemia K562 cells with different intracellular hemoglobin (Hb) concentrations. t-BuOOH-induced formation of oxoferryl-Hb-derived free radical species in cells was demonstrated by low temperature EPR spectroscopy. Intensity of the signals was proportional to Hb concentrations and was correlated with cell viability. Peroxidation of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, and cardiolipin metabolically labeled with oxidation-sensitive cis-parinaric acid was induced by t-BuOOH. An NO donor, (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)amino]-diazen-1-iu m-1, 2-diolate], produced non-heme iron dinitrosyl complexes and hexa- and pentacoordinated Hb-nitrosyl complexes in the cells. Nitrosylation of non-heme iron centers and Hb-heme protected against t-BuOOH-induced: (a) formation of oxoferryl-Hb-derived free radical species, (b) peroxidation of cis-parinaric acid-labeled phospholipids, and (c) cytotoxicity. Since NO did not inhibit peroxidation induced by an azo-initiator of peroxyl radicals, 2, 2'-azobis(2,4-dimethylvaleronitrile), protective effects of NO were due to formation of iron-nitrosyl complexes whose redox interactions with t-BuOOH prevented generation of oxoferryl-Hb-derived free radical species.

Attenuation of lethal graft-versus-host disease by inhibition of nitric oxide synthase

We have previously demonstrated that administration of the nitric oxide (NO) synthesis inhibitor aminoguanidine (AG) to mice undergoing nonlethal graft-versus-host disease (GVHD) results in less destruction of host tissue and enhanced proliferative responses of splenic lymphocytes. Subsequently, we have determined whether the amelioration of GVHD pathology associated with inhibition of NO synthesis affects survival in a lethal GVHD model. Utilizing a C57BL/6 to C57BL/6xDBA2JF1 model, administration of parental lymph node lymphocytes instead of splenocytes results in 80-90% lethality by week 4 after GVHD induction. Administration of AG resulted in significantly decreased lethality coincident with decreased serum NO2- + NO3- levels. AG therapy had no effect on donor anti-host cytolytic T cell activity, which indicates that destruction of host tissue via this pathway was unaffected by the therapy. Histological evaluation of spleen, small intestine, bone, and mesenteric lymph node did not reveal any difference in the histological correlates of disease in the treated mice. AG increased various hematopoietic indices, including red blood cell count, white blood cell count, and hemoglobin, which indicates that the disruption of hematopoiesis during acute GVHD is mediated in part by NO. In addition, the number of GVHD mice with endogenous bacterial infections in the spleen and liver was significantly decreased in mice receiving AG therapy. These data indicate that NO plays a detrimental role during GVHD that appears to result in decreased hematopoietic indices and concomitant susceptibility to bacterial infection.

Decreased urinary excretion of nitric oxide in acute rejection episodes in pediatric renal allograft recipients

Acute renal allograft rejection continues to have a negative effect on graft survival despite a better understanding of the molecular basis of renal allograft rejection. Nitric oxide (NO) has important biological functions in cell defense and injury and some evidence exists that it may act as an immunomodulator in allograft transplantation. To determine if NO has any role in acute renal allograft rejection in pediatric patients, acute rejection episodes in pediatric renal transplant recipients were evaluated. Four out of eleven patients who received a renal allograft in 1995 at Children's Kidney Center at The Children's Hospital of Buffalo had eight episodes of acute rejection. One patient received a living-related and three received cadaveric grafts. Stable metabolites of NO (NO-2 + NO-3 = NOx) were measured in the serum and urine samples of the patients daily. Serum levels of NO did not change significantly during acute rejection episodes. Urinary NOx levels decreased by 73+/-9% of the baseline values during episodes of acute rejection: mean +/-SE urinary nitric oxide/creatinine ratio (NOx/Cr) of 0.17+/-0.05 at baseline vs. 0.05+/-0.01 during rejection (P=0.02). Successful treatment of acute rejection by administration of high dose i.v. steroids or OKT-3 induced acute rises in the urinary NO levels to baseline values: NO/Cr = 0.17+/-0.04 (mean +/-SE). We conclude that urinary NO excretion decreases significantly during acute renal allograft rejection and that NOx concentration in the urine increases in response to successful antirejection therapy.

Nitric oxide synthase inhibition is associated with decreased survival of cardiac allografts in the rat

Nitric oxide is a biological mediator that regulates blood vessel wall tonus, enhances macrophage cytotoxicity, and inhibits cellular immune reactivity. Primary acute rejection is associated with increased intragraft production of NO but it is unknown whether this delays or enhances the loss of graft function. The aim of the current study was to determine the effect of L-NAME, a nitric oxide synthase inhibitor, on the course and histopathology of rat cardiac allografts with primary acute rejection. L-NAME decreased the graft survival time from 9.4+/-1.5 to 6.9+/-0.3 days; the histopathology at asystole showed predominantly ischemic necrosis. L-NAME combined with antihypertensive drugs restored the rejection time (from 8.6+/-0.4 to 14.2+/-3.2 days) and resulted in an acute rejection pattern. We conclude that blocking of nitric oxide formation during acute rejection of a vascularized cardiac graft results in a decreased graft survival time and ischemic graft necrosis, very likely secondary to unopposed vasoconstriction.

Differential localization of allograft nitric oxide synthesis: comparison of liver and heart transplantation in the rat model

Nitric oxide (NO) is a free radical with a diversity of cellular origins and potential functions. Within the realm of solid organ transplantation, NO has been the focus of much attention. Discordant reports have documented both suppression and potentiation of the alloimmune response. In addition to questions regarding its functional role, little is known of the cellular origins of NO in acute rejection of vascularized allografts. To address this question, acute rejection models of rat heterotopic heart and orthotopic liver transplantation were chosen. When compared with naive controls and isografted animals, acute rejection in both heart and liver transplantation was associated with elevated systemic levels of the NO metabolite, nitrite. This was accompanied by increased graft content of iNOS protein as determined by immunoblot analysis of protein extracts. Expression of iNOS mRNA was localized with in situ hybridization. In both heart and liver transplantation, iNOS mRNA was found in the inflammatory infiltrate accompanying acute rejection. In addition, hepatocytes also expressed iNOS mRNA in the rejecting liver allograft. In contrast, cardiac myocytes in the rejecting heart allograft did not stain for iNOS mRNA. These results indicate that organ-specific, differential cellular expression of iNOS occurs in the acutely rejecting allograft. Transcriptional regulation of iNOS may vary among various organs according to the local cellular milieu. In addition, there may be a variable allograft specific response to acute rejection which may modify the associated immunologic biology.

Bystander injury of host lymphoid tissue during murine graft-verus-host disease is mediated by nitric oxide

The suppressed lymphocyte proliferative responses characteristic of graft-versus-host disease (GVHD) are due, in part, to production of nitric oxide (NO). In order to more fully elucidate the role of NO during GVHD, an NO synthesis inhibitor, aminoguanidine (AG), was administered to unirradiated (C57BL/6J X DBA/2J)F1 mice injected with 5 X 10(7) C57BL/6J splenocytes. Administration of AG resulTed in abrogation of the elevation in serum NO2- + NO3- levels characteristic of GVHD. A significantly increased percentage of splenocytes of host phenotype (H2b/d, B220+, and THY1.2+) and a significantly higher hematocrit value were detected in GVHD animals receiving AG therapy. Additionally, the Con A-induced proliferative response of splenocytes obtained from GVHD mice receiving AG therapy was increased compared with the responses of splenocytes from animals that did not receive AG therapy. Parameters not affected by AG therapy included NO synthesis by recovered peritoneal macrophages, donor antihost cytolytic activity in splenocyte populations, serum GM-CSF levels and long-term engraftment of donor cells. These data indicate that NO may play a role in the destruction of both lymphoid and erythroid host tissue as well as the reduced lymphoproliferative responses associated with the acute phase of GVHD.

Cellular localization and effect of nitric oxide synthesis in a rat model of orthotopic liver transplantation

Nitric oxide (NO) is a multifunctional free radical with a variety of described biochemical and physiological roles. The immunologic relationships between organ transplantation and NO synthesis are unknown. While a number of in vitro and in vivo models have demonstrated an immunomodulatory role for NO, results suggest both an immunosuppressive and immunostimulatory function. In order to better delineate the role of NO in liver transplantation, the Kamada model of rat OLT with strain combinations simulating acute rejection and spontaneous hyporesponsiveness was chosen. In this setting, both acute rejection and spontaneous hyporesponsiveness were associated with increased levels of plasma NO metabolites and allograft expression of the enzyme, NO synthase (iNOS). The extent of expression was significantly greater with acute rejection. Using in situ hybridization, iNOS mRNA was localized to both infiltrating inflammatory cells and hepatocytes in the context of acute rejection. In contrast, iNOS mRNA expression was isolated to the hepatocytes in the hyporesponsive state. To specifically delineate the role of hepatocyte-derived NO, NO synthesis was ablated in the spontaneous hyporesponsiveness model and resulted in significant elevation of serum transaminase values with accompanying histologic evidence of increased periportal inflammatory infiltration. Our results suggest that the site of NO production varies according to the immunologic status of the liver allograft, and hepatocyte-derived NO may be protective in the hyporesponsive state.

Gas-generating systems in acute renal allograft rejection in the rat. Co-induction of heme oxygenase and nitric oxide synthase

Gases are now viewed as biologic messengers, and in this regard, carbon monoxide and nitric oxide are incriminated in signaling processes in neural tissue. Carbon monoxide is generated by heme oxygenase (HO), an enzyme inducible by heme, cytokines, and oxidative stress and considered an antioxidant response; nitric oxide is generated by nitric oxide synthase, an enzyme also inducible by cytokines. Since mononuclear cells infiltrate the acutely rejecting kidney, and foster within the kidney oxidative stress and a cytokine-enriched milieu, we examined the expression of these enzymes in acute renal allograft rejection (AR) (Brown Norway kidney to a Lewis rat; n = 17) and in control isografts (Lewis kidney to a Lewis rat; n = 17). No immunosuppressives were used. We found marked induction of HO mRNA and protein in renal allografts at day 5 after transplantation. Prominent expression of HO protein, as detected by immunofluorescence, was observed in the mononuclear cells infiltrating the renal allograft. More than 80% of these cells were macrophages, as identified by positive staining with ED1 antibody. ED1+ cells were rare in isografts and did not stain for HO. We also found co-expression of mRNA and protein for the inducible isoform of nitric oxide synthase (iNOS) in AR at day 5 after transplantation. Induction of HO and iNOS may reflect the cellular effect of diverse cytokines elaborated in the rejecting kidney. HO may enable the macrophage to degrade heme-containing proteins released from erythrocytes and other damaged cells; alternatively, induction of HO may defend the macrophage against oxidant injury. Increased nitric oxide, as a result of iNOS activity, may antagonize the vasoconstrictive effects of a number of mediators (i.e., thromboxane and endothelin) present in acute rejection; conversely, nitric oxide may prove cytotoxic through a number of recognized effects. Our studies provide the first demonstration of the induction of HO in the rejecting renal allograft as well as the first demonstration in vivo for the induction of HO in macrophages at the site of an inflammatory response. Such expression, linked as it is to the expression of iNOS, indicates that the macrophage mimics the behavior of neural cells by generating these gaseous messengers; thus, neural cells are not alone in deploying these mediators. Through a number of effects, these products of HO and iNOS may influence the nature and severity of tissue injury in AR.

Nitric oxide: what role does it play in inflammation and tissue destruction?

Large amount of nitric oxide (NO) are produced at sites of inflammation through the action of inducible nitric oxide synthase (iNOS) present in both infiltrating leucocytes and activated, resident tissue cells. However, the role of NO in inflammation remains unclear. NO is a vasodilator, which inhibits the adhesion of neutrophils to the vascular endothelium; it reduces the production of IL-6 by Kupffer cells and chondrocytes, and the production of gamma-IFN and TNF-alpha by splenocytes. The literature provides contradictory information on the effect of NO on vascular leakiness, chemotaxis, prostaglandin production and tissue damage. Increasingly, data suggest that NO is immunosuppressive. Inhibitors of NOS have potent prophylactic activity in several but not all, animal models of inflammatory disease. However, in rat adjuvant arthritis, therapeutic activity is weak. Whether inhibitors of iNOS will be therapeutically useful in human inflammatory diseases cannot be predicted on the basis of present information.

Nitric oxide and intracellular heme

Figure 2 depicts a working hypothesis for these results. Activation of .NO synthesis results in nitrogen oxide-induced loss of protein-bound heme from CYP proteins, which remain relatively intact. This heme liberation results in a decrease in heme synthesis (decreased ALAS) and an increase in heme degradation (increased HO). In addition, .NO synthesis results in direct inhibition of ferrochelatase, which further contributes to inhibition of heme synthesis. There also appears to be a mechanism to repair or resynthesize CYP after .NO synthesis is inhibited. Finally, a result of this effect may be protection against cellular injury, since increased HO is an important response against cellular injury from a variety of insults.