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

The arginine and nitric oxide metabolic pathway regulate the gut colonization and expansion of Ruminococcous gnavus

Ruminococcus gnavus is a mucolytic commensal bacterium whose increased gut colonization has been associated with chronic inflammatory and metabolic diseases in humans. Whether R. gnavus metabolites can modulate host intestinal physiology remains largely understudied. We performed untargeted metabolomic and bulk RNA-seq analyses using R. gnavus monocolonization in germ-free mice. Based on transcriptome-metabolome correlations, we tested the impact of specific arginine metabolites on intestinal epithelial production of nitric oxide (NO) and examined the effect of NO on the growth of various strains of R. gnavus in vitro and in nitric oxide synthase 2 (Nos2)-deficient mice. R. gnavus produces specific arginine, tryptophan, and tyrosine metabolites, some of which are regulated by the environmental richness of sialic acid and mucin. R. gnavus colonization promotes expression of amino acid transporters and enzymes involved in metabolic flux of arginine and associated metabolites into NO. R. gnavus induced elevated levels of NOS2, while Nos2 ablation resulted in R. gnavus expansion in vivo. The growth of various R. gnavus strains can be inhibited by NO. Specific R. gnavus metabolites modulate intestinal epithelial cell NOS2 abundance and reduce epithelial barrier function at higher concentrations. Intestinal colonization and interaction with R. gnavus are partially regulated by an arginine-NO metabolic pathway, whereby a balanced control by the gut epithelium may restrain R. gnavus growth in healthy individuals. Disruption in this arginine metabolic regulation will contribute to the expansion and blooming of R. gnavus.

Resolution of Inflammation in Acute Graft-Versus-Host-Disease: Advances and Perspectives

Inflammation is an essential reaction of the immune system to infections and sterile tissue injury. However, uncontrolled or unresolved inflammation can cause tissue damage and contribute to the pathogenesis of various inflammatory diseases. Resolution of inflammation is driven by endogenous molecules, known as pro-resolving mediators, that contribute to dampening inflammatory responses, promoting the resolution of inflammation and the recovery of tissue homeostasis. These mediators have been shown to be useful to decrease inflammatory responses and tissue damage in various models of inflammatory diseases. Graft-versus-host disease (GVHD) is a major unwanted reaction following allogeneic hematopoietic stem cell transplantation (allo-HSCT) and is characterized by an exacerbated inflammatory response provoked by antigen disparities between transplant recipient and donor. There is no fully effective treatment or prophylaxis for GVHD. This review explores the effects of several pro-resolving mediators and discusses their potential use as novel therapies in the context of GVHD.

Nitric oxide inhibition strategies

Nitric oxide is involved in many physiologic processes. There are efforts, described elsewhere in this volume, to deliver nitric oxide to tissues as a therapy. Nitric oxide also contributes to pathophysiologic processes. Inhibiting nitric oxide or its production can thus also be of therapeutic benefit. This article addresses such inhibitory strategies.

G-CSF mobilizes CD34 + regulatory monocytes that inhibit graft-versus-host disease

G-SCF–mobilized CD34 + monocytes inhibit graft-versus-host disease by the production of nitric oxide and the induction of regulatory T cells.

High pre-transplant serum nitrate levels predict risk of acute steroid-refractory graft-versus-host disease in the absence of statin therapy

Steroid-refractory graft-versus-host disease is a life-threatening complication after allogeneic stem cell transplantation. Evidence is accumulating that steroid-refractory graft-versus-host disease is associated with endothelial distress. Endothelial cell homeostasis is regulated by nitric oxide, and serum nitrates are derived from nitric oxide synthase activity or dietary sources. In this retrospective study based on 417 patients allografted at our institution we investigated whether quantification of serum nitrates could predict steroid-refractory graft-versus-host disease. Elevated pre-transplant levels of serum nitrates (>26.5 μM) predicted steroid-refractory graft-versus-host disease (P=0.026) and non-relapse mortality (P=0.028), particularly in combination with high pre-transplant angiopoietin-2 levels (P=0.0007 and P=0.021, respectively). Multivariate analyses confirmed serum nitrates as independent predictors of steroid-refractory graft-versus-host disease and non-relapse mortality. Differences in serum nitrate levels did not correlate with serum levels of tumor necrosis factor or C-reactive protein or expression of inducible nitric oxide synthase in blood cells. Patients with high pre-transplant nitrate levels had significantly reduced rates of refractory graft-versus-host disease (P=0.031) when pravastatin was taken. In summary, patients at high risk of developing steroid-refractory graft-versus-host disease could be identified prior to transplantation by serum markers linked to endothelial cell function. Retrospectively, statin medication was associated with a reduced incidence of refractory graft-versus-host disease in this endothelial high-risk cohort.

Immune Regulation and Oxidative Stress Reduction by Preimplantation Factor following Syngeneic or Allogeneic Bone Marrow Transplantation

Bone marrow transplantation (BMT), a well-established treatment for hematological diseases, is frequently hampered by graft-versus-host disease (GVHD) and/or by infections due to delay in immune restoration. Prelmplantation Factor (PIF) is an embryo-derived peptide whose physiological function is to regulate local and systemic immunity and promote transplant acceptance. Synthetic PIF’s effectiveness to regulate immune response following BMT was herein examined in murine model. PIF administration reduced GVHD following allogenic BMT, decreased skin, liver, and colon inflammation and down regulated GVHD-associated gene expression in the liver. iNOS gene expression was reduced both in liver and colon. In syngeneic BMT, PIF administration reduced proinflammatory genes expression and promoted mice weight recovery up to two months after transplantation. PIF immune-regulatory effects were mediated via interaction with monocytes, resulting in decreased iNOS expression and NO secretion in-vitro. Overall, we demonstrate that by regulating immune response after BMT, PIF reduces inflammation and oxidative stress, leading to transplant success.

The oxidative status of blood cells in a murine model of graft-versus-host disease

We studied the oxidative status of red and white blood cells during the development of graft vs host disease (GVHD) as well as the effects of treatment with antioxidants, both in vitro and in vivo. (BALB/c X C57BL/6) F1 mice were conditioned by total body radiation and, 1 day later, transplanted with semi-allogeneic C57BL/6 spleen cells. GVHD was followed by its clinical manifestations. Oxidative stress in red blood cells (RBC), neutrophils, and lymphocytes was assessed by measuring generation of reactive oxygen species and the content of reduced glutathione by flow cytometry after gating of the specific populations. Oxidative stress was noticed 3 weeks after transplantation. It was higher in mice receiving allogeneic spleen cells as compared with mice transplanted with syngeneic cells, suggesting that it was associated with GVHD. The results also demonstrated that treatment with the antioxidants N-acetylcysteine and a derivative of vitamin E (tocopherol succinate, propofol), both in vitro and in vivo, reduced the oxidative stress. The results indicate that various blood cells, including RBC, neutrophils, and lymphocytes, are under oxidative stress and that treatment with antioxidants reduced the stress and, thus, may be useful in ameliorating the severe consequences of GVHD.

Cytokine expanded myeloid precursors function as regulatory antigen-presenting cells and promote tolerance through IL-10-producing regulatory T cells

The initiation of graft-vs-host disease (GVHD) after stem cell transplantation is dependent on direct Ag presentation by host APCs, whereas the effect of donor APC populations is unclear. We studied the role of indirect Ag presentation in allogenic T cell responses by adding populations of cytokine-expanded donor APC to hemopoietic grafts that would otherwise induce lethal GVHD. Progenipoietin-1 (a synthetic G-CSF/Flt-3 ligand molecule) and G-CSF expanded myeloid dendritic cells (DC), plasmacytoid DC, and a novel granulocyte-monocyte precursor population (GM) that differentiate into class II+,CD80/CD86+,CD40- APC during GVHD. Whereas addition of plasmacytoid and myeloid donor DC augmented GVHD, GM cells promoted transplant tolerance by MHC class II-restricted generation of IL-10-secreting, Ag-specific regulatory T cells. Importantly, although GM cells abrogated GVHD, graft-vs-leukemia effects were preserved. Thus, a population of cytokine-expanded GM precursors function as regulatory APCs, suggesting that G-CSF derivatives may have application in disorders characterized by a loss of self-tolerance.

Nitric oxide synthesis contributes to inhibition of graft-versus-tumor-effects against intraperitoneal Meth A tumor

The role of nitric oxide (NO) in graft-versus-tumor-effect (GVT) was evaluated in the present study. GVT was induced by intravenous injection of C57BL/6J (H-2b) mouse splenocytes to {C57BL/6J (H-2b) x BALB/c (H-2d)} F1 mice bearing Meth A (H-2d) ascites tumors. Induction of GVT increased nitrite production and expression of inducible NO synthase by ascites cells. The increased nitrite production was inhibited by NG-monomethyl-L-arginine (MLA). Experiments employing immunomagnetic depletion of Mac-1+ cells from ascites indicated that macrophages were a major cellular source of the nitrite production. Interferon-gamma levels were increased in both serum and ascites fluid during GVT. Induction of GVT prolonged survival of ascites-bearing mice, and increased urinary nitrate excretion. MLA administration inhibited GVT-induced increase in urinary nitrate excretion, and further prolonged GVT-induced increase in survival. These results indicate that NO synthesis is induced in tumors during GVT, and the NO acts as an inhibitor of GVT.

Endogenous Nitric Oxide Protects against T Cell-Dependent Lethality during Graft-versus-Host Disease and Idiopathic Pneumonia Syndrome

The pathogenesis of idiopathic pneumonia syndrome (IPS), a noninfectious pulmonary complication of allogeneic bone marrow transplantation (BMT), has not been fully elucidated. However, several contributing factors have been proposed, including lung injury caused by reactive oxygen and nitrogen intermediates during preconditioning and development of graft-vs-host disease (GVHD). Studies on the role of reactive oxygen and nitrogen intermediates in IPS have yielded conflicting results. We have described a murine model of IPS, in which the onset of lung inflammation was delayed by several weeks relative to GVHD. This study evaluated whether the delay in onset of IPS was due to slow turnover of NO-producing, immunosuppressive alveolar macrophages (AM) following BMT. The results indicated that AM were immunosuppressive due to synthesis of NO. However, NO production and immunosuppressive activity by AM did not decline after BMT, but rather remained elevated throughout the 12-wk development of GVHD and IPS. In a 14-day model of IPS, continuous inhibition of NO with aminoguanidine (AG) reduced signs of IPS/GVHD, but also led to higher mortality. When AG treatment was initiated after onset of IPS/GVHD, rapid mortality occurred that depended on the severity of IPS/GVHD. AG-enhanced mortality was not due to inhibition of marrow engraftment, elevated serum TNF-alpha, liver injury, or hypertensive responses. In contrast, T cells were involved, because depletion of CD4(+) lymphocytes 24 h before AG treatment prevented mortality. Thus, NO production following allogeneic BMT affords a protective effect that helps down-regulate injury caused by T cells during GVHD and IPS.

The role of interferon-gamma, nitric oxide and lipopolysaccharide in intestinal graft-versus-host disease developing in F1-hybrid mice

(C57BL/6 x DBA/2)F1-hybrid mice injected with lymphoid cells from wild-type, C57BL/6 donors develop acute, lethal graft-versus-host disease (GVHD) in which the intestine is a major target. In its destructive phase intestinal GVHD is characterized by apoptosis of intestinal crypt epithelial cells and the development of endotoxaemia. Injection of as little as 10 microg endotoxin is lethal in mice with acute GVHD, and associated with the release of large amounts of tumour necrosis factor-alpha (TNF-alpha) into the serum. To explore the role of interferon-gamma (IFN-gamma) in the pathogenesis of intestinal GVHD we used IFN-gamma gene knockout (gko) mice as donors. Recipients of grafts from these donors did not develop intestinal GVHD and, unlike recipients of wild-type grafts, did not die when injected with lipopolysaccharide (LPS). We also found that injection 10 microg LPS into recipients of wild-type grafts induced apoptosis of intestinal epithelial crypt cells and was associated with a burst of nitric oxide production in the intestine. Administration of N(omega)nitro L-arginine methyl ester blocked this response. In contrast, LPS did not induce either intestinal epithelial cell apoptosis or increased nitric oxide production in recipients of IFN-gamma gko grafts. These findings indicate that donor-derived IFN-gamma is instrumental for the development of intestinal GVHD. In a previous study we showed that recipients of IFN-gamma gko grafts develop high levels of LPS-induced TNF-alpha release. When our current data are viewed in the context of this observation, they suggest that intestinal epithelial cell apoptosis in the parent-->F1-hybrid model of acute GVHD is mediated primarily by nitric oxide rather than TNF-alpha, and that this depends on donor-derived IFN-gamma.

Effects of oxidant stress on inflammation and survival of iNOS knockout mice after marrow transplantation

In a model of idiopathic pneumonia syndrome after bone marrow transplantation (BMT), injection of allogeneic T cells induces nitric oxide (.NO), and the addition of cyclophosphamide (Cy) generates superoxide (O.) and a tissue-damaging nitrating oxidant. We hypothesized that.NO and O. balance are major determinants of post-BMT survival and inflammation. Inducible nitric oxide synthase (iNOS) deletional mutant mice (-/-) given donor bone marrow and spleen T cells (BMS) exhibited improved survival compared with matched BMS controls. Bronchoalveolar lavage fluids obtained on day 7 post-BMT from iNOS(-/-) BMS mice contained less tumor necrosis factor-alpha and interferon-gamma, indicating that.NO stimulated the production of proinflammatory cytokines. However, despite suppressed inflammation and decreased nitrotyrosine staining, iNOS(-/-) mice given both donor T cells and Cy (BMS + Cy) died earlier than iNOS-sufficient BMS + Cy mice. Alveolar macrophages from iNOS(-/-) BMS + Cy mice did not produce.NO but persisted to generate strong oxidants as assessed by the oxidation of the intracellular fluorescent probe 2',7'-dichlorofluorescin. We concluded that.NO amplifies T cell-dependent inflammation and addition of Cy exacerbates.NO-dependent mortality. However, the lack of.NO during Cy-induced oxidant stress decreases survival of T cell-recipient mice, most likely by generation of.NO-independent toxic oxidants.

Bacterial translocation during graft-versus-host disease after small bowel transplantation is reduced following inhibition of inducible nitric oxide synthesis

BACKGROUND

Increased nitric oxide (NO) production may contribute to intestinal barrier dysfunction and increased bacterial translocation (BT). Since BT may play a major role in graft-versus-host disease (GVHD) after small bowel transplantation (SBTx), we evaluated the role of NO production in GVHD after SBTX in the rat.

METHODS

Using the standard model of semiallogeneic SBTx in the rat, we prepared three experimental groups. Recipients in group 1 received LBNF1-LBNF1 transplants and were treated with aminoguanidine (AG) (200 mg/kg), recipients in group 2 received Lewis-LBNF1 grafts and were injected with saline, and recipients in group 3 received Lewis-LBNF1 transplants and AG (200 mg/kg). Urine nitrite/nitrate levels were measured daily, and BT was determined by culturing peritoneal swabs, mesenteric lymph nodes, spleen, liver, and blood.

RESULTS

In group 1 we detected indefinite survival with normal histology. In group 2 a survival of 10.5 +/- 1.1 days was reached, and the typical histological features of acute GVHD were observed. The animals in group 3 showed a mean survival of 14.8 +/- 0.6 days (P<0.02 compared with group 2) and the histological features of acute GVHD, although with a prolonged time course. Comparing NO production and BT between groups 2 and 3 we detected significantly reduced NO production on postoperative days 2-9 (P<0.03) and significantly decreased BT on postoperative days 3 and 9 (P<0.03).

CONCLUSION

Inhibition of inducible NO synthesis with AG reduces NO production, decreases BT, and prolongs survival during GVHD after SBTx and therefore may be a useful addition to standard treatment protocols for GVHD.

Nitric oxide synthase in toxic epidermal necrolysis and Stevens-Johnson syndrome

Toxic epidermal necrolysis and Stevens-Johnson syndrome are severe cutaneous drug reactions of unknown mechanism. Nitric oxide can cause apoptosis and necrosis. The inducible form of nitric oxide synthase generates large amounts of nitric oxide and has been described in human skin. We propose that a large burst of nitric oxide in toxic epidermal necrolysis and Stevens-Johnson syndrome may cause the epidermal apoptosis and necrosis. Skin biopsies were taken from seven patients with actively progressing Stevens-Johnson syndrome or toxic epidermal necrolysis. Expression of inducible nitric oxide synthase was examined by reverse transcription-polymerase chain reaction and by immunoperoxidase staining for inducible nitric oxide synthase protein. Messenger RNA for inducible nitric oxide synthase was detected by reverse transcription-polymerase chain reaction and confirmed by the sequencing of polymerase chain reaction products. Strong staining for inducible nitric oxide synthase was observed in inflammatory cells in the lower epidermis and upper dermis. Diffuse, weaker staining was observed in keratinocytes. Expression of inducible nitric oxide synthase is consistent with the hypothesis that nitric oxide mediates the epidermal necrosis in toxic epidermal necrolysis and provides a potential target for therapeutic intervention.

High levels of peroxynitrite are generated in the lungs of irradiated mice given cyclophosphamide and allogeneic T cells. A potential mechanism of injury after marrow transplantation

In a murine bone-marrow transplant (BMT) model designed to determine risk factors for lung dysfunction in irradiated mice, we reported that cyclophosphamide (Cy)-induced injury and lethality depended on the infusion of donor spleen T cells. In the study reported here, we hypothesized that alveolar macrophage (AM)-derived reactive oxygen/nitrogen species are associated with lung dysfunction caused by allogeneic T cells, which stimulate nitric oxide (.NO) production, and by Cy, which stimulates superoxide production.NO reacts with superoxide to form peroxynitrite, a tissue-damaging oxidant. On Day 7 after allogeneic BMT, bronchoalveolar lavage fluid (BALF) obtained from mice injected with T cells contained increased levels of nitrite, which was associated with increased lactate dehydrogenase and protein levels, both of which are indices of lung injury. The injury was most severe in mice receiving both T cells and Cy. Messenger RNA (mRNA) for inducible nitric oxide synthase was detected only in murine lungs injected with T cells +/- Cy. AMs obtained on Day 7 after BMT from mice receiving T cells +/- Cy spontaneously generated between 20 and 40 microM nitrite in culture, versus < 2 microM generated by macrophages obtained from mice undergoing BMT but not receiving T cells. The level of 3-nitrotyrosine, the stable byproduct of the reaction of peroxynitrite with tyrosine residues, was increased in the BALF proteins of mice injected with both T cells and Cy. We conclude that allogeneic T cells stimulate macrophage-derived.NO, and that the addition of Cy favors peroxynitrite formation. Peroxynitrite generation clarifies the dependence of Cy-induced lung injury and lethality on the presence of allogeneic T cells.