Tracheal Allografts Research Articles

Transplantation of the cryopreserved tracheal allograft in growing rabbits

Background: Surgical treatment of the congenital tracheal stenosis is challenging, and the long-term fate of transplanted tracheal allograft remains unclear. The authors evaluated the morphologic changes of the cryopreserved tracheal allograft after transplantation in a growing rabbit model. Methods: Each allograft (n = 7) was harvested from 90- to 120-day-old Japanese rabbits, immersed in the preservation solution, and stored in a programmable freezer until reaching −80°c and then kept in liquid nitrogen for 1 week. Orthotopic tracheal transplantation of 7 tracheal rings in an end-to-end fashion was performed in age-matched young rabbits without immunosuppression. The grafts were assessed at 1 week, 1 month, and 3 months after transplantation. As controls, fresh autografts also were evaluated after the same procedure. Results: Two animals died of pneumonia. The body weight gain was similar in both groups. All grafts were patent, but no allografts showed normal growth in length or diameter. Microscopic findings of the allograft showed calcification of the tracheal cartilage without infiltration of inflammatory cells and marked lymphocyte proliferation in the subepithelium. Conclusions: Cryopreserved tracheal allografts without immunosuppressant showed favorable patency of the trachea at 3 months; however, no growth of the allograft occurred in this animal model. The problem of calcification of the allograft remains to be solved.

Reduction of recipient macrophages by gadolinium chloride prevents development of obliterative airway disease in a rat model of heterotopic tracheal transplantation.

Recent studies have shown the possible role of growth factors and the involvement of macrophages as a source of them in the pathogenesis of bronchiolitis obliterans (BO) after lung transplantation. The authors intended to determine whether depletion of recipient macrophages by gadolinium chloride (GdCl3) resulted in decreased obliterative airway disease (OAD) in a rat model of heterotopic tracheal transplantation. A tracheal segment of donor rats (Brown Norway) was transplanted into a subcutaneous pouch of fully major histocompatibility complex-incompatible recipient rats (Lewis). Recipients were injected intravenously with 80 mg/kg of GdCl3.6H2O or saline on days 0, 7, and 14 posttransplant. Allografts were harvested on days 7, 14, 17, and 21 and the degree of OAD resulting from fibroproliferative tissue was pathologically scored on a scale of 0 to 4. A portion of allografts was submitted to reverse-transcriptase polymerase chain reaction analysis to examine mRNA expression for platelet-derived growth factor (PDGF), basic fibroblast growth factor, and transforming growth factor-beta1. Immunohistochemical studies confirmed reduction in the number of ED2+ macrophages in tracheal allografts by GdCl3 injection. GdCl3 treatment significantly decreased OAD of allografts, with the histologic score of 1.4+/-0.3 in the treated animals compared with 3.0+/-0.5 in the controls (mean+/-SE, P=0.02) at day 21 posttransplant, and this was accompanied by decreased PDGF-A and PDGF-B gene expression in the GdCl3 group at day 17 posttransplant. Macrophage reduction by GdCl3 resulted in significantly decreased OAD development and reduced PDGF mRNA expression in allografts. This suggests a potential effectiveness of therapies targeting recipient macrophages in preventing BO after lung transplantation.

The role of the beta chemokines in experimental obliterative bronchiolitis

Beta chemokines have been implicated in cardiac and renal allograft rejection. This study determined if antibody antagonization of beta chemokines conferred protection against the development of experimental obliterative bronchiolitis (OB) in a heterotopic rat tracheal allograft model. Rat tracheas were transplanted from Brown-Norway or Lewis donors into Lewis recipients. Rats received 200 μg/day of either anti-RANTES or anti-MCP-1 antibody for 14 days. Luminal obstruction and epithelial loss were calculated. Northern blots for MCP-1 and RANTES mRNA expression were performed, and immunohistochemistry for chemokine protein localization. There was a significant increase in airway obstruction in allografts compared to isografts ( P < 0.001). Antibody-treated allografts demonstrated an amelioration of airway obstruction from 58% (vehicle allografts) to 26% (anti-RANTES) and 12% (anti-MCP-1), both of which were significant ( P < 0.001). Epithelial preservation was increased in both antibody-treated groups ( P < 0.001), and increased expression of MCP-1 and RANTES mRNA was present in tracheal allografts by Day 2 and maximal by Day 6. Beta chemokines are expressed during the development of experimental OB, as MCP-1 and RANTES mRNA expression increased with time from transplantation. Both MCP-1 and RANTES are functional in the formation of the fibroproliferative response that characterizes OB in this model, and their antagonization conferred protection against airway obstruction and epithelial loss.

Abrogation of obliterative airway disease in murine tracheal allografts by inhibition of matrix metalloproteinases-2 and -9

Abrogation of obliterative airway disease in murine tracheal allografts by inhibition of matrix metalloproteinases-2 and -9

Airway epithelium is the primary target of tracheal allograft rejection in obliterative airway disease

Airway epithelium is the primary target of tracheal allograft rejection in obliterative airway disease

Graft protective effects of heme oxygenase 1 in mouse tracheal transplant-related obliterative bronchiolitis.

Heme oxygenase (HO)-1, long believed to be a cytoprotective protein, has recently been identified as a graft survival gene. This study evaluates the role of HO-1 in a murine heterotopic tracheal allograft model for obliterative bronchiolitis. Mice with deficient or experimentally enhanced HO-1 expression underwent subcutaneous implantation of murine tracheal isografts and allografts. Grafts were excised after 9, 16, or 21 days and evaluated by histologic examination, immunohistochemistry for HO-1 and interleukin (IL)-10 proteins, and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling. To evaluate the relationships between IL-10 and HO-1, the effects of modulation of HO-1 expression on IL-10 expression were evaluated and HO-1 expression was examined in tracheal transplants from IL-10 null mice. Isografts demonstrated normal histology with minimal HO-1 staining, whereas allografts showed features of human airway rejection (loss of respiratory epithelium, luminal granulation tissue, lymphocytic tracheitis) with increased HO-1 staining in macrophages and mesenchymal cells. HO-1-deficient mice demonstrated a more rapid progression of the tracheal allograft injury as compared with control allografts, and this was associated with a decrease in the anti-inflammatory cytokine, IL-10. Tracheal transplants using IL-10-deficient mice also resulted in a more severe injury, and this was accompanied by a decrease in HO-1 staining. HO-1 protein expression is increased in murine heterotopic airway rejection, and deficiency of HO-1 accelerates the development of the obliterative bronchiolitis-like lesion. IL-10 protein expression parallels expression of HO-1, suggesting that IL-10 may participate in the genesis of HO-1's effects on the inflammatory processes triggered by allotransplantation.

Long-term outcomes of orthotopic tracheal allografts in the murine model

Otolaryngology–Head and Neck SurgeryVolume 129, Issue 2 p. P188-P188 Research Posters R095: Long-Term Outcomes of Orthotopic Tracheal Allografts in the Murine Model Houtan Chaboki presenter MD, Houtan Chaboki presenter MD New York, NYSearch for more papers by this authorHeidi Cleven, Heidi Cleven New York, NYSearch for more papers by this authorThomas Moran PhD, Thomas Moran PhD New York, NYSearch for more papers by this authorLloyd Mayer MD, Lloyd Mayer MD New York, NYSearch for more papers by this authorEric M Genden MD, Eric M Genden MD New York, NYSearch for more papers by this author Houtan Chaboki presenter MD, Houtan Chaboki presenter MD New York, NYSearch for more papers by this authorHeidi Cleven, Heidi Cleven New York, NYSearch for more papers by this authorThomas Moran PhD, Thomas Moran PhD New York, NYSearch for more papers by this authorLloyd Mayer MD, Lloyd Mayer MD New York, NYSearch for more papers by this authorEric M Genden MD, Eric M Genden MD New York, NYSearch for more papers by this author First published: 17 May 2016 https://doi.org/10.1016/S0194-59980301102-1Read the full textAboutPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume129, Issue2August 2003Pages P188-P188 RelatedInformation

R095: Long‐Term Outcomes of Orthotopic Tracheal Allografts in the Murine Model

Otolaryngology–Head and Neck SurgeryVolume 129, Issue 2 p. P188-P188 Research Posters R095: Long-Term Outcomes of Orthotopic Tracheal Allografts in the Murine Model Houtan Chaboki presenter MD, Houtan Chaboki presenter MD New York, NYSearch for more papers by this authorHeidi Cleven, Heidi Cleven New York, NYSearch for more papers by this authorThomas Moran PhD, Thomas Moran PhD New York, NYSearch for more papers by this authorLloyd Mayer MD, Lloyd Mayer MD New York, NYSearch for more papers by this authorEric M Genden MD, Eric M Genden MD New York, NYSearch for more papers by this author Houtan Chaboki presenter MD, Houtan Chaboki presenter MD New York, NYSearch for more papers by this authorHeidi Cleven, Heidi Cleven New York, NYSearch for more papers by this authorThomas Moran PhD, Thomas Moran PhD New York, NYSearch for more papers by this authorLloyd Mayer MD, Lloyd Mayer MD New York, NYSearch for more papers by this authorEric M Genden MD, Eric M Genden MD New York, NYSearch for more papers by this author First published: 17 May 2016 https://doi.org/10.1016/S0194-59980301102-1Read the full textAboutPDF ToolsExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume129, Issue2August 2003Pages P188-P188 RelatedInformation

The kinetics and pattern of tracheal allograft re-epithelialization.

Extensive tracheal defects may pose a life-threatening dilemma. Although tracheal transplantation may represent a reconstructive solution, very little is known regarding the immunobiology and behavior of tracheal allografts. The objective of this study was to assess the pattern and kinetics of re-epithelialization of orthotopic tracheal allografts in immunosuppressed recipients. Thirty-eight age-matched mice were randomly assigned to five experimental groups. BALB/c donor tracheal segments were orthotopically transplanted into either syngeneic BALB/c or MHC mismatched allogeneic C57BL/6 recipients with and without immunosuppression. On post-transplant days 7, 14, 28, 48, and 62, animals from each group were evaluated by serial histology, electron microscopy, and serial immunohistochemical analysis for mucosal phenotype, re-epithelialization pattern, and lymphocyte subpopulations. Nonimmunosuppressed recipients underwent recipient-derived basal cell re-epithelialization by Day 48, with differentiation into a sparse population of ciliated columnar epithelium by Day 62, whereas immunosuppressed recipients underwent basal cell re-epithelialization 28 d after transplantation and differentiation into a dense population of ciliated columnar epithelium by Day 48. The re-epithelialization process occurred in a definable pattern that was significantly enhanced with the addition of immunosuppression. Orthotopic tracheal transplants undergo progressive re-epithelialization with recipient-derived basal cells that differentiate into ciliated columnar epithelium in a definable pattern that is enhanced with the addition of immunosuppression.

CTLA4Ig-gene transfection inhibits obliterative airway disease in rats

Background Obliterative airway disease (OAD) is a major cause of long-term morbidity following lung transplantation. Its pathologic characteristics are small-airway inflammation and occlusion by fibrous tissue. However, the pathogenesis is uncertain and therapy is ineffective. This study presents the effects of CTLA4Ig-gene therapy on OAD in heterotopically transplanted rat tracheal allografts. Methods Dark Agouti (DA, RT1 a) allografts and Lewis (LEW, RT1 l) isografts were transplanted into Lewis recipients. The tracheal graft was transplanted heterotopically into the subcutaneous pocket into the back. Adenoviral vectors (1.0×10 9 pfu) containing the CTLA4Ig-gene (AdCTLA4Ig) or the LacZ-gene (AdLacZ) were injected into the tail vein immediately after grafting. Grafts were harvested and examined after more than 35 days for mononuclear cell infiltration development and lumen occlusion with fibrosis. Results Fully allogenic DA tracheas, treated with AdCTLA4Ig had significantly lower pathologic scores and infiltrating scores than the control allografts. The pathologic findings of the grafts, treated with AdCTLA4Ig, were very similar to those of the syngeneic grafts. The animals experienced no adverse events during follow-up. No evidence of vector-mediated tissue damage was seen in any graft. Conclusions Adenoviral vectors containing the CTLA4Ig-gene markedly inhibited the obliteration of the airway lumen. OAD may be associated with T-cell responses against graft tissue and alloimmune injury.

Recipient cells form the proliferative lesion in the rat heterotopic tracheal allograft model of obliterative airway disease

To determine the nature of the proliferative lesion in obliterative airway disease, heterotopic tracheal allograft transplantation was performed between fully disparate Brown Norway and Lewis rat strains. Four weeks after transplantation, the resulting lumenal occlusive lesion was stripped from the underlying tissue. The lesion was probed using immunohistochemical analysis with monoclonal antibodies and for DNA using strain-specific primers for Brown Norway or Lewis major histocompatibility complex Class I alleles. The lesions were α-actin positive, and polymerase chain reaction probing revealed only recipient DNA in the lesion tissue, regardless of the direction of transplantation, with no amplification of donor DNA. From this, we conclude that the proliferative lesion in the rat heterotopic tracheal model is of recipient origin a finding with important implications for the pathobiology of obliterative airway disease.

Segmental Tracheal Replacement in Mongrel Dogs

Objective—To evaluate the use of alcohol- and detergent-preserved tracheal allografts in dogs. Material and Methods—Experimental segmental tracheal replacement was performed in 18 adult mongrel dogs. Three different techniques were tried in three groups of dogs. In the first group, a four-ring cervical tracheal segment was dissected out and implanted in the trachea of another dog. In the second group, the procedure was performed using a tracheal allograft that had been preserved in 70% ethyl alcohol for 20 days. In the third group, tracheal grafts were previously impregnated in 10% povidone iodine for 72 h before being implanted. Allografts were harvested 60 days after transplantation and assessed both histologically and in terms of the percentage patency. Dogs that died within 60 days were also included in the analysis. Results—The best results were found in the group in which the tracheal allograft had been preserved in ethyl alcohol and this was explained by the reduced antigenicity of the graft in this group. Conclusions—The use of alcohol-preserved allografts is a practical method of tracheal transplantation, and the alcohol-preservation technique markedly reduces the immunogenicity of the grafts.

Airway-directed gene transfer of interleukin-10 using recombinant Sendai virus effectively prevents post-transplant fibrous airway obliteration in mice

Bronchiolitis obliterans (BO) after lung transplantation prevents a satisfactory prognosis, and recent studies suggested that interleukin-10 (IL-10) gene transfer to distant organs could inhibit BO in rodent models. Although delivery of the therapeutic gene to a local airway would be favored to minimize systemic effects, current limitations include lower gene transfer efficiency to airway epithelium. As recombinant Sendai virus (SeV) can produce dramatically efficient gene transfer to airway epithelium, we determined if SeV-mediated IL-10 gene transfer to the local airway would inhibit bronchial fibrous obliteration in murine tracheal allografts. Administration of cyclosporine A (CsA) significantly promoted not only recovery of the injured airway epithelium but also SeV-mediated IL-10 expression (CsA- versus CsA+ =228+/-78 versus 3627+/-1372 pg/graft with 5 x 10(7) pfu), thereby suggesting the requirement of epithelia for efficient gene transfer. Even at the highest expression, no significant leakage of IL-10 was evident in the systemic circulation, and the induction of interferon-gamma was completely diminished on day 7 by IL-10 gene transfer. As a result, luminal loss was significantly prevented in allografts treated with SeV-IL-10 (luminal opening, all control groups: 0% respectively, and SeV-IL-10 5 x 10(7) pfu: 25.7+/-10.5%), an effect that was enhanced by short-term CsA treatment (SeV-IL-10 5 x 10(7) pfu with CsA: 63.7+/-12.7%). We propose that SeV is a useful vector that can target airway epithelium to prevent BO avoiding putative systemic effect.

Induction of tolerance to a minor histocompatibility antigen prevents post-transplant obliterative airway disease in mouse tracheal allografts

Purpose: Development of obliterative airway disease in murine tracheal allografts is a suitable model for studying the pathogenesis of bronchiolitis obliterans syndrome after human lung transplantation. In this study, we used two congenic mouse strains which differs at the H13 locus that produces two H2Db-restricted minor histocompatibility antigens named SVL9 (H13a, SSVVGVWYL) and SIL9 (H13b, SSVIGVWYL).

Heterotopic transplantation of cryopreserved tracheae in a rat model.

The successful use of cryopreserved tracheal allografts in canine models suggests their use in humans. The grade of genetic difference, the mechanism of revascularisation and the method of cryopreservation are not clearly defined. The purpose of our study was to investigate the rejection of tracheal transplants in a standardised heterotopic rat model using different forms of cryopreservation. Tracheae from Brown Norway rats were implanted into the omentum from Brown Norway rats or Lewis rats. We transplanted fresh isografts or allografts and pretreated isografts or allografts. Cryopreservation was performed in a medium containing 10% dimethyl sulphoxide at -80 degrees C for 28 days (I) or -196 degrees C for 84 days (II) or without medium at -80 degrees C for 28 days (III). The transplants were excised after 7 and 21 days, respectively. Histological examinations revealed normal structure and function of isografts after 21 days. In the cryopreserved isograft, the epithelium had disappeared and the tracheal lumen was partially obstructed by a non-compact fibrous tissue. In the fresh allografts, the epithelium was replaced by aggressive fibrous tissue, infiltrating the membranous part of the trachea and occluding the tracheal lumen. The cartilage was vital without any sign of rejection. In the cryopreserved allografts, the tracheal lumen was obstructed by dense fibrous tissue with an inflammatory reaction. The cartilage of cryopreserved allografts (II) and (III) had lost the nuclei corresponding to non-vital tissue. Only in the cryopreserved allografts (I) did we find nodular regeneration at the edges of the cartilaginous bow. The heterotopic transplantation model allows the study of the mechanisms leading to tracheal obstruction. Cryopreservation was found to have no clear advantage in reducing transplant immunogenicity. Cryopreservation leads to significant damage to the cartilage, the intensity of which is dependent on the mode of cryopreservation.

Site of heterotopic tracheal allograft implantation affects graft rejection pattern

Background: Obliterative bronchiolitis (OB) is a major complication of lung transplantation, affecting about 50% of patients who survive beyond 3 months. It is a chronic inflammatory disease, leading to airway obliteration. Although the etiology and pathogenesis of OB are not completely understood, it is thought to be a manifestation of chronic rejection. The murine heterotopic tracheal transplantation model has been developed to investigate OB. Previous data showed no differences between different sites of tracheal graft implantation, but did not study an intramuscular placement. This pilot study compared 2 sites of heterotopic tracheal allograft transplantation in mice, to examine the role of ischemia in OB. Methods: 10 C3H/He (C3H; H-2) mice received tracheal grafts from C3H (isografts) and BALB/c (BALB; H-2) (allografts) implanted in a subcutaneous pouch (SC) (n 5), or within the pectoralis major muscle (IM) (n 6). 6 C3H mice were recipients of tracheal grafts from C3H, BALB, and C57BL/6 (B6; H-2) implanted in the SC pouch(MDSC). All grafts were removed 28 days after transplantation and examined for microscopic evidence of rejection. Results: No isografts showed intraluminal occlusion. Mean luminal occlusion in BALB allografts in the IM site was 37.7% ( 9.93) compared to 86.0% ( 20.4) in the SC site (P 0.0006). In MDSC group, mean luminal occlusion of BALB allografts was 80% ( 23.6), compared to 5.3% ( 7.9) in the B6 allografts, (P 0.001). Conclusions: In contrast to previous data, our pilot data suggest that site of airway allograft implantation affects OB development. Allografts implanted in the IM site showed significantly less airway obliteration compared to SC group, possibly due to rich muscle blood circulation. These results support a role of ischemia as a predisposing factor in development and progression of OB. In contrast to BALB allografts, B6 allografts showed significantly less occlusion by day 28, reflecting possible strain-related differences in immunogenicity.

Inhibition of poly (ADP) ribose synthetase ameliorates experimental obliterative bronchiolitis in rat tracheal allografts

Obliterative bronchiolitis (OB) affects over half of all chronic survivors following lung transplantation. Its course is typically progressive and fatal. Based on studies revealing the anti-inflammatory properties of poly (ADP) ribose synthetase (PARS) inhibitors, we hypothesized that their administration would be protective in a heterotopic model of experimental OB. We transplanted tracheas from Brown-Norway into Lewis rats, and treated two of four groups with the PARS inhibitor, INO-1001. The first group received 14 days of treatment, while the second group received drug beginning on post-transplant day 7. Both treatment groups were sacrificed on post-transplant day 14. Allograft controls were sacrificed on post-transplant days 7 and 14. Transplanted tracheas were analyzed by microscopy and computerized morphometry, and mechanistic studies were performed to analyze TNF and IL-1 mRNA expression and NFkB activity. Apoptosis was assessed by the TUNEL assay. PARS inhibition was statistically significant in decreasing luminal obstruction in the 14-day treated group, associated with 45% protection. The group treated on days 7 to 14 post-transplant revealed 30% protection when compared to 14-day controls. Day 7 controls confirmed the development of an obstructive lesion. While the 14-day control group demonstrated near complete loss of respiratory epithelium, allograft airways treated with INO-1001 revealed increased preservation of epithelial lining. The administration of a PARS inhibitor is both protective against the development of OB in an experimental rat model, and associated with increased preservation of respiratory epithelium. Delayed treatment with INO-1001 did not reverse the inflammatory lesion that had developed by day 7, but it did limit its progression when compared to 14-day controls. These studies suggest that activation of the nuclear enzyme PARS plays a critical role in obliterative bronchiolitis.

PX3.102, a novel immunosuppressant, slows tracheal allograft rejection in the mouse model of obliterative bronchiolitis

PX3.102, a novel immunosuppressant, slows tracheal allograft rejection in the mouse model of obliterative bronchiolitis

PG490-88, a derivative of triptolide, attenuates obliterative airway disease in a mouse heterotopic tracheal allograft model

The current treatment of obliterative bronchiolitis in lung transplant recipients is sub-optimal. Triptolide is a novel immunosuppressant that has a mechanism of action distinct from currently available immunosuppressants, including induction of T-cell apoptosis, blockade of fibroblast proliferation/maturation and inhibition of transforming growth factor-β (TGF-β) mRNA production. We hypothesized that triptolide may be helpful in blocking obliterative airway disease in lung transplant recipients. We investigated the effect of PG490-88, a water-soluble derivative of triptolide, in a mouse heterotopic tracheal allograft model of obliterative airway disease. We show that PG490-88 attenuates airway obliteration in this model and inhibits accumulation of inflammatory cells, and therefore may have preventive or therapeutic benefits for patients with obliterative airway disease (OAD) following lung transplantation.

Recipient iNOS but not eNOS deficiency reduces luminal narrowing in tracheal allografts.

Chronic airway rejection is characterized by prolonged inflammation, epithelial damage, and eventual luminal obliterative bronchiolitis (OB). In cardiac allografts, the inducible nitric oxide synthase (iNOS) promotes acute rejection but paradoxically reduces neointimal formation, the hallmark of chronic rejection. The specific roles of NOS isoforms in modulating lymphocyte traffic and airway rejection are not known. Using a double lumen mouse tracheal transplant model, tracheal grafts from B10.A (allo) or C57BL/6J (iso) mice were transplanted into cyclosporine-treated wild-type (WT) iNOS−/− or endothelial NOS (eNOS)−/− recipients. OB was observed in WT tracheal allografts at 3 weeks (53 ± 2% luminal occlusion vs. 17 ± 1% for isografts, P < 0.05) with sites of obstructive lesion formation coinciding with areas of CD3+ CD8+ T cell–rich lymphocytic bronchitis. In contrast, allografts in iNOS−/− recipients exhibited reductions in local expression of proinflammatory chemokines and cytokines, graft T cell recruitment and apoptosis, and luminal obliteration (29 ± 2%, P < 0.05 vs. WT allografts). Recipient eNOS deficiency, however, suppressed neither chemokine expression, lymphocyte infiltration, nor airway occlusion (54 ± 2%). These data demonstrate that iNOS exacerbates luminal obliteration of airway allografts in contrast with the known suppression by iNOS of cardiac allograft vasculopathy. Because iNOS−/− airways transplanted into WT allograft hosts are not protected from rejection, these data suggest that iNOS expressed by graft-infiltrating leukocytes exerts the dominant influence on airway rejection.