Lung Transplantation in the Fischer 344→Wistar Kyoto Rat Strain Combination is Not Suitable to Study Bronchiolitis Obliterans

A Comprehensive Review on the Surgical Aspect of Lung Transplant Models in Mice and Rats

Lung transplantation improves the outcome and quality of life of patients with end-stage pulmonary disease. However, the procedure is still hampered by the lack of suitable donors, the complexity of the surgery, and the risk of developing chronic lung allograft dysfunction. Over the past decades, translational experiments in animal models have led to a better understanding of physiology and immunopathology following the lung transplant procedure. Small animal models (e.g., rats and mice) are mostly used in experiments regarding immunology and pathobiology and are preferred over large animal models due to the ethical aspects, the cost-benefit balance, and the high throughput possibility. In this comprehensive review, we summarize the reported surgical techniques for lung transplantation in rodent models and the management of perioperative complications. Furthermore, we propose a guide to help identify the appropriate species for a given experiment and discuss recent experimental findings in small animal lung transplant models.

Bronchiolitis obliterans syndrome and restrictive allograft syndrome after lung transplantation: why are there two distinct forms of chronic lung allograft dysfunction?

Bronchiolitis obliterans syndrome (BOS) had been considered to be the representative form of chronic rejection or chronic lung allograft dysfunction (CLAD) after lung transplantation. In BOS, small airways are affected by chronic inflammation and obliterative fibrosis, whereas peripheral lung tissue remains relatively intact. However, recognition of another form of CLAD involving multiple tissue compartments in the lung, termed restrictive allograft syndrome (RAS), raised a fundamental question: why there are two phenotypes of CLAD? Increasing clinical and experimental data suggest that RAS may be a prototype of chronic rejection after lung transplantation involving both cellular and antibody-mediated alloimmune responses. Some cases of RAS are also induced by fulminant general inflammation in lung allografts. However, BOS involves alloimmune responses and the airway-centered disease process can be explained by multiple mechanisms such as external alloimmune-independent stimuli (such as infection, aspiration and air pollution), exposure of airway-specific autoantigens and airway ischemia. Localization of immune responses in different anatomical compartments in different phenotypes of CLAD might be associated with lymphoid neogenesis or the de novo formation of lymphoid tissue in lung allografts. Better understanding of distinct mechanisms of BOS and RAS will facilitate the development of effective preventive and therapeutic strategies of CLAD.

Diagnosis, Pathophysiology and Experimental Models of Chronic Lung Allograft Rejection

Chronic rejection is the Achilles heel of modern lung transplantation, characterized by a slow, progressive decline in allograft function. Clinically, this manifests as obstructive disease, restrictive disease, or a mixture of the 2 depending on the underlying pathology. The 2 major phenotypes of chronic rejection include bronchiolitis obliterans syndrome and restrictive allograft syndrome. The last decade of research has revealed that each of these phenotypes has a unique underlying pathophysiology which may require a distinct treatment regimen for optimal control. Insights into the intricate alloimmune pathways contributing to chronic rejection have been gained from both large and small animal models, suggesting directions for future research. In this review, we explore the pathological hallmarks of chronic rejection, recent insights gained from both clinical and basic science research, and the current state of animal models of chronic lung rejection.

LTB4 and montelukast in transplantation-related bronchiolitis obliterans in rats

Background

Lung transplantation is the only effective treatment for end-stage lung diseases. Bronchiolitis obliterans, which is known as non-infectious chronic lung allograft dysfunction (CLAD) in the new classification, is the greatest threat to long-term survival after lung transplantation. This study investigated the role of leukotriene B4 (LTB4) and montelukast in transplantation-related bronchiolitis obliterans and discussed the pathophysiological significance of LTB4 in chronic rejection.

Methods

Rats were randomly divided into an experimental group (montelukast), a positive control group (dexamethasone), and a blank control group (normal saline solution; NS). Each piece of trachea removed from a F344 rat was transplanted into a Lewis rat through a 5-mm incision at the episternum by subcutaneous embedding. The recipients were treated with gastric lavage with 3 mg/kg · d montelukast suspension, 1 mg/kg · d dexamethasone, and 1 mL/kg · d NS, respectively, in each group. On Day 28, peripheral blood was drawn to measure the white blood cell counts and plasma LTB4 levels. The donor specimens were stained by H-E and Masson, and their organizational structure and extent of fibrosis were visually assessed. The measurement data were compared using one-way analysis of variance, and the categorical data were compared using the chi-square test. A P value of less than 0.05 was considered to indicate statistical significance.

Results

The white blood cell counts of the montelukast, dexamethasone, and NS groups were (16.0 ± 4.2) × 10⁹/L, (19.5 ± 11.6) × 10⁹/L, and (25.8 ± 3.6) × 10⁹/L; no statistical significance was found (P = 0.101). The concentrations of LTB4 were 2230 ± 592 pg/mL, 1961 ± 922 pg/mL, and 3764 ± 1169 pg/mL, and statistical significance was found between the NS group and each of the others (P = 0.009). The percentages of tracheal occlusion were 73.6% ± 13.8%, 23.4% ± 3.2%, and 89.9% ± 11.3%, and statistical significance was found among the three groups (P = 0.000).

Conclusions

The study established a model to simulate bronchiolitis obliterans after clinical lung transplantation. Oral administration of montelukast reduced plasma LTB4 levels in rats and played a preventive role against tracheal fibrosis after transplantation. This suggests that LTB4 may be involved in bronchiolitis obliterans after pulmonary transplantation. This study indicates a new direction for research into the prevention and treatment of bronchiolitis obliterans after lung transplantation.

Technical Aspects and Benefits of Experimental Mouse Lung Transplantation

In recent years, the number of lung transplantations performed as the last option for many respiratory diseases has grown considerably, both in adults and children. However, the causes for the relatively short survival of lungs compared to other organ transplants still need to be studied. Techniques have improved since the 1950s when experimental lung transplantation began, and the different animal species used now include rodents. The advantage of using these small species is that the surgical model has been expanded and standardized, and different respiratory problems can be studied. In this review we examine the different technical strategies used in experimental transplantation in rats and mice, focusing on surgical techniques and anesthesia and monitoring methods, and highlighting the major contributions of mouse lung transplantation to the field.

A relevant experimental model for human bronchiolitis obliterans syndrome

BACKGROUND

The long-term success of human lung transplantation is limited by the development of bronchiolitis obliterans syndrome. Acute rejection episodes and infections are important risk factors and seem to play major pathogenic roles. We established a relevant experimental model that mimics important aspects of human bronchiolitis obliterans syndrome.

METHODS

The Fischer 344-to-Lewis rat strain combination was used for orthotopic left lung transplantation. Isogeneic transplantations were performed in the Lewis rat. Recipients were treated with ciclosporin for 10 days. Lipopolysaccharide or vehicle was instilled into the airways 28 days after transplantation. Grafts were monitored by computed tomography, and recipients were euthanized on Days 28-90. The messenger RNA expression of selected chemokines and their receptors was measured on Days 28, 29, 33, 40 after transplantation. Graft histopathology on Day 90 was compared with lungs from patients who underwent re-transplantation due to end-stage allograft dysfunction.

RESULTS

Lung allografts treated with ciclosporin and vehicle only sporadically displayed tissue remodeling. In contrast, lipopolysaccharide treatment induced severe inflammation. In the long-term, severe vascular remodeling, lung fibrosis, and fibroproliferative remodeling of airways were found that closely resemble the histopathologic changes in grafts from human patients with bronchiolitis obliterans syndrome. Chronic damage was virtually absent from pulmonary isografts and native right lungs. Chemokine (C-C motif) ligand 5 and chemokine (C-X-C motif) ligand 9-11, and their receptors, were over-expressed in allografts.

CONCLUSIONS

Our experimental model mirrors key aspects of human bronchiolitis obliterans syndrome. It will be useful to elucidate its pathogenesis and to develop therapeutic approaches improving the long-term outcome of human lung transplantation.

Early administration of FTY720 prevents chronic airway as well as vascular destruction in experimental rat lung transplantation

BACKGROUND

Chronic rejection (CR) in terms of bronchiolitis obliterans (BO) and vascular sclerosis (VS) still represents the major obstacle for pulmonary graft survival in the medium and long term course after lung transplantation (LTX). Aside from nonspecific stimuli, early acute rejection (AR) seems to be causative especially in cases of a late diagnosis or inadequate treatment. This study investigated the effects of FTY720, a new immunosuppressant that promotes lymphocyte sequestration into lymph nodes and Peyer's patches, on the development of CR after experimental LTX.

METHODS

A total of 50 rats underwent allogenic (F344-to-WKY) and syngenic (WKY-to-WKY) left LTX. Group 1 animals had no treatment. Group 2 animals were administered FTY720 (3 mg/kg body weight per day) at the maximum time of AR (day 14) and continued up to day 100 after LTX. Group 3 animals were treated with the same dosage of FTY720 from day 0 to 100. The grades of AR and CR were classified according to the criteria of the International Society for Heart and Lung Transplantation.

RESULTS

Within 14 days after allogenic LTX, all nontreated rats developed early AR followed by severe CR with VS and BO. Similar data were observed for FTY720 treatment of existing AR (group 2). Only early administration of FTY720 (at the time of LTX) significantly reduced the proportion of animals with severe acute vascular rejection (P < .001). However, all of these allografts showed high-grade acute airway inflammation. After long-term application, the chronic inflammatory response was absent; none of the allografts developed BO and VS.

CONCLUSION

Only application of FTY720 immediately after LTX prevented lymphocyte recirculation and lung injury.

Lentivirus IL-10 gene therapy down-regulates IL-17 and attenuates mouse orthotopic lung allograft rejection

The purpose of the study was to examine the effect of lentivirus-mediated IL-10 gene therapy to target lung allograft rejection in a mouse orthotopic left lung transplantation model. IL-10 may regulate posttransplant immunity mediated by IL-17. Lentivirus-mediated trans-airway luciferase gene transfer to the donor lung resulted in persistent luciferase activity up to 6 months posttransplant in the isograft (B6 to B6); luciferase activity decreased in minor-mismatched allograft lungs (B10 to B6) in association with moderate rejection. Fully MHC-mismatched allograft transplantation (BALB/c to B6) resulted in severe rejection and complete loss of luciferase activity. In minor-mismatched allografts, IL-10-encoding lentivirus gene therapy reduced the acute rejection score compared with the lentivirus-luciferase control at posttransplant day 28 (3.0 ± 0.6 vs. 2.0 ± 0.6 (mean ± SD); p = 0.025; n = 6/group). IL-10 gene therapy also significantly reduced gene expression of IL-17, IL-23, and retinoic acid-related orphan receptor (ROR)-γt without affecting levels of IL-12 and interferon-γ (IFN-γ). Cells expressing IL-17 were dramatically reduced in the allograft lung. In conclusion, lentivirus-mediated IL-10 gene therapy significantly reduced expression of IL-17 and other associated genes in the transplanted allograft lung and attenuated posttransplant immune responses after orthotopic lung transplantation.

Rattus model utilizing selective pulmonary ischemia induces bronchiolitis obliterans organizing pneumonia

Bronchiolitis obliterans organizing pneumonia (BOOP), a morbid condition when associated with lung transplant and chronic lung disease, is believed to be a complication of ischemia. Our goal was to develop a simple and reliable model of lung ischemia in the Sprague-Dawley rat that would produce BOOP. Unilateral ischemia without airway occlusion was produced by an occlusive slipknot placed around the left main pulmonary artery. Studies were performed 7 days later. Relative pulmonary and systemic flow to each lung was measured by injection of technetium Tc 99m macroaggregated albumin. Histological sections were examined for structure and necrosis and scored for BOOP. Apoptosis was detected by immunohistochemistry with an antibody against cleaved caspase 3. Pulmonary artery blood flow to left lungs was less than 0.1% of the cardiac output, and bronchial artery circulation was ∼2% of aortic artery flow. Histological sections from ischemic left lungs consistently showed Masson bodies, inflammation, and young fibroblasts filling the distal airways and alveoli, consistent with BOOP. In quantitative evaluation of BOOP using epithelial changes, inflammation and fibrosis were higher in ischemic left lungs than right or sham-operated left lungs. Apoptosis was increased in areas exhibiting histological BOOP, but there was no histological evidence of necrosis. Toll-like receptor 4 expression was increased in ischemic left lungs over right. An occlusive slipknot around the main left pulmonary artery in rats produces BOOP, providing direct evidence that ischemia without immunomodulation or coinfection is sufficient to initiate this injury. It also affords an excellent model to study signaling and genetic mechanisms underlying BOOP.

Ferret lung transplant: an orthotopic model of obliterative bronchiolitis

Obliterative bronchiolitis (OB) is the primary cause of late morbidity and mortality following lung transplantation. Current animal models do not reliably develop OB pathology. Given the similarities between ferret and human lung biology, we hypothesized an orthotopic ferret lung allograft would develop OB. Orthotopic left lower lobe transplants were successfully performed in 22 outbred domestic ferrets in the absence of immunosuppression (IS; n = 5) and presence of varying IS protocols (n = 17). CT scans were performed to evaluate the allografts. At intervals between 3-6 months the allografts were examined histologically for evidence of acute/chronic rejection. IS protects allografts from acute rejection and early graft loss. Reduction of IS dosage by 50% allowed development of controlled rejection. Allografts developed infiltrates on CT and classic histologic acute rejection and lymphocytic bronchiolitis. Cycling of IS, to induce repeated episodes of controlled rejection, promoted classic histologic hallmarks of OB including fibrosis-associated occlusion of the bronchiolar airways in all allografts of long-term survivors. In conclusion, we have developed an orthotopic lung transplant model in the ferret with documented long-term functional allograft survival. Allografts develop acute rejection and lymphocytic bronchiolitis, similar to humans. Long-term survivors develop histologic changes in the allografts that are hallmarks of OB.

Translational research: animal models of obliterative bronchiolitis after lung transplantation

Obliterative bronchiolitis (OB) or chronic graft dysfunction remains the major limitation to long-term success of lung transplantation. Investigation using animal models is a critical component of research to understand the underlying pathological mechanisms and to develop novel preventive and therapeutic strategies for OB. Multiple animal models of OB exist, including orthotopic lung transplantation in rodents and large animals, orthotopic tracheal transplantation and heterotopic transplantation of a trachea in variable sites such as subcutaneous, intraomental and intrapulmonary sites. The most important issue for researchers is not specifically which model is the best but which is the most appropriate model to test their scientific hypothesis. For example, while orthotopic lung transplantation best mimics the overall surgical procedure, a question regarding fibrotic processes of OB may be better answered using heterotopic tracheal transplant models because of their reliable reproducibility of allograft obliterative airway fibrosis. Animal models should be continuously refined, modified and sometimes combined to fit the particular research purpose. We review the available animal models, their modifications and possible applications to assist researchers in choosing the appropriate model for their intended research.

Bronchiolitis obliterans in lung transplantation: the good, the bad, and the future

Lung transplantation remains the hope for many incurable pulmonary diseases, such as cystic fibrosis, pulmonary fibrosis, and chronic obstructive pulmonary disease. Remarkable progress has been made in improving outcomes, although the incidence of acute rejection remains more than 50% in the 1st year, and the 5-year graft survival is still less than 50% primarily because of the development of chronic rejection and graft dysfunction. Chronic rejection is characterized by the development of obliterative bronchiolitis in allografts and manifests as bronchiolitis obliterans syndrome in humans with no effective treatment. Previous studies support a role for alloreactive T cells in the development of bronchiolitis obliterans syndrome, but the specific mechanisms are unknown. One major stumbling block to research in the field of lung transplantation has been the lack of physiologic models to study the disease in the laboratory. We will review the current understanding of the immunology of the pathogenesis of obliterative bronchiolitis and will discuss exciting new advances from the laboratory as well as the implications for future research in lung transplantation.

The orthotopic left lung transplantation in rats: a valuable experimental model without using cuff technique

Advances in the field of clinical lung transplantation must rely on observations made in animal models. In this study, we introduced a new procedure in the rat, orthotopic left lung transplantation without using the cuff technique, in which the donor pulmonary artery, pulmonary vein, and membranous parts of the bronchus were anastomosed continuously in the lumen using a mattress suture under a surgical microscope; meanwhile, a second, low-pressure perfusion through the pulmonary artery and turnover of the vascular stump were made, which also made the vessel anastomosis easy. Transplantations were completed in 68 rats (89.5%), the mean time used for suturing the left lung hilar structure was 23.5 +/- 4.6 min. All lung grafts had good life-sustaining function because of there being no cuff-induced granulation tissue in bronchial anastomotic stoma, and three out of 12 allografts were observed with active bronchiolitis obliterans lesions at 8 weeks after transplantation. This model is a simple, valuable experimental model for studying lung transplantation and new therapies for preventing acute or chronic rejection.

Treatment with keratinocyte growth factor does not improve lung allograft survival in the rat

PURPOSE

Lung allografts are threatened by primary graft dysfunction, infections, and rejection. Novel therapies protecting pulmonary allografts are badly needed. Keratinocyte growth factor (KGF) protects the lung against a variety of injurious stimuli and exerts anti-inflammatory effects. The aim of the study was to test the potential of recombinant truncated KGF (DeltaN23-KGF, palifermin) to attenuate pulmonary allograft rejection.

MATERIALS AND METHODS

Intratracheal instillation of 5 mg/kg DeltaN23-KGF was performed twice in donor rats on days 3 and 2 before explantation of the lung. In control animals, an equivalent volume of vehicle was instilled. Left lungs were transplanted in the fully allogeneic Dark Agouti to Lewis rat strain combination and in the less stringent Fischer 344 to Wistar Kyoto combination. Allograft recipients were additionally treated with DeltaN23-KGF post-transplantation. Graft outcome, leukocytic infiltration, and major histocompatibility complex (MHC) class II antigen expression was analyzed.

RESULTS

In both rat strain combinations, DeltaN23-KGF treatment did not improve pulmonary allograft outcome. Graft infiltration by macrophages and T lymphocytes remained unchanged. In addition, we demonstrated that MHC class II antigens were more abundant in KGF-treated allografts compared to control-treated grafts, which probably results in an increased alloreactivity.

CONCLUSION

In conclusion, intratracheal DeltaN23-KGF treatment is not effective to prevent acute pulmonary allograft rejection.