A mouse model of orthotopic vascularized aerated lung transplantation

Development of obliterative bronchiolitis in a murine model of orthotopic lung transplantation

Orthotopic lung transplantation in rats was first reported by Asimacopoulos and colleagues in 1971 ¹. Currently, this method is well accepted and standardized not only for the study of allo-rejection but also between syngeneic strains for examining mechanisms of ischemia-reperfusion injury after lung transplantation. Although the application of the rat and other large animal model ² contributed significantly to the elucidation of these studies, the scope of those investigations is limited by the scarcity of knockout and transgenic rats. Due to no effective therapies for obliterative bronchiolitis, the leading cause of death in lung transplant patients, there has been an intensive search for pre-clinical models that replicate obliterative bronchiolitis. The tracheal allograft model is the most widely used and may reproduce some of the histopathologic features of obliterative bronchiolitis ³. However, the lack of an intact vasculature with no connection to the recipient's conducting airways, and incomplete pathologic features of obliterative bronchiolitis limit the utility of this model ⁴. Unlike transplantation of other solid organs, vascularized mouse lung transplants have only recently been reported by Okazaki and colleagues for the first time in 2007 ⁵. Applying the basic principles of the rat lung transplant, our lab initiated the obliterative bronchiolitis model using minor histoincompatible antigen murine orthotopic single-left lung transplants which allows the further study of obliterative bronchiolitis immunopathogenesis⁶.

The technique of orthotopic mouse lung transplantation as a movie-improved learning by visualization

The introduction of the mouse model of orthotopic single-lung transplantation has significantly advanced research in experimental lung transplantation. Technically, this model is demanding and presents major challenges to the researcher. In order to facilitate and accelerate the successful acquisition of this technique, we provide here a narrated movie in which the full transplantation procedure is comprehensively illustrated, featuring all key operative steps, but also highlighting potential pitfalls.

Egr1: a novel target for ameliorating acute allograft rejection in an experimental lung transplant model

OBJECTIVES

Acute allograft rejection is one of the significant complications occurring in lung transplant recipients. Early growth response-1 (Egr-1), zinc-finger-type transcription factor, is known as a master switch regulator of diverse chemical mediators. We used an orthotopic mouse model of left lung transplant to elucidate the function of Egr-1 in acute pulmonary rejection.

METHODS

Left lung grafts retrieved from C57BL/6 wild mice or C57BL/6 Egr-1-null mice were orthotopically transplanted into BALB/c mice; the lungs were harvested at day 1, 3, 5 or 7 after lung transplantation. The grade of acute rejection was histopathologically evaluated. The intragraft gene expression levels of Egr-1 and downstream target mediators were quantitatively measured by real-time polymerase chain reaction. Immunohistochemical analysis was used to determine the location and distribution of the Egr-1 protein in the pulmonary graft.

RESULTS

Severe acute rejection was observed in allografts from wild-type mice at 5 days after transplantation. Only minimal rejection was seen in the lung graft from Egr-1-null donor mice at 5 days after transplantation. Strong upregulation of Egr-1 mRNA transcripts was observed at day 1, which then decreased during the next 5 days. The mRNA of Egr-1 target mediators [interleukin-1-beta (IL-1β), monocyte chemotactic protein-1 (MCP-1) and plasminogen activator inhibitor-1] reached maximal levels at day 5. Egr-1-null allografts exhibited significantly lower expressions of IL-1β and MCP-1 mRNA (P < 0.05).

CONCLUSIONS

Our study showed that deletion of Egr-1 in lung allografts ameliorates severe acute rejection with the reduction of expression levels of chemical mediators, implying a new possible strategy for treating acute pulmonary allograft rejection.

Donor lung derived myeloid and plasmacytoid dendritic cells differentially regulate T cell proliferation and cytokine production

Background

Direct allorecognition, i.e., donor lung-derived dendritic cells (DCs) stimulating recipient-derived T lymphocytes, is believed to be the key mechanism of lung allograft rejection. Myeloid (cDCs) and plasmacytoid (pDCs) are believed to have differential effects on T cell activation. However, the roles of each DC type on T cell activation and rejection pathology post lung transplantation are unknown.

Methods

Using transgenic mice and antibody depletion techniques, either or both cell types were depleted in lungs of donor BALB/c mice (H-2^d) prior to transplanting into C57BL/6 mice (H-2^b), followed by an assessment of rejection pathology, and pDC or cDC-induced proliferation and cytokine production in C57BL/6-derived mediastinal lymph node T cells (CD3+).

Results

Depleting either DC type had modest effect on rejection pathology and T cell proliferation. In contrast, T cells from mice that received grafts depleted of both DCs did not proliferate and this was associated with significantly reduced acute rejection scores compared to all other groups. cDCs were potent inducers of IFNγ, whereas both cDCs and pDCs induced IL-10. Both cell types had variable effects on IL-17A production.

Conclusion

Collectively, the data show that direct allorecognition by donor lung pDCs and cDCs have differential effects on T cell proliferation and cytokine production. Depletion of both donor lung cDC and pDC could prevent the severity of acute rejection episodes.

Cutting edge: paradoxical roles of BST2/tetherin in promoting type I IFN response and viral infection

Bone marrow stromal Ag 2 (BST2) is a transmembrane protein that prevents virus release from infected cells. It was also reported that BST2 inhibits type I IFN production by plasmacytoid dendritic cells. To determine BST2 impact on antiviral responses in vivo, we generated BST2(-/-) mice. Following infection with a murine retrovirus, BST2(-/-) mice had slightly elevated viral loads; however, infection with other enveloped viruses revealed unexpected roles of BST2. BST2(-/-) mice showed reduced type I IFN production by plasmacytoid dendritic cells. Moreover, BST2(-/-) mice had lower viral titers in lungs following intranasal infection with vesicular stomatitis virus expressing OVA and influenza B and increased numbers of virus-specific CD8 T cells in the lungs, suggesting that BST2 may facilitate entry and/or replication of enveloped viruses and modulate priming of CD8 T cells. These findings suggest complex roles of BST2 beyond retroviral control in vivo, possibly reflecting the involvement of BST2 in endocytosis and intracellular trafficking of viruses, viral nucleic acids, and Ags.

Chronic rejection pathology after orthotopic lung transplantation in mice: the development of a murine BOS model and its drawbacks

Almost all animal models for chronic rejection (CR) after lung transplantation (LTx) fail to resemble the human situation. It was our attempt to develop a representative model of CR in mice. Orthotopic LTx was performed in allografts receiving daily immunosuppression with steroids and cyclosporine. Controls included isografts and mice only undergoing thoracotomy (SHAM). Allografts were sacrificed 2, 4, 6, 8, 10 or 12 weeks after LTx. Pulmonary function was measured repeatedly in the 12w allografts, isografts and SHAM mice. Histologically, all allografts demonstrated acute rejection (AR) around the blood vessels and airways two weeks after LTx. This decreased to 50-75% up to 10 weeks and was absent after 12 weeks. Obliterative bronchiolitis (OB) lesions were observed in 25-50% of the mice from 4-12 weeks. Isografts and lungs of SHAM mice were normal after 12 weeks. Pulmonary function measurements showed a decline in FEV(0.1), TLC and compliance in the allografts postoperatively (2 weeks) with a slow recovery over time. After this initial decline, lung function of allografts increased more than in isografts and SHAM mice indicating that pulmonary function measurement is not a good tool to diagnose CR in a mouse. We conclude that a true model for CR, with clear OB lesions in about one third of the animals, but without a decline in lung function, is possible. This model is an important step forward in the development of an ideal model for CR which will open new perspectives in unraveling CR pathogenesis and exploring new treatment options.

Emergency granulopoiesis promotes neutrophil-dendritic cell encounters that prevent mouse lung allograft acceptance

The mechanisms by which innate immune signals regulate alloimmune responses remain poorly understood. In the present study, we show by intravital 2-photon microscopy direct interactions between graft-infiltrating neutrophils and donor CD11c(+) dendritic cells (DCs) within orthotopic lung allografts immediately after reperfusion. Neutrophils isolated from the airways of lung transplantation recipients stimulate donor DCs in a contact-dependent fashion to augment their production of IL-12 and expand alloantigen-specific IFN-γ(+) T cells. DC IL-12 expression is largely regulated by degranulation and induced by TNF-α associated with the neutrophil plasma membrane. Extended cold ischemic graft storage enhances G-CSF-mediated granulopoiesis and neutrophil graft infiltration, resulting in exacerbation of ischemia-reperfusion injury after lung transplantation. Ischemia reperfusion injury prevents immunosuppression-mediated acceptance of mouse lung allografts unless G-CSF-mediated granulopoiesis is inhibited. Our findings identify granulopoiesis-mediated augmentation of alloimmunity as a novel link between innate and adaptive immune responses after organ transplantation.

Neutrophils are required for 3-methylcholanthrene-initiated, butylated hydroxytoluene-promoted lung carcinogenesis

Multiple studies have shown a link between chronic inflammation and lung tumorigenesis. Inbred mouse strains vary in their susceptibility to methylcholanthrene (MCA)-initiated butylated hydroxytoluene (BHT)-promoted lung carcinogenesis. In the present study we investigated whether neutrophils play a role in strain dependent differences in susceptibility to lung tumor promotion. We observed a significant elevation in homeostatic levels of neutrophils in the lungs of tumor-susceptible BALB/cByJ (BALB) mice compared to tumor-resistant C57BL/6J (B6) mice. Additionally, BHT treatment further elevated neutrophil numbers as well as neutrophil chemoattractant keratinocyte-derived cytokine (KC)/chemokine (C-X-C motif) ligand 1 (Cxcl1) levels in BALB lung airways. Lung CD11c+ cells were a major source of KC expression and depletion of neutrophils in BALB mice resulted in a 71% decrease in tumor multiplicity. However, tumor multiplicity did not depend on the presence of T cells, despite the accumulation of T cells following BHT treatment. These data demonstrate that neutrophils are essential to promote tumor growth in the MCA/BHT two-step lung carcinogenesis model.

CD154 blockade abrogates allospecific responses and enhances CD4(+) regulatory T-cells in mouse orthotopic lung transplant

Acute cellular rejection (ACR) is a common and important clinical complication following lung transplantation. While there is a clinical need for the development of novel therapies to prevent ACR, the regulation of allospecific effector T-cells in this process remains incompletely understood. Using the MHC-mismatched mouse orthotopic lung transplant model, we investigated the short-term role of anti-CD154 mAb therapy alone on allograft pathology and alloimmune T-cell effector responses. Untreated C57BL/6 recipients of BALB/c left lung allografts had high-grade rejection and diminished CD4(+) : CD8(+) graft ratios, marked by predominantly CD8(+) >CD4(+) IFN-γ(+) allospecific effector responses at day 10, compared to isograft controls. Anti-CD154 mAb therapy strikingly abrogated both CD8(+) and CD4(+) alloeffector responses and significantly increased lung allograft CD4(+) : CD8(+) ratios. Examination of graft CD4(+) T-cells revealed significantly increased frequencies of CD4(+) CD25(+) Foxp3(+) regulatory T-cells in the lung allografts of anti-CD154-treated mice and was associated with significant attenuation of ACR compared to untreated controls. Together, these data show that CD154/CD40 costimulation blockade alone is sufficient to abrogate allospecific effector T-cell responses and significantly shifts the lung allograft toward an environment predominated by CD4(+) T regulatory cells in association with an attenuation of ACR.

Polarized alloantigen presentation by airway epithelial cells contributes to direct CD8+ T cell activation in the airway

Activated T lymphocytes are abundant in the airway during lung allograft rejection. Based on respiratory viral studies, it is the current paradigm that T cells cannot divide in the airway, and that their accumulation in the lumen of the respiratory tract is the exclusive result of recruitment from other sites, such as mediastinal lymph nodes. Here, we show that CD8(+) T cell activation and proliferation can occur in the airway after orthotopic lung transplantation. We also demonstrate that airway epithelium expresses major histocompatibility class I predominantly on the apical surface, both in vitro and in vivo, and initiates CD8(+) T cell responses in a polarized fashion, favoring luminal activation. Our data identify a unique site for CD8(+) T cell activation after lung transplantation, and suggest that attenuating these responses may provide a clinically relevant target.

Green tea epigallo-catechin-galleate ameliorates the development of obliterative airway disease

Lung transplantation has the worst outcome compared to all solid organ transplants due to chronic rejection known as obliterative bronchiolitis (OB). Pathogenesis of OB is a complex interplay of alloimmune-dependent and -independent factors, which leads to the development of inflammation, fibrosis, and airway obliteration that have been resistant to therapy. The alloimmune-independent inflammatory pathway has been the recent focus in the pathogenesis of rejection, suggesting that targeting this may offer therapeutic benefits. As a potent anti-inflammatory agent, epigallo-catechin-galleate (EGCG), a green tea catechin, has been very effective in ameliorating inflammation in a variety of diseases, providing the rationale for its use in this study in a murine heterotopic tracheal allograft model of OB. Mice treated with EGCG had reduced inflammation, with significantly less neutrophil and macrophage infiltration and significantly reduced fibrosis. On further investigation into the mechanisms, inflammatory cytokines keratinocyte (KC), interleukin-17 (IL-17), and tumor necrosis factor-α (TNF-α), involved in neutrophil recruitment, were reduced in the EGCG-treated mice. In addition, monocyte chemokine monocyte chemoattractant protein-1 (MCP-1) was significantly reduced by EGCG treatment. Antifibrotic cytokine interferon-γ-inducible protein-10 (IP-10) was increased and profibrotic cytokine transforming growth factor-β (TGF-β) was reduced, further characterizing the antifibrotic effects of EGCG. These findings suggest that EGCG has great potential in ameliorating the development of obliterative airway disease.

Maintenance of IKKβ activity is necessary to protect lung grafts from acute injury

BACKGROUND

Signaling pathways that target I-κB kinase β (IKKβ) activation stimulate the expression of nuclear factor (NF)-κB-dependent genes and are thus believed to primarily promote inflammation and injury in solid organ grafts.

METHODS

We examined the role of IKKβ in a mouse model of lung transplantation-mediated ischemia-reperfusion injury using NF-κB essential modulator (NEMO)-binding domain (NBD) peptide to pharmacologically inhibit IKK activation. As myeloid cells are primarily responsible for the production of acute inflammatory mediators after lung transplantation, we also investigated the effects of myeloid cell-specific IKKβ gene deletion on acute lung graft injury by transplanting mutant mice.

RESULTS

When NBD was administered at a dose that partially inhibits IKKβ activation, we observed attenuated lung graft injury and blunted expression of intragraft proinflammatory mediators. Surprisingly, when the dose of NBD was increased to a level that ablates intragraft IKKβ activation, graft inflammation, and injury were significantly worse compared with recipients treated with control peptide. Similar to lung recipients with pharmacologically ablated IKKβ activity, donor-recipient transplant combinations with a myeloid cell-specific IKKβ gene deletion had marked intragraft inflammation and poor lung function.

CONCLUSIONS

Our data show maintenance of IKKβ activity is critical for promoting graft homeostasis with important implications for targeting NF-κB-dependent signaling pathways for treating acute lung injury.

Neutralizing IL-17 prevents obliterative bronchiolitis in murine orthotopic lung transplantation

Obliterative bronchiolitis (OB) is the key impediment to the long-term survival of lung transplant recipients and the lack of a robust preclinical model precludes examining OB immunopathogenesis. In the current study, lungs from C57BL/10 H-2(b) mice that are MHC compatible, but minor histocompatability antigen incompatible, were transplanted into C57BL/6 mice. Histological features and cytokine profiles of OB were assessed. Moderate rejection (grade A3) developed by day 14, with evidence of OB at that time point. At 21 days, OB was present in 55% of grafts and moderate to severe rejection (grade A3-A4) was present in all mice. At 28 days, OB was present in 44% of mice and severe rejection (grade A4) was present in all. IL-17A, but not IL-17F, splenic mRNA transcripts and serum protein levels were increased only in mice that developed OB, whereas IL-10 transcripts and protein were increased only in non-OB mice. Neutralizing IL-17 prevented OB, down regulated acute rejection, and upregulated systemic IL-10. Collectively, these data show that transplantation of minor histoincompatible lungs from C57BL/10 mice into C57BL/6 mice results in a highly reproducible preclinical model of OB. In addition, these data indicate that neutralizing IL-17A or augmenting IL-10 could be therapeutic interventions to prevent OB.

Ischaemia-reperfusion injury in orthotopic mouse lung transplants - a scanning electron microscopy study

Lung ischaemia-reperfusion (I/R) injury remains a major cause of graft failure in lung transplantation (Tx). With the implementation of orthotopic lung Tx in mice, a physiological model on the base of a perfused and ventilated graft became available for the investigation of I/R injury. Using the scanning electron microscopy (SEM) technique, we here present an analysis of early and late morphological changes of pulmonary I/R injury. Syngeneic lungs were orthotopically transplanted between C57BL/6 mice. Grafts were exposed to 2 h of cold ischaemia. Transplants and right lungs were examined by SEM with corresponding haematoxylin-eosin histology 30 min and 4 h after reperfusion. Thirty minutes after reperfusion, the alveolar surface of transplants showed a discontinued lining of surfactant, while the lining of the non-transplanted lung was normal. Within the graft, leucocytes displayed an irregular surface with development of pseudopodia, and microvilli were detected on the membrane of pneumocytes. At 4 h after reperfusion, leucocytes significantly increased in numbers within the alveolar space. Also, the number of microvilli on pneumocytes increased significantly. Similar to these, the endothelium of vessels increasingly developed microvilli from 30 min towards 4 h after reperfusion. The airways of transplanted grafts showed mild changes with thickening of the bronchial epithelium and a destruction of kinocilia. Taken together, SEM detects pathological events of I/R that are previously not described in normal histology. These findings may influence the interpretation of studies investigating the I/R injury in the mouse model of lung Tx.

Bcl3 prevents acute inflammatory lung injury in mice by restraining emergency granulopoiesis

Granulocytes are pivotal regulators of tissue injury. However, the transcriptional mechanisms that regulate granulopoiesis under inflammatory conditions are poorly understood. Here we show that the transcriptional coregulator B cell leukemia/lymphoma 3 (Bcl3) limits granulopoiesis under emergency (i.e., inflammatory) conditions, but not homeostatic conditions. Treatment of mouse myeloid progenitors with G-CSF--serum concentrations of which rise under inflammatory conditions--rapidly increased Bcl3 transcript accumulation in a STAT3-dependent manner. Bcl3-deficient myeloid progenitors demonstrated an enhanced capacity to proliferate and differentiate into granulocytes following G-CSF stimulation, whereas the accumulation of Bcl3 protein attenuated granulopoiesis in an NF-κB p50-dependent manner. In a clinically relevant model of transplant-mediated lung ischemia reperfusion injury, expression of Bcl3 in recipients inhibited emergency granulopoiesis and limited acute graft damage. These data demonstrate a critical role for Bcl3 in regulating emergency granulopoiesis and suggest that targeting the differentiation of myeloid progenitors may be a therapeutic strategy for preventing inflammatory lung injury.

In vivo two-photon imaging reveals monocyte-dependent neutrophil extravasation during pulmonary inflammation

Immune-mediated pulmonary diseases are a significant public health concern. Analysis of leukocyte behavior in the lung is essential for understanding cellular mechanisms that contribute to normal and diseased states. Here, we used two-photon imaging to study neutrophil extravasation from pulmonary vessels and subsequent interstitial migration. We found that the lungs contained a significant pool of tissue-resident neutrophils in the steady state. In response to inflammation produced by bacterial challenge or transplant-mediated, ischemia-reperfusion injury, neutrophils were rapidly recruited from the circulation and patrolled the interstitium and airspaces of the lung. Motile neutrophils often aggregated in dynamic clusters that formed and dispersed over tens of minutes. These clusters were associated with CD115(+) F4/80(+) Ly6C(+) cells that had recently entered the lung. The depletion of blood monocytes with clodronate liposomes reduced neutrophil clustering in the lung, but acted by inhibiting neutrophil transendothelial migration upstream of interstitial migration. Our results suggest that a subset of monocytes serve as key regulators of neutrophil extravasation in the lung and may be an attractive target for the treatment of inflammatory pulmonary diseases.

Strain-specific susceptibility for pulmonary metastasis of sarcoma 180 cells in inbred mice

Most cancer deaths are a result of metastasis. To extend our understanding of the factors that influence the process, we aimed to develop a mouse model of pulmonary metastasis that can be assayed in multiple inbred mouse strains for further use in identification of host genetic variants that influence metastasis. We used i.v. injection of Sarcoma 180 (S180) cells, which can be tracked and quantified by bioluminescence imaging. We observed growth of S180 cells solely in the lung and observed a wide range of pulmonary metastasis among inbred mouse strains. Interestingly, we noted that the BTBRT+tf/J strain exhibited complete clearance and provide evidence that the mechanism of resistance may involve immune factors, as strains subjected to whole-body irradiation are significantly more susceptible to tumor growth. One possible mechanism of resistance to pulmonary metastasis in BTBRT+tf/J mice may require T-cell function. Our experiments present a new mouse model for further characterization of the genetics and mechanisms of pulmonary metastasis.

CCR2 regulates monocyte recruitment as well as CD4 T1 allorecognition after lung transplantation

Graft rejection remains a formidable problem contributing to poor outcomes after lung transplantation. Blocking chemokine pathways have yielded promising results in some organ transplant systems. Previous clinical studies have demonstrated upregulation of CCR2 ligands following lung transplantation. Moreover, lung injury is attenuated in CCR2-deficient mice in several inflammatory models. In this study, we examined the role of CCR2 in monocyte recruitment and alloimmune responses in a mouse model of vascularized orthotopic lung transplantation. The CCR2 ligand MCP-1 is upregulated in serum and allografts following lung transplantation. CCR2 is critical for the mobilization of monocytes from the bone marrow into the bloodstream and for the accumulation of CD11c(+) cells within lung allografts. A portion of graft-infiltrating recipient CD11c(+) cells expresses both recipient and donor MHC molecules. Two-photon imaging demonstrates that recipient CD11c(+) cells are associated with recipient T cells within the graft. While recipient CCR2 deficiency does not prevent acute lung rejection and is associated with increased graft infiltration by T cells, it significantly reduces CD4(+) T(h)1 indirect and direct allorecognition. Thus, CCR2 may be a potential target to attenuate alloimmune responses after lung transplantation.

Orthotopic vascularized right lung transplantation in the mouse

OBJECTIVES

Orthotopic left lung transplantation in the mouse, as recently developed by our laboratory, represents a physiologic model for studies in transplantation biology. However, because of the limited overall respiratory contribution of the murine left lung, left lung transplant recipients remain healthy despite immune-mediated graft necrosis. We sought to develop a lung transplantation model in which animal survival depends on graft function.

METHODS

Orthotopic vascularized right lung transplantations using cuff techniques were performed in syngeneic and allogeneic strain combinations. Grafts were assessed histologically or functionally by measuring arterial blood gases from 7 to 28 days after transplantation. In a parallel set of experiments, syngeneic and immunosuppressed allogeneic hosts underwent a left pneumonectomy 2 weeks after right lung transplantation, with assessment of graft function 1 week later.

RESULTS

We performed 40 right lung transplantations, with a survival rate of 87.5%. Syngeneic grafts remain free of inflammation as far as 28 days after transplantation. On day 7, arterial oxygen levels in syngeneic recipients (481 +/- 90 mm Hg) are equivalent to those in naive mice (503 +/- 59 mm Hg) after left hilar occlusion. Alternatively, allogeneic grafts develop histologic evidence of acute rejection, and arterial oxygen levels are significantly decreased after left hilar clamping (53.3 +/- 10.3 mm Hg). Both syngeneic and immunosuppressed allogeneic right lung recipients tolerate a left pneumonectomy.

CONCLUSIONS

Right lung transplantation followed by left pneumonectomy represents the first survival model of vascularized lung transplantation in the mouse and will therefore allow for the design of novel studies in experimental lung transplantation.

The role of adenosine A2A receptor signaling in bronchiolitis obliterans

BACKGROUND

Binding of adenosine to the anti-inflammatory Gs-coupled adenosine 2A receptor (A(2A)R) inhibits the activity of most inflammatory cells. Extensive preclinical evidence exists for the use of A(2A)R agonists in the prevention of acute ischemia-reperfusion injury. Activation of A(2A)Rs on lymphocytes and antigen-presenting cells also attenuates the alloimmune response. Because ischemia-reperfusion injury and alloimmunity are risk factors for the development of bronchiolitis obliterans (BO), the objective of this study was to determine the effect of A(2A)R signaling on tracheal rejection in a mouse model of BO.

METHODS

A non-revascularized heterotopic tracheal model across a total alloantigenic mismatch was used to study A(2A)R signaling in a mouse model of BO. Tracheal transplants were performed using Balb/c donors into wild-type or A(2A)R knockout C57BL/6 recipient mice. Another group of Balb/c transplants into C57BL/6 recipients were also treated with a selective A(2A)R agonist. Tracheas were assessed at 3, 7, 12, 21, and 28 days after transplantation by hematoxylin and eosin staining, immunohistochemical staining, and collagen staining.

RESULTS

Compared with allograft tracheas in wild-type recipients, allografts in A(2A)R knockout recipients had increased inflammation and more severe BO development. Recipient wild-type mice treated with a selective A(2A)R agonist were significantly protected from lymphocyte infiltration and luminal occlusion, but fibro-obliteration still developed by 28 days after transplantation.

CONCLUSIONS

Endogenous adenosine signals through the A(2A)R to attenuate inflammatory and immune factors involved in BO development. Synthetic A(2A)R agonists may provide a novel treatment strategy to prevent BO.

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.

Costimulatory blockade-mediated lung allograft acceptance is abrogated by overexpression of Bcl-2 in the recipient

Lung allografts are considered to be more immunogenic than other solid organs. Little is known about the effectiveness of immunosuppressive regimens after lung transplantation. Herein, we describe a novel model of murine vascularized orthotopic lung transplantation we used to study the effects of costimulatory blockade on lung rejection. Transplants were performed in the Balb --> B6 strain combination. Recipients were either not immunosuppressed or received perioperative CD40/CD40L and CD28/B7 costimulatory blockade. Nonimmunosupressed Balb/c --> B6 lung transplants had severe acute rejection 7 days after transplantation and CD8(+) T cells outnumbered CD4(+) T cells within the allografts. Alternatively, B6 recipients that received perioperative costimulatory blockade had minimal inflammation and there were nearly equal numbers of CD8(+) and CD4(+) T cells in these grafts. Approximately one third of graft-infiltrating CD4(+) T cells expressed Foxp3. CD4(+) T cells isolated from these grafts induced apoptosis of alloreactive CD8(+) T cells that were stimulated with donor splenocytes in vitro. In contrast with wild-type B6 recipient mice, we observed severe rejection of Balb/c lungs 7 days after transplantation into Bcl-2 transgenic B6 recipients that had received costimulatory blockade. CD8(+) T cells outnumbered CD4(+) T cells in these immunosuppressed Bcl-2 transgenic recipients and, compared with immunosuppressed wild-type B6 recipients, a lower percentage of graft-infiltrating CD4(+) T cells expressed Foxp3, and a higher percentage of graft-infiltrating CD8(+) T cells expressed intereferon-gamma. Thus, our results show that perioperative blockade of the CD40/CD40L and CD28/B7 costimulatory pathways markedly ameliorates acute rejection of lung allografts in wild type but not Bcl-2 transgenic recipients.

Monocyte differentiation is controlled by MyD88 after mouse orthotopic lung transplantation

In lung grafts, ischemia-reperfusion signals rapidly induce the recruitment and differentiation of host monocytes into macrophages and dendritic cells. The nature of ischemia-reperfusion signals are antigen independent, but have been hypothesized to initiate Toll-like receptor (TLR) and interleukin (IL)-1R-mediated signaling pathways that are thought to potentiate alloimmune responses. We wondered whether MyD88, an adaptor molecule critical for both TLR and IL-1R-mediated inflammatory responses, regulated monocyte differentiation in a mouse model of vascularized orthotopic lung transplantation. Orthotopic left lung transplants were performed in the following syngeneic combinations: CD45.1(+) B6 --> CD45.2(+) MyD88(-/-) and CD45.1(+) B6 --> CD45.2(+) B6. One day later, recipient-derived dendritic cells and macrophage numbers were assessed in the bronchiolar lavage by FACS analysis. Compared with the bronchiolar lavage of wildtype recipients, MyD88(-/-) recipients had lower numbers of dendritic cells in lung graft airways that were of recipient origin. Lower numbers of newly differentiated lung graft dendritic cells was coincident with the appearance of higher numbers of undifferentiated monocytes in the lung airways of MyD88(-/-) recipients as compared with wild-type recipients. Moreover, adoptive transfer experiments demonstrated that MyD88(-/-) monocytes were poorer at differentiating into lung dendritic cells as compared with wild-type monocytes. Taken together, these data show that MyD88 regulates graft-infiltrating monocyte differentiation and suggests a mechanism by which TLR/IL-1R-signaling pathways control adaptive responses in lung allografts through controlling monocyte fate.

Cutting edge: Acute lung allograft rejection is independent of secondary lymphoid organs

It is the prevailing view that adaptive immune responses are initiated in secondary lymphoid organs. Studies using alymphoplastic mice have shown that secondary lymphoid organs are essential to initiate allograft rejection of skin, heart, and small bowel. The high immunogenicity of lungs is well recognized and allograft rejection remains a major contributing factor to poor outcomes after lung transplantation. We show in this study that alloreactive T cells are initially primed within lung allografts and not in secondary lymphoid organs following transplantation. In contrast to other organs, lungs are acutely rejected in the absence of secondary lymphoid organs. Two-photon microscopy revealed that recipient T cells cluster predominantly around lung-resident, donor-derived CD11c(+) cells early after engraftment. These findings demonstrate for the first time that alloimmune responses following lung transplantation are initiated in the graft itself and therefore identify a novel, potentially clinically relevant mechanism of lung allograft rejection.

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.

Orthotopic mouse lung transplantation as experimental methodology to study transplant and tumor biology

Unlike transplantation of other solid organs, vascularized mouse lung transplantation has only recently been developed. In this protocol, we describe a detailed method for performing a vascularized and aerated mouse orthotopic lung transplant, which to date represents the most physiological mouse model of lung transplantation. The procedure is divided into two separate portions consisting of donor harvest followed by implantation using the cuff technique for bronchovascular anastomoses. After a training period spanning several months, the procedure can be successfully mastered and, in experienced hands, requires approximately 90 min to perform. After an initial learning curve, perioperative survival is close to 100%. As the donor hematopoietic cells in the transplanted lung are replaced by those of the host over time, thereby creating a 'chimeric lung,' this model represents a novel research tool for the study of transplantation biology as well as other disease processes, such as malignancies.

A mouse model of orthotopic, single-lung transplantation

OBJECTIVES

Progress in studying acute and chronic pulmonary allograft rejection has been hampered by the lack of feasible experimental animal transplantation models. Contemporary approaches are limited by anatomic applicability (heterotopic tracheal implantation) and lack of genetic variability (rat model). To utilize the breadth of available genetic modifications in a physiologic setup, we optimized and validated a procedure of orthotopically transplanted, perfused, and ventilated single pulmonary transplantation in mice.

METHODS

C57BL/6 mice served as recipient, with Balb/c as donor. At time of harvest, explanted lungs were perfused with Perfadex, and the heart-lung block excised. Under 30 to 40x magnification, vessels and bronchus were cuffed. Following left thoracotomy in the recipient, hilar structures were incised and cuff-anastomosed with the corresponding donor parts. Allogeneic and syngeneic transplantations (n = 12/group) were performed with a follow-up period of 5 days and up to 90 days, respectively.

RESULTS

The success rate of lung transplantation in mice was 87.5% (21/24). Mean cold ischemia time was 32.3 +/- 3.7 minutes, and warm ischemia time was 30.8 +/- 9.5 minutes. Deaths were due to bleeding during dissection of the hilus and/or caused by thrombosis postoperatively. Allogeneic grafts were rejected by day 5; syngeneic grafts were slightly congested but mainly unchanged up to day 90 posttransplantation.

CONCLUSIONS

Unilateral lung transplantation in mice can be performed in a standardized and controlled fashion with low mortality, comparable to the rat. Employing transgenic and knockout mice strains, this procedure holds great promise to advance the understanding of immunologic pathways in acute and chronic rejection in a physiologic model of pulmonary 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.

Pathogenesis, management, and consequences of primary graft dysfunction

Primary graft dysfunction continues to be a major contributing factor to morbidity and mortality after lung transplantation. This condition is presumed to be the result of ischemia-reperfusion injury, which is associated with the release of endogenous substances that can activate the innate immune system. Primary graft dysfunction has been shown to be an independent risk factor for the development of bronchiolitis obliterans syndrome indicating that it can shape alloimmune responses. In this review, we focus on the classification, pathogenesis, possible prevention strategies, management and consequences of primary graft dysfunction.

CD4+ T lymphocytes are not necessary for the acute rejection of vascularized mouse lung transplants

Acute rejection continues to present a major obstacle to successful lung transplantation. Although CD4(+) T lymphocytes are critical for the rejection of some solid organ grafts, the role of CD4(+) T cells in the rejection of lung allografts is largely unknown. In this study, we demonstrate in a novel model of orthotopic vascularized mouse lung transplantation that acute rejection of lung allografts is independent of CD4(+) T cell-mediated allorecognition pathways. CD4(+) T cell-independent rejection occurs in the absence of donor-derived graft-resident hematopoietic APCs. Furthermore, blockade of the CD28/B7 costimulatory pathways attenuates acute lung allograft rejection in the absence of CD4(+) T cells, but does not delay acute rejection when CD4(+) T cells are present. Our results provide new mechanistic insight into the acute rejection of lung allografts and highlight the importance of identifying differences in pathways that regulate the rejection of various organs.

Maintenance of airway epithelium in acutely rejected orthotopic vascularized mouse lung transplants

Lung transplantation remains the only therapeutic option for many patients suffering from end-stage pulmonary disease. Long-term success after lung transplantation is severely limited by the development of bronchiolitis obliterans. The murine heterotopic tracheal transplantation model has been widely used for studies investigating pathogenesis of obliterative airway disease and immunosuppressive strategies to prevent its development. Despite its utility, this model employs proximal airway that lacks airflow and is not vascularized. We have developed a novel model of orthotopic vascularized lung transplantation in the mouse, which leads to severe vascular rejection in allogeneic strain combinations. Here we characterize differences in the fate of airway epithelial cells in nonimmunosuppressed heterotopic tracheal and vascularized lung allograft models over 28 days. Up-regulation of growth factors that are thought to be critical for the development of airway fibrosis and interstitial collagen deposition were similar in both models. However, while loss of airway epithelial cells occurred in the tracheal model, airway epithelium remained intact and fully differentiated in lung allografts, despite profound vascular rejection. Moreover, we demonstrate expression of the anti-apoptotic protein Bcl-2 in airway epithelial cells of acutely rejected lung allografts. These findings suggest that in addition to alloimmune responses, other stimuli may be required for the destruction of airway epithelial cells. Thus, the model of vascularized mouse lung transplantation may provide a new and more physiologic experimental tool to study the interaction between immune and nonimmune mechanisms affecting airway pathology in lung allografts.