Indirect recognition and antibody production against a single mismatched HLA-A2-transgenic molecule precede the development of obliterative airway disease in murine heterotopic tracheal allografts

Murine orthotopic lung transplant models: A comprehensive overview of genetic mismatch degrees and histopathological insights into chronic lung allograft dysfunction

Currently, lung transplantation outcome remains inferior compared to other solid organ transplantations. A major cause for limited survival after lung transplantation is chronic lung allograft dysfunction. Numerous animal models have been developed to investigate chronic lung allograft dysfunction to discover adequate treatments. The murine orthotopic lung transplant model has been further optimized over the last years. However, different degrees of genetic mismatch between donor and recipient mice have been used, applying a single, minor, moderate, and major genetic mismatch. This review aims to reassess the existing murine mismatch models and provide a comprehensive overview, with a specific focus on their eventual histopathological presentation. This will be crucial to leverage this model and tailor it according to specific research needs.

Single-cell transcriptome mapping identifies a local, innate B cell population driving chronic rejection after lung transplantation

Bronchiolitis obliterans syndrome (BOS) is the main reason for poor outcomes after lung transplantation (LTx). We and others have recently identified B cells as major contributors to BOS after LTx. The extent of B cell heterogeneity and the relative contributions of B cell subpopulations to BOS, however, remain unclear. Here, we provide a comprehensive analysis of cell population changes and their gene expression patterns during chronic rejection after orthotopic LTx in mice. Of 11 major cell types, Mzb1-expressing plasma cells (PCs) were the most prominently increased population in BOS lungs. These findings were validated in 2 different cohorts of human BOS after LTx. A Bhlhe41, Cxcr3, and Itgb1 triple-positive B cell subset, also expressing classical markers of the innate-like B-1 B cell population, served as the progenitor pool for Mzb1⁺ PCs. This subset accounted for the increase in IgG_2c production within BOS lung grafts. A genetic lack of Igs decreased BOS severity after LTx. In summary, we provide a detailed analysis of cell population changes during BOS. IgG⁺ PCs and their progenitors — an innate B cell subpopulation — are the major source of local Ab production and a significant contributor to BOS after LTx.

Inhibition of B cell-dependent lymphoid follicle formation prevents lymphocytic bronchiolitis after lung transplantation

Lung transplantation (LTx) is the only therapeutic option for many patients with chronic lung disease. However, long-term survival after LTx is severely compromised by chronic rejection (chronic lung allograft dysfunction [CLAD]), which affects 50% of recipients after 5 years. The underlying mechanisms for CLAD are poorly understood, largely due to a lack of clinically relevant animal models, but lymphocytic bronchiolitis is an early sign of CLAD. Here, we report that lymphocytic bronchiolitis occurs early in a long-term murine orthotopic LTx model, based on a single mismatch (grafts from HLA-A2:B6-knockin donors transplanted into B6 recipients). Lymphocytic bronchiolitis is followed by formation of B cell-dependent lymphoid follicles that induce adjacent bronchial epithelial cell dysfunction in a spatiotemporal fashion. B cell deficiency using recipient μMT-/- mice prevented intrapulmonary lymphoid follicle formation and lymphocytic bronchiolitis. Importantly, selective inhibition of the follicle-organizing receptor EBI2, using genetic deletion or pharmacologic inhibition, prevented functional and histological deterioration of mismatched lung grafts. In sum, we provided what we believe to be a mouse model of chronic rejection and lymphocytic bronchiolitis after LTx and identified intrapulmonary lymphoid follicle formation as a target for pharmacological intervention of long-term allograft dysfunction after LTx.

Immunosuppression for lung transplantation

As a result of advances in surgical techniques, immunosuppressive therapy, and postoperative management, lung transplantation has become an established therapeutic option for individuals with a variety of end-stage lung diseases. The current 1-year actuarial survival rate following lung transplantation is approaching 80%. However, the 5- year actuarial survival rate has remained virtually unchanged at approximately 50% over the last 15 years due to the processes of acute and chronic lung allograft rejection (1). Clinicians still rely on a vast array of immunosuppressive agents to suppress the process of graft rejection, but find themselves limited by an inescapable therapeutic paradox. Insufficient immunosuppression results in graft loss due to rejection, while excess immunosuppression results in increased morbidity and mortality from opportunistic infections and malignancies. Indeed, graft rejection, infection, and malignancy are the three principal causes of mortality for the lung transplant recipient. One should also keep in mind that graft loss in a lung transplant recipient is usually a fatal event, since there is no practical means of long-term mechanical support, and since the prospects of re-transplantation are low, given the shortage of acceptable donor grafts. This chapter reviews the current state of immunosuppressive therapy for lung transplantation and suggests alternative paradigms for the management of future lung transplant recipients.

Animal models for bronchiolitis obliterans syndrome following human lung transplantation

Lung transplantation is the only viable treatment option that can improve survival and enhance the quality of life of patients with end-stage lung diseases such as emphysema, cystic fibrosis, idiopathic pulmonary fibrosis, and primary pulmonary hypertension. However, the long-term survival of lung allografts is still limited by the development of bronchiolitis obliterans syndrome (BOS), an irreversible condition unresponsive to therapy. BOS is the most significant cause of long-term morbidity and mortality after lung transplantation. Over the past decade, several animal models have been developed to investigate BOS. These are valuable to elucidate the immunologic and pathologic mechanisms that lead to BOS and to test treatment options for BOS. In this review, we discuss the advantages and disadvantages of different animal models and highlight work that has been done with each model.

Human and murine obliterative bronchiolitis in transplant

Obliterative bronchiolitis is a devastating illness that limits the long-term success of lung transplantation. Its high prevalence and overall poor response to current therapeutic measures demands further research to elucidate pathogenic mechanisms. Toward this goal, there is a role for animal models to study the mechanisms of obliterative bronchiolitis, such as the murine heterotopic tracheal allograft model. This review compares the tracheal allograft model to human obliterative bronchiolitis pathology and highlights the important mechanisms of airway rejection described using this model. Although certain limitations exist, the pursuit of proof-of-concept studies in this model, as well as other animal models, can provide the basis for genetic and cellular translational human studies directed toward post-transplant obliterative bronchiolitis pathogenesis. To meet these challenges, we call for the establishment of a National Institutes of Health-supported Lung Transplant Network to better orchestrate translational research efforts in obliterative bronchiolitis pathogenesis and treatment, and to advance the field of lung transplantation.

Molecular mechanisms of chronic rejection following transplantation

Although significant advances have been made in the field of organ transplantation, chronic rejection remains a major limiting factor for prolonged graft survival. The long-term survival and function of transplanted lungs are limited by the development of bronchiolitis obliterans syndrome (BOS). The 10-yr lung graft survival rate is only 18.6%. Aside from results of several clinical studies that strongly support the concept that BOS results from alloimmune-mediated injury, little is known regarding specific immune effectors or target molecules involved in the pathogenesis of BOS. Studies from our laboratory have provided evidence for the seminal role of CD4+ T-cells in the pathogenesis of obliterative airway disease (OAD) seen in BOS. Prior to any clinically detectable lesions, there is indirect antigen presentation of mismatched major histocompatibility complex (MHC) class I antigen and production of antibodies to these MHC antigens. Both MHC and minor histocompatibility antigen disparities can result in the development of OAD in animal models and preliminary results strongly suggest that peptide vaccination strategies may prevent OAD following heterotopic tracheal transplants. Using a newly developed orthotopic tracheal transplant model, we have obtained evidence for an important and probably exclusive role for airway epithelial cell injury as a primary mechanism for the immunopathogenesis of the development of OAD.

Inhibition of obliterative airway disease development following heterotopic murine tracheal transplantation by costimulatory molecule blockade using anti-CD40 ligand alone or in combination with donor bone marrow

INTRODUCTION

Obliterative airway disease (OAD) development in heterotopic murine tracheal allografts, a model of obliterative bronchiolitis after lung transplantation, is immunologically mediated. Whether tolerance induction by the administration of anti-CD40 ligand monoclonal anti-body (MR-1) alone or in conjunction with donor-derived bone marrow cells (BMCs) can prevent the development of OAD was tested in this study.

METHODS

BALB/c tracheal allografts were heterotopically transplanted into C57BL/6 recipients. Group 1 received no treatment. Group 2 received multiple infusions of donor BMCs intravenously. Group 3 was administered MR-1 intraperitoneally. Group 4 received donor BMCs and MR-1. Allografts were harvested at several time points post-transplantation and examined for the development of OAD.

RESULTS

Group 1 developed cellular infiltration and epithelial damage by Day 15 post-transplant and OAD by Day 28, evidenced by complete obliteration of the tracheal lumen. Group 2 developed OAD with similar kinetics to Group 1. Group 3 had no evidence of OAD at 28 days. At Days 45 to 90, moderate cellular infiltration, epithelial metaplasia, and a minimal narrowing of the tracheal lumen were evident. OAD developed by Day 120. Group 4 mice had patent tracheal lumens even at 120 days post-transplantation, with only mild epithelial metaplasia and luminal narrowing noted.

CONCLUSIONS

The administration of MR-1 alone in combination with infusions of donor bone marrow cells significantly attenuated the development of OAD. Tolerance-inducing regimens such as this deserve further investigation in the prevention of post-lung transplant obliterative bronchiolitis following human lung transplantation.

Pluripotent allospecific CD8+ effector T cells traffic to lung in murine obliterative airway disease

Long-term success in lung transplantation is limited by obliterative bronchiolitis, whereas T cell effector mechanisms in this process remain incompletely understood. Using the mouse heterotopic allogeneic airway transplant model, we studied T cell effector responses during obliterative airways disease (OAD). Allospecific CD8+ IFN-gamma+ T cells were detected in airway allografts, with significant coexpression of TNF-alpha and granzyme B. Therefore, using IFN-gamma as a surrogate marker, we assessed the distribution and kinetics of extragraft allo-specific T cells during OAD. Robust allospecific IFN-gamma was produced by draining the lymph nodes, spleen, and lung mononuclear cells from allograft, but not isograft recipients by Day 14, and significantly decreased by Day 28. Although the majority of allospecific T cells were CD8+, allospecific CD4+ T cells were also detected in these compartments, with each employing distinct allorecognition pathways. An influx of pluripotent CD8+ effector cells with a memory phenotype were detected in the lung during OAD similar to those seen in the allografts and secondary lymphoid tissue. Antibody depletion of CD8+ T cells markedly reduced airway lumen obliteration and fibrosis at Day 28. Together, these data demonstrate that allospecific CD8+ effector T cells play an important role in OAD and traffic to the lung after heterotopic airway transplant, suggesting that the lung is an important immunologic site, and perhaps a reservoir, for effector cells during the rejection process.

Induction of obliterative airway disease by anti-HLA class I antibodies

Anti-HLA class I Abs are associated with the development of bronchiolitis obliterans syndrome (BOS) after lung transplantation. BOS is characterized histologically by fibrosis and airway epithelial cell apoptosis. We have previously shown that anti-HLA class I Abs induce proliferation, growth factor production and apoptosis in airway epithelial cells in vitro. Thus, this study was designed to determine whether anti-HLA class I Abs alone could induce obliterative airway disease (OAD) in heterotopic murine tracheal allografts. Toward this, HLA-A*0201-transgenic tracheal allografts were transplanted into Rag1-deficient mice treated with the W6/32 anti-HLA class I mAb. Allografts were harvested at days +30, +45, +60 and +90. Allografts displayed epithelial metaplasia by day +45, epithelial destruction and mild cellular infiltration by day +60 and complete lumen obliteration and moderate cellular infiltration by day +90. Anti-HLA class I Abs induced the production of several growth factors and growth factor receptors and apoptosis of parenchymal cells in the allograft. In addition, anti-HLA class I Abs induced macrophages and granulocytes infiltration. The results from this study demonstrate that anti-HLA class I Abs play an important role in the pathogenesis of OAD by inducing growth factor production, apoptosis and chemotaxis of inflammatory cells.

Immunosuppression for lung transplantation

As a result of advances in surgical techniques, immunosuppressive therapy, and postoperative management, lung transplantation has become an established therapeutic option for individuals with a variety of end-stage lung diseases. The current 1-year actuarial survival rate following lung transplantation is approximately 75%. However, the processes of acute and chronic lung allograft rejection have limited the long-term success of lung transplantation. Clinicians currently rely on a vast armamentarium of immunosuppressive agents to ameliorate graft rejection, but find themselves limited by an inescapable therapeutic paradox. Insufficient immunosuppression results in graft loss due to rejection, while excess immunosuppression results in increased morbidity and mortality from opportunistic infections and malignancies. Indeed, graft rejection, infection, and malignancy are the three principal causes of mortality for the lung transplant recipient. One should also keep in mind that graft loss in a lung transplant recipient is usually a fatal event, since there is no practical means of long-term mechanical support, and since the prospects of re-transplantation are low, given the shortage of acceptable donor grafts. This chapter reviews the current state of immunosuppressive therapy for lung transplantation, and suggests alternative paradigms for the management of future lung transplant recipients.

Induction of obliterative airway disease in murine tracheal allografts by CD8+ CTLs recognizing a single minor histocompatibility antigen

The role of minor histocompatibility Ag (mHAg)-specific CD8+ CTLs in the pathogenesis of chronic lung allograft rejection (bronchiolitis obliterans syndrome) remains to be elucidated. Thus, the goal of this study was to define the role of a single mHAg mismatch at the polymorphic H13 allele in the development of obliterative airway disease (OAD) after murine heterotopic tracheal transplantation. The H13a and H13b alleles encode for the SSVVGVWYL (SVL9) and SSVIGVWYL (SIL9) mHAgs, respectively, presented in the context of the H2Db MHC class I molecule. Toward this, C56BL/10SnJ (H13a) tracheal allografts were transplanted into congenic B10.CE-H13b Aw(30NX)/Sn (H13b) recipients. The allografts were harvested at 30, 60, and 90 days after transplantation, and OAD lesions (epithelial damage, cellular infiltration, and luminal fibrosis) were confirmed histologically. Selected groups of mice were immunized (s.c.) or tolerized (i.v.) with the SVL9 peptide before transplantation. This single mHAg mismatch induced the development of OAD within 90 days. SVL9 immunization significantly accelerated the kinetics of the OAD lesions. In contrast, SVL9 tolerization completely abrogated the development of OAD. This was correlated with a complete abrogation of H13a-specific CD8+ CTL responses with a significant reduction in the frequency of IFN-gamma-producing CTLs and the activation of TGF-beta-producing CD8+ T cells. In conclusion, a single mHAg mismatch can induce the development of OAD. These data also suggest that mHAg-reactive CD8+ CTLs may play an important role in the pathogenesis of chronic lung allograft rejection in humans.

Bronchiolitis obliterans in children

Bronchiolitis obliterans (BO) in children is a relatively rare diagnosis. The increase in lung and bone marrow transplantation in children, however, has led to a heightened interest in BO, as this is one of the important complications of those procedures. This article will discuss BO as an entity that can follow any of several illnesses or toxic exposures, in addition to following allogeneic lung or bone marrow transplantation. The complex and incompletely understood pathology, pathogenesis, and molecular pathology involved in BO remain the subject of ongoing investigations. As the prognosis for BO is uncertain and treatment is often unsuccessful, the continued need for the recognition of surrogate markers for BO in patients at risk and the development of better forms of therapy are paramount. This review will describe our current understanding of BO, and will call attention to those research areas that require continuing efforts in order to prevent or treat this entity.

CMV-infected allogeneic endothelial cells initiate responder and bystander donor HLA class I release via the metalloproteinase cleavage pathway

Although cytomegalovirus (CMV) interferes with major histocompatibility expression in infected cells, both host and donor soluble human leukocyte antigen class I (sHLA-I) are often released into the serum of transplant recipients during CMV infection and may contribute to anti-HLA antibody production and graft rejection. We hypothesized that CMV infection of endothelial cells (EC) induces host T cells to release interferon (IFN)-gamma, which in turn drives the metalloproteinase (MPase)-cleavage pathway of sHLA-I generation in "bystander" uninfected ECs. To test this hypothesis, cultures of peripheral blood mononuclear cells (PBMCs) and either uninfected ECs or CMV-infected ECs (EC/CMV) were established and supernatants were tested in enzyme-linked immunosorbent assay for sHLA-I. Responder PBMC became activated and released sHLA-I via the MPase pathway when stimulated with allogeneic EC/CMV; the sHLA-I release was contact dependent and cytokine independent. In transwell cultures, IFN-gamma released by PBMCs in response to EC/CMV stimulated a release of sHLA-I from uninfected allogeneic ECs across the transwell; this release was also MPase dependent. This implies that CMV infection within the transplanted allograft will not only stimulate the release of self HLA from responding PBMCs, but will also stimulate the release of donor sHLA-I from uninfected bystander ECs, both via the class I MPase-pathway.

Immune mechanisms in the pathogenesis of bronchiolitis obliterans syndrome after lung transplantation

Lung transplantation is recognized as the only viable treatment option in a variety of end-stage pulmonary diseases. However, the long-term survival after lung transplantation is limited by the development of obliterative bronchiolitis, and its clinical correlate bronchiolitis obliterans syndrome (BOS), which is considered to represent chronic lung allograft rejection. Histopathologically, BOS is an inflammatory process that leads to fibrous scarring of the terminal and respiratory bronchioles and subsequent total occlusion of the airways. The specific etiology and pathogenesis of BOS are not well understood. The current premise is that BOS represents a common lesion in which different inflammatory insults such as ischemia-reperfusion, rejection, and infection can lead to a similar histological and clinical outcome. However, the low incidence of BOS in non-transplanted individuals and the observation that early development of BOS is predicted by the frequency and severity of acute rejection episodes indicate that alloimmune-dependent mechanisms play a crucial role in the pathogenesis of BOS. The evidence presented in this review indicates that BOS is the result of humoral and cellular immune responses developed against major histocompatibility complex molecules expressed by airway epithelial cells of the lung allograft. This process is aggravated by alloimmune-independent mechanisms such as ischemia-reperfusion and infection. Currently, treatment of BOS is frequently unsuccessful. Therefore, a better understanding of the immunopathogenesis of BOS is of paramount importance toward improving long-term patient and graft survival after lung transplantation.

[Bronchiolitis with airflow obstruction in adults]

PURPOSE

The purpose of this paper is twofold: to describe the clinical and anatomical characteristics of bronchiolitis associated with airflow obstruction in adults; to present through a clinical approach, a classification of the main aetiologies or pathological frames associated with that entity.

KEY POINTS

The constrictive bronchiolitis type is the most frequently encountered. On clinical grounds, cough, crackles, and a progressive dyspnea develop usually within a few weeks. Radiological signs of bronchiolar abnormalities are best visualized on high resolution expiratory CT scan. The decrease in maximal airflows and oxygen tension is of limited amplitude and poorly reversible with bronchodilators. Diagnosis is easily performed when a causative event, or the clinical context, can be delineated: inhalation of toxic fumes, diffuse bronchiectasis, rheumatoid arthritis, lung or bone marrow transplantation. Delayed formation of bronchiectasis in the central airways is common. The treatment is not standardized; corticosteroids are usually prescribed as a first line therapy; the benefit of the addition of, or substitution with immunosuppressive drugs has not been adequately evaluated, but is, on the mean, of limited amplitude.

PERSPECTIVES

Recent advances in the identification of inhaled agents toxic for the distal airways help in establishing appropriate measures of prevention. When the aetiology of the bronchiolitis cannot be suspected, extensive search of a causative agent should be performed, including microbial and mineral analysis of bronchoalveolar products. Negative results should lead to perform a surgical lung biopsy. The study of chronic rejection processes in animal models of lung transplantation, the identification of inhibitory factors of bronchiolar fibrogenesis, and the efficacy of some anti-cytokines on inflammatory processes could result in new therapeutic approaches.

Indirect Minor Histocompatibility Antigen Presentation by Allograft Recipient Cells in the Draining Lymph Node Leads to the Activation and Clonal Expansion of CD4+T Cells That Cause Obliterative Airways Disease

Ag recognition by OVA-reactive OT-II (I-Ab restricted) and DO11.10 (I-Ad restricted) TCR-Tg CD4+ T cells after heterotopic transplantation of OVA transgene-expressing tracheal grafts was examined as a model of minor histocompatibility Ag (mHAg)-induced chronic allograft rejection. In response to airway allotransplantation with grafts expressing the OVA transgene, these TCR-Tg CD4+ T cells expressed the activation markers CD69 and CD44, demonstrated evidence of blastogenesis, underwent multiple rounds of cell division leading to their clonal expansion in the draining lymph node, and proceeded to differentiate to a effector/memory T cell phenotype based on a reduction in the expression of CD45RB. These mHAg-specific TCR-Tg CD4+ T cells responded equally well to fully MHC-mismatched tracheas and to class II-deficient allografts, demonstrating that donor mHAg recognition by recipient CD4+ T cells does not rely on Ag presentation by donor-derived APC. The activation of mHAg-specific TCR-Tg CD4+ T cells after their adoptive transfer into recipient mice given MHC-matched, but mHAg-disparate, airway allografts was associated with their movement into the allograft and the near uniform destruction of the transplanted airway tissue secondary to the development of obliterative airways disease. These results demonstrate that an activation of mHAg-reactive CD4+ T cells in the draining lymph node by recipient APC that indirectly express graft mHAg-derived peptide/class II MHC complexes precedes responder T cell proliferation and differentiation, and leads to the eventual migration of these alloreactive T cells to the transplanted airway tissue and the promotion of chronic graft rejection.

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

Murine heterotopic tracheal allografts develop obliterative airway disease (OAD), a suitable model of chronic lung allograft rejection. This model, however, fails to account for the behavior of the allograft when adjacent to recipient airway tissues, particularly the epithelium. This study was performed to determine the immunologic role of the epithelium in development of OAD. BALB/c (H2d) tracheal allografts were transplanted orthotopically into C57BL/6 (H2b) mice and harvested 14-150 days post-transplantation. The phenotype of the allograft epithelium after orthotopic transplantation was determined with immunofluorescent staining. Orthotopic BALB/c tracheal allografts harvested at 28 days were re-transplanted heterotopically into BALB/c or C57BL/6 mice, harvested after 28 days, and assessed for OAD. Orthotopic allografts displayed mild cellular infiltration, no fibrosis and preserved epithelium at 28 days post-transplant. The presence of recipient-derived epithelium within the allograft was demonstrated with immunofluorescent staining at day 14. Significantly, BALB/c orthotopic allografts re-transplanted heterotopically into BALB/c mice developed OAD by day 28, whereas BALB/c orthotopic allografts re-transplanted heterotopically into C57BL/6 mice did not. Repopulation of orthotopic tracheal allografts with recipient-derived epithelium confers a protective effect against OAD after heterotopic re-transplantation. This indicates that the airway epithelium plays a crucial role in OAD development.

Specific immune responses against airway epithelial cells in a transgenic mouse-trachea transplantation model for obliterative airway disease

BACKGROUND

Immune injury to airway epithelium is suggested to play a central role in the pathogenesis of obliterative bronchiolitis (OB) after clinical lung transplantation. In several studies, a rejection model of murine trachea transplants is used, resulting in obliterative airway disease (OAD) with similarities to human OB. To focus on the role of an immune response specifically against airway epithelium, we transplanted tracheas from transgenic mice expressing human epithelial glycoprotein (hEGP) on epithelial cells. We hypothesized that the immune response against the hEGP-2 antigen would result in OAD in the trachea transplants.

METHODS

Tracheas from hEGP-2 transgenic and control nontransgenic FVB/N mice were heterotopically transplanted into FVB/N mice and harvested at week 1, 3, 6, and 9. Anti-hEGP-2 antibodies were determined in the recipient blood. The trachea grafts were analyzed for cellular infiltration, epithelial cell injury, and luminal obliteration.

RESULTS

Recipients of transgenic tracheal grafts gradually developed anti-hEGP-2 antibodies. In the transgenic grafts, the submucosa was infiltrated predominantly by CD4+ T cells. Epithelial cells remained present but showed progressive abnormality. The tracheal lumen showed a mild degree of obliteration. All these changes were absent in nontransgenic FVB/N trachea transplants.

CONCLUSION

The hEGP-2 antigen on the epithelial cells of transgenic trachea transplants induces specific humoral and cellular immune responses, leading to a mild form of OAD. It provides a suitable model for further investigation of the role of epithelial cells in the development of OAD in animals and OB in human-lung transplantation.

Anti-HLA class I antibody binding to airway epithelial cells induces production of fibrogenic growth factors and apoptotic cell death: a possible mechanism for bronchiolitis obliterans syndrome

Development of anti-HLA class I antibodies is associated with bronchiolitis obliterans syndrome (BOS) after lung transplantation. BOS is characterized histologically by significant fibrosis and airway epithelial cell (AEC) apoptosis. Thus, this study was designed to determine whether anti-HLA class I antibodies can activate AECs to produce growth factors and to undergo apoptosis. KCC-266 AECs were activated with the W6/32 anti-HLA class I monoclonal antibody. Proliferation and apoptosis levels were determined after 24, 48, and 72 hours. The induction of fibroblast and bronchial smooth muscle cell proliferation by anti-HLA class I activated AECs was assessed in the presence of neutralizing antibodies against various growth factors. The anti-HLA class I induced AEC proliferation after 24 hours followed by significant induction of apoptosis after 48 hours. Anti-HLA class I activated AECs produced soluble growth factors that stimulated fibroblasts but not bronchial smooth muscle cells. The stimulation of fibroblast proliferation was inhibited by antibodies against platelet-derived growth factor, heparin-binding epidermal growth factor, insulin-like growth factor 1, and basic fibroblast growth factor. The results from this study suggest that anti-HLA class I alloantibodies may play an important role in the pathogenesis of BOS by inducing proliferation, growth factor production, and apoptotic cell death in AECs.

Hierarchical contributions of allorecognition pathways in chronic lung rejection

The role of allorecognition in initiating lung graft rejection is not clearly defined. Using the heterotopic tracheal transplantation model, we examined the contributions of the indirect and direct allorecognition pathways in chronic airway rejection. Fully mismatched, wild-type grafts were transplanted into major histocompatibility complex (MHC) II-/-, class II-like accessory molecule (H2-DMalpha)-/- using MHC I-/- and wild-type allorecipients as control subjects. Similarly, MHC I-/-, MHC II-/-, or MHC I/II-/- allografts were transplanted into wild-type mice with appropriate control subjects. Grafts from nonimmunosuppressed recipients were evaluated at Weeks 2, 4, and 6. Grafts transplanted into MHC II-/- and H2-DMalpha-/- allorecipients showed a more intact epithelium and reduced lumen obliteration compared with grafts transplanted into wild-type or MHC I-/- allorecipients (p < 0.05 for each). These grafts exhibited abundant CD4+ and CD8+ cell infiltrates similar to control allografts. MHC I-/- and MHC I/II-/- but not MHC II-/- allografts placed in wild-type animals demonstrated less severe rejection compared with allograft control subjects (p < 0.05 for each). Although the indirect allorecognition pathway has the strongest influence on rejection, the direct pathway is sufficient to ultimately cause chronic airway rejection. In addition, these results suggest that MHC class I molecules are the principal alloantigens in the mouse heterotopic tracheal model of obliterative bronchiolitis.

Different kinetics of obliterative airway disease development in heterotopic murine tracheal allografts induced by CD4+ and CD8+ T cells

BACKGROUND

Both T and B cells have been shown to be implicated in the pathogenesis of bronchiolitis obliterans syndrome, which is considered to represent chronic lung allograft rejection. However, the relative contributions of T cells and alloantibodies in the pathogenesis of the disease are still unknown. In this study, we used an heterotopic murine tracheal transplantation model to determine the contribution of these components of the immune system in the pathogenesis of posttransplant obliterative airway disease (OAD).

METHODS

Tracheal allografts from BALB/c and HLA-A2-transgenic (HLA-A2+) mice were heterotopically transplanted into C57BL/6, CD4-knockout (KO), CD8-KO, Ig-KO, and Rag1-KO mice. In additional experiments, recipient mice were pretreated with depleting antibodies against CD4+, CD8+, and NK1.1+ cells. Development of OAD was determined by histopathology at days 10, 30, 60, 90, and 180 after transplantation.

RESULTS

HLA-A2+ allografts transplanted into C57BL/6, CD8-KO, and Ig-KO mice demonstrated OAD lesions by day 30. In contrast, allografts transplanted into CD4-KO mice showed no OAD lesions at day 30, partial OAD development by days 60 and 90, and complete OAD development by day 180. No OAD development was observed in allografts transplanted into Rag1-KO mice. Treatment with anti-NK1.1 antibody did not show any effect on posttransplant OAD development. In contrast, anti-CD4+ or anti-CD8+ antibody treatments partially reduced the OAD histopathology and combined anti-CD4/CD8 antibody treatment further abrogated the histopathology of the disease.

CONCLUSION

These results show that both CD4+ and CD8+ T cells have a role in the pathogenesis of OAD and that natural killer cells and alloantibodies are not necessary for the development of this disease.

Recent advances in the immunology of chronic rejection

Chronic allograft rejection is a slowly progressive, insidious process in which the host immune system continues to mount an immunological attack on a transplanted organ, ultimately resulting in the failure of the graft. To varying degrees, all solid organ grafts are at risk for chronic rejection and undergo a stereotypic process of injury and inflammation, eventually leading to parenchymal fibrosis. The clinical consequences of chronic rejection are particularly apparent in thoracic transplantation, where both patient and graft survival decline steadily over time and the opportunities for re-transplantation or long-term extracorporeal support are limited. A variety of antigen-dependent and antigen-independent factors are known to modulate the propensity for an organ to undergo chronic rejection. Recent clinical and laboratory research has suggested that distinct immunologic mechanisms may underlie the process of chronic rejection. Ultimately, strategies to induce long-term tolerance to alloantigens will be necessary to prevent chronic rejection and to abrogate the deleterious sequelae of chronic immunosuppression.