Lung transplantation

Human lung transplantation was first performed in 1963 but few patients survived and the operation was largely abandoned until the advent of ciclosporin and the development of combined heart and lung transplantation nearly 20 years later. Today over 30 000 lung transplants have been performed and the procedure is firmly established amongst the therapeutic options available for patients with severe lung disease.¹ Donor shortage is the main factor limiting its application and this drives the search for new strategies to increase the number of lungs available for transplantation. These include the use of live donors, easing of donor selection criteria and ex vivo techniques to recondition lungs of marginal viability.2, 3About 2% of lung transplants are repeat operations, performed because of graft failure. Patients receiving lung transplants have ranged from infants to the elderly, with the peak age being about 50 years. The principal conditions treated by lung transplantation in adults have been chronic obstructive pulmonary disease/emphysema (36%), idiopathic pulmonary fibrosis (21%), cystic fibrosis (16%), α₁-antitrypsin deficiency (7%) and primary pulmonary hypertension (4%). The remainder include sarcoidosis, lymphangioleiomyomatosis, connective tissue disease and, rarely, lung cancer.⁴ The commonest indication for lung transplantation in adolescents is cystic fibrosis and in children congenital heart disease.⁵

Types of lung transplant

Combined heart and lung transplantation, which was first performed in 1981, was followed by single-lung transplantation, then double-lung transplantation, and lastly sequential bilateral lung transplantation. The combined operation requires total cardiopulmonary bypass and if successful carries a risk of accelerated coronary atheroma and problems resulting from cardiac denervation. However, it is relatively simple technically, maintains coronary–tracheobronchial arterial anastomoses that help the tracheal anastomosis to heal, and is particularly suitable when both heart and lungs are damaged, as in pulmonary hypertension. In cystic fibrosis, it is necessary to replace both lungs to avoid the risk of spillover infection. Double-lung transplantation is a complex procedure but was initially used in emphysema because it was feared that with single-lung transplantation the native diseased lung would be preferentially ventilated. This proved not to be the case and single-lung transplantation is now widely used for both severe emphysema and pulmonary fibrosis. It is the commonest procedure, the simplest to perform, is associated with the fewest postoperative complications, requires the least amount of donor tissue and enables the greatest number of recipients to benefit from a single donor.⁶

Except for bronchial artery revascularisation, which is undertaken in only a few centres, no attempt is made to reanastomose the severed tracheal or bronchial blood vessels and nerves in any of these operations, or the lymphatics, which are also severed if the heart is not included. Loss of these structures promotes postoperative haemorrhage, breakdown of the tracheal or bronchial anastomosis, a reduction in the cough reflex and pulmonary oedema. A further aspect of lung transplantation is that some lymphatic tissue is inevitably included in the allograft, entailing a risk of graft-versus-host disease. This is greatest when the whole mediastinum is transferred, as in combined heart and lung transplantation, but in practice it is a rare complication.

The mortality associated with lung transplantation is constantly diminishing as techniques and immunosuppression improve. In 2009 the International Society of Heart–Lung Transplantation reported survival rates of 79%, 52% and 29% at 1, 5 and 10 years respectively for lung transplantation and 64%, 41% and 26% at the same periods for combined heart–lung transplantation (Fig. 11.1 ).⁴ In the first postoperative month mortality is chiefly due to sepsis, haemorrhage and poor lung preservation. After the first month the principal causes of death are infection and rejection in the form of obliterative bronchiolitis.

Figure 11.1.

Adult lung transplantation: actuarial survival by diagnosis.⁴ CF, cystic fibrosis; COPD, chronic obstructive pulmonary disease; IPF, idiopathic pulmonary fibrosis; PH, pulmonary hypertension.

Recipient selection

Lung transplantation is an operation of last resort. There are insufficient donors and patients are unlikely to be considered unless other measures have failed and their short-term prognosis is otherwise poor. The presence of uncontrolled systemic disease precludes consideration and good renal and hepatic function is essential, particularly in view of immunosuppressant drug toxicity. This is particularly important in α₁-antitrypsin deficiency and cystic fibrosis, both of which may affect the liver directly. Any infection that cannot be eliminated, either before or by the operation, is likely to disseminate postoperatively because of the immunosuppression and therefore militates against successful transplantation. An aspergilloma is a contraindication to any form of lung transplantation as its attempted removal inevitably leads to seeding of the pleural cavity and mediastinum. Previous thoracic surgery may make it difficult to operate and the possibility of a future lung transplant should therefore be borne in mind when considering the best treatment for conditions such as pneumothorax. Left ventricular function should be normal or near normal, although treatable coronary artery disease is not an absolute contraindication. Ventilator support prior to transplantation is a significant risk factor.⁷ Pulmonary neoplasia is generally regarded as a contraindication because of its early dissemination but bronchioloalveolar carcinoma (now termed adenocarcinoma-in-situ) is often limited to the lungs and double-lung transplantation has been employed to treat this form of lung cancer.⁸ Contraindications of less importance include age over 65 years and body mass index over 30.

In the USA, priority is given to those patients with rapidly progressive conditions such as idiopathic pulmonary fibrosis and who are in most need of a lung transplant. This is achieved with a numerical scoring system that takes into account the probability and duration of survival without a transplant as compared to that following transplantation. The system, which was introduced in 2005, has resulted in increased numbers of patients with idiopathic pulmonary fibrosis receiving transplants and a decrease in both waiting-list mortality and waiting-list time.9, 10 However, a high score predicts increased morbidity and mortality following transplantation, reflecting the poor clinical condition of many of the patients transplanted.11, 12 Such a system has yet to be widely adopted: in the UK organs are currently allocated on the basis of proximity and time on the waiting list.

Donor matching

The selection and management of donors and preservation of the harvested lung are crucial to reducing the impact of brainstem death on the lung and minimising reimplantation injury. Brainstem death leads to adrenergic systemic hypertension, which results in fluid shifting to the lesser circulation and neurogenic pulmonary oedema. Such injury is part of a generalised release of cytokines and other inflammatory mediators that damage many organs, including the lungs, where it may compound the other causes of injury, so that only about 20% of donated cadaver lungs can be used.¹³

Current donor selection criteria include age less than 65 years, ABO compatability, a clear chest X-ray, no airway mucosal inflammation at bronchoscopy and no history of lung contusion, severe chest trauma, previous cardiopulmonary surgery or aspiration.¹⁴ Donor seropositivity for cytomegalovirus is dangerous in a seronegative recipient.⁷ Persistent hepatitis B and C antigenaemia and human immunodeficiency virus infection are further contraindications. However mild asthma and smoking do not adversely affect outcome.¹⁵ Physiotherapy, bronchial toilet, prophylactic antibiotics as required and monitoring of blood gases and pulmonary wedge pressure are all important in managing a potential donor. Harvested lungs are best stored inflated, cooled and flushed through the pulmonary artery with Euro-Collins solution. Ischaemia is tolerated for up to 8 hours.¹⁴ Donor–recipient matching includes an approximate similarity in chest size, although volume reduction surgery may circumvent this if there is a significant mismatch. One lobe of an adult's lung may be used to replace a whole lung of a child.¹⁶ Tissue transplanted into a child grows at a normal rate.¹⁷

Potential recipients are screened for donor-specific lymphocytoxic antibodies against a panel of HLA-A, -B and -DR antigens.^17a The result is considered positive if the recipient's serum reacts to more than 10% of the HLA antigens in the panel, in which case donor-specific T- and B-lymphocytotoxic antibody cross-matching is required. A positive result here contraindicates transplantation from that donor as it is associated with accelerated graft rejection.¹⁸

Immunosuppression

The recipient's immunity is suppressed with a combination of ciclosporin, azathiaprine and prednisolone with backup provided by immunosuppressive drugs such as tacrolimus, mycophenolate mofetil, everolimus and sirolimus. All these drugs have side-effects, notably renal, hepatic and bone marrow toxicity and an increased risk of neoplasia. Antithymocyte globulin is now seldom used and cyclophosphamide and plasmaphoresis are reserved for use in antibody-mediated rejection. Low-dose Septrin is administered prophylactically against Pneumocystis infection.

Recipient monitoring

Frequent monitoring of the lung transplant recipient is essential in the first 6 months after transplantation when the risk of the main complications, acute rejection and infection, is greatest. Recipients undergo frequent clinical review and bronchoscopy is undertaken in response to symptoms, radiographic changes, unexplained fever or a sudden deterioration in lung function. Both bronchoalveolar lavage and transbronchial biopsy are generally performed. Lavage is particularly useful for detecting infection while transbronchial lung biopsy, despite a false-negative rate of about 20%, is the mainstay in the diagnosis of allograft rejection. Some centres schedule their biopsies whereas others only biopsy as symptoms dictate. Rejection of the heart in combined heart and lung transplant recipients is rare and endomyocardial biopsies provide an unreliable guide to pulmonary rejection.

Role of the histopathologist

Histopathology plays a major role in assessing the postoperative complications of transplantation, but can contribute more than this to the management of transplant recipients, both before and after the operation (Box 11.1 ).19, 20 An accurate preoperative diagnosis is important because histopathological reassessment of the potential recipient may affect the choice of transplantation procedure and identify conditions treatable by other means or that need to be eradicated before transplantation is undertaken. It may also predict the presence of systemic disease and the likelihood of the original disease recurring in the allograft, or detect malignant disease, which apart from bronchioloalveolar carcinoma (now adenocarcinoma-in-situ) is an absolute contraindication to transplantation. Explanted lungs should also be examined, and without delay, in case they show unsuspected diseases such as tuberculosis, sarcoidosis or malignancy that may be active elsewhere in the recipient.²¹ A major discrepancy between the pretransplant diagnosis and that apparent on examining the explant has been reported in up to 10% of cases.²² Similarly, if only one donor lung is used, the other should be examined as this may identify unsuspected infection or other disease that is likely also to be present in the graft. Thus, histopathologists should not be concerned solely in assessing complications, but this is undoubtedly their major role and the rest of this chapter will be largely confined to this aspect of transplantation.

Box 11.1. The role of the histopathologist in lung transplantation.

• Diagnosis of the underlying disease before transplantation

• Examination of the explanted lung to verify the diagnosis and identify any other diseases

• Examination of any unused donor tissue

• Biopsy assessment of posttransplant complications

• Postmortem examination

As well as contributing to the outcome of the transplantation, the histopathologist is well placed to undertake collaborative research.

Complications

To assess complications, at least five pieces of alveolated lung should be examined at a minimum of three levels each, using connective tissue and fungal stains in addition to haematoxylin and eosin. Because sequential sampling is common, the possibility of observed abnormalities being due to previous biopsies always has to be considered. It is also important to remember that some patterns of lung injury are non-specific. For example, diffuse alveolar damage may reflect severe reperfusion injury, infection or acute rejection. It may require special stains for infective agents and a consideration of the timing of events to distinguish these causes. There may be both rejection and infection and as their histological features overlap the biopsy findings should always be interpreted in the light of the clinical and radiological features, previous biopsy findings and results of current microbiological and serological investigations. The principal complications are listed in Box 11.2 and will now be considered.

Box 11.2. Complications of lung transplantation.

• •
  Postoperative

  • –
    Haemorrhage and other surgical complications
  • –
    Reimplantation syndrome
  • –
    Diffuse alveolar damage
• •
  Early and late airway complications

  • –
    Anastomotic dehiscence, granulation tissue overgrowth
  • –
    Bronchial stenosis, chondromalacia, ectasia
• •Rejection
• •Infection
• •Neoplasia
• •Posttransplantation lymphoproliferative disorders
• •Graft-versus-host disease
• •Recurrence of the underlying disease
• •Drug effects

Perioperative allograft injury

Even if the operation goes well and there are no surgical complications, the early postoperative period is often marked by a temporary period of dyspnoea. Chest radiographs at this time often show hilar opacification and if biopsy is undertaken this generally shows pulmonary oedema accompanied by a mild neutrophil exudate (Fig. 11.2 ).²³ These changes, termed reimplantation injury, are variously attributed to deterioration of the graft, surgical trauma, ischaemia, severance of the pulmonary lymphatics and the release of free radicals and chemokines from neutrophils interacting with the graft endothelium damaged by ischaemia.²⁴ They usually appear within 48 hours and peak at day 4, slowly settling as lymphatics regenerate and lung drainage is re-established.

Figure 11.2.

Reimplantation injury presenting as (A) hazy shadowing of left donor lung 3 days after single-lung transplantation for emphysema; (B) biopsy shows focal interstitial oedema, subacute inflammation, pneumocyte hyperplasia and aggregates of alveolar macrophages.

More severe changes include those of diffuse alveolar damage, to which the term ‘primary graft failure’ has been applied.²⁵ This may represent a reimplantation response, rejection or infection or occur for no obvious cause. The risk factors include those relating to the donor (advanced age, smoking history, prolonged ventilation and duration of ischaemia), the recipient (transplantation for pulmonary hypertension and pulmonary fibrosis) and the operation (cardiopulmonary bypass and excessive use of blood products).²⁶ Primary graft failure may progress to parenchymal fibrosis. It is associated with reduced survival at 1 year.²⁷

Airway infection promoted by slow recovery of mucociliary clearance may also complicate the early postoperative period,28, 29 while alveolar proteinosis and an unusual neutrophil-rich pattern of mixed interstitial pneumonitis have also been described.30, 31

Complications involving the airway anastomosis

Apart from the combined heart–lung procedure, lung transplantation inevitably involves severance of the tracheobronchial blood supply so that the donor airway receives only retrograde pulmonary arterial blood until new bronchial arterial anastomoses develop. It is not surprising therefore that poor healing at the airway anastomosis has been a serious complication, although bronchial artery revascularisation, performed at a few centres at the time of transplantation, have rendered it less common.32, 33 The anastomosis is also subject to infection until the surface epithelium heals and mucociliary clearance recovers.²⁹ Denervation contributes to the risk of infection by eliminating the cough reflex and promoting aspiration.³⁴ Dehiscence of the airway anastomosis is a devastating early complication, but one that is fortunately now rare. More common is the development of exuberant granulation tissue. This may seriously narrow the airway and require endoscopic removal or cryotherapy.³⁵ Late airway complications include stenosis and bronchomalacia (Figure 11.3, Figure 11.4 ), requiring the insertion of stents.³⁶

Figure 11.3.

Tracheal stenosis following double-lung transplantation.

Figure 11.4.

Bronchomalacia. There is erosion of the limiting plate of the bronchial cartilage.

Complications involving the vascular anastomosis

Postoperative obstruction of the pulmonary artery anastomosis is rare but kinking may occur when the vessels are long. This usually develops in the first week. It may be diagnosed by ventilation/perfusion scan or arteriography. Venous obstruction may also develop early after transplantation, especially when size mismatch leads to torsion of a small donor lung on its vascular pedicle. The result may be thrombosis of one or more of the pulmonary veins with infarction of the corresponding lobe (Fig. 11.5 ).37, 38, 39 Systemic embolisation may also occur if the thrombus extends into the left atrium and fragments. Diagnosis is by transoesophageal echocardiography.38, 40 Early recognition of this potentially catastrophic complication is essential if early surgical intervention with salvage of the graft (and patient) is to be achieved.

Figure 11.5.

(A) Thrombosis of pulmonary vein to left lower lobe following torsion of the vascular pedicle resulting in (B) infarction of the entire lower lobe.

Rejection

The mechanisms underlying lung allograft rejection are not fully understood but the process is probably initiated by T-cell recognition of histocompatibility antigens on the surface of donor cells.⁴¹ The brunt of the rejection damage is on the blood vessels and airways, presumably reflecting the increased expression of histocompatibility antigens by the pulmonary vascular endothelium and airway epithelium that has been shown to follow transplantation.⁴¹ Incompatibility is followed by the activation and sequestration of platelets, neutrophils and macrophages within the pulmonary capillaries, the release of reactive oxygen species and inflammatory cytokines, and increased expression of endothelin and inducible nitric oxide synthase.42, 43, 44 The rejection process probably involves both antibody and cell-mediated immune mechanisms but the extent to which these each contribute varies: hyperacute rejection is generally regarded as being antibody-mediated whereas acute and chronic rejection are thought to be predominantly cell-mediated.

Hyperacute allograft rejection

Hyperacute rejection occurs within 48 hours of transplantation and is caused by the reaction of preformed antibodies to donor endothelial cells. It is characterised by marked congestion and oedema, resulting in the production of copious frothy blood-stained fluid from the bronchial orifice of the allograft. Risk factors include multiple blood transfusions, pregnancy, surgery and previous transplantation, any of which necessitates donor-specific pretransplant T- and B-lymphocytotoxic antibody cross-matching (see above).

Microscopy shows marked pulmonary congestion and oedema, alveolar haemorrhage, vascular thrombosis, neutrophil infiltration, endothelial and epithelial damage, and ultimately diffuse alveolar damage (Fig. 11.6 ). The diagnosis is confirmed by the detection by immunofluorescence of C4d complement split product (as a marker of complement activation) and IgG deposition on endothelium, along vessel walls and in alveolar spaces, and by a strongly positive IgG-mediated lymphocytotoxic reaction to donor T and/or B lymphocytes.46, 47

Figure 11.6.

Hyperacute rejection. Neutrophils engorge the blood vessels and have passed into the alveolar interstitium and lumen. The patient died during the operation. Panel-reactive antibodies were positive and a donor-specific lymphocytotoxic cross-match was strongly positive for T and B lymphocytes.

Acute allograft rejection

Acute rejection is characterised clinically by dyspnoea, fever, hypoxaemia and pleuropulmonary opacification (Fig. 11.7 ). It is most frequent in the first postoperative year, typically developing within a few months of the operation but sometimes within a few weeks or several years later.

Figure 11.7.

Chest radiograph in acute rejection showing opacification of the lower zone of the right lung, which was transplanted because of emphysema.

Acute and chronic rejection are classified and graded according to internationally agreed histological criteria (Box 11.3 ).⁴⁸ Acute rejection may be centred on the blood vessels or the airways but predominantly affects the former and the surrounding lung parenchyma. The vascular changes (class A) are treated separately from those involving the airways (class B) because the latter may also reflect chronic infection, ischaemia, aspiration or chronic rejection.28, 34, 49, 50

Box 11.3. Revised working formulation for the classification and grading of pulmonary allograft rejection⁴⁸.

• A
  Acute rejection

  • Grade 0 None
  • 1 Minimal
  • 2 Mild
  • 3 Moderate
  • 4 Severe
• B
  Airway inflammation – lymphocytic bronchitis/bronchiolitis

  • Grade 0 None
  • 1R Low-grade
  • 2R High-grade
  • BX Ungradable (sampling problems, infection, tangential sectioning)
• C
  Chronic airway rejection – bronchiolitis obliterans

  • 0 Absent
  • 1 Present
• DChronic vascular rejection – accelerated graft vascular sclerosis

A and B may coexist, as may C and D

Histologically, the vascular changes are characterised by an infiltrate of variable intensity comprising small lymphocytes, plasma cells, histiocytes and occasional neutrophils surrounding small blood vessels and infiltrating the adjacent alveolar interstitium. They are graded as minimal (grade A1) if they can hardly be discerned without high magnification and mild (grade A2) if they are just evident at low magnification and consist of an infiltrate that includes large activated lymphocytes with pyroninophilic cytoplasm and angulated nuclei. Neutrophils and eosinophils are also seen and there is infiltration of the vascular intima (lymphocytic intimitis or endothelialitis). Endothelial cells show hyperplasia. Moderate acute rejection (grade A3) is characterised by extension of the infiltrate into the alveolar interstitium, while in severe (grade A4) acute rejection, which is usually fatal, the infiltrate is widespread and accompanied by haemorrhagic oedema, increased numbers of alveolar macrophages, fibrinous exudates, hyaline membranes and destructive changes typical of diffuse alveolar damage (Fig. 11.8 ). Grade A3 and A4 rejection may also be associated with organising pneumonia.49, 51, 52, 53 With successful suppression of the rejection, follow-up biopsies show a reduction in the infiltrate (termed resolving rejection/lower grade) and a change in its makeup to a mixture of small lymphocytes and haemosiderin-laden macrophages (termed resolved rejection or grade A0).⁵⁴

Figure 11.8.

Acute parenchymal rejection. (A) Grade A1 (minimal) changes are not discernible at low magnification but high power shows a sparse perivascular lymphoid infiltrate. (B) Grade A2 (mild) shows a more marked infiltrate but this is still restricted to the vessels and the alveolar septa are spared. (C) Grade A3 (moderate) acute rejection showing spread of the infiltrate into surrounding alveolar septa. (D) Grade A4 (severe) acute rejection is characterised by diffuse alveolar damage and a dense perivascular lymphoid infiltrate.

Immunohistochemistry shows that the majority of the lymphoid cells are CD8-suppressor lymphocytes (Fig. 11.9 ).⁵⁵ They are often pyroninophilic and may express the lymphocyte activation antigen CD30 and the proliferation antigen Ki-67. B lymphocytes are usually sparse; larger numbers may reflect rejection based on humoral rather than cellular mechanisms and therefore predict a poor response to the usual cell-based immunosuppressive regimes.⁵⁶ However, their presence should also prompt consideration of other entities such as eosinophilic pneumonia, fungal infection and lymphoproliferative disease.57, 58 The vascular endothelium and alveolar epithelium both show upregulation of class II HLA (HLA-DR) antigens.⁴¹

Figure 11.9.

Immunohistology of acute lung allograft rejection. The perivascular infiltrate stained by haematoxylin and eosin in (A) expresses the T-cell marker CD3 (B) and the proliferation marker Kiel-67 (C) while class II HLA-DR antigens are strongly expressed by alveolar epithelium and macrophages (D).

Although perivascular lymphoid cell infiltration is the hallmark of acute rejection, similar infiltrates may be seen in patients with infections and other conditions for which augmented immunosuppression is inappropriate. For this reason acute rejection should only be diagnosed and graded in the absence of infection.⁴⁸ The opportunistic infections may be viral, particularly herpes simplex and cytomegalovirus, the distinctive inclusions of which may be modified by prior antiviral treatment and therefore difficult to identify (Fig. 11.10 ).⁵⁹ However, the infiltrates of a viral infection are generally more extensive and the pattern is predominantly that of an interstitial pneumonitis with secondary involvement of vessels.⁵⁹ Perivascular lymphoid infiltrates may also be seen in Pneumocystis jirovecii pneumonia.⁶⁰ and in other fungal and bacterial infections, necessitating special stains, and in difficult cases immunocytochemistry and in situ hybridisation.61, 62 Eosinophilic pneumonia is an uncommon manifestation of graft rejection and, before accepting it as such, infection by organisms such as Aspergillus should be excluded.63, 64 A predominantly neutrophilic infiltrate, necrosis and granuloma formation all favour infection rather than rejection.

Figure 11.10.

Fragmented cytomegalovirus inclusions after treatment with ganciclovir.

Acute rejection should also be distinguished from the ischaemia–reperfusion injury described above and from the lymphoproliferative disorders described below.⁵⁸ Ischaemia–reperfusion injury lacks the perivascular and interstitial infiltration of rejection while a polymorphous, perivascular infiltrate of B and T lymphocytes with a predominance of the latter favours rejection rather than lymphoproliferative disease, which is characterised by a monomorphous infiltrate of large B lymphocytes.⁵⁷ Mass lesions also favour lymphoproliferative disease or infection rather than rejection. Recurrence in the allograft of diseases such as sarcoidosis and Langerhans cell histiocytosis may also be misinterpreted as acute rejection if their characteristic features are not well represented.

Airway inflammation in the form of lymphocytic bronchiolitis is the other major pattern of acute rejection. Evidence that it represents rejection includes its progression to chronic rejection (bronchiolitis obliterans) and frequent response to augmented immunosuppression.⁶⁵ It may accompany or succeed the perivascular infiltrates described above or be seen alone, but it most frequently follows the vascular changes.28, 50 Airway inflammation may be graded in the same way as the perivascular infiltration but in practice the small size of the biopsies generally precludes the precision required for this. The changes range from sparse airway cuffing by small lymphoid cells (grade B1R) to diffuse infiltration of the lamina propria and epithelium by medium and large lymphoid cells and epithelial apoptosis (grade B2R) (Fig. 11.11 ). In severe cases the lymphoid infiltrate is particularly dense and there is ulceration and fibrinopurulent exudation. The importance of large-airway inflammation in rejection is unclear as it is more often secondary to infection. Grading of airway inflammation is therefore currently restricted to the bronchioles.

Figure 11.11.

Acute airway rejection (lymphocytic bronchiolitis). (A) Grade B1 acute rejection is characterised by occasional small lymphocytes infiltrating the bronchiolar epithelium and lamina propria. (B) Grade B3 acute airway rejection shows extensive infiltration and obliteration of bronchiolar epithelium by lymphocytes.

The differential diagnosis of airway inflammation includes the presence of bronchus-associated lymphoid tissue of donor origin, low-grade infection and the consequences of aspiration. Bronchus-associated lymphoid tissue is distinguished by it comprising aggregates of B lymphocytes, sometimes with admixed anthracotic macrophages, in contrast to the T-cell-rich infiltrates of rejection. Prominent neutrophil or eosinophil infiltration is suggestive of infection or aspiration. However, it is important to note that airway or parenchymal infection, notably by cytomegalovirus, may accompany rejection.⁶⁶

Chronic allograft rejection

Chronic rejection of the transplanted lung is marked by progressive breathlessness, cough, which is often productive, fever and a decline in lung function.⁶⁷ The changes, which can be monitored by spirometry and imaging (Fig. 11.12 ), are centred on the bronchioles (class C) and blood vessels (class D).

Figure 11.12.

Computed tomography scan appearances in bronchiolitis obliterans syndrome. (A) The inspiratory image shows mild generalised hyperinflation with bronchial dilatation and wall thickening while (B) the expiratory image shows irregular dark areas of focal air-trapping.

The histological features are those of constrictive bronchiolitis obliterans. Typically there is concentric or eccentric hyaline fibrous thickening of the bronchiolar submucosa, encroaching on the airway lumen and eventually resulting in total occlusion (Fig. 11.13 ).68, 69, 70, 71 The changes may be subtle, in which case an elastin stain can be invaluable in identifying the fibrosed bronchioles. Residual bronchiolar epithelium may show squamous metaplasia and there is usually disruption of the muscle coat. The process may be active (i.e. associated with lymphocytic bronchiolitis) or inactive (consisting of dense fibrous scarring with minimal or no inflammation), although this is of no prognostic significance and does not predict response to treatment. The infiltrating cells are T lymphocytes.⁷² The process is patchy and therefore not always evident in small biopsies,73, 74 but its presence may be suggested by obstructive features such as the accumulation of foamy macrophages in distal air spaces. In the current classification constrictive bronchiolitis is simply recorded as being present (C1) or absent (C0).

Figure 11.13.

The histological spectrum of chronic airway rejection (bronchiolitis obliterans). (A) Partial occlusion of a bronchiole by plaque-like submucosal fibrous tissue with sparse chronic inflammation. (B) An incomplete ring of smooth muscle provides the only evidence that this focal pulmonary scar represents an obliterated bronchiole.

The recognition of bronchiolitis obliterans is the key discriminator between acute and chronic rejection. The adjacent lung may also show evidence of concomitant acute rejection (Fig. 11.14 ). Most patients with bronchiolitis obliterans also have bronchiectasis (Fig. 11.15 ), which probably results from a combination of factors, including rejection, infection and denervation. Concomitant acute airway inflammation suggests postobstructive infection.

Figure 11.14.

Concomitant acute and chronic rejection. Acute rejection is evidenced by a perivascular lymphoid infiltrate while a bronchiole totally obstructed by fibrous tissue is indicative of chronic rejection.

Figure 11.15.

Radiological and macroscopic appearances of chronic airway rejection complicated by bronchiectasis.

(Courtesy of Dr H Tazelaar, Rochester, Minnesota, USA.)

Unfortunately, sampling error renders the diagnosis of bronchiolitis obliterans unreliable on small biopsies and its identification generally depends upon recognition of a bronchiolitis obliterans syndrome. This is defined as ‘graft deterioration due to progressive airways disease for which there is no other cause’.75, 76 The principal role of transbronchial lung biopsy is therefore to exclude treatable causes of deterioration in lung function.

Bronchiolitis obliterans is seldom seen within 6 months of the operation and, while its incidence diminishes after 1–2 years, it affects 50% of recipients by 5 years.⁷³ Risk factors include earlier rejection episodes and histocompatibility mismatch,57, 70, 77, 78 supporting the view that it represents a late manifestation of rejection50, 79 with an immunological pathogenesis analogous to that occurring in rheumatoid and graft-versus-host disease (see p. 121).80, 81, 82, 83 The development of de novo anti-HLA antibodies following transplantation is also associated with an increased incidence of bronchiolitis obliterans, suggesting that humoral rejection may play a role.84, 85 Gastro-oesophageal reflux and chronic airway infection may also contribute to the development of obliterative bronchiolitis.86, 87, 88 Other causes include cytomegalovirus infection,28, 59, 77, 89, 90 community-acquired viral infection91, 92 and ischaemia.⁹³ Finally, it has been suggested that some early cases, characterised by acute inflammation on biopsy and lavage (in the absence of infection), respond well to azithromycin therapy (so-called neutrophilic reversible allograft dysfunction).94, 95, 96

The pathogenesis of the obliterative fibrosis has yet to be fully elucidated, but it is intriguing that some of the cells that participate in the fibrosis are derived from circulating fibroblast precursors of recipient bone marrow origin.⁹⁷ There is also evidence suggesting a role for epithelial–mesenchymal transition in the sclerosing process.98, 99

Chronic rejection may also involve the pulmonary vasculature, resulting in fibrointimal sclerosis of arteries and veins analogous to that occurring in the coronary arteries of heart and heart–lung allografts (Fig. 11.16 ).100, 101, 102, 103 This uncommon complication may be active or inactive and may be seen in association with obliterative bronchiolitis, which would then dominate the clinical picture. Sclerosis of small vessels may also represent chronic rejection but is generally disregarded as it may also follow ischaemia, acute rejection, non-rejection-related pulmonary inflammation and non-specific donor-related factors.

Figure 11.16.

Chronic vascular rejection. (a) Active chronic rejection affecting a muscular artery. (b) A pulmonary artery showing severe intimal fibrosis, the appearances of inactive alveolar infection.

Antibody-mediated rejection

Hyperacute rejection is the archetypal form of antibody-mediated rejection, in which preformed antibodies cross-react with graft antigens resulting in fulminant rejection. However, it is known from renal and cardiac transplantation that a less acute form of antibody-mediated rejection, may occur. The criteria for diagnosing this include the presence of de novo donor-specific antibodies, clinical allograft dysfunction, histological features of antibody-mediated rejection and the deposition of complement protein C4d, a stable biproduct of complement C4d activation, on capillary endothelium.^104,104a The role of antibody-mediated rejection in lung allograft rejection is far from clear but there are reports of lung transplant patients with many of the features of antibody-mediated rejection being treated successfully with plasmapheresis and intravenous immunoglobulin.105, 106, 107, 108, 109, 110, 111 While it has been suggested that capillaritis may represent antibody-mediated rejection,109, 112, 113 there is currently no consensus on what histological features define the condition in the lung.¹¹⁴ Furthermore C4d deposition may be seen in lungs with primary graft failure and infection,115, 116 limiting its use as a marker of rejection remains to be elucidated. Recent work has revealed complex interactions between components of the complement system, platelets and endothelial cells as well as evidence suggesting an influence of complement on T and B lymphocyte function.104a, 114 Endothelial activation appears to play a central role in the development of antibody-mediated rejection and further work may provide more specific and sensitive markers of this serious complication of lung transplantation humoral rejection.

Upper-lobe fibrosis

Progressive upper-lobe fibrosis has occasionally been described after lung transplantation. Imaging shows interlobular septal thickening and ground-glass change, progressing to traction bronchiectasis and honeycombing. Biopsy shows non-specific inflammation and fibrosis.¹¹⁷ Further research is required to determine whether this provisional clinicopathological entity is related to rejection or other factors.

Infection

Infection is a major hazard for the immunosuppressed recipient of a lung allograft.¹¹⁸ It is often multiple and may affect the native lung and other organs as well as the allograft. Its recognition in the lungs largely depends upon the microbiological examination of bronchoalveolar lavage fluid but the addition of transbronchial lung biopsy increases the detection rate.¹¹⁹ Problems in biopsy interpretation include distinguishing rejection from infection, which is compounded by the atypical host response of the immunosuppressed patient. Distinguishing colonisation, subclinical infection and clinically significant disease may also be difficult. The pathology of pulmonary infection is described in Chapter 5 but the special circumstances associated with transplantation warrant further comment on some infective agents here.

Viruses

Cytomegalovirus infection of a seronegative recipient, transmitted by blood transfusion or the graft, increases the incidence of rejection, probably by promoting the expression of the major histocompatibility antigens in the alveolar epithelium.41, 120, 121 The recognition of clinically significant disease (as opposed to mere carriage of the virus) is based upon identification of blood cytomegalovirus pp65 antigen,122, 123 biopsy evidence of interstitial pneumonitis and bronchiolitis¹²⁴ (Fig. 11.17 ) and the detection of the virus. The latter often requires immunohistochemistry or molecular techniques such as the polymerase chain reaction because in the early stages of infection typical viral inclusions are often sparse (Fig. 11.18 ). Only in the later stages are there numerous intranuclear and cytoplasmic inclusions (Fig. 11.19 ). Other patterns of cytomegalovirus disease include poorly formed granulomas, diffuse alveolar damage and mass lesions simulating a tumour.¹²⁵ Viral prophylaxis may result in fragmented inclusions and an acute neutrophilic pneumonitis (see Fig. 11.10).

Figure 11.17.

Cytomegalovirus antigenaemia. Positive immunohistochemical staining of peripheral blood leukocytes for cytomegalovirus pp65 antigen.

Figure 11.18.

Early cytomegalovirus pneumonitis. (A) Focal acute interstitial pneumonitis with several possible cytomegaloviral inclusions, (B) confirmed by immunohistochemistry.

Figure 11.19.

Severe cytomegalovirus pneumonitis in a cytomegalovirus-negative recipient of a cytomegalovirus-positive lung.

Infection by herpes simplex virus is infrequent in lung allograft recipients but may cause necrotising tracheobronchitis and pneumonia in these patients.¹²⁶ Similarly, infection by influenza and respiratory syncytial viruses may cause significant morbidity and contribute to the development of bronchiolitis obliterans.92, 127 Other viral infections of note in transplant patients include pulmonary and systemic infection by varicella-zoster (Fig. 11.20 ).

Figure 11.20.

Fatal chickenpox pneumonitis. Foci of necrosis are surrounded by extensive alveolar haemorrhage.

Community-acquired respiratory viruses such as rhinovirus, enterovirus, coronavirus, respiratory syncytial virus, parainfluenza virus, influenza A and B and adenovirus infect up to 57% of lung transplant recipients. The severity varies from mild upper respiratory tract disease to severe pneumonia complicated by bacterial or fungal superinfection. Some studies have linked such infection with the onset of acute and chronic rejection. Features on biopsy are often of a non-specific interstitial pneumonitis. Occasionally, a specific viral cytopathic effect may be seen on biopsy, such as with adenovirus (see p. 161), but usually viral culture is required to identify the organism. Molecular techniques applied to lavage material greatly enhance the sensitivity and specificity of viral diagnosis.

Bacteria

Opportunistic mycobacteria originating in either the donor or recipient infect lung allograft recipients on rare occasions, or merely colonise the graft without causing disease.128, 129, 130 Other bacterial infections of these patients include nocardiosis and Legionella pneumonia.

Fungi

Pneumocystis jirovecii pneumonia is rare in lung transplantation because of chemoprophylaxis; the reported incidence is less than 1%.²⁰ It mostly follows the immunosuppression being increased because of acute rejection.¹³¹ The histological patterns include a predominantly lymphoplasmacytic interstitial pneumonitis with scanty exudate, and a granulomatous pneumonitis. The fungal cysts are sparse and the classic foamy alveolar exudate is rarely encountered. The perivascular component of the lymphoplasmacytic pattern resembles that of acute rejection, hence the mandatory use of silver staining techniques for all transbronchial lung allograft biopsies and the routine screening of all bronchoalveolar lavage specimens using immunocytochemistry or the polymerase chain reaction.¹³²

Other fungi that infect lung allograft patients include Candida and Aspergillus. These agents may merely colonise the airways¹³³ or cause ulcerative tracheobronchitis, dehiscence of the anastomosis, bronchocentric granulomatosis, cavitating pneumonia, mediastinitis and multiple haematogenous abscesses.134, 135, 136, 137

Toxoplasmosis

Infection of the lung allograft by Toxoplasma gondii is rare, having been largely eliminated in seronegative solid-organ transplant recipients given a positive organ by prophylaxis with pyrimethamine. It usually occurs as part of systemic infection following graft mismatch.¹³⁸ Identification of the organism in biopsy material may be difficult as the cysts of T. gondii are sparse and extracellular tachyzoites may be mistaken for haematoxyphil debris (see Fig. 5.5.1, p. 252). The diagnosis may be confirmed by immunohistochemistry or by the polymerase chain reaction applied to tissue, body fluids or peripheral blood.¹³⁹

Aspiration

Gastro-oesophageal reflux and aspiration are fairly common following lung transplantation and are being increasingly recognised as risk factors for the development of obliterative bronchiolitis through repeated episodes of epithelial injury.86, 140, 141, 142 The histological features are generally those of non-specific active chronic inflammation, although a foreign-body giant cell reaction to aspirated material occasionally allows a definitive diagnosis (see Fig. 5.2.16, p. 190). Bronchoalveolar lavage studies suggest that it is the aspiration of bile salts rather than gastric acid that is related to the subsequent development of obliterative bronchiolitis.⁸⁸

Neoplasia

After rejection, neoplasia is the most significant factor limiting long-term survival in solid-organ transplantation.7, 143 It ranges in incidence from 6% to 11%.144, 145 The tumours may arise de novo or be inadvertently introduced within the allograft. They are often particularly aggressive. Predisposing factors in addition to immunosuppression include ultraviolet irradiation and activation of oncogenic viruses such as Epstein–Barr virus, papillomavirus and herpesvirus. Molecular techniques have shown that those tumours apparently arising de novo may be of donor rather than recipient origin.¹⁴⁶

The most frequently encountered de novo tumours include the posttransplantation lymphoproliferative disorders dealt with see and cutaneous tumours, notably squamous carcinoma, preceded by premalignant skin lesions, which are often multiple. The carcinomas frequently metastasise and may contain papillomavirus.143, 147 Intraepithelial neoplasia and squamous carcinomas of the uterine cervix, vulva and perineum, also associated with human papillomavirus infection, may occur. Other tumours reputed to occur with greater frequency than in the general population include carcinomas of the kidney,¹⁴⁸ hepatobiliary tract and lung,118, 149 the last of these often presenting at an advanced stage and being of donor origin.¹⁴⁶ Cytological specimens of the bronchi often show epithelial atypia but this is more often reactive than malignant.¹⁵⁰

Other tumours seen more commonly in lung transplant recipients than the general population include Kaposi's sarcoma¹⁵¹ and low-grade leiomyosarcoma.152, 153 Between 3% and 7% of single-lung transplant recipients develop bronchogenic carcinoma within their native lung. This may reflect the large number of transplants performed for smoking-related diseases such as emphysema and idiopathic pulmonary fibrosis.¹⁵⁴ The role of immunosuppression in the development of these tumours is unclear but it is notable that they behave aggressively.

Posttransplantation lymphoproliferative disorders

A spectrum of lymphoproliferative disorders may complicate transplantation of many organs, including the lungs.155, 156, 157, 158 Immunosuppression would appear to underlie this escape from normal control as similar disease is seen in other conditions characterised by profound immunosuppression – the acquired immunodeficiency syndrome (AIDS), for example. Epstein–Barr virus is suspected of playing a part in the causation of these lesions, having been identified in many of them by immunostaining and the application of molecular probes.159, 160, 161 The incidence of lymphoproliferative disease is higher in recipients who are sero-negative for Epstein–Barr virus before the operation,¹⁶² suggesting that the viral infection is primary rather than representing reinfection or reactivation of latent disease.¹⁵⁹ The virus is detectable more often with early than late disease. A notable feature is that the lesions are potentially reversible once immunosuppression is reduced.157, 163

Posttransplantation lymphoproliferative disorders have been described in association with virtually all currently used immunosuppressive agents and are thought to result from uncontrolled proliferation of Epstein–Barr virus-immortalised B cells due to loss of cytotoxic T-cell control.160, 164 As with most solid-organ transplants, the lymphoproliferation usually involves cells of recipient origin, in contrast to bone marrow transplantation where the lymphoproliferative disorders generally involve cells of donor origin.165, 166, 167, 168, 169, 170 Nevertheless they often first present in the allograft itself.

Risk factors include the type of transplant, young age, multiple rejection episodes, multiagent immunosuppression, anti-T-cell therapy and pretransplant Epstein–Barr virus sero-negativity.162, 166 The interval between lung transplantation and the development of the lymphoproliferative process is generally short (median time 7 months), as is the median time to death (5 months).¹⁶¹

The incidence of posttransplantation lymphoproliferative disorders ranges from about 1% with bone marrow and kidney transplants to nearly 10% with lung and heart–lung and 17% with intestine.¹⁶⁷ This is thought to reflect the relatively higher levels of immunosuppression required for lung and intestinal transplantation.

Posttransplantation lymphoproliferative disease may involve the lung in isolation or as part of disseminated disease.158, 159, 161 The lungs may also be involved by posttransplantation lymphoproliferative disease in recipients of other solid-organ transplants.171, 172 The disease may be asymptomatic or cause cough, fever and malaise. Radiographs may show diffuse infiltrates or single or multiple nodules. High serum levels of Epstein–Barr virus DNA predict the development of posttransplant lymphoproliferative disease. Tissue should be examined for Epstein–Barr virus by immunocytochemistry or in situ hybridisation, cell phenotype and immunoglobulin clonality, and the lesion classified according to the scheme advocated by the World Health Organization ( Box 11.4 ).¹⁷³ Clinical suspicion of posttransplantation lymphoproliferative disease should be conveyed to the pathologist so that some tissue can be snap-frozen for appropriate molecular studies. Synchronous and metachronous lesions should all be investigated as variations in clonality and morphology occur both within an individual lesion and between simultaneous or subsequent lesions. Difficulties encountered in diagnosis, especially on core biopsies, stem from the small size of the sample, crush artefact and extensive necrosis caused by angioinvasion. The differential diagnosis includes infections such as cytomegalovirus and Pneumocystis, inflammation caused by previous biopsies, the presence of bronchus-associated lymphoid tissue and, in the transplanted lung, acute allograft rejection and, rarely, graft-versus-host disease.

Box 11.4. World Health Organization classification of posttransplantation lymphoproliferative disorders (PTLD)¹⁷³.

• 1
  Early lesions

  • • Reactive plasmacytic hyperplasia
    • Infectious mononucleosis-like lesion
• 2
  PTLD – polymorphic

  • • Polyclonal (rare)
    • Monoclonal
• 3
  PTLD – monomorphic (classify according to lymphoma they resemble)

  • B-cell neoplasms

    • Diffuse large B-cell lymphoma
    • Burkitt lymphoma
    • Plasma cell myeloma
    • Plasmacytoma-like lesion
    • Other
  • T-cell neoplasms

    • Peripheral T-cell lymphoma, not otherwise categorised
    • Hepatosplenic T-cell lymphoma
    • Other
• 4Classic Hodgkin lymphoma-type PTLD

The chief pulmonary manifestations of posttransplantation lymphoproliferative disease are lymphoid hyperplasia, lymphoid interstitial pneumonia and the full spectrum of lymphomas,159, 173 all of which are described in Chapter 12.4. They predominantly involve B lymphocytes whereas T lymphocytes predominate in the infiltrates that characterise rejection.¹⁵⁸ High-grade lymphomas often show angioinvasion and extensive necrosis, thereby resembling lymphomatoid granulomatosis. Early lesions may be rich in mature plasma cells or show changes suggestive of infectious mononucleosis. Polymorphous infiltrates often include plasmacytoid cells (Fig. 11.21 ). Monomorphous infiltrates may consist of transformed blasts, centroblast-like, centrocyte-like cells or immunoblasts showing a high proliferation index (Fig. 11.22 ). Single-cell necrosis is common. Histiocytes and T lymphocytes often cuff the lesion and infiltrate the bronchiolar epithelium while the uninvolved lung may show patchy organising pneumonia. Immunoglobulin clonality is variable. Early lesions tend to be polyclonal. Monomorphous lesions are usually monoclonal while polymorphous lesions may be either polyclonal or monoclonal.167, 174

Figure 11.21.

Pulmonary posttransplantation lymphoproliferative disorder (polymorphous, lymphoplasmacytoid). (A) Plasma cells, plasmacytoid cells and medium-sized lymphocytes surround an alveolar duct. (B) CD79a-positivity shows that the infiltrate is of B-cell origin.

Figure 11.22.

Pulmonary posttransplantation lymphoproliferative disorder (monomorphous, large B-cell lymphoma). (A) Large pleomorphic lymphoid cells, some with immunoblastic features, show angiocentricity. (B) Some of the lymphoid cells infiltrating the vessel wall show immunoblastic features. (C) CD20-positivity shows that the infiltrate is of B-cell origin. (D) In situ hybridisation for Epstein–Barr virus-encoded RNA shows strong nuclear positivity.

(D courtesy of Dr JA Thomas, London, UK.)

A reduction of immunosuppression and treatment with antiviral agents usually leads to rapid resolution¹⁵⁹ but relapse is frequent and may require radiation treatment or chemotherapy. Newer, less toxic treatments such as cytotoxic T-lymphocyte infusions, interferon-α and antibodies to B cells (anti-CD20; Rituximab) are now available.175, 176, 177

Graft-versus-host disease

The transplanted lung or heart–lung block contains a significant amount of lymphoid tissue and on rare occasions a rash, colitis, pancytopenia and liver dysfunction may suggest that graft-versus-host disease has developed.178, 179, 180, 181 The identification of donor lymphocytes in the blood and bronchoalveolar lavage fluid of such patients supports this possibility. Graft-versus-host disease tends to develop in the early postoperative period. It may be severe but is fortunately rare and usually responds to increased immunosuppression. The graft-versus-host disease of bone marrow transplantation may cause constrictive obliterative bronchiolitis that is indistinguishable from that seen in lung rejection81, 82, 182, 183 but of course in lung transplantation the donor lung is spared this complication.

Biopsy diagnosis is generally dependent upon the histological appearances in the skin rather than the lung. The diagnosis may be confirmed by demonstrating donor and recipient chimerism on HLA typing of peripheral blood lymphocytes and bronchoalveolar lavage fluid.

Recurrent disease in the allograft

The systemic effects of underlying diseases such as cystic fibrosis may have a significant impact on recovery but only recurrence in the allograft will be considered here.

The aetiology of many diseases treated by lung transplantation is imperfectly understood but our knowledge of them has been broadened by their behaviour following transplantation. Some of these diseases are prone to recur in the new lungs whereas others are not. Of particular interest is the finding that lungs transplanted into patients with cystic fibrosis do not appear to develop the basic defect of membrane transport that underlies this disease.184, 185 It is also notable that whereas asthma appears to be cured by lung transplantation, it may also be transferred by transplanting the lungs of asthmatic donors to non-atopic patients.¹⁸⁶ Sarcoidosis has recurred in the graft in up to 60% of cases187, 188, 189 but is seldom of clinical significance.¹⁸⁹ Other diseases that have recurred in the graft include emphysema in an α₁-antitrypsin-deficient recipient,¹⁹⁰ the giant cell interstitial pneumonia of hard-metal workers,¹⁹¹ desquamative interstitial pneumonia,192, 193 panbronchiolitis,¹⁹⁴ alveolar proteinosis,¹⁹⁵ Langerhans cell histiocytosis196, 197, 198 and lymphangioleiomyomatosis.199, 200, 201 This last disease is almost entirely confined to women and it is notable that the donors of some lungs affected by recurrent disease were men; the recurrent smooth-muscle proliferation is of recipient origin,202, 203 as are the immune cells that contribute to recurrent sarcoidosis.²⁰⁴ Recurrent bronchioloalveolar cell carcinoma has been shown to have originated in the recipient and the recurrence to have taken place despite both lungs having been replaced and there being no evidence of extrapulmonary involvement.²⁰⁵ Idiopathic pulmonary fibrosis and emphysema are examples of diseases that have not yet been reported to recur in transplanted lungs.

Drug injury

Some of the immunosuppressive drugs used to maintain a transplant may harm other organs. For example, ciclosporin may cause renal damage and rhabdomyolysis while azathioprine may damage the liver and bone marrow. Several patterns of lung disease have been described in the recipients of lung and other organ transplants following treatment with sirolimus: these include lymphocytic interstitial pneumonia, organising pneumonia, diffuse alveolar haemorrhage and pulmonary haemorrhage.206, 207, 208 Sirolimus is also contraindicated postoperatively as it may cause dehiscence of the bronchial anastomosis. Amiodarone may be used to treat atrial fibrillation, which is a common postoperative complication, and rare cases of amiodarone lung have been reported in transplant recipients.

Future prospects

With increasing numbers of patients awaiting lung transplantation and a limited donor pool, the pressure on transplant centres is immense. Advances in the medical treatment of those pulmonary conditions for which patients are often transplanted (such as cystic fibrosis, pulmonary hypertension and fibrosing lung disease) will reduce demand on transplant services but new strategies are required. Those being explored include attempts to improve the condition of borderline lungs, the use of mechanical assist devices, xenotransplantation and the construction of new lungs by bioengineering.

So far the only strategy to enter the clinical arena is reconditioning ex vivo, whereby the condition of borderline donor lungs is improved by mechanical ventilation and perfusion in a specially designed circuit.209, 210, 211 Stem cell therapy may also prove of benefit in reconditioning borderline lungs.

While mechanical assist devices have a well-established role in end-stage heart failure, the use of such devices to support patients awaiting lung transplantation is limited. Extracorporeal membrane oxygenation has been used successfully as a short-term bridge to transplantation in some patients, but the development of a portable artificial lung for longer support outside hospital is still awaited. Pulmonary xenotransplantation is an area of active research, with several groups exploring the use of porcine lungs.