Lung transplantation may offer longer survival and improved quality of life to patients with end-stage lung disease. Common indications for single lung, bilateral (double) lung, and heart-lung transplantation are listed in Table 12-1. Bilateral lung transplantation is the norm for cystic fibrosis, but of interest, the proportion of bilateral lung transplantation procedures has been rising for other major indications as well.¹ Living donor single lobe transplantation may be a viable alternative to cadaveric lung transplantation for selected patients.² Benchmark survival rates for adult lung transplant recipients are 88% at 3 months, 78% at 1 year, 63% at 3 years, 51% at 5 years, and 28% at 10 years after transplantation.¹

Table 12-1 Most Common Indications for Lung Transplant Procedures

Transplant Procedure	Most Common Indications
Adult single lung	Chronic obstructive pulmonary disease Idiopathic pulmonary fibrosis α₁-Anti-trypsin deficiency emphysema
Adult bilateral/double lung	Cystic fibrosis Chronic obstructive pulmonary disease Idiopathic pulmonary fibrosis α₁-Anti-trypsin deficiency emphysema Idiopathic pulmonary arterial hypertension
Adult heart-lung	Congenital heart disease Idiopathic pulmonary arterial hypertension Cystic fibrosis
Pediatric lung	Cystic fibrosis “Primary pulmonary hypertension” Congenital heart disease “Interstitial pneumonitis” Surfactant protein B deficiency

Unfortunately, the number of patients who can benefit from lung transplantation is limited by the availability of donor organs. Historically, waiting time was the main determinant of donor lung allocation in the United States. In 2005, a lung allocation score (LAS) was implemented, dramatically changing the way donor lungs are distributed.³ Under the new system, priority for transplantation is determined by medical urgency and expected outcome. Early evaluations of the new system indicate that the waiting time and waitlist mortality rate are decreased, the number of transplants is increased, and the post-transplantation survival is unchanged.⁴

Complications of lung transplantation may be related to (1) the operation itself (primary graft dysfunction, anastomotic complications), (2) the host’s immunologic response to the allograft (rejection), and (3) the immunosuppressive therapy used to prevent rejection (infection, post-transplantation lymphoproliferative disorders [PTLDs]). Other complications, such as organizing pneumonia and recurrence of the original disease, may also occur. To aid the differential diagnosis, post-transplant time intervals can be divided arbitrarily into immediate (within 4 days), early (4 days to 1 month), and late (beyond 1 month) post-transplantation periods.⁵ Differential diagnostic possibilities for each of these periods are listed in Table 12-2.

Table 12-2 Complications of Lung Transplantation

Post-transplantation transbronchial biopsy may be performed for a specific clinical indication or for surveillance of acute rejection. The role of surveillance biopsy in lung transplant patients remains controversial.^6,⁷ At least five pieces of well-expanded alveolated lung parenchyma are required for the assessment of acute rejection.⁸ The histopathologic findings most commonly encountered in a post-transplantation transbronchial biopsy include acute rejection, cytomegalovirus infection, airway-centered inflammation, pneumonia, bronchiolitis obliterans, harvest injury, invasive aspergillosis, and PTLDs.^6,⁹

Operation-Related Complications

Primary Graft Dysfunction

Despite many advances in organ preservation, surgical technique, and perioperative care, primary graft dysfunction, also known as harvest injury, ischemia-reperfusion injury, early graft dysfunction, and reimplantation response, contributes significantly to both the morbidity and mortality for lung transplantation. Primary graft dysfunction affects an estimated 10% to 25% of pulmonary allografts and can range in clinical severity from transient decrease in oxygenation to complete graft failure.¹⁰ The International Society for Heart and Lung Transplantation (ISHLT) has proposed a definition and a grading scheme based on the chest film and Pao₂/Fio₂ ratio.¹¹

Pathologic Findings

Mild cases may show alveolar and interstitial edema with scattered neutrophils.¹³ The histologic correlate of severe primary graft dysfunction is diffuse alveolar damage.¹² The acute phase of diffuse alveolar damage is characterized by hyaline membranes, interstitial edema, occasional fibrin thrombi, and scattered neutrophils in the alveolar septa (Fig. 12-1). In the organizing phase, hyaline membranes are incorporated into the alveolar septa, which become thickened by fibroblast-rich connective tissue (Fig. 12-2).

Figure 12-1 Acute diffuse alveolar damage due to harvest injury. The key to the diagnosis is the presence of hyaline membranes.

Figure 12-2 Organizing diffuse alveolar damage resulting from harvest injury. In the absence of residual hyaline membranes, the history can aid the diagnosis.

Histologic Differential Diagnosis

Diffuse alveolar damage is a nonspecific histologic pattern that can be elicited by various insults in the post-transplantation setting (Box 12-1). Immunofluorescent studies are helpful in separating primary graft dysfunction from acute antibody-mediated rejection. Acute antibody-mediated rejection is characterized by alveolar septal deposits of IgG and complement (particularly C4d), which are absent in primary graft dysfunction. Acute rejection is not a major concern during the immediate post-transplantation period. Any infection can manifest as diffuse alveolar damage in an immunocompromised patient; therefore, it is always prudent to perform special stains to rule out acid-fast bacilli and fungal organisms.

Box 12-1 Common Causes of Diffuse Alveolar Damage in the Pulmonary Allograft

Harvest injury

Acute antibody-mediated rejection

Severe acute rejection

Infection

Treatment, Prognosis, and Prevention

The treatment is supportive and may include mechanical ventilation. A retrospective analysis by Christie and associates showed that in patients with and those without primary graft dysfunction, 30-day mortality rates are 42.1% and 6.1%, respectively.¹⁴ Primary graft dysfunction is also associated with an increased risk of obliterative bronchiolitis.¹⁵ For prevention of primary graft dysfunction, research studies have focused on improving lung preservation techniques by optimizing the volume, temperature, pressure and components of preservation solutions, and inflation and ventilation parameters of the organs during transport.¹⁰ So far, these studies have had modest clinical impact.

Arterial Anastomotic Obstruction

The incidence of pulmonary arterial anastomotic obstruction after lung transplantation is relatively low.¹⁶ Causes include narrowed anastomosis, with or without thrombus formation resulting from suboptimal surgical anastomoses and excessive length of donor or recipient pulmonary artery with kinking or torsion of the anastomosis.¹⁶

Time Period

Arterial anastomotic obstruction usually occurs during the first week after transplantation.

Clinical Presentation

Signs and symptoms include dyspnea, hypoxemia, and elevated pulmonary arterial pressure.

Diagnosis

The diagnosis is suggested by reduced perfusion in the allograft by ventilation-perfusion (V/Q) scan and can be confirmed by echocardiogram or pulmonary angiography. Large areas of infarction may be present. Pathologic confirmation is usually not required.

Venous Anastomotic Obstruction

Minor abnormalities of the pulmonary venous anastomosis are relatively common complications of lung transplantation.¹⁷ Occlusive thrombus formation is relatively rare but may have catastrophic consequences, including allograft failure and stroke.

Time Period

Venous anastomotic obstruction usually presents in the immediate post-transplantation period but has been reported to occur as late as the eighth postoperative day.¹⁶

Clinical Presentation

Pulmonary venous obstruction after lung transplantation should be suspected in every case of persistent pulmonary edema in the first postoperative days, often associated with a frothy blood-stained secretion from the endotracheal tube.¹⁸

Radiologic Findings

Chest radiograph reveals diffuse unilateral interstitial edema.

Diagnosis

Transesophageal echocardiography with color-flow Doppler imaging is virtually diagnostic, demonstrating a marked reduction of the flow in the affected pulmonary vein.¹⁸

Pathologic Findings

The specimen from the surgical revision may include a thrombus. Biopsy of the lung obtained at the same time may show congestion and venous engorgement.

Treatment

Venous anastomotic obstruction is considered a surgical emergency, and revision of the anastomosis with removal of any associated thrombus is required to prevent irreversible injury to the lung allograft.

Airway Dehiscence

In the early years of lung transplantation, airway dehiscence due to ischemia of the donor bronchus was a major cause of morbidity and death. Improved surgical techniques, reduced immunosuppression, and better allograft preservation have reduced the incidence of airway complications.¹⁹ Currently, most centers report a 7% to 18% complication rate with a related mortality rate of 2% to 4%.¹⁹

Time Period

Airway dehiscence may develop in the first few weeks after transplantation.

Diagnosis

Ischemia and necrosis of the bronchus can be diagnosed by direct visualization with a bronchoscope.

Pathologic Findings

Biopsies show coagulation necrosis of the bronchial mucosa, submucosa, and cartilage. Superimposed bacterial or fungal infection may produce neutrophilic infiltrates, thereby enhancing necrosis and dehiscence of the anastomosis.

Treatment

Surgical options are limited, but may include anastomotic revision, retransplantation, or stent placement.

Large Airway Stenosis

Large airway (bronchial) stenosis is the most common airway complication. The incidence is estimated to be between 1.6% and 32%.¹⁹ It is usually seen after necrosis or dehiscence or in healing or treated infections. “Telescoped” anastomosis is associated with a 7% incidence of airway stenosis.

Nonanastomotic large airway stenosis has also been described.^20,²¹ The pathogenesis of this lesion is unclear, but it may represent a response to ischemic damage, alloreactive injury, or infection.

Time Period

Bronchial stenosis usually occurs a few months after the transplantation procedure but has been described as early as 8 days.²²

Clinical Presentation

Clinical findings include dyspnea, retained secretions, recurrent pneumonia, and decline in spirometry, all of which can mimic chronic airway rejection.

Diagnosis

Bronchoscopic examination provides the diagnosis, with biopsies providing confirmatory histology.

Pathologic Findings

Common findings include prominent granulation tissue, fibrosis, and squamous metaplasia.

Treatment

Airway stenting is often used as the primary management option for airway complications after lung transplantation.²³

Rejection

With the exception of monozygotic twins, donors and recipients are genetically different and express different histocompatibility antigens. As a result, allografts are rejected by the recipient’s immune system. Multiple immunologic processes are involved, creating a spectrum of rejection responses. A “working formulation for the classification of pulmonary allograft rejection” was introduced by the ISHLT in 1990.²⁴ The working formulation was first revised in 1996.²⁵ The currently accepted scheme for grading pulmonary allograft rejection was approved by the ISHLT board of directors in 2007 (Box 12-2).⁸ The differences between the 1996 and 2007 schemes are relatively minor and are related to airway inflammation and chronic airway rejection. Acute antibody-mediated rejection is a controversial subject and is not included in the 2007 working formulation. For the sake of completeness, however, it is discussed at the end of this section.

Acute (Cellular) Rejection

The term acute rejection without a qualifier is used to describe acute cellular rejection. This is a cell-mediated process, in contrast to the antibody-mediated process of acute antibody-mediated (humoral) rejection. Most lung transplant recipients experience episodes of acute rejection.

Time Period

Acute rejection may occur as early as 3 days and as late as several years after transplantation. The majority of acute rejection episodes begin within the first 3 months after transplantation.

Clinical Presentation

Clinical features may include low-grade fever, cough, dyspnea, crackles, and adventitious sounds on auscultation. Features suspicious for rejection include a more than 10% decrease in the forced expiratory volume in 1 minute (FEV₁) and hypoxemia.

Radiologic Findings

Radiologic abnormalities include perihilar or lower lung zone alveolar and interstitial infiltrates, septal lines, subpleural edema, peribronchial cuffing, and pleural effusion. In cases of a single-lung transplant, the V/Q lung scan will show decreased perfusion to the allograft.

Diagnosis

Clinical features may suggest acute rejection, but a transbronchial biopsy usually is required to confirm the diagnosis and rule out infection. If biopsy from multiple sites is technically impossible, lower lobe biopsies are preferred because they appear to be more informative.²⁶

Pathologic Findings

The hallmark of acute rejection is the presence of perivascular mononuclear cell infiltrates. If small airway inflammation is present, it should be noted (see later discussion).

Acute rejection is graded according to the density and extent of the perivascular infiltrates and the presence or absence of secondary pneumocyte damage (Table 12-3). Rejection-type infiltrates usually involve more than one vessel, but a single perivascular infiltrate should be evaluated by the same criteria as for multiple infiltrates, as follows:

1. Minimal acute rejection (grade A1) is characterized by infrequent two- to three-cell-thick perivascular mononuclear cell infiltrates (Fig. 12-3).

2. In mild acute rejection (grade A2), the perivascular mononuclear cell infiltrates become thicker, denser, and usually more frequent (Fig. 12-4).

3. In moderate acute rejection (grade A3), the infiltrates extend into the alveolar septa and air spaces (Fig. 12-5).

4. In severe acute rejection (grade A4), the mononuclear cell infiltrates are associated with pneumocyte damage. The latter often manifests as diffuse alveolar damage with hyaline membranes (Fig. 12-6).

Table 12-3 Grading Acute Rejection

Figure 12-3 Minimal acute rejection (A1) with sparse perivascular mononuclear cell infiltrate.

Figure 12-4 In mild acute rejection (A2), the mononuclear cell infiltrate is denser and is more than three cell layers thick. However, it is limited to the perivascular area.

Figure 12-5 In moderate acute rejection (A3), the perivascular infiltrate extends into the alveolar septa.

Figure 12-6 In severe acute rejection (A4), the perivascular infiltrates lead to lung injury. The latter manifests as fibrinous exudates and hyaline membranes in this case. A, Lower magnification. B, Higher magnification.

The composition of the cellular infiltrates also changes with increasing severity of rejection. In minimal acute rejection, the perivascular infiltrates are composed predominantly of small, round, plasmacytoid, and transformed lymphocytes. As the rejection advances in intensity, the infiltrates contain more activated lymphocytes, macrophages, eosinophils, and neutrophils. Subendothelial and peribronchiolar infiltrates become more pronounced.

In higher-grade rejection, the inflammatory cells permeate through the vessels with extension to the endothelium, giving rise to endothelialitis. In 30% of mild and 60% of moderate acute rejection, there is also associated airway inflammation.

A rare form of acute rejection also exists, characterized by abundant eosinophils, which may obscure the mononuclear cells in the perivascular infiltrates.

Histologic Differential Diagnosis

Perivascular and interstitial mononuclear cell infiltrates are not specific for acute rejection.⁸ Differential diagnostic considerations include infections, especially cytomegalovirus pneumonia and Pneumocystis jiroveci pneumonia,²⁷^–²⁹ and PTLDs.³⁰ Some histologic features may favor infection over acute rejection (Table 12-4). Cultures and special stains may be helpful in the diagnosis of mycobacterial, fungal, and Pneumocystis jiroveci infections. Viral pneumonias can be confirmed by cultures as well as serologic, immunohistochemical, or molecular hybridization techniques.

Table 12-4 Histologic Features Favoring Infection over Acute Rejection

Histologic Features	Infection Favored
Predominant alveolar septal infiltrates as compared with perivascular infiltrates	Any infection
Abundant neutrophils	Bacterial pneumonia, CMV pneumonia, or candidiasis
Abundant eosinophils	Fungal infection
Nuclear or cytoplasmic inclusions	Viral pneumonia
Multinucleation	Respiratory syncytial virus or parainfluenza virus pneumonia
Punctate zones of necrosis	Herpes simplex virus, varicella-zoster virus, or CMV pneumonia
Granulomatous inflammation	Mycobacterial, fungal, or Pneumocystis jiroveci infection
Frothy intra-alveolar exudates	Pneumocystis jiroveci pneumonia

CMV, cytomegalovirus.

In some cases, histologic features of acute rejection and infection coexist. In these cases, the pathologist should attempt to decide which is dominant and guide the clinician by favoring one over the other. Follow-up biopsy after appropriate antimicrobial therapy is also recommended so that any acute rejection component can be reassessed.⁸ The differential diagnosis between acute rejection and PTLDs is discussed later.

Treatment and Prognosis

The treatment of acute rejection typically consists of bolus therapy with intravenous steroids, which may be supplemented by temporary increases in the maintenance immunosuppression regimen. In at least 80% of the cases, acute rejection is successfully treated. However, 15% to 20% of acute rejection episodes persist or recur, presenting a particularly difficult management problem for the clinician. When this occurs, intensified immunosuppression with one or more agents is usually attempted. However, it has been shown that patients with persistent, recurrent, or late (occurring at least 3 months after transplantation) acute rejection are at increased risk for developing chronic airway rejection.³¹ Recent studies have indicated that an increased risk may exist even with minimal acute rejection.^32,³³

Airway Inflammation: Lymphocytic Bronchiolitis

The 2007 working formulation has collapsed the four previous B grades into two (grade 1R—low grade and grade 2R—high grade) and has retained B0 (no airway inflammation) and BX (ungradable). Another change from the previous working formulation is that the B grade designation applies only to small airways (bronchioles). Airway inflammation may be a harbinger of chronic airway rejection.^34,³⁵

Pathologic Findings

Criteria for grading airway inflammation are listed in Table 12-5.

Table 12-5 Grading Airway Inflammation

Grade	Airway Inflammation
B0—no airway inflammation	None
B1R—low-grade small airway inflammation	Mononuclear cells in the submucosa (can be infrequent and scattered or forming band-like infiltrates) Occasional eosinophils may be seen
B2R—high-grade small airway inflammation	Mononuclear cells in the submucosa with greater numbers of eosinophils Epithelial damage and intraepithelial lymphocytic infiltration Ulceration and fibrinopurulent exudates may occur
BX—ungradable	Sampling problems, infection, tangential cutting, other problems

Histologic Differential Diagnosis

Infection, particularly that caused by viral, bacterial, mycoplasmal, fungal, and chlamydial organisms, may mimic the airway inflammation related to acute rejection.²⁸

Chronic Airway Rejection: Obliterative Bronchiolitis

Obliterative bronchiolitis is the most significant long-term complication of lung transplantation, with a prevalence of 30% to 50% and an associated mortality rate of 25%.³⁶ The terminology is somewhat confusing, because obliterative bronchiolitis of chronic airway rejection is sometimes referred to as bronchiolitis obliterans or bronchiolitis obliterans syndrome in the clinical lung transplantation literature. It is important to recognize that obliterative bronchiolitis or bronchiolitis obliterans of chronic airway rejection is both clinically and histologically distinct from the (sub)acute lung injury pattern once known as bronchiolitis obliterans organizing pneumonia (BOOP). To make this distinction clear, the nomenclature has been changed, and the currently preferred term for BOOP is organizing pneumonia.³⁷

Time Period

Obliterative bronchiolitis is most frequently diagnosed between 9 and 15 months after transplantation.³⁸ It rarely develops during the first 3 months but has been reported as early as 2 months after transplantation.³⁸

Clinical Presentation

Obliterative bronchiolitis often develops insidiously with vague general symptoms, and nonproductive cough. Later, progressive dyspnea on exertion becomes the dominant complaint. At this later stage, pulmonary function tests show a decline in the FEV₁ as compared to a previously established post-transplantation baseline.

Radiologic Findings

Chest radiographs are typically unremarkable until later in the disease, when a variable pattern of bronchiectasis is accompanied by airway tapering/obliteration and zones of hyperinflation. These changes reflect the peculiar nature of chronic airway rejection—proximal bronchiectasis (dilatation) with distal obliterative bronchiolitis (constriction).

Diagnosis

Transbronchial biopsy is an insensitive method for the detection of obliterative bronchiolitis.⁸ An ad hoc ISHLT working group has concluded that FEV₁ is the most reliable and consistent indicator of chronic airway rejection.³⁹

Pathologic Findings

The term obliterative bronchiolitis refers to hyalinized fibrous plaques present in the submucosa of small airways.^8,¹³ They lead to partial or complete luminal compromise (Fig. 12-7). The scar tissue may be concentric or eccentric and may be associated with destruction of the smooth muscle wall. The 1996 working formulation retained the designation of active versus inactive obliterative bronchiolitis, depending on the presence and degree of accompanying inflammation.²⁵ However, the consensus in 2007 was that the distinction between active and inactive is no longer useful, and the condition should be designated merely as C0, indicating a biopsy with no evidence of obliterative bronchiolitis, and C1, indicating that obliterative bronchiolitis is present in the biopsy.⁸ Obliterative bronchiolitis often produces mucostasis or postobstructive (endogenous lipid) pneumonia.^8,¹³

Figure 12-7 Bronchiolitis obliterans. Scar tissue obliterates the lumen of a bronchiole, which can be recognized by the presence of smooth muscle and elastic fibers in the wall. A, Hematoxylin-eosin stain. B, Elastic–van Gieson stain.

Histologic Differential Diagnosis

Transplant-related obliterative bronchiolitis involves the small airways. Large airway fibrosis is a nonspecific finding and should not be considered as evidence of chronic airway rejection. Organizing pneumonia pattern is manifested as fibromyxoid connective tissue plugs within the lumina of bronchioles and alveoli.⁴⁰ These loose edematous airspace-filling plugs should be distinguished from the densely eosinophilic submucosal scars of transplant bronchiolitis obliterans.

Treatment and Prognosis

Augmented immunosuppression appears to be of some benefit in treating bronchiolitis obliterans, but it is far from optimal. Avoiding this complication of lung transplantation may require better use of current immunosuppressive medications, or the development of novel immunosuppressive strategies.³⁸

Chronic Vascular Rejection: Accelerated Graft Vascular Sclerosis

The clinicopathologic significance of chronic vascular rejection is not entirely clear. However, chronic vascular changes may coincide with the presence of obliterative bronchiolitis in lung transplant recipients and with the presence of accelerated coronary artery disease in combined heart-lung transplant recipients.^41,⁴²

Diagnosis

Chronic vascular rejection is not applicable with transbronchial biopsies but may be noted in surgical lung samples.⁸

Pathologic Findings

In chronic vascular rejection, there is fibrointimal thickening in arteries and veins (Fig. 12-8). There may also be an “active” inflammatory component consisting of subendothelial, intimal, or medial, predominantly lymphoid, mononuclear cell infiltrates.

Figure 12-8 Chronic vascular rejection (accelerated vascular sclerosis). Intimal proliferation occludes the lumen of a muscular pulmonary artery, which can be recognized by the presence of two elastic laminae. A, Hematoxylin-eosin stain. B, Elastic–van Gieson stain.

Acute Antibody-Mediated (Humoral) Rejection

Acute antibody-mediated (humoral) rejection is mediated by antibodies specific for donor antigens, particularly those of the human leukocyte antigen (HLA) system. These antibodies, which may develop before and after transplantation, bind to target antigens and activate the complement system, leading to tissue injury.^43,⁴⁴ Improvements in anti-HLA antibody detection have increased recognition of antibody-mediated rejection following renal, heart, and lung transplantation.^43,^45,⁴⁶ Early observations of acute antibody-mediated rejection were based on the phenomenon of hyperacute rejection, in which preexistent antibodies lead to complement activation and rapid graft loss. With improved cross-matching before transplantation, the incidence of hyperacute rejection has decreased.

Time Period

Hyperacute rejection is noticeable within minutes to hours after transplantation. However, humoral sensitization and acute antibody-mediated rejection may also occur beyond the immediate post-transplantation period.

Clinical Presentation

Clinical findings include progressive respiratory failure.

Radiologic Findings

In hyperacute rejection, complete opacification of the pulmonary allograft is seen.

Diagnosis

The presence of donor-specific anti-HLA antibodies in the context of vascular C4d deposition and refractory acute rejection fulfills the criteria for antibody-mediated rejection.⁴⁷ Assays used for HLA antibody screening and identification include complement-dependent cytotoxicity (CDC) and solid-phase technologies such as enzyme-linked immunosorbent assay (ELISA), flow cytometry, and Luminex analysis.⁴³

Pathologic Findings

The recent ISHLT report remains very cautious in discussing the pathologic appearance of acute antibody-mediated rejection.⁸ The consensus is that capillaritis, along with immunofluorescent or immunohistochemical staining for C4d, should raise clinical suspicion for acute antibody-mediated rejection.^48,⁴⁹ Histologic features of hyperacute rejection also include diffuse alveolar damage.⁵⁰^–⁵²

Histologic Differential Diagnosis

Capillaritis is a nonspecific histologic finding that may occur in infection, diffuse alveolar damage, and severe acute cellular rejection.⁸ However, the presence of capillaritis and C4d staining as well as anti-HLA antibodies should be seen as strong evidence for acute antibody-mediated rejection.

Prevention, Treatment, and Prognosis

One of the major goals in donor selection is to avoid HLA antigens against which the potential recipient has preformed antibodies.⁴³

Optimal treatment of acute antibody-mediated rejection remains uncertain. Intravenous immunoglobulin (IVIG) is one of the most common therapies used to decrease antibody-mediated immunity.⁵³ Rituximab, an anti-CD20 monoclonal antibody that causes B cell depletion, has been proved effective in the treatment of presensitized renal transplant recipients in conjunction with IVIG.⁴³ Plasmapheresis has been shown to lead to clinical improvement in lung transplant recipients with pulmonary capillaritis unresponsive to steroids.⁴⁸ However, it is usually reserved for severe cases of suspected acute antibody-mediated rejection, given the side effects and difficulties of administration.

Infection

Pulmonary infections are the most common cause of morbidity in the lung transplant population. Prompt recognition and treatment are necessary to prevent poor outcomes.

Bacterial Infections

Cystic fibrosis patients frequently show airway colonization with gram-negative bacteria both before and after lung transplantation. Recent data suggest that colonization with gram-negative bacteria may play a role in the pathogenesis of chronic airway rejection.⁵⁴

Bacterial infections of the lower respiratory tract may manifest as acute bronchitis or bronchopneumonia. Gram-negative infections, especially those caused by Pseudomonas species, account for about 75% of bacterial pneumonias. Other reported bacterial pathogens include a wide range of nosocomial organisms. Legionellosis is rarely reported.⁵⁵

Time Period

Bacterial infections can occur shortly after transplantation presumably due to transmission of bacteria from the donor. Nevertheless, the risk of bacterial infection persists throughout the lifetime of the allograft.

Clinical Presentation

The clinical findings include fever, cough, purulent sputum, shortness of breath, rales on auscultation, hypoxemia, leukocytosis, and decline in spirometry.

Radiologic Findings

New or increasing infiltrates on chest radiograph are common manifestations of bacterial pneumonias.

Diagnosis

Most of the clinical features of transplant-associated pneumonia are nonspecific and largely modified by the patient’s immunocompromised status. Bronchoalveolar lavage (BAL) and transbronchial biopsy are often performed in the evaluation of new infiltrates. Culture results are often an important part of the diagnostic workup.

Pathologic Findings

In acute bronchitis, neutrophils infiltrate the bronchial mucosa. This pathologic change may be associated with mucosal ulceration and intraluminal neutrophils. As in the normal host, acute pneumonia is recognized by the presence of neutrophils within the alveolar spaces (Fig. 12-9).

Figure 12-9 Acute pneumonia. Neutrophil granulocytes are present in the alveolar spaces.

Histologic Differential Diagnosis

The composition of inflammatory infiltrates distinguishes bacterial infection from acute rejection. Bacterial infection is characterized by the presence of neutrophils, whereas mononuclear cells (mainly lymphoid cells) are seen predominantly in acute rejection.

Treatment and Prognosis

Organism-specific management is essential. Any regimen of broad-spectrum antibiotics instituted before identification of an organism should include agents effective against Pseudomonas species.

Viral Infections

Cytomegalovirus (CMV) infection remains a serious problem in lung transplant recipients. Donor-recipient mismatch, with the donor being seropositive and the recipient seronegative for CMV, poses the highest risk for the development of CMV pneumonia. Seropositive recipients of a seropositive or seronegative donor are at intermediate risk of acquiring active CMV pneumonia, and seronegative recipients of a seronegative donor are at lowest risk. Universal ganciclovir prophylaxis is a strategy aimed at reducing CMV infection and delaying the development of obliterative bronchiolitis. However, the optimal duration of ganciclovir prophylaxis remains unclear. If the prophylaxis is discontinued, the incidence of CMV pneumonia is around 57%.⁵⁶ A recent study has suggested that indefinite ganciclovir prophylaxis may prevent CMV pneumonia in 98% of lung transplant recipients.⁵⁶

Herpes simplex virus (HSV) infections are also a potential problem in lung transplantation. The frequency of HSV infections has also been reduced remarkably with the routine use of ganciclovir prophylaxis.

Other viruses responsible for respiratory infections include adenovirus, respiratory syncytial virus, influenzavirus, parainfluenza virus, and varicella-zoster virus.^57,⁵⁸

Time Period

Before universal ganciclovir prophylaxis, CMV infection generally occurred between 2 weeks and 4 months after transplantation. HSV infection typically began as oral ulcers or tracheitis during the first month after transplantation.

Clinical Presentation

Fever, malaise, myalgias, chills, abdominal discomfort, cough, and shortness of breath are frequent symptoms of CMV pneumonia. Physical examination may reveal crackles or may be normal. Other features include hypoxemia and decline in spirometry values. Fortunately, pneumonia caused by HSV is now rare, thanks to routine prophylaxis. The clinical features are similar to those of CMV pneumonia.

Radiologic Findings

Chest radiographs may show reticular or reticulonodular infiltrates but may be clear in up to two thirds of patients.

Diagnosis

The diagnosis of viral pneumonia is often impossible on clinical grounds alone. BAL and transbronchial biopsy play important roles in establishing the diagnosis.

Pathologic Findings

Recognizing tissue responses and cytopathic effects may help in identifying viral infections (see Chapter 6). Tissue responses to viral pathogens range from minimal nonspecific inflammation to diffuse alveolar damage. Most cases of CMV infection show interstitial pneumonia with a mixed lymphocytic and polymorphonuclear cell infiltrate²⁷ (Figs. 12-10 and 12-11). Zonal necrosis may be seen with herpes simplex, varicella-zoster, and CMV pneumonia (Figs. 12-12 and 12-13). CMV may also be associated with neutrophilic microabscesses. Necrotizing bronchiolitis may be a feature of adenovirus, influenzavirus, and respiratory syncytial virus infections. Some characteristic viral cytopathic effects are listed in Table 12-6. These cytopathic effects, however, can be sparse or absent.

Figure 12-10 Cytomegalovirus pneumonia. Mononuclear cells infiltrate the alveolar septa diffusely, with no perivascular accentuation.

Figure 12-11 Cytopathic effects characteristic of cytomegalovirus infection. Both nuclear and cytoplasmic inclusions are present, but the latter are less apparent.

Figure 12-12 Herpes simplex virus pneumonia with an area of necrosis.

Figure 12-13 Higher-power view of involved lung in herpes simplex pneumonia showing a nuclear inclusion.

Table 12-6 Viruses and Their Cytopathic Effects

Virus	Cytopathic Effects
Cytomegalovirus	Cytomegaly, nuclear and cytoplasmic inclusions
Herpes simplex virus	Nuclear inclusions
Varicella-zoster virus	Nuclear inclusions
Adenovirus	Smudge cells, nuclear inclusions
Respiratory syncytial virus	Occasional multinucleation, cytoplasmic inclusions
Influenzavirus	None
Parainfluenza virus	Occasional multinucleation, cytoplasmic inclusions

Immunohistochemistry, in situ hybridization, and polymerase chain reaction (PCR) techniques can be used to identify many viruses and have largely replaced electron microscopy in this role (Fig. 12-14).

Figure 12-14 Herpes simplex virus infection. Paraffin immunoperoxidase studies reveal herpes simplex virus–positive cells.

Histologic Differential Diagnosis

The histologic differential diagnosis of viral pneumonia includes acute rejection.²⁷ Both processes can exhibit perivascular and interstitial mononuclear cell infiltrates. However, perivascular infiltrates predominate in acute rejection, and alveolar septal infiltrates are more prominent in viral infection (see Table 12-4). The presence of CMV inclusions is indicative of CMV pneumonia, but attention to other histologic details is necessary to exclude concurrent acute rejection and bronchiolitis obliterans, which are frequently associated with CMV infection.

Fungal Infections

Fungal infections are less frequent than other infections in the transplant recipient but carry a high mortality rate when they do occur. The fungal species most commonly encountered in lung transplant biopsies include Aspergillus and Candida.⁵⁹ Cryptococcosis, histoplasmosis, coccidioidomycosis, and mucormycosis have also been reported.^59,⁶⁰ Fungal organisms may colonize the respiratory tract or cause overt infection.⁶⁰ Prolonged antibiotic therapy predisposes patients to disseminated candidiasis.

Time Period

Fungal infections have a bimodal presentation: early onset between 2 weeks and 2 months after transplantation, secondary to difficult postsurgical periods and prior colonizations; and late onset, primarily secondary to chronic rejection and terminal renal insufficiency.⁶⁰

Clinical Presentation

The clinical picture is not specific. Fungal pneumonias may manifest with fever, leukocytosis, and hypoxemia.

Radiologic Findings

Radiographically, pulmonary infiltrates with consolidation or cavitary nodules may be seen.

Diagnosis

The diagnosis is most often made by a combination of clinical features and the recovery of fungal organisms from BAL, transbronchial biopsy, blood, or other body fluids.

Pathologic Findings

Fungal species may be a source of bronchial anastomotic infections (Figs. 12-15 and 12-16). Aspergillus pneumonia is characterized by hemorrhagic infarction, and sparse inflammatory cell infiltrates (Figs. 12-17 and 12-18). Long, septate hyphae, with 45-degree branching points, invade blood vessels and permeate alveolar septa. Candida infection produces neutrophilic infiltrates and is associated with abscess formation. Clusters of pseudohyphae and yeast forms are often found in the center of abscesses.

Figure 12-15 Bronchial mucosal necrosis and associated Aspergillus infection. There is no significant inflammation.

Figure 12-16 Grocott methenamine silver stain of material from the same case as in Figure 12-15 reveals fungal organisms compatible with Aspergillus species.

Figure 12-17 Aspergillus pneumonia with an area of infarction.

Figure 12-18 Grocott methenamine silver stain of material from the same case as in Figure 12-17 shows vasoinvasive fungal elements compatible with Aspergillus species.

Pneumocystis jiroveci Pneumonia

Although recent studies have strongly suggested that Pneumocystis jiroveci (formerly known as Pneumocystis carinii) is a fungus, we discuss Pneumocystis jiroveci pneumonia separately from other fungal infections for didactic purposes. Without prophylaxis, Pneumocystis jiroveci pneumonia occurs in nearly all lung transplant recipients.⁶¹ Thanks to the routine use of prophylaxis, it is rarely seen today in this patient population.

Time Period

Historically, infections were most common around the seventh post-transplantation week.

Clinical Presentation

The clinical presentation is nonspecific and includes cough, fever, dyspnea, and hypoxemia.

Radiologic Findings

Radiographically, diffuse pulmonary infiltrates are seen.

Diagnosis

Because Pneumocystis jiroveci cannot be grown in culture, the diagnosis is usually made by identification of the organisms in lavage fluid. Rarely, transbronchial lung biopsy is required.

Pathologic Findings

The classic histologic picture of interstitial pneumonia with frothy intra-alveolar exudates, seen in patients with acquired immunodeficiency syndrome, is rarely encountered in lung transplant recipients (Figs. 12-19 and 12-20). In these patients, Pneumocystis jiroveci pneumonia more often manifests as diffuse alveolar damage and the organisms are typically embedded in the prominent hyaline membranes (Fig. 12-21). Granulomatous inflammation is a less common manifestation of infection with Pneumocystis jiroveci.

Figure 12-19 Pneumocystis jiroveci pneumonia, bronchoalveolar lavage. Frothy exudate can be seen.

Figure 12-20 Grocott methenamine silver stain of material from the same case as in Figure 12-19 reveals Pneumocystis jiroveci organisms.

Figure 12-21 Pneumocystis jiroveci pneumonia. Both frothy intra-alveolar exudates and hyaline membranes are visible.

Post-Transplantation Lymphoproliferative Disorders

PTLD lesions are lymphoid or plasmacytic proliferations that develop as a consequence of immunosuppression in an allograft recipient.⁶² Characteristics of PTLDs vary somewhat with allograft types and with immunosuppressive regimens. PTLDs are relatively more common among pulmonary allograft recipients as a result of higher levels of immunosuppression.³⁰ In this population, the occurrence rate for PTLDs may be as high as 5%.

A majority of PTLDs are associated with primary or reactivated Epstein-Barr virus (EBV) infection and appear to represent EBV-induced B cell or rarely T cell proliferations. EBV-seronegative recipients who develop primary EBV infection have a higher incidence of PTLD. Approximately 20% of patients with PTLDs are EBV-seronegative. The etiology of EBV-negative cases is unknown, but the fact that some of them respond to decreased immunosuppression suggests that they are also related to decreased immune competence.

Time Period

PTLD most commonly develops in the first year after lung transplantation.⁶³

Clinical Presentation

Primary EBV infection often presents as a mononucleosis-like illness with fever and sore throat. Pulmonary involvement by PTLD may cause shortness of breath or may be discovered incidentally on a routine chest radiograph. Simultaneous involvement of extrapulmonary sites may result in diarrhea, due to involvement of the gastrointestinal tract, and dysphagia, due to involvement of the tonsils. Physical examination may reveal lymphadenopathy, enlarged tonsils, splenomegaly, and crackles on chest auscultation. In some cases, the physical examination may be entirely normal.

Radiologic Findings

Thoracic abnormalities are present in most lung transplant recipients with PTLD.⁶⁴ The most common radiologic finding is multiple pulmonary nodules. Other manifestations include a solitary nodule, multifocal alveolar infiltrates, and hilar or mediastinal adenopathy.

Diagnosis

The diagnosis is usually suspected on the basis of the clinical and radiologic findings, but histologic diagnosis is required.

Pathologic Findings

The spectrum of PTLDs ranges from early lesions to polymorphic PTLD to lymphomas.^65,⁶⁶ Several classification schemes have been proposed, but the World Health Organization (WHO) Classification is now widely accepted and is presented in Box 12-3.⁶²

Specimen evaluation for the diagnosis of PTLD should include routine morphologic examination, immunophenotyping, preservation of tissue for potential molecular genetic studies, and detection of EBV infection.^62,⁶⁷ Flow cytometry or frozen section immunohistochemistry is more useful in determining cell lineage and clonality than paraffin section immunohistochemistry. If immunophenotyping studies show polytypic immunoglobulin, clonality can be further assessed by molecular genetic studies that are capable of identifying polyclonal or monoclonal gene rearrangement. EBV infection can be detected using immunohistochemistry for latent membrane protein (LMP-1), but in situ hybridization for EBV-encoded nuclear RNA (EBER) is considered the gold standard.

Early Lesions

Early lesions of PTLD include plasmacytic hyperplasia and infectious mononucleosis–like lesions. These lesions usually arise in lymph nodes or Waldeyer’s ring and only rarely involve true extranodal sites such as the lung. They are characterized by some degree of architectural preservation of the involved tissue,⁶² but they differ from typical reactive follicular hyperplasia in having a diffuse proliferation of plasma cells. Plasmacytic hyperplasia is distinguished by numerous plasma cells and rare immunoblasts, and infectious mononucleosis–like lesion has the typical morphologic features of infectious mononucleosis in the lymph node, namely paracortical expansion and numerous immunoblasts in a background of T cells and plasma cells.

Immunophenotypic studies show an admixture of polyclonal B cells, plasma cells, and T cells. EBV-positive immunoblasts are typically present.

Polymorphic PTLDs

Polymorphic PTLDs are destructive lesions that efface the architecture of lymph nodes or form destructive extranodal masses.⁶² In contrast with most lymphomas, polymorphic PTLDs show the full extent of B cell maturation and are composed of immunoblasts, plasma cells, small and intermediate-sized lymphocytes, and centrocyte-like cells. Scattered large, bizarre cells (atypical immunoblasts) and areas of necrosis may also be present. Polymorphic PTLDs were at one time subdivided into “polymorphic B cell hyperplasia” and “polymorphic B cell lymphoma.” Today this separation is not deemed necessary, because both have similar clinical features. Immunophenotyping studies typically show a mixture of B and T cells. Most of the cases are monoclonal, at least by molecular genetic studies. EBV-positive immunoblasts are present in a majority of the cases.

Monomorphic B Cell PTLDs

Monomorphic B cell PTLDs are characterized by nodal architectural effacement or tumoral growth in extranodal sites, with confluent sheets of large, transformed cells.⁶² These tumors should be diagnosed as B cell lymphomas and should be classified according to lymphoma classification guidelines. However, PTLD should also appear in the differential diagnosis. A majority of B cell PTLDs have morphologic features of diffuse large B cell lymphoma (Figs. 12-22 to 12-24). A minority may be classified as Burkitt lymphoma, plasma cell myeloma, or plasmacytoma-like lesions. Immunophenotyping studies of monomorphic B cell PTLD show B cell–associated antigen expression (CD19, CD20, CD79a). Many cases co-express antigens usually associated with T cells (CD43, CD45RO). A majority of cases are monoclonal and EBV-positive. Monomorphic B cell PTLDs often contain oncogene or tumor suppressor gene alterations (N-ras gene codon 61 point mutation, p53 gene mutation, or c-myc gene rearrangement).^68,⁶⁹

Figure 12-22 Panorama view of monomorphic B cell post-transplantation lymphoproliferative disorder showing a mass-like lesion in the pulmonary parenchyma.

Figure 12-23 Higher magnification of involved lung in B cell post-transplantation lymphoproliferative disorder showing morphologic features of a diffuse large B cell lymphoma.

Figure 12-24 In situ hybridization studies performed on involved lung in B cell post-transplantation lymphoproliferative disorder reveal numerous cells that are positive for Epstein-Barr virus–encoded nuclear RNA.

Monomorphic T Cell PTLDs

T cell lymphomas have been reported in allograft recipients. Similar to monomorphic B cell PTLDs, monomorphic T cell PTLDs have sufficient atypia to be recognized as neoplastic and should be classified according to the standard lymphoma classification. Monomorphic T cell PTLDs express pan–T cell antigens. Most of the reported cases are EBV-negative.

Classic Hodgkin Lymphoma–Type PTLD

Classic Hodgkin lymphoma–type PTLD is the least common form of PTLD.⁶² The diagnosis is based on classic morphologic and immunophenotypic features, preferably including both CD15 and CD30 expression. This type of PTLD is almost always EBV-positive. Because Reed-Sternberg–like cells may also be seen in some polymorphic and monomorphic PTLDs, the distinction between classic Hodgkin lymphoma–type PTLD and Hodgkin lymphoma–like PTLD may be difficult in some cases. However, the latter are better categorized as either polymorphic or monomorphic PTLDs.

Histologic Differential Diagnosis

Acute rejection may be considered in the differential diagnosis for PTLDs, especially in small biopsy samples. Detection of EBV infection is very helpful in this respect. A sheet-like monomorphous infiltrate with a mononuclear composition of more than 25% B cells and more than 30% large lymphoid cells also favors PTLD over acute rejection.⁷⁰

Treatment and Prognosis

Therapy of PTLD must be tailored to the individual patient. Newer modalities such as anti-CD20 monoclonal antibody therapy (with rituximab) complement the standard stepwise approach that begins with a reduction of immunosuppression.⁷¹ The role of chemotherapy continues to be defined, and in some cases early recourse to this approach may be desirable. Survival varies by age and extent of disease, with pediatric patients and those with localized disease tending to fare better.

Other Complications

Cryptogenic Organizing Pneumonia

Cryptogenic organizing pneumonia, previously known as idiopathic BOOP, occurs as a response to acute lung injury. In lung transplant recipients, it is commonly associated with aspiration, infection, and acute rejection.^40,⁷²^–⁷⁴ However, organizing pneumonia is not a component of, and does not necessarily predispose to, chronic airway rejection (obliterative bronchiolitis).

In organizing diffuse alveolar damage, the fibroblastic proliferation involves the interstitium rather than the airspaces and remnants of hyaline membranes may be seen.⁴⁰ However, organizing pneumonia and diffuse alveolar damage are both acute lung injury patterns and features of both may be present in a given case. Airspace fibromyxoid tissue can also be seen in organizing infectious pneumonia and healing rejection, especially higher-grade rejection following steroid therapy. Separation of organizing pneumonia from transplant obliterative bronchiolitis has been discussed earlier.