Lung transplantation–related pathology encompasses a spectrum of disorders that include, but are not limited to, indications for lung transplantation (seen in explanted lungs), surgical complications (airway anastomotic and vascular complications), ischemia-reperfusion injury, rejection (acute and chronic), infections, and posttransplantation lymphoproliferative disorders (PTLDs).
Over the last two decades, lung transplantation has become an accepted modality of treatment for many end-stage lung diseases. With greater experience and analysis of significant numbers of cases, lung transplant recipients are living longer, and follow-up regimens are becoming streamlined. Thus, a comprehensive understanding of lung transplantation–related pathology is now necessary for both tertiary care pathologists dealing with highly specialized lung transplantation teams and a much larger spectrum of healthcare providers who may be involved in the care of lung transplant recipients or candidates for lung transplantation.
The Registry of the International Society for Heart and Lung Transplantation (ISHLT) monitors and reports statistics on clinical lung transplantation. Data accrued from 1989 through 2009 report a total of 32,652 lung transplantations from centers around the world. In 2010, 3519 lung transplantation procedures were reported to the ISHLT. 
In the ISLHT 2012 report, lung transplantation–related pathology, specifically acute rejection, was reported to affect 34% of lung transplant recipients within the first postoperative year.
The main indications for single-lung transplantation, as reported by ISHLT 2012, include the following:
Chronic obstructive pulmonary disease (COPD) (45.6%)
Idiopathic pulmonary fibrosis (33.4%)
Cystic fibrosis (1.7%)
Bilateral lung transplantation is indicated for COPD (26.6%) and cystic fibrosis (26.3%), followed by idiopathic pulmonary fibrosis (16.8%) and alpha 1 antitrypsin deficiency emphysema (6.4%). 
The etiology of lung transplantation–related pathology is almost always immune-mediated. The transplantation of a ”foreign” organ, despite meticulous matching for donor-recipient compatibility, leads to a complex adaptive and innate immune system–mediated injury. Although aimed at protecting the host from infection, in the context of transplantation, this injury leads to allograft rejection. Such chronic injury is usually mediated by anti–human leukocyte antigen antibodies.
As noted above, advances in tissue typing and cross-match testing deter severe rejections. Rejection may result from direct cellular attack (cellular rejection) or be mediated by antibodies (humoral rejection). Both are discussed below.
Clinical Features and Imaging
Clinical features can range from an absence of symptoms to signs and symptoms of respiratory distress and/or infection and are not detailed in this article.
Of note, clinical findings do not play a role in the pathologic grading of rejection.
A quick intraoperative gross examination of the lung allograft may reveal previously unrecognized lesions, the nature of which could be addressed with frozen section analysis. Although rare, pulmonary embolism, infections, or neoplasms in the allograft may be diagnosed in this manner, which would affect the decision to use the donor lung.
Explanted lungs should be fixed in formalin (preferably by inflating through the main bronchus) for 8-12 hours and sliced at 1-cm intervals. The cut surface shows the characteristic findings of the disease for which lung transplantation was indicated.
Emphysema is end stage and generalized by the time the patient needs transplantation. Patchy fibrosis with greater involvement of lower lobes and subpleural regions is readily apparent in idiopathic pulmonary fibrosis. In cystic fibrosis, the dilated bronchi are usually plugged by yellow-green mucopurulent material. The largest pulmonary arteries have small yellow atherosclerotic plaques in pulmonary arterial hypertension.
In the explanted lung, the classic findings of the primary disease are readily apparent; in addition, there may be superimposed foci of acute inflammation, organizing pneumonia, and mucostasis.
In the transplanted lung, material for histologic evaluation is obtained via transbronchial biopsy and is analyzed after formalin fixation and paraffin processing. Tissue is obtained either as part of surveillance protocols or as clinically indicated by new onset of symptoms.
This section describes the microscopic findings of the following lung transplantation–related pathologies:
Lung allograft rejection
Lung allograft rejection
With current immunosuppressive regimes, the incidence of acute rejection is low, but chronic rejection is still the main complication that limits long-term survival. The ISHLT grading of lung transplant rejection was updated in 1996  and revised in 2007  (see Table 1). Grades B and C are simplified in the latter; however, many clinicians and pathologists still use the former since it offers more information on which treatment decisions can be based.
Table 1. ISHLT Grading of Lung Transplant Rejection—1996 and 2007. (Open Table in a new window)
|A. Acute Rejection||A. Acute Rejection|
A0 - Normal
A1 - Minimal
A2 - Mild
A3 - Moderate
A4 - Severe
Grade 0 - None
Grade 1 - Minimal
Grade 2 - Mild
Grade 3 - Moderate
Grade 4 - Severe
|B. Airway Inflammation||B. Airway Inflammation|
BX - Inadequate sampling
B0 - None
B1 - Minimal
B2 - Mild
B3 - Moderate
B4 - Severe
Grade 0 - None
Grade 1R - Low grade*
Grade 2R - High grade*
Grade X - Ungradeable
|C. Chronic Airway Rejection - Bronchiolitis Obliterans||C. Chronic Airway Rejection - Obliterative Bronchiolitis|
0 - Absent
1 - Present
|D. Chronic Vascular Rejection – Accelerated Graft Vascular Sclerosis||D. Chronic Vascular Rejection – Accelerated Graft Vascular Sclerosis|
|*”R" denotes revised grade to avoid confusion with the 1996 scheme.|
Given the patchy nature of rejection, a consensus statement by the Lung Rejection Study Group recommends 5 fragments of well-expanded alveolated lung (with bronchioles and >100 alveolar spaces) be examined. This may require more than 5 transbronchial biopsies, especially to recognize features of bronchiolitis obliterans (BO). Specimens may be gently agitated in formalin to inflate the fragments and require sensitive handling to avoid crush artifacts.
Histologic examination should include sections from 3 levels of the paraffin block for hematoxylin and eosin (H&E) staining. Connective-tissue stains may help evaluate any submucosal fibrosis. Silver stains can be performed for fungi. Immunohistochemical stain for cytomegalovirus is very helpful and strongly recommended. Concomitant bronchoalveolar lavage (BAL) fluid may be analyzed to exclude infection but plays no role in the diagnosis of rejection.
If the biopsy samples contain diagnostic material but do not meet the minimum assessable criteria, grading should be carried out as usual, with a comment describing the number of lung fragments with emphasis that the material may not be representative of the overall allograft. Similarly, if no alveolated lung or no airway is present, the type of rejection should be indicated by the appropriate letter, suffixed by an "X" (see below). Biopsies taken for rejection surveillance should always be evaluated for histopathologic features indicative of infection, aspiration, organizing pneumonia, recurrent disease (eg, sarcoidosis), and PTLD.
Acute rejection may occur at any time (days to years) following transplantation but is primarily seen between 2 and 9 months. The diagnosis is based on the presence of perivascular or airway inflammatory infiltrates that are composed mainly of activated lymphocytes but also contain few eosinophils, neutrophils, and plasma cells. The intensity and the distribution of the infiltrate form the basis of the histologic grade. With multiple foci of rejection, the grade reflects the most advanced pattern of rejection rather than the predominant or average pattern.
It is imperative to note evidence of aspiration (see image below) or concomitant infection, since these may precipitate rejection. Clinical findings play no role in histopathologic grading of lung transplant rejection.
Acute cellular rejection, grade A
Perivascular inflammation forms the basis of grade A acute cellular rejection. The cellular components are the same in each of the following grades, which are determined by the extent of the inflammatory infiltrate:
Grade AX: No alveolated lung tissue or no arteriole or venule, precluding determination of the presence or absence of perivascular infiltrates
Grade A0 (no acute rejection): Normal pulmonary parenchyma without perivascular inflammation
Grade A1 (minimal acute rejection): Scattered perivascular infiltrates forming an incomplete or a 2-or 3-cell–layer–thick cuff (see image below); rare eosinophils; endothelialitis usually absentMinimal acute cellular rejection: incomplete perivascular cuff of inflammatory cells, grade A1 (H&E, 200x)
Grade A2 (mild acute rejection): More frequent perivascular infiltrates (may be densely compacted or loose) consisting of round lymphocytes, activated lymphocytes, plasmacytoid lymphocytes, and macrophages (see image below); eosinophils and endothelialitis may be presentMild acute perivascular rejection: thick complete cuff around blood vessel, grade A2 (H&E, 200x)
Grade A3 (moderate acute rejection): Dense perivascular mononuclear cell infiltrates with frequent foci of endothelialitis and extension into alveolar walls (see image below); eosinophils and neutrophils frequently presentModerate acute perivascular rejection: the inflammation extends into adjacent alveolar walls and is accompanied by fibrinous exudates, grade A3 (H&E, 200x)
Grade A4 (severe acute rejection): Diffuse perivascular, interstitial and alveolar infiltrates of mononuclear inflammatory cells; prominent pneumocyte damage and endothelialitis; intraalveolar necrotic epithelial cells, hyaline membranes, hemorrhage, and neutrophils may be seen
Acute cellular rejection, grade B
Grade B cellular rejection (lymphocytic bronchiolitis in the rejection classification) was described in the 1996 update according to a scale of B0 to B4, (see Table 1) with B1 being treated only if accompanied by clinical symptoms of rejection, and B2 and above being treated regardless of symptoms. The 2006 revision of the B grades collapsed the 4 previous grades into two and retains B0 (no airway inflammation) and BX (ungradeable). Airway inflammation may be present in the absence of perivascular infiltrates. Grade B infiltrates represent a clinically important type of acute rejection since they have been shown to be a very strong risk factor for chronic rejection, necessitating their identification in allograft biopsy samples. Thus, many clinicians and pathologists prefer to use the 1996 grading system, which can be easily translated to the 2007 grading (but not vice versa), if needed, for comparison.
Grade BX: Ungradeable because of insufficient sample, artifact, infection, etc
Grade B0: No evidence of airway inflammation
Grade B1 (B1R) (minimal acute airway rejection): Few activated lymphocytes and plasma cells within the submucosa of the bronchioles (see image below); rare eosinophils may be present; epithelial damage or intraepithelial lymphocytes are not seenMinimal acute airway rejection: there is focal inflammation in the submucosa, grade B1 (H&E, 200x)
Grade B2 (B1R) (mild acute airway rejection): Bandlike infiltrate of activated, larger mononuclear cells in the submucosa (see image below); more eosinophils, few neutrophils and plasma cells are presentMild acute airway rejection: there is a band-like infiltrate in the submucosa, grade B2 (H&E, 200x)
Grade B3 (B2R): The infiltrate extends from the submucosa into the epithelium but without epithelial damage (see image below)Moderate acute airway rejection: the inflammatory infiltrate extends into the overlying epithelium, grade B3 (H&E, 200x)
Grade B4 (B2R): Epithelial damage (necrosis and metaplasia) and numerous intraepithelial lymphocytes are seen in addition to the B3 findings; this is rare with current immunosuppression regimens
Bronchial associated lymphoid tissue is often present and should not be interpreted as rejection (see image below).
Antibody-mediated rejection, also called humoral or hyperacute rejection, is not included in the ISHLT classification owing to the scarcity of reports in the literature. There is no consensus on its recognition and diagnosis by either histopathologic or immunologic methods, nor on its significance and treatment.
Antibody-mediated rejection is characterized by the activation of inflammatory pathways along with the complement and coagulation cascades due to binding of preformed antibody to endothelial cells. Progressive respiratory failure within minutes to hours after transplantation suggests antibody-mediated rejection. It is also thought to occur weeks or months after transplantation upon development of serum antibodies. Histologic evaluation reveals diffuse alveolar damage (DAD), fibrin thrombi, vasculitis, intraalveolar hemorrhage, and interstitial neutrophilia.
Chronic rejection usually occurs after 1 year posttransplantation but may be seen within as early as a few weeks. It is the most significant complication limiting long-term survival, affecting up to 50% of patients at 3 years posttransplantation. A previous episode of acute rejection is the most well-recognized risk factor.
Persistent significant decreases on lung function tests are the most reliable method to diagnose chronic rejection (clinically termed bronchiolitis obliterans syndrome). It manifests pathologically as BO, a chronic, progressive fibroinflammatory occlusion of small airways.
The pathogenic mechanism of chronic rejection is poorly understood but is thought to result from the monocyte/macrophage system with the airway epithelium, with up-regulation of major histocompatibility complex (MHC) antigens, costimulatory molecules, and adhesion molecules, leading to an outpouring of inflammatory and fibroproliferative mediators. Chronic rejection should be suspected with an insidious onset of generalized symptoms, including cough and dyspnea with a progressive decline on pulmonary function tests.
Chronic airway rejection, obliterative bronchiolitis (grade C)
Although often difficult to diagnose on a transbronchial biopsy, BO is the characteristic histologic feature of grade C chronic rejection. In the early phase, the fibrosis is loose and often contains mononuclear inflammatory cells. Later, dense eosinophilic fibrosis is present in the bronchiolar submucosa, resulting in luminal occlusion. Destruction of smooth muscle of the airway wall may be present. Distal airspaces often reveal mucostasis with foamy histiocytes and lipoid pneumonia, which may be a histologic clue when BO is not seen in biopsy material. Transbronchial biopsies are insufficiently sensitive for the detection of obliterative bronchiolitis, and a diagnosis of BO may require a wedge biopsy. Many clinicians want to know if the fibrosis contains inflammatory cells so that they can be targeted by appropriate treatment; thus, the 1996 grading is preferred.
C0: No BO seen
Ca (C1R): Asymmetric or concentric submucosal fibrosis causing partial or complete airway obstruction with inflammation (see image below)Bronchiolitis obliterans (chronic rejection): this patient underwent retransplantation for chronic rejection which is seen here as eccentric fibrosis partially occluding the airway lumen. Note scant inflammatory cells and plump fibroblasts in the lesion (H&E, 100x)
Cb (C1R): Asymmetric or concentric submucosal fibrosis causing partial or complete airway obstruction without inflammation
Chronic vascular rejection (grade D)
Chronic vascular rejection (grade D rejection) is characterized by fibrointimal thickening of arteries and veins, with or without inflammatory infiltrates. Chronic vascular rejection is not diagnosed with transbronchial biopsies. Overall, grade D rejection is rare and has not been reported to cause significant allograft dysfunction. It is most often seen in the setting of BO.
Because of immunosuppressant therapy, transplant recipients are at an increased risk for both common and opportunistic infections. Bacterial infections tend to occur in the first month posttransplantation, with viral and fungal infections usually following within the first 3-6 months. Lung allograft infection is usually diagnosed based on clinical signs and symptoms, microbiologic cultures, and serum viral testing, but only occasionally with transbronchial biopsy.
Histologically, the presence of neutrophils in airway epithelium (see image below) raises the differential of bacterial infection versus grade B cellular rejection. Similar to other immunocompromised situations, gram-negative infections in transplant recipients may not elicit significant inflammatory responses. Bacterial cultures are prudent in light of clinical suspicion. Mycobacterial infection is very rare. It may cause granulomatous inflammation in the lung allograft recipient and can result from either Mycobacterium tuberculosis or atypical acid-fast organisms.
Cytomegalovirus (CMV) infection can be diagnosed on H&E sections; however, immunohistochemistry for CMV antigens allows for earlier detection and treatment, especially in the first posttransplantation year (see image below). Histologically, CMV is characterized by enlarged cells (cytomegaly) with single large basophilic intranuclear inclusions and a clear halo. Viral infections may also be diagnosed with serologic techniques or culture. Matching the CMV status (donor and recipient), widespread use of prophylaxis, and the monitoring of viral load with polymerase chain reaction (PCR) have significantly reduced the incidence of CMV pneumonitis in transplant recipients. Viral infection places patients at a higher risk of acute and chronic rejection and death.
Silver stains help highlight fungal yeast/hyphae. Culprit organisms include Candida species, Aspergillus, and Mucor. Owing to routine prophylaxis, Pneumocystis infection is extremely rare.
Posttransplantation lymphoproliferative disease
PTLD is an Epstein-Barr virus (EBV)–driven condition that has been described in all transplant populations and is a particular problem in lung allograft recipients, in whom it usually involves the allograft itself, as well as nodal and extranodal sites. Improved immunosuppression modalities have decreased the overall incidence of PTLD, reported to vary between 1.8% and 5%.
Histologically, PTLD may range from a collection of cytologically benign polyclonal lymphocytes to a monoclonal proliferation with features akin to a high-grade B-cell lymphoma (see image below). PTLD confined to the allograft has a better overall outcome than disseminated disease. 
In most cases, the diagnosis of acute and chronic rejection is made on H&E-stained sections.
The role of C4d immunostaining for antibody-mediated rejection in the lung is still controversial.
Immunostaining for lymphocyte markers (B- and T-cells) and in situ hybridization for EBV mRNA (EBER) and CMV may be performed as appropriate.
Prognosis and Predictive Factors
According to the ISHLT 2012 report, risk factors for 1-year mortality are related to the indication for transplantation. Poor prognostic factors include older recipient age, lower transplant-center volume, lower cardiac output, lower forced vital capacity, higher pretransplantation bilirubin levels, higher supplemental oxygen need at rest, and increasing negative donor-recipient height difference.
Graft failure and non-CMV infections are reported to be the major causes of death within this period. Data vary, with some centers reporting greater than 85% 1-year survival rates. Substantial improvements in quality of life and functional capacity of lung transplant recipients have been reported.