Pathology of lung transplantation

Time period

Acute rejection may occur as early as 3 days and as late as several years after transplantation. Most 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. Patients might also have low serum levels of immunosuppressive medication such as sirolimus or tacrolimus.

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 ventilation-perfusion (V/Q) lung scan will show decreased perfusion to the allograft.

Diagnosis

Clinical features may suggest acute cellular rejection, but a transbronchial biopsy is usually required to confirm the diagnosis and rule out mimickers such as infection, reperfusion injury, aspiration, or abnormal medication effect among others. If biopsy from multiple sites is technically impossible, lower lobe biopsies are preferred because they appear to be more informative.

Pathologic findings

The hallmark of acute rejection is the presence of perivascular mononuclear cell infiltrates. Acute rejection is graded according to the density and extent of the perivascular infiltrates and the presence or absence of secondary pneumocyte damage ( Table 13.4 ). Rejection-type infiltrates usually involve more than one vessel; the final grade is given based on the vessel with the highest-grade rejection. Acute cellular rejection is graded as follows:

• Minimal acute rejection (grade A1) is characterized by infrequent two- to three-cell-thick perivascular mononuclear cell infiltrates ( Fig. 13.1 ).

  

• In mild acute rejection (grade A2), the perivascular mononuclear cell infiltrates become thicker, denser, and usually more frequent ( Fig. 13.2 ).

  

• In moderate acute rejection (grade A3), the infiltrates extend into the adjacent interalveolar and peribronchiolar septa and airspaces ( Fig. 13.3 ).

  

• 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, fibrinous organizing pneumonia or organizing pneumonia might also be seen ( Fig. 13.4 ). There might be a paradoxical diminishing of the perivascular infiltrates (see Fig 13.4 B).

  

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, and plasmacytoid lymphocytes. As the rejection advances in intensity, the infiltrates contain more activated lymphocytes, macrophages, plasma cells, eosinophils, and occasionally neutrophils (see Figs. 13.3 C and 13.4 C). Subendothelial infiltrates become more pronounced. In higher-grade rejection, the inflammatory cells permeate the vessels with extension to the endothelium, giving rise to endothelialitis ( Figs. 13.3 C and 13.5 ). In 30% of mild and 60% of moderate acute rejection, there is also associated airway inflammation.

A rare form of acute rejection also exists; it is 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. Differential diagnostic considerations include infections, especially CMV pneumonia and Pneumocystis jirovecii pneumonia, and posttransplant lymphoproliferative disorders (PTLDs). Some histologic features may favor infection over acute rejection ( Table 13.5 ). Cultures and special stains may be helpful in the diagnosis of mycobacterial and fungal infections. P. jirovecii infections might be identified by fungal stains of tissue sections and reverse transcriptase- polymerase chain reaction (RT-PCR) of the bronchoalveolar lavage (BAL) because this organism does not grow in culture. Viral pneumonias can be confirmed by molecular techniques as well as serologic or immunohistochemical techniques. Eosinophils might also be seen in abnormal medication reaction.

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. 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 with an increased risk existing even with minimal acute rejection. ^,

Pathologic findings

Criteria for grading airway inflammation are listed in Table 13.6 .

Histologic differential diagnosis

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

Time period

Obliterative bronchiolitis is most frequently diagnosed between 9 and 15 months after transplantation. It rarely develops during the first 3 months but has been reported as early as 2 months after transplantation. The median BOS-free survival time is 4 years for bilateral lung transplanted patients and is significantly shorter at 3.2 years for single lung transplant recipients. Almost all patients who live 10 years posttransplantation will have developed BOS. Obliterative bronchiolitis/BOS is the most common cause of death in patients after 1 year of transplantation accounting for 21% to 29% of all deaths in these patients.

Clinical presentation

Obliterative bronchiolitis often develops insidiously with vague general symptoms and nonproductive cough. Later, progressive dyspnea on exertion becomes the dominant complaint.

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. However, bronchiectasis can also be seen in patients with RAS or patients with combined features of BOS and RAS. These changes reflect the peculiar nature of chronic airway rejection: proximal bronchiectasis (dilatation) with distal obliterative bronchiolitis (constriction).

Diagnosis

Given the patchy nature of obliterative bronchiolitis the sensitivity of transbronchial biopsies is relatively low. Therefore clinicians use a decline in FEV1 to diagnose BOS clinically. ^, Specifically, CLAD including BOS is defined as a decline in FEV1 of 20% or more when compared with a previously established posttransplantation baseline.

Pathologic findings

The term obliterative bronchiolitis refers to hyalinized fibrous plaques present in the submucosa of small airways. ^, In early obliterative bronchiolitis there is usually subepithelial fibrous tissue ( Fig. 13.8 ), in late obliterative bronchiolitis the fibrous tissue leads to partial occlusion of the lumen of the small airway and eventually possibly to its obliteration ( Fig. 13.9 ). The scar tissue may be concentric or eccentric and may be associated with destruction of the smooth muscle wall. An elastic stain such as a Verhoeff-Van Gieson stain might help to visualize the elastic lamina of the airway (see Figs. 13.8 B and 13.9 B); a Masson Trichrome stain highlights the scar/fibrotic tissue. Sometimes the scar tissue can be accompanied by chronic inflammation or there might be a prominence of fibroblasts to suggest a more active fibrosis ( Figs. 13.8 and 13.10 ). Overall, obliterative bronchiolitis is designated as grade C1; C0 indicates there is no evidence of obliterative bronchiolitis; CX is used if no small airway is identified in the biopsy or small airways obliteration cannot be assessed due to another reason. Obliterative bronchiolitis often produces morphologic signs of obstruction such as mucostasis or postobstructive (endogenous lipid) pneumonia. ^,

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 is manifested as plugs of fibromyxoid connective tissue within the lumina of bronchioles and alveoli. 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 obliterative bronchiolitis, but it is far from optimal. It is suggested that cyclosporine be switched to tacrolimus, and a trial of azithromycin is also recommended. ^, Referral to an experienced surgeon to evaluate the gastroesophageal junction for fundoplication is suggested for patients who also have confirmed gastroesophageal reflux. ^, Other reported therapeutic options might include total lymphoid irradiation or extracorporeal photosynthesis. However, overall, pharmacologic therapy appears to be of limited efficacy and retransplantation might be the only effective therapy in selected patients. Survival is higher among patients who undergo retransplantation for obliterative bronchiolitis than for those who undergo retransplantation for other reasons, but it is lower compared with patients undergoing primary lung transplantation. Overall, as mentioned earlier, BOS is the most common cause of death of patients after 1 year posttransplantation. Therefore, prevention of BOS is important, which may include prevention of infections, prophylactic treatment with arythromycin, and de novo use of tacrolimus as compared to cyclosporine might be promising. ^{, ,}

Diagnosis

Accelerated graft vascular sclerosis is not applicable to transbronchial biopsies but may be noted in surgical lung samples.

Pathologic findings

In accelerated graft vascular sclerosis, there is fibrointimal thickening in arteries and veins ( Figs. 13.11 and 13.12 ). There may also be an “active” inflammatory component consisting of subendothelial, intimal, or medial, predominantly lymphoid mononuclear cell infiltrates.

Time period

AMR is arbitrarily divided into hyperacute (occurring intraoperatively or within 24 hours of surgery), acute (often mimicking acute rejection), and chronic forms.

Clinical presentation

AMR can present clinically with measurable allograft dysfunction, such as hypoxemia and decreased FEV1, or might be subclinical, with normal allograft function. ^, Patients with clinical AMR may have symptoms such as dyspnea, cough, fever, and malaise, or they may be asymptomatic.

Radiologic findings

Imaging studies may show lung infiltrates.

Diagnosis

The diagnosis of pulmonary AMR remains a challenge and requires the correlation of clinical, radiologic, pathologic (including immunostain), serologic, and microbiologic findings. Key diagnostic criteria include lung biopsy findings consistent with AMR (see later), deposition of C4d on the endothelium of capillaries as evaluated by immunostaining, and detection of circulating DSAs. The ISHLT consensus of pulmonary AMR differentiates between clinical AMR, which is associated with allograft dysfunction, and subclinical AMR without dysfunction of the allograft. Each of these two categories is further subdivided into definite, probable and possible AMR based on the presence of all 3 criteria including histologic features suggestive of AMR, C4d deposition, circulating DSA (de novo or preexisting) (definite), 2 of the 3 criteria (probable) or 1 of the 3 criteria (possible) after exclusion of other causes of allograft dysfunction. Therefore, overall allograft dysfunction may bring the attention to AMR but is not required for the diagnosis (clinical vs. subclinical AMR). When there is measurable pulmonary allograft dysfunction, other potential causes of the dysfunction such as infection need to be excluded.

Pathologic findings

The most common histologic finding described in association with AMR is capillary inflammation, which encompasses neutrophilic capillaritis and neutrophilic margination. ^{, ,} In 2012 the Pathology Council of the ISHLT proposed some definitions to refine the morphologic spectrum of AMR including neutrophilic margination (increased numbers of neutrophils within septal capillaries but lacking capillary injury, specifically no karyorrhexis), neutrophilic capillaritis (patchy or diffuse septal capillary neutrophilic collections associated with cellular karyorrhectic debris and fibrinous exudates) and acute lung injury with or without hyaline membranes ( Fig. 13.13 A–C). Microvascular thrombi, alveolar hemorrhage, and/or accumulations of neutrophilic infiltrates within adjacent alveolar airspaces might also be seen. Widening of the alveolar septa, as recognized on low magnification, is a first hint of possible AMR and has been observed in all cases of AMR in a recent multi-institutional study. Other patterns reported in AMR include persistent or recurrent high grade acute cellular rejection, lymphocytic bronchiolitis, and obliterative bronchiolitis. However, all of these morphologic findings are insensitive and not specific for AMR by itself.

Although it has low sensitivity and specificity in the setting of lung transplantation, immunohistochemistry for C4d may provide supportive evidence for AMR (see Fig. 13.13 D). ^{, ,} C4d deposition on the endothelium of interstitial capillaries in the lung can be studied using immunoperoxidase and immunofluorescence techniques. Diffuse staining of greater than 50% of the alveolar septal capillaries is considered positive. The interpretation of C4d in the lung might be hampered by nonspecific staining of elastic fibers of large vessels or hyaline membranes. ^, Furthermore, reproducibility of the interpretation of C4d deposition can be problematic.

Ultrastructural analysis showed higher scores of endothelial cell swelling, endothelial vacuolization, and neutrophilic margination in patients with AMR in contrast to acute cellular rejection and normal lung, however, validation studies are needed.

Histologic differential diagnosis

No histologic findings are specific for AMR. For example, neutrophilic capillaritis, neutrophilic margination, and acute lung injury can also be seen in infection and severe acute cellular rejection. ^, Morphologic findings described in AMR can also be seen in aspiration, abnormal drug reaction, and preservation injury shortly after transplantation. Therefore it is important that histologic findings be interpreted in a clinicopathologic context and infection be excluded before a diagnosis of AMR is made.

Capillary C4d deposition is also not specific to AMR. While diffuse (>50% of capillaries) or focal (10%–50% of capillaries) staining has been shown in patients with morphologic findings suggestive of AMR, it has also been found in some cases of acute lung injury from other causes including fibrinous pneumonia and diffuse alveolar damage (DAD), in infection, and preservation injury ( Fig. 13.14 ). ^,

Prevention, treatment, and prognosis

One of the major goals of donor selection is to avoid hyperacute rejection due to preexistent antibodies. Because harmful DSAs can also develop after transplantation, DSA testing should be performed promptly if AMR is suspected.

Overall, the optimal treatment regimen of AMR in the lung is still unknown. Treatment of AMR is focused on antibody depletion. Rituximab, an anti-CD20 monoclonal antibody that causes B cell depletion, might be applied in conjunction with intravenous immunoglobulin (IVIg) and other multimodality treatment. ^, Proteasome inhibitors have also been used. Plasmapheresis, possibly in conjunction with IVIg with or without bortezomib can also lead to reduction of de novo DSA and clinical improvements. ^, However, outcomes have been poor with high mortality and high incidence of early CLAD among survivors.

Time period

Pulmonary allograft recipients surviving for at least 3 months were included in the study of Sato et al. Similar to obliterative bronchiolitis, PPFE is unlikely to occur during the first 3 months after transplantation.

Clinical presentation

Patients present with insidious or acute onset of shortness of breath, fever, nonproductive cough, pleurisy, chest tightness, and weight loss. A subset of patients presents with acute hypoxemic respiratory failure similar to acute respiratory distress syndrome (ARDS) with rapid deterioration leading to death or requiring retransplantation; other patients might present with recurrent acute exacerbations followed by periods of clinical stability; another subset is identified only on imaging studies, but will follow a steady decline in lung function afterwards.

Radiologic findings

Chest x-ray and CT scan show persistent opacities. On CT scans, opacities may include ground glass opacities, consolidation, small linear and reticular opacities, and/or volume loss. Overall, persistent (≥3 months) multilobar opacities with or without pleural changes are the hallmark of RAS. When RAS is suspected imaging studies are recommended to be performed as inspiratory thin section HRCT scans without contrast media. Initial reports in 2003 were those of opacities and honeycombing predominantly in the upper lobes on HRCT. Upper lobe fibrosis was also described in a subsequent study in 2005. However, later reports showed an upper lobe predominance of these findings only in a subset of patients (64%); another study did not identify a particular distribution of parenchymal changes between various lung regions. ^, A subset of patients also showed radiologic signs of small airways disease including air trapping, “tree-in-bud” opacities, centrilobular nodules, airway thickening, and bronchiectasis suggestive of a mixed BOS and RAS phenotype.

Diagnosis

RAS is defined as a consistent 20% or more decline in FEV1 (with or without a decline in FVC) with a concomitant 10% or more decline in TLC (also compared with posttransplantation baseline) and persistent opacities on chest x-ray or CT. ^,