Although PTLD is not typically thought of as an infectious process, it is discussed here because of the strong association with Epstein Barr virus (EBV). In short PTLD is a lymphoproliferation of EBV infected lymphocytes (usually of B-cell origin) arising in the background of overimmunosuppression. The incidence ranges from 3.5% to 7.8% in long term lung transplant recipients. In lung transplant recipients, there is a relatively high incidence (60%) of PTLD occurring in the allograft. The reason for this may be multifactorial and postulations include 1) the lung being the primary site of inoculation (since PTLD is recognized as evolving from a primary EBV infection, not reactivation), 2) relatively high doses of immunosuppression when compared to other organ transplants, and 3) bronchial associated lymphoid tissue acting as "homing" sites for EBV infected host B-cells.

Morphologically, PTLD typically grows as a mass lesion with some showing angioinvasion. Histologically they may divided into monomorphous and polymorphous subtypes. The majority of the former are monoclonal proliferations whereas the latter are polyclonal. As the mass expands, foci of necrosis may appear, leaving viable lymphoid cells at the periphery. When these areas are biopsied transbronchially, distinction from acute cellular rejection may be difficult. In these instances, demonstrating the presence of Epstein Barr virus latent protein (EBV-LMP) by immunohistochemistry or Epstein Barr virus encoded RNA (EBER) by in-situ hybridization have been shown to be a very specific for PTLD. Caution should be exercised because extremely sensitive techniques such as Polymerase Chain Reaction may detect very small quantities of EBV genomes in patients without evidence of PTLD.

VI. Histologic Tissue Reactions and their Differential Diagnosis

Miliary pattern of acute inflammation

CMV and HSV infections and tuberculosis may produce a miliary pattern of neutrophilic infiltrates particularly in the interstitium. The process is suspected at low power magnification and diagnostic cytopathic changes are identified at higher magnification. When the cytopathic changes of these viral infections are not well developed, immunohistochemical stains aid in their recognition.

Perivascular Inflammation

While perivascular mononuclear lymphoplasmacytic infiltrate is considered the hallmark of acute cellular rejection, it may be seen in infection due to CMV or pneumocystis. Therefore, the interpretation of a perivascular mononuclear cell infiltrate must be undertaken with caution. Search for CMV inclusions should be performed on every biopsy and Grocott stains are recommended as routine stains. As a rule, if the perivascular infiltrate is due to either of these two infections, clues are often present which would guide one to the correct diagnosis.

Acute Bronchitis/ Bronchiolitis

An acute inflammation of the airways would suggest an infectious process. However, in the context of transplantation, the finding of acute bronchitis/ bronchiolitis (ABB) suggests other differential diagnostic possibilities. In addition to acute infection, other graft syndromes producing ABB include harvest injury, acute cellular rejection, and bronchiolitis obliterans. Distinction between these entities requires careful clinicopathologic correlation and the recognition of histologic clues favoring one process over another. For example, ABB secondary to harvest injury presents with a background of diffuse alveolar damage. In acute cellular rejection, the predominant inflammatory cell population is lymphoplasmacytic and the neutrophilic component is relatively minor. Nevertheless, secondary infections must be considered and BAL culture should be obtained at the time of biopsy. Within the grades of acute rejection, moderate and severe ACR would potentially give problems in interpretation. One of the hallmarks of chronic rejection is bronchiolitis obliterans (OB). The current Working Formulation divides OB into active and inactive lesions. Active lesions are characterized by an "activated" mononuclear cell infiltrate permeating through the scarred airway. With increased intensity of the infiltrate, bronchiolar epithelial cell necrosis is followed by a neutrophilic infiltrate, producing the lesion of ABB. On the other hand, scarring of the airways results in cylindrical bronchiectasis with alternating areas of dilatation and constriction. With such altered anatomy, airways distal to constricted foci would develop obstruction and become susceptible to microbial growth. Therefore, ABB in the syndrome of bronchiolitis obliterans may be secondary to rejection, post obstructive infection or both.

Lymphocytic Bronchitis/ Bronchiolitis

The finding of a mononuclear cell infiltrate involving the airways as well as the vasculature is characteristic of acute cellular rejection, as long as the corresponding BAL cultures are sterile. When airway mononuclear cell infiltrates are seen alone, the term lymphocytic bronchitis/bronchiolitis is applied. While this lesion has been documented as the "cellular phase" of airway rejection, it should be noted that an identical lesion may be produced by infection. In that instance, correlation with culture results becomes critical.

Interstitial, Airway and Airspace Granulation Tissue

Fibromyxoid plugs of granulation tissue within airways, airspaces, and interstitium are seen in three clinicopathologic settings: acute cellular rejection, harvest injury, and infection. At least a moderate grade of acute cellular rejection is generally necessary to produce significant airway injury and the ingrowth of loose myxoid granulation tissue. This reaction should be distinguished from bronchiolitis obliterans which demonstrates scarred airways with dense eosinophilic collagen. The myxoid material is generally reversible and does not necessarily progress to bronchiolitis obliterans. Similar changes are seen in organizing pneumonia secondary to infection. Search for active bronchopneumonic foci and/or the identification of the microorganism would assist in its distinction from rejection or harvest injury. The airway injury secondary to an infectious pneumonia manifests as young myxoid granulation tissue which is to be distinguished from the dense fibrosis seen in obliterative bronchiolitis. Nonspecific airspace granulation tissue is often seen adjacent to a nidus of infection which may not be sampled by transbronchial biopsy. Localized processes such as Nocardia abscesses and PTLD are better sampled by fine needle aspiration and biopsy (FNAB).

Granulomas and Giant Cells

While mycobacterial and fungal infections need to considered in the differential diagnosis of granulomas and giant cells, one should be aware of noninfectious etiologies for this type of tissue reaction. Aspiration with secondary giant cell reaction is not uncommon and in such cases, the identification of the foreign material leads one to the correct diagnosis. Some recurrences of primary diseases manifest with granulomas or giant cells. Recurrent cases of sarcoidosis have been documented by transbronchial biopsy and at autopsy with the documentation of nonnecrotizing granulomas. This diagnosis is one of exclusion as there is no specific marker for sarcoidosis. When one encounters nonnecrotizing granulomas on transbronchial biopsy in a patient with a primary diagnosis of sarcoidosis, grocott and acid fast bacilli stains should be ordered on the sections. If they are negative, the findings would be consistent with recurrent sarcoidosis. Giant cells have also been documented to reappear in single lung transplant recipients with the diagnosis of Giant Cell Interstitial Pneumonia which is currently thought to be an occupational pneumoconiosis secondary to inhalation of hard metal dust particles. In the cases reported, the recipients had not returned to work and it is postulated that the remaining native lung may "seed" the allograft with hard metal particles and subsequently evoke a giant cell reaction.

Bronchial Cartilage Alterations

Anastomosis of the major airways currently utilizes the telescoping procedure and in the past omentopexy. While the bronchi and pulmonary arteries and veins are reanastomosed, the bronchial circulation is interrupted. Since the bronchial artery supplies the large airways, some degree of airway ischemia is expected in all transplants. Changes in the bronchial cartilage with interruption of the cartilaginous contours, perichondrial fibrosis, and ossification have been demonstrated. While ischemia may produce these changes, rejection and infection may be superimposed on the ischemic process, adding further injury to the airways. Therefore, the airway wall demonstrating inflammation, necrosis, and scarring should be evaluated for an infectious process and rejection as well. If an infectious agent is identified, the distinction between airway colonization, bronchitis, and invasive disease should be made.

Graft Eosinophilia

While eosinophils are known to be a cellular component of acute cellular rejection, parenchymal eosinophilia is associated with a variety of nonrejection related conditions including asthma, drug reactions, and infections. The infectious organisms identified include Aspergillus, coxsackie virus, and Pseudomonas maltophilia. The histopathologic tissue reaction demonstrates acute eosinophilic pneumonia which is not a form of acute cellular rejection. Although rare, parasitic infections producing eosinophilia should also be added to the differential diagnosis. The finding of marked eosinophilia on BAL or a pattern of eosinophilic pneumonia on transbronchial biopsy should alert one to search for an infectious etiology.