BRONCHIOLITIS OBLITERANS
Chronic rejection of the lung allograft is defined as a
fibrosing process affecting the lung, which primarily affects the conducting
airways
and the pulmonary vasculature. The process affecting the conducting airways has
been labeled bronchiolitis obliterans, while that affecting the pulmonary arteries
and veins has been named graft atherosclerosis/graft phlebosclerosis.
Bronchiolitis Obliterans:
The concept of bronchiolitis obliterans is a difficult one
because the term has been utilized as both a morphologic descriptor and
clinicopathologic syndrome. Using the lexicon of pathologists, bronchiolitis
obliterans(OB) describes an intraluminal polypoid plug of granulation tissue found
within the terminal and respiratory bronchioles. This granulation tissue polyp
is a
very non-specific histologic finding as it is seen in most infectious pneumonias,
diffuse alveolar damage, aspiration, usual interstitial pneumonia, cryptogenic
organizing pneumonia, among other conditions. To the pulmonologist however
bronchiolitis obliterans implies a chronic scarring process affecting the small
airways of the lung which results in progressive obliteration of the small airways
with resultant obstructive lung disease. In the setting of lung transplantation,
we
utilize the term bronchiolitis obliterans in a hybrid fashion - in the lung
allograft, bronchiolitis obliterans represents dense irreversible eosinophilic
scarring of the terminal and respiratory bronchioles which may either partially or
totally obliterate the lumen of the airway. By utilizing this definition
bronchiolitis obliterans is usually an irreversible process (in contrast to the
reversibility of granulation tissue) and therefore has a strong correlation with
diminished pulmonary function test scores, primarily forced expiratory volume at
one
second. It is strongly correlated with the recently proposed clinical grade
for the
bronchiolitis obliterans syndrome (BOS), which is based on obstructive functional
alterations.
Bronchiolitis obliterans may develop in the allograft lung
through several means. In the usual proposed sequence (which is assumed but not
necessarily mandatory) the bronchioles of the lung have their submucosa infiltrated
by a mononuclear cell population, primarily small round, small cleaved, and
occasional large lymphoid cells. These lymphocytes home to the basement membrane
of
the airways and migrate through the basement membrane into the overlying
respiratory
mucosa. This infiltration may occur as part of acute rejection reactions,
lymphocytic bronchitis/ bronchiolitis, or through other pathways. Because of
cytotoxic alloreactive injury to the epithelium, individual cell necrosis occurs.
When confluent areas of necrosis develop, a fibropurulent exudate forms within the
lumen of the airway. This may either be focal and segmental or diffuse throughout
the length of the airway, and frequently is associated with skip regions of
preserved epithelium. Regions of ulceration are accompanied by the proliferation
of fibroblasts, myofibroblasts, and endothelial cells which migrate from the
denuded
submucosa into the fibropurulent exudate forming intraluminal polypoid masses of
loose fibromyxoid tissue. This tissue frequently contains lymphocytes,
occasional
neutrophils, macrophages, hemosiderin, and foamy histiocytes. Over time this
myxoid
tissue is re-epithelialized through the growth of the adjacent metaplastic
epithelium which incorporates the loose granulation tissue into the wall of the
airway.
Several consequences of this inflammatory reaction are
possible. If there is severe epithelial damage and the granulation tissue tethers
opposite sides of the airway, the bronchiole may be completely occluded by
connective tissue. Over time this granulation tissue may convert into dense
eosinophilic scar tissue with total obliteration of the airway lumen. With partial
injury to the airway an eccentric or concentric plaque of dense eosinophilic
connective tissue may be interposed between the submucosa and the regenerating
epithelium. This may require trichrome or elastic tissue stains to highlight these
submucosal plaques, whose compromise of the airway diameter increases airway
resistance and induces obstructive airway disease. In other instances the
granulation tissue of the airway my completely resorb and the airway may return to
normal with little evidence of residual scarring. In any scenario, the diagnosis
of bronchiolitis obliterans by a pathologist, should be restricted to those cases
containing dense eosinophilic scar tissue, not loose myxoid granulation tissue.
What determines whether granulation tissue converts into dense scar tissue is
unclear. We believe it is in part related to the severity of injury to the airway,
to the persistence of the injury, and to the presence of severe basement membrane
damage to the respiratory mucosa. Healing may also be affected by the absence of a
bronchial circulation.
The development of clinical bronchiolitis obliterans is
related to the number of airways affected and the extent of airway injury. For
example, if a biopsy demonstrates only one injured airway out of several thousand,
one would not expect significant pulmonary function abnormalities. This may
account
for some instances of "bronchiolitis obliterans" developing in the setting of
normal
pulmonary function tests. Other times the observation of bronchiolitis obliterans
represents a harbinger of a subsequent poor clinical response if for example 100
airways are affected and 120 affected airways are required for detection of
pulmonary function abnormalities. Obviously the extent of injury to the airways
would have some effect on pulmonary function alterations.
It should be noted that, while the small airways are
destroyed, the bronchi (large airways) become dilated, chronically inflamed, and
fibrotic. Why bronchi dilate and small airways scar down is unclear - it may
simply
be due to luminal diameter, but this paradox exists. Ectasia accounts for
exuberant
production of mucus and its potential fungal colonization. Bronchocentric
granulomatosis like reactions to fungi may occur in these cylindrically dilated
air passages.
The development of bronchiolitis obliterans is associated
with several conditions. First it is related to the number, frequency, and
intensity of acute rejection episodes. Second there is a suggestion that
bronchiolitis obliterans may be related to previous pulmonary infection,
particularly cytomegalovirus infection. Third, there is some evidence that early
ischemic damage to the lung may be associated with the development of bronchiolitis
obliterans. Milne et al and Yousem have also shown that organizing pneumonia like
reactions in airways and airspaces may induce bronchiolitis obliterans.
The development of bronchiolitis obliterans in the
allograft lung does not necessarily mean that it is due to immunologic activity.
Numerous causes of small airway scarring have been recognized in lung transplant
and
non-transplant patients. Particularly relevant to the lung allograft recipient is
the role of infection. Numerous infectious agents (to which the immunocompromised
lung recipient is exposed) have been documented to cause bronchiolitis obliterans.
These include many bacterial infections, viral infections, and some atypical
organisms, including mycoplasma and chlamydia. In our practice, if we note
histologic OB after a non-rejection related inflammatory process, we are certain to
note this etiology in our pathology reports so that patients are not treated as for
rejection induced OB. Second the development of acute harvest (ischemic)
injury to
the lung parenchyma has been associated with airway and interstitial scarring.
This
is particularly relevant in individuals who experience severe diffuse alveolar
damage with secondary airway scarring similar to that in pediatric bronchopulmonary
dysplasia. Third, because of the loss of their cough reflex lung transplant
recipients are predisposed to aspiration. Aspiration has been recognized as one
cause of bronchiolitis obliterans and obstructive lung disease, however it is
extremely rare to identify aspirated food stuffs in the airways of lung recipients
who are several months post-transplant or who have OB.
In the differential diagnosis of bronchiolitis obliterans
one must recognize histologic mimics especially to the intra-airway granulation
tissue phase of the OB reaction. First acute cellular rejection with small airway
inflammation can have perivascular mononuclear infiltrates accompanied by
intraairway granulation tissue polyps. It is important to recognize that this
granulation tissue reaction is part of the acute lung injury associated with the
acute rejection reaction, rather than bronchiolitis obliterans. Second, a few lung
transplant recipients have developed bronchiolitis obliterans organizing
pneumonia.
Again these individuals have granulation tissue within their airways and airspaces
and have a rather dramatic response to steroid therapy, much as one would
anticipate
in an acute rejection reaction. Finally in patients who experience diffuse
alveolar
damage, the proliferative phase of this reaction is frequently associated with
intraluminal granulation tissue polyps. It is particularly important to highlight
the need to distinguish loose granulation tissue reactions from dense eosinophilic
scarring of the airways, an essential distinction in predicting prognosis.
Numerous immunohistochemical studies have been performed
on
patients with bronchiolitis obliterans. It has been noted these patients have
increased expression of class II antigens by the epithelial cells of the bronchial
mucosa. In addition, the cellular infiltrate appears rich in cytotoxic T cells
rather than helper T cells, and is accompanied by a significant infiltrate of S100
positive Langerhans cells. Some individuals have also noted that Leu 7 positive
natural killer cells are more prominent in bronchiolitis obliterans than other
conditions. Recent studies have also indicated that granzyme and perforin
producing
T cells are more commonly noted in chronic airway rejection.
One of the most difficult problems in the diagnosis of OB
is its means of histologic confirmation. The role of transbronchial biopsy(TBBx)
has been controversial however at the University of Pittsburgh, TBBx is the method
of choice for the diagnosis of OB, having at least a 60% sensitivity rate and 95%
specificity. The utilization of TBBx requires several components, the most
important being a pulmonologist who is willing to obtain the necessary five
pulmonary parenchymal fragments to achieve good results. Second, a pathologist
committed to the interpretation of TBBx is essential. Third, adequate sectioning
is necessary - at least three H&E slides at three levels is necessary, and
should be accompanied by trichrome, elastic and Grocott stains.
The pathologist must be aware that a slide displaying a
well oriented cross section of a membranous or respiratory bronchiole is the
exception rather than the rule in a TBBx. One needs to learn to recognize the
interrupted fascicles of smooth muscle which define the respiratory bronchiole and
to identify the amount of "normal" connective tissue surrounding the muscle and
within the submucosa. This is essential because OB frequently is identified as
bridging bands of scar tissue crisscrossing airway lumens and obliterating the
conducting passage. Bands of blue collagen on trichrome connecting cords of red
smooth muscle provide the key to early diagnosis. In our experience, the vast
majority of histologic OB cases will have a clinical correlate in new onset
pulmonary function abnormalities. In the absence of TBBx confirmation, a repeat
TBBx will be performed which increases yield by 10 - 20%. If the diagnosis is
still
unconfirmed after two TBBx sessions, the patient is either given a clinical
diagnosis of OB using the BOS criteria or taken to thoracoscopic wedge biopsy.
It should be emphasized that scarring of the respiratory
bronchioles or even alveolar ducts requires that one assumption be made about the
donor lung - that it is normal when harvested. In fact, many donor lungs are from
cigarette smokers and show evidence of smokers (respiratory) bronchiolitis with
fibrosis of the alveolar ducts and airways. This needs to be factored when a
diagnosis of OB is considered. Be certain to compare the current biopsy to
previous
ones, especially the first after transplant where OB is rarely seen.
In the experience of the University of Pittsburgh
approximately 30% of our patient population develops clinical and pathologic
bronchiolitis obliterans usually late in the first year after transplant. The
response of these patients to interventional therapy has been disheartening.
Approximately two thirds of the patients experience a progressive unrelenting loss
of pulmonary function which may continue for several years. The development of
progressive obstruction is frequently accompanied by repetitive infectious episodes,
primarily bacterial and fungal, which exacerbates this progressive decline.
Ultimately patients expire of an infectious complication. Bronchiolitis obliterans
develops on an average of 11 months post-transplant, however it has been noted to
develop as early as two months after transplant and as late as several years after
transplantation. New therapies are currently being developed to combat this
chronic process, and at the University of Pittsburgh we have focused largely on the
use of aerosolized cyclosporine.
Graft Atherosclerosis:
Developing hand-in-hand with bronchiolitis obliterans is a
progressive myointimal thickening of the pulmonary arteries and veins which
corresponds to the graft atherosclerosis seen in the coronary arteries of heart
allografts. In fact the development of pulmonary atherosclerosis in heart/lung
recipients correlates strongly with the development of coronary artery changes in
the cardiac allograft. The pulmonary vascular changes which develop seem to have
little functional impact however and at this point in time are histologic
curiosities. I have seen rare cases of marked vasculitis in acute rejection, which
caused disproportionate vascular destruction and in these cases pulmonary
hypertension was noted. We are unclear what isolated atherosclerosis in a TBBx
means, in the absence of OB. It may occasionally be due to donor disease perhaps
previous vascular damage from acute rejection, or a harbinger of OB. This finding
needs to be further investigated.
Philosophical Aside:
Although we classify acute rejection by the degree of
perivascular infiltrates, it is important to recognize that the long term survival
of the graft is tied to a progressive airway obliterative process. To many of us,
this paradox highlights the need to grade the degree of airway inflammation in
acute
rejection reactions (B1-B4) and to study airway damage over time in lung
allografts.
With mononuclear cell damage to epithelium and mesenchyme, there comes a point in
time where scarring develops and bronchiolitis obliterans is diagnosed. How this
fibrosis develops is a question. Is bronchiolitis obliterans simply an
uncontrolled
acute rejection? Does bronchiolitis obliterans develop some time after
transplantation when specific lymphocytes and mononuclear cells are stimulated and
become alloreactive? Is there a low level of airway rejection in all transplant
patients that smolders along until enough airways are damaged to cause obstructive
function tests? Probably each hypothetical scenario accounts for some cases of
chronic rejection.
We also use fibrosis to differentiate bronchiolitis
obliterans from the airway damage in acute rejection. In future grading schemes,
it
may be worthwhile to develop three scales of rejection grade: a perivascular
inflammation grade, and airway inflammation grade, and a degree of fibrosis grade
which, in the context of the number of vessels and bronchi/bronchioles affected,
could generate a numerical score of rejection.
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