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Antibody-Mediated Rejection in Lung Transplant: Pathologist's Perspective

Prodipto Pal, MD, PhD

Rex Michael Santiago, MD
University Health Network
University of Toronto
Toronto, ON, Canada

Antibody-mediated rejection (AMR) is a complex process, well-recognized in kidney and cardiac allografts, but remains ill-defined in lung transplantation [1]. There are no agreed-upon histopathologic diagnostic criteria in pulmonary allografts. AMR as a cause of chronic lung allograft dysfunction (CLAD) is well-established in the literature; yet, the true incidence of pulmonary AMR remains unknown due to lack of comprehensive diagnostic criteria. The consequences of AMR mediated by donor HLA-specific antibodies (donor specific antibodies - DSA) range from persistent/recurrent acute cellular rejection (ACR) to lymphocytic bronchiolitis, and chronic rejection manifesting as bronchiolitis obliterans syndrome (BOS) to the irreversible changes of chronic rejection and CLAD [2, 3].

The pathologic features of AMR are non-specific. The key concept of AMR stems from immune activation and antibody production directed against donor lung antigens by allospecific B-cells and plasma cells, whereby antigen-antibody complexes result in an amplified immune response, mediated by both complement dependent and independent pathways and manifests as transplant associated changes - the pathologic morphological features are reviewed in detail by Berry et al in an International Society for Heart and Lung Transplant (ISHLT) publication [4]. C4d plays a central role in the classification scheme of AMR, which can be detected by immunohistochemistry (IHC) or by immunofluorescence techniques. The spectrum of histologic findings in biopsies, when present, is an indication for further work up for C4d status. The AMR histologic patterns, as mentioned earlier, can be quite diverse. Historically, AMR had been associated with the so-called "hyperacute rejection" where primary graft failure occurred very early post-transplant; morphologically, the findings included fibrin thrombi, fibrinoid necrosis of alveolar septal wall and hemorrhage [5]. Pulmonary capillaritis was the histological mainstay of AMR, although with poor reproducibility [5]. The possible/probable histological findings were later expanded and range from neutrophilic capillaritis to neutrophilic septal margination, high grade ACR or persistent/recurrent ACR, diffuse alveolar damage, high grade lymphocytic bronchiolitis or persistent low grade lymphocytic bronchiolitis (ISHLT grade B2R or persistent B1R), obliterative bronchiolitis and arteritis in the absence of infection or cellular rejection [4]. Interestingly, two additional indications for C4d status included graft dysfunction without morphologic explanation and any histologic finding in a setting of de novo DSA positivity. It should be noted that the aforementioned histologic patterns can be seen in a variety of clinical settings including infection (bacterial and viral), spectrum of ACR, graft preservation injury or reperfusion injuries and secondary to drug reactions.

C4d status determination deserves special mention, as detection of this marker is central to all the recent AMR classification systems. IHC for C4d is an excellent marker for AMR in cardiac and renal transplant patients; however, the interpretation of C4d in lung tissue presents a unique challenge. This can be partly explained by the distinctive microanatomy of lung with multitude of small capillaries, an abundance of pulmonary macrophages, which makes the other markers of AMR detection (e.g., kidney and cardiac allografts) such as CD31 (endothelial marker), CD68 (macrophage marker) extremely difficult to often impossible to interpret. Moreover, since the lung is more often exposed to immunologic challenges (air-borne or hematogenous) as well as infectious or injurious insults, more so than other solid organ transplants (such as the kidney or the heart) further limits an accurate interpretation of C4d. In fact, this hypothesis is indirectly supported by the observation of conflicting results on C4d IHC in pulmonary allograft biopsies [6]. Briefly, C4d staining is reported in interstitial capillaries, vascular and airway elastic fibers, septal walls, venules/arterioles/muscular arteries, peribronchial capillaries, alveolar lining cells and hyaline membranes [7, 8]. The ISHLT recommendation is that the assessment of C4d expression should be made only within interstitial alveolar capillaries, with an immunoreactivity threshold of > 50% to be considered positive [4]. There is limited data in the literature on C4d detection by immunofluorescence on frozen tissue with no specific ISHLT recommendation. The primary objective is to assign accurate subclassification of AMR, without compromising sensitivity; the current recommendation is a multidisciplinary approach using a combination of findings with regard to clinical allograft dysfunction, serologic evidence of DSA, C4d expression and pathologic biopsy findings, reviewed in detail by Levite et al. in a recent article, wherein clinical/subclinical AMR were diagnostically categorized in three categories, viz., definite, probable and possible, based on aforementioned clinicopathologic parameters [1].

With evolving technologies coupled with considerable clinical interest in defining pulmonary AMR, the research opportunities are immense, specifically in elucidating a more comprehensive assessment of biological pathways and towards facilitating more reproducible diagnosis. Newer approaches include gene expression data in a panel of immune regulatory genes using microarray technologies [9]. DSA-induced immune responses operate by activating complements, recruiting natural killer (NK) cells and monocytes, while, the complement-independent DSA operates predominantly via monocyte recruitment (reference). An approach interrogating the biological pathways, specifically the role of NK cells, typable by CD16 IHC, along with downstream interferon-? induced increased endothelial expression of major histocompatibility complex (MHC) is appears quite promising. Other studies have employed a combinatorial approach of flow cytometric characterization of peripheral blood mononuclear cells (viz., monocytes) in combination with tissue based morphologic findings [10].

In summary, a practical definition and consensus pathologic criteria towards facilitating reproducible diagnosis is critical to further our understanding of AMR. ■

Disclosure Statement: The author has no conflicts of interest to disclose.


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