Pulmonary Apical Cap as a Potential Risk Factor for Pleuroparenchymal Fibroelastosis

Pleuroparenchymal fibroelastosis (PPFE) is a progressive and frequently fatal interstitial lung disease that involves the upper lobes. Although its cause remains unknown, the histopathologic evidence underlying PPFE bears striking resemblance to that of the pulmonary apical cap (PAC), a relatively common and benign entity. We describe the case of a patient with PAC that evolved into distinctly asymmetric PPFE over 6 years after unilateral surgical lung injury. Given the histologic similarity between these two conditions, we propose that these two entities underlie common biologic pathways of abnormal response to lung injury, with the presence of a PAC increasing susceptibility to the development of PPFE in the face of ongoing inflammatory insults. This case describes the histopathologic evolution of PAC to PPFE before and after an inciting injury.

Significant increases in the density and number of lymphatic vessels in pleuroparenchymal fibroelastosis

AIMS

Some investigators have detected fibrinous exudate or immature organisation in the alveolar spaces prior to the development of subpleural elastofibrosis in patients with pleuroparenchymal fibroelastosis (PPFE). We hypothesised that PPFE progress is associated with an impaired lymphatic drainage system, resulting in the failed resolution of intra-alveolar exudate. The aim of this study is to investigate the pulmonary lymphatic vessels in PPFE, histologically.

METHODS AND RESULTS

We retrospectively reviewed our medical records from 1995 to 2017, and selected autopsied or surgically biopsied patients with PPFE (n = 18), pulmonary apical cap (n = 18), and IPF (n = 26). We detected lymphatic endothelial cells by using immunostained specimens, calculating the percentage of lymphatic vessel area in the non-aerated area (lymphatic vessel density) and the number of lymphatic vessels per non-aerated area (per mm² ) (lymphatic vessel number). These parameters in PPFE were compared with those in apical cap, IPF, and normal lung tissue. The lymphatic vessel density in PPFE patients [2.97%; interquartile range (IQR) 2.61-3.86] was significantly higher than that in normal lung (0.91%; IQR 0.84-1.07), pulmonary apical cap (0.67%; IQR 0.58-0.83), and IPF (0.91%; IQR 0.68-1.25) (P < 0.01 in any comparison). The lymphatic vessel number in PPFE was also significantly higher than that in normal lung, pulmonary apical cap, and IPF. Among PPFE patients, the increase in lymphatic vessel density was found to be correlated with the characteristic physiology of PPFE, such as a flattened chest cage on computed tomography and high residual volume/total lung capacity ratio on spirometry.

CONCLUSIONS

Significant increase in the density and number of lymphatic vessels is a supportive characteristic that enables the differentiation of PPFE from IPF and apical cap.

Podoplanin-positive myofibroblasts: a pathological hallmark of pleuroparenchymal fibroelastosis

Pathological differential diagnoses of pleuroparenchymal fibroelastosis (PPFE) include usual interstitial pneumonia (UIP) and pulmonary apical cap (PAC); however, there are no specific immunostaining makers to distinguish between these diseases. We performed immunohistochemistry using several pleural mesothelial cell-related markers, including cytokeratin-5/6, CAM5.2, WT-1, calretinin, desmin and podoplanin, for PPFE (n = 4), UIP (n = 10) and PAC (n = 3) lung sections. Among the examined markers, in PPFE and PAC lungs podoplanin commonly showed positivity for spindle cells both in thickened pleura and subpleural fibroelastosis lesions; these cells were also stained with α-smooth muscle actin, a marker of myofibroblasts. However, even in elastic fibre-rich cases, UIP lungs did not show such podoplanin-positive myofibroblasts in pleura/subpleura and fibroblastic foci. These findings were also verified using immunofluorescence. By contrast, immunohistochemically as well as morphologically, the difference between PPFE and PAC was not apparent. The presence of podoplanin-positive myofibroblasts could be a pathological hallmark of PPFE, suggesting a pathogenic process distinct from UIP but common to PAC.