Original contributionCharacterization of lymphangiogenesis in various stages of idiopathic diffuse alveolar damage
Introduction
Diffuse alveolar damage (DAD) is a histologic manifestation of acute lung injury and is associated clinically with acute respiratory distress syndrome [1], [2], [3], [4]. The condition also is manifested as an idiopathic disease, acute interstitial pneumonia [5], [6], [7], [8], [9]. These conditions generally are refractory to anti-inflammatory treatment and often cause lethal respiratory failure [1], [5], [6], [7], [8], [9]. On the basis of histopathologic evidence, DAD is divided into 3 consecutive stages: exudative, defined by the formation of a hyaline membrane and edema of the alveolar wall; proliferative, defined by the presence of intra-alveolar fibrosis; and fibrotic, defined by the shrunken interstitium neighboring dilated air spaces (honeycomb formation) [1], [4]. The mechanism by which DAD occasionally progresses to irreversible fibrosis of the lung is unclear.
Inflammatory fibrosis has been hypothesized as the explanation for the tissue destruction and fibrogenic remodeling associated with pulmonary fibrosis [10]. However, findings from experimental pulmonary fibrosis do not necessarily agree with this hypothesis. Mice deficient in the β6-integrin chain (β6−/−) develop exaggerated inflammation. However, they are protected from bleomycin-induced pulmonary fibrosis, although a similar concentration of transforming growth factor-β was detectable in both β6−/− and β6+/+ mice [11]. A fibrogenic response in the lung has been demonstrated in a mouse model of hyperoxic stress under blood-free conditions where the development of inflammation is limited [12]. Clinical outcomes in patients poorly compliant with anti-inflammatory therapy for DAD indicate that a pathogenic mechanism other than inflammatory fibrosis causes progression of the disease [10].
The lymphatic system plays various roles in fluid homeostasis and the activation of adaptive immunity by fluid drainage and cell transport [13]. Exploiting the ability to detect lymphatics by immunohistochemistry techniques, lymphangiogenesis has been discovered in various pathophysiological conditions [14], [15], [16], [17], [18], and its significance in tumor metastasis and wound healing has been postulated [19], [20], [21], [22], [23]. It has been demonstrated that in wound healing, lymphangiogenesis occurs after angiogenesis by the sprouting of existing lymphatics, and the resultant lymphatic system exists as a transient structure [22]. Lymphatics were observed to be less developed in a prolonged healing condition such as chronic skin ulcers than in normal healing [22]. It is reasonable to hypothesize that newly formed lymphatics play important roles in the later processes of wound healing, such as fibrosis.
Pulmonary fibrosis is believed to be a pathological consequence of wound healing in the lung [10]. We have focused on lymphangiogenesis in pulmonary fibrosis. The principal aim of this study was to characterize lymphangiogenesis in a fibrogenic condition associated with idiopathic DAD.
Section snippets
Materials
We examined autopsy lung specimens of 11 men and 2 women with idiopathic DAD that were obtained from the archives of the Department of Pathology at Saka General Hospital, Ishinomaki Red Cross Hospital, Sendai Medical Center, or Iwate Medical University. Patients with idiopathic DAD fulfilled the clinical and pathological criteria for acute interstitial pneumonia described by Katzenstein et al [5]. None had any evidence of systemic infection, iatrogenic causes of immunosuppression, toxic
Stage-specific histopathologic characteristics of lymphatic and blood vessels in DAD progression
In the exudative stage, characterized by the formation of the hyaline membrane and mild fibrosis (Fig. 1A), greater numbers of capillaries were observed in the alveolar wall than in normal lung (Fig. 1B and C, inset). Lymphatics were observed around large blood vessels and in the lobular septa, as generally observed in a normal lung (Fig. 1C). Hereafter, the lymphatics with these characteristics are termed “existing lymphatics.” In the proliferative stage, characterized by the fibrous
Discussion
The present study confirmed stage-specific characteristics during the progression of idiopathic DAD, particularly with regard to lymphangiogenesis and capillary angiogenesis. The results lead to new insights, revealing that robust lymphangiogenesis occurs in intra-alveolar fibrotic lesions in the proliferative stage of DAD. On the other hand, capillary angiogenesis was observed to occur early in the exudative stage and subsided thereafter. Notably, capillary angiogenesis was induced to a
Acknowledgments
We would like to thank Dr Keiichi Saito, Mrs Naoko Shibata, Mr Koichi Sato, and Dr Toshio Kumasaka for their excellent help.
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