Collagen and elastic system in the remodelling process of major types of idiopathic interstitial pneumonias (IIP) G F Rozin, M M Gomes, E R Parra, R A Kairalla, 1 C R R de Carvalho 1 & V L Capelozzi Department of Pathology and 1 Division of Pulmonary Diseases, Heart Institute (InCor), Universidade de Sa ˜o Paulo, Sa ˜o Paulo, Brazil Date of submission 1 March 2004 Accepted for publication 18 June 2004 Rozin G F, Gomes M M, Parra E R, Kairalla R A, de Carvalho C R R & Capelozzi V L (2005) Histopathology 46, 413–421 Collagen and elastic system in the remodelling process of major types of idiopathic interstitial pneumonias (IIP) Aims: Structural remodelling in acute and chronic idiopathic interstitial pneumonia (IIP) has been exten- sively investigated, but little attention has been directed to the elastic tissue in these situations. The aim of this study was to determine whether elastic deposition accompanies collagen deposition in the four major histological patterns of IIP: diffuse alveolar damage (DAD), organizing pneumonia (OP), non-specific inter- stitial pneumonia (NSIP) and usual interstitial pneu- monia (UIP). Methods and results: We measured, by image analysis, the content of fibres of the collagenous and elastic systems of the alveolar septum in histological slides of open lung biopsies, using the picrosirius-polarization method and Weigert’s resorcin–fuchsin stain, respect- ively. Five groups were studied: 10 cases of DAD; nine cases of OP; nine cases of NSIP; and 10 cases of UIP. Four normal lungs were used for comparison. The content of collagen fibres was significantly higher in UIP when compared to DAD, NSIP, OP and normal lung. The content of elastic fibres was increased in comparison with normal lungs but this was not significantly different among the histological patterns. Conclusion: Acute and chronic IIP cause a similar increase in the collagen and elastic contents of the lungs, representing a process of ‘fibroelastosis’ rather than an exclusive process of fibrosis. A profibrogenic mechanism is responsible for the unparallelled collagen augmentation observed in UIP subjects, the nature of which is yet to be determined. Keywords: collagen, diffuse alveolar damage, elastic fibres, extracellular matrix, idiopathic interstitial pneumonia, interstitial lung diseases, lung remodelling, non-specific interstitial pneumonia, organizing pneumonia, usual interstitial pneumonia Abbreviations: COP, cryptogenic organizing pneumonia; DAD, diffuse alveolar damage; IIP, idiopathic interstitial pneumonia; NSIP, non-specific interstitial pneumonia; OP, organizing pneumonia; UIP, usual interstitial pneumonia Introduction Idiopathic interstitial pneumonias (IIPs) comprise a heterogeneous group of diseases of unknown aetiology, which have different outcomes. In acute interstitial pneumonia (AIP), patients develop a rapid onset of dyspnoea with a dramatic and frequently lethal deteri- oration of pulmonary function. 1,2 In contrast, in cryptogenic organizing pneumonia (COP) patients pre- sent with subacute onset of dyspnoea responsive to corticosteroid therapy, achieving complete clinical and morphological recovery. 3,4 The non-specific pneu- monia pattern, besides its idiopathic form, is associated with a multitude of diseases of different aetiopathogen- esis. It most probably represents a non-specific response to lung injury in which the outcome depends mainly on the primary disease. 4–6 Finally, in idiopathic Address for correspondence: Vera Luiza Capelozzi MD, Departamento de Patologia Faculdade de Medicina da Universidade de Sa ˜o Paulo, Av. Dr Arnaldo 455, 01246-903, Sa ˜o Paulo, SP, Brazil. e-mail: vcapelozzi@lim05.fm.usp.br Ó 2005 Blackwell Publishing Limited. Histopathology 2005, 46, 413–421. DOI: 10.1111/j.1365-2559.2005.02103.x