Interstitial lung diseases-can pathologists arrive at an etiology-based diagnosis? A critical update

[Interstitial lung disease associated with connective tissue disease]

Interstitial lung disease (ILD) is the most frequent organ manifestation in rheumatic autoimmune disease. Depending on the underlying autoimmune disease, differently pronounced affections of small airways, interstitial parenchyma, and vessels are found. The group of rheumatic autoimmune diseases mainly includes connective tissue diseases (CTDs), also known as collagen vascular diseases, such as rheumatoid arthritis (RA), systemic sclerosis, (SSc), systemic lupus erythematosus, primary Sjögren's syndrome, idiopathic inflammatory myositis (IIM), and interstitial pneumonia with autoimmune features (IPAF). Frequency and manifestations of parenchymal lung disorders are described clinically, radiologically, and morphologically in these entities. For the precise diagnosis and for the differentiation between the wide range of parenchymal disorders with known possible cause or with unknown origin, also called unclassifiable or idiopathic interstitial pneumonias (IIPs), high resolution computed tomography (HRCT) findings represent the diagnostic gold standard. A transbronchial biopsy, surgical biopsy, or cryobiopsy will be used in unclassifiable findings to confirm a definitive histological confirmation. A precise diagnosis of these ILDs is crucial since the different pathologies that encompass ILD have different therapeutic options. In this sense, the participation of a pneumologist, rheumatologist, radiologist, and pathologist become essential in the multidisciplinary evolution of ILD.

[Pulmonary Langerhans cell histiocytosis]

Pulmonary Langerhans cell histiocytosis is regarded as a reactive proliferation of the dendritic Langerhans cell population stimulated by chronic tobacco-derived plant proteins due to incomplete combustion but can also occur in childhood as a tumor-like systemic disease. Currently, both these forms cannot be morphologically distinguished. In the lungs a nodular proliferation of Langerhans cells occurs in the bronchial mucosa and also peripherally in the alveolar septa with an accompanying infiltration by eosinophilic granulocytes and destruction of the bronchial wall. Langerhans cells can be selectively detected with antibodies against CD1a and langerin. In the reactive isolated pulmonary form, abstinence from tobacco smoking in most patients leads to regression of infiltration and improvement of symptoms. In high-resolution computed tomography (HRCT) the small star-like scars can still be detected even after complete cessation of tobacco smoking.