Abstract
Hypersensitivity pneumonitis (HP) is one of the most common interstitial lung diseases (ILD), that presents unique challenges for a confident diagnosis and limited therapeutic options. The disease is triggered by exposure to a wide variety of inciting antigens in susceptible individuals which results in T-cell hyperactivation and bronchioloalveolar inflammation. However, the genetic risk and the pathogenic mechanisms remain incompletely elucidated. Revised diagnostic criteria have recently been proposed, recommending to classify the disease in fibrotic and non-fibrotic HP which has strong therapeutic and outcome consequences. Confident diagnosis depends on the presence of clinical features of ILD, identification of the antigen(s), typical images on high-resolution computed tomography (HRCT), characteristic histopathological features, and lymphocytosis in the bronchoalveolar lavage. However, identifying the source of antigen is usually challenging, and HRCT and histopathology are often heterogeneous and not typical, supporting the notion that diagnosis should include a multidisciplinary assessment. Antigen removal and treating the inflammatory process is crucial in the progression of the disease since chronic persistent inflammation seems to be one of the mechanisms leading to lung fibrotic remodeling. Fibrotic HP has a few therapeutic options but evidence of efficacy is still scanty. Deciphering the molecular pathobiology of HP will contribute to open new therapeutic avenues and will provide vital insights in the search for novel diagnostic and prognostic biomarkers.
Highlights
Hypersensitivity Pneumonitis (HP) is an immune-mediated disease that manifests as interstitial lung disease (ILD) in susceptible individuals after exposure to identified or unidentified inciting agent(s) [1]
From ATS/JRS/ALAT [1], and from CHEST [31] recommended diagnostic algorithms based in three domains: exposure identification, high-resolution computed tomography (HRCT) findings, and bronchioalveolar lavage (BAL) lymphocytosis, which in the case of the ATS/JRS/ALAT diagnostic criteria is strengthened by histopathologic findings
Several studies have reported acute exacerbation (AE) in patients with fibrotic Hypersensitivity pneumonitis (HP), following the same definition used in IPF, which results in poor prognosis [61]
Summary
Hypersensitivity Pneumonitis (HP) is an immune-mediated disease that manifests as interstitial lung disease (ILD) in susceptible individuals after exposure to identified or unidentified inciting agent(s) [1]. At the initial stages of fibrosis, the disease may stabilize or even improve in the pulmonary functional status, a subset of patients develops an aggressive phenotype called progressive pulmonary fibrosis that results in the destruction of the lung architecture [24] The mechanisms triggering this devastating phenotype are unclear but may include the type of fibrosis (UIP vs non-UIP pattern), the aberrant composition and stiffness of the extracellular matrix, and the emergence of some unique profibrotic cell subsets [25]. Since antigen T-cell mediated immune response plays a pivotal role in the pathogenesis of HP it has been proposed that lymphocyte proliferation testing may be a diagnostic tool [30, 35, 36] Studies using this method are scant, usually performed in small cohorts, and primarily in patients suspected to have bird-related HP. UIP-like pattern is related with worst survival [1, 45, 46]
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