Antemortem cytologic diagnosis of pulmonary Langerhans cell histiocytosis in a cat

Abstract

Feline pulmonary Langerhans cell histiocytosis (FPLCH) is a rare histiocytic proliferative disease of middle‐aged to older domestic cats. Langerhans cells in the terminal airways proliferate and infiltrate the interstitium and the airways to a lesser degree, widely effacing normal parenchyma. Historically, definitive diagnosis has required postmortem evaluation where pulmonary lesions have a classic gross and histologic morphology. Here, we present the first documented antemortem diagnosis of FPLCH using bronchoalveolar lavage (BAL) cytology and immunocytochemistry (ICC) in a 9‐year‐old British shorthair mix. The cat had a 3‐month history of respiratory difficulty that was refractory to steroids and antimicrobials. Pulmonary radiographs had marked diffuse changes with a complex bronchointerstitial and micronodular pattern. BAL cytology revealed neutrophilic inflammation and markedly increased histiocytes with morphology distinct from typical pulmonary macrophages. ICC characterized histiocytes as CD1a+/E‐cadherin+/CD11b−/PanCK−, consistent with a Langerhans cell phenotype. The cat was humanely euthanized due to poor prognosis and presented for necropsy. Gross, histopathologic, immunophenotypic, and ultrastructural findings confirmed a diagnosis of FPLCH. Proliferative cells were E‐cadherin+/Iba‐1+/CD18+/CD1a+/CD5+/MHCII+/CD204−/CD4−; transmission electron microscopy identified the presence of Birbeck granules in the proliferating histiocytes, consistent with previous reports of FPLCH.