Fig. 5— Immunohistochemical detection of serotonin (5-hydroxytryptamine; 5-HT)2B receptor. In bleomycin-treated mouse lungs, the 5-HT2B receptor was expressed by a) bronchial epithelial cells and b) alveolar macrophages (arrows), c) weakly by smooth muscle cells and focally by endothelial cells, and d) strongly by inflammatory cells within the lesions (macrophages, lymphocytes and polymorphonuclear cells). Mesothelial cells remained negative for this receptor. e) In control murine lungs, 5-HT2B was mainly expressed by bronchial epithelial cells and f) focally by pneumocytes along the alveolar wall. g) Negative control using mouse immunoglobulin G1 shown for comparison. h) Immunohistochemical localisation of the human 5-HT2B receptor in cryosections of lungs from idiopathic pulmonary fibrosis patients, using the VECTASTAIN® ABC alkaline phosphatase kit system (Vector Labs, Abcys, Paris, France) and the Fast Red substrate (Dako, Trappes, France) showed, in contrast to 5-HT2A, that the 5-HT2B receptor was strongly expressed by fibroblasts in the fibroblastic focus (ff) and in the surrounding fibrotic tissue (arrows), as well as by hyperplastic pneumocytes. br: bronchiole; bv: blood vessel; as: alveolar space. a–d, g and h) Scale bars = 100 µm. e and f) Scale bars = 50 µm.