Pulmonary histiocytosis X. Immunoperoxidase staining for HLA-DR antigen and S100 protein

Immunoperoxidase staining for S100 protein and HLA-DR antigen was used to identify histiocytosis X (HX) cells in 23 cases of pulmonary histiocytosis X (PHX), three cases of idiopathic pulmonary fibrosis, and one case of hypersensitivity pneumonitis. S100 protein was present in HX cells in 22 of the PHX cases; HLA-DR antigen was present in HX cells from 16 cases. Varying numbers of peribronchiolar and interstitial cells were positive for either S100 or HLA-DR in two of the three cases of idiopathic pulmonary fibrosis, and in the case of hypersensitivity pneumonitis. Immunoperoxidase staining for chromogranin showed isolated neuroendocrine cells within the mucosa and wall or airways, sites in which HX cells were occasionally found. As other types of dendritic cells, as well as some neuroendocrine cells, may contain S100 protein, positive staining for S100 is not specific for HX cells.