In vivo localization of CD1a⁺ PPARγ⁻ and CD1a⁺ PPARγ⁺ dendritic cells. Double IF or IP stainings for PPARγ-expressing cells in human reactive lymph nodes (A-E) and lung tissues with histiocytosis-X (F-J) that may coexpress DC markers with characteristic antigen-presenting cell morphologies. (A,F-G) Hematoxylin-eosin (HE)–stained sections used for immunolabeling. (B-C) Lymph node with increased number of CD1a⁺ DCs (green cytoplasmic fluorescence) but negative for PPARγ expression (red nuclear fluorescence), both located predominantly at the interfollicular compartments (compare the position of germinal center [GC] in panel A with the cells in panels B and C separately stained for green and red). (D-E) Lymph node normally harbors both DC-SIGN–positive (D) and S100-positive (E) cells with characteristic cytoplasmic projections (green) and both may coexpress PPARγ nucleoprotein (red, arrowheads) or not contain detectable amounts of PPARγ (arrow). Note that few DC-like PPARγ⁺ cells with characteristic S100⁺ cytoplasmic projections can be seen within the germinal centers (not shown). (F-G) HE of disseminated pulmonary Langerhans-cell histiocytosis showing nodular infiltrates along the alveolar spaces (F, asterisks) and the characteristic morphology of proliferating LCs with oval vesiculated nuclei among the reactive lymphocytes and few eosinophils (G). The detailed clinicopathology of this case was reported earlier.²⁶  Double IF (H) and IP on serial sections with identical fields (I-J) demonstrate that LCs with characteristic CD1a positivity (arrows) do not express PPARγ, which confirms our in vitro data and indicates that a double-positive antigen-presenting cell phenotype in human tissues is unlikely. Few PPARγ⁺ cells, however, are found in such lesions, likely corresponding to inflammatory macrophages (arrowheads). Identical bronchioles are indicated by the letter b. IF-stained sections have DAPI and IP-stained sections have methyl-green nuclear counterstainings. Objectives used: × 4/0.1 NA (F); × 20/0.4 NA (A-B,I-J); × 40/0.65 NA (E,G,H); × 100/1.3 NA oil (C-D).