The monoclonal anti-HERV-W Env antibody 6A2B2 recognizes an antigen expressed in placenta and acute inflammatory MS lesions. Immunohistochemistry was performed with mAb 6A2B2 on human placenta (A) and an acute MS lesion (B). Arrowheads in A point to syncytiotrophoblast cell layer. Strong staining with 6A2B2 was seen in a case of fulminant MS in activated microglia/macrophages, mononuclear cells, and endothelial cells (B). Magnification × 200 (A), × 400 (B). Double immunofluorescence was carried out on placenta tissue (C-F), with mAb 6A2B2 (C, green) and anti-Syncytin-1 pAb (D, red). Cell nuclei were counterstained with DAPI (E, blue). Merged image (F). White arrowheads in D highlight membrane-associated staining. Magnification × 400 (C-F).