IL-4 expression in primary CNS lymphoma. (A) Immunohistochemical localization of IL-4 (brown color) to vessels associated with angiotropic CNS lymphoma cells at the infiltrating edge of a primary CNS lymphoma. (B) IL-4 immunoreactivity was not detected in the vessels of normal brain. (C) IL-4 immunoreactivity was absent or less prominent in association with the vessels of large B-cell lymphoma isolated from lymph nodes. Original magnification × 200 for panels A-C. (D-E) Dual-color immunofluorescence demonstrates colocalization of IL-4 (D-E; Alexa 594 red immunofluorescence) and von Willebrand factor (E; FITC immunofluorescence) on tumor vessels in primary CNS lymphomas. Original magnification × 400 for panels D-E. (F-I) Endothelial cells in CNS lymphoma are immunoreactive for IL-4 as demonstrated by confocal microscopy. Diffuse cytoplasmic and perinuclear IL-4 expression by an endothelial cell did not exhibit significant spectral overlap with von Willebrand factor expression, which appeared to be localized to specialized Weibel-Palade bodies. Dimensions of displayed images are 68.3 μm × 31.8 μm each.