(A and B) Frozen sections from nodular lymphocyte predominance HD stained with the BCL-6 MoAb (A) and the syn-1 MoAb (B). L&H cells show a strong nuclear positivity for BCL-6 (A), whereas no syn-1 expression is detectable (arrows). Residual plasma cells show cytoplasmic staining for syn-1 MoAb (B). (C and D) Frozen sections from classic (nodular sclerosis) HD stained with the BCL-6 MoAb (C) and the syn-1 MoAb (D). No BCL-6 expression is detectable; a BCL-6⁻ RS cell is shown (arrow) (C). Conversely, several RS cells show cytoplasmic and membrane staining for syn-1 (D). (E and F) Serial frozen sections from a case of classic (nodular sclerosis) HD stained with the BCL-6 MoAb (E) and the syn-1 MoAb (F). Few RS cells show a nuclear positivity for BCL-6 (E), whereas the majority of RS cells express syn-1 (F); the intensity of staining of RS cells for syn-1 (arrows) is lower than that of reactive plasma cells in the background (asterisk). (G and H) Frozen sections from cases of classic (nodular sclerosis) HD tested by two-color staining with BCL-6 MoAb and syn-1 MoAb (see Materials and Methods). In both cases BCL-6⁺ (nuclear, blue) RS cells and syn-1⁺ (cytoplasmic and membranous, red) RS cells (arrows) are present. Note positive staining of bystander plasma cells (asterisks). No coexpression of BCL-6 protein is detectable in the syn-1⁺ RS cells. APAAP immunostaining, hematoxylin counterstain (A through F). Two-color staining, no counterstain (G and H). Original magnification × 180 (A and B), × 250 (C through G), × 400 (H).