Detection of activated CTLs. (A) Biopsy specimen of an ALCL patient with ≥ 15% activated CTLs, presenting with stage 1 disease who died 13 months later as a result of the disease. Brown cytoplasmic staining indicates GrB expression. Tumor cells are negative for GrB. (B) Biopsy specimen of an ALCL patient with a cytotoxic phenotype of the tumor cells and ≥ 15% activated CTLs. Both GrB⁺ tumor cells (arrowheads) and activated CTLs (thick arrows) show brown cytoplasmic staining; thin arrow indicates GrB⁻ reactive lymphocytes. (C) Double-staining for CD8 and GrB in a biopsy specimen of an ALCL patient. The majority of activated CTLs express both CD8 (brown membranous staining) and GrB (black cytoplasmic staining). Tumor cells are CD8⁻ and GrB⁻. (D) Double-staining for CD8 and GrB in a biopsy specimen of an ALCL patient with cytotoxic phenotype of the tumor cells. Tumor cells (arrowheads) show expression of GrB (black cytoplasmic staining), but not CD8, whereas activated CTLs (thick arrows) show expression of both GrB (black cytoplasmic staining) and CD8 (brown membranous staining).