Immunohistochemistry for JAK2 and p-STAT6. IHC for JAK2 with an lpHL case (A) and a tonsil (B) is shown. Inserts show higher magnifications of an L&H cell in panel A and the mantle zone/GC border in panel B. While JAK2 amounts in normal B cells were below the detection limit of IHC, 40 of 47 cases of lpHL showed positivity in L&H cells. (C) IHC with an lpHL case and a p-JAK2 specific antibody is shown. Fractions of L&H cells were positive in 13 of 33 analyzed cases. (D) IHC for p-STAT6 with an lpHL case is shown. Twenty-one of 43 analyzed lpHL cases showed positivity for p-STAT6. All cases were from the files of the Department of Pathology of the University of Frankfurt. Bound antibodies were detected using the Envision system (Dako, Hamburg, Germany) for anti-JAK2 and the CSAII signal amplification system for anti–p-JAK2 and anti–p-STAT6 with horseradish peroxidase and 3.3-diaminobenzidine as substrate. Specificity of antibodies was verified by comparison of IHC with Western blot analysis (JAK2 and p-STAT6; Figures S1,S2) and/or by the use of 2 different antibodies directed against the same antigen (JAK2 and p-JAK2). Specificity of anti–p-JAK2 and anti–p-STAT6 antibody for phosphorylated JAK2 and STAT6 was tested by pretreatment of several cases with T-cell phosphatase,⁵  which abolished staining completely (Figure S3). (E) Western blot analysis of JAK2 expression and activation in the lpHL-derived cell line DEV is shown. Whereas JAK2 was expressed in the MLBCL-derived cell line Karpas1106P, the cHL-derived cell line KMH2, and DEV, activation of JAK2 was observed only in Karpas1106P and DEV, in line with the presence of SOCS1 mutation only in Karpas1106P and DEV⁶ .