Expression of phosphorylated STAT6 and MHC-I proteins in cHL. Expression of MHC-I (A-C), and pSTAT6 (E-G) was examined by immunohistochemistry in formalin-fixed sections of cHL. (A) HRS cells with absent expression of MHC-I in a exemplificative B2M mutated cHL case. (B) HRS cells with absent expression of MHC-I in a exemplificative B2M wild-type (WT) cHL case, in which alternative genetic mechanisms not explored by our assay, may be responsible for the loss of MHC-I expression. (C) HRS cells with normal expression of MHC-I in a exemplificative B2M WT cHL case. Staining for MCH-I was absent on the HRS cell surface of 11/18 patients with cHL, as clearly shown by the lack of surface staining at the cell boundaries in the cluster of HRS cells, including 5/5 cases harboring B2M mutations and 6/13 cases lacking B2M mutations (D). (E) HRS cells with a nuclear overexpression of p-STAT6 in a cHL case harboring STAT6 mutation. (F) HRS cells with a nuclear overexpression of p-STAT6 in a exemplificative STAT6 WT cHL case, consistent with the plethora of genetic mechanisms affecting the cytokine signaling program in this tumor, some of which are not explored by our assay and may be responsible for the overexpression of p-STAT6. (G) HRS cells with absent nuclear expression of p-STAT6 in a exemplificative STAT6 WT cHL case. Staining for nuclear pSTAT6 was positive in HRS cells of 14/18 patients with cHL, including 8/8 cases harboring STAT6 mutations and 6/10 cases lacking STAT6 mutations (H). Original magnifications ×630.