ICOS expression of malignant cells in the blood of patients with SS. (A) Percentages of lymphoid cell populations within peripheral blood of 13 patients with SS and 12 healthy donors using flow cytometry. NK, natural killer. (B) Gating strategy for Sézary cells (CD4⁺CD158k⁺CD7⁻) by flow cytometry for a representative patient. (C) Flow cytometric evaluation of ICOS expression in patients with SS and healthy donors. Percentage of ICOS⁺ cells among Sézary CD4⁺ T cells and nontumoral CD4⁺ T cells. Sézary cells were defined as CD4⁺, KIR3DL2⁺, CD7⁻, or CD4⁺, KIR3DL2⁺, CD26⁻. Nontumoral CD4⁺ T cells are KIR3DL2⁻. Approximately 70% of Sézary cells expressed ICOS compared with <40% of nontumoral CD4⁺ cells in patients and CD4⁺ cells in healthy donors. (D) Gating strategy for Treg cells (CD4⁺FoxP3⁺CD25⁺) by flow cytometry for a representative healthy donor and patient. (E) Percentage of ICOS⁺ Treg cells in patients with SS and healthy donors. ***P < .001; **P =.001-.01; *P = .01-.05. IC, isotypic control.