CD137L expression on malignant T cells in CTCL. (A-C) CD137L and CD137 expression was analyzed by flow cytometry in CD4⁺CD7⁻, CD4⁺CD7⁺, and CD8⁺ T cells from 10 SS patients, CD4⁺ and CD8⁺ T cells from 14 healthy controls (A-B), human CTCL cell lines (Hut78, MyLa, SeAx, HH, and MJ cells), and murine lymphoma cell line EL-4 cells (C). Representative flow cytometric plots are shown (A, C). The percentage of CD137L⁺ or CD137⁺ cells was counted in CD4⁺CD7⁻ T cells from SS patients or CD4⁺ T cells from healthy controls. Correlations between the percentage of CD137L⁺ and CD137⁺ cells in CD4⁺CD7⁻ T cells were examined (B). (D) CD137L expression in lesional skin of CTCL was determined by immunohistochemistry (original magnification ×400). Diaminobenzidine was used for visualizing the staining and counterstaining with Mayer hematoxylin was performed. Representative results are shown. (E) Quantitative RT-PCR was performed to measure expression levels of CD137L and CD137 using mRNA extracted from CTCL lesional skin (n = 57; 51 MF cases and 6 SS cases) and healthy skin (n = 20). (F) Correlations between CD137L and CD137 mRNA levels in CTCL lesional skin. (G) Kaplan-Meier analysis for disease-specific survival of 57 CTCL patients with high CD137L mRNA levels (>0.0014) and those with low CD137L mRNA levels (≤0.0014). (H) Serum sCD137L and sCD137 levels in CTCL patients (n = 43). (I) Correlations between serum sCD137L levels and sIL-2 receptor (sIL-2R) or lactate dehydrogenase levels (LDH) levels in CTCL patients. Means are presented as bars. *P < .05, **P < .01. GAPDH, glyceraldehyde-3-phosphate dehydrogenase; Max, maximum.