Expression of SD-4 by CTCL lines and PBMCs from SS patients. (A) Total RNA isolated from 3 CTCL or 2 non-CTCL T-cell leukemia (TCL) lines was examined by reverse transcription-PCR for mRNA expression of SD-4, coinhibitors (CD160, CTLA-4, BTLA, and PD-1) or glyceraldehyde-3-phosphate dehydrogenase. (B) The 5 cell lines and normal in vitro–activated CD4⁺ T cells (CD4-T) were also assayed by flow cytometry for surface expression of SD-4. Positive (open histogram) and control (shaded histogram) staining are shown. (C-D) PBMCs freshly isolated from 7 patients with SS in different stages of lymphomalignancy (Table 1) or 4 normal donors (ND) were examined by flow cytometry for surface expression of SD-4 on CD4⁺ T cells. Dot plots shown are representative for healthy donors and patients with SS (Pt. 6). (C). Frequency of SD-4⁺CD4⁺ cells in total PBMCs of each patient and normal donor is shown as a percentage and plotted in a scatter chart as mean (± SD). P was evaluated by the Student t test. (D-E) PBMCs from 3 patients with SS (Pt. 1 to Pt. 3B) were also examined for SD-4 expression on clonal malignant T cells (TCR-Vβ subtype-positive). (F) PBMCs from Pt. 7 and Pt. 8 (with low tumor burden) contained CD4^low and CD4^high subpopulations, each of which was examined for SD-4 expression. CD4^low and CD4^high cells were negative and positive, respectively, for CD26 expression (data not shown). (G) PBMCs freshly isolated from patients with atopic dermatitis, psoriasis, or MF (each 3 patients) were also assayed for SD-4 expression on CD4⁺ T cells. Data shown represent each disease group.