Malignant T cells function as superantigen- and alloantigen-presenting cells. (A) SE presentation by malignant to nonmalignant T cells is MHC class II dependent. Nonmalignant T cells (P1119) from healthy donors were grown in microtiter plates with or without malignant T cells (MyLa 2059) or EBV-LCLs and with or without SEA (wt) or SEA (F47A, D227A) with mutations in the HLA class II–binding sites. Malignant T cells and EBV-LCLs were irradiated (22 Gy, as indicated by *) to avoid proliferation. Twelve hours prior to harvest, ³H-thymidine (1 μCi [0.037 MBq]/well) was added, the cells were harvested onto glass fiber filters, and ³H-thymidine incorporation was measured. The proliferation was expressed as mean counts per minute of triplicate cultures. The data are representative of 3 independent experiments. (B) Malignant T cells present HLA-DP allospecific T cells. HLA-DPB*0301–, HLA-DPB*0401–, HLA-DPB*0501–, and HLA-DPB*0601–specific CD4⁺ T-cell lines from healthy donors were grown in microtiter plates with or without HLA-DPB0401–positive irradiated (to avoid proliferation) malignant T cells (MyLa 2039). ³H-thymidine uptake was performed and measured. The data are representative of 3 independent experiments. (C) HLA-DP presentation is blocked by HLA-DP mAb. HLA-DPB*0401–positive malignant T cells (MyLa 2039) were irradiated and incubated with or without HLA-DP, HLA-DQ, or HLA-DR framework mAbs (10 μg/mL) prior to coculture for 48 hours with HLA-DPB*0401–specific nonmalignant T cells in microtiter plates. The data are representative of 3 independent experiments. (D) Alloantigen stimulation by malignant T cells is inhibited by LFA-3 mAb. HLA-DPB*04.01–positive malignant T cells (MyLa 2039) were irradiated and incubated with or without CD5, CD8, LFA-3 mAb (10 μg/mL), or CTLA4-Ig (1 μg/mL) prior to coculture for 48 hours with HLA-DPB*0401–specific nonmalignant T cells in microtiter plates. The data are representative of 3 independent experiments. Error bars represent SD.