Induction of proliferative T-helper cell responses by DCs transfected with CLL-derived RNA. CD4⁺ T lymphocytes were isolated by using magnetic bead technology. T-helper cell induction was performed by coincubating DCs electroporated with total RNA isolated from CLL cells and autologous CD4⁺ T lymphocytes. Two restimulations with autologous RNA-transfected DCs were performed on days 7 and 14 after T-cell induction. The induced proliferation was determined by a 16-hour pulse with ³H-thymidine and subsequent measurement of thymidine incorporation. Autologous unstimulated CLL cells, B lymphocytes, or monocyte-derived DCs electroporated with the cognate CLL RNA were used as stimulating cells. Unstimulated CD4⁺ T lymphocytes and stimulation with DCs electroporated with EGFP RNA served as negative controls. Inhibition of HLA class I or class II molecules was performed by incubating DCs for 1 hour prior to the assay either with the mAb W06/32 (directed against HLA class I) or Tü39 (directed against HLA class II molecules).