Peripheral blood mononuclear cells were isolated from leukapheresis products of healthy volunteers by density gradient centrifugation over Ficoll/Hypaque (Biochrom AG, Germany). Collection of blood cells from healthy donors was performed in compliance with the Helsinki Declaration. All donors signed an informed consent. The leukapheresis procedure and subsequent purification of peripheral blood cells was approved by the local ethical committee (reference number 92-1782 and 09/066c). CD4+ cells were enriched using magnetically labeled human CD4 MicroBeads (Miltenyi Biotec,Germany) and the Midi-MACS system (Miltenyi Biotec). The CD4+ fraction was stained with CD4 FITC (Becton Dickinson, cat no. 345768), CD25 PE (Becton Dickinson, cat no. 341011) and CD45RA APC and CD3+CD4+CD25− T cells were sorted on a FACS-Aria high-speed cell sorter (BD Biosciences, Germany). CD8+ cells were enriched using magnetically labeled human CD8 MicroBeads (Miltenyi). The CD8+ fraction was stained with CD3 FITC (Becton Dickinson, cat no 345763) and CD8 APC (Becton Dickinson, cat no. 345775) and sorted for CD3+CD8+ T cells. CD19+ and CD56+ cells were enriched from the CD8− fraction using magnetically labeled human CD19 and CD56 MicroBeads (Miltenyi). Enriched cells were
