Nicoletta Pella

Thymic origin of some natural killer cells: clonal proliferation of human CD3−16+ cells from CD3−4−8− thymocyte precursors requires the presence of H9 leukemic cells

SummaryPurified CD3−4− thymocyte populations were cultured in the presenceof interleukin-2 (IL-2) and peripheral blood lymphocytes (PBL) and/or tumor cell lines as a source of irradiated feeder cells. Maximal cell proliferation was obtained in the presence of a mixture of H9 leukemic cells and normal PBL. More importantly, under these culture conditions 30%–50% of these cells were found to express CD16 surface antigen after 1–2 weeks of culture. Similar proportions of CD16+ cells could be detected in CD3−4− thymocyte populations that had been further depleted of CD16+ cells. Cloning of CD3−4−16− thymocytes under limiting dilution conditions resulted, in the presence of H9 cells, in more than 50% of CD16+ clones (cloning efficiency 3%–8%). Since some of the surface CD3− clones expressed cytoplasmic CD3 antigen, it has been possible to identify four distinct phenotypic groups of clones (CD16+ cyCD3+, CD16+ cyCD3−, CD16− cyCD3+, CD16− cyCD3−). Independently of their phenotype, all thymus-derived CD3− clones expressed a strong cytolytic activity against natural killer (NK)-sensitive and NK-resistant tumour target cells. In addition, following stimulation with phytohemagglutinin (PHA) and phorbol 12-myristate 13-acetate (or PHA alone) all clones released interferon-γ and tumour necrosis factor-α, but not IL-2. Taken together, our data provide evidence that cells which share their phenotypic and functional properties with CD3− CD16+ NK cells can be derived from thymic precursors.