CD8+CD11b+ peripheral blood T lymphocytes contain lymphokine-activated killer cell precursors

CD3⁺CD8⁺ cells, purified from peripheral blood T cells by depletion of CD4⁺ lymphocytes, were tested for their cytotoxic activity against K-562 or HL-60 cells. Freshly prepared cells had no cytotoxic function, but upon exposure to recombinant interleukin 2 (rIL2) in vitro they acquired a lymphokine-activated killer (LAK) activity. Fractionation of CD3⁺CD8⁺ cells into CD11b⁺ and CD11b^- cells demonstrated that all the cells with the potential of developing LAK cell functions were within the CD8⁺CD11b⁺ subset. These cells lacked natural killer cell markers such as CD16 or NKH1, but a proportion of them stained for Leu-7. Furthermore, they expressed the α/β chains, but not the γ/δ chains, of the T cell receptor, as could be determined by staining with the appropriate monoclonal antibodies. CD8⁺CD11b⁺ cells had a large granular lymphocyte morphology, as shown by both cytochemical and electron microscopy analyses. They proliferated in response to IL2 in vitro and developed cytotoxic functions against a number of natural killer resistant targets. Their response to phytohemagglutinin or pokeweed mitogen was very weak or absent. By contrast, CD8⁺CD11b^- cells failed to generate LAK cells in response to rIL2, did not show a large granular lymphocyte morphology, but responded vigorously to phytohemagglutinin or pokeweek mitogen. Purified CD8⁺CD11b⁺ cells were cloned by limiting dilution in the presence of rIL2. The observed cloning efficiency of 19 ± 0.3% indicates that a fraction of the cells only could respond to IL2. Furthermore, only 50% of the clones obtained had a LAK cell function. These data show that CD8⁺CD11b⁺ cells represent a heterogeneous cell population. Nevertheless this cell subset probably represents the major source of LAK cell progenitors within the circulating T cells.