Preservation of cytotoxic function during multi-cycle immunomagnetic cell separations of human NK cells using a new type of magnetic bead.

The isolation from human peripheral blood lymphocytes of natural killer (NK) cell populations by a novel magnetic cell sorting (MACS) procedure yielded large amounts of viable cells with active cytotoxic function. Non-adherent B cell-depleted lymphocytes were sequentially labelled with specific monoclonal antibodies, biotin-conjugated second antibody, FITC-conjugated streptavidin and biotin-conjugated magnetic particles (diameter 50-150 nm). In the magnetic field of a permanent magnet, positively labelled cells were retained on columns with a ferromagnetic matrix. When OKT3 was used for the depletion, 96-99% of the T cells were removed. The resulting non-labelled NK cell population contained 78-89% Leu11b+ and 87-96% Leu19+ cells. Magnetic retention of NK cells mediated by anti-Leu19 yielded about 81% and retention mediated by anti-Leu11b about 80% of total cells as determined by positive fluorescence. The resulting labelled and unlabelled cell subpopulations maintained their full NK activity as determined in 4 h cytotoxicity assays against human K562 tumor cells. The viability of non-labelled cells was fully preserved, whereas that of labelled cells slowly decreased with increasing numbers of preparative cycles. Furthermore, the ability of the isolated NK cells to show enhancement of their NK cytotoxicity after preincubation with IL-2 was maintained. The cytotoxic function of NK cells was also preserved when two or more MACS cycles using the same or different antibodies were carried out. The saving of time and the physiological condition of the isolated cells offer valuable advantages over FACS procedures.