COSTIMULATION OF HUMAN NATURAL-KILLER-CELL PROLIFERATION - ROLE OF ACCESSORY CYTOKINES AND CELL CONTACT-DEPENDENT SIGNALS

Despite the importance of natural killer (NK) cells in the immune resp onse, the regulation of human NK cell growth has not been well charact erized. We have hypothesized that, similar to the proliferation of T a nd B lymphocytes, optimal proliferation of NK cells requires costimula tory signals as well as a primary mitogenic stimulus. Evidence for cos timulation by both soluble cytokines and cell contact-dependent factor s is presented. Soluble IL-1 and TNF were found to augment NK cell pro liferation in response to primary mitogenic cytokines, including IL-2, IL-4, IL- 7, and IL-12. The costimulatory effect of IL-1 and TNF is s trongly enhanced by the calcium ionophore ionomycin. Coculture of NK c ells with irradiated K562 cells can largely substitute for the costimu latory signal provided by ionomycin. Costimulation by K562 requires in timate cell contact and is not reconstituted by cell-free supernatants . Activated T lymphocytes can also mediate contact-dependent costimula tion of NK cells; resting PBMC, several NK-sensitive cell lines, and a ll NK-resistant cell lines tested were not found to be costimulatory. Engagement of CD16 did not augment NK cell proliferation. Thus, trigge ring of natural killing or antibody-dependent cell-mediated cytotoxici ty (ADCC) does not consistently provide a costimulatory signal for NK cell proliferation. Cell contact-dependent costimulation of NK cells d oes not appear to involve known receptors that can costimulate T cells , including CD2, CD27, CD28, CD29, or LFA-1. The molecular nature of t he putative NK cell costimulatory receptor remains to be elucidated. N evertheless, human NK cells could be expanded in vitro using leukocyte -conditioned medium (LCM) as a source of IL-2 and accessory cytokines and ionomycin to bypass the putative receptor for cell contact-depende nt costimulation. NK cells expanded in LCM and ionomycin express typic al NK cell antigens and mediate natural killing and ADCC. Further char acterization of the Costimulation costimulatory signals for NK cell pr oliferation may elucidate the physiologic regulation of NK cell growth and may ultimately allow more effective manipulation of these lymphoc ytes in the immunotherapy of human diseases.