Natural killer cell activity and natural antibody to K-562 cells

Natural killer (NK) cell cytotoxicity is initiated by recognition of an appropriate target cell with subsequent killing. The K-562 cell line has been used in many laboratories as a target cell for NK activity but the molecular site(s) of recognition is not known. We find that human NK cells from peripheral blood competitively reduce a specific antibody-dependent, complement-mediated cytotoxicity (ADCMC). The antibody used was present in the gamma globulin fraction of goat antiserum to K-562 cells that had been extensively absorbed with normal human peripheral blood leukocytes and normal bone marrow cells. Furthermore, extracts of K-562 cells, from which a specific antigen can be isolated, specifically and competitively inhibited NK-lysis of K-562 cells as well as the ADCMC. These findings suggest that NK cells recognize the same antigen as does the antibody. Lymphocyte populations from human donors, containing NK cells, were fractionated by their adherence to plastic, nylon wool, and K-562 monolayers. Cells nonadherent to plastic or nylon wool but adherent to K-562 monolayers and eluted gave NK cytolysis of K-562 cells. Goat immune globulin added to unfractionated human lymphocytes gave antibody-dependent, cellular cytotoxicity (ADCC) and it was possible that NK activity could be interpreted as antibody-dependent. However, this ADCC activity was associated with Fc receptor positive (sensitized E-rosettes) cells that showed partial, non-selective adherence to K-562 monolayers. Furthermore, trypsin treatment of the lymphocytes removed the Fc receptor so that no E-rosettes formed or ADCC occurred, hut trypsin treatment did not affect NK activity. From 80 normal humans, 52 (65%) showed NK cell activity and a level of natural antibody (ADCMC) to K-562, 24 additional showed antibody only, and 4 had neither activity. The natural antibody from several donors did not bind to protein A-Sepharose, migrated as 19S in sucrose gradients, and was rendered inactive by 2-mercaptoethanol. These criteria identified it as IgM.