Tumor target cells (TC) are lysed by natural killer (NK) cells provided that they (1) form conjugates with the effector cells, (2) activate effector cells to release cytotoxic factors, and (3) they are susceptible to the lytic effect of these factors. While this cascade of events that leads to TC killing has been defined, the signal molecules responsible for each of the steps remain largely undetermined.
A variety of human leukemia-derived TC lines and clones were analyzed for their sensitivity to NK cell-mediated lysis and for their ability to bind and activate NK cells. These characteristics have been correlated with TC surface expression of differentiation antigens and carbohydrate residues. Of the cell lines and clones tested, K562, SPI-802, MOLT-4, MOLT-4/C8-1, ZS, KG-1/A-3, and HL-60S were sensitive to NK cell-mediated lysis, while KG-1, THP-1-0, HL-60R, and LFM were resistant. KG-1, THP-1-0, HL-60R, and LFM cells were further studied to determine mechanisms responsible for their resistance to NK cells. It was found that HL-60R and LFM cells were unable to bind NK cells. In contrast, KG-1 and THP-1-0 cells were able to bind to and activate NK cells. Therefore, it is likely that the NK-resistance of KG-1 and THP-1-0 cells may be related to their lack of sensitivity to cytotoxic factors released by bound NK cells.
All of the TC cell lines and clones capable of binding NK cells expressed the 3-fucosyl-N-acetyl-lactosamine hapten (Lex or SSEA-1 antigen) recognized by the monoclonal antibody Leu M1. These TC consistently lacked surface L-fucose residues, as shown by lack of Ulex europaeus agglutinin binding. In contrast, HL-60R and LFM which did not form conjugates with NK cells, did not express surface Lex determinants and avidly bound the Ulex agglutinin. Distinct subpopulations of NK-resistant KG-1 cells expressed Lex antigens or bound Ulex. We compared KG-1/A-3, a NK-sensitive cell clone, with the parental NK-resistant KG-1 cell line. KG-1/A-3 lost the ability to bind the Ulex lectin displayed by the parental cell line and showed increased expression of Lex determinants. Results from these phenotypic analyses suggest that expression of Lex determinants and Ulex binding sites on the TC membrane are mutually exclusive and their expression or absence may correlate with mechanisms which regulate TC-NK cell interactions.
This work has been supported by a Core Facility Grant (CA-13148) to the Immunocytology Laboratory of the Comprehensive Cancer Center, by VA research funds, and by a grant of the Italian National Research Council (Progetto Finalizzato: Oncologia), 84.00621.44, to C. E. G.