Protein-bound polysaccharide (PSK) induces cytotoxic activity in the NKL human natural killer cell line

We studied the effect of protein-bound polysaccharide PSK on the activation of the human natural killer cell line NKL. We observed an increased natural killer cytotoxic activity against different tumor cells (K562, Daudi, and U937) when a standard 2- to 3-h 51chromium release assay was performed. The results parallel those obtained after treatment of the NKL cell line with interleukin-2. The highest cytotoxic activity was reached at a concentration of 100 microg/ml of PSK. This natural killer activation was inhibited when the PSK dose was 1,000 microg/ml. None of the cell surface markers that were analyzed by fluorescence-activated cell sorting showed variations after PSK or interleukin-2 treatment of NKL cells. These markers included CD2, CD11b, CD11c, CD18, CD16, CD54, CD56, CD98, CD25, CD122, HLA class I, HLA class II, CD94, ILT2, p58.1, p70, and NKp46. One of these markers (NKp46) is a major triggering receptor reported to be involved in the natural cytotoxicity of fresh or cultured human natural killer cells. In our study, another triggering receptor must be implicated in PSK-induced natural killer lysis. Our data suggest that PSK is an important biological response modifier of natural killer cells in vitro and may prove to be useful for the study of human natural killer cell biology.