The polysaccharide K (PSK) potentiates in vitro activation of the cytotoxic function in human blood lymphocytes by autologous tumour cells

We have studied the effect of polysaccharide K (PSK) in the in vitro recognition of ex vivo carcinoma, sarcoma and lymphoma cells by the autologous blood lymphocytes. In 4/25 experiments PSK treatment activated the lymphocytes for auto-tumour lysis. Tumour cells alone generated lytic activity both in short- (16 h) and in longterm (6 days) mixed lymphocyte/tumour cell cultures (MLTC), in 2/12 and 3/13 cases respectively. The tumours that activated the lymphocytes expressed high levels of major histocompatibility complex (MHC) class I molecules. In vitro cytokine (interferon γ and tumour necrosis factor α) treatment of the tumour cells elevated the amounts of class I antigens and the treated cells acquired stimulatory potential. When PSK was added to the MLTC, in which untreated tumour cells were used, lytic potential was induced in 9/13 short-term and in 11/12 long-term cultures. It is noteworthy that in the presence of PSK the untreated, negative or low-class-I-expressor tumours also activated the cytotoxic function of the lymphocytes in 4/5 long-term and in 6/7 short-term cultures. Even in the case of those lymphocytes that could be activated by PSK or tumour cells alone, the simultaneous exposure was more efficient. The effect of PSK was dose-dependent, being optimal at 1 µg/ml and 10 µg/ml. The presence of EDTA and/or cytochalasin B in the cytotoxic test performed with the activated effectors abrogated the lysis, indicating the requirement of contacts with the effector cells.