Anti-tumour cytotoxin from macrophages: no correlation between cytotoxin adsorption by tumour cell lines and their cytotoxin susceptibility.

Mononuclear phagocytes can synthesize a cytotoxin, similar to that in tumour necrosis serum, which is cytotoxic to certain tumour cell lines in vitro. This study has investigated whether susceptibility and resistance to the cytotoxin can be explained in terms of the amount of cytotoxin receptor expressed on the tumour cell surface. Binding of cytotoxin has been quantitated both by direct adsorption and competitive inhibition assays with cytotoxin-resistant or -susceptible tumour lines and with sublines of susceptible lines, selected for resistance to the cytotoxin. For both rabbit and human cytotoxin, there was no correlation between cytotoxin adsorption by tumour cell lines and their cytotoxin susceptibility, suggesting that resistance to the cytotoxin is expressed at a post-receptor stage. Preliminary studies on the cytotoxin receptor of K562 cells have shown that it is probably not the transferrin receptor, and that protein but not carbohydrate is essential for its function.