Enhancement of macrophage tumoricidal activity by a serum glycoprotein that increased in OK-432-treated mice

An antitumor protein that increased in the serum of OK-432-treated mice showed an enhancing effect on the tumor cell killing by OK-432-elicited macrophages as measured by the 51Cr release test. This protein, designated as LB, also enhanced the cytolytic effect of macrophages on normal target cells, although the activity was lower toward normal cells than tumor cells. On the other hand, LB exhibited no effect on normal, thioglycolate-elicited, or heat-inactivated OK-432-elicited macrophages. The effect of LB on the binding of tumor cells to macrophage monolayers was also examined. On the addition of LB, OK-432-elicited macrophages became capable of binding more tumor cells than before. As in the cytolytic assay, the increased binding by the macrophages was not selective for tumor cells, and normal macrophages did not increase the binding of tumor cells upon the addition of LB. These results indicate that LB enhances macrophage-mediated cytotoxicity by increasing intimate binding between the OK-432-elicited macrophages and their target cells.