SPECIFIC BINDING OF BACILLUS-CALMETTE-GUERIN TO UROTHELIAL TUMOR-CELLS IN-VITRO

Intravesical immunotherapy with bacillus Calmette-Guerin (BCG) against recurrences of superficial bladder cancer and carcinoma in situ is a highly effective regimen in urology. Despite intensive efforts to clar ify the immunological mechanisms of the most successful immunotherapy known today, the cellular mechanism of its antitumor activity remains unknown. In our approach to elucidate the way of action of intravesica l BCG, we applied an in vitro adhesion assay to investigate the intera ction of radiolabeled BCG with urothelial bladder-tumor cells. We demo nstrated a BCG dose-dependent binding to bladder-tumor cell lines deri ved from tumors of different gradings. The binding of BCG is apparentl y specific, since competition experiments showed an inhibition by nonr adioactive BCG but not by Escherichia coli. We also found that there w as no difference between the binding of living or heat-killed mycobact eria. Control experiments showed only a low affinity of BCG for fibrob lasts, smooth-muscle cells, and endothelial cells in comparison with t he tumor cells. Furthermore, we investigated the role of fibronectin a s an adhesion molecule that is also present in the bladder wall. We de monstrated that BCG was capable of binding to fibronectin-coated surfa ces in a dose-dependent manner. However, competitive binding assays fa iled to reveal an inhibition of the binding of BCG to bladder-tumor ce lls by anti-fibronectin. Furthermore, binding was not influenced by so luble fibronectin. These data suggest that the in vitro attachment of BCG to bladder-tumor cells appears not to be mediated by fibronectin. In electron microscope studies an adhesion of BCG to bladder-tumor cel ls was observed after an incubation period of only 30 min. After 24 h of incubation, we saw in addition that tumor cell lines of all differe nt gradings had phagocytosed the mycobacteria. The phagocytosed mycoba cteria within vacuoles were in various states of structural integrity ranging from completely intact to almost fully disintegrated. In contr ast, fibroblasts were incapable of engulfing BCG. We conclude that BCG can bind to bladder-tumor cells in a specific manner and can be phago cytosed by these cells. Tumor cells of all gradings showed this behavi or, but fibroblasts did not. The specific interaction observed between BCG and bladder-tumor cells of all gradings might be an important ste p in the development of antitumor defense mechanisms in bladder cancer patients.