Antitumor effect of cationic liposome incorporating keto-mycolic acid from Mycobacterium bovis bacillus Calmette-Guérin in mouse model.

Abstract

461 Background: Intravesical instillation of live bacillus Calmette-Guérin (BCG) is an established immunotherapy for non-muscle invasive bladder cancer. However, development of non-infectious agents has been expected, because there are several problems due to instillation of live bacteria. BCG cell wall skeleton is known to stimulate host immune system, but the mechanism is still unknown. Mycolic acid (MA), which consist of three subclasses (α, keto, methoxy), is the most abundant lipid component of BCG cell wall and is expected to be one of essential active components. Objective: In order to identify active component from BCG cell wall and develop non-infectious BCG-derived immunotherapy, anti-tumor activities of cationic liposomes incorporating each subclass of MA were assessed using mouse syngeneic graft model. Methods: Heat killed packed cells of M. bovis BCG Tokyo 172 were fully hydrolyzed and MA was obtained. MA was separated to three subclasses by thin layer chromatography. Cationic and hydrophilic liposomes incorporating each subclass of MA were prepared. C57BL/6 mice were subcutaneously inoculated with a mixture of MB49 cells and PBS or liposome with/without MA followed by additional injection of PBS or liposome. Tumor growth was monitored. Infiltration of CD4/8 lymphocytes was examined by immunostaining. Results: Cationic liposomes were internalized into cytoplasm of MB49 cells, a mouse bladder cancer cell line. Cell proliferation was not affected by liposome incorporating MA. Tumor growth was significantly suppressed in mice treated with keto MA-liposome (vs PBS, p = 0.007), but not in mice treated withα-or methoxy-MA liposomes. Number of infiltrating CD8 lymphocytes was higher in tumor treated with keto MA-liposome than control. Antitumor effect of keto MA-liposome disappeared in nude mouse, while the effect existed in beige mouse with NK activity deficiency (vs PBS, p = 0.045). Conclusions: Keto MA-liposome showed the strongest antitumor effects in mouse model among three subclasses. T cell immunity may contribute to those effects.