Regression mechanisms of mouse fibrosarcoma cells after in vitro exposure to quercetin: Diminution of tumorigenicity with a corresponding decrease in the production of prostaglandin E2

Summary We have previously reported that both regressor (QR) and progressor (metastatic, QP) clones were obtained after the in vitro exposure of a mouse fibrosarcoma BMT-11 cl-9 to quercetin 17]. In this study, we investigated possible mechanisms of spontaneous regression of QR clones as compared with tumorigenic QP and BMT-11 cl-9 tumor clones. We observed that BMT-11 cl-9 cells produced relatively high amounts of prostaglandin E₂ (PGE₂) during in vitro culture. The average production by 11 subclones of BMT-11 cl-9 cells was 9236±2829 pg/ml whereas that by 9 QR clones was 3411±2213 pg/ml (P <0.02). Indomethacin not only inhibited in vitro PGE₂ synthesis by QP clones (high-PGE₂ producers) but also the s.c. growth of QP clones in mice. Chronological changes in host immune responses to tumor-associated antigen were measured by cytotoxic T lymphocyte (CTL) activity examined after mixed lymphocyte/tumor cell culture of spleen cells obtained from tumor-bearing mice. The CTL activity disappeared abruptly in the spleen of QP-clonebearing mice 21 days after the inoculation of tumors, whereas the spleen cells of QR-clone-inoculated mice retained their CTL activity. We determined that the mechanism responsible for the regression of these regressor clones is not due to any qualitative or quantitative increase in pre-existing membrane antigens, nor the emergence of new antigen(s) on the cell surface of the QR clones; nor was it due to enhanced susceptibility of QR clones to natural killer cells, lymphokine-activated killer cells and macrophages. These finding suggest that the regression mechanism of QR clones may be the diminished inhibition of host response to tumor-associated antigen caused by the reduced production of PGE₂ by QR clones.