Evidence for a role for calcium in immunosuppression by tumor cells in vitro

Abstract

The anti-SRBC response of normal, syngeneic splenocytes in the presence of cells from various tumors (Moloney leukemia spleen cells and methylcholanthrene-induced rhabdomyosarcoma cells (MC)) was tested in vitro in different culture media: RPMI 1640, BME with Hanks balanced salt solution (MEM), and CMRL 1066. The tumor-associated cells expressed an immunosuppressive effect, the degree of which varied with the culture medium used. Whereas spleen cells cultured in RPMI in the presence of tumor-associated cells were highly inhibited in their response to SRBC, those cultured in MEM were not. A full 5 to 10 times more tumor cells were required to achieve the same degree of immunosuppression in MEM. There appeared to be a correlation between the degree of immunosuppression obtained and the Ca 2+ concentration of the medium. Thus the immunosuppressive effect of tumor-associated cells was greatest in cultures with RPMI 1640 (0.4 m M Ca 2+), lesser in MEM (1.27 m M Ca 2+), and least in CMRL 1066 (1.8 m M Ca 2+). Furthermore, if the Ca 2+ content of RPMI 1640 was increased to 1.4 m M Ca 2+ by the addition of CaCl 2, the percent suppression to the anti-SRBC response in vitro mediated by the addition of tumor cells decreased to the level found in MEM. Increasing the Mg 2+ content of RPMI had no effect on tumor-mediated immunosuppression. Tumor cell replication and RNA synthesis were comparable in all media tested, regardless of Ca 2+ concentration. In view of the increasing evidence for a role for Ca 2+ in lymphocyte activation, we postulate herein that the Ca 2+ content of the medium plays a role in the manifestation of immunosuppression by tumorassociated cells in vitro.