Immunostimulation of highly immunogenic target tumor cells by lymphoid cells in vitro.

A highly immunogenic mouse tumor was studied with the use of microcytotoxicity test, performed at lymphoid cell:target tumor cell ratios ranging from 1:1 to 1,000:1 at various times during tumour growth and after tumor excision. Although the tumor was highly immunogenic, in vitro lymphoid cell-mediated cytotoxicity was shown only with lymphoid cells from hyperimmunized mice, i.e., mice that had "seen" the tumor more than once. Instead, the lymphoid cells harvested 2 and 3 weeks after tumor inoculation, as well as after excision of the tumor, stimulated the target cells. At 4 weeks after inoculation of the tumor, there appeared to be neither stimulation nor inhibition of the target tumor cells, though at this point the growth of this highly immunogenic tumor had been arrested in vivo.