In Vitro Studies on Mechanism of the “Eclipse” of Cell-Mediated Immunity in Mice Bearing Advanced Tumors

A characteristic pattern of evolution of cell-mediated antitumor immunity was confirmed in 3 mouse host-tumor systems. This pattern, as defined through the in vitro colony inhibition test after confrontation of target tumor cells with peritoneal cells (PC), included an immunologic “eclipse” period observed regularly in mice bearing tumors ≥ 10 mm in diameter. This eclipse was immunologically specific, not seen in mice with an antigenically unrelated tumor. The mechanism determining the eclipse could be partially explained by experiments in which sera from tumor-bearing mice, incubated with sensitized PC, significantly suppressed their activity. The same sera incubated with the target cells were much less efficient in protecting them against sensitizedPC. The possibility of a central mechanism suppressing the immunologic activity of sensitized PC by circulating tumor antigens or antigen-antibody complexes was discussed.