Mutual deactivation of memory/effector T cells and tumor cells. (A) Memory cells from tumor-bearing mice are desensitized to activation in vitro: IL-2 secretion by memory CD4 A18 T cells from tumor-bearing mice (—) or from mice not inoculated with tumor (- - -) following stimulation with C5-transfected LK35 tumor cells (left panel; •) or C5 peptide-pulsed tumor cells (right panel; ○). (B) Ex vivo tumor cells from mice with T cells cannot stimulate T-cell responses in vitro: IL-2 secretion by A18 T cells (memory T cells isolated from mice not inoculated with tumor) following stimulation with: ex vivo tumors (▴ and ▵ representing tumor cells from 2 different donors), compared with C5-transfected LK35 tumor cells cultured in vitro (•). (C) Ex vivo tumor cells fail to process C5 protein: IL-2 secretion by the C5-specific A18 hybridoma following stimulation with ex vivo tumors, from mice with T cells (▴ and ▵ from 2 different mice) or lacking T cells (•). Exogenous C5 protein (left panel) or peptide (right panel) was added to the cultures to assess the processing capacity of the tumor cells. All figures show T-cell activation as assessed by proliferation of triplicate cultures of IL-2—dependent CTLL cells using an alamar blue—based assay. Tumors were removed at day 34 after inoculation. All memory or effector cells were derived from mice that had been injected with naive A18 TCR transgenic T cells and syngeneic DCs.