Loss of antigen-expressing tumor cells. (A) RT-PCR for C5 in ex vivo tumors (lanes 1-3) untransfected LK35, and C5-transfected LK35 tumor cells cultured under selection in vitro. (B) Percentage of antigen-positive (▪) and -negative (□) single cell clones from C5-transfected LK35 tumor cells maintained in vitro and ex vivo tumors (small and large). Single cell clones were assessed for their ability to activate the C5-specific A18 hybridoma to IL-2 secretion, measured by proliferation of IL-2—dependent CTLL cells using an alamar blue—based assay.