Involvement of CD28 expression in the A-LAK–mediated lysis of B7-1 expressing tumor cells. (A) The lytic activity of AKR (H-2^k haplotype), (B) B10.BR (H-2^k haplotype), (C, E) C57BL/6 (H-2^b haplotype), and (D, F) CD28^−/− knockout mice (H-2^bhaplotype)–derived IL-2/IL-12 A-LAK cells was tested against (A through D) parental and B7-1 transfected BW-Li (H-2^khaplotype) and (E through F) RMA-S (H-2^b haplotype) tumor cell variants in an [¹¹¹In]-release assay. The specific lysis of YAC-1 was used as a reference for the lytic capability of the IL-2/IL-12 A-LAK. Spontaneous release was ≤10% for all cell lines. One representative of three experiments is shown, indicating the mean percentage specific release of targets in triplicate (±SD). Standard deviations less than ±3% are not shown for the sake of clarity. (▪), BW-Li; (⧫), BW-Li(B7-1); (*), YAC-1; (▴), RMA-S; (×), RMA-S(B7-1).