Cell death (A), DNA fragmentation (B), and FasL mRNA (C) are induced by B7.1 expressing cells. (A) DWT6.11 cells were seeded for 12 hours in a 24-well plate in the presence of MoAbs or in the presence of either L cells (L) or L cells expressing a transfected B7.1 cDNA (LB7.1), as indicated, with a 3:1 (DWT6.11:L) cell ratio. After this incubation period, cells were collected and PI stained followed by cytofluorometry as described in the Materials and Methods. PI staining versus the number of nuclei is shown. Numbers above histograms indicate the percentage of apoptotic nuclei. (B) DNA was extracted from the collected cells at 8 hours of incubation and analyzed onto a 1% agarose gel containing ethidium bromide. (C) mRNA was extracted from cells collected at 6 hours of incubation, followed by reverse transcription and PCR using FasL, IL-2, or β2 microglobulin primer pairs as described in the Materials and Methods. Amplification products were analyzed on a 1% agarose gel, followed by ethidium bromide staining. The results from one representative experiment out of three are shown. Where indicated, the CTLA4 Ig recombinant protein was added 15 minutes before treatment.