Physiologic consequence of CD80 acquisition by T cells in vivo. (A) The acquisition of CD80 by CD4 T cells was examined after PCC TCR-Tg mice were vaccinated with peptide/adjuvant (PCC in alum) and killed 1, 3, or 5 days after vaccination. T cells from LNs were analyzed for CD80 acquisition by FACS. (B) PCC-vaccinated PCC TCR-Tg mice were treated either with anti-CD28 antibody, anti-CD80 antibody, or isotype antibody (100 μg intraperitoneally and 100 μg subcutaneously) at the time of vaccination. Animals were killed 1 day later and their LNs were analyzed by FACS. (C) PCC-vaccinated PCC TCR-Tg mice were killed on indicated days after vaccination and T cells from LNs were analyzed for CD4, CD80, and annexin V (to identify the apoptotic cells) by FACS. (D) PCC-vaccinated PCC TCR-Tg mice were killed 1 day after vaccination and T cells from LNs were analyzed for CD69 and CD25 expression on CD4/CD80^acq (open histogram) or CD4/CD80⁻ (closed gray histogram) T cells by FACS.