APC-expressed C3 and DAF influence T-cell apoptosis after antigen stimulation. (A) Flow cytometry plots showing annexin V staining of CFSE-labeled Mar T cells on day 3 after stimulation with WT, Daf1^−/−, C3^−/−, or Daf1/C3^−/− macrophages plus HYDby peptide. Percentages of annexin V⁺ cells are given in the upper left. Data are representative of 4 individual experiments. *P < .05 versus WT APCs. Similar results were found when TUNEL staining was used as a readout (Figure S2). (B) Percentage of annexin V⁺ polyclonal H-2^b T cells on day 4 of in vitro culture with allogeneic H-2^d WT, Daf1^−/−, or C3^−/− macrophages. Values are means plus or minus SD of 3 individual experiments. *P < .05. (C) CFSE-labeled WT or C3^−/− H-2^d T cells were stimulated in vitro with WT, C3^−/−, or Daf1/C3^−/− allogeneic H-2^b macrophages for 4 days and stained with annexin V. Values are means plus or minus SD of 3 individual experiments. *P < .05 versus WT T cells. (D) Flow cytometry plots depicting annexin V⁺ Mar T cells in the spleens of recipient mice on day 3 after adoptive transfer (10 × 10⁶ Mar T cells per mouse) into groups of male WT, Daf1^−/−, or C3^−/− recipients (n = 3 per group). *P < .05 versus WT APCs. (E) Groups of male bone marrow chimeric mice (n = 3) were injected with 5 × 10⁶ CFSE-labeled Mar T cells and Vβ6⁺ splenic Mar cells were assayed for apoptosis by annexin V staining on day 3. *P < .05 versus WT→WT controls.