C5a binding to T cell–expressed C5aR regulates T-cell apoptosis and expansion. (A) Representative flow plots of annexin V⁺ Mar T cells after in vitro stimulation with H-2^b bone marrow dendritic cells plus HYDby peptide⁺ recombinant mouse C5a. The experiment was repeated with similar results. Numbers in upper left quadrant represent percentage annexin V⁺ (n = 3 per group). *P < .05 versus no C5a control. (B) Left: Flow plots of polyclonal H-2^b WT or C5aR^−/− CD4 T cells (CFSE dilution) after injection into (B6xBalb/c) F1 WT or Daf1^−/− recipients (day 3, n = 2 or 3/group). The percentages of injected cells per recipient spleen on day 3 are shown. Right: Annexin V expression on the injected cells within the gate. *P < .05 versus WT control. (C) Polyclonal H-2^b WT or C5aR^−/− CD4 T cells were injected into (B6xBalb/c) F1 WT or Daf1^−/− recipients. Total numbers of donor T cells in recipient spleens were determined on day 4 after injection. Data are representative of 2 experiments. *P < .05.