Optimal in vivo clonal expansion of P1CTL requires CD28 on the T cells.

Purified CD28^+/− (blue lines) or CD28^−/−(red lines) CD8⁺ P1CTL were labeled with CFSE and were adoptively transferred into RAG-2^−/− mice that bore either J558-B7 (C) or J558-Neo (B) tumors or into RAG-2^−/− mice that received no tumor cells (A). Mononuclear cells were harvested on day 4 after transfer. Division of P1CTL was analyzed by flow cytometry. Data shown are CFSE intensity of gated Vα8⁺ T cells, as measured by flow cytometry and analyzed by Flowjo software, as detailed in the legend to Figure 1. Five thousand gated P1CTL cells were analyzed. The increased proliferation of CD28^+/− T cells over that of the CD28^−/− T cells was reproduced in 3 independent experiments.