In vitro CTL suppression by the CD11b⁺Gr-1^int iMC subset does not correlate with CTL suppression in vivo. Polyclonally activated B6 T cells or antigen-specifically activated transgenic P14 T cells were cocultured with 24%, 12%, 6%, or 3% CD11b⁺ cells or CD11b⁺Gr-1^int iMC from young LoxP-Tag mice (Tg [young]), LoxP-Tag mice bearing sporadic tumors (Tg), HCC-bearing LoxP-Tag × Alb-Cre mice (Tg × Alb-Cre), and Ad.Cre-infected LoxP-Tag mice bearing virus-induced HCC (Tg/Ad.Cre). After 72 hours, CTL were stained with anti-mouse CD8α antibodies and CFSE dilution of CD8⁺ T cells was analyzed by flow cytometry. (A, B) Total CD11b⁺ cells suppress polyclonally activated and antigen-specifically activated P14 T cells in vitro. One representative example (12% CD11b⁺ cells) per experimental group is shown for polyclonally activated T cells (A) and suppression at different CD11b⁺/T cell ratios is shown for antigen-specifically activated P14 T cells (B). (C, D) CTL suppression is mediated by the CD11b⁺Gr-1^int iMC subset that inhibits proliferation of polyclonally activated (C) and antigen-specifically activated P14 T cells (D) in vitro. Overall significance for each graph was tested by Kruskal–Wallis 1-way analysis of variance.