Transfer of Id-specific CD4⁺ T cells confers tumor protection. (A) TCR-transgenic CD4⁺ cells, but not CD4^- cells, protect. SCID mice were reconstituted with purified TCR-transgenic CD4⁺ or CD4^- cells, or BALB/c CD4+ T cells, injected intravenously (2 × 10⁶). Three days later, the mice were challenged with 1.25 × 10⁵ Id+ F9 cells. TCR-transgenic CD4⁺ cells versus TCR-transgenic CD4^- cells: P = .018. TCR-transgenic CD4+ cells versus BALB/c CD4+ cells: P = .0014. TCR transgenic CD4^- versus BALB/c: P = 0.59. (B) Number of Id-specific CD4⁺ T cells required for transfer of protection. Groups of Rag2^-/- mice were injected intravenously with the indicated titrated amounts of purified TCR-transgenic CD4⁺ T cells, or left nonreconstituted, and challenged 3 days later with 1.25 × 10⁵ F9 cells. (C) Id-specific CD4+ T cells purified from recombination deficient TCR-transgenic SCID mice confer protection. Groups of SCID mice were reconstituted with 2.5 × 10⁵ CD4⁺ cells purified from either TCR-transgenic SCID or BALB/c mice, or left nonreconstituted. Three days later, the mice were challenged with 1.25 × 10⁵ F9 cells. TCR-transgenic SCID CD4⁺ cells versus BALB/c CD4⁺ cells: P = .014. BALB/c CD4+ versus nonreconstituted mice: P = .44.