CD8⁺T lymphocytes determine the threshold of the MHC class II molecule requirement for CD4⁺T-cell survival. CFSE-labeled lymph node CD4⁺ T cells (5 × 10⁶) from C57BL/6 CD45.1 mice were transferred into CD3ϵ^-/-mice and CD3ϵ^-/-MHC II^Δ/Δ mice. In some cases, host mice were injected with 50 × 10⁶lymph node CD45.2 CD8⁺ or CD4⁺ T cells 3 days before CD45.1 CD4⁺ T-cell transfer. (A) Absolute numbers of recovered CD45.2 CD8⁺ and CD4⁺ T cells are shown 28 days after CD45.1 CD4⁺ T-cell transfer. (B) Absolute numbers of CFSE^- CD45.1 CD4⁺ TCR^hicells are shown 28 days after their transfer as a function of the CD4/8 phenotype of preinjected CD45.2 T cells. (C) CFSE fluorescence histograms of CFSE⁺ CD45.1 CD4⁺ TCR^hicells are shown 28 days after their transfer as a function of the CD4/8 phenotype of preinjected CD45.2 T cells. (D) Absolute numbers of CFSE⁺ CD45.1 CD4⁺ TCR^hicells are shown 28 days after their transfer as a function of the CD4/8 phenotype of preinjected CD45.2 T cells. (A-B, D) Data represent mean ± SD.