Survival of CD4⁺T cells in a lymphopenic environment does not depend on interactions with MHC class II molecules. CFSE-labeled lymph node CD4⁺ T cells (5 × 10⁶) from normal C57BL6 mice were injected intravenously into CD3ϵ^-/-mice and CD3ϵ^-/-MHC II^Δ/Δ mice. At various times after transfer, lymph nodes and spleen were recovered and pooled, and single-cell suspensions were prepared. (A) CFSE fluorescence histograms of CD4⁺ TCR^hicells are shown as a function of time after transfer. Histograms on the right are gated on CFSE⁺ CD4⁺ TCR^hicells. (B) Absolute numbers of total, CFSE⁺, and CFSE^- CD4⁺ TCR^hicells are shown as a function of time after transfer. Data represent mean ± SD. (C) CD4, TCR, and CD44 fluorescence histograms of CFSE⁺ and CFSE^- CD4⁺ T cells recovered 28 days after transfer are shown in comparison with CD4, TCR, and CD44 fluorescence histograms of control CD4⁺ T cells from normal C57BL/6 mice.