Survival of CD4⁺ T cells depends on interactions with MHC class II molecules in a nonlymphopenic environment. CFSE-labeled lymph node CD4⁺ T cells (5 × 10⁶) from C57BL/6 CD45.1 mice were injected intravenously into MHC II^Δ/Δ mice and C57BL/6 CD45.2 mice. (A) CFSE fluorescence histograms of CD45.1 CD4⁺ TCR^hi cells are shown as a function of time after transfer. (B) Absolute numbers of recovered CFSE⁺ CD45.1 CD4⁺ TCR^hicells are shown as a function of time after transfer. Data represent mean ± SD. Statistically significant differences were assessed using Student t test. ^**P < .01; ^***P < .001.• indicates C57BL/6; □, MHC II^Δ/Δ.