CFSE^- CD4⁺ T-cell generation in the absence of MHC class II molecule expression is not related to preferential expansion of memory CD4⁺ T cells. (A) Lymph node CD4⁺ T cells from C57BL/6 mice were purified and naive CD4⁺ T cells were electronically sorted on the basis of their nonexpression of CD25 and their low or absent expression of CD44. (B) CFSE-labeled CD4⁺ T cells (10⁶) or 10⁶purified naive CFSE-labeled CD4⁺ T cells (CD4⁺ CD44^-CD25^-) were injected intravenously into CD3ϵ^-/-mice and CD3ϵ^-/-MHC II^Δ/Δ mice. CFSE fluorescence histograms of CD4⁺ TCR^hicells recovered 14 days after transfer are shown. (C) Fourteen days after transfer the absolute numbers of CFSE^- CD4⁺ TCR^hicells were calculated. Data represent mean ± SD