The extent of the memory phenotype of H-Y CD8 T cells determines their ability to respond to cytokines in vitro as well as their ability to undergo homeostatic expansion in vivo. (A) Purified CD8⁺ T cells from female H-2^b H-Y (CD8^hi), male H-2^b H-Y (CD8^lo), or male H-2^b/d H-Y (CD8^int) mice were labeled with CFSE and cultured with IL-2 (200 U/mL), IL-15 (100 ng/mL), or H-Y peptide (1 μM), B6-Tap1^-/- splenocytes (1 × 10⁷), and IL-2 (20 U/mL). Proliferation of gated H-Y TCRα⁺ CD8⁺ cells was analyzed by FACS at 48 (filled histogram), 72 (dark line), and 96 (light line) hours with each CFSE peak representing one cell division. (B) Purified CD8^hi, CD8^int, and CD8^lo cells were CFSE labeled and transferred into sublethally irradiated B6 female (filled histogram) or male (unfilled histogram) recipients. CFSE profiles of gated CD8⁺ H-Y TCR⁺ cells 7 days after transfer are shown in the histograms.