Requirement for CD4 T-cell help in the memory response is reproduced when mice are restimulated by skin transplantation. Mice that had been previously immunized with male H60 cells were transplanted with skin from male or female H60 congenic mice. (A) Mice were observed longer than 90 days after the transplantation. The data are pooled from 3 independent experiments (n = 6, P < .001). Mean survival time (MST) for each group is indicated (UD, undefined). (B) The mice were eye-bled on day 0, before skin transplantation, and on day 10 after skin transplantation. PBLs were stained with H60-tetramer-PE, anti–CD8-APC, and anti–CD11a-FITC. Flow cytometric analyses were done (gated for CD8⁺ cells); the data are representative of 3 independent experiments.