Donor CD4⁺ and CD8⁺ T-cell expansion requires the presence of host-derived hematopoietic APCs. B6 CD45.1 SCs (1 × 10⁷) and 2C CD8⁺ T cells (1 × 10⁶) were CFSE-labeled and then transferred to B6 CD45.2 + BALB/c → BALB/c MCs or B6 CD45.2 → BALB/c FCs 10 weeks following BMT. (A) Histograms show log CFSE staining of gated CD4⁺CD45.2^– and CD8⁺CD45.2^– T-cell populations within the spleen on day +6 following DLI. Results are representative of 3 independent experiments. (B) Absolute numbers of DLI-derived CD4⁺ and CD8⁺ T cells in recipient spleens of MCs and FCs on day +12 following DLI (n = 6 each group). Data are shown as mean ± SEM.