Graft-versus-host reactive donor CD8⁺ T-cell expansion following delayed DLI is dependent on CD4⁺ T-cell help. B6 CD45.1 CD8⁺ T cells (1 × 10⁶) and 2C CD8⁺ T cells (1 × 10⁶) with or without 2 × 10⁶ B6 CD45.1 CD4⁺ T cells were transferred to B6 CD45.2 + BALB/c → BALB/c MCs or B6 CD45.2 → BALB/c FCs 10 weeks following BMT. (A-C) Histograms show absolute numbers of CD4⁺CD45.2^–, CD8⁺CD45.2^–, and CD8⁺1B2⁺ populations within the spleen on day 12 following transfer (n = 3 each group). Open bars indicate CD8⁺ T cells transferred without CD4⁺ T cells; closed bars, CD4⁺ and CD8⁺ T cells cotransferred. Note that in Figure 6A, only closed bars are shown (CD4⁺ and CD8⁺ T cells cotransferred). Data are shown as mean ± SEM.