Host hematopoietic APCs prime host-specific donor T-cell expansion. 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 CD8⁺1B2⁺ T-cell populations within the spleen on day +6 following transfer. 1B2 is a clonotypic marker that identifies cells bearing the 2C TCR. Data are representative of 3 independent experiments. (B) Representative flow cytometric contour plots of recipient spleens of MCs and FCs on day +12 following DLI (x-axis, CD8-PE; y-axis, 1B2-APC). Data are representative of 3 independent experiments. (C) Percentage of gated CD8⁺1B2⁺ T cells that were CD44^high (left) or CD62L⁺ (right) in recipient spleens of MCs and FCs on day +12 following DLI (naive controls shown for comparison, n = 3 each group). Data shown represent 1 of 2 independent experiments. (D) Representative flow cytometric plots of intracellular staining of IFN-γ in gated CD8⁺1B2⁺ T cells in recipient spleens of MCs and FCs on day +12 following DLI after brief ex vivo stimulation. Percentages of gated CD8⁺1B2⁺ cells that were IFN-γ⁺ were 40% ± 9% in MCs versus 37% ± 8% in FCs (P = NS; mean ± SEM; n = 4 each group). (E) Representative flow cytometric contour plots of recipient spleens of MCs and FCs on day +12 following DLI on gated CD45.2^– cells (x-axis, CD4-PE; y-axis, Vβ3-APC or Vβ8.1/2-APC). (F) Percentage of gated CD4⁺Vβ3⁺ or CD4⁺Vβ8.1/2⁺ T cells that were CD44^high (top) or CD62L⁺ (bottom) in recipient spleens of MCs and FCs on day +12 following DLI (naive phenotype shown for comparison, n = 3 each group). (C,F) Data are shown as mean ± SEM.