MHC class II expression by bone marrow–derived APCs is required for maximal GVL effects in MCs. (A) Peripheral blood chimerism analysis was performed in various cell lineages by flow cytometry 6 weeks after BMT. Representative contour plots show B10.A donor MHC class I (34-2-12, H2-D^d) and B6 recipient MHC class II (H2-A^b) expression in B cells of MCs generated by reconstitution with B10.A and either B6 MHC class II^+/+ or B6 MHC class II^–/– bone marrow. (B) MCs received either 3 × 10⁷ B10.A DLIs on day 56 after BMT, EL4 on day 63, or both. B6 MHC class II^+/+ plus B10.A MCs: no DLI (▪, n = 4), DLI (•, n = 7), DLI + EL4 (▾, n = 18), EL4 (○, n = 8). B6 MHC class II^–/– plus B10.A MCs (CII^– MCs): no DLI (⋄, n = 3), DLI (▿, n = 5), DLI + EL4 (▵, n = 25), EL4 (*, n = 11). Data were pooled from 2 similar, independent experiments. (C) Comparisons of GVL activities of DLI in MCs versus CII^– MCs versus FCs. MCs: DLI (• n = 7), DLI + EL4 (▾, n = 11). CII^– MCs: DLI (▿, n = 2), DLI + EL4 (▵, n = 14). FCs: DLI (♦, n = 6), DLI + EL4 (□, n = 8).