Effect of IL-18 on serum proinflammatory cytokines, donor CD8⁺ T-cell expansion, and CTL activity after BMT. bm1 mice were injected with IL-18 or diluent and underwent transplantation with TCD BM and CD8⁺ T cells from syngeneic or allogeneic donors, as in Figure 1B. Sera from the recipient animals (n = 3/group) were obtained on day +7 after BMT and were analyzed as described in “Materials and methods.” Syngeneic plus IL-18 (□), allogeneic plus diluent (▪), and allogeneic plus IL-18 (▤). Serum levels of (A) IFN-γ and (B) TNF-α were significantly elevated in the IL-18-injected allogeneic recipients compared with allogeneic control animals (▤ vs ▪; ★P < .01). (C) Splenocytes were harvested from the recipients (n = 4/group) and labeled with anti-CD8⁺ PE and anti-CD45.1⁺ FITC. The number of donor T cells (CD45.1⁺CD8⁺) was determined by flow cytometry. IL-18 significantly increased donor CD8⁺ expansion (▪, allogeneic controls vs ▤, allogeneic plus rmL-18) on days 7 and 14 after BMT (★P = .01). (D) IL-18 augments allospecific CD8⁺ T-cell activity. B6 CD8⁺ T cells were cultured with bm1 stimulators in a primary mixed lymphocyte culture (MLC) with IL-18 (20 ng/mL) (▪) or the diluent (□), as described in “Materials and methods.” After 4 days of culture, CD8⁺ cells were assessed for cytotoxicity against bm1 concanavalin A-stimulated lymphoblasts in a ⁵¹Cr release assay at the indicated effector-target ratios. CTL activity was significantly enhanced by IL-18 (▪ vs □; ★P < .01; ★★P < .05). Error bars represent means ± SE.