Fig. 2. IL-15 induces snap arming and occurs in the naïve CD8⁺ T cells lacking phenotypic markers of memory T cells.

Ex vivo splenocytes were depleted of GR-1⁺ memory-phenotype CD8⁺ T cells and cultured for 24 hours with or without 10 ng/ml IL-15. The cultures were harvested and cytotoxic potential evaluated as described in Figure 1. (A) CD44⁺CD8⁺ T cells were depleted from ex vivo splenocytes. Magnetic beads coupled with anti-Gr1 mAb were used to deplete the memory phenotype CD8⁺ T cells. The frequency of memory phenotype effector cells (CD3⁺CD8⁺CD44^hi) was reduced from 27% to 7%. (B) IL-15 induced snap arming and facilitated allogeneic killing. A reaction between C57BL/6 splenocytes (H2^d) against DBA P815 tumor targets (H2^b) demonstrated moderate allogeneic killing capacity. Depletion of T cells with memory effector phenotype has limited effect. (C) IL-15 induced cytotoxic potential was maximized by CD3 redirection and occurred in the absence of CD8⁺CD44^hi T cells. Anti-CD3 redirected lysis of splenocytes after IL-15 treatment was markedly greater than allogeneic killing alone. Furthermore, the cytotoxic potential observed was independent of the action of memory phenotype T killers.