By promoting (i) long-term survival of NK and memory CD8 + T cells and (ii) type 1 function (IFN-␣ production and cytotoxicity) in NK, NK T cells and memory CD8 + T cells, IL-15 plays an important role in antiviral immune response (Biron et al., 1999; Carson et al., 1997; Kennedy et al., 2000; Liu et al., 2002; Zhang et al., 1998) .