IL-15-independent antiviral function of primary and memory CD8(+) T cells

Memory CD8(+) T cells are comprised of CD122(hi) IL-15-dependent and CD122(lo) IL-15-independent subsets. Induction and retention of IL-15-independent memory CD8(+) T cells was assessed in IL-15(−/−) and wild-type (wt) mice immunized with recombinant vaccinia virus (rVV) or Sindbis virus (rSIN) vectors expressing the identical foreign epitope. Both vectors induced epitope-specific CD8(+) T cell expansion and function, independent of IL-15. Similar kinetics of rVV clearance confirmed effective CD8(+) T cell function in IL-15(−/−) mice. CD44(hi) CD122(hi) CD8(+) T cells, mainly of the CD62L(−/lo) phenotype, increased more dramatically and declined more rapidly in IL-15(−/−) mice, independent of the vector. Rapid IL-15-independent memory CD8(+) T cell expansion following challenge of immune mice compensated for the limited memory CD8(+) populations in IL-15(−/−) mice. However, despite expansion and expression of potent effector function, viral clearance was delayed in the absence of IL-15, coinciding with a rapid loss in cytolytic function.