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

Memory CD8+ T cells are comprised of CD122hi IL-15-dependent and CD122lo 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. CD44hi CD122hi 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.