Interleukin 15 Controls both Proliferation and Survival of a Subset of Memory-Phenotype CD8+ T Cells

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Abstract
Previous work has shown that memory-phenotype CD44hi CD8+ cells are controlled by a cytokine, interleukin (IL)-15. However, the dependency of CD44hi CD8+ cells on IL-15 is partial rather than complete. Here, evidence is presented that CD44hi CD8+ cells comprise a mixed population of IL-15–dependent and IL-15–independent cells. The major subset of CD122hi CD44hi CD8+ cells is heavily dependent on IL-15 by three different parameters, namely (1) “bystander” proliferation induced via IFN-induced stimulation of the innate immune system, (2) normal “background” proliferation, and (3) T cell survival; IL-15 dependency is most extreme for the Ly49+ subset of CD122hi CD44hi CD8+ cells. In contrast to CD122hi cells, the CD122lo subset of CD44hi CD8+ cells is IL-15 independent; likewise, being CD122lo, CD44hi CD4+ cells are IL-15 independent. Thus, subsets of memory-phenotype T cells differ radically in their sensitivity to IL-15.