References

Unlike naive lymphocytes, murine T cells with a memory (CD44hi) phenotype have a relatively rapid rate of turnover (proliferation) in vivo and can survive in the absence of MHC ligands. Evidence will be presented that, in marked contrast to CD44hi CD4+ cells, CD44hi CD8+ cells are heavily dependent on contact with a cytokine, IL-15. This finding applies to 1) T cell proliferation induced via interferon-mediated stimulation of the innate immune system ("bystander " proliferation), 2) normal "background " T cell turnover, and 3) T cell survival. By all of these parameters, dependency on IL-15 is restricted to a subset of CD44hi CD8+ cells expressing a high density of CD122 (IL-2Rb), a component of the receptor for IL-15; for CD122hi cells, IL-15 dependency is especially prominent for cells expressing Ly49, a marker also found on natural killer cells. In contrast to CD122hi cells, the CD122lo subset of CD44hi CD8+ cells is IL-15 independent; likewise, being devoid of CD122hi cells, the large population of CD44hi CD4+ cells is IL-15 independent. Thus, both for proliferation and survival, subsets o