A major unanswered question is what distinguishes the majority of activated CD8 T cells that die after an acute viral infection from the small fraction (5-10%) that survive to become long-lived memory cells. In this study we show that increased expression of the interleukin 7 receptor alpha-chain (IL-7Ralpha) identifies the effector CD8 T cells that will differentiate into memory cells. IL-7R(hi) effector cells contained increased amounts of antiapoptotic molecules, and adoptive transfer of IL-7R(hi) and IL-7R(lo) effector cells showed that IL-7R(hi) cells preferentially gave rise to memory cells that could persist and confer protective immunity. Thus, selective expression of IL-7R identifies memory cell precursors, and this marker may be useful in predicting the number of memory T cells generated after infection or immunization.

The peripheral mature T cell pool is regulated by complex homeostatic mechanisms. Naive T cells are maintained by interleukin-7 (IL-7) and T cell receptor (TCR) signaling from contact with major histocompatibility complex (MHC), which sustain expression of antiapoptotic molecules and allow the cells to survive in interphase. Competition for these ligands declines when T cell numbers are reduced and causes residual naive T cells to proliferate and differentiate into memory-like cells. This memory cell population is thus heterogeneous and comprised of cells derived from responses to both foreign and self-antigens. Typical memory cells are kept alive and induced to divide intermittently by a mixture of IL-7 and IL-15. This review highlights recent advances in how naive and memory T cell homeostasis is regulated.

Interleukin-2 (IL-2), which is a growth factor for T lymphocytes, can also sometimes be inhibitory. Thus, the proliferation of CD8+ T cells in vivo is increased after the injection of a monoclonal antibody that is specific for IL-2 (IL-2 mAb), perhaps reflecting the removal of IL-2-dependent CD4+ T regulatory cells (T regs). Instead, we show here that IL-2 mAb augments the proliferation of CD8+ cells in mice simply by increasing the biological activity of preexisting IL-2 through the formation of immune complexes. When coupled with recombinant IL-2, some IL-2/IL-2 mAb complexes cause massive (>100-fold) expansion of CD8+ cells in vivo, whereas others selectively stimulate CD4+ T regs. Thus, different cytokine-antibody complexes can be used to selectively boost or inhibit the immune response.

In T cell-deficient conditions, naive T cells undergo spontaneous "homeostatic" proliferation in response to contact with self-MHC/peptide ligands. With the aid of an in vitro system, we show here that homeostatic proliferation is also cytokine-dependent. The cytokines IL-4, IL-7, and IL-15 enhanced homeostatic proliferation of naive T cells in vitro. Of these cytokines, only IL-7 was found to be critical; thus, naive T cells underwent homeostatic proliferation in IL-4(-) and IL-15(-) hosts but proliferated minimally in IL-7(-) hosts. In addition to homeostatic proliferation, the prolonged survival of naive T cells requires IL-7. Thus, naïve T cells disappeared gradually over a 1-month period upon adoptive transfer into IL-7(-) hosts. These findings indicate that naive T cells depend on IL-7 for survival and homeostatic proliferation.