The naïve and memory T lymphocyte pools are maintained through poorly understood homeostatic mechanisms that may include signaling via cytokine receptors. We show that interleukin-7 (IL-7) plays multiple roles in regulating homeostasis of CD8+ T cells. We found that IL-7 was required for homeostatic expansion of naïve CD8+ and CD4+ T cells in lymphopenic hosts and for CD8+ T cell survival in normal hosts. In contrast, IL-7 was not necessary for growth of CD8+ T cells in response to a virus infection but was critical for generating T cell memory. Up-regulation of Bcl-2 in the absence of IL-7 signaling was impaired after activation in vivo. Homeostatic proliferation of memory cells was also partially dependent on IL-7. These results point to IL-7 as a pivotal cytokine in T cell homeostasis.

T cell functional differentiation is mediated by lineage-specific transcription factors. T helper 17 (Th17) has been recently identified as a distinct Th lineage mediating tissue inflammation. Retinoic acid receptor-related orphan receptor gamma (ROR gamma) was shown to regulate Th17 differentiation; ROR gamma deficiency, however, did not completely abolish Th17 cytokine expression. Here, we report Th17 cells highly expressed another related nuclear receptor, ROR alpha, induced by transforming growth factor-beta and interleukin-6 (IL-6), which is dependent on signal transducer and activator of transcription 3. Overexpression of ROR alpha promoted Th17 differentiation, possibly through the conserved noncoding sequence 2 in Il17-Il17f locus. ROR alpha deficiency resulted in reduced IL-17 expression in vitro and in vivo. Furthermore, ROR alpha and ROR gamma coexpression synergistically led to greater Th17 differentiation. Double deficiencies in ROR alpha and ROR gamma globally impaired Th17 generation and completely protected mice against experimental autoimmune encephalomyelitis. Therefore, Th17 differentiation is directed by two lineage-specific nuclear receptors, ROR alpha and ROR gamma.

After activation, CD4+ helper T (T(H)) cells differentiate into distinct effector subsets that are characterized by their unique cytokine expression and immunoregulatory function. During this differentiation, T(H)1 and T(H)2 cells produce interferon-gamma and interleukin (IL)-4, respectively, as autocrine factors necessary for selective lineage commitment. A distinct T(H) subset, termed T(HIL-17), T(H)17 or inflammatory T(H) (T(H)i), has been recently identified as a distinct T(H) lineage mediating tissue inflammation. T(H)17 differentiation is initiated by transforming growth factor-beta and IL-6 (refs 5-7) and reinforced by IL-23 (ref. 8), in which signal transduction and activators of transcription (STAT)3 and retinoic acid receptor-related orphan receptor (ROR)-gamma mediate the lineage specification. T(H)17 cells produce IL-17, IL-17F and IL-22, all of which regulate inflammatory responses by tissue cells but have no importance in T(H)17 differentiation. Here we show that IL-21 is another cytokine highly expressed by mouse T(H)17 cells. IL-21 is induced by IL-6 in activated T cells, a process that is dependent on STAT3 but not ROR-gamma. IL-21 potently induces T(H)17 differentiation and suppresses Foxp3 expression, which requires STAT3 and ROR-gamma, which is encoded by Rorc. IL-21 deficiency impairs the generation of T(H)17 cells and results in protection against experimental autoimmune encephalomyelitis. IL-21 is therefore an autocrine cytokine that is sufficient and necessary for T(H)17 differentiation, and serves as a target for treating inflammatory diseases.

Regulatory T (Treg) and T helper 17 (Th17) cells were recently proposed to be reciprocally regulated during differentiation. To understand the underlying mechanisms, we utilized a Th17 reporter mouse with a red fluorescent protein (RFP) sequence inserted into the interleukin-17F (IL-17F) gene. Using IL-17F-RFP together with a Foxp3 reporter, we found that the development of Th17 and Foxp3(+) Treg cells was associated in immune responses. Although TGF-beta receptor I signaling was required for both Foxp3 and IL-17 induction, SMAD4 was only involved in Foxp3 upregulation. Foxp3 inhibited Th17 differentiation by antagonizing the function of the transcription factors RORgammat and ROR*. In contrast, IL-6 overcame this suppressive effect of Foxp3 and, together with IL-1, induced genetic reprogramming in Foxp3(+) Treg cells. STAT3 regulated Foxp3 downregulation, whereas STAT3, RORgamma, and ROR* were required for IL-17 expression in Treg cells. Our data demonstrate molecular antagonism and plasticity of Treg and Th17 cell programs.

Evidence has accumulated that cytokines have a fundamental role in the differentiation of memory T cells. Here, we follow the CD8+ T cell from initial activation to memory-cell generation, indicating the checkpoints at which cytokines determine the fate of the T cell. Members of the common cytokine-receptor gamma-chain (gammac)-cytokine family--in particular, interleukin-7 (IL-7) and IL-15--act at each stage of the immune response to promote proliferation and survival. In this manner, a stable and protective, long-lived memory CD8+ T-cell pool can be propagated and maintained.