David F. Tough

Potent and Selective Stimulation of Memory-Phenotype CD8+ T Cells In Vivo by IL-15

Proliferation of memory-phenotype (CD44hi) CD8+ cells induced by infectious agents can be mimicked by injection of type I interferon (IFN I) and by IFN I-inducing agents such as lipopolysaccharide and Poly I:C; such proliferation does not affect naive T cells and appears to be TCR independent. Since IFN I inhibits proliferation in vitro, IFN I-induced proliferation of CD8+ cells in vivo presumably occurs indirectly through production of secondary cytokines, e.g., interleukin-2 (IL-2) or IL-15. We show here that, unlike IL-2, IL-15 closely mimics the effects of IFN I in causing strong and selective stimulation of memory-phenotype CD44hi CD8+ (but not CD4+) cells in vivo; similar specificity applies to purified T cells in vitro and correlates with much higher expression of IL-2Rbeta on CD8+ cells than on CD4+ cells.

Induction of bystander T cell proliferation by viruses and type I interferon in vivo

T cell proliferation in vivo is presumed to reflect a T cell receptor (TCR)-mediated polyclonal response directed to various environmental antigens. However, the massive proliferation of T cells seen in viral infections is suggestive of a bystander reaction driven by cytokines instead of the TCR. In mice, T cell proliferation in viral infections preferentially affected the CD44hi subset of CD8+ cells and was mimicked by injection of polyinosinic-polycytidylic acid [poly(I:C)], an inducer of type I interferon (IFN I), and also by purified IFN I; such proliferation was not associated with up-regulation of CD69 or CD25 expression, which implies that TCR signaling was not involved. IFN I [poly(I:C)]-stimulated CD8+ cells survived for prolonged periods in vivo and displayed the same phenotype as did long-lived antigen-specific CD8+ cells. IFN I also potentiated the clonal expansion and survival of CD8+ cells responding to specific antigen. Production of IFN I may thus play an important role in the generation and maintenance of specific memory.

Type i interferons potently enhance humoral immunity and can promote isotype switching by stimulating dendritic cells in vivo.

Type I interferons (IFN-I) are rapidly induced following infection and play a key role in nonspecific inhibition of virus replication. Here we have investigated the effects of IFN-I on the generation of antigen-specific antibody responses. The data show that IFN-I potently enhance the primary antibody response to a soluble protein, stimulating the production of all subclasses of IgG, and induce long-lived antibody production and immunological memory. In addition, endogenous production of IFN-I was shown to be essential for the adjuvant activity of CFA. Finally, IFN-I enhanced the antibody response and induced isotype switching when dendritic cells were the only cell type responding to IFN-I. The data reveal the potent adjuvant activity of IFN-I and their important role in linking innate and adaptive immunity.

Cross-priming of CD8+ T cells stimulated by virus-induced type I interferon.

CD8+ T cell responses can be generated against antigens that are not expressed directly within antigen-presenting cells (APCs), through a process known as cross-priming. To initiate cross-priming, APCs must both capture extracellular antigen and receive specific activation signals. We have investigated the nature of APC activation signals associated with virus infection that stimulate cross-priming. We show that infection with lymphocytic choriomeningitis virus induces cross-priming by a mechanism dependent on type I interferon (IFN-α/β). Activation of cross-priming by IFN-α/β was independent of CD4+ T cell help or interaction of CD40 and CD40 ligand, and involved direct stimulation of dendritic cells. These data identify expression of IFN-α/β as a mechanism for the induction of cross-priming during virus infections.

Links between innate and adaptive immunity via type I interferon.

Type I interferon (IFN-alpha/beta) is expressed rapidly following exposure to a wide variety of infectious agents and plays a key role in innate control of virus replication. Recent studies have demonstrated that dendritic cells both produce IFN-alpha/beta and undergo maturation in response to IFN-alpha/beta. Moreover, IFN-alpha/beta has been shown to potently enhance immune responses in vivo through the stimulation of dendritic cells. These findings indicate that IFN-alpha/beta serves as a signal linking innate and adaptive immunity.

Initial T cell frequency dictates memory CD8+ T cell lineage commitment.

Memory T cells can be divided into central memory T cell (TCM cell) and effector memory T cell (TEM cell) subsets based on homing characteristics and effector functions. Whether TEM and TCM cells represent interconnected or distinct lineages is unclear, although the present paradigm suggests that TEM and TCM cells follow a linear differentiation pathway from naive T cells to effector T cells to TEM cells to TCM cells. We show here that naive T cell precursor frequency profoundly influenced the pathway along which CD8+ memory T cells developed. At low precursor frequency, those TEM cells generated represented a stable cell lineage that failed to further differentiate into TCM cells. These findings do not adhere to the present dogma regarding memory T cell generation and provide a means for identifying factors controlling memory T cell lineage commitment.

The Development, Maturation, and Turnover Rate of Mouse Spleen Dendritic Cell Populations

Three distinct subtypes of dendritic cells (DC) are present in mouse spleen, separable as CD4−8α−, CD4+8α−, and CD4−8α+ DC. We have tested whether these represent stages of development or activation within one DC lineage, or whether they represent separate DC lineages. All three DC subtypes appear relatively mature by many criteria, but all retain a capacity to phagocytose particulate material in vivo. Although further maturation or activation could be induced by bacterially derived stimuli, phagocytic capacity was retained, and no DC subtype was converted to the other. Continuous elimination of CD4+8− DC by Ab depletion had no effect on the levels of the other DC subtypes. Bromodeoxyuridine labeling experiments indicated that all three DC subtypes have a rapid turnover (half-life, 1.5–2.9 days) in the spleen, with none being the precursor of another. The three DC subtypes showed different kinetics of development from bone marrow precursors. The CD8α+ spleen DC, apparently the most mature, displayed an extremely rapid turnover based on bromodeoxyuridine uptake and the fastest generation from bone marrow precursors. In conclusion, the three splenic DC subtypes behave as rapidly turning over products of three independent developmental streams.

IL-15 Is Expressed by Dendritic Cells in Response to Type I IFN, Double-Stranded RNA, or Lipopolysaccharide and Promotes Dendritic Cell Activation

Cytokines that are induced by infection may contribute to the initiation of immune responses through their ability to stimulate dendritic cells (DCs). In this paper, we have addressed the role of IL-15 in DC activation, investigating its expression by DCs in response to three different signals of infection and examining its ability to stimulate DCs. We report that the expression of both IL-15 and the IL-15 receptor α-chain are increased in splenic DCs from mice inoculated with dsRNA (poly(I:C)), LPS, or IFN-αβ, and in purified murine splenic DCs treated with IFN-αβ in vitro. Furthermore, IL-15 itself was able to activate DCs, as in vivo or in vitro exposure of splenic DCs to IL-15 resulted in an up-regulation of costimulatory molecules, markedly increased production of IFN-γ by DC and an enhanced ability of DCs to stimulate Ag-specific CD8+ T cell proliferation. The magnitude of all of the IL-15-induced changes in DCs was reduced in mice deficient for the IFN-αβ receptor, suggesting a role for IFN-αβ in the stimulation of DCs by IL-15. These results identify IL-15 as a stimulatory cytokine for DCs with the potential for autocrine activity and link its effects to expression of IFN-αβ.

IL-15 promotes the survival of naive and memory phenotype CD8+ T cells.

IL-15 stimulates the proliferation of memory phenotype CD44highCD8+ T cells and is thought to play a key role in regulating the turnover of these cells in vivo. We have investigated whether IL-15 also has the capacity to affect the life span of naive phenotype (CD44low) CD8+ T cells. We report that IL-15 promotes the survival of both CD44low and CD44high CD8+ T cells, doing so at much lower concentrations than required to induce proliferation of CD44high cells. Rescue from apoptosis was associated with the up-regulation of Bcl-2 in both cell types, whereas elevated expression of Bcl-xL was observed among CD44high but not CD44low CD8+ cells. An investigation into the role of IL-15R subunits in mediating the effects of IL-15 revealed distinct contributions of the α- and β- and γ-chains. Most strikingly, IL-15Rα was not essential for either induction of proliferation or promotion of survival by IL-15, but did greatly enhance the sensitivity of cells to low concentrations of IL-15. By contrast, the β- and γ-chains of the IL-15R were absolutely required for the proliferative and pro-survival effects of IL-15, although it was not necessary for CD44highCD8+ cells to express higher levels of IL-15Rβ than CD44low cells to proliferate in response to IL-15. These results show that IL-15 has multiple effects on CD8 T cells and possesses the potential to regulate the life span of naive as well as memory CD8+ T cells.

Cutting Edge: Enhancement of Antibody Responses Through Direct Stimulation of B and T Cells by Type I IFN

Type I IFN (IFN-αβ) is induced rapidly by infection and plays a key role in innate antiviral defense. IFN-αβ also exerts stimulatory effects on the adaptive immune system and has been shown to enhance Ab and T cell responses. We have investigated the importance of B and T cells as direct targets of IFN-αβ during IFN-α-mediated augmentation of the Ab response against a soluble protein Ag. Strikingly, the ability of IFN-α to stimulate the Ab response and induce isotype switching was markedly reduced in mice in which B cells were selectively deficient for the IFN-αβR. Moreover, IFN-α-mediated enhancement of the Ab response was also greatly impaired in mice in which T cells were selectively IFN-αβR-deficient. These results indicate that IFN-αβR signaling in both B and T cells plays an important role in the stimulation of Ab responses by IFN-αβ.