Potent and selective stimulation of memory-phenotype CD8+ T cells in vivo by IL-15

Cytotoxic T-lymphocyte memory, virus clearance and antigenic heterogeneity

Cytotoxic T-lymphocyte (CTL) memory to viruses has traditionally been studied in an isolated setting. However, recent experiments have indicated that the presence of antigenically heterologous challenges can result in the attrition of CTL memory. Here we use mathematical models in order to explore the consequence of these dynamics for the ability of the immune system in controlling multiple infections. Mathematical models suggest that antigen-independent persistence of CTL memory is required in order to resolve and clear an infection. This ensures strong immunological pressure at low loads when the virus population declines towards extinction. If the number of antigenic stimuli exposed to the immune system crosses a threshold, we find that immunological pressure is significantly reduced at low loads and this can prevent virus clearance and reduces overall control of viral replication. Hence, exposure to many heterologous challenges reduces the ability of CTL memory to contribute to virus control. The higher the number of infections present in the host, the higher the overall virus load and the higher the total number of memory CTLs. Beyond a given threshold, addition of new viruses to the system results in accelerated loss of virus control which eventually leads to a reduction in the overall memory CTL population. These dynamics might contribute to the progressively weaker immunity observed as a result of ageing. In this context, antigenically variable pathogens expose the immune system to many heterologous challenges within a short period of time and this could result in accelerated ageing of the immune system. These results have important implications for vaccination and treatment strategies directed against viral infections.

The growth of the very large CD8+ T cell clones in older mice is controlled by cytokines

Older humans and mice frequently contain very large clones of CD8(+) T cells. In mice these cells are phenotypically very similar to memory CD8(+) T cells. Like memory CD8(+) T cells, most members of the clones are in continuous slow division, apparently independently of Ag stimulation. Proliferation of the CD8(+) clonal T cells is inhibited in mice treated with Ab to the IL-2R beta-chain that blocks signaling by either IL-2 or IL-15. However, inhibition of IL-2 increases the numbers of dividing clonal cells. Therefore, like normal memory CD8(+) T cells, expansion of the clones is driven by IL-15 and inhibited by IL-2 and is probably limited by the amounts of IL-15 and IL-2 present in the host. Control by these two cytokines may account for the fact that, although the clones can be very large, they do not overwhelm or kill their hosts. Nevertheless the clonal cells compete successfully with normal memory CD8(+) T cells for growth. Perhaps the clonal cells use IL-15 more effectively or are more resistant to the inhibitory effects of IL-2. Thus they might affect the immune response of their hosts by competing for factors that stimulate and inhibit normal CD8(+) memory T cells.

Human T cell lymphotropic virus type I Tax activates IL-15R alpha gene expression through an NF-kappa B site

IL-15 mRNA levels are increased in diseases caused by human T cell lymphotropic virus type I (HTLV-I). In this study, we demonstrated that IL-15Ralpha, the IL-15-specific binding receptor, mRNA and protein levels were also elevated in HTLV-I-infected cells. We showed that transient HTLV-I Tax expression lead to increased IL-15Ralpha mRNA levels. In addition, by using a reporter construct that bears the human IL-15Ralpha promoter, we demonstrated that Tax expression increased promoter activity by at least 4-fold. Furthermore, using promoter deletion constructs and gel shift analysis, we defined a functional NF-kappaB-binding motif in the human IL-15Ralpha promoter, suggesting that Tax activation of IL-15Ralpha is due, in part, to the induction of NF-kappaB. These data indicate that IL-15Ralpha is transcriptionally regulated by the HTLV-I Tax protein through the action of NF-kappaB. These findings suggest a role for IL-15Ralpha in aberrant T cell proliferation observed in HTLV-I-associated diseases.

Overexpression of IL-15 in vivo enhances Tc1 response, which inhibits allergic inflammation in a murine model of asthma

IL-15, a pleiotropic cytokine, is involved in the inflammatory responses in various infectious and autoimmune diseases. We have recently constructed IL-15-transgenic (Tg) mice, which have an increased number of memory-type CD8+ T cells in the peripheral lymphoid tissues. In the present study, we found that eosinophilia and Th2-type cytokine production in the airway were severely attenuated in OVA-sensitized IL-15-Tg mice following OVA inhalation. IL-15-Tg mice preferentially developed Tc1 responses mediated by CD8+ T cells after OVA sensitization, and in vivo depletion of CD8+ T cells by anti-CD8 mAb aggravated the allergic airway inflammation in IL-15-Tg mice following OVA inhalation. Adoptive transfer of CD8+ T cells from OVA-sensitized IL-15-Tg mice into normal mice before OVA sensitization suppressed Th2 response to OVA in the normal mice. These results suggest that overexpression of IL-15 in vivo suppresses Th2-mediated-allergic airway response via induction of CD8+ T cell-mediated Tc1 response.

Mice lacking interleukin-2 (IL-2)/IL-15 receptor beta chain are susceptible to infection with avirulent Salmonella enterica subsp. enterica serovar choleraesuis but mice lacking IL-2 are resistant

Interleukin-2 (IL-2)/IL-15 receptor beta (IL-15R beta)(-/-) mice were susceptible to infection with avirulent Salmonella enterica subsp. enterica serovar Choleraesuis, whereas IL-2(-/-) mice were resistant. A natural killer cell response was not evident for both types of deficient mice. A Th1 response was detected in IL-2(-/-) but not in IL-2/IL-15R beta(-/-) mice infected with Salmonella, suggesting that IL-2/IL-15R beta signaling is important for the generation of protective Th1 cells.

Predicting the dynamics of antiviral cytotoxic T-cell memory in response to different stimuli: cell population structure and protective function

This paper examines the numerical and functional consequences of various stimuli on antiviral CD8+ T-cell memory using a mathematical model. The model is based upon biological evidence from the murine model of infection with lymphocytic choriomeningitis virus (LCMV) that the phenotype of immunological memory represents low-level responses driven by various stimuli, and the memory CTL population is partitioned between resting, cycling and effector cells. These subpopulations differ in their lifespan, their potential to mediate antiviral protection and in the stimuli needed for their maintenance. Three types of maintenance stimuli are examined: non-antigen-specific (bystander) stimulation, persisting antigen stimulation and reinfection-mediated stimulation. The modelling predicts that: (i) stable persistence of CTL memory requires the presence of either bystander or antigen-specific stimulation above a certain threshold depending on the sensitivity of memory CTL to stimulation and their life-span; (ii) a relatively low level of stimuli (approximately 10(4) fold less on a per CTL basis compared to acute infection) is needed to stabilize the expanded memory CTL population; (iii) the presence of CTL subsets in the memory pool of different activation states and lifespans ensures the robustness of memory persistence in the face of temporal variation in the low-level stimuli and; (iv) an 'optimal' population structure of the memory CTL pool, in terms of immediate protection, requires the presence of both activated cycling and effector CTL. For this, persisting antigen alone or synergistically with bystander signals provide the appropriate stimulation, so that the stimuli equivalent to approximately 30 p.f.u. of LCMV in the spleen are sufficient to maintain approximately 10(5)-10(6) specific CTL in the memory pool. These observations are relevant both to our understanding of natural protective immunity and to vaccine design.

Fatal leukemia in interleukin 15 transgenic mice follows early expansions in natural killer and memory phenotype CD8+ T cells

Inflammation likely has a role in the early genesis of certain malignancies. Interleukin (IL)-15, a proinflammatory cytokine and growth factor, is required for lymphocyte homeostasis. Intriguingly, the expression of IL-15 protein is tightly controlled by multiple posttranscriptional mechanisms. Here, we engineered a transgenic mouse to overexpress IL-15 by eliminating these posttranscriptional checkpoints. IL-15 transgenic mice have early expansions in natural killer (NK) and CD8+ T lymphocytes. Later, these mice develop fatal lymphocytic leukemia with a T-NK phenotype. These data provide novel evidence that leukemia, like certain other cancers, can arise as the result of chronic stimulation by a proinflammatory cytokine.

Bystander activation of CD8+ T cells contributes to the rapid production of IFN-gamma in response to bacterial pathogens

The bacterium Burkholderia pseudomallei causes a life-threatening disease called melioidosis. In vivo experiments in mice have identified that a rapid IFN-gamma response is essential for host survival. To identify the cellular sources of IFN-gamma, spleen cells from uninfected mice were stimulated with B. pseudomallei in vitro and assayed by ELISA and flow cytometry. Costaining for intracellular IFN-gamma vs cell surface markers demonstrated that NK cells and, more surprisingly, CD8(+) T cells were the dominant sources of IFN-gamma. IFN-gamma(+) NK cells were detectable after 5 h and IFN-gamma(+) CD8(+) T cells within 15 h after addition of bacteria. IFN-gamma production by both cell populations was inhibited by coincubation with neutralizing mAb to IL-12 or IL-18, while a mAb to TNF had much less effect. Three-color flow cytometry showed that IFN-gamma-producing CD8(+) T cells were of the CD44(high) phenotype. The preferential activation of NK cells and CD8(+) T cells, rather than CD4(+) T cells, was also observed in response to Listeria monocytogenes or a combination of IL-12 and IL-18 both in vitro and in vivo. This rapid mechanism of CD8(+) T cell activation may be an important component of innate immunity to intracellular pathogens.

Unusual cytotoxic activities of thymus-independent, self-antigen-specific CD8(+) T cells

We compared the cytotoxic activities of thymus-dependent and thymus-independent CD8(+) T cells. Thymus-dependent CD8(+) T cells, which are foreign antigen specific, acquired cytotoxic activity to tumor cells with a basal dose of the antigen peptides and to hybridoma cells expressing anti-TCR mAb only after differentiation into effector cytotoxic T lymphocytes (CTL). In contrast, thymus-independent CD8(+) T cells, which have been shown to be self-antigen specific, never showed cytotoxic activity to the target cells with a basal dose of the self-antigen peptide, while they could lyse hybridoma cells expressing anti-TCR mAb even without prior antigenic stimulation. Furthermore, the ex vivo cytotoxic activity of thymus-independent CD8(+) T cells was also observed against the target cells with high doses of the antigen peptides, which were not lysed by freshly isolated thymus-dependent CD8(+) T cells. Thus it is revealed that thymus-independent, self-antigen-specific CD8(+) T cells already acquire mature CTL functions in situ but have an increased threshold of TCR-mediated signaling for activation. These differences in cytotoxic activities between thymus-dependent and thymus-independent CD8(+) T cells suggest distinct roles of the two subsets of CD8(+) T cells in vivo.

How does interleukin 15 contribute to the pathogenesis of HTLV type 1-associated myelopathy/tropical spastic paraparesis?

HTLV-1 is the etiological agent of a neurological disease named HAM/TSP that has clinical characteristics similar to those of multiple sclerosis (MS). The PBMC obtained from HAM/TSP patients undergo spontaneous proliferation in the absence of addition of any exogenous cytokines in an ex vivo culture. This spontaneous proliferation has been thought to be due to the proliferation of T cells. It was demonstrated that this proliferation is, in part, due to the production of IL-2 and its receptor (IL-2Ralpha) by HTLV-1-infected T cells. In this review, we demonstrate that IL-15 production also contributes to the spontaneous proliferation of T cells obtained from HAM/TSP PBMC. We provide data indicating that IL-15 expression is elevated in HAM/TSP PBMC when compared to that of the normal donors. Furthermore, we demonstrate that IL-15 overexpression by HTLV-1 is due to Tax trans-activation of its promoter and induction of NF-kappaB transcription factors. On the basis of these studies, we propose a model in which HTLV-1 infection of T cells results in the production of both IL-2 and IL-15 cytokines, growth factors that support the proliferation of T cells.

CD8(+) T cell-mediated skin disease in mice lacking IRF-2, the transcriptional attenuator of interferon-alpha/beta signaling

The balanced action of cytokines is known to be critical for the maintenance of homeostatic immune responses. Here, we report the development of an inflammatory skin disease involving CD8(+) T cells, in mice lacking the transcription factor, interferon regulatory factor-2 (IRF-2). CD8(+) T cells exhibit in vitro hyper-responsiveness to antigen stimulation, accompanied with a notable upregulation of the expression of genes induced by interferon-alpha/beta (IFN-alpha/beta). Furthermore, both disease development and CD8(+) T cell abnormality are suppressed by the introduction of nullizygosity to the genes that positively regulate the IFN-alpha/beta signaling pathway. IRF-2 may represent a unique negative regulator, attenuating IFN-alpha/beta-induced gene transcription, which is necessary for balancing the beneficial and harmful effects of IFN-alpha/beta signaling in the immune system.

Interleukin-7 mediates the homeostasis of naïve and memory CD8 T cells in vivo

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.

Review article: molecular signals and genetic reprogramming in peripheral T-cell differentiation

Rearrangement of gene segments occurs in T lymphocytes during thymic development as the T-cell receptor (TCR) is first expressed, allowing T cells to become central regulators of antigen specificity in the acquired immune system. However, further development of T cells occurs after population of peripheral lymphoid tissues, which can result in T-cell expansion and differentiation into effectors of various immune function, or progression to memory T cells, anergic cells or death by apoptosis. This review focuses on more recent developments concerning the choices that peripheral T cells make between first encountering antigen through TCR recognition and death. These decisions are associated with a process of genetic reprogramming that alters the behaviour of cells so that immune responses are appropriately regulated.

Memory-type CD8+ T cells protect IL-2 receptor alpha-deficient mice from systemic infection with herpes simplex virus type 2

IL-2Ralpha-deficient (IL-2Ralpha(-/-)) mice exhibit an impaired activation-induced cell death for T cells and develop abnormal T cell activation with age. In our study, we found that IL-2Ralpha(-/-) mice at the age of 5 wk contained an increased number of CD44(+)CD69(-)CD8(+) T cells in lymph nodes, which expressed a high intensity of IL-2Rbeta and vigorously proliferated in response to a high dose of IL-15 or IL-2. The T cells produced a large amount of IFN-gamma in response to IL-15 plus IL-12 in a TCR-independent bystander manner. When IL-2Ralpha(-/-) mice were inoculated i.p. with HSV type 2 (HSV-2) 186 strain, they showed resistance to the infection accompanied by an increased level of serum IL-15. The depletion of CD8(+) T cells by in vivo administration of anti-CD8 mAb rendered IL-2Ralpha(-/-) mice susceptible to HSV-2-induced lethality. These results suggest that memory-type CD8(+) T cells play a novel role in the protection against HSV-2 infection in IL-2Ralpha(-/-) mice.

Selective bystander proliferation of memory CD4+ and CD8+ T cells upon NK T or T cell activation

Ag-experienced or memory T cells have increased reactivity to recall Ag, and can be distinguished from naive T cells by altered expression of surface markers such as CD44. Memory T cells have a high turnover rate, and CD8(+) memory T cells proliferate upon viral infection, in the presence of IFN-alphabeta and/or IL-15. In this study, we extend these findings by showing that activated NKT cells and superantigen-activated T cells induce extensive bystander proliferation of both CD8(+) and CD4(+) memory T cells. Moreover, proliferation of memory T cells can be induced by an IFN-alphabeta-independent, but IFN-gamma- or IL-12-dependent pathway. In these conditions of bystander activation, proliferating memory (CD44(high)) T cells do not derive from activation of naive (CD44(low)) T cells, but rather from bona fide memory CD44(high) T cells. Together, these data demonstrate that distinct pathways can induce bystander proliferation of memory T cells.

IL-2-induced activation-induced cell death is inhibited in IL-15 transgenic mice

A transgenic (Tg) mouse expressing human IL-15 was generated to define the role of IL-15 in the normal immune response. Overexpression of IL-15 resulted in an increase of NK, CD44(hi)CD8 memory T cells, and gammadelta T cells. Additionally, we observed the emergence of a novel type of NK-T cells with CD8alphaalpha' expression. Due to the expansion and activation of NK cells, the IL-15Tg mouse showed enhanced innate immunity. In adaptive T cell immunity, the roles of IL-15 contrasted with those of IL-2. IL-15 inhibited IL-2-induced T cell death, which plays a role in the maintenance of peripheral self-tolerance. IL-15 thus seems to contribute to enhanced immune memory by selectively propagating memory T cells and by blocking T cell death mediated by IL-2.

Dynamics of T lymphocyte responses: intermediates, effectors, and memory cells

The immune response is initiated in organized lymphoid tissues where antigen-loaded dendritic cells (DCs) encounter antigen-specific T cells. DCs function as packets of information that must be decoded by the T cell before an appropriate immune response can be mounted. We discuss how the dynamics of DC-T cell encounter and the mechanism of T cell differentiation make the decoding of this information stochastic rather than determinate. This results in the generation of both terminally differentiated effector cells and intermediates that play distinctive roles in protection, immunoregulation, and immunological memory.

Cellular interactions involved in Th cell memory

The cellular interactions involved in maintaining CD4+ T cell memory have hitherto not been identified. In this report, we have investigated the roles played by B cells and dendritic cells (DCs) in this process. We show that long-lasting Th cell memory depends on the presence of B cells, but that direct Ag presentation by B cells is not required. Instead, Ag presentation by DCs is critical for the survival of memory Th cells. DCs presenting specific Ag can be detected in animals long after immunization. These findings support a model in which B cells provide an environment in which Ags may be trapped and retained. This Ag is periodically presented to memory CD4+ T cells by DCs, providing an essential survival signal.

The dual role of IL-2 in the generation and maintenance of CD8+ memory T cells

The mechanisms responsible for the generation and maintenance of T cell memory are unclear. In this study, we tested the role of IL-2 in allospecific CD8+ T cell memory by analyzing the long-term survival, phenotype, and functional characteristics of IL-2-replete (IL-2+/+) and IL-2-deficient (IL-2-/-) CD8+ TCR-transgenic lymphocytes in an adoptive transfer model. We found that IL-2 is not essential for the in vivo generation, maintenance, or recall response of CD8+ memory T cells. However, IL-2 increased the size of the CD8+ memory pool if present at the time of initial T cell activation but reduced the size of the pool if present during memory maintenance by inhibiting the proliferation of CD8+ memory T cells. Thus, IL-2-based vaccine strategies or immunosuppressive regimens that target IL-2 should take into account the divergent roles of IL-2 in CD8+ T cell immunity.

A protective role of interleukin-15 in a mouse model for systemic infection with herpes simplex virus

To define the role of cytokine binding to the IL-2/IL-15R beta chain in protective immunity against systemic infection with herpes simplex virus type 2 (HSV-2), IL-2/IL-15 receptor(R)beta knock-out mice were inoculated intraperitoneally with HSV-2 strain 186. IL-2/IL-15R beta-deficient mice were susceptible to systemic HSV-2 infection compared with their heterozygous littermates. The emergence of natural killer (NK) cells was impaired in IL-2/IL-15R beta knock-out mice, but CD4(+) T cell receptor (TCR) alpha beta(+) T cells were normally detected in the peritoneal cavity after infection with HSV-2. However, the generation of HSV-2-specific CD4(+) T helper (Th) 1 cells producing interferon-gamma was impaired in IL-2/IL-15R beta knock-out mice following HSV-2 infection. The serum IL-15 level in control mice was increased in the early stage after HSV-2 infection but was not detectable in IL-2/IL-15R beta knock-out mice. In vivo administration of recombinant IL-15 protected normal mice from HSV-2-induced lethality, accompanied by increases in numbers of NK cells and HSV-2-specific Th1 cells. Taken together, these results suggest that IL-15, using the IL-2/IL15R beta chain, plays an important role in mounting protective immunity during the course of systemic HSV-2 infection.

Poly-guanosine motifs costimulate antigen-reactive CD8 T cells while bacterial CpG-DNA affect T-cell activation via antigen-presenting cell-derived cytokines

Pathogen-derived pattern recognition ligands like lipopolysaccharide (LPS) and bacterial cytidine-guanosine (CpG)-DNA not only activate dendritic cells and macrophages but are also mitogenic for B cells. Less clear are the claimed effects of CpG-DNA on T cells, which range from direct activation, costimulation, or indirect transient activation via antigen-presenting cell (APC)-derived interferon type I (IFN type I). Here we demonstrate that CpG-DNA sequence specifically triggers macrophages to produce IFN type I, interleukin (IL)-12, IL-6 and tumour necrosis factor (TNF), but lacks the ability to directly costimulate T cells. Strikingly, poly-guanosine (poly-G) extensions to CpG-containing oligonucleotides (ODN) abolished the macrophage stimulatory potential yet generated T-cell costimulatory activities. In fact, independently of CpG-motifs, poly-G-ODN displayed the ability to costimulate T cells. Costimulation was operative on CD8 T cells but not CD4 T cells. Poly-G-mediated costimulation resulted in IL-2-driven T-cell proliferation and induced cytolytic T cells. Overall the data imply that poly-G motifs costimulate antigen reactive CD8 T cells, while CpG-DNA motifs fail to do so but may affect T-cell activation via APC derived cytokines such as IFN type I.

Naive T cells transiently acquire a memory-like phenotype during homeostasis-driven proliferation

In a depleted lymphoid compartment, naive T cells begin a slow proliferation that is independent of cognate antigen yet requires recognition of major histocompatibility complex-bound self-peptides. We have followed the phenotypic and functional changes that occur when naive CD8(+) T cells undergo this type of expansion in a lymphopenic environment. Naive T cells undergoing homeostasis-driven proliferation convert to a phenotypic and functional state similar to that of memory T cells, yet distinct from antigen-activated effector T cells. Naive T cells dividing in a lymphopenic host upregulate CD44, CD122 (interleukin 2 receptor beta) and Ly6C expression, acquire the ability to rapidly secrete interferon gamma, and become cytotoxic effectors when stimulated with cognate antigen. The conversion of naive T cells to cells masquerading as memory cells in response to a homeostatic signal does not represent an irreversible differentiation. Once the cellularity of the lymphoid compartment is restored and the T cells cease their division, they regain the functional and phenotypic characteristics of naive T cells. Thus, homeostasis-driven proliferation provides a thymus-independent mechanism for restoration of the naive compartment after a loss of T cells.

Vaccines against intracellular infections requiring cellular immunity

Vaccines against a variety of infectious diseases represent one of the great triumphs of medicine. The immune correlates of protection induced by most current vaccines seem to be mediated by long-lived humoral immune responses. By contrast, there are no currently available vaccines that are uniformly effective for diseases such as HIV, malaria and tuberculosis, in which the cellular immune response might be crucial in mediating protection. Here we examine the mechanisms by which long-lived cellular immune responses are generated and maintained in vivo. We then discuss current approaches for vaccination against diseases in which cellular immune responses are important for protection.

Cytokines and T cells in host defense

Soluble and cell-bound ligands profoundly influence the differentiative fate of lymphocytes during an immune response. Recent advances have been made in understanding the role of cytokine signals and costimulatory signals in the regulation of T cell responses associated with resistance or susceptibility to infection. There has also been recent progress in defining the requirements for the generation and maintenance of immunologic memory, a critical component of adaptive immunity.

Innate immune response of the human host to exposure with herpes simplex virus type 1: in vitro control of the virus infection by enhanced natural killer activity via interleukin-15 induction

Infections with herpes simplex virus type 1 (HSV-1) in humans and in animal models are accompanied by enhanced natural killer (NK) activity. In vitro, HSV-1 also enhances the NK activity of human peripheral blood mononuclear cells (PBMC). The molecular basis of this enhanced NK activity, however, is not well characterized. We investigated the role of human interleukin-15 (IL-15) in this phenomenon and report here that HSV-1-mediated enhanced NK activity was abrogated by neutralizing antibodies for IL-15 but not for other cytokines (i.e., IL-2, IL-12, gamma interferon [IFN-gamma], tumor necrosis factor alpha, or IFN-alpha). Anti-CD122 antibodies which block signaling through IL-2 receptor beta chain, and therefore neutralize the effects of IL-15 (and IL-2), also abrogated this enhancement. Furthermore, HSV-1 increased the levels of IL-15 mRNA and the production of IL-15 in HSV-1-infected PBMC cultures. The neutralization of IL-15 in cocultures of PBMC with HSV-1-infected cells significantly increased HSV-1 production. These results strongly suggest a role for IL-15 in the HSV-1-mediated in vitro enhancement of NK activity and in the PBMC-mediated suppression of HSV-1 replication.

Homeostasis of alpha beta TCR+ T cells

Cytokines contribute to T cell homeostasis at all stages of T cell existence. However, the particular cytokine involved varies as T cells progress from a naïve through an activated to a memory state. In many cases the important cytokines are members of the interleukin 2 subfamily of the short-chain type I cytokines. A case is made for the idea that the evolutionary divergence of the short-chain family allowed for concurrent divergence in leukocytes.

Viral activation of interleukin-15 (IL-15): characterization of a virus-inducible element in the IL-15 promoter region

We identified an interferon regulatory factor motif (IRF-E) upstream of an NF-kappaB binding site in the interleukin-15 (IL-15) promoter. Since these two motifs are part of the virus-inducible enhancer region of the beta interferon promoter, we speculated that there might be similar responses of these two genes to stimuli such as viruses. To test this hypothesis, L929 cells were infected with Newcastle disease virus (NDV), which led to the induction of IL-15 mRNA and protein expression. Using IL-15 promoter-reporter deletion constructs, a virus-inducible region, encompassing IRF-E, NF-kappaB, and a 13-nucleotide sequence flanked by these two motifs, was mapped to the -295-to--243 position relative to the transcription initiation site. Using cotransfection studies, it was demonstrated that all three motifs were essential to achieve the maximum promoter activity induced by IRF-1 and NF-kappaB expression plasmids. The presence of a virus-inducible region in the IL-15 promoter suggests a role for IL-15 as a component of host antiviral defense mechanisms.

Accessing complexity: the dynamics of virus-specific T cell responses

The cellular dynamics of the immune system are complex and difficult to measure. Access to this problematic area has been greatly enhanced by the recent development of tetrameric complexes of MHC class I glycoprotein + peptide (tetramers) for the direct staining of freshly isolated, antigen-specific CD8(+ )T cells. Analysis to date with both naturally acquired and experimentally induced infections has established that the numbers of virus-specific CD8(+) T cells present during both the acute and memory phases of the host response are more than tenfold in excess of previously suspected values. The levels are such that the virus-specific CD8(+) set is readily detected in the human peripheral blood lymphocyte compartment, particularly during persistent infections. Experimentally, it is now possible to measure the extent of cycling for tetramer (+)CD8(+) T cells during the acute and memory phases of the host response to viruses. Dissection of the phenotypic, functional, and molecular diversity of CD8(+) T cell populations has been greatly facilitated. It is hoped it will also soon be possible to analyze CD4(+) T cell populations in this way. Though these are early days and there is an enormous amount to be done, our perceptions of the shape of virus-specific cell-mediated immunity are changing rapidly.

IL-12 is required to maintain a Th1 response during Leishmania major infection

IL-12 initiates Th1 cell development and cell-mediated immunity, but whether IL-12 contributes to the maintenance of a Th1 response is unclear. To address this question, we infected IL-12 p40-/- C57BL/6 mice with Leishmania major, an intracellular protozoan parasite controlled by a cell-mediated immune response, and simultaneously administered IL-12. Whereas untreated p40-/- mice developed an uncontrolled infection, p40-/- mice treated with IL-12 for the first 2 or 4 wk of infection developed a Th1 response and resolved their lesions. However, the induction of this protective Th1 cell response by IL-12 treatment was not associated with long term immunity. We observed that on rechallenge in the absence of IL-12, the mice exhibited a susceptible phenotype. In addition, without rechallenge, lesions in the IL-12-treated p40-/- mice developed several weeks after cessation of IL-12 treatment. In both cases, disease was associated with the loss of a Th1 response and the development of a Th2 response. Our observations are not limited to the C57BL/6 strain, because IL-12 treatment was also unable to provide lasting protection to p40-/- BALB/c mice. Finally, we found that although Th1 cells from healed wild-type C57BL/6 mice adoptively transferred protection to L. major-infected RAG-/- mice, they were unable to protect p40-/- mice. In conclusion, these studies provide the first demonstration that IL-12 is required not only to initiate Th1 cell development but also throughout infection to maintain a Th1 cell response and resistance to L. major.

Population biology of lymphocytes: the flight for survival

In this essay we suggest that the primary goal of the cells of the immune system is to ensure their own growth and survival. In adults, in steady-state conditions, the number and distribution of lymphocyte populations is under homeostatic control. New lymphocytes that are continuously produced in primary and secondary lymphoid organs must compete with resident cells for survival. We discuss recent findings supporting lymphocyte survival as a continuous active process and implicating cognate receptor engagement as fundamental survival signals for both T and B lymphocytes. The conflict of survival interests between different cell types gives rise to a pattern of interactions that mimics the behavior of complex ecological systems. In their flight for survival and in response to competition, lymphocytes use different survival signals within different ecological niches during cell differentiation. This is the case for T and B lymphocytes and also for naive and memory/activated T and B cells. We discuss how niche differentiation allows the co-existence of different cell types and guarantees both repertoire diversity and efficient immune responses.

Loss of CD28 expression on CD8(+) T cells is induced by IL-2 receptor gamma chain signalling cytokines and type I IFN, and increases susceptibility to activation-induced apoptosis

CD8(+)CD28(-) T cells are selectively expanded during viral infections, indicating their importance in anti-viral immune responses. Since little is known about the differentiation of CD8(+)CD28(-) cells, we investigated the generation, function and survival characteristics of this subset. In healthy individuals CD8(+)CD28(-) T cells contained more elevated levels of perforin and IFN-gamma than the CD8(+)CD28(+) subset, indicating that they can have an effector function. CD8(+)CD28(-) cells were selectively expanded when activated CD8(+)CD28(+) T cells were cultured in IL-2, IL-7 or IL-15. Moreover, the generation of CD8(+)CD28(-) cells was accelerated by type I IFN suggesting that these cytokines which are released during viral infections influence CD8(+) T cell differentiation. We did not observe re-expression of CD28 by CD8(+)CD28(-) T cells in any of the experiments performed. Activated T cells are susceptible to activation-induced cell death (AICD) if re-stimulated in the absence of co-stimuli. AICD was induced in both CD28(+) and CD28(-) subsets of activated T cells when stimulated with anti-CD3 antibody in the absence of co-stimuli but the magnitude of death was greater in the CD28(-) subset. While co-stimulation through LFA-1 (CD11a and CD18) significantly reduced AICD in the CD8(+)CD28(+) subset, death was not prevented in CD8(+)CD28(-) cells. These results suggest that CD8(+)CD28(-) T cells are more functionally differentiated than the CD8(+)CD28(+) subset and indicate they may represent a terminally differentiated effector population which is destined for clearance by apoptosis at the end of the immune response.

Evolution of the CD8 T-cell repertoire during infections

CD8 T cells exist in a dynamic network whose repertoire remains static in the absence of infection but changes in the presence of foreign antigens. Individuals each have unique T-cell repertoires that continually evolve in the presence of antigen and of cross-reactive heterologous antigens, and homeostatic forces drive deletions in T-cell memory pools to accommodate the entry of new memory cells into a finite immune system.

IFN-alpha and IFN-beta: a link between immune memory and chronic inflammation

The majority of expanded T cells generated during an immune response are cleared by apoptosis. Prevention of death in some activated T cells enables the persistence of a memory T-cell pool. Here, observations that IFN-alpha and IFN-beta inhibit activated T-cell apoptosis are described. Although this enables memory T cells to persist without antigen, excessive IFN-alpha or IFN-gamma secretion might lead to chronic inflammation.

DNA vaccination to treat autoimmune diabetes

In numerous animal models, DNA immunization has been shown to induce protective immunity against infectious diseases (viral, bacterial and protozoan) and cancers (1, 2). In these situations it is desirable to induce a strong immune response to the DNA-encoded antigen in order to generate an immune memory that enables the vaccine to respond more rapidly to subsequent challenge. The success of DNA vaccination in this regard has led to its rapid introduction into several human clinical trials (3, 4). However, in autoimmunity, undesirable immune responses to autoantigens are thought to lead to the destruction of target cells or organs, resulting in diseases such as myasthenia gravis, diabetes or multiple sclerosis. Thus, at first sight, it appears that immunization would more likely trigger autoimmunity than ameliorate it. Nevertheless, clinical experience has shown that certain immune-mediated diseases may be countered by low-dose antigen administration ('desensitization'), although the underlying mechanisms remain somewhat conjectural. Here, we will describe an intriguing approach to the prevention of autoimmune disease, in which we use a DNA vaccine encoding a self-antigen to abrogate autoimmune diabetes. The success of this strategy relies on the nature of the immune response induced by the DNA vaccine.

Identification of a previously unknown antigen-specific regulatory T cell and its mechanism of suppression

Despite increasing evidence for the existence of antigen-specific regulatory T cells, the mechanisms underlying suppression remain unclear. In this study we have identified and cloned a novel subset of antigen-specific regulatory T cells and demonstrated that these T cells possess a unique combination of cell surface markers and array of cytokines. The regulatory T cells are able to inhibit the function of T cells carrying the same T-cell receptor specificity and prevent skin allograft rejection in an antigen-specific, dose-dependent manner. The regulatory T cells are able to acquire alloantigen from antigen-presenting cells, present the alloantigen to activated syngeneic CD8+ T cells and then send death signals to CD8+ T cells. These findings provide a novel mechanism of regulatory T-cell-mediated, antigen-specific suppression.

Interferon-α and -β inhibit the in vitro differentiation of immunocompetent human dendritic cells from CD14+ precursors

Dendritic cell (DC) precursors and immature DC reside in epithelium where they encounter pathogens and cytokines, which stimulate their differentiation. We hypothesized that type-I interferons (IFN- and -β), cytokines that are produced early in the innate immune response against viruses and some bacteria, may influence DC differentiation and function. To examine this possibility, we used an in vitro model of DC differentiation in which initial culture of human CD14+monocytes with granulocyte–macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 generates immature DC, and subsequent culture with tumor necrosis factor (TNF)- drives the final development into mature DC. We found in this model that IFN-/β, added from the initiation of the culture on, significantly reduced the survival and altered the morphology and differentiation of DC. TNF-–dependent maturation of IFN-β–treated immature DC led to cells with reduced expression of CD1a, CD40, CD54, and CD80 when compared with mature DC controls. IFN-/β–treated DC further had a reduced capacity to induce naive Th-cell proliferation through allostimulation or anti-CD3 monoclonal antibody stimulation. In addition, IFN-/β–treated DC secreted less IL-12 upon stimulation with Staphylococcus aureus Cowan strain or with CD4+ T cells, and this decrease correlated directly with their inability to support CD4+ T-cell secretion of IFN-γ, even though T-cell lymphotoxin production was unaffected. These findings indicate that type-I IFNs can influence the generation of acquired immune responses by modifying T-helper cell differentiation through the regulation of DC differentiation and function.

Interferon-α and -β inhibit the in vitro differentiation of immunocompetent human dendritic cells from CD14+ precursors

Dendritic cell (DC) precursors and immature DC reside in epithelium where they encounter pathogens and cytokines, which stimulate their differentiation. We hypothesized that type-I interferons (IFN- and -β), cytokines that are produced early in the innate immune response against viruses and some bacteria, may influence DC differentiation and function. To examine this possibility, we used an in vitro model of DC differentiation in which initial culture of human CD14+monocytes with granulocyte–macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 generates immature DC, and subsequent culture with tumor necrosis factor (TNF)- drives the final development into mature DC. We found in this model that IFN-/β, added from the initiation of the culture on, significantly reduced the survival and altered the morphology and differentiation of DC. TNF-–dependent maturation of IFN-β–treated immature DC led to cells with reduced expression of CD1a, CD40, CD54, and CD80 when compared with mature DC controls. IFN-/β–treated DC further had a reduced capacity to induce naive Th-cell proliferation through allostimulation or anti-CD3 monoclonal antibody stimulation. In addition, IFN-/β–treated DC secreted less IL-12 upon stimulation with Staphylococcus aureus Cowan strain or with CD4+ T cells, and this decrease correlated directly with their inability to support CD4+ T-cell secretion of IFN-γ, even though T-cell lymphotoxin production was unaffected. These findings indicate that type-I IFNs can influence the generation of acquired immune responses by modifying T-helper cell differentiation through the regulation of DC differentiation and function.

Interleukin-15 up-regulates the expression of CD154 on synovial fluid T cells

To investigate the role of the CD40-CD154 interaction in rheumatoid arthritis (RA), we analysed the expression of CD154 on CD3+ and CD4+ T cells in synovial fluid (SF) from patients with RA and in peripheral blood (PB) from patients and normal controls. As interleukin (IL)-15 is a potent activator of synovial T cells we wanted to study whether IL-15 also regulated the expression of CD154 on these T cells. Freshly isolated synovial T cells did not express significant levels of CD154, as evaluated using flow cytometry, whereas the expression of CD86 and human leucocyte antigen (HLA)-DR was significantly elevated on SF T cells when compared with PB T cells from patients or controls. Synovial T cells could up-regulate their CD154 expression following activation with phorbol 12-myristate 13-acetate (PMA) + ionomycin or anti-CD3 + anti-CD28 monoclonal antibodies (mAbs), but the maximal level of expression remained lower than in control T cells. IL-15 significantly increased the expression of CD154 on SF and PB T cells from patients, whereas IL-2 had minimal effects. Furthermore, IL-15 induced extensive proliferation in SF T cells. Our results show that SF T cells up-regulate the expression of CD154 in the presence of IL-15, a cytokine present in the synovium of patients with RA. These results further emphasize the role of IL-15 in the pathogenesis of RA.

Constitutive expression of a chimeric receptor that delivers IL-2/IL-15 signals allows antigen-independent proliferation of CD8+CD44high but not other T cells

We have prepared transgenic mice whose T cells constitutively express a chimeric receptor combining extracellular human IL-4R and intracellular IL-2Rbeta segments. This receptor can transmit IL-2/IL-15-like signals in response to human, but not mouse, IL-4. We used these animals to explore to what extent functional IL-2R/IL-15R expression controls the capacity of T cells to proliferate in response to IL-2/IL-15-like signals. After activation with Con A, naive transgenic CD8+ and CD4+ T cells respond to human IL-4 as well as to IL-2. Without prior activation, they failed to proliferate in response to human IL-4, although human IL-4 did prolong their survival. Thus, IL-2-induced proliferation of activated T cells requires at least one other Ag-induced change apart from the induction of a functional IL-2R. However, a fraction of CD8+CD44high T cells proliferate in human IL-4 without antigenic stimulation or syngeneic feeder cells. In contrast, CD4+CD44high T cells are not constitutively responsive to human IL-4. We conclude that although all transgenic T cells express a functional chimeric receptor, only some CD8+CD44high T cells contain all molecules required for entry into the cell cycle in response to human IL-4 or IL-15.

Role of interferon-alpha and clonally expanded T cells in the immunotherapy of chronic myelogenous leukemia

Twenty five percent of patients in the chronic phase of chronic myelogenous leukemia (CML) are treated with interferon-alpha (IFN-alpha) to induce a cytogenic remission. In addition to its direct effects on leukemic cells, IFN-alpha has been shown to induce immunologic alterations, including upregulation of the expression of major histocompatibility (MHC) antigens in antigen-presenting cells (APCs), as well as augmentation of the activity of the lymphocytes against tumor cells. However, there has been little direct evidence supporting a causal interaction between cellular immunoreactivity and clinical responsiveness to IFN-alpha. We have shown that one approach to elucidate the immunological mechanisms by which IFN-alpha exerts its anti-CML activity is by analyzing therapy-induced modulation in T-cell receptor (TCR) Vbeta chain usage, using the reverse transcription-polymerase chain reaction (RT-PCR) followed by single-strand conformation (SSCP) analysis. This method is particularly attractive, since it provides an index of antigen-specific T cell expansion, but does not require the extraction and purification of the antigens involved in the T-cell response. T cell clones that express the Vbeta 10, 12, and 14 families predominate in the peripheral blood (PB) of CML patients. The enhanced expression of the Vbeta 9 and 20 families has been detected in IFN-alpha responsive patients but not patients who are poorly responsive to this agent. This suggests that expansion of T cells expressing these TCR Vbeta gene families may serve as a prognostic factors of the clinical responsiveness of CML patients to IFN-alpha. In addition, since T cell clones that express certain Vbeta families may react with a discrete set of antigenic peptides presented on the surface of malignant cells, a better understanding of the immunobiology of T cells in CML may allow for the design of increasing efficacious immune therapy for this disease.

Differences in interleukin-12 and -15 production by dendritic cells at the early stage of Listeria monocytogenes infection between BALB/c and C57 BL/6 mice

The mechanisms responsible for the resistance of C57BL/6 mice and for the susceptibility of BALB/c mice to infection with Listeria monocytogenes were studied by comparing early IL-12 and IL-15 production by dendritic cells (DC) after infection with L. monocytogenes. Splenic DC expressing CD11b(low) and CD11c(+) obtained from C57BL/6 mice at 3 and 6 h after L. monocytogenes infection expressed higher levels of IL-12 p40 mRNA and IL-12 p40 protein than did those from BALB/c mice. Concurrently, a larger amount of IFN-gamma was produced by the splenic T cells from C57BL/6 mice in response to immobilized anti-TCRalphabeta mAb than by those from BALB/c mice, while the splenic T cells from BALB/c mice produced a higher level of IL-4 upon TCR alphabeta stimulation than did those of C57BL/6 mice. IL-15 mRNA and intracellular IL-15 protein were detected more abundantly in the DC from C57BL/6 mice than in those from BALB/c mice on day 3 after infection. CD3(+) IL2Rbeta (+) cells in the spleen were increased in C57BL/6 mice but not in BALB/c mice at the early stage after infection. Furthermore, IL-12Rbeta2 gene expression was up-regulated in T cells from C57BL/6 mice but not in those from BALB/c mice at the early stage after listerial infection. These results suggest that the difference in early production of IL-12 and IL-15 by DC may at least partly underlie the difference in susceptibility to L. monocytogenes between C57BL/6 and BALB/c mice.

The role of Stat5a and Stat5b in signaling by IL-2 family cytokines

The activation of Stat5 proteins (Stat5a and Stat5b) is one of the earliest signaling events mediated by IL-2 family cytokines, allowing the rapid delivery of signals from the membrane to the nucleus. Among STAT family proteins, Stat5a and Stat5b are the two most closely related STAT proteins. Together with other transcription factors and co-factors, they regulate the expression of the target genes in a cytokine-specific fashion. In addition to their activation by cytokines, activities of Stat5a and Stat5b, as well as other STAT proteins, are negatively controlled by CIS/SOCS/SSI family proteins. The outcome of Stat5 activation in regulating expression of target genes varies, depending upon the complexity of the promoter region of target genes and the other signaling pathways that are activated by each cytokine as well. Here, we mainly focus on the IL2-/IL-2 receptor system, as it is one of the best-studied systems that depend on Stat5-mediated signals. We will summarize what we have learned about the molecular mechanisms of how Stat5 is activated by IL-2 family cytokines from in vitro biochemical studies as well as the role that is played by Stat5 in each of the cytokine signaling pathways from in vivo gene-targeting analyses. Oncogene (2000).

Type I interferon as a powerful adjuvant for monocyte-derived dendritic cell development and activity in vitro and in Hu-PBL-SCID mice

Type I interferons (IFNs) are cytokines exhibiting antiviral and antitumor effects, including multiple activities on immune cells. However, the importance of these cytokines in the early events leading to the generation of an immune response is still unclear. Here, we have investigated the effects of type I IFNs on freshly isolated granulocyte/macrophage colony-stimulating factor (GM-CSF)-treated human monocytes in terms of dendritic cell (DC) differentiation and activity in vitro and in severe combined immunodeficiency mice reconstituted with human peripheral blood leukocytes (hu-PBL-SCID) mice. Type I IFNs induced a surprisingly rapid maturation of monocytes into short-lived tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL)-expressing DCs endowed with potent functional activities, superior with respect to the interleukin (IL)-4/GM-CSF treatment, as shown by FACS((R)) analyses, mixed leukocyte reaction assays with allogeneic PBLs, and lymphocyte proliferation responses to HIV-1-pulsed autologous DCs. Type I IFN induced IL-15 production and strongly promoted a T helper cell type 1 response. Notably, injection of IFN-treated HIV-1-pulsed DCs in SCID mice reconstituted with autologous PBLs resulted in the generation of a potent primary immune response, as evaluated by the detection of human antibodies to various HIV-1 antigens. These results provide a rationale for using type I IFNs as vaccine adjuvants and support the concept that a natural alliance between these cytokines and monocytes/DCs represents an important early mechanism for connecting innate and adaptive immunity.

T-cell memory: You must remember this

While antigen-inexperienced (naive) T cells appear to require constant tickling of their receptor by self-antigens for homeostasis, antigen-experienced (memory) T cells have no such requirement. An implication is that long-term T-cell memory does not depend on persisting antigen.