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

Differential effects of denileukin diftitox IL-2 immunotoxin on NK and regulatory T cells in nonhuman primates

Denileukin diftitox (DD), a fusion protein comprising IL-2 and diphtheria toxin, was initially expected to enhance antitumor immunity by selectively eliminating regulatory T cells (Tregs) displaying the high-affinity IL-2R (α-β-γ trimers). Although DD was shown to deplete some Tregs in primates, its effects on NK cells (CD16(+)CD8(+)NKG2A(+)CD3(-)), which constitutively express the intermediate-affinity IL-2R (β-γ dimers) and play a critical role in antitumor immunity, are still unknown. To address this question, cynomolgus monkeys were injected i.v. with two doses of DD (8 or 18 μg/kg). This treatment resulted in a rapid, but short-term, reduction in detectable peripheral blood resting Tregs (CD4(+)CD45RA(+)Foxp3(+)) and a transient increase in the number of activated Tregs (CD4(+)CD45RA(-)Foxp3(high)), followed by their partial depletion (50-60%). In contrast, all NK cells were deleted immediately and durably after DD administration. This difference was not due to a higher binding or internalization of DD by NK cells compared with Tregs. Coadministration of DD with IL-15, which binds to IL-2Rβ-γ, abrogated DD-induced NK cell deletion in vitro and in vivo, whereas it did not affect Treg elimination. Taken together, these results show that DD exerts a potent cytotoxic effect on NK cells, a phenomenon that might impair its antitumoral properties. However, coadministration of IL-15 with DD could alleviate this problem by selectively protecting potentially oncolytic NK cells, while allowing the depletion of immunosuppressive Tregs in cancer patients.

The role of interleukin-15 in inflammation and immune responses to infection: implications for its therapeutic use

Interleukin-15 (IL-15) is a pleiotropic cytokine with a broad range of biological functions in many diverse cell types. It plays a major role in the development of inflammatory and protective immune responses to microbial invaders and parasites by modulating immune cells of both the innate and adaptive immune systems. This review provides an overview of the mechanisms by which IL-15 modulates the host response to infectious agents and its utility as a cytokine adjuvant in vaccines against infectious pathogens.

NK cell development, homeostasis and function: parallels with CD8⁺ T cells

Natural killer (NK) cells survey host tissues for signs of infection, transformation or stress and, true to their name, kill target cells that have become useless or are detrimental to the host. For decades, NK cells have been classified as a component of the innate immune system. However, accumulating evidence in mice and humans suggests that, like the B and T cells of the adaptive immune system, NK cells are educated during development, possess antigen-specific receptors, undergo clonal expansion during infection and generate long-lived memory cells. In this Review, we highlight the many stages that an NK cell progresses through during its remarkable lifetime, discussing similarities and differences with its close relative, the cytotoxic CD8(+) T cell.

How flaviviruses activate and suppress the interferon response

The flavivirus genus includes viruses with a remarkable ability to produce disease on a large scale. The expansion and increased endemicity of dengue and West Nile viruses in the Americas exemplifies their medical and epidemiological importance. The rapid detection of viral infection and induction of the innate antiviral response are crucial to determining the outcome of infection. The intracellular pathogen receptors RIG-I and MDA5 play a central role in detecting flavivirus infections and initiating a robust antiviral response. Yet, these viruses are still capable of producing acute illness in humans. It is now clear that flaviviruses utilize a variety of mechanisms to modulate the interferon response. The non-structural proteins of the various flaviviruses reduce expression of interferon dependent genes by blocking phosphorylation, enhancing degradation or down-regulating expression of major components of the JAK/STAT pathway. Recent studies indicate that interferon modulation is an important factor in the development of severe flaviviral illness. This suggests that an increased understanding of viral-host interactions will facilitate the development of novel therapeutics to treat these viral infections and improved biological models to study flavivirus pathogenesis.

Herpes simplex virus (HSV)-specific T cells activated in the absence of IFN-gamma express alternative effector functions but are not protective against genital HSV-2 infection

Interferon gamma (IFNgamma) is important for immune resistance to herpes simplex virus (HSV) infection. To examine the influence of IFNgamma on the development of HSV-specific immune responses and test for IFNgamma-independent adaptive immune mechanisms of protection, IFNgamma-deficient mice (IFNgamma(-/-)) were immunized with thymidine kinase-deficient HSV-2 (HSV-2 333tk(-)). HSV-specific cellular and humoral responses were elicited in immunized IFNgamma(-/-) mice resulting in increased resistance relative to non-immune C57BL/6J (B6) mice following challenge with fully virulent HSV-2. CD8(+) T cells from IFNgamma(-/-) mice displayed cytotoxic activity and secreted TNFalpha. HSV-specific CD4(+) T cells from immunized IFNgamma(-/-) mice secreted IL-4, TNFalpha, and IL-17, but unlike T cells from HSV-immune B6 mice, could not clear virus from genital tissue following adoptive transfer. HSV-immune IFNgamma(-/-) mice produced predominantly IgG(1) HSV-specific antibodies while immune B6 mice produced predominantly IgG(2c) antibodies. Transfer of equivalent amounts of HSV-specific antibodies from either strain to naïve mice imparted equivalent early resistance against infection of the genital epithelia. However, protection against neurological symptoms mediated by immune B6 antibodies was superior late in infection. Taken together, these results demonstrate that the limited resistance of HSV-immune IFNgamma(-/-) mice to HSV-2 infection resulted from the action of HSV-specific Ab rather than IFNgamma-independent effector functions of T cells. Further, protection against neurological manifestations of HSV-2 infection was superior in mice receiving Ab from immune B6 mice suggesting that Ab-mediated protective mechanisms involving IFNgamma-induced IgG subclasses were more effective once virus had spread to neural tissues.

Interleukin-15 mediates potent antiviral responses via an interferon-dependent mechanism

Interleukin-15 (IL-15) is a potent growth factor for activated T and natural killer (NK) cells, stimulator of memory T cells and plays an important role in viral immunity. To investigate mechanisms underlying the antiviral activity of IL-15, a recombinant vaccinia virus (rVV) encoding murine IL-15 (VV-IL-15) was constructed. Following infection of mice with VV-IL-15, virus titres in the ovaries were significantly reduced compared to mice infected with control VV. Growth of VV-IL-15 was also reduced in nude athymic mice, indicating the antiviral activity of IL-15 does not require T cells. Additionally, VV-IL-15 augmented the cytolytic activity of natural NK cells in the spleen and enhanced interferon (IFN) mRNA expression and transcription factors associated with IFN induction. Using knockout mice and antibody depletion studies, we showed for the first time that the control of VV-IL-15 replication in mice is dependent on NK cells and IFNs and, in their absence, the protective role of IL-15 is abolished.

Safety and immunologic effects of IL-15 administration in nonhuman primates

The administration of cytokines that modulate endogenous or transferred T-cell immunity could improve current approaches to clinical immunotherapy. Interleukin-2 (IL-2) is used most commonly for this purpose, but causes systemic toxicity and preferentially drives the expansion of CD4(+)CD25(+)Foxp3(+) regulatory T cells, which can inhibit antitumor immunity. IL-15 belongs to the gamma(c) cytokine family and possesses similar properties to IL-2, including the ability to induce T-cell proliferation. Whereas IL-2 promotes apoptosis and limits the survival of CD8(+) memory T cells, IL-15 is required for the establishment and maintenance of CD8(+) T-cell memory. However, limited data are available to guide the clinical use of IL-15. Here, we demonstrate in nonhuman primates that IL-15 administration expands memory CD8(+) and CD4(+) T cells, and natural killer (NK) cells in the peripheral blood, with minimal increases in CD4(+)CD25(+)Foxp3(+) regulatory T cells. Daily administration of IL-15 resulted in persistently elevated plasma IL-15 levels and transient toxicity. Intermittent administration of IL-15 allowed clearance of IL-15 between doses and was safe for more than 3 weeks. These findings demonstrate that IL-15 has profound immunomodulatory properties distinct from those described for IL-2, and suggest that intermittent administration of IL-15 should be considered in clinical studies.

Herpes simplex virus infection downmodulates NKG2D ligand expression

Herpes simplex virus (HSV) type 1 infection may cause orofacial infections in humans. The virus resides in a latent form in neural ganglia and occasionally reactivates and infects epithelial cells. Natural killer (NK) cells have been implicated in immune control of herpes virus infections, possibly by downmodulating major histocompatibility complex (MHC) class I and by other, as yet unidentified, mechanisms. Upon HSV-1 infection of cell lines, surface levels of NKG2D ligands MHC class I related proteins (MIC) A and UL16 binding protein 2 were downmodulated due to late viral gene product(s). As also MHC class I levels were reduced by HSV-1, NK cell recognition of HeLa cells was not affected by infection. Total cellular MICA contents remained unchanged, suggesting masking, internalization or intracellular retention of MICA as possible mechanisms of viral downregualtion of MICA surface levels. Furthermore, NK cells from patients with active HSV-1 infection had a tendency towards increased expression level of the activating receptor NKG2D. These data support a role for NKG2D-MICA interactions in immune responses to HSV-1 reactivation.

Dendritic cells are required for optimal activation of natural killer functions following primary infection with herpes simplex virus type 1

Natural killer (NK) cells play an important role in the optimal clearance of herpes simplex virus type 1 (HSV-1) infection in mice. Activated NK cells function via cytokine secretion or direct cytolysis of target cells; dendritic cells (DCs) are thought to make critical contributions in the activation of both of these functions. Yet, the magnitude and physiological relevance of DC-mediated NK cell activation in vivo is not completely understood. To examine the contribution of DC help in regulating NK cell functions after infection with HSV-1, we utilized a transgenic mouse model that allows the transient ablation of DCs. Using this approach, it was found that the gamma interferon (IFN-gamma) expression potential of NK cells is quantitatively and qualitatively impaired in the absence of DCs. With regard to priming of NK cytolytic functions, the ablation of DCs did not significantly affect cytotoxic protein expression by NK cells. An in vivo cytolytic assay did, however, reveal impairments in the magnitude of NK cell cytotoxicity. Overall, this study provides direct evidence that functional DCs are required for optimal IFN-gamma expression and cytolytic function by NK cells following infection with HSV-1.

Requirement of TLR2-mediated signaling for the induction of IL-15 gene expression in human monocytic cells by HSV-1

Exposure of human monocytic cells to herpes simplex virus type 1 (HSV-1) results in immediate up-regulation of interleukin (IL)-15 gene expression. However, the receptor involved in this induction is not known. Here, we provide evidence that this induction depends on TLR2-mediated signaling pathway. Through the use of small interfering RNAs (siRNAs), we demonstrate that HSV-1-induced up-regulation of IL-15 gene expression in monocytic THP1 cells requires the presence of the adaptors MyD88, IRAK1, and TRAF6. Interestingly, TIRAP/Mal, an adaptor molecule specifically recruited to TLR2 and TLR4, was also required for maximal up-regulation of IL-15. This response was completely abrogated by anti-TLR2, but not anti-TLR4, blocking mAbs in both primary monocytes and THP1 cells. Furthermore, THP1 cells rendered defective in TLR2 expression by disrupting the expression of Sp1, a major transcription factor involved in TLR2 promoter activity, were unable to up-regulate IL-15 gene expression in response to HSV-1. In addition, HSV-1-induced NF-kappaB activation was significantly reduced after neutralization of TLR2 and the adaptor proteins. Altogether, these results unequivocally show that HSV-1 induces TLR2-dependent activation of IL-15 gene expression, which requires the recruitment of both MyD88 and TIRAP/Mal and the activation of IRAK1 and TRAF6 leading to NF-kappaB translocation to the nucleus.

Disruption of the gamma c cytokine network in T cells during HIV infection

The common gamma chain (gammac)-sharing cytokines (IL's-2, 4, 7, 9, 15, and 21) play a vital role in the survival, proliferation, differentiation and function of T lymphocytes. As such, disruption of their signaling pathways would be expected to have severe consequences on the integrity of the immune system. Indeed, it appears that the signaling network of these cytokines is both disrupted and exploited by HIV at various stages of infection. IL-2 secretion and signaling downstream of its receptor are impaired in T cells from chronically-infected HIV+ patients. Elevated plasma IL-7 levels and decreased IL-7Ralpha expression in patient T cells results in significantly decreased responsiveness to this critical cytokine. Interestingly, IL-2 and IL-15 are also able to render CD4+ T cells permissive to HIV infection through their influence on the activity of the APOBEC3G deaminase enzyme. Herein, we describe the current state of knowledge on how the gammac cytokine network is affected during HIV infection, with a focus on how this impairs CD4+ and CD8+ T cell function while also benefiting the virus itself. We also address the use of cytokines as adjuncts to highly active antiretroviral therapy to bolster immune reconstitution in infected patients.

Subcellular expression pattern and role of IL-15 in pneumococci induced lung epithelial apoptosis

Streptococcus pneumoniae is the leading causative agent of community-acquired pneumonia. Induction of apoptosis in pulmonary epithelial cells by bacteria during pneumonia might be harmful to the host. Interleukin-15 (IL-15) has been demonstrated as an effective inhibitor of apoptosis and is expressed in lung epithelium on the mRNA and protein level. Therefore, we characterized the sub-cellular expression pattern of the short and long IL-15 isoforms in lung epithelial cells in vitro as well as its role in pneumococci-related lung epithelial cell apoptosis. We found an expression pattern for both IL-15 signal peptides in the pulmonary epithelial cell lines A549 and Beas-2B. Moreover, a strong co-localization of IL-15 and IL-15Ralpha was detected on cell surfaces. Compared to pro-inflammatory cytokine stimulation, neither IL-15 nor its trimeric receptor complex was up-regulated after pneumococcal infection. However, overexpression of IL-15 isoforms revealed IL-15LSP and IL-15Vkl as inhibitors of pneumococci induced apoptosis in pulmonary epithelial cells. Thus, IL-15 may act as an anti-apoptotic molecule in pneumococci infection, thereby suggesting IL-15 as a benefical cytokine in pulmonary host defense against infection.

Production and utilization of interleukin-15 in malignant catarrhal fever

Malignant catarrhal fever (MCF) is an often fatal lymphoproliferative disease of ungulates caused by either alcelaphine herpesvirus-1 (AlHV-1) or ovine herpesvirus-2 (OvHV-2). The pathogenesis of MCF is poorly understood, but appears to involve an auto-destructive pathology whereby cytotoxic lymphocytes destroy areas of a variety of tissues. The cytokine interleukin-15 (IL-15) is involved in the development and maintenance of cytotoxic lymphocytes and may therefore have a role in the pathogenesis of MCF. Virus-infected large granular lymphocytes (LGLs) were obtained from the tissues of rabbits infected with AlHV-1 or OvHV-2. These cells exhibited a similar proliferative response to IL-15 and to IL-2 in culture, but their content of the activated cytotoxic enzyme (BLT-esterase) was maintained at higher levels in the presence of IL-15 compared with IL-2. The LGLs did not express IL-15 mRNA or produce IL-15 protein. By contrast, there was abundant expression of IL-15 mRNA and protein in affected tissues. IL-15 production was associated with necrotic lesions of the mesenteric lymph node and appendix of OvHV-2-infected rabbits, but was not found in the same tissues of rabbits infected with AlHV-1 in which there were no necrotic lesions. The cellular source of the IL-15 was predominantly lymphoid cells that did not express B cell or monocyte-macrophage markers. Only a few IL-15+ cells (<10%) co-localized with pan-T cells or CD8+ T cells. The abundance of IL-15 in tissue with lesions of MCF suggests that this cytokine may have a role in the pathogenesis of MCF.

An intense form of homeostatic proliferation of naive CD8+ cells driven by IL-2

In conditions of T lymphopenia, interleukin (IL) 7 levels rise and, via T cell receptor for antigen–self–major histocompatibility complex (MHC) interaction, induce residual naive T cells to proliferate. This pattern of lymphopenia-induced “homeostatic” proliferation is typically quite slow and causes a gradual increase in total T cell numbers and differentiation into cells with features of memory cells. In contrast, we describe a novel form of homeostatic proliferation that occurs when naive T cells encounter raised levels of IL-2 and IL-15 in vivo. In this situation, CD8⁺ T cells undergo massive expansion and rapid differentiation into effector cells, thus closely resembling the T cell response to foreign antigens. However, the responses induced by IL-2/IL-15 are not seen in MHC-deficient hosts, implying that the responses are driven by self-ligands. Hence, homeostatic proliferation of naive T cells can be either slow or fast, with the quality of the response to self being dictated by the particular cytokine (IL-7 vs. IL-2/IL-15) concerned. The relevance of the data to the gradual transition of naive T cells into memory-phenotype (MP) cells with age is discussed.

Cell-specific interleukin-15 and interleukin-15 receptor subunit expression and regulation in pneumococcal pneumonia--comparison to chlamydial lung infection

Interleukin (IL)-15 has critical impact on the homeostasis and activation of natural killer cells, natural killer T cells, gammadeltaT cells, and CD8(+)T cells, and contributes to antimicrobial defenses particularly at mucosal sites. The respiratory tract comprises a large mucosal surface and harbors significant amounts of lymphocytes, however the expression pattern of IL-15 in the lung and its role in local immune responses are largely unknown. We therefore analyzed the differential expression of IL-15 and the IL-15 receptor (IL-15R) complex in the lungs of mice and demonstrated substantial constitutive expression in bronchial and alveolar epithelial cells, alveolar macrophages, and vascular smooth muscle cells, implicating contribution to pulmonary immune cell homeostasis already under normal conditions. The induction of pneumococcal pneumonia but not the infection with Chlamydophila pneumoniae evoked a significant up-regulation of IL-15 on alveolar macrophages and bronchial epithelial cells, with the latter presenting de-novo expression of IL-15 on their basolateral surface and additional up-regulation of IL-15Ralpha. Moreover, transcriptome analysis as well as semi-quantitative PCR indicated at least partial transcriptional regulation in mice lungs. In conclusion IL-15 is suggested being of functional importance in the pulmonary immune response against pneumococcal pneumonia.

Combined IL-15/IL-15Ralpha immunotherapy maximizes IL-15 activity in vivo

IL-15 has substantial potential as an immunotherapeutic agent for augmenting immune responses. However, the activity of IL-15 is mediated by a unique mechanism in which the cytokine is transpresented by cell-bound high-affinity IL-15Ralpha to target cells expressing the IL-15Rbeta and the common gamma-chain. Thus, the efficacy of administered IL-15 alone may be limited by the availability of free IL-15Ralpha. We now show that administration of soluble IL-15/IL-15Ralpha complexes greatly enhanced IL-15 half-life and bioavailability in vivo. Treatment of mice with this complex, but not with IL-15 alone, resulted in robust proliferation of memory CD8 T cells, NK cells, and NK T cells. The activity of the complex required IL-15Rbeta, but not IL-15Ralpha, expression by the responding cells and was IL-7-independent. Interestingly, IL-15/IL-15Ralpha immunotherapy also caused naive CD8 T cell activation and development into effector cells and long-term memory T cells. Lastly, complexed IL-15, as compared with IL-15 alone, dramatically reduced tumor burden in a model of B16 melanoma. These findings hold significant importance for the use of IL-15 as a potential adjuvant/therapeutic and inducer of homeostatic proliferation, without the necessity for prior immunodepletion.

Interleukin-15 mediates protection against experimental tuberculosis: a role for NKG2D-dependent effector mechanisms of CD8+ T cells

CD8+ T cells are involved in protection against Mycobacterium tuberculosis infection and represent a promising target for new vaccine strategies. Because IL-15 is important for the homeostasis of CD8+ T cells, we studied the immune response in IL-15-deficient mice during tuberculosis. In the absence of IL-15, CD8+ T cells failed to efficiently accumulate in draining lymph nodes and at the site of infection. The expression of antigen-specific effector functions, such as the production of interferon-gamma and cytotoxicity, were impaired in CD8+ T cells, but not CD4+ T cells, from IL-15-deficient mice. This defect was associated with an increased mortality of IL-15-deficient mice during the chronic phase of infection. The lectin-like stimulatory receptor natural killer group 2D (NKG2D) was up-regulated on CD8+ T cells only from wild-type mice, but not from IL-15-deficient mice. Mechanistically, blocking NKG2D function with an mAb inhibited M. tuberculosis-directed CD8+ T cell responses in vitro. We conclude that in addition to regulating the expansion of CD8+ T cells, IL-15 is also necessary for inducing effector mechanisms in CD8+ T cells that depend on NKG2D expression. Hence, our results implicate IL-15 and NKG2D as promising targets for modulating CD8+ T cell-mediated protection against tuberculosis.

Herpesviruses and the innate immune response

Herpesvirus infection leads to the rapid induction of an innate immune response. A central aspect of this host response is the production and secretion of type I interferon. The current model of virus-mediated interferon production includes three stages: sensitization, induction, and amplification. A key mediator of all three stages is the cellular transcription factor interferon regulatory factor 3 (IRF3). Although the precise details of IRF3 activation and interferon production in response to herpesvirus infection are still being elucidated, viral proteins that block components of the interferon pathway, particularly IRF3, have been identified and characterized. In vivo studies have shown that in addition to type I interferon, interleukin-15 (IL-15) and natural killer (NK) cells also play an important role in mediating resistance to herpesvirus infection. Recent investigations have demonstrated a strong association between IRF3, interferon, IL-15, and NK cells. This review will focus on herpesvirus-mediated induction of innate immunity, the central role of the type I interferon response and mechanisms used by herpesviruses to block host antiviral immunity.

NK and NKT cell-independent contribution of interleukin-15 to innate protection against mucosal viral infection

Interleukin-15 (IL-15) is essential for the development, maturation, and function of NK and NKT cells, which are critical components of the innate immune defense against viral infections. We recently showed that mice lacking IL-15 and/or NK/NKT cells are significantly more susceptible to intravaginal (IVAG) herpes simplex virus type 2 (HSV-2) infection than control mice. For this study, we examined whether IL-15 has any direct antiviral activity, independent of NK/NKT cells, in innate protection against HSV-2 infection. A sensitive enzyme-linked immunosorbent assay for murine IL-15 was developed and used to show that IVAG HSV-2 infection induces IL-15 in vaginal washes. Using immunohistochemistry, we detected IL-15-positive cells in the submucosa and vaginal epithelium following IVAG HSV-2 infection. Local, but not systemic, delivery of murine recombinant IL-15 (mrIL-15) to the genital mucosae of IL-15(-/-) and RAG-2(-/-) gamma(c)(-/-) mice, which both lack NK and NKT cells, resulted in significant reductions in HSV-2 titers in genital washes and 60% survival following IVAG HSV-2 challenge. Furthermore, we showed that IL-15 is important for CpG oligodeoxynucleotide (ODN)-induced innate protection against genital HSV-2 infection. While 100% of CpG ODN-treated RAG2(-/-) gamma(c)(-/-) mice, which are capable of producing IL-15 but lack NK/NKT cells, survived an IVAG HSV-2 challenge, only 60% of CpG ODN-treated IL-15(-/-) mice survived, and all of these mice had similar vaginal viral titers to those in control mice by day 3 postchallenge. Lastly, a treatment of RAW264.7 cells with mrIL-15 induced the production of tumor necrosis factor alpha and beta interferon (IFN-beta), but not IFN-alpha, and significantly protected them against HSV-2 infection in vitro. The results of these studies indicate that IL-15 can act independently of NK/NKT cells in mediating the innate defense against viral infection.

Induction and regulation of IFNs during viral infections

Interferons (IFN)s are involved in numerous immune interactions during viral infections and contribute to both induction and regulation of innate and adaptive antiviral mechanisms. IFNs play a pivotal rule in the outcome of a viral infection, as demonstrated by the impaired resistance against different viruses in mice deficient for the receptors IFNAR-2 and IFNGR. During viral infections, IFNs are involved in numerous immune interactions as inducers, regulators, and effectors of both innate and adaptive antiviral mechanisms. IFN-alpha/beta is produced rapidly when viral factors, such as envelope glycoproteins, CpG DNA, or dsRNA, interact with cellular pattern-recognition receptors (PRRs), such as mannose receptors, toll-like receptors (TLRs), and cytosolic receptors. These host-virus interactions signal downstream to activate transcription factors needed to achieve expression from IFN-alpha/beta genes. These include IFN regulatory factor-3 (IRF-3), IRF-5, IRF-7, c-Jun/ATF-2, and NF-kappaB. In contrast, IFN-gamma is induced by receptor-mediated stimulation or in response to early produced cytokines, including interleukin-2 (IL-12), IL-18, and IFN-alpha/beta, or by stimulation through T cell receptors (TCRs) or natural killer (NK) cell receptors. IFNs signal through transmembrane receptors, activating mainly Jak-Stat pathways but also other signal transduction pathways. Cytokine and TCR-induced IFN-gamma expression uses distinct signal transduction pathways involving such transcription factors as NFAT, Stats and NF-kappaB. This results in induction and activation of numerous intrinsic antiviral factors, such as RNA-activated protein kinase (PKR), the 2-5A system, Mx proteins, and several apoptotic pathways. In addition, IFNs modulate distinct aspects of both innate and adaptive immunity. Thus, IFN-alpha/beta and IFN-gamma affect activities of macrophages, NK cells, dendritic cells (DC), and T cells by enhancing antigen presentation, cell trafficking, and cell differentiation and expression profiles, ultimately resulting in enhanced antiviral effector functions. This review focuses on the latest findings regarding induction and regulation of IFNs, primarily during the early phase of an antiviral immune response. Both cellular and molecular aspects are discussed from the perspective of host-virus interactions.

IL-15 induces IFN-beta and iNOS gene expression, and antiviral activity of murine macrophage RAW 264.7 cells

The effects of interleukine-15 (IL-15) on macrophage activation and antiviral activity have been investigated in this study. We have provided evidence that IL-15 stimulates murine macrophage RAW 264.7 cells to release nitric oxide (NO) and inhibit vaccinia virus (VV) replication in bystander human 293 cells in a dose-dependent manner. The IL-15-induced antiviral activity was partially mediated by NO, as blocking NO production by NO synthase (iNOS) inhibitor NG-monomethyl-L-arginine acetate (L-NMA) partially restored the virus replication. Interferon-gamma (IFN-gamma) was not detectable by ELISA in the cell supernatant of IL-15-activated macrophages or in the co-cultures of macrophages and infected bystander cells. Neutralizing anti-IFN-gamma, anti-IFN-gamma receptor R2, anti-TNF-alpha, or anti-IL-12 antibodies had no effect on NO production or antiviral activity. In contrast, neutralizing anti-IFN-alpha/beta antibody completely restored the VV replication and reduced the NO level to one third of that in the control. Elevated mRNA levels of IFN-beta and iNOS genes were detected in IL-15-activated RAW 264.7 cells by RT-PCR. Our data suggest that IL-15 is capable of inducing IFN-beta, which could participate in NO-mediated antiviral effect.

Interleukin-15 enhances immune reconstitution after allogeneic bone marrow transplantation

Interleukin-15 (IL-15) is a gamma-common cytokine that plays an important role in the development, survival, and proliferation of natural killer (NK), NK T, and CD8+ T-cells. We administered IL-15 to recipients of an allogeneic bone marrow transplantation (allo BMT) to determine its effects on immune reconstitution. Posttransplantation IL-15 administration significantly increased donor-derived CD8+ T (mostly CD122(+)CD44(+)CD8+ T-cells), NK, and NK T-cells at day +28 in young and old recipients of allo BMT. This was associated with enhanced T-cell and NK-cell function. IL-15 stimulated homeostatic proliferation of donor CD8+ T-cells in recipients of carboxyfluorescein diacetate succinimidyl ester-labeled donor T-cell infusions. Posttransplantation IL-15 administration also resulted in a decrease in apoptotic CD8+ T-cells, an increase in Bcl-2-expressing CD8+ T-cells, and an increase in the fraction of Ki67+ proliferative NK and CD8+ T-cells in recipients of allo BMT. IL-15 did not exacerbate graft-versus-host disease (GVHD) in recipients of T-cell-depleted BMT but could aggravate GVHD in some cases in recipients of a T-cell-repleted BMT. Finally, we found that IL-15 administration could enhance graft-versus-leukemia activity. In conclusion, IL-15 can be administered safely to recipients of a T-cell-depleted allo BMT to enhance CD8+ T, NK, and NK T-cell reconstitution.

Bioluminescence imaging reveals systemic dissemination of herpes simplex virus type 1 in the absence of interferon receptors

Herpes simplex virus type 1 (HSV-1) can produce disseminated, systemic infection in neonates and patients with AIDS or other immunocompromising diseases, resulting in significant morbidity and mortality in spite of antiviral therapy. Components of host immunity that normally limit HSV-1 to localized epithelial and neuronal infection remain incompletely defined. We used in vivo bioluminescence imaging to determine effects of type I and II interferons (IFNs) on replication and tropism of HSV-1 infection in mice with genetic deficiency of type I, type II, or both type I and II IFN receptors. Following footpad or ocular infection of mice lacking type I IFN receptors, HSV-1 spread to parenchymal organs, including lung, liver, spleen, and regional lymph nodes, but mice survived. Deletion of type I and II IFN receptors produced quantitatively greatest and most widespread dissemination of virus to visceral organs and the nervous system, and these mice invariably died after ocular or footpad infection. Type II receptor knockout and wild-type mice had comparable viral replication and localization, with no systemic spread of HSV-1 or lethality. Therefore, while isolated deficiency of type II IFN receptors did not affect pathogenesis, loss of these receptors in combination with genetic deletion of type I receptors had a profound effect on susceptibility to HSV-1. These data demonstrate different effects of type I and II IFNs in limiting systemic dissemination of HSV-1 and further validate the use of bioluminescence imaging for studies of viral pathogenesis.

Interleukin-15 and natural killer and NKT cells play a critical role in innate protection against genital herpes simplex virus type 2 infection

Interleukin-15 (IL-15), natural killer (NK) cells, and NK T (NKT) cells, components of the innate immune system, are known to contribute to defense against pathogens, including viruses. Here we report that IL-15(-/-) (NK(-) and NKT(-/+)) mice and RAG-2(-/-)/gamma(c)(-/-) (NK(-) and NKT(-)) mice that lack all lymphoid cells were very susceptible to vaginal infection with a low dose of herpes simplex virus type 2 (HSV-2). IL-15(-/-) and RAG-2(-/-)/gamma(c)(-/-) mice were 100-fold more susceptible and RAG-2(-/-), CD-1(-/-) (NKT(-)), and gamma interferon (IFN-gamma)(-/-) mice were 10-fold more susceptible to vaginal HSV-2 infection than control C57BL/6 mice. NK and/or NKT cells were the early source of IFN-gamma in vaginal secretions following genital HSV-2 infection. This study demonstrates that IL-15 and NK-NKT cells are critical for innate protection against genital HSV-2.

Coordinated and distinct roles for IFN-alpha beta, IL-12, and IL-15 regulation of NK cell responses to viral infection

NK cell cytotoxicity, IFN-gamma expression, proliferation, and accumulation are rapidly induced after murine CMV infections. Under these conditions, the responses were shown to be elicited in overlapping populations. Nevertheless, there were distinct signaling molecule requirements for induction of functions within the subsets. IL-12/STAT4 was critical for NK cell IFN-gamma expression, whereas IFN-alphabeta/STAT1 were required for induction of cytotoxicity. The accumulation/survival of proliferating NK cells was STAT4-independent but required IFN-alphabeta/STAT1 induction of IL-15. Taken together, the results define the coordinated interactions between the cytokines IFN-alphabeta, IL-12, and IL-15 for activation of protective NK cell responses during viral infections, and emphasize these factors' nonredundant functions under in vivo physiological conditions.

Genetic control of innate immune responses against cytomegalovirus: MCMV meets its match

Cytomegalovirus (CMV) is a widespread pathogen that is responsible for severe disease in immunocompromised individuals and probably, associated with vascular disease in the general population. There is increasing evidence that cells of the innate immune system play a key role in controlling this important pathogen. This is particularly evident in the experimental murine CMV (MCMV) model of infection which has revealed an important role for natural killer (NK) cells in controlling early viral replication after infection with MCMV. In this model, different strains of inbred mice exhibit striking differences in their level of susceptibility to MCMV infection. Genetic studies, performed almost 10 years ago, revealed that this pattern of susceptibility/resistance can be attributed to a single genetic locus termed Cmv1 and recently several groups that have been working on the mapping and identification of Cmv1 have met with success. Interestingly, Cmv1 is allelic to a member of the Ly49 gene family, which encode activating or inhibitory transmembrane receptors present on the surface of NK cells. All Ly49 receptors characterized to date interact with MHC class I molecules on potential target cells, resulting in the accumulation of signals to the NK to either 'kill' or 'ignore' the cell based upon the repertoire of MHC class I molecules expressed. The identification of Cmv1 as Ly49H, a stimulatory member of the Ly49 family, adds an interesting twist to the Ly49 story. Although the ligand of Ly49H is not yet known, there is already compelling evidence that the ligand is upregulated on virally infected cells, resulting in specific activation of Ly49H-expressing NK cells. This review provides an historical perspective of the MCMV infection model from its inception to the discovery of the gene responsible for the phenotype and provides a basis for further experiments aimed at understanding the role of NK cells, in general, and Ly49H, in particular, in mediating resistance to cytomegalovirus.

Interleukin-2 and interleukin-15: immunotherapy for cancer

Interleukin (IL)-2 and IL-15 are two cytokine growth factors that regulate lymphocyte function and homeostasis. Early clinical interest in the use of IL-2 in the immunotherapy of renal cell carcinoma and malignant melanoma demonstrated the first efficacy for cytokine monotherapy in the treatment of neoplastic disease. Advances in our understanding of the cellular and molecular biology of IL-2 and its receptor complex have provided rationale to better utilize IL-2 to expand and activate immune effectors in patients with cancer. Exciting new developments in monoclonal antibodies recognizing tumor targets and tumor vaccines have provided new avenues to combine with IL-2 therapy in cancer patients. IL-15, initially thought to mediate similar biological effects as IL-2, has been shown to have unique properties in basic and pre-clinical studies that may be of benefit in the immunotherapy of cancer. This review first summarizes the differences between IL-2 and IL-15 and highlights that better understanding of normal physiology creates new ideas for the immunotherapy of cancer. The application of high, intermediate, and low/ultra low dose IL-2 therapy in clinical trials of cancer patients is discussed, along with new avenues for its use in neoplastic diseases. The growing basic and pre-clinical evidence demonstrating that IL-15 may be useful in immunotherapy approaches to cancer is also presented.

Adenoviral vectors stimulate murine natural killer cell responses and demonstrate antitumor activities in the absence of transgene expression

Adenoviral vector-mediated gene delivery is currently the focus of many efforts to administer therapeutic gene products for the treatment of cancer. Although these vectors are replication deficient, they can induce specific immune responses against both vector- and transgene-encoded proteins. We have extended these findings to determine the level of innate natural killer (NK) cell responses to adenoviral vector administration in vivo. Similar to many replicating viruses, the vectors induce prominent NK cell activation in mouse spleens within 2 days of injection. We also observed these NK cell responses regardless of the route of administration. Furthermore, stimulation of NK cells by adenoviral vectors is independent of viral gene transcription, as UV inactivation of the vectors does not reduce the NK cell response. In contrast, heat treatment of the vectors destroys their ability to activate NK cells, demonstrating the necessity for intact vector particles. In addition, we found that administration of "empty" (no transgene) adenoviral vectors delays tumor growth in mice bearing B16 melanomas, and this effect is abrogated by depletion of NK cells. Collectively, these results demonstrate in a murine system that the adenoviral vector gene delivery system itself stimulates NK cells, and this in turn can nonspecifically enhance antitumor immunity.

Fighting infection using immunomodulatory agents

The last decade has seen the emergence of immunomodulators as promising therapeutic agents in infectious diseases. A diverse array of recombinant, synthetic and natural immunomodulatory preparations for prophylaxis and treatment of various infections are available today. Some of these substances, such as granulocyte colony-stimulating factor (G-CSF), interferons, imiquimod and bacterial-derived preparations are already licensed for use in patients. Others including IL-12, various chemokines, synthetic cytosine phosphate-guanosine (CpG) oligodeoxynucleotides and glucans are being investigated extensively in clinical and preclinical studies. Immunomodulatory regimens offer an attractive approach as an adjunct modality for control of microbial diseases in the era of antibiotic resistance. Practical application of the advances in molecular biology, bioinformatics, genomic mining and high-throughput peptide synthesis should foster future discovery and development of novel immunomodulators contingent upon scientific evidence rather than dictates of discursive empiricism.