Interleukin 15 is required for proliferative renewal of virus-specific memory CD8 T cells

Interleukin‑15: a potent adjuvant enhancing the efficacy of an autologous whole‑cell tumor vaccine against Lewis lung carcinoma

Lung cancer is a major cause of cancer‑associated mortality worldwide due to its limited response rate to current chemotherapy and radiation, thus immunotherapy is rapidly becoming the most promising approach. Although the highly specific tumor‑associated antigen of lung cancer has been found, autologous whole‑cell tumor vaccines remain indispensable in the development of therapeutic cancer vaccines. Interleukin (IL)‑15 is a T helper type 1 cytokine that has been demonstrated to have a marked antitumor immune response and the potential ability to reverse the host tolerance of tumor antigens in certain preclinical trials. In the present study, a cationic liposome encapsulating IL‑15 gene‑loaded plasmid acted as an adjuvant of an autologous whole‑cell tumor vaccine by subcutaneous injection. The combination immunotherapy resulted in significant inhibition of tumor growth without side effects in the preventive tumor inhibition and adoptive therapy study. Cytotoxic lymphocyte assay detection of the serum antigen and cytokines using an enzyme‑linked immunosorbent assay suggested that the IL‑15 gene can significantly improve the cellular immune response and humoral immune response provoked by autologous whole‑cell tumor vaccines. These results demonstrated that the IL‑15 gene was an effective adjuvant of autologous whole‑cell tumor vaccines against mouse lung cancer and may provide an attractive vaccine strategy for cancer immunotherapy.

Cutting edge: CXCR4 is critical for CD8+ memory T cell homeostatic self-renewal but not rechallenge self-renewal

Central memory (CM) CD8(+) T cells "remember" prior encounters because they maintain themselves through cell division in the absence of ongoing challenge (homeostatic self-renewal), as well as reproduce the CM fate while manufacturing effector cells during secondary Ag encounters (rechallenge self-renewal). We tested the consequence of conditional deletion of the bone marrow homing receptor CXCR4 on antiviral T cell responses. CXCR4-deficient CD8(+) T cells have impaired memory cell maintenance due to defective homeostatic proliferation. Upon rechallenge, however, CXCR4-deficient T cells can re-expand and renew the CM pool while producing secondary effector cells. The critical bone marrow-derived signals essential for CD8(+) T cell homeostatic self-renewal appear to be dispensable to yield self-renewing, functionally asymmetric cell fates during rechallenge.

A population biological approach to understanding the maintenance and loss of the T-cell repertoire during aging

The adaptive immune system requires a diverse T-cell repertoire to be able to respond to a wide variety of pathogens. Worryingly, the repertoire diversity declines dramatically in old age. As thymic output generates novel T cells, the conventional view holds that a decrease in this output with age is responsible for the loss in the repertoire. However, many additional factors affect the repertoire such as homeostatic turnover and antigen-dependent expansion in response to infection. Mathematical models taking a population biology perspective are important tools for understanding how the interplay between these factors affects the immune repertoire. These models suggest that thymic decline is not a major factor but rather that some combination of virus-induced proliferation and T-cell-intrinsic genetic or epigenetic changes gives rise to the oligoclonal expansions that cause the decline in T-cell diversity. We also discuss consequences for strategies to rejuvenate the immune repertoire in old age.

Prolonged antigen presentation by immune complex-binding dendritic cells programs the proliferative capacity of memory CD8 T cells

Antibodies can regulate the quality and functionality of a subset of antiviral CD8⁺ T cell memory responses to influenza by promoting sustained DC antigen presentation during the contraction phase of primary responses.

The commitment of naive CD8 T cells to effector or memory cell fates can occur after a single day of antigenic stimulation even though virus-derived antigens (Ags) are still presented by DCs long after acute infection is resolved. However, the effects of extended Ag presentation on CD8 T cells are undefined and the mechanisms that regulate prolonged Ag presentation are unknown. We showed that the sustained presentation of two different epitopes from influenza virus by DCs prevented the premature contraction of the primary virus-specific CD8 T cell response. Although prolonged Ag presentation did not alter the number of memory CD8 T cells that developed, it was essential for programming the capacity of these cells to proliferate, produce cytokines, and protect the host after secondary challenge. Importantly, prolonged Ag presentation by DCs was dependent on virus-specific, isotype-switched antibodies (Abs) that facilitated the capture and cross-presentation of viral Ags by FcγR-expressing DCs. Collectively, our results demonstrate that B cells and Abs can regulate the quality and functionality of a subset of antiviral CD8 T cell memory responses and do so by promoting sustained Ag presentation by DCs during the contraction phase of the primary T cell response.

IL-15: a central regulator of celiac disease immunopathology

Interleukin-15 (IL-15) exerts many biological functions essential for the maintenance and function of multiple cell types. Although its expression is tightly regulated, IL-15 upregulation has been reported in many organ-specific autoimmune disorders. In celiac disease, an intestinal inflammatory disorder driven by gluten exposure, the upregulation of IL-15 expression in the intestinal mucosa has become a hallmark of the disease. Interestingly, because it is overexpressed both in the gut epithelium and in the lamina propria, IL-15 acts on distinct cell types and impacts distinct immune components and pathways to disrupt intestinal immune homeostasis. In this article, we review our current knowledge of the multifaceted roles of IL-15 with regard to the main immunological processes involved in the pathogenesis of celiac disease.

Role of tumor suppressor TSC1 in regulating antigen-specific primary and memory CD8 T cell responses to bacterial infection

The serine/threonine kinase mammalian/mechanistic target of rapamycin (mTOR) integrates various environmental cues such as the presence of antigen, inflammation, and nutrients to regulate T cell growth, metabolism, and function. The tuberous sclerosis 1 (TSC1)/TSC2 complex negatively regulates the activity of an mTOR-containing multiprotein complex called mTOR complex 1. Recent studies have revealed an essential cell-intrinsic role for TSC1 in T cell survival, quiescence, and mitochondrial homeostasis. Given the emerging role of mTOR activity in the regulation of the quantity and quality of CD8 T cell responses, in this study, we examine the role of its suppressor, TSC1, in the regulation of antigen-specific primary and memory CD8 T cell responses to bacterial infection. Using an established model system of transgenic CD8 cell adoptive transfer and challenge with Listeria monocytogenes expressing a cognate antigen, we found that TSC1 deficiency impairs antigen-specific CD8 T cell responses, resulting in weak expansion, exaggerated contraction, and poor memory generation. Poor expansion of TSC1-deficient cells was associated with defects in survival and proliferation in vivo, while enhanced contraction was correlated with an increased ratio of short-lived effectors to memory precursors in the effector cell population. This perturbation of effector-memory differentiation was concomitant with decreased expression of eomesodermin among activated TSC1 knockout cells. Upon competitive adoptive transfer with wild-type counterparts and antigen rechallenge, TSC1-deficient memory cells showed moderate defects in expansion but not cytokine production. Taken together, these findings provide direct evidence of a CD8 T cell-intrinsic role for TSC1 in the regulation of antigen-specific primary and memory responses.

Human monocytes have increased IFN-γ-mediated IL-15 production with age alongside altered IFN-γ receptor signaling

IL-15 is involved in regulating host defense and inflammation. Monocytes produce the biologically active cell surface IL-15 in response to IFN-γ. Although aging can alter the immune system, little is known about whether and how aging affects IFN-γ-mediated IL-15 production in human monocytes. We showed that monocytes of healthy older adults (age ≥ 65) had increased cell surface IL-15 expression in response to IFN-γ compared to those of healthy young adults (age ≤ 40). This finding stems in part from increased IFN-γ receptor (R)1/2 expression on monocytes in older adults, leading to enhanced STAT1 activation and interferon regulatory factor 1 synthesis with increased IL15 gene expression. Our study suggests that with aging the IFN-γ-mediated IL-15 production pathway in human monocytes is uncompromised, but rather augmented, and could be considered as a therapeutic target point to modulate host defense and inflammation in older adults.

Increasing the biological activity of IL-2 and IL-15 through complexing with anti-IL-2 mAbs and IL-15Rα-Fc chimera

IL-2 and IL-15 are structurally relative cytokines that share two receptor subunits, CD132 (γ(c) chain) and CD122 (β chain). However, the expression pattern and physiological role of IL-2 and IL-15 private receptor α chains CD25 and IL-15Rα, respectively, are strikingly different. CD25, together with CD122 and CD132, forms a trimeric high affinity IL-2 receptor that is expressed and functions on cells acquiring an IL-2 signal. Conversely, IL-15Rα is expressed and binds IL-15 with high affinity per se already in the endoplasmic reticulum of the IL-15 producing cells and it presents IL-15 to cells expressing CD122/CD132 dimeric receptor in trans. Thus, while IL-2 is secreted almost exclusively by activated T cells and acts as a free molecule, IL-15 is expressed mostly by myeloid cells and works as a cell surface-associated cytokine. Interestingly, the in vivo biological activity of IL-2 can be dramatically increased through complexing with certain anti-IL-2 mAbs; such IL-2/anti-IL-2 mAbs immunocomplexes selectively stimulate the proliferation of a distinct population of immune cells, depending on the clone of the anti-IL-2 mAb used. IL-2/S4B6 mAb immunocomplexes are highly stimulatory for CD122(high) populations (memory CD8(+) T and NK cells) and intermediately also for CD25(high) populations (Treg and activated T cells), while IL-2/JES6-1 mAb immunocomplexes enormously expand only CD25(high) cells. Although IL-2 immunocomplexes are much more potent than IL-2 in vivo, they show comparable to slightly lower activity in vitro. The in vivo biological activity of IL-15 can be dramatically increased through complexing with recombinant IL-15Rα-Fc chimera; however, IL-15/IL-15Rα-Fc complexes are significantly more potent than IL-15 both in vivo and in vitro. In this review we summarize and discuss the features and biological relevance of IL-2/anti-IL-2 mAbs and IL-15/IL-15Rα-Fc complexes, and try to foreshadow their potential in immunological research and immunotherapy.

Asymptomatic memory CD8+ T cells: from development and regulation to consideration for human vaccines and immunotherapeutics

Generation and maintenance of high quantity and quality memory CD8(+) T cells determine the level of protection from viral, bacterial, and parasitic re-infections, and hence constitutes a primary goal for T cell epitope-based human vaccines and immunotherapeutics. Phenotypically and functionally characterizing memory CD8(+) T cells that provide protection against herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) infections, which cause blinding ocular herpes, genital herpes, and oro-facial herpes, is critical for better vaccine design. We have recently categorized 2 new major sub-populations of memory symptomatic and asymptomatic CD8(+) T cells based on their phenotype, protective vs. pathogenic function, and anatomical locations. In this report we are discussing a new direction in developing T cell-based human herpes vaccines and immunotherapeutics based on the emerging new concept of "symptomatic and asymptomatic memory CD8(+) T cells."

Transcriptional profiling of recall responses to Francisella live vaccine strain

Global gene expression profile changes were monitored in human peripheral blood mononuclear cells (PBMCs) after challenge with the live vaccine strain (LVS) of Francisella tularensis. Because these PBMCs were from individuals previously immunized with LVS, stimulating these cells with LVS should activate memory responses. The Ingenuity Pathway Analysis tool identified pathways, functions, and networks associated with this in vitro recall response, including novel pathways triggered by the memory response. Dendritic cell (DC) maturation was the most significant among the more than 25 relevant pathways discovered. Interleukin 15, granulocyte-macrophage colony-stimulating factor, and triggering receptor expressed on myeloid cells 1 signaling pathways were also significant. Pathway analysis indicated that Class 1 antigen presentation may not be optimal with LVS vaccination. The top three biological functions were antigen presentation, cell-mediated and humoral immune responses. Network analysis revealed that the top network associated with these functions had IFNγ and TNFα in central interactive positions. Our results suggest that DC maturation is a key factor in the recall responses and that more effective antigen processing and presentation is needed for cytotoxic T lymphocyte responses. Taken together, these considerations are critical for future tularemia vaccine development studies.

Antigen-specific T-cell-mediated immunity after HIV-1 infection: implications for vaccine control of HIV development

The definition of immune correlates of protection in HIV-1 infection is pivotal to the design of successful vaccine candidates and strategies. Although significant methodological and conceptual strides have been made in our understanding of HIV-specific cellular immunity, we have not yet defined those parameters that have a role in controlling the spread of HIV infection. This review discusses the basis of our understanding of HIV-specific cellular immunity and identifies its shortcomings. Furthermore, potential protective characteristics will be proposed that may ultimately be required for an effective vaccine designed to stimulate cellular immunity against HIV-1.

Hoxb4 overexpression in CD4 memory phenotype T cells increases the central memory population upon homeostatic proliferation

Memory T cell populations allow a rapid immune response to pathogens that have been previously encountered and thus form the basis of success in vaccinations. However, the molecular pathways underlying the development and maintenance of these cells are only starting to be unveiled. Memory T cells have the capacity to self renew as do hematopoietic stem cells, and overlapping gene expression profiles suggested that these cells might use the same self-renewal pathways. The transcription factor Hoxb4 has been shown to promote self-renewal divisions of hematopoietic stem cells resulting in an expansion of these cells. In this study we investigated whether overexpression of Hoxb4 could provide an advantage to CD4 memory phenotype T cells in engrafting the niche of T cell deficient mice following adoptive transfer. Competitive transplantation experiments demonstrated that CD4 memory phenotype T cells derived from mice transgenic for Hoxb4 contributed overall less to the repopulation of the lymphoid organs than wild type CD4 memory phenotype T cells after two months. These proportions were relatively maintained following serial transplantation in secondary and tertiary mice. Interestingly, a significantly higher percentage of the Hoxb4 CD4 memory phenotype T cell population expressed the CD62L and Ly6C surface markers, characteristic for central memory T cells, after homeostatic proliferation. Thus Hoxb4 favours the maintenance and increase of the CD4 central memory phenotype T cell population. These cells are more stem cell like and might eventually lead to an advantage of Hoxb4 T cells after subjecting the cells to additional rounds of proliferation.

Gamma interferon regulates contraction of the influenza virus-specific CD8 T cell response and limits the size of the memory population

The factors that regulate the contraction of the CD8 T cell response and the magnitude of the memory cell population against localized mucosal infections such as influenza are important for generation of efficient vaccines but are currently undefined. In this study, we used a mouse model of influenza to demonstrate that the absence of gamma interferon (IFN-γ) or IFN-γ receptor 1 (IFN-γR1) leads to aberrant contraction of antigen-specific CD8 T cell responses. The increased accumulation of the effector CD8 T cell population was independent of viral load. Reduced contraction was associated with an increased fraction of CD8 T cells expressing the interleukin-7 receptor (IL-7R) at the peak of the response, resulting in enhanced numbers of memory/memory precursor cells in IFN-γ(-/-) and IFN-γR(-/-) compared to wild-type (WT) mice. Blockade of IL-7 within the lungs of IFN-γ(-/-) mice restored the contraction of influenza virus-specific CD8 T cells, indicating that IL-7R is important for survival and is not simply a consequence of the lack of IFN-γ signaling. Finally, enhanced CD8 T cell recall responses and accelerated viral clearance were observed in the IFN-γ(-/-) and IFN-γR(-/-) mice after rechallenge with a heterologous strain of influenza virus, confirming that higher frequencies of memory precursors are formed in the absence of IFN-γ signaling. In summary, we have identified IFN-γ as an important regulator of localized viral immunity that promotes the contraction of antigen-specific CD8 T cells and inhibits memory precursor formation, thereby limiting the size of the memory cell population after an influenza virus infection.

Signal transducer and activator of transcription 3 (STAT3) mutations underlying autosomal dominant hyper-IgE syndrome impair human CD8(+) T-cell memory formation and function

BACKGROUND

The capacity of CD8(+) T cells to control infections and mediate antitumor immunity requires the development and survival of effector and memory cells. IL-21 has emerged as a potent inducer of CD8(+) T-cell effector function and memory development in mouse models of infectious disease. However, the role of IL-21 and associated signaling pathways in protective CD8(+) T-cell immunity in human subjects is unknown.

OBJECTIVE

We sought to determine which signaling pathways mediate the effects of IL-21 on human CD8(+) T cells and whether defects in these pathways contribute to disease pathogenesis in patients with primary immunodeficiencies caused by mutations in components of the IL-21 signaling cascade.

METHODS

Human primary immunodeficiencies resulting from monogenic mutations provide a unique opportunity to assess the requirement for particular molecules in regulating human lymphocyte function. Lymphocytes from patients with loss-of-function mutations in signal transducer and activator of transcription 1 (STAT1), STAT3, or IL-21 receptor (IL21R) were used to assess the respective roles of these genes in human CD8(+) T-cell differentiation in vivo and in vitro.

RESULTS

Mutations in STAT3 and IL21R, but not STAT1, led to a decrease in multiple memory CD8(+) T-cell subsets in vivo, indicating that STAT3 signaling, possibly downstream of IL-21R, regulates the memory cell pool. Furthermore, STAT3 was important for inducing the lytic machinery in IL-21-stimulated naive CD8(+) T cells. However, this defect was overcome by T-cell receptor engagement.

CONCLUSION

The IL-21R/STAT3 pathway is required for many aspects of human CD8(+) T-cell behavior but in some cases can be compensated by other signals. This helps explain the relatively mild susceptibility to viral disease observed in STAT3- and IL-21R-deficient subjects.

Aggregatibacter actinomycetemcomitans lipopolysaccharide stimulated epithelial cells produce interleukin-15 that regulates T cell activation

OBJECTIVE

Oral epithelial cells act not only as mechanical barriers but also as immunological barriers by producing various mediators such as cytokines. Since, in periodontal disease, limited information is available regarding the role of oral epithelial cell-derived cytokines on T cell activation, we investigated the responses of human T cells (Jurkat cell) to cytokines in KB cells (an oral epithelial cell line) that had been stimulated with Aggregatibacter actinomycetemcomitans lipopolysaccharide (LPS).

DESIGN

To evaluate T cell activation in response to the culture supernatant of KB cells, we examined cell proliferation and interferon gamma (IFN-γ) production, which is closely related to periodontal disease, in Jurkat cells. Culture supernatant of LPS-stimulated KB cells enhanced cell proliferation and IFN-γ production in Jurkat cells. To determine the active component within the culture supernatant, the production of epithelial cell-derived cytokines, interleukin-12 (IL-12), IL-15 and IL-18, in LPS-stimulated KB cells was analysed.

RESULTS

IL-15, but not IL-18, was significantly increased in the culture supernatant of LPS-stimulated KB cells. Moreover, additional anti-IL-15 neutralizing antibody abolished culture supernatant-induced IFN-γ expression in Jurkat cells.

CONCLUSION

These results suggest that periodontal pathogens induce the production of IL-15 from epithelial cells, and leading the activation of T cells in periodontal lesions.

Safety and comparative immunogenicity of an HIV-1 DNA vaccine in combination with plasmid interleukin 12 and impact of intramuscular electroporation for delivery

BACKGROUND

DNA vaccines have been very poorly immunogenic in humans but have been an effective priming modality in prime-boost regimens. Methods to increase the immunogenicity of DNA vaccines are needed.

METHODS

HIV Vaccine Trials Network (HVTN) studies 070 and 080 were multicenter, randomized, clinical trials. The human immunodeficiency virus type 1 (HIV-1) PENNVAX®-B DNA vaccine (PV) is a mixture of 3 expression plasmids encoding HIV-1 Clade B Env, Gag, and Pol. The interleukin 12 (IL-12) DNA plasmid expresses human IL-12 proteins p35 and p40. Study subjects were healthy HIV-1-uninfected adults 18-50 years old. Four intramuscular vaccinations were given in HVTN 070, and 3 intramuscular vaccinations were followed by electroporation in HVTN 080. Cellular immune responses were measured by intracellular cytokine staining after stimulation with HIV-1 peptide pools.

RESULTS

Vaccination was safe and well tolerated. Administration of PV plus IL-12 with electroporation had a significant dose-sparing effect and provided immunogenicity superior to that observed in the trial without electroporation, despite fewer vaccinations. A total of 71.4% of individuals vaccinated with PV plus IL-12 plasmid with electroporation developed either a CD4(+) or CD8(+) T-cell response after the second vaccination, and 88.9% developed a CD4(+) or CD8(+) T-cell response after the third vaccination.

CONCLUSIONS

Use of electroporation after PV administration provided superior immunogenicity than delivery without electroporation. This study illustrates the power of combined DNA approaches to generate impressive immune responses in humans.

PD-1 regulates T cell proliferation in a tissue and subset-specific manner during normal mouse pregnancy

The regulation of T cell homeostasis during pregnancy has important implications for maternal tolerance and immunity. Evidence suggests that Programmed Death-1 (PD-1) participates in regulation of T cell homeostasis and peripheral tolerance. To examine the contribution of PD-1 signaling on T cell homeostasis during normal mouse pregnancy, we examined T cell number or proportion, PD-1 expression, proliferation, and apoptosis by flow cytometry, BrdU incorporation, and TUNEL assay in pregnant mice given anti-PD-1 blocking antibody or control on days 10, 12, and 14 of gestation. We observed tissue, treatment, and T cell-specific differences in PD-1 expression. Both pregnancy and PD-1 blockade increased T cell proliferation in the spleen, yet this effect was limited to CD4 T cells in the uterine- draining nodes. In the uterus, PD-1 blockade markedly altered the composition of the T cell pool. These studies support the idea that pregnancy is a state of dynamic T cell homeostasis and suggest that this state is partially supported by PD-1 signaling.

Promoting immunity during chronic infection--the therapeutic potential of common gamma-chain cytokines

The continued global burden wrought by chronic infectious disease is unrelenting. Current therapies have curbed the severity of disease for patients, but Human Immunodeficiency Virus (HIV) and Hepatitis B (HBV) infection remain incurable and Mycobacterium tuberculosis (MTB) is rapidly becoming resistant to our existing antibiotics. Much attention has been given to enhancing T cell immunity through the use of certain common gamma-chain cytokines, which have proven to be essential and necessary for T cell survival and function. This article reviews the pre-clinical and clinical literature surrounding IL-2, IL-7, IL-15 and IL-21 and we comment on the potential therapeutic promise of these cytokines as adjuvant treatments for chronic infectious diseases.

The survival of memory CD8 T cells that is mediated by IL-15 correlates with sustained protection against malaria

Ag-specific memory T cell responses elicited by infections or vaccinations are inextricably linked to long-lasting protective immunity. Studies of protective immunity among residents of malaria endemic areas indicate that memory responses to Plasmodium Ags are not adequately developed or maintained, as people who survive episodes of childhood malaria are still vulnerable to either persistent or intermittent malaria infections. In contrast, multiple exposures to radiation-attenuated Plasmodium berghei sporozoites (Pb γ-spz) induce long-lasting protective immunity to experimental sporozoite challenge. We previously demonstrated that sterile protection induced by Pb γ-spz is MHC class I-dependent and CD8 T cells are the key effectors. IFN-γ(+) CD8 T cells that arise in Pb γ-spz-immunized B6 mice are found predominantly in the liver and are sensitive to levels of liver-stage Ag depot and they express CD44(hi)CD62L(lo) markers indicative of effector/effector memory phenotype. The developmentally related central memory CD8 T (TCM) cells express elevated levels of CD122 (IL-15Rβ), which suggests that CD8 TCM cells depend on IL-15 for maintenance. Using IL-15-deficient mice, we demonstrate in this study that although protective immunity is inducible in these mice, protection is short-lived, mainly owing to the inability of CD8 TCM cells to survive in the IL-15-deficient milieu. We present a hypothesis consistent with a model whereby intrahepatic CD8 TCM cells, being maintained by IL-15-mediated survival and basal proliferation, are conscripted into the CD8 effector/effector memory T cell pool during subsequent infections.

Transcriptomic analysis identifies candidate genes and functional networks controlling the response of porcine peripheral blood mononuclear cells to mitogenic stimulation

It is difficult to obtain phenotypic data on disease susceptibility directly from swine in an industry setting. The magnitude of the proliferative response of peripheral blood mononuclear cells (PBMC) to the T cell mitogen concanavalin A (Con A) has long been used as an indirect measure of the responsiveness of the immune system to antigenic stimulation. This trait is known to exhibit moderate heritability in swine, but little is known about the identity of the genes that control the response. In this study, we carried out a time-course microarray experiment to measure gene expression at 3 different stages (3, 20, and 68 h) poststimulation of PBMC with Con A. A total of 46, 452, and 418 differentially expressed (DifEx) genes were identified at each time point, respectively. Expression changes for a subset of these genes were subsequently confirmed by real-time PCR. Functional annotation analyses of the microarray results successfully identified sets of genes involved in processes associated with multiple aspects of cell division, such as DNA and protein synthesis, and control of mitosis. However, the discovery of genes that controlled the response of PBMC to mitogen was limited with this approach, because the drastic changes in the transcriptional program necessitated by cells undergoing division masked changes in smaller immune response gene sets. Pathway and network analyses that focused on immune cells proved to be a more effective strategy for the identification of genes that coordinate aspects of the mitogenic response that are specific to PBMC. The cytokine gene IL15 was shown to be central to the highest scoring network at 20 h and affect the expression of 16 other DifEx genes, including some genes known to regulate T cell activation, such as IL7R, JUN, TNFRSF9, and ZAP70. The IL15 gene maps to a previously identified QTL interval for immune responsiveness to Con A on SSC 8, which also contains the related IL2 gene. At 68 h, a distinct downregulation of major histocompatibility complex class II antigen presentation genes was observed. Overall, the gene expression profile of the Con A-stimulated porcine PBMC points to a Th(1) bias in immune activation. Further work is required to determine whether polymorphisms linked to genes identified in this study affect this immune response trait in pig populations and whether the trait itself correlates with decreased susceptibility to intracellular pathogens in swine.

Cooperativity between CD8+ T cells, non-neutralizing antibodies, and alveolar macrophages is important for heterosubtypic influenza virus immunity

Seasonal epidemics of influenza virus result in ∼36,000 deaths annually in the United States. Current vaccines against influenza virus elicit an antibody response specific for the envelope glycoproteins. However, high mutation rates result in the emergence of new viral serotypes, which elude neutralization by preexisting antibodies. T lymphocytes have been reported to be capable of mediating heterosubtypic protection through recognition of internal, more conserved, influenza virus proteins. Here, we demonstrate using a recombinant influenza virus expressing the LCMV GP33-41 epitope that influenza virus-specific CD8+ T cells and virus-specific non-neutralizing antibodies each are relatively ineffective at conferring heterosubtypic protective immunity alone. However, when combined virus-specific CD8 T cells and non-neutralizing antibodies cooperatively elicit robust protective immunity. This synergistic improvement in protective immunity is dependent, at least in part, on alveolar macrophages and/or other lung phagocytes. Overall, our studies suggest that an influenza vaccine capable of eliciting both CD8+ T cells and antibodies specific for highly conserved influenza proteins may be able to provide heterosubtypic protection in humans, and act as the basis for a potential “universal” vaccine.

Influenza virus continues to pose a significant risk to global health and is responsible for thousands of deaths each year in the United States. This threat is largely due to the ability of the influenza virus to undergo rapid changes, allowing it to escape from immune responses elicited by previous infections or vaccinations. Certain internal determinants of the influenza virus are largely conserved across different viral strains and represent attractive targets for potential “universal” influenza vaccines. Here, we demonstrated that cross-subtype protection against the influenza virus could be obtained through simultaneous priming of multiple arms of the immune response against conserved elements of the influenza virus. These results suggest a novel strategy that could potentially form a primary component of a universal influenza vaccine capable of providing long-lasting protection.

Anti-viral CD8 T cells and the cytokines that they love

Viral infections cause an immunological disequilibrium that provokes CD8 T cell responses. These cells play critical roles in purging acute infections, limiting persistent infections, and conferring life-long protective immunity. At every stage of the response anti-viral CD8 T cells are sensitive to signals from cytokines. Initially cytokines operate as immunological warning signs that inform of the presence of an infection, and also influence the developmental choices of the responding cells. Later during the course of the response other sets of cytokines support the survival and maintenance of the differentiated anti-viral CD8 T cells. Although many cytokines promote virus-specific CD8 T cells, other cytokines can suppress their activities and thus favor viral persistence. In this review we discuss how select cytokines act to regulate anti-viral CD8 T cells throughout the response and influence the outcome of viral infections.

CD8α+ dendritic cell trans presentation of IL-15 to naive CD8+ T cells produces antigen-inexperienced T cells in the periphery with memory phenotype and function

Various populations of memory phenotype CD8(+) T cells have been described over the last 15-20 y, all of which possess elevated effector functions relative to naive phenotype cells. Using a technique for isolating Ag-specific cells from unprimed hosts, we recently identified a new subset of cells, specific for nominal Ag, but phenotypically and functionally similar to memory cells arising as a result of homeostatic proliferation. We show in this study that these virtual memory (VM) cells are independent of previously identified innate memory cells, arising as a result of their response to IL-15 trans presentation by lymphoid tissue-resident CD8α(+) dendritic cells in the periphery. The absence of IL-15, CD8(+) T cell expression of either CD122 or eomesodermin or of CD8a(+) dendritic cells all lead to the loss of VM cells in the host. Our results show that CD8(+) T cell homeostatic expansion is an active process within the nonlymphopenic environment, is mediated by IL-15, and produces Ag-inexperienced memory cells that retain the capacity to respond to nominal Ag with memory-like function. Preferential engagement of these VM T cells into a vaccine response could dramatically enhance the rate by which immune protection develops.

Decisions on the road to memory

A fundamental property of the adaptive immune system is the ability to generate antigen-specific memory, which protects against repeated infections with the same pathogens and determines the success of vaccination. Immune memory is built up alongside a response providing direct protection during the course of a primary immune response. For CD8 T cells, this involves the generation of two distinct types of effector cells. Short lived effector cells (SLECs) confer immediate protection, but contribute little to the memory repertoire. Memory precursor effector cells (MPECs) have the ability to respond to survival signals and develop into memory cells. These two types of cells can be distinguished on the basis of surface markers and express distinct genetic programs. A single naive CD8 T cell can give rise to both MPEC and SLEC daughter cells. This may involve an initial asymmetric division or depend on later instructive signals acting on equipotent daughter cells. Strong inflammatory signals favor the generation of SLECs and weaker inflammation favors the generation of MPECs. A distinguishing feature of MPECs is their ability to persist when most effector cells die. This survival depends on signals from the IL-7 receptor, which induce expression of anti-apoptotic factors. MPECs are therefore characterized by expression of the IL-7 receptor as well as the CCR7 chemokine receptor, which allows homing to areas in lymphoid organs where IL-7 is produced. Critical for persistence of MPECs is further their responsiveness to myeloid cell derived IL-15, which instructs these cells to switch their metabolic programs from glycolysis associated with rapid proliferation to fatty acid oxidation required during a more resting state. As the mechanisms determining generation of immunological memory are unraveled, opportunities will emerge for the improvement of vaccination strategies.

Early dysregulation of the memory CD8+ T cell repertoire leads to compromised immune responses to secondary viral infection in the aged

BACKGROUND

Virus-specific memory CD8+ T cells persist long after infection is resolved and are important for mediating recall responses to secondary infection. Although the number of memory T cells remains relatively constant over time, little is known about the overall stability of the memory T cell pool, particularly with respect to T cell clonal diversity. In this study we developed a novel assay to measure the composition of the memory T cell pool in large cohorts of mice over time following respiratory virus infection.

RESULTS

We find that the clonal composition of the virus-specific memory CD8+ T cell pool begins to change within months of the initial infection. These early clonal perturbations eventually result in large clonal expansions that have been associated with ageing.

CONCLUSIONS

Maintenance of clonal diversity is important for effective long-term memory responses and dysregulation of the memory response begins early after infection.

IL-7 downregulates IL-7Rα expression in human CD8 T cells by two independent mechanisms

Interleukin (IL)-7 is an essential nonredundant cytokine, and throughout the lifespan of a T-cell signaling via the IL-7 receptor influences cell survival, proliferation and differentiation. It is therefore no surprise that expression of the IL-7 receptor alpha-chain (CD127) is tightly regulated. We have previously shown that IL-7 downregulates expression of CD127 at the cell surface and now elucidate the kinetics of that suppression and demonstrate that IL-7 downregulates CD127 transcripts and surface protein in primary human CD8 T cells by two separate pathways. We show that IL-7 induces the initial reduction in cell-surface CD127 protein independent of transcriptional suppression, which is delayed by 40-60 min. Although IL-7-mediated downregulation of CD127 transcripts is dependent on Janus kinase (JAK)/STAT5, the early downregulation of surface CD127 protein is independent of JAK activity. The data further illustrate that low levels of IL-7 induce smaller and transient decreases in CD127 transcripts and surface protein, whereas higher concentrations induce more profound and sustained suppression. Such flexibility in receptor expression likely allows for fine-tuned immune responses in human CD8 T cells in different microenvironments and in response to different immunological challenges.

Technical Advance: Fluorescent reporter reveals insights into eomesodermin biology in cytotoxic lymphocytes

The T-box transcription factor Eomes is expressed in cytotoxic immune cells and plays an important role in development, maintenance, and function of these cell types. Identification and separation of cells with differential Eomes expression would allow for better understanding of the transcriptional program governing these cytotoxic lymphocytes. Here, we report the use of an Eomes(gfp)-targeted mouse allele that displays robust fidelity to Eomes protein expression in NK and T cells. Use of this reporter mouse revealed that Eomes expression in antiviral effector cells did not correlate with enhanced cytotoxicity but rather was associated with more efficient central memory differentiation. Weakening of reporter activity in Eomes-deficient CD8(+) T cells revealed a critical role for Eomes protein in maintaining central memory cells that have activated the Eomes locus. Characterization of reporter activity in NK lineage cells also permitted identification of a novel intermediate of NK cell maturation. Thus, the murine Eomes(gfp)-targeted allele provides a novel opportunity to explore Eomes biology in cytotoxic lymphocytes.

Contribution of 4-1BBL on radioresistant cells in providing survival signals through 4-1BB expressed on CD8⁺ memory T cells in the bone marrow

The persistence of memory lymphocytes is a critical feature of adaptive immunity. The TNF family ligand 4-1BBL supports the antigen-independent survival of CD8⁺ memory T cells. Here, we show that mice lacking 4-1BB only on αβ T cells show a similar defect in CD8⁺ T-cell recall responses, as previously shown in 4-1BBL-deficient mice. We show that 4-1BB is selectively expressed on BM CD8⁺ but not CD4⁺ memory T cells of unimmunized mice. Its ligand, 4-1BBL, is found on VCAM-1⁺ stromal cells, CD11c⁺ cells, and a Gr1(lo) myeloid population in unimmunized mice. Adoptive transfer of in vitro generated memory T cells into mice lacking 4-1BBL only on radioresistant cells recapitulates the defect in CD8⁺ T-cell survival seen in the complete knockout mice, with smaller effects of 4-1BBL on hematopoietic cells. In BM, adoptively transferred DsRed CD8⁺ memory T cells are most often found in proximity to VCAM-1⁺ cells or Gr1⁺ cells, followed by B220⁺ cells and to a much lesser extent near CD11c⁺ cells. Thus, a VCAM-1⁺CD45(-) stromal cell is a plausible candidate for the radioresistant cell that provides 4-1BBL to CD8⁺ memory T cells in the BM.

Immunological pathways in virus-induced COPD exacerbations: a role for IL-15

BACKGROUND

Chronic obstructive pulmonary disease (COPD) is characterized by high levels of antiviral type 1 cytokine interferon-γ and activated CD8(+) T cells. COPD exacerbations are the major cause of morbidity and mortality, have a prolonged and intense effect on quality of life and may result in accelerated loss of lung function. Respiratory virus infections, frequently within a state of colonization by bacteria, are the major cause of COPD exacerbations, and there is also evidence of virus latency in 'stable' disease, suggesting that latent infection might be a cause of chronic inflammation in COPD.

DESIGN

This is an update of current literature concerning the role of interleukin-15 and major histocompatibility complex class I-related chain A and B molecules in type 1 immune responses, particularly to respiratory virus infections, which are the main cause of COPD exacerbations. We also present data from our own group suggesting a role for interleukin-15 in virus-induced COPD exacerbations.

RESULTS

Type 1 cytokine interleukin-15 is produced by resident airway cells (epithelial cells and macrophages) in response to virus infection and bacteria. Virus infections modulate major histocompatibility complex class I-related chain A and B molecules in respiratory epithelial cells.

CONCLUSIONS

Interleukin-15 could play a major role in the airway inflammation in COPD directly, via its own receptors, by amplifying the type 1 immune responses and decreasing apoptosis or indirectly, via modulating molecules associated with cytotoxic activity of natural killer and CD8(+) T cells, such as major histocompatibility complex class I-related chain A and B.

Cathepsin B controls the persistence of memory CD8+ T lymphocytes

The persistence of memory T lymphocytes confers lifelong protection from pathogens. Memory T cells survive and undergo homeostatic proliferation (HSP) in the absence of Ag, although the cell-intrinsic mechanisms by which cytokines drive the HSP of memory T cells are not well understood. In this study we report that lysosome stability limits the long-term maintenance of memory CD8(+) T cell populations. Serine protease inhibitor (Spi) 2A, an anti-apoptotic cytosolic cathepsin inhibitor, is induced by both IL-15 and IL-7. Mice deficient in Spi2A developed fewer memory phenotype CD44(hi)CD8(+) T cells with age, which underwent reduced HSP in the bone marrow. Spi2A was also required for the maintenance of central memory CD8(+) T cell populations after acute infection with lymphocytic choriomeningitis virus. Spi2A-deficient Ag-specific CD8(+) T cell populations declined more than wild-type competitors after viral infection, and they were eroded further after successive infections. Spi2A protected memory cells from lysosomal breakdown by inhibiting cathepsin B. The impaired maintenance of Spi2A-deficient memory CD8(+) T cells was rescued by concomitant cathepsin B deficiency, demonstrating that cathepsin B was a physiological target of Spi2A in memory CD8(+) T cell survival. Our findings support a model in which protection from lysosomal rupture through cytokine-induced expression of Spi2A determines the long-term persistence of memory CD8(+) T cells.

Rapidly generated multivirus-specific cytotoxic T lymphocytes for the prophylaxis and treatment of viral infections

Severe and fatal viral infections remain common after hematopoietic stem cell transplantation. Adoptive transfer of cytotoxic T lymphocytes (CTLs) specific for Epstein–Barr virus (EBV), cytomegalovirus (CMV), and adenoviral antigens can treat infections that are impervious to conventional therapies, but broader implementation and extension to additional viruses is limited by competition between virus-derived antigens and time-consuming and laborious manufacturing procedures. We now describe a system that rapidly generates a single preparation of polyclonal (CD4⁺ and CD8⁺) CTLs that is consistently specific for 15 immunodominant and subdominant antigens derived from 7 viruses (EBV, CMV, Adenovirus (Adv), BK, human herpes virus (HHV)-6, respiratory syncytial virus (RSV), and Influenza) that commonly cause post-transplant morbidity and mortality. CTLs can be rapidly produced (10 days) by a single stimulation of donor peripheral blood mononuclear cells (PBMCs) with a peptide mixture spanning the target antigens in the presence of the potent prosurvival cytokines interleukin-4 (IL4) and IL7. This approach reduces the impact of antigenic competition with a consequent increase in the antigenic repertoire and frequency of virus-specific T cells. Our approach can be readily introduced into clinical practice and should be a cost-effective alternative to common antiviral prophylactic agents for allogeneic hematopoietic stem cell transplant (HSCT) recipients.

Regulating the immune system via IL-15 transpresentation

Transpresentation has emerged as an important mechanism mediating IL-15 responses in a subset of lymphocytes during the steady state. In transpresentation, cell surface IL-15, bound to IL-15Rα is delivered to opposing lymphocytes during a cell-cell interaction. The events most dependent on IL-15 include the development and homeostasis of memory CD8 T cells, Natural Killer cells, invariant Natural Killer T cells, and intraepithelial lymphocytes. As lymphocyte development and homeostasis involve multiple steps and mechanisms, IL-15 transpresentation can have diverse roles throughout. Moreover, distinct stages of lymphocyte differentiation require IL-15 transpresented by different cells, which include both hematopoietic and non-hematopoietic cell types. Herein, we will describe the points where IL-15 transpresentation impacts these processes, the specific cells thought to drive IL-15 responses, as well as their role in the course of development and homeostasis.

T cell memory in the context of persistent herpes viral infections

The generation of a functional memory T cell pool upon primary encounter with an infectious pathogen is, in combination with humoral immunity, an essential process to confer protective immunity against reencounters with the same pathogen. A prerequisite for the generation and maintenance of long-lived memory T cells is the clearance of antigen after infection, which is fulfilled upon resolution of acute viral infections. Memory T cells play also a fundamental role during persistent viral infections by contributing to relative control and immuosurveillance of active replication or viral reactivation, respectively. However, the dynamics, the phenotype, the mechanisms of maintenance and the functionality of memory T cells which develop upon acute/resolved infection as opposed to chronic/latent infection differ substantially. In this review we summarize current knowledge about memory CD8 T cell responses elicited during α-, β-, and γ-herpes viral infections with major emphasis on the induction, maintenance and function of virus-specific memory CD8 T cells during viral latency and we discuss how the peculiar features of these memory CD8 T cell responses are related to the biology of these persistently infecting viruses.

A push-pull vaccine strategy using Toll-like receptor ligands, IL-15, and blockade of negative regulation to improve the quality and quantity of T cell immune responses

We have developed a strategy to optimize the efficacy of vaccines to induce T-cell immunity against chronic viral infections and cancer based on a "push-pull" approach in which we first optimize the antigen structure by increasing the affinity of epitopes for major histocompatibility complex molecules ("epitope enhancement"), then push the response not only in magnitude but also in quality toward the desired response phenotype, using synergistic combinations of cytokines, Toll-like receptor ligands, and costimulatory molecules, and then pull the response by removing the brakes exerted by negative regulatory mechanisms, including regulatory cells, cell surface molecules, and cytokines. Components of this approach show promise in macaque models of AIDS virus infection and in murine models of cancer, and are being developed for human clinical trials.

Functions of IL-15 in anti-viral immunity: multiplicity and variety

An effective immune response to an invading viral pathogen requires the combined actions of both innate and adaptive immune cells. For example, NK cells and cytotoxic CD8 T cells are capable of the direct engagement of infected cells and the mediation of antiviral responses. Both NK and CD8 T cells depend on common gamma chain (γc) cytokine signals for their development and homeostasis. The γc cytokine IL-15 is very well characterized for its role in promoting the development and homeostasis of NK cells and CD8 T cells, but emerging literature suggests that IL-15 mediates the anti-viral responses of these cell populations during an active immune response. Both NK cells and CD8 T cells must become activated, migrate to sites of infection, survive at those sites, and expand in order to maximally exert effector functions, and IL-15 can modulate each of these processes. This review focuses on the functions of IL-15 in the regulation of multiple aspects of NK and CD8 T cell biology, investigates the mechanisms by which IL-15 may exert such diverse functions, and discusses how these different facets of IL-15 biology may be therapeutically exploited to combat viral diseases.

Strategies to use immune modulators in therapeutic vaccines against cancer

Cancers so much resemble self that they prove difficult for the immune system to eliminate, and those that have already escaped natural immunosurveillance have gotten past the natural immune barriers to malignancy. A successful therapeutic cancer vaccine must overcome these escape mechanisms. Our laboratory has focused on a multistep "push-pull" approach in which we combine strategies to overcome each of the mechanisms of escape. If tumor epitopes are insufficiently immunogenic, we increase their immunogenicity by epitope enhancement, improving their binding affinity to major histocompatibility complex (MHC) molecules. If the anti-tumor response is too weak or of the wrong phenotype, we use cytokines, costimulatory molecules, Toll-like receptor ligands, and other molecular adjuvants to increase not only the quantity of the response but also its quality, to push the response in the right direction. Finally, the tumor invokes multiple immunosuppressive mechanisms to defend itself, so we need to overcome those as well, including blocking or depleting regulatory cells or inhibiting regulatory molecules, to pull the response by removing the brakes. Some of these strategies individually have now been translated into human clinical trials in cancer patients. Combinations of these in a push-pull approach are promising for the successful immunotherapy of cancer.

Lack of IL-7 and IL-15 signaling affects interferon-γ production by, more than survival of, small intestinal intraepithelial memory CD8+ T cells

Survival of antigen-specific CD8(+) T cells in peripheral lymphoid organs during viral infection is known to be dependent predominantly on IL-7 and IL-15. However, little is known about a possible influence of tissue environmental factors on this process. To address this question, we studied survival of memory antigen-specific CD8(+) T cells in the small intestine. Here, we show that 2 months after vaccinia virus infection, B8R(20-27) /H2-K(b) tetramer(+) CD8(+) T cells in the small intestinal intraepithelial (SI-IEL) layer are found in mice deficient in IL-15 expression. Moreover, SI-IEL and lamina propria lymphocytes do not express the receptor for IL-7 (IL-7Rα/CD127). In addition, after in vitro stimulation with B8R(20-27) peptide, SI-IEL cells do not produce high amounts of IFN-γ neither at 5 days nor at 2 months postinfection (p.i.). Importantly, the lack of IL-15 was found to shape the functional activity of antigen-specific CD8(+) T cells, by narrowing the CTL avidity repertoire. Taken together, these results reveal that survival factors, as well as the functional activity, of antigen-specific CD8(+) T cells in the SI-IEL compartments may markedly differ from their counterparts in peripheral lymphoid tissues.

Division-linked generation of death-intermediates regulates the numerical stability of memory CD8 T cells

Infection or successful vaccination results in the formation of long-lived memory CD8 T-cell populations. Despite their numerical stability, memory CD8 T-cell populations are thought to completely turn over through proliferation within a 2- to 3-mo period. Therefore, steady-state memory cell proliferation must be balanced by a precisely regulated and equivalent death rate. However, the mechanisms regulating this balancing process remain completely undefined. Herein, we provide evidence for "death-intermediate memory cells" (T(DIM)) within memory CD8 T-cell populations generated by infection. Importantly, CD62L(Lo)/CD27(Lo) T(DIM)s are functionally characterized by an inability to produce cytokines, the failure to internalize T-cell receptor following antigenic stimulation, and signatures of apoptotic death. Furthermore, we demonstrate that, mechanistically, T(DIM) are directly generated from dividing "central memory" T-cell populations undergoing memory turnover in vivo. Collectively, these results demonstrate that as central memory CD8 T cells proliferate, they continuously generate a population of CD8 T cells that are nonfunctional and apoptotic; thus, our data support a model wherein division-linked generation of T(DIM) contributes to numerically stable CD8 T-cell memory.

Homeostatic maintenance of T cells and natural killer cells

Homeostasis in the immune system encompasses the mechanisms governing maintenance of a functional and diverse pool of lymphocytes, thus guaranteeing immunity to pathogens while remaining self-tolerant. Antigen-naïve T cells rely on survival signals through contact with self-peptide-loaded major histocompatibility complex (MHC) molecules plus interleukin (IL)-7. Conversely, antigen-experienced (memory) T cells are typically MHC-independent and they survive and undergo periodic homeostatic proliferation through contact with both IL-7 and IL-15. Also, non-conventional γδ T cells rely on a mix of IL-7 and IL-15 for their homeostasis, whereas natural killer cells are mainly dependent on contact with IL-15. Homeostasis of CD4(+) T regulatory cells is different in being chiefly regulated by contact with IL-2. Notably, increased levels of these cytokines cause expansion of responsive lymphocytes, such as found in lymphopenic hosts or following cytokine injection, whereas reduced cytokine levels cause a decline in cell numbers.

Selective dependence of H2-M3-restricted CD8 responses on IL-15

We studied whether CD8 T cell responses that are mediated by unconventional MHC class Ib molecules are IL-15 dependent in mice. CD8(+) T cell responses to Listeria monocytogenes infection that are restricted by the MHC class Ib molecule H2-M3 decreased in the absence of IL-15, whereas other primary MHC class Ib- and MHC class Ia-restricted responses were IL-15 independent. This result was confirmed in MHC class Ia-deficient mice in which IL-15 deficiency also reduced H2-M3-restricted but not all CD8 T cell responses to L. monocytogenes. IL-15 deficiency did not affect proliferation or survival of responding H2-M3-restricted CD8(+) T cells, but IL-15 was necessary to detect H2-M3-restricted CD8(+) T cells in naive mice. This finding suggests that these CD8(+) T cells require IL-15 during development, but become IL-15 independent after activation. IL-15 was necessary for the survival of most class Ib-restricted CD8(+) T cells, starting at the mature thymocyte stage in naive mice, but does not affect a distinct CD44(low)/CD122(low) subpopulation. These data suggest that the nature of the selecting MHC class Ib molecule determines whether CD8(+) T cells acquire IL-15 dependence during thymic development.

Therapeutic cancer vaccines: current status and moving forward

Concurrent with U.S. Food and Drug Administration (FDA) approval of the first therapeutic cancer vaccine, a wide spectrum of other cancer vaccine platforms that target a diverse range of tumor-associated antigens is currently being evaluated in randomized phase II and phase III trials. The profound influence of the tumor microenvironment and other immunosuppressive entities, however, can limit the effectiveness of these vaccines. Numerous strategies are currently being evaluated both preclinically and clinically to counteract these immunosuppressive entities, including the combined use of vaccines with immune checkpoint inhibitors, certain chemotherapeutics, small-molecule targeted therapies, and radiation. The potential influence of the appropriate patient population and clinical trial endpoint in vaccine therapy studies is discussed, as well as the potential importance of biomarkers in future directions of this field.

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.