IL-15 mediates anti-tumor effects after cyclophosphamide injection of tumor-bearing mice and enhances adoptive immunotherapy: the potential role of NK cell subpopulations

Myxoma virus expressing interleukin-15 fails to cause lethal myxomatosis in European rabbits

Myxoma virus (MYXV) is a poxvirus pathogenic only for European rabbits, but its permissiveness in human cancer cells gives it potential as an oncolytic virus. A recombinant MYXV expressing both the tdTomato red fluorescent protein and interleukin-15 (IL-15) (vMyx-IL-15-tdTr) was constructed. Cells infected with vMyx-IL-15-tdTr secreted bioactive IL-15 and had in vitro replication kinetics similar to that of wild-type MYXV. To determine the safety of this virus for future oncolytic studies, we tested its pathogenesis in European rabbits. In vivo, vMyx-IL-15-tdTr no longer causes lethal myxomatosis. Thus, ectopic IL-15 functions as an antiviral cytokine in vivo, and vMyx-IL-15-tdTr is a safe candidate for animal studies of oncolytic virotherapy.

The anticancer immune response: indispensable for therapeutic success?

Although the impact of tumor immunology on the clinical management of most cancers is still negligible, there is increasing evidence that anticancer immune responses may contribute to the control of cancer after conventional chemotherapy. Thus, radiotherapy and some chemotherapeutic agents, in particular anthracyclines, can induce specific immune responses that result either in immunogenic cancer cell death or in immunostimulatory side effects. This anticancer immune response then helps to eliminate residual cancer cells (those that fail to be killed by chemotherapy) or maintains micrometastases in a stage of dormancy. Based on these premises, in this Review we address the question, How may it be possible to ameliorate conventional therapies by stimulating the anticancer immune response? Moreover, we discuss the rationale of clinical trials to evaluate and eventually increase the contribution of antitumor immune responses to the therapeutic management of neoplasia.

Interleukin-15/interleukin-15R alpha complexes promote destruction of established tumors by reviving tumor-resident CD8+ T cells

Tumors often escape immune-mediated destruction by suppressing lymphocyte infiltration or effector function. New approaches are needed that overcome this suppression and thereby augment the tumoricidal capacity of tumor-reactive lymphocytes. The cytokine interleukin-15 (IL-15) promotes proliferation and effector capacity of CD8(+) T cells, natural killer (NK) cells, and NKT cells; however, it has a short half-life and high doses are needed to achieve functional responses in vivo. The biological activity of IL-15 can be dramatically increased by complexing this cytokine to its soluble receptor, IL-15R alpha. Here, we report that in vivo delivery of IL-15/IL-15R alpha complexes triggers rapid and significant regression of established solid tumors in two murine models. Despite a marked expansion of IL-2/IL-15R beta(+) cells in lymphoid organs and peripheral blood following treatment with IL-15/IL-15R alpha complexes, the destruction of solid tumors was orchestrated by tumor-resident rather than newly infiltrating CD8(+) T cells. Our data provide novel insights into the use of IL-15/IL-15R alpha complexes to relieve tumor-resident T cells from functional suppression by the tumor microenvironment and have significant implications for cancer immunotherapy and treatment of chronic infections.

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.

The biology of interleukin-2 and interleukin-15: implications for cancer therapy and vaccine design

Interleukin-2 and interleukin-15 have pivotal roles in the control of the life and death of lymphocytes. Although their heterotrimeric receptors have two receptor subunits in common, these two cytokines have contrasting roles in adaptive immune responses. The unique role of interleukin-2 is in the elimination of self-reactive T cells to prevent autoimmunity. By contrast, interleukin-15 is dedicated to the prolonged maintenance of memory T-cell responses to invading pathogens. As discussed in this Review, the biology of these cytokines will affect the development of novel therapies for malignancy and autoimmune diseases, as well as the design of vaccines against infectious diseases.

Gene therapy that safely targets and kills tumor cells throughout the body

The authors studied the therapeutic value of Sindbis vectors for advanced metastatic cancer by using a variety of clinically accurate mouse models and demonstrated through imaging, histological, and molecular data that Sindbis vectors systemically and specifically infect/detect and kill metastasized tumors in vivo, leading to significant suppression of tumor growth and enhanced survival. Use of two different bioluminescent genetic markers for the IVIS Imaging System (Xenogen Corp., Alameda, CA) permitted demonstration of an excellent correlation between vector delivery and metastatic locations in vivo. Sindbis tumor specificity is not attributable to a species difference between human tumor and mouse normal cells. Sindbis virus is known to infect mammalian cells using the Mr 67,000 laminin receptor, which is elevated in tumor versus normal cells, and downregulated expression of laminin receptor with small interfering RNA significantly reduces the infectivity of Sindbis vectors. Tumor overexpression of the laminin receptor may explain the specificity and efficacy that Sindbis vectors demonstrate for tumor cells in vivo. Laser capture microdissection of mouse tumor implants showed equivalent laminin receptor expression levels in the different tumor metastases in the peritoneal cavity. Incorporation of antitumor cytokine genes such as interleukin-12 and interleukin-15 genes enhances the efficacy of the vector. These results suggest that Sindbis viral vectors may be promising agents for both specific detection and growth suppression of metastatic ovarian cancer.

Using sindbis viral vectors for specific detection and suppression of advanced ovarian cancer in animal models

We studied the therapeutic value of Sindbis vectors for advanced metastatic ovarian cancer by using two highly reproducible and clinically accurate mouse models: a SCID xenograft model, established by i.p. inoculation of human ES-2 ovarian cancer cells, and a syngenic C57BL/6 model, established by i.p. inoculation of mouse MOSEC ovarian cancer cells. We demonstrate through imaging, histologic, and molecular data that Sindbis vectors systemically and specifically infect/detect and kill metastasized tumors in the peritoneal cavity, leading to significant suppression of the carcinomatosis in both animal models. Use of two different bioluminescent genetic markers for the IVIS Imaging System permitted demonstration, for the first time, of an excellent correlation between vector delivery and metastatic locations in vivo. Sindbis vector infection and growth suppression of murine MOSEC tumor cells indicate that Sindbis tumor specificity is not attributable to a species difference between human tumor and mouse normal cells. Sindbis virus is known to infect mammalian cells using the Mr 67,000 laminin receptor. Immunohistochemical staining of tumor cells indicates that laminin receptor is elevated in tumor versus normal cells. Down-regulated expression of laminin receptor with small interfering RNA significantly reduces the infectivity of Sindbis vectors. Tumor overexpression of the laminin receptor may explain the specificity and efficacy that Sindbis vectors demonstrate for tumor cells in vivo. We show that incorporation of antitumor cytokine genes such as interleukin-12 and interleukin-15 genes enhances the efficacy of the vector. These results suggest that Sindbis viral vectors may be promising agents for both specific detection and growth suppression of metastatic ovarian cancer.

Role of trans-cellular IL-15 presentation in the activation of NK cell-mediated killing, which leads to enhanced tumor immunosurveillance

Interleukin 15 (IL-15) is a critical factor for the proliferation and activation of natural killer (NK) and CD8 T cells. Recently, we demonstrated that IL-15Ralpha expressed on monocytes/dendritic cells captures and presents IL-15 to neighboring cells in trans (trans-presentation of IL-15) through cell-cell contact. In the current study, we provide evidence that the IL-15 presented in trans, but not soluble IL-15 at physiologic concentrations, augments the killing activity mediated by NK cells in vitro. In addition, transfection of IL-15Ralpha into a colon carcinoma cell line (MC38) enabled these cells to present IL-15 in trans to NK cells and augmented their killing activity, resulting in the efficient lysis of MC38 cells by NK cells in vitro. Furthermore, these transfected MC38 cells no longer form fatal pulmonary metastases in mice. It was also shown that NK cells play an important role in the rejection of MC38 cells under these circumstances. These results collectively suggest that the IL-15 trans-presentation mechanism operates in vivo to augment the tumor immune surveillance mechanism. Furthermore, our observation provides the scientific basis for a novel strategy to prevent cancer development/metastasis.

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.

Turnover and proliferation of NK cells in steady state and lymphopenic conditions

To gain insight into NK cell dynamics, we investigated the turnover and proliferation rates of NK cells in normal and lymphopenic conditions. In contrast to previous reports suggesting a very rapid turnover of NK cells, continuous 5-bromo-2'-deoxyuridine (BrdU)-labeling studies demonstrated that the time necessary for labeling 50% of splenic NK cells in mature mice was 17 days, similar to the rate of labeling of memory T cells. In contrast, in young mice, splenic NK cells labeled very rapidly with BrdU, although cell cycle analyses and BrdU pulse-labeling studies suggested that most of this proliferation occurred in a precursor population. A somewhat larger percentage of bone marrow NK cells was cycling, suggesting that these proliferating cells are the precursors of the mostly nondividing or slowly dividing splenic NK cells. Splenic NK cells from mature mice also did not proliferate significantly when transferred to normal mice, but did proliferate when transferred to irradiated mice. Thus, NK cells, like T cells, undergo homeostatic proliferation in a lymphopenic environment. Homeostatic proliferation of NK cells was not dependent on host cell class I molecules or host production of IL-15. Nevertheless, the number of recovered NK cells was much lower in IL-15(-/-) hosts. These results suggest that IL-15 is not essential for homeostatic proliferation of NK cells, but is necessary for survival of the NK cells. Our results provide important basic information concerning the production and replacement of NK cells.

IL-15-mediated induction of LFA-1 is a late step required for cytotoxic differentiation of human NK cells from CD34+Lin- bone marrow cells

Optimal differentiation of cytotoxic NK cells is important to provide protective innate immunity to patients after bone marrow transplantation. In vitro differentiation of CD56(+)CD3(-) NK cells takes weeks and is supported by several cytokines, including IL-2, IL-7, and IL-15, and thus can be useful for immunotherapy. However, IL-2 therapy is problematic in vivo, and NK cells differentiated in vitro with only IL-7 lack cytotoxicity. We assessed whether human NK cells initially differentiated in vitro from CD34(+)Lin(-) bone marrow cells with IL-7 could acquire cytotoxicity after exposure to additional cytokines and what changes promoted cytotoxicity. The cells cultured with IL-7 already had granzyme B as well as perforin, as previously reported, the proteins of cytotoxic granules. The cells also lacked LFA-1. After 1 wk of secondary culture with either IL-2 or IL-15, but not with IL-12 or IL-18, the IL-7-cultured cells acquired cytotoxicity. IL-2 or IL-15 also induced LFA-1. Ab to the LFA-1 subunits CD11a and CD18 blocked lysis by the NK cells, indicating that the new LFA-1 correlated with, and was essential for, the cytotoxic function of the in vitro generated cells. The LFA-1 also participated in target cell binding by the in vitro differentiated cells. In this study, we demonstrated a new function for IL-15, the induction of LFA-1 in NK progenitor cells, and that IL-15 does more than merely support NK progenitor cell proliferation. The efficacy after only 1 wk of IL-15 administration is a positive practical feature that may apply to human therapy.

Overexpression of interleukin-15 in vivo enhances antitumor activity against MHC class I-negative and -positive malignant melanoma through augmented NK activity and cytotoxic T-cell response

Interleukin (IL)-15, a pleiotropic cytokine, is involved in the development and maintenance of NK cells and memory CD8+ T cells. We examined the effects of in vivo overexpression of IL-15 on protection against 2 types of murine B16 melanoma lines, MHC class I-negative B16.44 and MHC class I-positive B16F10 cells, using IL-15 transgenic (Tg) mice that we have recently constructed. The tumor growth was severely retarded in IL-15 Tg mice after subcutaneous (s.c.) inoculation with B16.44 or B16F10 cells. IL-15 Tg mice showed an augmented NK cell activity against B16.44 cells, and in vivo depletion of NK cells by anti-asialoGM1 Ab treatment abrogated the antitumor activity in IL-15 Tg mice. On the other hand, IL-15 Tg mice inoculated with B16F10 cells developed a significant level of CTL response against B16F10 cells, and in vivo depletion of CD8+ T cells by anti-CD8 MAb treatment abrogated the antitumor activity. Thus, overexpression of IL-15 augmented antitumor activity against different tumors via augmentation of different antitumor mechanisms. These results suggest a possible therapeutic application of IL-15 for human neoplasms expressing a wide range of MHC class molecules.

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.

Effect of IL-15 on the secretion of IL-1beta, IL-1Ra and sIL-1RII by PMN from cancer patients

In the present study, we demonstrate an effect of rhIL-15 on the simultaneous secretion of IL-1beta and its natural inhibitors IL-1Ra and sIL-1RII by human neutrophils isolated from normal and tumour-bearing hosts (oral cavity cancer and melanoma patients) compared with serum IL-15 levels. We found an rhIL-15 influence on IL-beta and IL-1Ra secreted by PMN from healthy controls. In contrast, the PMNs from cancer patients were not sensitive to rhIL-15 stimulation. However, we found a priming effect of rhIL-15 on IL-1beta production by LPS-stimulated cells in oral cavity cancer. We also found no effect on sIL-1RII release by PMN from cancer patients.

Chemo-immunotherapy and chemo-adoptive immunotherapy of cancer

The chemo-immunotherapy (CIT) and chemo-adoptive immunotherapy (CAIT) regimens tested in the past decade are summarized. From them we have learned a great deal about the interactions between various chemotherapeutic agents, immune modulating agents and effector cells. The most commonly reported result in multi-modality experiments with CAIT has been a synergistic enhancement in antitumor activity. Clinical trials usually demonstrated improvement in patient quality of life, an extension of survival time, and occasional complete regression of tumor. In many animal models, this enhancement often meant the complete regression and apparent cure of tumor in the animal. One mechanism by which this synergistic enhancement takes place appears to be a suppression of tumor-associated suppressor T cell activity by the chemotherapeutic agents, thereby inducing enhanced cytolytic activity against tumor by the adoptively transferred, activated effector cells. In CAIT the most commonly used drug has been cyclophosphamide. In CIT a wide variety of chemotherapy agents have been used but none of the clinical trials made use of cyclophosphamide. Thus, direct comparisons are not possible. Suggestive of the intricate regulatory processes involved, many CIT studies indicate a synergy only when specific doses of chemotherapy and immunotherapy agents are given, and in a specific sequence. CIT has become less toxic, is being handled on a cost-effective outpatient basis, while maintaining similar objective response rates to earlier inpatient treatments. In the future, CAIT and CIT will probably have an increasing role in the management of patients with specific cancers.

GM-CSF and IL-2 induce specific cellular immunity and provide protection against Epstein-Barr virus lymphoproliferative disorder

Epstein-Barr virus-associated lymphoproliferative disease (EBV-LPD) is a potentially life-threatening complication in immune-deficient patients. We have used the severe combined immune deficient (SCID) mouse engrafted with human leukocytes (hu-PBL-SCID) to evaluate the use of human cytokines in the prevention of EBV-LPD in vivo. Daily low-dose IL-2 therapy can prevent EBV-LPD in the hu-PBL-SCID mouse, but protection is lost if murine natural killer (NK) cells are depleted. Here we demonstrate that combined therapy with human GM-CSF and low-dose IL-2 is capable of preventing EBV-LPD in the hu-PBL-SCID mouse in the absence of murine NK cells. Lymphocyte depletion experiments showed that human NK cells, CD8(+) T cells, and monocytes were each required for the protective effects of GM-CSF and IL-2 combination therapy. This treatment resulted in a marked expansion of human CD3(+)CD8(+) lymphocytes in vivo. Using HLA tetramers complexed with EBV immunodominant peptides, a subset of these lymphocytes was found to be EBV-specific. These data establish that combined GM-CSF and low-dose IL-2 therapy can prevent the immune deficiencies that lead to fatal EBV-LPD in the hu-PBL-SCID mouse depleted of murine NK cells, and they point to a critical role for several human cellular subsets in mediating this protective effect.

NK cells recover early and mediate cytotoxicity via perforin/granzyme and Fas/FasL pathways in umbilical cord blood recipients

Umbilical cord blood (UCB) is now widely accepted as a source of stem cells in patients with malignant hematologic and genetic disorders. We have recently reported that in a series of 30 pediatric UCB transplant recipients comparable outcome to that anticipated with other unrelated stem cell sources. In our series, however, the probability of GVHD for grade III-IV was 9% and no UCB recipient developed chronic GVHD. The reason for the low incidence of GVHD after UCB transplantation is not fully understood. Because functional NK cells are among the first population of lymphocytes to be detected in UCB transplant recipients, 2 months post-transplant on average, we wanted to establish whether NK cells could be implicated in reducing the risk of GVHD. Here, we confirm that early NK cells detected in UCB transplant recipients activate the granzyme/perforin lytic pathway and, in addition, they can mediate Fas/Fas ligand (FasL) activity, a finding not previously reported. Both pathways develop simultaneously and are detectable months before the other lymphocytes, notably CD8 are fully functional. Our contention, therefore, is that the low GVHD observed in UCB recipients may be partially due to early NK cells.

Therapeutic activity of NK cells against tumors

While it is generally accepted that natural killer (NK) cells, by killing tumor cells in the circulation, represent a first line of defense against metastases, their therapeutic activity against established tumors has been limited. In this review, we describe studies to improve the therapeutic effectiveness of activated NK cells in both animal models and clinical trials to better understand the biological problems that limit their effectiveness.

Ontogeny and expansion of human natural killer cells: clinical implications

Our knowledge of NK cells and their critical role in the innate immune system has increased enormously since their discovery several decades ago. However, it is only within the last 10 years that rational cytokine therapies, such as those utilizing low doses of IL-2, have been successful in expanding NK cells in patients with cancer and/or immunodeficiency. Such experiences in vivo have highlighted the importance of basing immunotherapeutic strategies on the known cellular and molecular properties of the targeted cell population. Recent advances in our understanding of the physiologic factors and events that orchestrate NK cell ontogeny, including IL-15 and receptor tyrosine kinase ligands to c-kit and flt3, provide novel therapeutic possibilities for cytokine therapy. This review summarizes our current understanding of human NK cell ontogeny, and links this knowledge to ongoing and future clinical strategies for the endogenous expansion of NK cells in patients with cancer and/or immunodeficiency.

Murine models for experimental therapy of pediatric solid tumors with poor prognosis

Novel therapeutic strategies are required for pediatric solid tumors with poor prognosis such as metastasizing neuroblastoma, rhabdomyosarcoma and Ewing's sarcoma. A prerequisite for the development of such new therapies is the availability of murine models. To be useful for therapeutic studies, these models should not only recapitulate the genetic alterations characteristic of the human disease but should also mimic the metastatic process and the response to current therapy, both of which ultimately determine the fate of children with these tumors. This review scrutinizes the utility of existing murine models of neuroblastoma, rhabdomyosarcoma and Ewing's sarcoma for investigating novel therapies. Much experience has been gained with both syngeneic and xenogeneic transplantable models of these tumors, while transgenic and knockout mice are just beginning to be available for therapeutic investigations. Modeling the genetic aberrations characterizing these tumors may provide faithful models for therapeutic studies in the future.

Enhancement of natural killer cytotoxicity delayed murine carcinogenesis by a probiotic microorganism

Regulation of innate immunity may be an effective means of cancer control. Delaying cancer onset is regarded as an important mode of action in cancer prevention. We have been investigating the chemopreventive mechanisms of Lactobacillus casei Shirota (LcS), a probiotic strain. In this study, we evaluated the effect of LcS on tumor onset and the involvement of natural killer (NK) cells using a 3-methylcholanthrene-induced carcinogenesis model. C3H/HeN mice were divided into three groups, according to treatment: vehicle-treated, treated with vehicle only; control, 3-methylcholanthrene treated; LcS, 3-methylcholanthrene and LcS treated. 3-Methylcholanthrene was injected intradermally at 7 weeks of age. LcS was mixed into the diet (0.05%, w/w), which the mice were fed from the day of 3-methylcholanthrene injection onward. Tumor incidence was observed weekly. Profiles of splenic NK cells, in vitro cytotoxicity and the proportion, in the early stage of carcinogenesis were analyzed at 5 weeks after the injection. The tumor suppressive effect of LcS was also evaluated in a beige mouse model that is genetically deficient in NK cells. LcS delayed tumor onset and reduced tumor incidence in the results with C3H/HeN mice (P< 0.05). More specifically, tumor incidence in the control group was 33% at 6 weeks after the injection and 83% at 11 weeks as opposed to 0 and 42%, respectively, in the LcS group. NK cell cytotoxicity was significantly higher than in the control group, and the number of NK cells also increased in the LcS group of C3H/HeN mice. However, LcS failed to suppress tumorigenesis in the beige mouse. These findings suggest that enhancement of the cytotoxicity of NK cells by LcS delays tumor onset.

In vitro effect of lithium chloride on interleukin-15 production by monocytes from IL-breast cancer patients

Lithium salt compounds are used to limit the degree and duration of neutropenia in patients receiving chemotherapy for cancer. Interleukin-15 (IL-15) is a cytokine which possesses promoting activities on hematopoiesis and is also involved in antitumor response, activating NK, CTL and LAK cells. In this study we analyzed IL-15 production by monocyte cultures treated with lithium chloride (LiCl). Monocytes were obtained from patients affected by non-metastatic and metastatic breast cancer. LiCl treatment induced IL-15 production by monocytes mainly from non-metastatic patients. Combined lipopolysaccharide/LiCl treatment of monocyte cultures up-regulated IL-15 release compared to those treated with LPS alone (p<0.0001). The modulation of LiCl-induced IL-15 could counteract the immunosuppression state of cancer patients, which should be taken into account when developing new immunotherapeutic strategies.

Reversible defects in natural killer and memory CD8 T cell lineages in interleukin 15-deficient mice

C57BL/6 mice genetically deficient in interleukin 15 (IL-15(-/-) mice) were generated by gene targeting. IL-15(-/-) mice displayed marked reductions in numbers of thymic and peripheral natural killer (NK) T cells, memory phenotype CD8(+) T cells, and distinct subpopulations of intestinal intraepithelial lymphocytes (IELs). The reduction but not absence of these populations in IL-15(-/-) mice likely reflects an important role for IL-15 for expansion and/or survival of these cells. IL-15(-/-) mice lacked NK cells, as assessed by both immunophenotyping and functional criteria, indicating an obligate role for IL-15 in the development and functional maturation of NK cells. Specific defects associated with IL-15 deficiency were reversed by in vivo administration of exogenous IL-15. Despite their immunological defects, IL-15(-/-) mice remained healthy when maintained under specific pathogen-free conditions. However, IL-15(-/-) mice are likely to have compromised host defense responses to various pathogens, as they were unable to mount a protective response to challenge with vaccinia virus. These data reveal critical roles for IL-15 in the development of specific lymphoid lineages. Moreover, the ability to rescue lymphoid defects in IL-15(-/-) mice by IL-15 administration represents a powerful means by which to further elucidate the biological roles of this cytokine.

Immunogene therapy of murine fibrosarcoma using IL-15 gene with high translation efficiency

We have recently found that translational efficiency is up-regulated by an alternative exon in IL-15 mRNA in mice. In a malignancy model using BALB/c mice and syngeneic Meth A fibrosarcoma (Meth A), we successfully applied immunological gene therapy with IL-15 protein using alternative IL-15 cDNA with high translational efficiency. Two expression vectors carrying the murine IL-15 gene were constructed for use in tumor immunotherapy, one utilizing IL-15 cDNA with alternative exon 5 and the second utilizing IL-15 cDNA with normal exon 5. The first vector induced the production of a large amount of IL-15 protein in Meth A cells, whereas tumor cells transfected by the second vector produced only a marginal level of IL-15 protein. Although cell growth of both transfectants in vitro remained unchanged, inoculation of clones transfected with normal IL-15 cDNA resulted in progressive tumor growth, while clones transfected with alternative IL-15 cDNA led to the rejection of the tumor. The clone producing high levels of IL-15 grew progressively in nude mice and mice treated with anti-CD4 monoclonal antibodies (mAb), whereas the growth of the transfectants was retarded in anti-CD8 mAb- or anti-asialo GM1 antibody-treated mice. Cured mice were shown to have generated immunity against a subsequent challenge with the wild type of Meth A but not against Meth 1 tumor cells, another type of fibrosarcoma derived from BALB/c mice. Thus, tumor therapy based on IL-15 gene transfection was effective against Meth A tumor cells, suggesting a possible application to human neoplasms.

Combination Chemotherapy and IL-15 Administration Induce Permanent Tumor Regression in a Mouse Lung Tumor Model: NK and T Cell-Mediated Effects Antagonized by B Cells

Previous studies have demonstrated that IL-15 administration after cyclophosphamide (CY) injection of C57BL/6J mice bearing the i.m. 76-9 rhabdomyosarcoma resulted in a significant prolongation of life. In the present study, we investigated the immune response against the 76-9 experimental lung metastases after CY + IL-15 therapy. Administration of CY + IL-15, but not IL-15 alone, induced prolongation of life and cures in 32% of mice bearing established experimental pulmonary metastases of 76-9 tumor. The CY + IL-15 therapy resulted in increased levels of NK1.1+/LGL-1+ cells, and CD8+/CD44+ T cells in PBL. In vitro cytotoxic assay of PBL indicated the induction of lymphokine-activated killer cell activity, but no evident tumor-specific class I-restricted lytic activity. Survival studies showed that the presence of NK and T lymphocytes is necessary for successful CY + IL-15 therapy. Experiments using knockout mice implied that either αβ or γδ T cells were required for an antitumor effect induced by CY + IL-15 therapy. However, mice lacking in both αβ and γδ T cells failed to respond to combination therapy. Cured B6 and αβ or γδ T cell-deficient mice were immune to rechallenge with 76-9, but not B16LM tumor. B cell-deficient mice showed a significant improvement in the survival rate both after CY and combination CY + IL-15 therapy compared with normal B6 mice. Overall, the data suggest that the interaction of NK cells with tumor-specific αβ or γδ T lymphocytes is necessary for successful therapy, while B cells appear to suppress the antitumor effects of CY + IL-15 therapy.