Interleukin 15: biology and relevance to human disease

Strategies to enhance T-cell reconstitution in immunocompromised patients

Immune deficiency, together with its associated risks such as infections, is becoming an increasingly important clinical problem owing to the ageing of the general population and the increasing number of patients with HIV/AIDS, malignancies (especially those treated with intensive chemotherapy or radiotherapy) or transplants (of either solid organs or haematopoietic stem cells). Of all immune cells, T cells are the most often affected, leading to a prolonged deficiency of T cells, which has important clinical consequences. Accordingly, strategies to improve the recovery and function of T cells, as we discuss here, should have a direct impact on reducing the morbidity and mortality of many patients and should increase the efficacy of therapeutic and prophylactic vaccinations against microbial pathogens or tumours.

Interleukin-15-deficient mice develop protective immunity to Toxoplasma gondii

Previous studies have suggested an important role for interleukin-15 (IL-15) in resistance to and memory for Toxoplasma gondii infection. The studies presented here reveal that IL-15 is not required for infection-induced expansion of NK or CD8+ T cells. Furthermore, IL-15-/- mice develop long-term protective immunity to this pathogen.

Targeting IL-15 receptor-bearing cells with an antagonist mutant IL-15/Fc protein prevents disease development and progression in murine collagen-induced arthritis

It has been suggested that the inflammatory cytokine IL-15 plays an important role in the development of several autoimmune diseases, including rheumatoid arthritis. We have generated a unique lytic and antagonistic IL-15 mutant/Fcgamma2a fusion protein (CRB-15) that targets the IL-15R. In the present study we examined the effects of targeting the IL-15R on the prevention and treatment of collagen-induced arthritis (CIA) in mice and probed the possible mechanisms of action of this IL-15 mutant/Fcgamma2a protein. Upon immunization with type II collagen, DBA/1 mice develop severe articular inflammation and destruction. Treatment of DBA/1 mice with a brief course of CRB-15 at the time of type II collagen challenge markedly inhibited the incidence and severity of arthritis. Moreover, in animals with ongoing established arthritis, treatment with CRB-15 effectively blocked disease progression compared with that in control-treated animals. The therapeutic effect of CRB-15 on either disease development or disease progression is remarkably stable, because withdrawal of treatment did not lead to disease relapse. A detailed analysis revealed that treatment with CRB-15 decreased synovitis in the joints; reduced bone erosion and cartilage destruction; reduced in situ production of the proinflammatory cytokines TNF-alpha, IL-1beta, IL-6, and IL-17; and decreased the responder frequency of autoreactive T cells. Our study suggests that the effective targeting of IL-15R-triggered events with CRB-15 can be of therapeutic importance in the treatment of rheumatoid arthritis.

Coordinated induction by IL15 of a TCR-independent NKG2D signaling pathway converts CTL into lymphokine-activated killer cells in celiac disease

A major function of NKG2D linking innate and adaptive immunity is to upregulate antigen-specific CTL-mediated cytotoxicity in tissues expressing stress-induced NKG2D ligands, such as MIC, by coactivating TCR signaling. Here, we show that, under conditions of dysregulated IL15 expression in vivo in patients with celiac disease and in vitro in healthy individuals, multiple steps of the NKG2D/DAP10 signaling pathway leading to ERK and JNK activation are coordinately primed to activate direct cytolytic function independent of TCR specificity in effector CD8 T cells. These findings may not only explain previous reports of transformation of CTL into NK-like "lymphokine-activated killers" (LAK cells) under high doses of IL2 (a substitute for IL15) but may also have significant implications for understanding and treating immunopathological diseases.

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

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

Human neutrophils produce interferon gamma upon stimulation by interleukin-12

Interferon gamma (IFNgamma) is a Th1 cytokine mainly produced by T cells, NK cells and macrophages in response to interleukin (IL)-12. As polymorphonuclear neutrophils (PMN) have been shown to produce and to release numerous cytokines, in particular upon IL-12 stimulation, we investigated the ability of highly purified PMN to secrete IFNgamma. We found that PMN contained a small store of IFNgamma, and that this store was rapidly secreted upon stimulation by degranulating agents such as formyl peptides. Moreover, after a few hours of stimulation with appropriate agents, PMN synthesized IFNgamma. The effect of IL-12 was time- and concentration-dependent, and IL-12 combinations with IL-2, IL-15, IL-18 or LPS were highly synergistic. Cycloheximide inhibited IFNgamma release in such optimal conditions, confirming the ability of PMN to synthesize IFNgamma. IFNgamma synthesis was associated with an increase in specific mRNA content, pointing to a transcriptional mechanism. The IFNgamma produced by PMN was biologically active, as demonstrated by its ability to induce TNFalpha synthesis by PMN themselves or to induce IL-10 synthesis by peripheral blood mononuclear cells. These findings reveal a novel pathway of autocrine and paracrine PMN activation. They also identified a new role for IFNgamma, bridging innate and adaptive immune responses.

IL-15Ralpha is a negative regulator of TCR-activated proliferation in CD4+ T cells

Although IL-15 is known to be a T cell growth factor, the function in T cells of IL-15Ralpha, its high affinity receptor, remains unclear. We found that murine IL-15Ralpha(-/-) CD4(+) T cells hyperproliferated in response to TCR stimulation, in vitro and in vivo, and displayed a lower TCR activation threshold than wild-type CD4(+) T cells. TCR-induced activation of Zap70 and of the phospholipase C-gamma1-NFATp, Ras-ERK-c-Fos, and Rac-JNK-c-Jun pathways was all augmented in IL-15Ralpha(-/-) CD4(+) T cells. This in turn led to earlier IL-2Ralpha induction and higher IL-2 production, which most likely contribute to the hyperproliferation of IL-15Ralpha(-/-) CD4(+) T cells. Exogenous IL-15 reduced levels of TCR-activated signals, transcription factors, IL-2, and IL-2Ralpha, and division in wild-type CD4(+) T cells. These results reveal IL-15Ralpha to be a negative regulator for CD4(+) T cell activation and demonstrate a novel layer of regulation of TCR signaling by a cytokine system.

Mature myeloid dendritic cell subsets have distinct roles for activation and viability of circulating human natural killer cells

Natural killer (NK) cells are important effectors of innate immunity. In contrast to many studies of interleukin-2 (IL-2)-activated NK cells, the physiologic requirements for stimulating resting NK cells have only recently received attention. Given the emerging variety of dendritic cell (DC) types and their division of labor for stimulating immunity, we compared the capacity of monocyte-derived DCs (moDCs) with that of CD34+ hematopoietic progenitor cell (HPC)-derived dermal-interstitial DCs (DDC-IDCs) and Langerhans cells (LCs) to stimulate resting NK cells. MoDCs, and to a lesser extent CD34+ HPC-derived DDC-IDCs, directly stimulate NK-cell proliferation, CD56 up-regulation, and cytotoxicity. LCs, on the contrary, require exogenous IL-2 or IL-12 to activate NK cells, but they can maintain resting NK-cell viability and sustain NK-cell proliferation induced by moDCs. LCs do not secrete bioactive IL-12p70 but do produce significantly higher concentrations of IL-15 and IL-18 than either of the other 2 DC types. Despite secretion of IL-15, LCs lack IL-15R-alpha for surface presentation of IL-15. This together with the deficiency of IL-12p70 undermines any direct NK-cell activation by LCs. Hence, the principal myeloid DCs differ in critical ways regarding the stimulation of NK and T lymphocytes and could be used or targeted accordingly in DC-based immunotherapies.

Enhancement of the host immune responses in cutaneous T-cell lymphoma by CpG oligodeoxynucleotides and IL-15

Patients with advanced cutaneous T-cell lymphoma (CTCL) exhibit profound defects in cell-mediated immunity. Host immune functions appear to play an integral role in mediating disease-controlling responses in CTCL, therefore we investigated the effects of synthetic oligode-oxynucleotides with CpG motifs (CpG ODN), which have been recognized as immune stimulatory by virtue of activation of dendritic cells (DCs) following binding to Toll-like receptor (TLR) 9. Peripheral blood mononuclear cells (PBMCs) from patients with advanced CTCL (erythroderma with circulating malignant T cells) and healthy volunteers were cultured with either CpG-A or CpG-B ODN. Patients' PBMCs exhibited marked induction of interferon-alpha (IFN-alpha) release following culture with CpG-A. Similarly significant activation of NK cells and CD8 T cells occurred as assessed by up-modulation of CD69 expression and by natural killer lytic activity. Nevertheless, the PBMCs of patients exhibited blunted responses to CpG-A compared to healthy volunteers. In such cases, IL-15 was capable of producing levels of NK activation that were superior to CpG-A, while the combined effects of CpG-A plus IL-15 induced maximal activation of NK cells and further enhanced activation of CD8 T cells. These findings have important implications for the potential enhancement of antitumor immunity among patients with advanced CTCL.

Distinct homeostatic requirements of CD4+ and CD4- subsets of Valpha24-invariant natural killer T cells in humans

CD1d-restricted Valpha24-invariant natural killer T cells (iNKTs) are important in immunoregulation. CD4(+) and CD4(-) iNKTs develop with similar frequencies in murine thymus and depend on interleukin-15 (IL-15) in periphery. However, homeostatic requirements of iNKTs have not been analyzed in humans. We evaluated thymic production, peripheral dynamics, and functional maturation of human iNKTs. CD4(+) subset comprises 90% of iNKTs in mature thymocytes and cord blood (CB) but only 40% in adult blood. Using T-cell receptor excision circle (TREC) analysis, we directly measured in vivo replicative history of CD4(+) and CD4(-) iNKT cells. Compared to CD4(+), CD4(-) iNKTs contain fewer TRECs, express higher levels of IL-2Rbeta, and proliferate with higher rate in response to IL-15. In contrast, CD4(+) cells express higher levels of IL-7Ralpha and better respond to IL-7. Neither thymic nor CB iNKTs are able to produce cytokines unless they are induced to proliferate. Therefore, unlike in the mouse, human CD4(+) iNKTs are mainly supported by thymic output and limited peripheral expansion, whereas CD4(-) cells undergo extensive peripheral expansion, and both subsets develop their functions in periphery. These findings reveal important differences in homeostatic requirements and functional maturation between murine and human iNKTs that are to be considered for clinical purposes.

Suppressor of cytokine signaling-1 overexpression protects pancreatic beta cells from CD8+ T cell-mediated autoimmune destruction

In type 1 diabetes, cytokine action on beta cells potentially contributes to beta cell destruction by direct cytotoxicity, inducing Fas expression, and up-regulating class I MHC and chemokine expression to increase immune recognition. To simultaneously block beta cell responsiveness to multiple cytokines, we overexpressed suppressor of cytokine signaling-1 (SOCS-1). This completely prevented progression to diabetes in CD8(+) TCR transgenic nonobese diabetic (NOD) 8.3 mice without affecting pancreas infiltration and partially prevented diabetes in nontransgenic NOD mice. SOCS-1 appeared to protect at least in part by inhibiting TNF- and IFN-gamma-induced Fas expression on beta cells. Fas expression was up-regulated on beta cells in vivo in prediabetic NOD8.3 mice, and this was inhibited by SOCS-1. Additionally, IFN-gamma-induced class I MHC up-regulation and TNF- and IFN-gamma-induced IL-15 expression by beta cells were inhibited by SOCS-1, which correlated with suppressed 8.3 T cell proliferation in vitro. Despite this, 8.3 T cell priming in vivo appeared unaffected. Therefore, blocking beta cell responses to cytokines impairs recognition by CD8(+) T cells and blocks multiple mechanisms of beta cell destruction, but does not prevent T cell priming and recruitment to the islets. Our findings suggest that increasing SOCS-1 expression may be useful as a strategy to block CD8(+) T cell-mediated type 1 diabetes as well as to more generally prevent cytokine-dependent tissue destruction in inflammatory diseases.

HIV-specific CD8+ T cells exhibit markedly reduced levels of Bcl-2 and Bcl-xL

Human immunodeficiency virus-specific CD8(+) T cells are highly sensitive to spontaneous and CD95/Fas-induced apoptosis, and this sensitivity may impair their ability to control HIV infection. To elucidate the mechanism behind this sensitivity, in this study we examined the levels of antiapoptotic molecules Bcl-2 and Bcl-x(L) in HIV-specific CD8(+) T cells from HIV-infected individuals. Bcl-2 expression was markedly decreased in HIV-specific CD8(+) T cells compared with CMV-specific and total CD8(+) T cells from HIV-infected individuals as well as total CD8(+) T cells from healthy donors. CD8(+) T cell Bcl-2 levels inversely correlated with spontaneous and CD95/Fas-induced apoptosis of CD8(+) T cells from HIV-infected individuals. HIV-specific CD8(+) T cells also had significantly lower levels of Bcl-x(L) compared with CMV-specific CD8(+) T cells. Finally, IL-15 induces both Bcl-2 and Bcl-x(L) expression in HIV-specific and total CD8(+) T cells, and this correlated with apoptosis inhibition and increased survival in both short- and long-term cultures. Our data indicate that reduced Bcl-2 and Bcl-x(L) may play an important role in the increased sensitivity to apoptosis of HIV-specific CD8(+) T cells and suggest a possible mechanism by which IL-15 increases their survival.

Monocyte surface-bound IL-15 can function as an activating receptor and participate in reverse signaling

IL-15 is a short chain, four-alpha helix cytokine that shares some biological function with IL-2. One striking difference between IL-2 and IL-15 is the ability of monocytes to express IL-15 on their cell surface after activation. In the current study we have investigated the ability of human monocyte cell surface IL-15 to participate in reverse signaling. Cross-linking anti-IL-15 Abs were used as a surrogate ligand for surface IL-15 engagement. Ligation of cell surface-expressed IL-15 induced monocyte adhesion that required the activity of small m.w. GTPases. Reverse signals through surface IL-15 activated the Rho-GTPase Rac3. In addition, engagement of cell surface IL-15 was found to activate a number of signaling pathways, including both extracellular signal-regulated kinase 1/2 and p38, and resulted in the secretion of IL-8. IL-8 production required mitogen-activated protein kinase activity. Thus, the current study has established that cell surface IL-15 is more than just a ligand; it can function as a receptor and participate in reverse signaling that results in cellular adhesion and production of inflammatory cytokines.

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.

IL-2 and IL-15 receptor alpha-subunits are coexpressed in a supramolecular receptor cluster in lipid rafts of T cells

The private alpha-chains of IL-2 and IL-15 receptors (IL-2R and IL-15R) share the signaling beta- and gamma(c)-subunits, resulting in both common and contrasting roles of IL-2 and IL-15 in T cell function. Knowledge of the cytokine-dependent subunit assembly is indispensable for understanding the paradox of distinct signaling capacities. By using fluorescence resonance energy transfer and confocal microscopy, we have shown that IL-2R alpha, IL-15R alpha, IL-2/15R beta and gamma(c)-subunits, as well as MHC class I and II glycoproteins formed supramolecular receptor clusters in lipid rafts of the T lymphoma line Kit 225 FT7.10. Fluorescence crosscorrelation microscopy demonstrated the comobility of IL-15R alpha with IL-2R alpha and MHC class I. A model was generated for subunit switching between IL-2R alpha and IL-15R alpha upon the binding of the appropriate cytokine resulting in the formation of high-affinity heterotrimeric receptors. This model suggests a direct role for the alpha-subunits, to which no definite function has been assigned so far, in tuning cellular responses to IL-2 or IL-15. In addition, both alpha-chains were at least partially homodimerized/oligomerized, which could be the basis of distinct signaling pathways by the two cytokines.

Effect of levodopa on interleukin-15 and RANTES circulating levels in patients affected by Parkinson's disease

Parkinson's disease (PD) is an extra-pyramidal neurodegenerative disorder, in which alterations of the immune system are involved. Interleukin (IL)-15 stimulates cellular immune response and induces growth and differentiation of various immune cells. RANTES, promoting leukocyte infiltration to sites of inflammation, mediates the trafficking and homing of immune cells. To clarify the potential effect of levodopa on the immunological network of PD, we analyzed IL-15 and RANTES serum levels in PD patients, treated or not with levodopa, and in healthy donors. Levodopa-treated patients showed significantly higher IL-15 and RANTES circulating levels with respect to healthy controls and higher, although not significantly, levels with respect to untreated patients. So, we hypothesize that the immunological alterations found in PD may be linked, at least in part, to levodopa therapy.

Natural, Proteolytic Release of a Soluble Form of Human IL-15 Receptor α-Chain That Behaves as a Specific, High Affinity IL-15 Antagonist

IL-15 and IL-2 are two structurally and functionally related cytokines whose high affinity receptors share the IL-2R beta-chain and gamma-chain in association with IL-15R alpha-chain (IL-15R alpha) or IL-2R alpha-chain, respectively. Whereas IL-2 action seems restricted to the adaptative T cells, IL-15 appears to be crucial for the function of the innate immune responses, and the pleiotropic expression of IL-15 and IL-15R alpha hints at a much broader role for the IL-15 system in multiple cell types and tissues. In this report, using a highly sensitive radioimmunoassay, we show the existence of a soluble form of human IL-15R alpha (sIL-15R alpha) that arises from proteolytic shedding of the membrane-anchored receptor. This soluble receptor is spontaneously released from IL-15R alpha-expressing human cell lines as well as from IL-15R alpha transfected COS-7 cells. This release is strongly induced by PMA and ionomycin, and to a lesser extent by IL-1 beta and TNF-alpha. The size of sIL-15R alpha (42 kDa), together with the analysis of deletion mutants in the ectodomain of IL-15R alpha, indicates the existence of cleavage sites that are proximal to the plasma membrane. Whereas shedding induced by PMA was abrogated by the synthetic matrix metalloproteinases inhibitor GM6001, the spontaneous shedding was not, indicating the occurrence of at least two distinct proteolytic mechanisms. The sIL-15R alpha displayed high affinity for IL-15 and behaved as a potent and specific inhibitor of IL-15 binding to the membrane receptor, and of IL-15-induced cell proliferation (IC(50) in the range from 3 to 20 pM). These results suggest that IL-15R alpha shedding may play important immunoregulatory functions.

Differential diagnosis of pleural effusions by fuzzy-logic-based analysis of cytokines

Pleural effusions can be caused by highly different underlying diseases and are characterized by complex interactions of various local and circulating cells as well as numerous soluble parameters like interleukins (IL). Knowledge about this complex network could help to indicate underlying disease. Therefore, we have investigated immunoreactive concentrations of IL-4, IL-6, IL-11, IL-15, IL-17, IL-18, and tumor necrosis factor-alpha (TNF-alpha) in pleural effusions and peripheral blood from patients with tuberculosis, bronchial carcinoma and other carcinomas as well as congestive heart failure (CHF) and pneumonias. To determine the value of cytokine measurement for differential diagnosis, statistical and fuzzy-logic methods were applied. Quantitative analysis showed high concentrations of IL-6 and IL-11 only in pleural effusions. IL-15, IL-17, IL-18 and TNF-alpha could be detected also in blood plasma. Lowest amounts were detected in CHF indicating the non-inflammatory origin of effusions. Statistical analysis did not provide evidence for diagnostic relevance of singular cytokines. Fuzzy-logic analysis was able to assign patients to the correct diseases with 80% accuracy using IL-6 and IL-15 measurement. Our results confirm the pathogenetic role of these cytokines in pleural effusions. Fuzzy-logic-based procedures may help to characterize and distinguish effusions of unknown origin even in small patient groups.

Unique phenotype of human uterine NK cells and their regulation by endogenous TGF-β

Natural killer (NK) cells are a major population of lymphocytes in the human endometrium (EM), and NK cells can be a significant source of cytokines that alter local immune responses. The aim of this study was to determine the expression of NK cell receptors in situ and to test whether uterine NK (uNK) cells produce cytokines and how this activity may be regulated by transforming growth factor-beta (TGF-beta). We observed that human uNK cells were CD56+, CD3-, CD57-, CD9+, CD94+, killer inhibitory receptor+, and CD16+/- in situ by confocal microscopy. We examined cytokine production by uNK cells and uNK cell clones derived from human EM. Stimulation of uNK cells with interleukin (IL)-12 and IL-15, both of which are expressed in the human EM, induced interferon-gamma (IFN-gamma) and IL-10 production. IFN-gamma production by uNK cell clones was completely inhibited by TGF-beta1 in a dose-dependent manner with an inhibitory concentration 50% value of 20 pg/ml. IL-10 secretion by uNK cell clones was also inhibited by TGF-beta1 at similar concentrations. Furthermore, blocking endogenous TGF-beta in fresh human endometrial cell cultures increased the production of IFN-gamma by uNK cells. These data indicate that uNK cells have a unique phenotype that is distinct from blood NK cells. Further, data demonstrate that uNK cells can produce immunoregulatory cytokines and that inhibition of uNK cells by locally produced TGF-beta1 is a likely mechanism to regulate NK cell function in the human EM.

Enhancement of human immunodeficiency virus type 1-DNA vaccine potency through incorporation of T-helper 1 molecular adjuvants

It is clear that the development of a safe and effective vaccine for human immunodeficiency virus type 1 (HIV-1) remains a crucial goal for controlling the acquired immunodeficiency syndrome epidemic. At present, it is not clear what arm of the immune response correlates with protection from HIV-1 infection or disease. Therefore, a strong cellular and humoral immune response will likely be needed to control this infection. Among different vaccine alternatives, DNA vaccines appeared more than a decade ago, demonstrating important qualities of inducing both humoral and cellular immune responses in animal models. However, after several years and various clinical studies in humans, supporting the safety of the HIV-DNA vaccine strategies, it has become clear that their potency should be improved. One way to modulate and enhance the immune responses induced by a DNA vaccine is by including genetic adjuvants such as cytokines, chemokines, or T-cell costimulatory molecules as part of the vaccine itself. Particularly, vaccine immunogenicity can be modulated by factors that attract professional antigen-presenting cells, provide additional costimulation, or enhance the uptake of plasmid DNA. This review focuses on developments in the coadministration of molecular adjuvants for the enhancement of HIV-1 DNA-vaccine potency.

Identification of an Interleukin-15α Receptor-binding Site on Human Interleukin-15

To identify the epitopes in human interleukin-15 (IL-15) that are responsible for binding to the interleukin-15 receptor alpha chain, antibody and receptor mapping by peptide scanning and site-directed mutagenesis was used. By using peptide scanning, we identified four regions in IL-15. The first region ((85)CKECEELEEKN(95)) is located in the C-D loop and is recognized by a set of non-inhibitory antibodies. The second region ((102)SFVHIVQMFIN(112)) is located in helix D and is recognized by two antibodies that are inhibitory of IL-15 bio-activity but not of IL-15 binding to IL-15Ralpha. The two remaining regions react with a recombinant soluble form of the IL-15Ralpha; the first ((44)LLELQVISL(52), peptide 1) corresponds to a sequence located in the B-helix and the second ((64)ENLII(68), peptide 2) to a sequence located in helix C. The latter is also contained in the epitope recognized by an antibody (monoclonal antibody B-E29) that prevents IL-15 binding to IL-15Ralpha. By site-directed mutagenesis, we confirmed that residues present in peptide 1 (Leu-45, Glu-46, Val-49, Ser-51, and Leu-52) and peptide 2 (Leu-66 and Ile-67) are involved in the binding of IL-15 to IL-15Ralpha. Furthermore, the results presented indicate that residues in the second peptide (Glu-64, Asn-65, and Ile-68) participate in IL-2Rbeta recruitment. This finding could have implications for the dynamics of receptor assembly. These results also indicate that the modes of interaction of IL-15 and IL-2 with their respective alpha chains are not completely analogous. Finally, some of the IL-15 mutants generated in this study displayed agonist or antagonist properties and may be useful as therapeutic agents.

Interleukin-15 enhances human neutrophil phagocytosis by a Syk-dependent mechanism: importance of the IL-15Rα chain

Interleukin-15 (IL-15) is a cytokine that possesses interesting, potential therapeutic properties. However, based on several parameters including activation of neutrophils, it is also recognized as a proinflammatory cytokine. The mechanisms by which IL-15 activates human neutrophil functions are not fully understood. Although these cells express a functional IL-15 receptor (IL-15R) composed of IL-15Ralpha, IL-2/15Rbeta (CD122), and gamma(c) (CD132) subunits, the role of each receptor component has not been investigated in IL-15-induced human neutrophil responses. In the present study, fluorescein-activated cell sorter analysis revealed that the ability of IL-15 to enhance neutrophil phagocytosis is not a result of increased expression of IL-15Ralpha, CD122, or CD132 on the neutrophil cell surface. Pretreatment of neutrophils with specific antibodies to IL-15Ralpha, CD122, or CD132 was found to inhibit phagocytosis of opsonized-sheep red blood cells by nearly 40%, 21%, and 27%, respectively. As expected, pretreatment of neutrophils with anti-IL-2Ralpha (CD25) had no effect. Pretreatment of cells with the Syk inhibitor piceatannol was found to significantly inhibit the ability of IL-15 to enhance phagocytosis. In addition, IL-15 was found to induce tyrosine phosphorylation of Syk that was largely inhibited by pretreating cells with piceatannol. Moreover, we found that Syk kinase is physically associated with IL-15Ralpha. We conclude that IL-15R enhances neutrophil phagocytosis by a Syk-dependent mechanism and that the IL-15Ralpha chain plays a key role in mediating this response, at least by interacting with Syk kinase.

Antiviral activity against CMV-infected fibroblasts in pediatric patients transplanted with CD34+-selected allografts from alternative donors

Human cytomegalovirus (HCMV) remains a cause of serious infectious complications after allogeneic transplantation of hematopoietic stem cells, especially in recipients of T-cell-depleted grafts. Here we investigated the antiviral activity of natural killer (NK) cells from healthy donors (n = 8) as well as of mononuclear cells (MNC) from transplanted pediatric patients (n = 11) who had received CD34(+) selected (and thus T-cell-depleted) stem cells from unrelated and mismatched related donors. Allogeneic human fibroblasts infected with HCMV laboratory strain AD169 for 5 days were used as targets in a 2-h cytotoxicity assay. Downregulation of human leukocyte antigen class I and upregulation of the adhesion molecules CD54 (ICAM-1) and CD58 (LFA-3) were observed after infection. In this experimental setting, NK cells from healthy donors exerted no specific lysis. However, antibody-dependent cellular cytotoxicity (ADCC) mediated by human anti-CMV IgG (cytoglobin) as well as stimulation with low-dose interleukin-(IL)-2 or IL-15 enhanced lysis markedly. MNC from two thirds of the patients (7/11) were capable of lysing infected targets without stimulation. Here also, lytic activity was significantly increased by IL-2 or IL-15, used in combination with ADCC. In contrast, 4/11 patients exerted no lysis. The observed antiviral activity may contribute to the low incidence of CMV DNAemia (29% at day 100, detected by polymerase chain reaction) in the whole group of our patients who have been transplanted with CD34(+)-selected allografts since 1995. Furthermore, our data suggest a potential benefit of using low-dose IL-2 or IL-15, also combined with anti-CMV immunoglobulinG, for immune modulation in CMV disease.

Blood mononuclear cell gene expression profiles characterize the oxidant, hemolytic, and inflammatory stress of sickle cell disease

In sickle cell disease, deoxygenation of intra-erythrocytic hemoglobin S leads to hemoglobin polymerization, erythrocyte rigidity, hemolysis, and microvascular occlusion. Ischemia-reperfusion injury, plasma hemoglobin-mediated nitric oxide consumption, and free radical generation activate systemic inflammatory responses. To characterize the role of circulating leukocytes in sickle cell pathogenesis we performed global transcriptional analysis of blood mononuclear cells from 27 patients in steady-state sickle cell disease (10 patients treated and 17 patients untreated with hydroxyurea) compared with 13 control subjects. We used gender-specific gene expression to validate human microarray experiments. Patients with sickle cell disease demonstrated differential gene expression of 112 genes involved in heme metabolism, cell-cycle regulation, antioxidant and stress responses, inflammation, and angiogenesis. Inducible heme oxygenase-1 and downstream proteins biliverdin reductase and p21, a cyclin-dependent kinase, were up-regulated, potentially contributing to phenotypic heterogeneity and absence of atherosclerosis in patients with sickle cell disease despite endothelial dysfunction and vascular inflammation. Hydroxyurea therapy did not significantly affect leukocyte gene expression, suggesting that such therapy has limited direct anti-inflammatory activity beyond leukoreduction. Global transcriptional analysis of circulating leukocytes highlights the intense oxidant and inflammatory nature of steady-state sickle cell disease and provides insight into the broad compensatory responses to vascular injury.

IL-15 and cognate antigen successfully expand de novo-induced human antigen-specific regulatory CD4+ T cells that require antigen-specific activation for suppression

An important prerequisite in using regulatory T cells for immunotherapy is their ex vivo expansion without loss of suppressor function. Human anergic regulatory T cells are expandable by Ag-specific stimulation in the presence of IL-2. IL-15, like IL-2, is a T cell growth factor that, in contrast to IL-2, stimulates survival of T cells. In this study, we examined whether IL-15 could be exploited as a superior growth factor of human CD4(+) anergic regulatory T cells that were generated by costimulation blockade. Next, IL-15, as compared with IL-2, was investigated with respect to expansion and function of these regulatory T cells. Optimal expansion required cognate allogeneic stimulation in the presence of exogenous IL-15. IL-15 resulted in enhanced survival that was paralleled by an increased number of Bcl-2-expressing cells. Moreover, IL-15 induced a distinct type of anergy characterized by hyperreactivity to IL-15, resulting in improved expansion. This is likely attributed to increased propensity of these cells to up-regulate both alpha- and gamma-chains of the IL-2 and IL-15 receptor. Notably, IL-15-expanded regulatory CD4(+) T cells suppressed both naive and memory T cells in a superior way. Immunosuppression required alloantigen-specific stimulation and appeared gamma-irradiation resistant and independent of IL-10, TGFbeta, or CTLA-4 interactions. These regulatory T cells were stable suppressors, mediating bystander suppression upon TCR stimulation, but leaving recall responses unaffected in the absence of cognate Ag. Finally, human naturally occurring regulatory CD4(+)CD25(+) T cells appeared important in generating regulatory T cells by costimulation blockade. In conclusion, IL-15-expanded, de novo-induced human anergic regulatory CD4(+) T cells are of interest in Ag-specific immunotherapy.

Interleukin-15 production by monocyte-derived dendritic cells and T cell proliferation in HIV-infected patients with discordant response to highly active antiretroviral therapy

A discordant response to highly active antiretroviral therapy (HAART) occurs when CD4 T cell counts are stable or increased over time despite persistently detectable HIV-RNA levels. In order to identify immunological factors affecting discordant treatment responses, a total of 27 HIV-infected patients were studied: (a) 10 naive patients (mean CD4+ = 101.5 cells/microl; mean HIV-RNA = 4.8 log10 copies/ml); (b) seven responder patients (mean CD4+ = 908.9 cells/microl); and (c) 10 discordant patients (mean CD4+ = 396.1 cells/microl; mean HIV-RNA = 5.4 log10 copies/ml). Five healthy blood donors were included as HIV-seronegative controls. The following parameters were evaluated: interleukin (IL)-15 production by monocyte-derived dendritic cells (MDDC) after stimulation with lypopolysaccaride (LPS) and Candida albicans; recall and HIV-1-specific antigen lymphocyte proliferation (LP). Increased levels of IL-15 production by MDDC after stimulation with LPS and C. albicans were found both in discordant patients and responder patients. Conversely, a strong reduction of IL-15 levels was observed in naive patients. Discordant patients developed positive LP responses to C. albicans and HIV-1 p24. LP in response to C. albicans and HIV-1 p24 was also positive in responder patients. Decreased LP response was found in naive patients. In conclusion, HIV-infected patients with discordant viro-immunological responses to HAART present increased levels of IL-15 production by MDDC and enhanced recall and HIV-1-specific antigen LP responses, suggesting an improvement in indices of immune function.

Expansion of EBV latent membrane protein 2a specific cytotoxic T cells for the adoptive immunotherapy of EBV latency type 2 malignancies: influence of recombinant IL12 and IL15

BACKGROUND

EBV-associated malignancies with a Type II latency gene expression pattern, such as EBV-positive HD, or nasopharyngeal carcinoma, frequently express the EBV latency Ag LMP2a. Hence, they provide a potential target for adoptive immunotherapy using in vitro-generated LMP2a-specific cytotoxic T lymphocytes (CTL). In this study, LMP2a-specific CTL were specifically amplified and the influence of rIL12 and rIL15 on the culture outcome was tested.

METHODS

PBMC from donors were stimulated twice with autologous DC transduced with an adenovirus vector expressing LMP2a. This led to a significant expansion of LMP2a-tetramer-specific CTL, which were subsequently further expanded with autologous EBV-transformed B-lymphoblastoid cells (LCL). The addition of rIL12 and rIL15 to the standard IL2-containing culture medium enhanced the proliferation of LMP2a-specific CTL.

RESULTS

While rIL15 did not change the pattern of cytokines secreted by LMP2a-CTL, rIL12 enhanced the production of Th1/Tc1 cytokines, such as IFN-n, while suppressing the production of the Th2/Tc2 cytokine IL5.

DISCUSSION

Stimulation of CTL cultures with rIL12 or rIL15 will generate CTL more rapidly, facilitating the application of this approach for patients with these EBV-associated disorders.

IL-2 is not required for the initiation of CD8 T cell cycling but sustains expansion

Based primarily on in vitro data, IL-2 is believed to be the key cytokine for initiation of the cell cycle of activated T cells. However, the role of IL-2 remains unresolved for T cell responses in vivo. We examined whether the absence of IL-2-mediated signaling in CD8 T cells affected initiation of proliferation. Our results conclusively demonstrated that initial division of Ag-specific CD8 T cells following priming was IL-2 independent, regardless of the context in which Ag was presented. In contrast, the latter stage of the proliferative phase was IL-2-dependent, particularly in nonlymphoid tissues. Thus, activated CD8 T cells initially undergo IL-2-independent proliferation, but reach a critical juncture where the requirement for IL-2 as a growth factor gains prominence.

Differential expression of interleukins IL-13 and IL-15 in normal ovarian tissue and ovarian carcinomas

OBJECTIVE

To determine the temporal and spatial expression of interleukins (IL)-13 and IL-15 in ovarian carcinoma compared to normal ovarian tissue.

METHODS

Quantitative RT-PCR, ELISA and immunohistochemistry.

RESULTS

Q-RT-PCR, ELISA and immunohistochemical analysis indicates that IL-13 and IL-15 mRNA and protein are expressed in normal ovary at various phases of the menstrual cycle with immunoreactive proteins detected in granulosa/theca and luteal cells and to a lesser extent in stromal cells and surface epithelial cells. Compared to normal ovary, ovarian carcinoma expresses elevated levels of IL-13 and IL-15 mRNA, with higher IL-13 expression in primary vs. metastatic tumors. IL-13 and IL-15 protein expression was also higher in the tumor tissues compared to ascites. In normal ovary, ovarian tumors and ascites, the ratio of IL-13/IL-15 favored IL-13. Immunoreactive IL-13 and IL-15 proteins were localized primarily in the tumor cells and infiltrated inflammatory cells with increased intensity with disease stage.

CONCLUSION

Normal ovary and ovarian tumors express IL-13 and IL-15 and pattern of their expression in carcinomas suggests that these cytokines may function in various ovarian cellular activities including inflammatory/immune responses that are integrated cellular events taking place in normal ovary and ovarian tumors.

Interleukin-15 delays human neutrophil apoptosis by intracellular events and not via extracellular factors: role of Mcl-1 and decreased activity of caspase-3 and caspase-8

Interleukin-15 (IL-15) induces the de novo protein synthesis of intracellular polypeptides and delays neutrophil apoptosis by a mechanism that is still unclear. Herein, we investigated the potential antiapoptotic role of newly synthesized proteins released into the external milieu in IL-15-induced neutrophils. We found that IL-15 induces the de novo synthesis of an approximately 23-kDa protein, representing the predominant protein detected in the milieu, and identified it as IL-1 receptor antagonist (IL-1Ra) by Western blot and immunoprecipitation. We quantified IL-1Ra, IL-1alpha, and IL-1beta concentrations by enzyme-linked immunosorbent assay in intracellular and extracellular fractions from IL-15-induced neutrophils and found that IL-15 does not increase IL-1alpha or IL-1beta production but induces IL-1Ra release. Also, we demonstrated that IL-1Ra does not modulate apoptosis, even at a concentration 250 times greater than that measured in the external milieu. In contrast to granulocyte macrophage-colony stimulating factor, the supernatant harvested from IL-15-induced neutrophils was devoid of antiapoptotic activity. Addition of cycloheximide demonstrates that IL-15 delays apoptosis via de novo synthesis of intracellular proteins and that it increases myeloid cell differentiation factor-1 stability. We demonstrated also that IL-15 decreases the activity of caspase-3 and caspase-8, resulting in an inhibition of vimentin cleavage. Our results indicate that IL-15 can activate an anti-inflammatory loop, based on its ability to induce the synthesis of IL-1Ra by neutrophils. We conclude that IL-15 delays human neutrophil apoptosis by intracellular events and not via extracellular factors.

Autocrine/paracrine IL-15 that is required for type I IFN-mediated dendritic cell expression of MHC class I-related chain A and B is impaired in hepatitis C virus infection

We previously reported that monocyte-derived dendritic cells activate resting NK cells by expressing MHC class I-related chain A and B (MICA/B), ligands for NKG2D, in response to IFN-alpha, but the MICA/B expression was severely impaired in patients with chronic hepatitis C virus (HCV) infection. In the present study, we examined induction of MICA/B on DCs by various innate cytokines and found that DCs from either healthy donors or HCV-infected individuals, upon IL-15 stimulation, express MICA/B and can activate NK cells, which is solely dependent on MICA/B-NKG2D interaction. Of interest is the finding that IL-15- and type I IFN-mediated induction of MICA/B in healthy donors is completely inhibited when DCs are incubated in the presence of anti-IFN-alpha/betaR or anti-IL-15Ralpha, respectively, suggesting interdependent roles of these cytokines in MICA/B expression. Indeed, DCs produced IL-15 in response to type I IFN, whereas they directly produced IFN-beta, in response to IL-15, which was followed by the production of IFN-alpha. In HCV-infected individuals, type I IFN-mediated production of IL-15 was virtually absent, but IL-15-mediated production of type I IFN was not compromised, which is consistent with the distinct ability of these cytokines to induce MICA/B in these patients. The present study demonstrates that IL-15 and type I IFN lead to DC expression of MICA/B and subsequent DC activation of NK cells, which is critically dependent on each other's autocrine/paracrine effect, and suggests that impaired IL-15 production is one of the mechanisms of the aberrant response of DC to type I IFN in HCV-infected patients.

Interleukin-1 inhibits interleukin-15 production by progesterone during in vitro decidualization in human

Interleukin (IL)-15 is a novel cytokine that plays important roles in uterine natural killer cell function and one of the candidate genes that is upregulated during the window of implantation for human endometrium. IL-15 expression and production by human endometrial stromal cells (ESCs) is elevated during in vitro decidualization by progesterone (P). In the present study, we evaluated the effects of IL-1beta, a proinflammatory cytokine, on IL-15 production in ESCs. By enzyme-linked immunosorbent assay (ELISA), IL-1beta had no effect on IL-15 production from ESCs in short-term culture (for 24 h), whereas IL-1beta stimulated production of IL-8. However, using ELISA and Northern blot analyses we found that IL-1beta significantly inhibited P-induced IL-15 production and mRNA expression in long-term culture (for 12 days) of ESCs in vitro (P<0.01). This inhibition was not due to IL-1beta-mediated cytotoxicity, as ESCs cultured in the presence of IL-1beta showed no evidence of significant change in their viability. These results suggest that ovarian steroid hormones and IL-1beta regulate IL-15 mRNA expression and protein production in long-term culture, and that IL-1beta plays a role as a negative regulator of IL-15 production during decidualization in human endometrium.

IL-21: a novel IL-2-family lymphokine that modulates B, T, and natural killer cell responses

IL-21 is a recently described type I cytokine produced by activated CD4(+) T cells that profoundly affects the growth, survival, and functional activation of B, T, and natural killer lymphocytes in concert with other cytokines or activating stimuli. Structurally, IL-21 is predicted to display a 4-helix-bundle-type fold with significant homology to IL-2, IL-4, and IL-15 and mediates its biologic effects through a novel type I cytokine receptor, IL-21R, in conjunction with the common cytokine receptor gamma chain (gammac) of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors. As a new member of the gammac-dependent cytokine family, there is significant interest in IL-21, in part because of its potential to provide new insights into the immunologic phenotype caused by gammac deficiency. IL-21R knockout mice have been generated that have normal lymphoid cell development yet exhibit impaired production of the immunoglobulin IgG(1) and increased IgE responses after immunization. As expected for cytokines that use gammac, recent studies indicate that IL-21 induces Janus kinase 1 (JAK1) and JAK3 activation to initiate signal transduction, but unlike these other gammac-dependent cytokines, which predominantly activate signal transducer and activator of transcription 5 (STAT5), IL-21 preferentially activates STAT1 and STAT3. IL-21 potently enhances primary antigen responses and the effector functions of T and natural killer cells and stimulates IFN-gamma production alone or in concert with other cytokines. Thus, on the basis of primary structure, receptor composition, and biologic activities, IL-21 is a new IL-2-family cytokine that participates in both innate and adaptive immunity and might be important for the development of a T(H)1 immune response.

Resolution of psoriasis upon blockade of IL-15 biological activity in a xenograft mouse model

Psoriasis is a chronic inflammatory disease of the skin characterized by epidermal hyperplasia, dermal angiogenesis, infiltration of activated T cells, and increased cytokine levels. One of these cytokines, IL-15, triggers inflammatory cell recruitment, angiogenesis, and production of other inflammatory cytokines, including IFN-gamma, TNF-alpha, and IL-17, which are all upregulated in psoriatic lesions. To investigate the role of IL-15 in psoriasis, we generated mAb's using human immunoglobulin-transgenic mice. One of the IL-15-specific antibodies we generated, 146B7, did not compete with IL-15 for binding to its receptor but potently interfered with the assembly of the IL-15 receptor alpha, beta, gamma complex. This antibody effectively blocked IL-15-induced T cell proliferation and monocyte TNF-alpha release in vitro. In a human psoriasis xenograft model, antibody 146B7 reduced the severity of psoriasis, as measured by epidermal thickness, grade of parakeratosis, and numbers of inflammatory cells and cycling keratinocytes. These results obtained with this IL-15-specific mAb support an important role for IL-15 in the pathogenesis of psoriasis.

Correlation between interleukin-15 and granzyme B expression and acute lung allograft rejection

The levels of interleukin (IL)-15 and granzyme B mRNA expression have been correlated with acute rejection episodes of kidney and heart allografts. Thus, the purpose of this study was to determine whether a correlation exists between the expression of IL-15 and granzyme B and acute lung allograft rejection. Toward this, the levels of IL-15 and granzyme B mRNA expression were determined in bronchoalveolar lavage-derived alveolar macrophages and total cells, respectively, from lung transplant patients with stable lung allograft function and patients undergoing acute rejection episodes. The expression levels of IL-15 mRNA was significantly higher in the patients undergoing acute rejection as compared to patients with stable lung function (P=0.02). The expression levels of granzyme B mRNA was also significantly higher in the patients undergoing acute rejection as compared to patients with stable lung function (P=0.005). The Receiver-Operating-Characteristic curve demonstrated that acute rejection can be predicted with a sensitivity of 94% and specificity of 67% with the use of a cutoff value of 3.1 fg of granzyme B mRNA per microgram of total RNA (or 71% sensitivity and 75% specificity of a cutoff value of 9.1 fg/microg). These data indicate that IL-15 secreted by activated alveolar macrophages and granzyme B secreted by activated CD8+ cytotoxic T lymphocytes play important roles in the process of acute lung allograft rejection.

The novel JAK-3 inhibitor CP-690550 is a potent immunosuppressive agent in various murine models

JAK-3 has been shown to play a key role in cytokine signaling via gammac, e.g. IL-2, 4, 7, 9, 15, 21. The current study describes the immunosuppressive effects of CP-690550, a novel, small molecule inhibitor of JAK-3, in various murine models. In vitro, CP-690550 effectively inhibited a murine mixed lymphocyte reaction (MLR) (IC50= 91 nm). Mice chronically dosed with CP-690550 (1.5-15 mg/kg/day) demonstrated dose- and time-dependent alterations in lymphocyte subsets when examined by flow cytometry. The most dramatic change observed was a 96% reduction in splenic NK1.1 + TCRbeta- cell numbers following 21 days of treatment. Delayed-type hypersensitivity (DTH) responses in sensitized mice were reduced in a dose-dependent manner following treatment with the JAK-3 inhibitor (1.87-30 mg/kg, s.c.). Extended survival of neonatal Balb/c hearts implanted into the ear pinna of MHC mismatched C3H/HEN mice was observed with CP-690550 monotherapy (10-30 mg/kg/day), but improved upon combination with cyclosporin (10 mg/kg/day). These data support the participation of JAK-3 in various lymphocyte homeostatic functions in mature mice. Furthermore, the ability of CP-690550 to extend cardiac allograft survival in murine models suggests it may afford a new treatment for prevention of transplant rejection.

Mature dendritic cells can enhance CD8+ cell noncytotoxic anti-HIV responses: the role of IL-15

The CD8+ cell noncytotoxic anti-HIV response (CNAR) is associated with a long-term healthy clinical state in HIV-infected individuals. Over time CNAR is reduced concomitant with progression to disease. In studies to evaluate whether the interaction between CD8+ cells and dendritic cells (DCs) could increase CNAR, CD8+ cells from individuals who showed a decrease in this antiviral activity were cocultured with monocyte-derived dendritic cells matured with CD40 ligand. After coculture with these mature DCs, the CD8+ cells showed an increase in CNAR greater than that observed with CD8+ cells costimulated with CD3/CD28 antibodies. This antiviral response appeared to be mediated primarily by production of interleukin-15 (IL-15) by the mature DCs. Purified IL-15 also enhanced CNAR, whereas IL-12 showed no substantial effect. These studies provide another potential approach by which the immune system in HIV infection could be restored by cytokine therapy, particularly IL-15 administration.

Elevated serum interleukin-15 level in acute graft-versus-host disease after hematopoietic cell transplantation

OBJECTIVE

To correlate serum cytokine levels and the development of acute graft-versus-host disease (GVHD), the authors conducted a prospective study on serial measurements of interferon (IFN)-gamma and interleukin (IL)-10, IL-12 and IL-15.

METHODS

The cytokines were measured in 27 subjects by enzyme-linked immunosorbent assay serially for the first 2 months after hematopoietic cell transplantation.

RESULTS

Nineteen subjects with acute GVHD had significantly higher mean peak serum levels of IFN-gamma and IL-15 than the baseline levels at the start of conditioning. The peak level occurred soon after stem cell infusion and returned to the pretransplantation state in the second month. In contrast, there was lesser difference between the mean peak serum levels of IFN-gamma, IL-10, and IL-15 and the baseline level in the eight subjects without GVHD. The peak serum level for IL-15 was, in addition, significantly higher among GVHD subjects than those without GVHD in the first month posttransplantation. However, the level of IL-15 showed no correlation with the severity of GVHD.

CONCLUSIONS

These changes point to a possible role of systemic cytokine secretion in the development of acute GVHD. Elevated levels of IL-15 early in the posttransplant period could be a helpful laboratory predictor of acute GVHD.

Natural killer cells: emerging concepts in immunity to infection and implications for assessment of immunodeficiency

PURPOSE OF REVIEW

As the molecular networks that connect innate and adaptive immunity are untangled, the prominence of natural killer (NK) cells in host defense continues to emerge. Herein we highlight recent findings pertaining to NK cell development, trafficking, and interactions with other innate and adaptive immune cells in the context of predicting how NK cells may be involved in a wider range of clinical immunodeficiency.

RECENT FINDINGS

NK cells contribute vital roles in innate and adaptive immunity, especially in collaboration with dendritic cells (DC). Fascinating new details have been reported about cell surface integrins and receptors that regulate NK functions, as well as the cytokine/chemokine networks that provide for NK-DC interactions. Moreover, NK cells appear to play an important role in the attenuation or resolution of an immune response through either action against CD8 T cells or indirect control of certain DC. These findings shed important insights as to how NK cells and DC cooperate to control primary infections and shape the subsequent adaptive immune responses.

SUMMARY

Natural killer cells are heterogeneous lymphocytes that provide an essential function in host defense. NK cells respond early to microbial assault and interact with other cells of the innate immune system, but they recognize and intercept pathogenic infections through highly specific mechanisms that are similar to T cells. Thus, NK cells are positioned as a cellular bridge between innate and adaptive immunity. It is imperative, then, to include a careful assessment of NK cell populations and functions in most cases of suspected immunodeficiency.

IL-15 and IL-2 oppositely regulate expression of the chemokine receptor CX3CR1

The chemokine receptor CX3CR1 (CX3C chemokine receptor 1) is expressed in mouse blood on natural killer (NK) cells and on monocytes. Because interleukin-15 (IL-15) is an essential cytokine for NK cell development and maintenance, we hypothesized that it may induce CX3CR1 expression on this cell type. In contrast, we found that in primary mouse bone marrow-derived NK cells IL-15 specifically inhibited CX3CR1 protein and mRNA accumulation, whereas the related cytokine IL-2 did not inhibit but instead increased CX3CR1 expression. Consistent with this finding, intravenous injection of a single dose of recombinant IL-15 into C57BL/6 mice decreased steady-state CX3CR1 levels 24 hours after injection in freshly isolated peripheral blood mononuclear cells (PBMCs), splenocytes, and bone marrow cells, and treatment of mouse PBMCs with IL-15 in vitro inhibited CX3CL1 (ligand for CX3CR1)-induced chemotaxis. These data suggest that IL-15 may be a negative regulator of innate immunity by inhibiting CX3CR1 expression. These data also suggest that IL-15 inhibition of CX3CR1 may subvert potential cell immunotherapy strategies in which IL-15 is used to expand NK cell populations in vivo or ex vivo. Finally, our results provide additional evidence for differential signaling by IL-2 and IL-15, despite usage of common beta gamma c receptor chains.

Differential Roles for IL-15R α-Chain in NK Cell Development and Ly-49 Induction

IL-15Ralpha-deficient (IL-15Ralpha(-/-)) mice lack NK cells. However, when bone marrow (BM) progenitors from IL-15Ralpha(-/-) mice were cultured with IL-7, stem cell factor and flt3 ligand, followed by IL-15, they were able to differentiate into functional NK cells, indicating that IL-15Ralpha is not critical for NK cell development. Whereas NK cells generated in vitro from IL-15Ralpha(-/-) BM progenitors expressed CD94/NKG2, they failed to express Ly-49 receptors. In keeping with this, when IL-15Ralpha(-/-) BM cells were transferred into wild type recipients, they gave rise to NK cells in vivo, but with greatly reduced expression of Ly-49 receptors. Furthermore, the small numbers of NK cells found in IL-15(-/-) as well as IL-15Ralpha(-/-) but not flt3 ligand(-/-) mice expressed much lower levels of Ly-49 receptors than those from wild type mice. These results indicate a novel role for IL-15Ralpha-chain in Ly-49 induction on developing NK cells.

Reduced cumulative incidence of diabetes but not insulitis following administration of chimeric human IL-15-murine IgG2b in NOD mice

BACKGROUND

It has been recently demonstrated that apoptosis is involved in beta-cell destruction in the NOD mouse model of diabetes. The aim of the present study was to investigate whether IL-15, a cytokine involved in the modulation of the apoptotic process, is capable of modifying the natural history of diabetes and/or insulitis in pre-diabetic NOD mice. The rationale for the use of IL-15-IgG2b recombinant cytokine is related to its long half-life (28 +/- 4 h).

METHODS

At 10 weeks of age, 2 groups of 24 female mice were treated with single or multiple i.p. doses of IL-15-IgG2b respectively. As control, 2 groups of 24 age- and litter-matched female mice were injected intra-peritoneally with single or multiple doses of IgG2b immunoglobulin.

RESULTS

Diabetes incidence at 33 weeks of age was lower in the group of mice treated with multiple doses than in the control group (p = 0.03). The cumulative incidence of diabetes at 33 weeks of age between single-dose treated mice and the control group was similar. No significant differences in the calculated index of insulitis were observed in all treated and control mice.

CONCLUSIONS

We conclude that IL-15-IgG2b reduces the cumulative incidence of diabetes, without affecting the extent and severity of the insulitis process. Considering this and the well-defined anti-apoptotic effects of IL-15, we suggest that the reduction of diabetes incidence could be due to a down-regulation of beta-cell apoptosis.

IL-15 induces antigen-independent expansion and differentiation of human naive CD8+ T cells in vitro

Recent studies in mice have shown that although interleukin 15 (IL-15) plays an important role in regulating homeostasis of memory CD8+ T cells, it has no apparent function in controlling homeostatic proliferation of naive T cells. We here assessed the influence of IL-15 on antigen-independent expansion and differentiation of human CD8+ T cells. Both naive and primed human T cells divided in response to IL-15. In this process, naive CD8+ T cells successively down-regulated CD45RA and CD28 but maintained CD27 expression. Concomitant with these phenotypic changes, naive cells acquired the ability to produce interferon gamma (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha), expressed perforin and granzyme B, and acquired cytotoxic properties. Primed CD8+ T cells, from both noncytotoxic (CD45RA-CD27+) and cytotoxic (CD45RA+CD27-) subsets, responded to IL-15 and yielded ample numbers of cytokine-secreting and cytotoxic effector cells. In summary, all human CD8+ T-cell subsets had the ability to respond to IL-15, which suggests a generic influence of this cytokine on CD8+ T-cell homeostasis in man.

Cytokine tissue levels as markers of disease activity in pediatric Crohn disease

The mucosal immune system is overactivated in Crohn disease (CD) and viral infections have been associated with clinical exacerbations. To investigate the potential association between mucosal inflammation and the cytokines involved in the early response to viruses, we analyzed colonic tissue levels of IL-2Ralpha, interferon-alpha, and IL-15 in CD. Patients undergoing diagnostic colonoscopy were classified into controls (n = 22) and three CD groups based on the histologic severity of inflammation and clinical activity: a) severely active CD (n = 3); b) mild to moderately active CD (n = 14); and c) quiescent CD (n = 23). Rectal biopsies (two per patient) were homogenized and cytokine levels determined by ELISA kits. Statistical analysis was performed by ANOVA with Tukey and Scheffé tests. IL-2Ralpha levels were increased in the active CD group compared with the quiescent CD group: a) 405 +/- 87, b) 159 +/- 31, and c) 33 +/- 15 pg/mg DNA (p < 0.001). The latter group was similar to controls (39 +/- 20 pg/mg DNA). Furthermore, a linear correlation (r = 0.98) between IL-2Ralpha and disease activity (Van Hees index) was observed. IL-15 levels were also higher in active compared with quiescent CD and controls: a) 0.69 +/- 0.23 and b) 0.72 +/- 0.31 versus c) 0.28 +/- 0.21 and 0.28 +/- 0.14 pg/mg DNA for controls (p < 0.05). Interferon-alpha levels were undetectable in all samples. Our data suggest that IL-2Ralpha tissue levels correlate with CD activity. IL-15 is also overproduced in inflamed CD tissue. The lack of a parallel elevation of interferon-alpha does not support a role for viral induction of IL-15 in inflamed CD samples.

In vivo antitumor activity of NKT cells activated by the combination of IL-12 and IL-18

Interleukin-12 and IL-18 have been demonstrated to potentiate innate immunity in a variety of experimental tumor models, but the functional roles of NK and/or NKT cells and their mechanism of action in these models have not been fully addressed. Through adoptive transfer of NKT cells activated in vitro with a combination of IL-12 plus IL-18 (IL-12/IL-18 NKT) into syngeneic animals, we demonstrate in this study that IL-12/IL-18 NKT cells are essential and collaborate with the host's own NK cells in natural host immunity against the growth of ALC and MC57X syngeneic tumors. The relative roles of the adoptively transferred IL-12/IL-18 NKT cells and endogenous NK cells in host protection were first shown in normal C57BL/6 (B6) mice treated with anti-asialo GM1 Ab that selectively depletes NK cells; second, in B6.TCRJalpha281(-/-) mice specifically deficient for NKT cells; and third, in B6.scid mice that also lack NKT cells. Furthermore, by injecting normal B6 mice with anti-IL-2 and/or anti-IFN-gamma mAb, we could demonstrate that effective innate immunity against both types of syngeneic tumors was dependent on the production of IL-2 and IFN-gamma by the adoptively transferred NKT cells. In vitro studies confirmed both the secretion of IL-2 and IFN-gamma by the IL-12/IL-18-activated NKT cells and their collaborative role with NK cells for lysis of ALC and MC57X syngeneic tumor targets. This is the first description of an antitumor function of IL-12/IL-18 NKT cells adoptively transferred into syngeneic hosts that provides the basis for a new modality in the cellular immunotherapy of cancer.

Recent developments in the biology and therapy of T-cell and natural killer-cell lymphomas

PURPOSE OF REVIEW

T-cell/natural killer (T/NK)-cell lymphomas represent a group of poor-risk lymphoproliferative disorders that have only recently been recognized as distinct clinicopathologic entities. The average outcome with currently available therapy is substantially inferior to that of aggressive B-cell lymphomas. Significant gaps remain in our knowledge of their origin, diagnosis, and clinical spectrum. This review outlines recent developments in the biology and molecular genetics of these disorders, current diagnostic challenges, and future avenues for therapy.

RECENT FINDINGS

Several cancer-prone transgenic mouse models that develop predominantly T/NK-cell lymphomas have been produced in the past 2 to 3 years. These models point to an important role for chronic cytokine stimulation and for disruption of genes involved in the control of chromatin remodeling and maintenance of genome integrity in the pathogenesis of T-cell lymphomas. The recognition of T/NK-cell lymphomas has been greatly facilitated by the broad acceptance of standard diagnostic criteria and by the increasing availability of assays for the analysis of T-cell receptor rearrangement and a more precise definition of functional T/NK-cell subsets. New drugs with potential for use in T/NK-cell lymphomas, including monoclonal antibodies, tyrosine kinase inhibitors, synthetic retinoids, immunoconjugates, and immunosuppressive molecules with novel mechanisms of action are in the early phase of clinical investigation.

SUMMARY

Much remains to be learned in the pathogenesis, clinical spectrum, and optimal therapy of T/NK-cell lymphomas. The availability of animal models of disease, new diagnostic tools, and targeted drugs with novel mechanisms of action should lead to rapid progress in this group of malignancies in the near future.