Immune responses in interleukin-2-deficient mice

IFN-gamma decreases CTL generation by limiting IL-2 production: A feedback loop controlling effector cell production

IFN-gamma is produced by cytotoxic T lymphocytes (CTL) but can also decrease CTL generation. We used IFN-gamma-R1-deficient (GRKO) and IFN-gamma-deficient (GKO) mice to study the effects of IFN-gamma in MLC on the generation of CTL activity and CTL number, IL-2 production and cell proliferation. CTL activity was increased in MLC when GRKO responders or GKO stimulators and responders were used, compared to wild-type (WT) MLC. The number of cells displaying the CTL phenotype (CD3+, CD8+, CD25+) was also increased, accompanied by increased IL-2 production and proliferation. Combinations of WT or GRKO CD4+ T cells with WT or GRKO CD8+ T cells as responders showed that IFN-gamma mostly affects CD4+ T cells to limit CTL generation. Intracellular staining indicated that IL-2 production was largely by CD4+ T cells. Moreover, addition of IL-2 to WT responders mimicked GKO CTL generation and activity, whereas neutralizing IL-2 decreased CTL activity in GRKO and WT responders. Thus IFN-gamma reduces CTL generation in alloimmune responses largely by limiting proliferation of IL-2 producing CD4+ T cells. This creates a feedback loop in which effectors produce IFN-gamma that limits IL-2 production which in turn limits CTL generation.

IL-15-independent antiviral function of primary and memory CD8+ T cells

Memory CD8⁺ T cells are comprised of CD122^hi IL-15-dependent and CD122^lo IL-15-independent subsets. Induction and retention of IL-15-independent memory CD8⁺ T cells was assessed in IL-15^−/− and wild-type (wt) mice immunized with recombinant vaccinia virus (rVV) or Sindbis virus (rSIN) vectors expressing the identical foreign epitope. Both vectors induced epitope-specific CD8⁺ T cell expansion and function, independent of IL-15. Similar kinetics of rVV clearance confirmed effective CD8⁺ T cell function in IL-15^−/− mice. CD44^hi CD122^hi CD8⁺ T cells, mainly of the CD62L^−/lo phenotype, increased more dramatically and declined more rapidly in IL-15^−/− mice, independent of the vector. Rapid IL-15-independent memory CD8⁺ T cell expansion following challenge of immune mice compensated for the limited memory CD8⁺ populations in IL-15^−/− mice. However, despite expansion and expression of potent effector function, viral clearance was delayed in the absence of IL-15, coinciding with a rapid loss in cytolytic function.

Roles for common cytokine receptor gamma-chain-dependent cytokines in the generation, differentiation, and maturation of NK cell precursors and peripheral NK cells in vivo

NK cells differentiate in adult mice from bone marrow hemopoietic progenitors. Cytokines, including those that signal via receptors using the common cytokine receptor gamma-chain (gamma(c)), have been implicated at various stages of NK cell development. We have previously described committed NK cell precursors (NKPs), which have the capacity to generate NK cells, but not B, T, erythroid, or myeloid cells, after in vitro culture or transfer to a fetal thymic microenvironment. NKPs express the CD122 Ag (beta chain of the receptors for IL-2/IL-15), but lack other mature NK markers, including NK1.1, CD49b (DX5), or members of the Ly49 gene family. In this report, we have analyzed the roles for gamma(c)-dependent cytokines in the generation of bone marrow NKP and in their subsequent differentiation to mature NK cells in vivo. Normal numbers of NKPs are found in gamma(c)-deficient mice, suggesting that NK cell commitment is not dependent on IL-2, IL-4, IL-7, IL-9, IL-15, or IL-21. Although IL-2, IL-4, and IL-7 have been reported to influence NK cell differentiation, we find that mice deficient in any or all of these cytokines have normal NK cell numbers, phenotype, and effector functions. In contrast, IL-15 plays a dominant role in early NK cell differentiation by maintaining normal numbers of immature and mature NK cells in the bone marrow and spleen. Surprisingly, the few residual NK cells generated in absence of IL-15 appear relatively mature, expressing a variety of Ly49 receptors and demonstrating lytic and cytokine production capacity.

Interleukins 2 and 15 Regulate Ets1 Expression via ERK1/2 and MNK1 in Human Natural Killer Cells

Interleukins (IL)-2 and IL-15 regulate natural killer (NK) cell proliferation, survival, and cytolytic activity. Ets1 is a transcription factor expressed early in NK cell differentiation. Because IL-2Rbeta, IL-2Rgamma, IL-15, and Ets1 knock-out mice similarly lack NK cells, we explored a molecular connection between IL-2R signaling and Ets1. Here we report the post-transcriptional regulation of Ets1 by IL-2R signaling in human NK cells. IL-2 and IL-15 stimulation leads to increased Ets1 protein levels with no significant change in mRNA levels. Pulse and pulse-chase experiments show that IL-2 stimulation results in both a marked increase in the nascent translation of Ets1 and an increased protein half-life. Pharmacological inhibition of MEK specifically blocks IL-2- and IL-15-induced translation, whereas p38, phosphatidylinositol 3-kinase, and mTOR inhibitors had no effect on Ets1 levels. Fli1, an Ets family member, exhibited a different mechanism of regulation, illustrating the specificity of IL-2R beta and gamma subunit signaling on the regulation of Ets1 expression. Expression of a dominant negative form of MNK1, a regulator of the translation initiation factor eIF4E, blocks the expression of Ets1 as do the dominant negative forms of the common IL-2R beta and gamma chains. Expression of Ets1 is regulated similarly in normal peripheral human NK cells. Taken together, our findings provide a direct link between IL-2R subunit signaling and Ets1 expression and helps to explain the interdependence of the IL-2R subunits and Ets1 for NK cell development and function.

Jak3, severe combined immunodeficiency, and a new class of immunosuppressive drugs

The recent elucidation of the multiple molecular mechanisms underlying severe combined immunodeficiency (SCID) is an impressive example of the power of molecular medicine. Analysis of patients and the concomitant generation of animal models mimicking these disorders have quickly provided great insights into the pathophysiology of these potentially devastating illnesses. In this review, we summarize the discoveries that led to the understanding of the role of cytokine receptors and a specific tyrosine kinase, Janus kinase 3 (Jak3), in the pathogenesis of SCID. We discuss how the identification of mutations of Jak3 in autosomal recessive SCID has facilitated the diagnosis of these disorders, offered new insights into the biology of this kinase, permitted new avenues for therapy, and provided the rationale for a generation of a new class of immunosuppressants.

Reduced expression of IL-12 receptor beta2 and IL-18 receptor alpha genes in natural killer cells and macrophages derived from B6-mi/mi mice

The mi transcriptional factor (MITF) is a basic helix-loop-helix leucine zipper-type transcriptional factor. The mi mutant allele encodes an abnormal MITF, in which one out of four consecutive arginines is deleted in the basic domain. The VGA-9-tg (tg) allele is another mutant allele and considered to be a null mutant allele. C57BL/6 (B6)-mi/mi mice showed abnormal phenotypes of natural killer (NK) cells and macrophages, whereas B6-tg/tg mice did not. The expression levels of the genes for the interleukin-12 receptor (IL-12R) beta2 and IL-18Ralpha were reduced in both the NK cells and macrophages of B6-mi/mi mice, while the expression levels of the corresponding genes in B6-tg/tg mice were unaffected. The B6-mi/mi NK cells and B6-mi/mi macrophages showed impaired responses to stimulation with IL-12, IL-18, and IL-12 plus IL-18 stimulation. The abnormal NK cell and macrophage of B6-mi/mi mice appear to be due to decreased expression of the IL-12Rbeta2 and IL-18Ralpha genes.

Interleukin-2, Interleukin-15, and Their Roles in Human Natural Killer Cells

Natural killer (NK) cells are CD56+CD3- large granular lymphocytes that constitute a key component of the human innate immune response. In addition to their potent cytolytic activity, NK cells elaborate a host of immunoregulatory cytokines and chemokines that play a crucial role in pathogen clearance. Furthermore, interactions between NK and other immune cells are implicated in triggering the adaptive, or antigen-specific, immune response. Interleukin-2 (IL-2) and IL-15 are two distinct cytokines with partially overlapping properties that are implicated in the development, homeostasis, and function of NK cells. This review examines the pervasive effects of IL-2 and IL-15 on NK cell biology, with an emphasis on recent discoveries and lingering challenges in the field.

Frontline: An in-depth evaluation of the production of IL-2 by antigen-specific CD8 T cells in vivo

IL-2 is an important cytokine that is capable of inducing both proliferation and apoptosis of activated T cells. CD4 T cells are thought to be the major producers of IL-2, but CD8 T cells also produce copious amounts of this cytokine. However, our current understanding regarding the kinetics of IL-2 production by antigen-specific CD8 T cells, and the proportion of these cells that produce IL-2 in vivo, is extremely limited. We now demonstrate that virus-specific CD8 T cells initiate IL-2 production by 6 h post-infection and prior to cell division in vivo. Interestingly, peak levels of IL-2 production were achieved very early during the response and prior to the proliferative peak. We also show -- using transgenic mice expressing herpes simplex virus-1 thymidine kinase under the control of the IL-2 promoter -- that, unlike what has been reported for antigen-specific CD4 T cells, the majority of antigen-specific CD8 T cells produce IL-2 during primary as well as secondary immune responses in vivo.

The quantal theory of how the immune system discriminates between "self and non-self"

In the past 50 years, immunologists have accumulated an amazing amount of information as to how the immune system functions. However, one of the most fundamental aspects of immunity, how the immune system discriminates between self vs. non-self, still remains an enigma. Any attempt to explain this most intriguing and fundamental characteristic must account for this decision at the level of the whole immune system, but as well, at the level of the individual cells making up the immune system. Moreover, it must provide for a molecular explanation as to how and why the cells behave as they do. The "Quantal Theory", proposed herein, is based upon the "Clonal Selection Theory", first proposed by Sir McFarland Burnet in 1955, in which he explained the remarkable specificity as well as diversity of recognition of everything foreign in the environment. The "Quantal Theory" is built upon Burnet's premise that after antigen selection of cell clones, a proliferative expansion of the selected cells ensues. Furthermore, it is derived from experiments which indicate that the proliferation of antigen-selected cell clones is determined by a quantal, "all-or-none", decision promulgated by a critical number of cellular receptors triggered by the T Cell Growth Factor (TCGF), interleukin 2 (IL2). An extraordinary number of experiments reported especially in the past 20 years, and detailed herein, indicate that the T cell Antigen Receptor (TCR) behaves similarly, and also that there are several critical numbers of triggered TCRs that determine different fates of the T cells. Moreover, the fates of the cells appear ultimately to be determined by the TCR triggering of the IL2 and IL2 receptor (IL2R) genes, which are also expressed in a very quantal fashion. The "Quantal Theory" states that the fundamental decisions of the T cell immune system are dependent upon the cells receiving a critical number of triggered TCRs and IL2Rs and that the cells respond in an all-or-none fashion. The "Quantal Theory" accounts fully for the development of T cells in the thymus, and such fundamental cellular fates as both "positive" and "negative" selection, as well as the decision to differentiate into a "Regulatory T cell" (T-Reg). In the periphery, the "Quantal Theory" accounts for the decision to proliferate or not in response to the presence of an antigen, either non-self or self, or to differentiate into a T-Reg. Since the immune system discriminates between self and non-self antigens by the accumulated number of triggered TCRs and IL2Rs, therapeutic manipulation of the determinants of these quantal decisions should permit new approaches to either enhance or dampen antigen-specific immune responses.

Reflections on CD8 T-cell activation and memory

CD8 T cells contribute to clearance and long-term protection following acute infection with certain viruses, bacteria, and protozoa, and may play an important role in tumor immunity. Primary adaptive CD8 T-cell responses have been conceptually divided into four phases: activation, expansion, contraction, and memory. We summarize each phase of the response, and discuss recent advances in our understanding of the development and maintenance of CD8 T-cell memory.

Oncolysis of multifocal hepatocellular carcinoma in the rat liver by hepatic artery infusion of vesicular stomatitis virus

Hepatocellular carcinoma (HCC) is a lethal malignancy with poor prognosis and few effective treatments, as well as ever-increasing frequencies in the Western world. Viruses that replicate selectively in cancer cells hold considerable promise as novel therapeutic agents for the treatment of malignancy. Vesicular stomatitis virus (VSV) is a negative-strand RNA virus with intrinsic oncolytic specificity due to significantly attenuated antiviral responses in many tumor cells. The aim of this study was to evaluate the potential of VSV, administered via the hepatic artery, as an effective and safe therapeutic agent for treating "multifocal" HCC in the rat liver. Recombinant VSV vector expressing beta-galactosidase (rVSV-beta-gal) was generated by reverse genetics and infused into the hepatic artery of Buffalo rats bearing orthotopically implanted multifocal HCC. Access by the virus to multifocal HCC lesions in the liver, as well as the kinetic profiles of intratumoral viral replication and spread, was established by X-gal staining of liver and tumor sections. Plaque assays were also performed to determine the infectious viral yields in tumor and normal liver tissues. Pharmacotoxicology studies, including serum chemistries and proinflammatory cytokine production, as well as organ histopathology, were performed. Buffer- or vector-treated tumor-bearing rats were followed for survival and the results were analyzed by the Kaplan-Meier method and the log-rank test. Hepatic arterial infusion of rVSV-beta-gal at the maximum tolerated dose in tumor-bearing rats resulted in efficient viral transduction of multifocal HCC lesions in their livers, tumor-selective viral replication, and extensive oncolysis. Importantly, no significant vector-associated toxicities were noted and, in particular, no damage to the hepatic parenchyma was seen. Finally, survival of vector-treated rats was substantially prolonged over that of animals in the control treatment group (p < 0.028). Thus, hepatic arterial administration of VSV is both effective and safe in an orthotopic animal model of multifocal HCC. The results suggest that oncolytic VSV can be developed into an effective and safe therapeutic modality for patients with multifocal HCC in the future.

IL-2 induces a competitive survival advantage in T lymphocytes

The acquisition of long-term survival potential by activated T lymphocytes is essential to ensure the successful development of a memory population in the competitive environment of the lymphoid system. The factors that grant competitiveness for survival to primed T cells are poorly defined. We examined the role of IL-2 signals during priming of CD4(+) T cells in the induction of a long-lasting survival program. We show that Ag-induced cycling of CD4(+) IL-2(-/-) T cells is independent of IL-2 in vitro. However, IL-2(-/-) T cells failed to accumulate in large numbers and develop in effector cells when primed in the absence of IL-2. More importantly, Ag-activated IL-2(-/-) T cells were unable to survive for prolonged periods of time after adoptive transfer in unmanipulated, syngeneic mice. IL-2(-/-) T cells exposed to IL-2 signals during priming, however, acquired a robust and long-lasting survival advantage over cells that cycled in the absence of IL-2. Interestingly, this IL-2-induced survival program was required for long-term persistence of primed IL-2(-/-) T cells in an intact lymphoid compartment, but was unnecessary in a lymphopenic environment. Therefore, IL-2 enhances competitiveness for survival in CD4(+) T cells, thereby facilitating the development of a memory population.

Tolerance, not immunity, crucially depends on IL-2

Interleukin-2 (IL-2) was identified based on its potent T-cell growth-factor activity and is widely considered to be a key cytokine in T-cell-dependent immune responses. However, the main non-redundant activity of this cytokine centres on the regulation of T-cell tolerance, and recent studies indicate that a failure in the production of CD4(+)CD25(+) regulatory T cells is the underlying cause of autoimmunity in the absence of IL-2. In marked contrast to the importance of IL-2 in peripheral T-cell tolerance, T-cell immunity is readily elicited to various agents in the absence of IL-2 in vivo. Here, we discuss these findings and, in particular, the action of IL-2 on regulatory T cells and effector cells, and the targeting of IL-2 and/or the IL-2 receptor in clinical settings.

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.

IL-2, regulatory T cells, and tolerance

IL-2 is a potent T cell growth factor that for many years was assumed to amplify lymphocyte responses in vivo. Accordingly, IL-2 has been used clinically to enhance T cell immunity in patients with AIDS or cancer, and blocking Abs to the IL-2R are used to inhibit T cell responses against transplanted tissues. It was later shown in mice that, unexpectedly, disruption of the IL-2 pathway results in lymphoid hyperplasia and autoimmunity rather than immune deficiency, indicating that the major physiological function of IL-2 is to limit rather than enhance T cell responses. This apparent paradox has recently been resolved with the discovery that IL-2 is critical for the development and peripheral expansion of CD4(+)CD25(+) regulatory T cells, which promote self-tolerance by suppressing T cell responses in vivo. Our new understanding of IL-2 biology prompts a re-evaluation of how best to clinically manipulate this important immunoregulatory pathway.

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-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 expansion of umbilical cord blood mononuclear cell–derived natural killer cells dependent on the dose of interleukin-15 with Flt3L

OBJECTIVE

We investigated the effect of interleukin-15 (IL-15) with Flt3 ligand (Flt3L) on the expansion and activation of NK cells derived from umbilical cord blood mononuclear cells (UCB-MNCs).

MATERIALS AND METHODS

UCB-MNCs were cultured at 1 to 100 ng/mL of IL-15 + Flt3L (10 ng/mL) compared with 1 to 500 ng/mL of IL-2 + Flt3L (10 ng/mL). Cultured cells were assessed for surface marker and we calculated absolute number of NK cells and T cells. The cytotoxic activity was analyzed with purified NK cells.

RESULTS

After 2 weeks culture with 5 ng/mL of IL-15 + Flt3L, the fold inductions of absolute number of NK cells significantly increased to 20.9-fold +/- 9.3-fold of the number of NK cells on day 0 (p < 0.05), with 24.4-fold +/- 16.1-fold of T cells. But with 50 ng/mL of IL-15 + Flt3L, fold induction of NK cells decreased to 5.1-fold +/- 3.9-fold, while T cells showed 34.8-fold +/- 18.7-fold (n = 8). The proportion of NK vs T cells showed to be significantly higher (1.61 +/- 0.91) with 5 ng/mL of IL-15 than with 50 ng/mL of IL-15 (0.12 +/- 0.03). Such proportional change of NK/T cells could not be observed with IL-2. Immunophenotypes of CD56, CD16, LFA1, CD94, CD8, and perforin of cultured NK cells with 10 ng/mL of IL-15 + Flt3L showed the same pattern of those with 50 ng/mL of IL-2 + Flt3L. Cytotoxic activity against K562 of cultured NK cells resulted in the same level as adult peripheral blood (PB)-derived NK cells.

CONCLUSIONS

Higher induction of NK cells derived from UCB-MNCs was achieved by low dose (5 to 10 ng/mL) rather than high dose (> 50 ng/mL) of IL-15.

The immunobiology of natural killer cells and bone marrow allograft rejection

Natural killer (NK) cells mediate the acute rejection of bone marrow cell (BMC) allografts, but not solid tissue grafts, in lethally irradiated mice. However, the mechanisms underlying this capability for rejecting BMC remain unclear. NK cells express (1) inhibitory receptors specific for major histocompatibility complex (MHC) class I molecules and (2) activating receptors with diverse specificities. Inhibitory NK receptors confer to NK cells the ability to discriminate between MHC class I-positive and -negative target cells and are therefore involved in the control of NK cell tolerance to self, as well as in the elimination of cells that have downregulation of MHC class I molecules. Preclinical studies in mice have provided good evidence that subsets of NK cells that bear different combinations of both inhibitory and activating Ly49 receptors can interact with each other and target specific BMC rejection, as well as NK cell responses toward tumor cells. Recent clinical studies have also shown that the use of killer cell immunoglobulin-like receptor ligand incompatibility in patients with leukemia who received hematopoietic stem cell transplants correlated not only with the elimination of graft rejection, but also with eradication of tumor and prevention of graft-versus-host disease; this offers a significant advantage for survival. In this review, we attempt to bring together literature regarding the biology of NK cells and discuss the current issues in bone marrow transplantation and the potential clinical role of NK cell alloreactivity in the efficacy of this procedure for immunotherapy of cancer and infectious states.

Distinct pathways involving the FK506-binding proteins 12 and 12.6 underlie IL-2-versus IL-15-mediated proliferation of T cells

The molecular basis for the different roles of IL-2 and IL-15 in lymphocyte function has been poorly defined. Searching for differences that underlie the distinct T cell responses to the two cytokines, we observed a marked susceptibility of the IL-15-induced but not of the IL-2-induced proliferation to rapamycin despite a decrease of p70S6 kinase (p70S6K) activation by the drug in response to both cytokines. Activated splenic T lymphocytes deficient in the FK506-binding protein (FKBP) 12, a target of rapamycin activity, had reduced proliferation in response to IL-15 but not to IL-2. This decreased proliferation was accompanied by reduced activation of p70S6K and of the extracellular signal-regulated kinases (ERK) after IL-15 treatment. In contrast to FKBP12-/- cells, splenic FKBP12.6-/- T cells exhibited a decreased proliferative response to IL-2 in the presence of rapamycin without affecting p70S6K or ERK activation. Thus, IL-15 induces T cell proliferation mainly via FKBP12-mediated p70S6K activation. In contrast, IL-2 signaling involves multiple pathways that include at least one additional pathway that depends on FKBP12.6.

Cytokine requirements for the growth and development of mouse NK cells in vitro

Natural killer (NK) cells arise from immature progenitors present in fetal tissues and adult bone marrow, but the factors responsible for driving the proliferation and differentiation of these progenitors are poorly understood. Mouse NK cells had previously been thought not to express interleukin (IL)-2Ralpha chains, but we show here that immature and mature mouse NK cells express IL-2Ralpha chain mRNA and that low levels of IL-2Ralpha chains can be detected on the surface of immature and mature NK cells provided they are cultured in the absence of IL-2. Despite their potential expression of high-affinity IL-2 receptors, immature NK cells only proliferate if IL-2 is present at extremely high concentrations. Surprisingly, IL-15 can also only support the growth of immature NK cells at high, presumably nonphysiological concentrations. Although NK cells express mRNA for the high-affinity IL-15Ralpha chain, they also express a variety of alternately spliced transcripts whose protein products could potentially disrupt signaling through IL-15 receptors. The requirement for high concentrations of IL-2 and IL-15 suggests that if these cytokines play any role in the proliferative expansion of NK cells in vivo, they act indirectly via other cells or in cooperation with other factors. In support of the latter possibility, we report that the recently described cytokine IL-21 can markedly enhance the proliferation of immature (and mature) NK cells in the presence of doses of IL-2 and IL-15 that by themselves have little growth-promoting activity.

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.

Development and functions of natural killer cells

Over the last decade, progress in molecular and cellular biology and gene targeting techniques has removed veils from the mysteries of natural killer (NK) cell development and function. NK cells are derived from hematopoietic stem cells, for which stem cell factor or Flt3 ligand is required in the early stage of differentiation to NK cell progenitors. Interleukin 15 then plays a crucial role for differentiation and/or maturation of NK progenitors into functional NK cells. Several members of the zinc finger, ETS, and interferon regulatory factor transcription factor families are also involved in the lineage commitment of hematopoietic stem or progenitors into NK cells. Animal models as well as patients deficient in NK cells have provided formal evidence that NK cells play an important role in vivo for innate immunity against tumors and viral infections and for linkage to adaptive immunity. Moreover, recent studies have revealed novel human NK cell subsets in peripheral blood that have the phenotypical characteristics CD3- CD16+ CD56+ and CD3- CD16- CD56bright, which are mainly involved in cytotoxicity and cytokine-mediated immunoregulation, respectively.

Characterization and analysis of the proximal Janus kinase 3 promoter

Janus kinase 3 (Jak3) is a nonreceptor tyrosine kinase essential for signaling via cytokine receptors that comprise the common gamma-chain (gammac), i.e., the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. Jak3 is preferentially expressed in hemopoietic cells and is up-regulated upon cell differentiation and activation. Despite the importance of Jak3 in lymphoid development and immune function, the mechanisms that govern its expression have not been defined. To gain insight into this issue, we set out to characterize the Jak3 promoter. The 5'-untranslated region of the Jak3 gene is interrupted by a 3515-bp intron. Upstream of this intron and the transcription initiation site, we identified an approximately 1-kb segment that exhibited lymphoid-specific promoter activity and was responsive to TCR signals. Truncation of this fragment revealed that core promoter activity resided in a 267-bp fragment that contains putative Sp-1, AP-1, Ets, Stat, and other binding sites. Mutation of the AP-1 sites significantly diminished, whereas mutation of the Ets sites abolished, the inducibility of the promoter construct. Chromatin immunoprecipitation assays showed that histone acetylation correlates with mRNA expression and that Ets-1/2 binds this region. Thus, transcription factors that bind these sites, especially Ets family members, are likely to be important regulators of Jak3 expression.

Stochastic model of T cell proliferation: a calculus revealing IL-2 regulation of precursor frequencies, cell cycle time, and survival

The outcome of Ag exposure is dictated by complex regulation of T cell proliferation. The rates of proliferation and survival are altered by numerous signals that the cell receives and integrates to achieve a net response. We have illustrated previously how small changes in kinetic parameters can lead to large differences, even under conditions of saturating IL-2. In this study, we examine the effect of varying IL-2 concentration on T cell response and develop a model incorporating additional parameters of proliferation and survival. Strikingly, the proportion of cells that enter the first division, but not the time at which they enter, is dramatically altered by IL-2. Furthermore, the survival and average division time of cells in later divisions are also altered by IL-2 concentration. Together, the small simultaneous effects on these parameters result in large differences in total cell number. These results reveal how in vitro systems may exaggerate the contribution of IL-2, and thus how costimuli or additional helper cells that alter IL-2 concentration, even by relatively small amounts, will generate large in vitro differences in cell number and therefore appear obligatory. Furthermore, they illustrate how a quantitative model of T cell activation can clarify how complex signal integration is handled by T cells in situ, and therefore more appropriately aid development of a theory of behavior.

What does it take to make a natural killer?

We know how B and T cells develop, what they 'see' and the receptors they 'see with'. By contrast, and despite an unprecedented increase in the number of receptors and ligands known to regulate the activity of natural killer (NK) cells, we still have many questions regarding how these cells develop. Nevertheless, we are beginning to understand the transcriptional programmes of NK-cell maturation and the role of the effector functions of NK cells in the regulation of immune responses. An improved knowledge of NK-cell development in mice and humans might be useful to harness the power of these natural killers in the clinic to fight autoimmune diseases, infection and cancer.

Comparison of adjuvant efficacy of herpes simplex virus type 1 recombinant viruses expressing TH1 and TH2 cytokine genes

The adjuvant effects of cytokines in humoral and cell-mediated immunity to herpes simplex virus type 1 (HSV-1) have been examined in mice using HSV-1 recombinant viruses expressing murine interleukin-2 (IL-2), IL-4, or gamma interferon (IFN-gamma) gene. Groups of naive BALB/c mice were immunized intraperitoneally with one or three doses of the HSV-1 recombinant viruses expressing IL-2, IL-4, or IFN-gamma or with parental control virus. Despite similar replication kinetics, these three recombinant viruses elicited different immune responses to HSV-1 on immunization. Immunization with the recombinant virus expressing IL-4 elicited a humoral response of greater magnitude than immunization with the recombinant viruses expressing IL-2 or IFN-gamma or with parental virus. In contrast, immunization with recombinant virus expressing IL-2 elicited a higher cytotoxic T-cell response than immunization with viruses expressing IL-4 or IFN-gamma. Stimulation in vitro of splenocytes obtained from the mice immunized with UV-inactivated HSV-1 McKrae resulted in a T(H)1 pattern of cytokine expression irrespective of the recombinant virus used in the immunization. As observed for the parental virus, both CD4(+) and CD8(+) T cells contributed equally to the production of IL-2 by the splenocytes of mice immunized with any of the three recombinant viruses. However, the pattern of IFN-gamma production by CD4(+) and CD8(+) T cells differed according to the recombinant virus used. After lethal ocular challenge, all immunized mice were protected completely against death and manifestations of eye disease caused by HSV-1, which are typical responses in unimmunized mice. Mice immunized with IL-4-expressing virus cleared the virus from their eyes more rapidly than mice immunized with IL-2- or IFN-gamma-expressing virus. Taken together, our results suggest that, in contrast to IFN-gamma which did not exhibit an adjuvant effect, both IL-4 and IL-2 act as adjuvants in immunization with HSV, with IL-4 showing greater efficacy.

Cytokine control of memory T-cell development and survival

Evidence has accumulated that cytokines have a fundamental role in the differentiation of memory T cells. Here, we follow the CD8+ T cell from initial activation to memory-cell generation, indicating the checkpoints at which cytokines determine the fate of the T cell. Members of the common cytokine-receptor gamma-chain (gammac)-cytokine family--in particular, interleukin-7 (IL-7) and IL-15--act at each stage of the immune response to promote proliferation and survival. In this manner, a stable and protective, long-lived memory CD8+ T-cell pool can be propagated and maintained.

The pathophysiological role of cytokines

Cytokines are important hormonal mediators, produced in tissues undergoing defence, growth and repair processes. Infection and inflammation in particular result in a cascade of cytokine induction that acts to maintain tissue homeostasis. Most cytokines act within the injured tissues, although some have an endocrine action, recruiting distant tissues in defence of the tissue producing the cytokine and many are important for regulating acquired immunity in secondary lymphoid tissues. Induction of interleukin-1 (IL-1), tumour necrosis factor (TNF) and IL-6 highlight the way in which local tissue cytokine responses are induced and act. Since most cytokines act locally, cytokine measurement presents several difficulties. Only where cytokines (such as IL-6) have a systemic action are plasma cytokine concentrations really meaningful. The presence of cytokine antagonists and soluble cytokine receptors, often released in concert with their respective cytokine agonists, presents additional complexity to interpretation. Measurement and manipulation of cytokines can contribute towards an understanding of their pathophysiological role in both experimental and clinical settings. This includes measurement of plasma IL-6, which has striking relationships to tissue inflammation. Its value is exemplified by some recent studies of stroke patients, where IL-6 reflects not only the initial response but also clinical outcome and survival.

Efficient induction of primary and secondary T cell-dependent immune responses in vivo in the absence of functional IL-2 and IL-15 receptors

IL-2 and IL-15 are thought to be important cytokines for T cell-dependent immune responses. Mice deficient in IL-2, IL-2Ralpha, and IL-2Rbeta are each characterized by a rapid lethal autoimmune lymphoproliferative disorder that complicates their use in studies aimed at investigating the role of these cytokines and receptors for immune responses in vivo. We have previously characterized a novel transgenic (Tg) mouse on the IL-2Rbeta-/- genetic background (Tg-/- mice) that lacks autoimmune disease but still contains peripheral T cells that are nonresponsive to IL-2 and IL-15. In the present study, these mice were used to investigate the extent by which IL-2 and IL-15 are essential for T cell immunity in vivo. Tg-/- mice generated near normal primary and secondary Ab responses to OVA, readily mounted first and second set allogeneic skin graft rejection responses, and developed primary and recall CD8 T cell responses to vaccinia virus. However, Tg-/- mice generated a slightly lower level of IgG2a Abs to OVA, exhibited a somewhat delayed first set skin graft rejection response with lower allo-specific CTL, and developed a significantly lower number of IFN-gamma-producing vaccinia-specific CD8+ T cells. Thus, although T effector function is somewhat impaired, T cell immunity is largely functional in the absence of IL-2- and IL-15-induced signaling through IL-2Rbeta.

Regulation of lymphoid homeostasis by interleukin-15

Interleukin (IL)-15 is a member of the common gamma chain family of cytokines, and is closely related to IL-2. While these two cytokines share several important biological functions in vitro, recent mouse models have demonstrated unique roles for these two cytokines in supporting lymphoid homeostasis in vivo. IL-15 has been shown to regulate the homeostasis of both innate and adaptive immune cells, and this review will discuss several ways in which this pleiotropic cytokine may support lymphoid homeostasis.

Interleukin-15 and the regulation of lymphoid homeostasis

Interleukin-15 (IL-15) is a cytokine that plays unique roles in both innate and adaptive immune cell homeostasis. While early studies suggested that IL-15 resembled IL-2, more recent work suggests that IL-15 may play multiple unique roles in immune homeostasis befitting its pleiotropic expression pattern. This review will focus on recent studies that highlight some of these functions.

In vivo evidence for a dependence on interleukin 15 for survival of natural killer cells

Cellular homeostasis requires a balance between cell production, cell survival, and cell death. Production of natural killer (NK) cells from bone marrow precursor cells requires interleukin 15 (IL-15); however, very little is known about the factors controlling survival of mature NK cells in vivo. Because mice deficient in IL-15 (IL-15(-/-) mice) fail to develop NK cells, it is not known whether mature NK cells can survive in an environment lacking IL-15. We hypothesized that IL-15 might indeed be required for survival of mature NK cells in vivo. Freshly isolated NK cells labeled with 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester (CFSE) were adoptively transferred into IL-15(-/-) mice and littermate control (IL-15(+/-)) mice. Within 36 hours after transfer, NK cells were detected in both IL-15(-/-) and IL-15(+/-) mice; however, significantly more (P <.003) CFSE-positive (CFSE(+)) NK cells were found in control mice than in IL-15(-/-) mice. By 5 days, similar numbers of CFSE(+) NK cells were still easily detected in IL-15(+/-) mice, whereas no CFSE(+) NK cells survived in IL-15(-/-) mice. Furthermore, mice with severe combined immunodeficiency treated with the Fab fragment of a blocking antibody recognizing a signaling subunit of the IL-15 receptor, IL-2/15Rbeta, had a significant ( approximately 90%) loss of NK cells compared with control mice. Finally, NK cells from Bcl-2 transgenic mice that were adoptively transferred into IL-15(-/-) mice did survive. These results show conclusively that IL-15 is required for mature NK cell survival in vivo and suggest that IL-15 mediates its effect on NK cell survival by means of Bcl-2.

Relationships between early B- and NK-lineage lymphocyte precursors in bone marrow

Recent studies have demonstrated that lineage marker-negative (Lin(-)) c-kit(Lo) Flk-2/Flt3(+) IL-7R(+) Sca-1(Lo) CD27(+) Ly-6C(-) Thy-1(-)CD43(+) CD16/32(Lo/-) terminal deoxynucleotidyl transferase (TdT)(+) cells in murine bone marrow are functional lymphocyte precursors. However, it has not been clear if this is an obligate intermediate step for transit of multipotential hematopoietic stem cells to natural killer (NK) cells. We have now used serum-free, stromal cell-free cultures to determine that NK progenitors are enriched among an estrogen-regulated, c-kit(Lo) subset of the Lin(-) fraction. However, several experimental approaches suggested that this population is heterogeneous and likely represents a stage where B and NK lineages diverge. Although most B-cell precursors were directly sensitive to estrogen in culture, much of the NK-cell precursor activity in that fraction was hormone resistant. B-lineage potential was largely associated with interleukin 7 receptor alpha (IL-7R(alpha)) expression and was selectively driven in culture by IL-7. In contrast, many NK precursors did not display detectable amounts of this receptor and their maturation was selectively supported by IL-15. Finally, single-cell experiments showed that the Lin(-) c-kit(Lo) fraction contains a mixture of B/NK, B-restricted, and NK-restricted progenitors. Two-step culture experiments revealed that NK precursors become hormone resistant on or before acquisition of CD122, signaling commitment to the NK lineage. CD45R is preferentially, but not exclusively, expressed on maturing B-lineage cells. Production of these 2 blood cell types is regulated in bone marrow by common and then independent mechanisms that can now be studied with greater precision.

T helper cells, IL-2 and the generation of cytotoxic T-cell responses

CD8 T-cell immunity is thought to require helper activity derived from CD4 T cells. Nevertheless, under some circumstances, effective CD8-dependent T-cell responses occur in vivo without CD4 T-cell help. Several recent papers help to explain this paradox and lead to a refined view concerning the role of T helper cells and interleukin-2 receptor signaling in the production of cytotoxic T lymphocytes.

IL-15Ralpha recycles and presents IL-15 In trans to neighboring cells

We report intriguing aspects of the contribution of IL-15Ralpha to IL-15 functions. Consistent with high-affinity interactions between IL-15 and IL-15Ralpha, these two molecules form stable complexes on the cell surface of activated monocytes. The formation of IL-15/IL-15Ralpha complexes on cell surfaces induces a trans-endosomal recycling of IL-15 leading to the persistence of surface-bound IL-15 due to the constant reappearance of IL-15 on plasma membranes. This complex contributes to the long survival of T cells expressing IL-15Ralpha after IL-15 withdrawal. Finally, these complexes on activated monocytes present IL-15 in trans to target cells such as CD8(+) T cells that express only IL-2/15Rbeta and gammac upon cell-cell interaction.

Enforced expression of Bcl-2 restores the number of NK cells, but does not rescue the impaired development of NKT cells or intraepithelial lymphocytes, in IL-2/IL-15 receptor beta-chain-deficient mice

IL-2/IL-15Rbeta-deficient mice display impaired development of NK cells, NKT cells, and intraepithelial lymphocytes of the intestine and skin. To determine the role of survival signals mediated by IL-2/IL-15R in the development of these innate lymphocytes, we introduced a bcl-2 transgene into IL-2/IL-15Rbeta-deficient mice. Enforced expression of Bcl-2 restored the number of NK cells in IL-2/IL-15Rbeta-deficient mice, but the rescued NK cells showed no cytotoxic activity. The numbers of NKT cells and intestinal intraepithelial lymphocytes did not increase significantly, and skin intraepithelial lymphocytes remained undetectable in the bcl-2 transgenic IL-2/IL-15Rbeta-deficient mice. These results indicate an essential role of IL-2/IL-15R-mediated survival signals in the development of NK cells, but they also show that additional nonsurvival signals from IL-2/IL-15R are necessary for innate lymphocyte development.

Pentoxifylline functions as an adjuvant in vivo to enhance T cell immune responses by inhibiting activation-induced death

Modalities for inducing long-lasting immune responses are essential components of vaccine design. Most currently available immunological adjuvants empirically used for this purpose cause some inflammation, limiting clinical acceptability. We show that pentoxifylline (PF), a phosphodiesterase (PDE) inhibitor in common clinical use, enhances long-term persistence of T cell responses, including protective responses to a bacterial immunogen, Salmonella typhimurium, via a cAMP-dependent protein kinase A-mediated effect on T cells if given to mice for a brief period during immunization. PF inhibits activation-mediated loss of superantigen-reactive CD4 as well as CD8 T cells in vivo without significantly affecting their activation, and inhibits activation-induced death and caspase induction in stimulated CD4 as well as CD8 T cells in vitro without preventing the induction of activation markers. Consistent with this ability to prevent activation-induced death in not only CD4 but also CD8 T cells, PF also enhances the persistence of CD8 T cell responses in vivo. Thus, specific inhibition of activation-induced T cell apoptosis transiently during immune priming is likely to enhance the persistence of CD4 and CD8 T cell responses to vaccination, and pharmacological modulators of the cAMP pathway already in clinical use can be used for this purpose as immunological adjuvants.

Transgenic overexpression of human IL-17E results in eosinophilia, B-lymphocyte hyperplasia, and altered antibody production

We have identified and cloned a novel human cytokine with homology to cytokines of the interleukin-17 (IL-17) family, which we have termed human IL-17E (hIL-17E). With the identification of several IL-17 family members, it is critical to understand the in vivo function of these molecules. We have generated transgenic mice overexpressing hIL-17E using an apolipoprotein E (ApoE) hepatic promoter. These mice displayed changes in the peripheral blood, particularly, a 3-fold increase in total leukocytes consisting of increases in eosinophils, lymphocytes, and neutrophils. Splenomegaly and lymphoadenopathy were predominant and included marked eosinophil infiltrates and lymphoid hyperplasia. CCR3(+) eosinophils increased in the blood and lymph nodes of the transgenic mice by 50- and 300-fold, respectively. Eosinophils also increased 8- to 18-fold in the bone marrow and spleen, respectively. In the bone marrow, most of the eosinophils had an immature appearance. CD19(+) B cells increased 2- to 5-fold in the peripheral blood, 2-fold in the spleen, and 10-fold in the lymph nodes of transgenic mice, whereas CD4(+) T lymphocytes increased 2-fold in both blood and spleen. High serum levels of the cytokines IL-2, IL-4, IL-5, granulocyte colony-stimulating factor, eotaxin, and interferon gamma were observed. Consistent with B-lymphocyte increases, serum immunoglobulin (Ig) M, IgG, and IgE were significantly elevated. Antigenic challenge of the transgenic mice with keyhole limpet hemocyanin (KLH) resulted in a decrease in anti-KLH IgG accompanied by increases of anti-KLH IgA and IgE. In situ hybridization of transgenic tissues revealed that IL-17Rh1 (IL-17BR/Evi27), a receptor that binds IL-17E, is up-regulated. Taken together, these data indicate that IL-17E regulates hematopoietic and immune functions, stimulating the development of eosinophils and B lymphocytes. The fact that hIL-17E overexpression results in high levels of circulating eosinophils, IL-4, IL-5, eotaxin, and IgE suggests that IL-17E may be a proinflammatory cytokine favoring Th2-type immune responses.

Susceptibility of interleukin-2-deficient mice to Toxoplasma gondii is associated with a defect in the production of gamma interferon

Costimulation through the B7-CD28 interaction is an important second signal for T-cell activation, and previous studies have shown that CD28(-/-) mice infected with Toxoplasma gondii generate suboptimal CD4(+) T-cell responses, associated with a defect in production of the T-cell growth factor interleukin-2 (IL-2). To address the role of IL-2 in the expansion of T cells during toxoplasmosis, IL-2(-/-) mice were infected with T. gondii and their ability to generate a protective T-cell response was assessed. Although IL-2(-/-) mice produced normal levels of IL-12p40, they had reduced levels of gamma interferon (IFN-gamma) in serum, had an increased parasite burden, and succumbed to infection with T. gondii within 20 days. Fluorescence-activated cell sorter analysis revealed that, although uninfected IL-2(-/-) mice had an increased number of activated T cells compared with uninfected IL-2(+/+) mice, following infection they were unable to further upregulate this population. Examination of the ability of splenocytes from uninfected and infected mice to produce IFN-gamma revealed that IL-2(-/-) mice were hyporesponsive to stimulation with anti-CD3 or parasite antigen compared with wild-type mice, and the addition of IL-2 alone or in combination with IL-12 or stimulation with phorbol myristate acetate and ionomycin did not restore the production of IFN-gamma. Together, these studies reveal that IL-2(-/-) mice are unable to generate a protective IFN-gamma response following infection with T. gondii and suggest that IL-2(-/-) mice have an intrinsic defect in their ability to activate and expand IFN-gamma-producing T cells required for resistance to T. gondii.

CD25+CD4+ T cells contribute to the control of memory CD8+ T cells

Previously we demonstrated that IL-15 and IL-2 control the number of memory CD8+ T cells in mice. IL-15 induces, and IL-2 suppresses the division of these cells. Here we show that CD25+CD4+ regulatory T cells play an important role in the IL-2-mediated control of memory phenotype CD8+ T cell number. In animals, the numbers of CD25+CD4+ T cells were inversely correlated with the numbers of memory phenotype CD8+ T cells with age. Treatment with anti-IL-2 caused CD25+CD4+ T cells to disappear and, concurrently, increased the numbers of memory phenotype CD8+ T cells. This increase in the numbers of CD8+ memory phenotype T cells was not manifest in animals lacking CD4+ cells. Importantly, adoptive transfer of CD25+CD4+ T cells significantly reduced division of memory phenotype CD8+ T cells. Thus, we conclude that CD25+CD4+ T cells are involved in the IL-2-mediated inhibition of memory CD8+ T cell division and that IL-2 controls memory phenotype CD8+ T cell numbers at least in part through maintenance of the CD25+CD4+ T cell population.

Essential role for IL-2 in the regulation of antiviral extralymphoid CD8 T cell responses

IL-2 is a cytokine produced primarily by activated T cells and is thought to be the quintessential T cell growth factor. The precise role of IL-2 in the regulation of CD8 T cell responses to foreign Ag in vivo however remains enigmatic. Using an adoptive transfer system with IL-2- or IL-2R-deficient TCR transgenic CD8 T cells and MHC class I tetramers, we demonstrated that the expansion of antiviral CD8 T cells in secondary lymphoid tissues was IL-2 independent, whereas IL-2 played a more significant role in supporting the continued expansion of these cells within nonlymphoid tissues. Paradoxically, autocrine IL-2 negatively regulated the overall magnitude of the CD8 T cell response in nonlymphoid tissues via a Fas-independent mechanism. Furthermore, autocrine IL-2 did not regulate the contraction or memory phase of the response. These experiments identified a novel role for IL-2 in regulation of antiviral CD8 T cell responses and homeostasis in nonlymphoid tissues.

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.

IL-21 limits NK cell responses and promotes antigen-specific T cell activation: a mediator of the transition from innate to adaptive immunity

IFNalpha/beta, IL-12, and IL-15 regulate NK cell activation and expansion, but signals triggering resolution of the NK response upon induction of adaptive immunity remain to be defined. We now report that IL-21, a product of activated T cells, may serve this function. Mice lacking IL-21R (IL-21R(-/-)) had normal NK cell development but no detectable responses to IL-21. IL-21 enhanced cytotoxic activity and IFNgamma production by activated murine NK cells but did not support their viability, thus limiting their duration of activation. Furthermore, IL-21 blocked IL-15-induced expansion of resting NK cells, thus preventing the initiation of further innate responses. In contrast, IL-21 enhanced the proliferation, IFNgamma production, and cytotoxic function of CD8(+) effector T cells in an allogeneic MLR. These observations suggest that IL-21 promotes the transition between innate and adaptive immunity.

The influence of gammadelta T cells on the CD4+ T cell and antibody response during a primary Plasmodium chabaudi chabaudi infection in mice

A primary infection with Plasmodium chabaudi chabaudi (AS) is characterized by an expansion of gammadelta cells after the acute phase of infection in mice. This is particularly marked during chronic infections in B cell-deficient mice. Infections in gammadelta T cell-deficient mice suggest that, although these cells play some role in the control of parasitaemia and can produce interferon-gamma, they do not appear to be involved in the development of hypoglycaemia, loss of weight and temperature during a P. c. chabaudi infection. However, gammadelta T cells do influence the nature of the CD4+ T cell response during infection since, in their absence, Th2-like responses, such as interleukin (IL)-4 production and help for malaria-specific antibody responses, are more pronounced. This alteration in CD4+ T cells is reflected in a more rapid and greater immunoglobulin (Ig)G1 and IgG3 antibody response to the parasite. The large gammadelta T cell expansion normally observed in infected B cell-deficient mice did not take place in the absence of IL-2, and double-knockout mice lacking both B cells and functional IL-2 were highly susceptible to lethal infection with P. c. chabaudi. The majority of the single IL-2 knockout mice, in contrast, were able to control and clear a primary infection, suggesting that for the CD4+ T cell and antibody response, IL-2 could be replaced by other cytokines.