Control of homeostasis of CD8+ memory T cells by opposing cytokines

The contrasting roles of IL-2 and IL-15 in the life and death of lymphocytes: implications for the immunotherapy of rheumatological diseases

Interleukin-15 (IL-15) is a 14-15-kDa member of the 4alpha helix bundle family of cytokines that stimulate T and NK (natural killer) cells. IL-15 and IL-2 utilize heterotrimeric receptors that include the cytokine-specific private receptors IL-2Ralpha and IL-15Ralpha, as well as two receptor elements that they share, IL-2Rbeta and gammac. Although IL-2 and IL-15 share two receptor subunits and many functions, at times they provide contrasting contributions to T-cell-mediated immune responses. IL-2, through its pivotal role in activation-induced cell death (AICD), is involved in peripheral tolerance through the elimination of self-reactive T cells. In contrast, IL-15 in general manifests anti-apoptotic actions and inhibits IL-2-mediated AICD. IL-15 stimulates the persistence of memory phenotype CD8+ T cells, whereas IL-2 inhibits their expression. Abnormalities of IL-15 expression have been described in patients with rheumatoid arthritis or inflammatory bowel disease and in diseases associated with the retrovirus HTLV-I (human T-cell lymphotropic virus I). Humanized monoclonal antibodies that recognize IL-2Ralpha, the private receptor for IL-2, are being employed to inhibit allograft rejection and to treat T-cell leukemia/lymphoma. New approaches directed toward inhibiting the actions of the inflammatory cytokine, IL-15, are proposed for an array of autoimmune disorders including rheumatoid arthritis as well as diseases associated with the retrovirus HTLV-I.

T-cell dynamics after high-dose chemotherapy in adults: elucidation of the elusive CD8+ subset reveals multiple homeostatic T-cell compartments with distinct implications for immune competence

Recovery of total T cell numbers after in vivo T-cell depletion in humans is accompanied by complex perturbation within the CD8+ subset. We aimed to elucidate the reconstitution of CD8+ T cells by separate analysis of putative naïve CD95- CD28+, memory CD95+ CD28+ and CD28- T cell compartments after acute maximal depletion by high-dose chemotherapy (HD-ChT) in women with high-risk breast cancer. We found that recovery of putative naïve CD8+ CD95- CD28+ and CD4+ CD95- CD28+ T cells, was compatible with a thymus-dependent regenerative pathway since their recovery was slow and time-dependent, their values were tightly related to each other, and their reconstitution patterns were inversely related to age. By analysing non-naïve T cells, a striking diversion between putative memory T cells and CD28- T cells was found. These latter increased early well beyond normal values, thus playing a pivotal role in total T-cell homeostasis, and contributed to reduce the CD4 : CD8 ratio. In contrast, putative memory T cells returned to values not significantly different from those seen in patients at diagnosis, indicating that this compartment may recover after HD-ChT. At 3-5 years after treatment, naïve T cells persisted at low levels, with expansion of CD28- T cells, suggesting that such alterations may extend further. These findings indicate that CD28- T cells were responsible for 'blind' T-cell homeostasis, but support the notion that memory and naïve T cells are regulated separately. Given their distinct dynamics, quantitative evaluation of T-cell pools in patients undergoing chemotherapy should take into account separate analysis of naïve, memory and CD28- T cells.

Vaccinology for control of apicomplexan parasites: a simplified language of immune programming and its use in vaccine design

Most mammalian immune systems and parasites have co-evolved over the millennia, interacting within a common environment and communicating through a common language. This language is comprised of copious dialects in which a variety of host innate and acquired immune pathways actively interact with a multitude of parasite-specific survival strategies. Nonetheless, a simplified language is likely present since the same basic molecular and cellular mechanisms are associated with resistance or susceptibility to parasite infection. Protective immunity against protozoa within the phylum Apicomplexa (e.g. Cryptosporidia, Eimeria, Neospora, Plasmodia and Toxoplasma) is generally CD4+ T cell-dependent and elicited along the IL-12/IFN-gamma/iNOS effector axis. This simplified language can be decoded in part by significant advances in understanding naïve T cell activation, differentiation and generation of immunologic memory. Vaccine adjuvants and new immunisation strategies for generation of more potent immunity can also be viewed through this common language lens. The aim of this paper is to summarise recently published fundamental immunology studies, their relevance through examples in specific coccidian-host immune dialects, and how this simplified language can be used for the more rationale design of parasite vaccine control strategies.

IL-15 mimics T cell receptor crosslinking in the induction of cellular proliferation, gene expression, and cytotoxicity in CD8+ memory T cells

Generation of CD8(+) memory T cells requires antigenic stimulation through T cell receptor (TCR); however, maintenance of CD8(+) memory T cells seems to be mediated by cytokines, such as IL-15, in a TCR-independent manner. Compared with the TCR-induced activation, less is known about the mechanisms of IL-15 action. We report here a comparative and kinetic analysis of the responses of memory phenotype CD8(+) T cells to IL-15 or TCR (anti-CD3) stimulation in vitro. These two stimuli induce highly similar responses in memory phenotype CD8(+) T cells as measured by cellular proliferation, gene expression changes, synthesis of effector molecules (IFNgamma, tumor necrosis factor beta, granzyme B, and perforin), and induction of cytotoxicity. From 189 genes/expressed sequence tags (ESTs) whose expression changed in CD8(+) memory T cells after IL-15 and anti-CD3 stimulation identified by cDNA microarray analysis, 77% of the genes/ESTs exhibit a highly similar pattern of expression between IL-15 and anti-CD3-treated cells, and only 16% and 7% of the genes/ESTs are differentially expressed in response to IL-15 and anti-CD3 treatments, respectively. These results show that IL-15 and anti-CD3 stimulation induced remarkably similar gene expression and effector function. Thus, IL-15 acts not only as a crucial growth factor but also as an antigen-independent activator of effector functions for CD8(+) memory T cells.

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.

Pathways of apoptosis in lymphocyte development, homeostasis, and disease

Apoptosis plays a critical role in lymphocyte development and homeostasis. Enhanced lymphocyte apoptosis can cause immunodeficiency through cell loss. Conversely, inhibition of apoptosis can lead to the development of autoimmunity or lymphoma. Two major pathways contribute to the regulation of lymphocyte cell death, death-by-neglect and death-by-instruction.

Effector and memory T-cell differentiation: implications for vaccine development

Recent work shows that after stimulation with antigen, CD4+ and CD8+ T cells embark on a programme of proliferation that is closely linked with the acquisition of effector functions and leads ultimately to memory-cell formation. Here, we discuss the signals required for commitment to this programme of development and the factors that might influence its progression. Models of the pathways of effector and memory T-cell differentiation are discussed, and we highlight the implications of this new understanding for the optimization of vaccine strategies.

Engineering N-glycosylation mutations in IL-12 enhances sustained cytotoxic T lymphocyte responses for DNA immunization

Interleukin-12 (IL-12), consisting of p40 and p35 subunits, produces both p70 heterodimer and free p40. p70 is essential for the induction of T-helper 1 (Th1) and cytotoxic T-cell (CTL) immunity, whereas p40 inhibits p70-mediated function. Here, we found that mutations introduced into N-glycosylation sites (N220 of murine p40 and N222 of human p40) reduced secretion of p40 but not p70. Co-immunization of N220 mutant mIL-12 gene with hepatitis C virus (HCV) E2 DNA significantly enhanced long-term E2-specific CD8+ T-cell response and protection against tumor challenge compared with that of wild type. Our results indicate that the ratio of p70 to p40 is important for generating sustained long-term cell-mediated immunity. Thus, the mutant IL-12 could be utilized for the development of DNA vaccines as an adjuvant for the generation of long-term memory T-cell responses.

Maintaining the status quo: T-cell homeostasis

Lymphocyte numbers and subset composition are maintained at relatively constant levels throughout adulthood. Fluctuations in cell numbers due to infection, loss of cells, or other influences are smoothed out by unknown mechanisms that return the system to homeostasis. Regulation of lymphocyte numbers could be controlled at several points: rate of production, alteration in life span by diminution or enhancement of survival, or by division of cells within the compartment. Cell numbers within the immune system appear to be controlled by homeostatic mechanisms that influence each of these points. This discussion will focus on the regulation of peripheral cell numbers by the antigen-independent proliferation of T cells, which has recently been demonstrated to contribute to the maintenance and recovery of lymphocyte homeostasis.

IL-15, a survival factor for kidney epithelial cells, counteracts apoptosis and inflammation during nephritis

IL-15, a T cell growth factor, has been linked to exacerbating autoimmune diseases and allograft rejection. To test the hypothesis that IL-15-deficient (IL-15-/-) mice would be protected from T cell-dependent nephritis, we induced nephrotoxic serum nephritis (NSN) in IL-15-/- and wild-type (IL-15+/+) C57BL/6 mice. Contrary to our expectations, IL-15 protects the kidney during this T cell-dependent immunologic insult. Tubular, interstitial, and glomerular pathology and renal function are worse in IL-15-/- mice during NSN. We detected a substantial increase in tubular apoptosis in IL-15-/- kidneys. Moreover, macrophages and CD4 T cells are more abundant in the interstitia and glomeruli in IL-15-/- mice. This led us to identify several mechanisms responsible for heightened renal injury in the absence of IL-15. We now report that IL-15 and the IL-15 receptor (alpha, beta, gamma chains) are constitutively expressed in normal tubular epithelial cells (TECs). IL-15 is an autocrine survival factor for TECs. TEC apoptosis induced with anti-Fas or actinomycin D is substantially greater in IL-15-/- than in wild-type TECs. Moreover, IL-15 decreases the induction of a nephritogenic chemokine, MCP-1, that attracts leukocytes into the kidney during NSN. Taken together, we suggest that IL-15 is a therapeutic for tubulointerstitial and glomerular kidney diseases.

Homeostatic T cell proliferation in a T cell-dendritic cell coculture system

Naive T cells do not proliferate in normal individuals in the absence of antigen stimulation, but they proliferate spontaneously when T cells are severely depleted. We show here that coculture of syngeneic dendritic cells (DC) with naive T cells expressing a single T cell receptor also results in T cell proliferation in the absence of foreign antigen. As in lymphopenic mice, where T cell proliferation depends upon DC, this response in the coculture system requires interaction of the T cells' T cell receptor with self-peptide-MHCs on DC. This in vitro proliferation also requires soluble factors, including IL-15 secreted by DC, and can be inhibited potently by cell-cell contact with CD4+CD25+ regulatory T cells. The coculture system described may illuminate mechanisms that maintain stable numbers of T cells in normal individuals.

Enhanced signaling through the IL-2 receptor in CD8+ T cells regulated by antigen recognition results in preferential proliferation and expansion of responding CD8+ T cells rather than promotion of cell death

Multiple cytokines, including IL-2, can affect T cell proliferation and survival. However, IL-2 can lead to apoptosis as well as proliferation, making unclear whether IL-2 receptor (IL-2R) signals ultimately have a predominantly positive or negative effect. To address this issue, we examined the effect of enhancing IL-2R signals in CD8(+) T cells after antigen stimulation by engineering a transgenic (Tg) mouse strain with CD8(+) T cells capable of augmented, regulated, autocrine IL-2R signaling after target recognition by means of expression of a chimeric granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-2R. The Tg CD8(+) T cells can bind the granulocyte-macrophage colony-stimulating GM-CSF produced by antigen stimulation, but the GM-CSF binding results in delivery of an IL-2R signal. After antigen stimulation in vivo, the Tg T cells demonstrated marked increases in the initial proliferative response and cell expansion and displayed continued increases in cell expansion after repeated antigen exposure. These data suggest that the predominant role of IL-2R signals delivered to responding CD8(+) T cells is to set the size of the initial response to antigen by promoting T cell proliferation and survival and not cell death.

Different contributions of thymopoiesis and homeostasis-driven proliferation to the reconstitution of naive and memory T cell compartments

Following transfer into lymphopenic hosts, naive CD8 T cells proliferate and acquire memory phenotype. Although the acquired phenotype is stable in recombination activating gene-1-deficient (RAG-/-) recipients, in sublethally irradiated mice naive CD8 T cells of donor origin gradually accumulate. The naive cells have been attributed to phenotypic reversion of homeostatic memory cells, implying instability of memory phenotype and restoration of the naive T cell compartment by homeostasis-driven proliferation. We show here that (i) the accumulation of naive CD8 T cells of donor origin only occurs in recipients that have been irradiated and have an intact thymus; (ii) the apparent reversion of memory to naive cells actually results from de novo T cell development of hematopoietic stem cells, present in the donor spleen or lymph node cell populations, in the thymus of irradiated recipients; and (iii) the number of homeostatic memory cells generated in both RAG-/- and irradiated hosts reaches a plateau value and their phenotype is stably maintained even after retransfer into nonirradiated normal mice for 30 days. These findings demonstrate that homeostatic memory T cells do not revert to naive cells. After severe T cell depletion homeostasis-driven proliferation restores only the memory T cell compartment, whereas thymopoiesis is required for the reconstitution of the naive T cell compartment.

The IL-2/IL-15 receptor systems: targets for immunotherapy

Although interleukin-2 (IL-2) and -15 (IL-15) share two receptor subunits and many functions, at times they provide contrasting contributions to T-cell-mediated immune responses. IL-2, through its pivitol role in activation-induced cell death (AICD), is involved in peripheral tolerance through the elimination of self-reactive T cells. In contrast, in general IL-15 manifests antiapoptotic actions and inhibits IL-2-mediated AICD. IL-15 stimulates persistence of memory phenotype CD8+ T cells, whereas IL-2 inhibits their expression. Humanized monoclonal antibodies that recognize IL-2Ra, the private receptor for IL-2, are being employed to inhibit allograft rejection and to treat T-cell leukemia/lymphoma. Therapies directed toward inhibiting the actions of the inflammatory cytokine, IL-15, are proposed for an array of autoimmune disorders as well as diseases associated with the retrovirus human T-cell lymphotrophic virus 1.

Repeated antigen exposure is necessary for the differentiation, but not the initial proliferation, of naive CD4(+) T cells

The mechanisms that regulate CD4(+) T cells responses in vivo are still poorly understood. We show here that initial Ag stimulation induces in CD4(+) T cells a program of proliferation that can develop, for at least seven cycles of division, in the absence of subsequent Ag or cytokine requirement. Thereafter, proliferation stops but can be reinitiated by novel Ag stimulation. This initial Ag stimulation does not however suffice to induce the differentiation of naive CD4(+) T cells into effector Th1 cells which requires multiple contacts with Ag-loaded APC. Thus, recurrent exposure to both Ag and polarizing cytokines appears to be essential for the differentiation of IFN-gamma-producing cells. Ag and cytokine availability therefore greatly limits the differentiation, but not the initial proliferation, of CD4(+) T cells into IFN-gamma-producing cells.

Differential SLP-76 expression and TCR-mediated signaling in effector and memory CD4 T cells

We present in this study novel findings on TCR-mediated signaling in naive, effector, and memory CD4 T cells that identify critical biochemical markers to distinguish these subsets. We demonstrate that relative to naive CD4 T cells, memory CD4 T cells exhibit a profound decrease in expression of the linker/adapter molecule SLP-76, while effector T cells express normal to elevated levels of SLP-76. The reduced level of SLP-76 is memory CD4 T cells is coincident with reduced phosphorylation overall, yet the residual SLP-76 couples to a subset of TCR-associated linker molecules, leading to downstream mitogen-activated protein (MAP) kinase activation. By contrast, effector CD4 T cells strongly phosphorylate SLP-76, linker for activation of T cells, and additional Grb2-coupled proteins, exhibit increased associations of SLP-76 to phosphorylated linkers, and hyperphosphorylate downstream Erk1/2 MAP kinases. Our results suggest distinct coupling of signaling intermediates to the TCR in naive, effector, and memory CD4 T cells. Whereas effector CD4 T cells amplify existing TCR signaling events accounting for rapid effector responses, memory T cells engage fewer signaling intermediates to efficiently link TCR triggering directly to downstream MAP kinase activation.

Aging leads to disturbed homeostasis of memory phenotype CD8(+) cells

Examining the rate of in vivo T cell turnover (proliferation) in aged mice revealed a marked reduction in turnover at the level of memory-phenotype CD44(hi) CD8(+) cells relative to young mice. Based on adoptive transfer experiments, the reduced turnover of aged CD44(hi) CD8(+) cells reflected an inhibitory influence of the aged host environment. Aged CD44(hi) CD8(+) cells also showed poor in vivo responses to IL-15 and IL-15-inducing agents, but responded well to IL-15 in vitro. Two mechanisms could account for the reduced turnover of aged CD44(hi) CD8(+) cells in vivo. First, aging was associated with a prominent and selective increase in Bcl-2 expression in CD44(hi) CD8(+) cells. Hence, the reduced turnover of aged CD44(hi) CD8(+) cells may in part reflect the antiproliferative effect of enhanced Bcl-2 expression. Second, the impaired in vivo response of aged CD44(hi) CD8(+) cells to IL-15 correlated with increased serum levels of type I interferons (IFN-I) and was largely reversed by injection of anti-IFN-I antibody. Hence the selective reduction in the turnover of aged CD44(hi) CD8(+) cells in vivo may reflect the combined inhibitory effects of enhanced Bcl-2 expression and high IFN-I levels.

IL-15 induces type 1 and type 2 CD4+ and CD8+ T cells proliferation but is unable to drive cytokine production in the absence of TCR activation or IL-12 / IL-4 stimulation in vitro

Interleukin 15 (IL-15) is a pleiotropic cytokine produced principally by monocytes and affects both innate and acquired immunity. It has been shown that IL-15 is essential for the proliferation and maintenance of CD8+ memory cells but has little or no effect on naive CD8+ cells or CD4+ T cells. We report here, using an in vitro culture system of antigen-specific OVA TCR transgenic T cells as well as normal mouse T cell activated with anti-CD3 antibody that IL-15, at high concentrations, induced proliferation of both naive and memory CD4+ and CD8+ cells. IL-15 also enhanced the differentiation of type 1 (IFN-gamma-producing) and type 2 (IL-5-producing) CD4+ and CD8+ T cells under IL-12 and IL-4 driving conditions, respectively. However, IL-15 alone was not efficient in stimulating cytokine production of these cells in the absence of T cell subset driving cytokines (IL-12 or IL-4) and / or simultaneous TCR activation. Together, these results demonstrate that IL-15, at high dose, is a pan-T cell growth factor. The apparent requirement of IL-15 for the maintenance of memory CD8+ cell in vivo may reflect the exceptionally restricted nature of this subpopulation of cells for IL-15. The inability of IL-15 alone to stimulate cytokine synthesis also suggests that IL-15 on its own does not drive antigen-specific T cells to exhaustion. The levels of these cells are maintained by IL-15 and they are only mobilized to carry out effector functions when subsequently confronted with specific pathogens.

Exhaustion of cytotoxic T cells during adoptive immunotherapy of virus carrier mice can be prevented by B cells or CD4+ T cells

Rapid disappearance of antiviral CTL after transfusion into persistently infected individuals is a serious limitation of adoptive immunotherapy protocols. In the mouse model of persistent infection with lymphocytic choriomeningitis virus (LCMV) naive or immune virus-specific donor CD8+ T cells are exhausted after transfusion into carrier recipients with similar kinetics. Here we show that cotransfusion of immune CD4+ T cells prevents exhaustion of immune CD8+ T cells. Interestingly, cotransfer of primed B cells also prevented CD8+ T cell exhaustion in carriers even in the absence of T helper cells. This effect required the presence of immune B cells as repetitive treatment with hyperimmune serum led to the generation of antibody escape mutants. A combination of primed CD4+ T cells and primed B cells enhanced antiviral effects and prevented exhaustion also of naive CD8+ T cells. One key factor for prevention of CD8+ T cell exhaustion was the antiviral effect of the cotransfused cells thus reducing the time that CD8+ T cells are confronted with a high systemic viral load. These findings have implications for improving adoptive immunotherapy for persistent human viral infections.

Overexpression of IL-15 in vivo increases antigen-driven memory CD8+ T cells following a microbe exposure

To elucidate potential roles of IL-15 in the maintenance of memory CD8+ T cells, we followed the fate of Ag-specific CD8+ T cells directly visualized with MHC class I tetramers coupled with listeriolysin O (LLO)(91-99) in IL-15 transgenic (Tg) mice after Listeria monocytogenes infection. The numbers of LLO(91-99)-positive memory CD8+ T cells were significantly higher at 3 and 6 wk after infection than those in non-Tg mice. The LLO(91-99)-positive CD8+ T cells produced IFN-gamma in response to LLO(91-99), and an adoptive transfer of CD8+ T cells from IL-15 Tg mice infected with L. monocytogenes conferred a higher level of resistance against L. monocytogenes in normal mice. The CD44+ CD8+ T cells from infected IL-15 Tg mice expressed the higher level of Bcl-2. Transferred CD44+ CD8+ T cells divided more vigorously in naive IL-15 Tg mice than in non-Tg mice. These results suggest that IL-15 plays an important role in long-term maintenance of Ag-specific memory CD8+ T cells following microbial exposure via promotion of cell survival and homeostatic proliferation.

IL-2 and related cytokines can promote T cell survival by activating AKT

The regulated elimination of T cells serves to maintain normal immune function and prevents autoimmune responses. IL-2 family cytokines play an important role in controlling the survival of immature and mature T cells. These molecules activate the protein kinase, AKT/PKB. AKT has been shown to transduce an antiapoptotic signal in numerous cell types. In this study, we show that an active form of AKT can protect T cells from apoptosis following growth factor withdrawal and that IL-2 family cytokines can promote T cell survival by activating this kinase. We also provide evidence that AKT does not block death receptor-mediated killing of lymphocytes. These data suggest that AKT may serve as a common signaling element by which members of the IL-2 family of cytokines promote T cell survival.

Reduced expression of Bcl-2 in CD8+ T cells deficient in the IL-15 receptor alpha-chain

Mice that lack IL-15 or the IL-15R alpha-chain (IL-15Ralpha) are deficient in peripheral CD8(+), but not in CD4(+), T cells. This CD8(+) T cell-specific deficiency has now been investigated further by characterization of a new strain of IL-15Ralpha(-/-) mice. The adult mutant mice exhibited a specific reduction in the percentage of CD8-single positive TCR(high) thymocytes. The expression of Bcl-2 was reduced in both CD8(+) thymocytes and naive T cells of the mutant animals, and the susceptibility of these cells to death was increased. Memory CD8(+) cells were profoundly deficient in IL-15Ralpha(-/-)mice, and the residual memory-like CD8(+) cells contained a high percentage of dead cells and failed to up-regulate Bcl-2 expression compared with naive CD8(+) cells. Moreover, exogenous IL-15 both up-regulated the level of Bcl-2 in and reduced the death rate of wild-type and mutant CD8(+) T cells activated in vitro. These results indicate that IL-15 and IL-15Ralpha regulate the expression of Bcl-2 in CD8(+) T cells at all developmental stages. The reduced Bcl-2 content in CD8(+) cells might result in survival defect and contribute to the reduction of CD8(+) cells in IL-15Ralpha(-/-)mice.

Dendritic cells: immune regulators in health and disease

Dendritic cells (DCs) are bone marrow-derived cells of both lymphoid and myeloid stem cell origin that populate all lymphoid organs including the thymus, spleen, and lymph nodes, as well as nearly all nonlymphoid tissues and organs. Although DCs are a moderately diverse set of cells, they all have potent antigen-presenting capacity for stimulating naive, memory, and effector T cells. DCs are members of the innate immune system in that they can respond to dangers in the host environment by immediately generating protective cytokines. Most important, immature DCs respond to danger signals in the microenvironment by maturing, i.e., differentiating, and acquiring the capacity to direct the development of primary immune responses appropriate to the type of danger perceived. The powerful adjuvant activity that DCs possess in stimulating specific CD4 and CD8 T cell responses has made them targets in vaccine development strategies for the prevention and treatment of infections, allograft reactions, allergic and autoimmune diseases, and cancer. This review addresses the origins and migration of DCs to their sites of activity, their basic biology as antigen-presenting cells, their roles in important human diseases and, finally, selected strategies being pursued to harness their potent antigen-stimulating activity.

Regulation of naïve and memory T-cell homeostasis

Recent work has confirmed the existence of homeostatic mechanisms that regulate the overall size and composition of the mature T-cell pool. Homeostatic mechanisms not only control total T-cell numbers but appear to act differently on naïve vs. memory cells. The roles of self-MHC/peptide ligands and certain cytokines in T-cell homeostasis are discussed.

Homeostatic competition among T cells revealed by conditional inactivation of the mouse Cd4 gene

Absence of CD4 impairs the efficiency of T cell receptor (TCR) signaling in response to major histocompatibility complex (MHC) class II-presented peptides. Here we use mice carrying a conditional Cd4 allele to study the consequences of impaired TCR signaling after the completion of thymocyte development. We show that loss of CD4 decreases the steady-state proliferation of T cells as monitored by in vivo labeling with bromo-deoxyuridine. Moreover, T cells lacking CD4 compete poorly with CD4-expressing T cells during proliferative expansion after transfer into lymphopenic recipients. The data suggest that T cells compete with one another during homeostatic proliferation, and indicate that the basis of this competition is TCR signaling.

Cytokine-driven proliferation and differentiation of human naive, central memory, and effector memory CD4(+) T cells

Memory T lymphocytes proliferate in vivo in the absence of antigen maintaining a pool of central memory T cells (T(CM)) and effector memory T cells (T(EM)) with distinct effector function and homing capacity. We compared human CD4(+) naive T, T(CM), and T(EM) cells for their capacity to proliferate in response to cytokines, that have been implicated in T cell homeostasis. Interleukin (IL)-7 and IL-15 expanded with very high efficiency T(EM), while T(CM) were less responsive and naive T cells failed to respond. Dendritic cells (DCs) and DC-derived cytokines allowed naive T cells to proliferate selectively in response to IL-4, and potently boosted the response of T(CM) to IL-7 and IL-15 by increasing the expression of the IL-2/IL-15Rbeta and the common gamma chain (gamma(c)). The extracellular signal regulated kinase and the p38 mitogen-activated protein (MAP) kinases were selectively required for TCR and cytokine-driven proliferation, respectively. Importantly, in cytokine-driven cultures, some of the proliferating T(CM) differentiated to T(EM)-like cells acquiring effector function and switching chemokine receptor expression from CCR7 to CCR5. The sustained antigen-independent generation of T(EM) from a pool of T(CM) cells provides a plausible mechanism for the maintenance of a polyclonal and functionally diverse repertoire of human CD4(+) memory T cells.

A ligand for the chemokine receptor CCR7 can influence the homeostatic proliferation of CD4 T cells and progression of autoimmunity

Homeostasis of T cell numbers in the periphery implies an ability of lymphocytes to sense cell numbers. Although the mechanisms are unknown, we find that the chemokine CCL21 (also known as TCA4, SLC, 6Ckine), a ligand for the chemokine receptor CCR7, can regulate homeostasis of CD4 (but not CD8) T cells. In the absence of CCR7 ligands, transferred CD4 T cells failed to expand in lymphopenic hosts, whereas in the presence of CCL21 overexpression, homeostatic CD4 T cell proliferation occurred even in nonlymphopenic recipients. Ag-specific CD4 T cells transferred into Ag-expressing mice proliferated and induced autoimmunity only in lymphopenic recipients. Pancreatic expression of CCL21 was sufficient to replace the requirement for lymphopenia in the progression of autoimmune disease. These results suggest that CD4 T cells use local concentrations of CCR7 ligands as an index of T cell steady state numbers and that homeostatic expansion of the T cell population may be a contributing factor in the development of autoimmune disease.

IL-2, -4, and -15 differentially regulate O-glycan branching and P-selectin ligand formation in activated CD8 T cells

The glycosyltransferase core 2 beta1-6 N-acetylglucosaminyl transferase (C2GnT1 or C2GlcNAcT1) is responsible for formation of branched structures on O-glycans present on cell surface glycoproteins. The O-glycan branch created by C2GnT1 is physiologically important insofar as only this structure can be extended and modified to yield P-selectin ligands that promote initial interactions between extravasating lymphocytes and endothelia. In mature T cells, C2GnT1 activity is thought to be induced as an intrinsic consequence of T cell activation. Through analysis of C2GnT1-dependent epitopes on CD43 and CD45RB we have found that in activated CD8(+) T cells expression of C2GnT1 was dependent upon exposure to specific cytokines rather than being induced as a direct consequence of activation. Activated CD8(+) cells became receptive to strong induction of C2GnT1 expression and P-selectin ligand expression in response to IL-2, moderate induction by IL-15, and minimal induction in response to IL-4. Our observations clarify the relationship between T cell activation and C2GnT1 expression, demonstrate the differential impact of distinct cytokines on expression of C2GnT1 activity and P-selectin ligand, and reinforce the concept that the cytokine milieu subsequent to activation can influence adhesion systems that dictate lymphocyte homing properties.

IL-15 plays a major role in the persistence of Tax-specific CD8 cells in HAM/TSP patients

IL-15 is a critical cytokine for the maintenance of memory-phenotype CD8 cells in mice. Here, we investigated the role of IL-15 in the neurological disease termed human T cell lymphotropic virus I-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The high number of viral-specific CD8 cells in these patients is associated with inflammatory responses in the central nervous system. Because IL-15 is overexpressed in these patients, we asked whether IL-15 contributes to the persistence of human T cell lymphotropic virus I viral-specific CD8 cells. Using ex vivo cultures of HAM/TSP peripheral blood mononuclear cells, we demonstrated that in the majority of patients examined here blocking IL-15 action resulted in a decrease in the number of viral-specific CD8 cells. This decrease was caused by both inhibition of proliferation and induction of apoptosis in these cells. The data indicate that IL-15 plays a major role in the maintenance of viral-specific CD8 cells in HAM/TSP.

IL-15-induced conversion of monocytes to mature dendritic cells

IL-15 is produced by a wide variety of tissues in response to inflammatory stimuli. We examined the effect of IL-15 in supporting the maturation of monocytes to dendritic cells in ex vivo culture. IL-15 transformed CD14(+) monocytes to mature dendritic cells. These dendritic cells were similar to those obtained from monocyte cultures treated with a combination of the cytokines GM-CSF, IL-4 and TNF-alpha. The effects of IL-15 did not depend on endogenously produced GM-CSF. The IL-15-induced dendritic cells also expressed chemokines and stimulated strong allo-responses that were characteristic of mature dendritic cells. These data indicate that CD14(+) monocytes respond to IL-15 by undergoing morphological transformation and acquiring characteristic dendritic cell features that facilitate antigen-specific responses of T cells. Thus, the release of IL-15 by inflammatory stimuli may induce the conversion of monocytes to immuno-stimulatory dendritic cells to support primary immune responses against pathogens.

Naive and memory B cells in T-cell-dependent and T-independent responses

This review focuses on the properties and roles of distinct subsets among the primary and the memory B lymphocytes regarding their contribution to helper-T-cell-dependent and -independent antibody responses. The naive/memory B cell functions are explained in the context of current concepts on the basic mechanisms of humoral immunity. Differences between murine and human B cells are also discussed.

Learning to remember: generation and maintenance of T-cell memory

Immunologic memory results from a carefully coordinated interplay between cells of the immune system. In this review, we explore various aspects of the nature, generation, and maintenance of T lymphocyte-mediated immunologic memory. In light of the demonstrated heterogeneity of the memory T-cell pool, we hypothesize that subsets of memory T cells instructed to mature to distinct differentiation stages may differ, not only in functional and homing properties, but also in the conditions they require for survival, including antigen persistence and cytokine environment. Hence, according to this hypothesis, distinct memory T-cell subsets result from the nature and timing of the signals provided by the immune environment and occupy distinct niches. Intracellular and extracellular molecular mechanisms that underlie and modulate T-cell memory are discussed.

Divergence in the degree of clonal expansions in inflammatory T cell subpopulations mirrors HLA-associated risk alleles in genetically and clinically distinct subtypes of childhood arthritis

Clinically distinct forms of childhood arthritis are associated with different risk alleles of polymorphic loci within the MHC, which code for the antigen-presenting class I or class II molecules. We have compared the TCR diversity of synovial T cells from children with enthesitis-related (HLA-B27(+)) arthritis and oligoarticular arthritis (with class II MHC risk allele associations) in parallel with peripheral blood T cells from each child, using a high-resolution heteroduplex TCR analysis. We demonstrate that multiple clonal T cell expansions are present and persistent within the joint in both groups, but that there is disease-specific divergence in the dominant T cell subset containing these expansions. Thus, the largest clonotypes within the inflamed joints of children with class II-associated arthritis are within the CD4(+) synovial T cell population, while the dominant clones from children with enthesitis-related arthritis (associated with a class I allele) are within the CD8(+) synovial T cell population. These data provide powerful data to support the concept that recognition of MHC-peptide complexes by T cells plays a role in the pathogenesis of juvenile arthritis.

Strategies for designing and optimizing new generation vaccines

Although the field of immunology developed in part from the early vaccine studies of Edward Jenner, Louis Pasteur and others, vaccine development had largely become the province of virologists and other microbiologists, because the model for classic vaccines was to isolate the pathogen and prepare a killed or attenuated pathogen vaccine. Only recently has vaccinology returned to the realm of immunology, because a new understanding of immune mechanisms has allowed translation of basic discoveries into vaccine strategies.

Interleukin 15-mediated induction of cytotoxic effector cells capable of eliminating Epstein-Barr virus-transformed/immortalized lymphocytes in culture

BACKGROUND

Interleukin 15 (IL-15) activates cytotoxic lymphocytes and drives the expansion of memory T cells. Its role in immune control of virus-transformed cells and other tumor cells remains to be elucidated. We investigated the role of IL-15 in controlling Epstein-Barr virus (EBV)-transformed/immortalized lymphocytes in culture. EBV is a highly potent lymphocyte-transforming and opportunistic oncogenic herpesvirus associated with several human tumors.

METHODS

Peripheral blood mononuclear cells (PBMCs) from healthy donors were infected with EBV and cultured with either IL-15 or IL-15 plus anti-IL-15 antibodies for 3-4 weeks. We monitored EBV-induced transformation by assessing the clearly visible cell clusters by microscopy and analyzing the expression of EBV-encoded latent membrane oncoprotein-1 (LMP-1) and the EBV nuclear antigen (EBNA) complex by immunoblotting and immunofluorescence techniques, respectively. We depleted EBV-infected cultures of PBMCs of specific effector cell populations to investigate the effector cells involved in mediating IL-15 effect.

RESULTS

The presence of IL-15 resulted in the complete elimination of EBV-transformed cells in PBMC cultures. Western blot and immunofluorescence analyses performed 3-4 weeks after infection showed no detectable levels of LMP-1 and EBNA in IL-15-treated EBV-infected cultures, whereas IL-15-untreated EBV-infected cultures and IL-15/anti-IL-15-treated cultures expressed both proteins. IL-15 mediated its anti-EBV effect through early and late response mechanisms, i.e., by first activating natural killer (NK) cells and subsequently inducing cytolytic NK-T cells. The presence of anti-IL-15 neutralizing antibodies abrogated IL-15's effect on both mechanisms.

CONCLUSION

In vitro, IL-15 mediated complete elimination of EBV-infected/transformed lymphocytes via successive activation of NK and NK-T cytotoxic effectors. If these in vitro findings reflect in vivo mechanisms, then IL-15 might be considered for cytokine-based immunotherapy in patients with EBV-associated lymphoproliferative disorders/malignancies.

NKG2D receptors induced by IL-15 costimulate CD28-negative effector CTL in the tissue microenvironment

Unlike primary T cells in lymph nodes, effector CD8(+) CTL in tissues do not express the costimulatory receptor CD28. We report that NKG2D, the receptor for stress-induced MICA and MICB molecules expressed in the intestine, serves as a potent costimulatory receptor for CTL freshly isolated from the human intestinal epithelium. Expression and function of NKG2D are selectively up-regulated by the cytokine IL-15, which is released by the inflamed intestinal epithelium. These findings identify a novel CTL costimulatory pathway regulated by IL-15 and suggest that tissues can fine-tune the activation of effector T cells based on the presence or absence of stress and inflammation. Uncontrolled secretion of IL-15 could lead to excessive induction of NKG2D and thus contribute to the development of autoimmune disease by facilitating the activation of autoreactive T cells.

Role of CD28-B7 interactions in generation and maintenance of CD8 T cell memory

Although the role of CD28-B7 interaction in the activation of naive T cells is well established, its importance in the generation and maintenance of T cell memory is not well understood. In this study, we examined the requirement for CD28-B7 interactions in primary T cell activation and immune memory. Ag-specific CD8 T cell responses were compared between wild-type (+/+) and CD28-deficient (CD28(-/-)) mice following an acute infection with lymphocytic choriomeningitis virus (LCMV). During the primary response, there was a substantial activation and expansion of LCMV-specific CD8 T cells in both +/+ and CD28(-/-) mice. However, the magnitude of the primary CD8 T cell response to both dominant and subdominant LCMV CTL epitopes was approximately 2- to 3-fold lower in CD28(-/-) mice compared with +/+ mice; the lack of CD28-mediated costimulation did not lead to preferential suppression of CD8 T cell responses to the weaker subdominant epitopes. As seen in CD28(-/-) mice, blockade of B7-mediated costimulation by CTLA4-Ig treatment of +/+ mice also resulted in a 2-fold reduction in the anti-LCMV CD8 T cell responses. Loss of CD28/B7 interactions did not significantly affect the generation and maintenance of CD8 T cell memory; the magnitude of CD8 T cell memory was approximately 2-fold lower in CD28(-/-) mice as compared with +/+ mice. Further, in CD28(-/-) mice, LCMV-specific memory CD8 T cells showed normal homeostatic proliferation in vivo and also conferred protective immunity. Therefore, CD28 signaling is not necessary for the proliferative renewal and maintenance of memory CD8 T cells.

Sensory adaptation in naive peripheral CD4 T cells

T cell receptor interactions with peptide/major histocompatibility complex (pMHC) ligands control the selection of T cells in the thymus as well as their homeostasis in peripheral lymphoid organs. Here we show that pMHC contact modulates the expression of CD5 by naive CD4 T cells in a process that requires the continued expression of p56(lck). Reduced CD5 levels in T cells deprived of pMHC contact are predictive of elevated Ca(2)+ responses to subsequent TCR engagement by anti-CD3 or nominal antigen. Adaptation to peripheral pMHC contact may be important for regulating naive CD4 T cell responsiveness.

Type I interferon is required to mount an adaptive response to immunostimulatory DNA

Immunostimulatory DNA sequences (ISS, CpG motifs) potently stimulate Th1 and cytotoxic T lymphocyte (CTL) responses to antigens and have thus generated considerable interest due to their potential use in immunotherapeutics. An array of cytokines are produced in response to ISS exposure, but the relative importance of each of these mediators in the stimulation of innate and adaptive ISS-induced immunity has yet to be fully investigated. To address this issue, we measured immune responses in mice with targeted deletions of the ISS-induced genes encoding IL-12 (IL-12(-/-)), IFN-gamma (IFN-gamma(-/-)), the IFN-gamma receptor (IFN-gammaR(-/-)), and the IFN-alpha/beta receptor (IFN-alpha/betaR(-/-)) after immunization with ISS-containing oligodeoxynucleotides and model antigens. IL-12(-/-) and IFN-alpha/betaR(-/-) mice were compromised in their ability to develop a cross-primed CTL response, whereas IFN-gamma(-/-) and IFN-gammaR(-/-) mice were not. In addition, lymphocytes from immunized IFN-alpha/betaR(-/-) mice had defective IFN-gamma responses to antigen restimulation. Antigen nonspecific ISS-induced B cell proliferation was normal in the four deficient strains; however, innate IL-6 production was reduced in IFN-gamma(-/-) and IFN-gammaR(-/-) splenocytes and eliminated in IFN-alpha/betaR(-/-) cells. While IL-12 production was defective in only the IFN-gamma(-/-) splenocytes, innate natural killer cell IFN-gamma synthesis was virtually absent in the IL-12(-/-) and IFN-alpha/betaR(-/-) mice. Thus, while IFN-alpha/beta, IFN-gamma, and IL-12 each play important and distinct roles in the development of the innate and adaptive immune responses to ISS, IFN-alpha/beta is a particularly crucial and currently under-appreciated factor in this system.

T cell-independent interleukin 15Ralpha signals are required for bystander proliferation

Cytokine driven or "bystander" proliferation of T cells occurs in vivo independently of major histocompatibility complex-T cell receptor interactions. This process may be important for supporting T cell homeostasis and facilitating T cell responses to microbial antigens, and may involve the cytokine interleukin (IL)-15. In this study, we find that IL-15Ralpha-deficient (IL-15Ralpha(-/-)) mice fail to undergo poly I:C or IL-15 driven bystander proliferation of CD8(+) T cells. Surprisingly, IL-15Ralpha(-/-) CD8(+) T cells proliferate in response to poly I:C when adoptively transferred into normal mice, and normal CD8(+) T cells fail to proliferate in IL-15Ralpha(-/-) mice. Normal mice reconstituted with IL-15Ralpha(-/-) bone marrow cells also fail to exhibit bystander responses. Thus, CD8(+) T cell independent IL-15Ralpha signals from radiation sensitive hematopoietic cells are likely required for bystander responses. Moreover, normal CD8(+) T cells proliferate in IL-15Ralpha(-/-) mice after treatment with IL-15. Therefore, IL-15Ralpha signals may mediate a positive feedback loop involving the further physiological production of IL-15. These findings provide new insights into how IL-15Ralpha supports memory phenotype CD8(+) T cell proliferation, and suggest novel mechanisms by which memory CD8(+) T cells are maintained in vivo.

IL-2 during in vitro priming promotes subsequent engraftment and successful adoptive tumor immunotherapy by persistent memory phenotypic CD8(+) T cells

Adoptive T cell tumor immunotherapy potentially consists of two protective components by the transferred effector cells, the immediate immune response and the subsequent development of memory T cells. The extent by which adoptively transferred CD8(+) CTL are destined to become memory T cells is ambiguous as most studies focus on the acute effects on tumor shortly following adoptive transfer. In this study we show that a substantial fraction of the input CTL develop into memory cells that reject a s.c. tumor challenge. The use of exogenous IL-2 or a combination of IL-2 and IL-4, but not solely IL-4, during the ex vivo culture for the CTL inoculation was necessary for efficient development of CD8(+) memory T cells. Thus, an important component of adoptive immunotherapy using CTL is the production of CD8(+) Ag-specific memory cells which is primarily favored by IL-2 receptor signaling during ex vivo generation of the effector CTL.

Fatal leukemia in interleukin-15 transgenic mice

The role of inflammation in the early genesis of certain malignancies has recently been appreciated. Interleukin (IL)-15, a proinflammatory cytokine and growth factor, is required for lymphocyte homeostasis. Intriguingly, the expression of IL-15 protein is tightly controlled by multiple posttranscriptional mechanisms, suggesting that inappropriate expression of IL-15 may be detrimental to the host. We recently engineered a transgenic mouse in which the normal posttranscriptional control of IL-15 is eliminated, thereby overexpressing the murine IL-15 protein. IL-15 transgenic mice have early expansions in NK and CD8+ T lymphocytes and later develop fatal lymphocytic leukemia with a T-NK phenotype. This article recapitulates the phenotype of these IL-15 transgenic mice and discusses the utility of this model as a tool to further our understanding of leukemogenesis.

Interleukin-10 promotes the maintenance of antitumor CD8(+) T-cell effector function in situ

Interleukin-10 (IL-10) is a multifunctional cytokine that can exert suppressive and stimulatory effects on T cells. It was investigated whether IL-10 could serve as an immunostimulant for specific CD8(+) cytotoxic T cell (CTL) in vivo after vaccination and, if so, under what conditions. In tumor prevention models, administration of IL-10 before, or soon after, peptide-pulsed primary dendritic cell immunization resulted in immune suppression and enhanced tumor progression. Injection of IL-10, however, just after a booster vaccine significantly enhanced antitumor immunity and vaccine efficacy. Analysis of spleen cells derived from these latter animals 3 weeks after IL-10 treatment revealed that the number of CD8(+) CD44(hi) CD122(+) T cells had increased and that antigen-specific proliferation in vitro was enhanced. Although cytotoxicity assays did not support differences between the various treatment groups, 2 more sensitive assays measuring antigen-specific interferon-gamma production at the single-cell level demonstrated increases in the number of antigen-specific responder T cells in animals in the vaccine/IL-10 treatment group. Thus, IL-10 may maintain the number of antitumor CD8(+) T cells. In adoptive transfer studies, the ability of IL-10 to maintain CTL function could be enhanced by the depletion of CD4(+) T cells. This suggests that IL-10 mediates contrasting effects on both CD4(+) and CD8(+) T cells that result in either immune dampening or immune potentiation in situ, respectively. Appreciation of this dichotomy in IL-10 immunobiology may allow for the design of more effective cancer vaccines designed to activate and maintain specific CD8(+) T-cell effector function in situ.