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

The Bcl-2 protein family: sensors and checkpoints for life-or-death decisions

Members of the Bcl-2 family are crucial integrators of survival and death signals in higher eukaryotes. Although recent studies have provided novel and quite unexpected insights into the mechanisms by which these proteins might issue life permits or death sentences in cells, we are still on the way to fully understand their modes of action. This review provides a snapshot on where we are on this journey and how we may exploit our knowledge on this family of proteins to unveil the mysteries of immune regulation.

Neonates support lymphopenia-induced proliferation

T cells expand without intentional antigen stimulation when transferred into adult lymphopenic environments. In this study, we show that the physiologic lymphopenic environment existing in neonatal mice also supports CD4 T cell proliferation. Strikingly, naive CD4 T cells that proliferate within neonates acquire the phenotypic and functional characteristics of memory cells. Such proliferation is inhibited by the presence of both memory and naive CD4 T cells, is enhanced by 3-day thymectomy, is independent of IL-7, and requires a class II MHC-TCR interaction and a CD28-mediated signal. CD44(bright) CD4 T cells in neonates have a wide repertoire as judged by the distribution of Vbeta expression. Thus, lymphopenia-induced T cell proliferation is a physiologic process that occurs during the early postnatal period.

A role for Stat5 in CD8+ T cell homeostasis

Cytokine signals are known to contribute to CD8+ memory T cell homeostasis, but an exact understanding of the mechanism(s) has remained elusive. We have now investigated the role of Stat5 proteins in this process. Whereas Stat5a and Stat5b KO mice have decreased numbers of CD8+ T cells, Stat5-transgenic mice have an increased number of these cells. Stat5b-transgenic mice exhibit increased Ag-induced cell death of CD4+ T cells and augmented proliferation and Bcl-2 expression in CD8+ T cells, providing a basis for this finding. Moreover, CD8+ memory T cells are substantially affected by Stat5 levels. These findings identify Stat5 proteins as critical signaling mediators used by cytokines to regulate CD8+ T cell homeostasis.

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.

Evidence for a major psoriasis susceptibility locus at 6p21(PSORS1) and a novel candidate region at 4q31 by genome-wide scan in Chinese hans

Psoriasis is a heterogeneous disease with seven major psoriasis susceptibility loci reported so far on chromosomes 1p, 1q, 3q, 4q, 6p, 17q, and 19p, respectively. To investigate the psoriasis susceptibility loci in Chinese Hans, a genome-wide scan was performed with two-point and multipoint parametric and nonparametric linkage analyses in 61 multiplex families. These families were Chinese Hans residing in east and south-east China, comprising 189 affected and 166 unaffected individuals. We detected evidence for linkage at 6p21 (PSORS1) with nonparametric linkage scores > 3 in the range of 39.9-62.3 cM and a maximum multipoint nonparametric linkage score of 4.58 (p=0.000032). Parametric analysis revealed a maximum two-point heterogeneity lod score of 4.30 with 58% as the proportion of linked families (alpha) and a maximum multipoint heterogeneity lod score of 4.25 (alpha=53%) under the assumption of a dominant model. We could not confirm a previous reported locus (PSORS3) on distal chromosome 4q; however, a region of highly suggestive linkage was identified proximal to this proposed locus. Multipoint nonparametric analysis demonstrated nonparametric linkage scores > 3 throughout a region between 152.5 cM and 165.1 cM (from pter) with a maximum peak of 3.69 (p=0.00033) at 157.9 cM, which locates D4S413. A maximum multipoint heterogeneity lod score of 2.31 (alpha=46%) was reached at 163.1 cM. With two-point parametric linkage analysis, we observed the highest lod score of 2.43 and heterogeneity lod score of 3.94 (alpha=77%) at marker D4S1597. Our results showed that chromosomes 6p and 4q may contain genes involved in the susceptibility to psoriasis vulgaris in a Chinese Han population. Other regions with weaker evidence for linkage could also hide minor susceptibility genes.

A cell-based artificial antigen-presenting cell coated with anti-CD3 and CD28 antibodies enables rapid expansion and long-term growth of CD4 T lymphocytes

We compared the ability of two genetically modified myeloid cells, K562 and U937, to serve as artificial antigen-presenting cells (aAPC). Both aAPC were stably transfected with the low-affinity Fcgamma receptor CD32 (K32/U32 cells). K32 cells loaded with anti-CD3 and anti-CD28 Ab (K32/CD3/28) induced more rapid CD4 T-cell expansion than CD3/28-coated beads. In contrast, U32/CD3/28 induced high levels of CD4 T-cell thymidine uptake but were unable to sustain long-term T-cell expansion. K32 cells, but not U32 cells, loaded with anti-CD3 alone also stimulated CD4 T-cell growth and IL-2 secretion, indicating the expression of additional costimulatory molecules on K32 cells. We found constitutive expression of B7-H3 and a strong upregulation of mRNA encoding for IL-15, PD-L1, and PD-L2 after coculture with CD4 T cells activated by K32/CD3/28 but not U32/CD3/28. We conclude that K32 aAPCs are a robust system for clinical scale ex vivo expansion of CD4 T cells.

Dendritic cells: making progress with tumour regression?

Due to their potent ability to activate the immune system, dendritic cells (DC) are showing promise as potential adjuvants for tumour immunotherapy of cancer patients. However, little is known about the effect tumour cells can have on DC function. Indeed, the discovery of different DC subsets with different immunological functions indicates that the relationship between tumour cells and tumour-infiltrating DC subtypes is likely to be complex. There remains a lot to be understood about the effects of tumours on DC before we can expect to benefit from DC-based tumour immunotherapy of cancer patients. Here we review the recent advances being made in understanding DC phenotype and function in relation to interactions with different types of tumours.

Expansion of activated human naïve T-cells precedes effector function

Naïve T-cells divide and mature, both functionally and phenotypically, upon stimulation through the T-cell receptor. Although much is known about the overall changes that occur in naïve cells upon TCR stimulation, and the different memory/effector populations that arise following stimulation, the relationship between cell division and functional and phenotypical changes that occur after activation is poorly understood. Here, we examine the early stages of human naïve and antigen-experienced T-cell activation, and the relationship between cell division and acquisition of effector function during the transition from resting antigen-experienced or naïve T-cells into effector cells. Stimulated naïve T-cells proliferate prior to acquisition of effector function, as measured by cytokine production and expression of effector-associated cell surface molecules. Additionally, we show that interlukin-7 (IL-7) can drive proliferation of naïve T-cells without TCR:MHC peptide interactions. IL-7 alone does not, however, drive the proliferation of antigen-experienced T-cells. Memory T-cells will divide in response to exogenous IL-7 but only in the presence of naïve T-cells and IL-2. This study contributes to the current understanding of the mechanistic differences between naïve and memory T-cell responses by defining the functional and phenotypic changes that occur to T-cells after stimulation.

IL-15 is a growth factor and an activator of CD8 memory T cells

Memory lymphocytes, arising from naïve lymphocytes after antigenic stimulation and being long-lived, are the cellular basis for immunological memory. Recent studies of CD8 T cells suggest that generation of CD8 memory T cells requires the engagement of T cell antigen receptors (TCR) with antigen, yet the maintenance of CD8 memory T cells appears to be dependent on cytokines, such as IL-15, independent of TCR. Although considerable progress has been made in understanding the molecular and cellular events of TCR-induced differentiation and proliferation in the past decade, less is known about the mechanisms of IL-15 action. From a kinetic and comparative analysis of the responses of memory phenotype CD8 T cells to IL-15 and TCR stimulation in vitro, we found that IL-15 and anti-CD3 induce highly similar responses in memory phenotype CD8 T cells as measured by general gene expression profiles, synthesis of effector molecules (IFNgamma, TNFbeta, granzyme B and perforin), induction of cytotoxicity, and cellular proliferation. These findings indicate that IL-15 is not only a growth factor but also an antigen-independent activator for CD8 memory T cells.

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.

IL-15 induces unspecific effector functions in human peptide-specific CD8+ T-cell cultures

Antigen (Ag)-specific CD8+ T cells are a major host defence against viral infections. In the present study, we generated human CD8+ T-cell lines specific towards influenza matrix peptide (IMP)-pulsed Ag-presenting cells. We compared the effect of interleukin-2 (IL-2) and IL-15 on the proliferation and cytotoxic activity of primary and secondary IMP-specific cytotoxic T lymphocyte (CTL) culture. In primary CTL cultures, IL-15-induced cell expansion was considerably reduced as compared with IL-2-induced cell expansion, and IL-15 favoured the outgrowth of CTLs without peptide specificity in these cultures. Secondary IMP-specific CD8+ T cells were generated by the addition of IL-2 during two cycles of restimulation. From the third restimulation, identical CTL cultures were expanded with either IL-2 or IL-15 in parallel. Cell expansion as well as Ag specificity was considerably reduced after a 5 day culture period in the presence of IL-15. No or low CD69 expression was observed in IL-15-cultured CTLs, whereas IL-2-cultured CTLs contained high fractions of CD69+ cells. Furthermore, a high fraction of these latter cells coexpressed the cytotoxic marker CD56. However, IL-15-cultured CTLs exhibited cytotoxic activity without detectable expression of CD56, suggesting that CD56 is not essential for cytotoxic activity. Thus, the results presented suggest that IL-15 favours the outgrowth of unspecific cytotoxic effector T cells.

Enhanced ELISPOT detection of antigen-specific T cell responses from cryopreserved specimens with addition of both IL-7 and IL-15--the Amplispot assay

The importance of the enzyme-linked immunosorbent spot (ELISPOT) assay as a tool for studying immune responses in vitro is becoming increasingly apparent. However, there remains a need for enhanced sensitivity for the detection of low frequency antigen-specific T cell responses. We reasoned that the addition of a combination of the cytokines interleukin (IL)-7 and IL-15 would selectively increase interferon-gamma (IFN-gamma) production from antigen-stimulated CD4+ and CD8+ effector memory T cells. Freshly isolated or cryopreserved peripheral blood mononuclear cells (PBMC) from four healthy donors were analysed by ELISPOT for the frequency of purified protein derivative (PPD)-specific CD4+ T cells or cytomegalovirus (CMV) peptide-specific CD8+ T cells. Addition of IL-7 and IL-15 increased the number of PPD-specific CD4+ T cells up to 2.4-fold in fresh PBMC and up to 18-fold in cryopreserved PBMC. The cytokines also increased the number of CMV peptide-specific CD8+ T cells in fresh PBMC up to 7.5-fold. No additional increases were seen when antibodies to co-stimulatory molecules CD28 and CD49d were applied together with the cytokine combination. These data demonstrate that the sensitivity of the ELISPOT assay may be significantly augmented by addition of the cytokines IL-7 and IL-15 to antigen-stimulated cells. This method will be particularly useful for the assessment of antigen-stimulated cytokine production by T cells in cryopreserved biological specimens.

Cytokine memory of T helper lymphocytes

Memory is one of the key features of the adaptive immune system. Specific T and B lymphocytes are primed for a particular antigen and upon challenge with it will react faster than naive lymphocytes. They also memorize the expression of key effector molecules, in particular cytokines, which determine the type and scale of an immune reaction. While in primary activations differential expression of cytokine genes is dependent on antigen-receptor signaling and differentiation signals, in later activations the expression is triggered by antigen-receptor signaling and dependent on the cytokine memory. The molecular basis of the cytokine memory implies differential expression of transcription factors and epigenetic modifications of cytokine genes and gene loci. GATA-3 for Th2 and T-bet for Th1 cells expressing interleukin-4 or interferon-gamma, respectively, are prime candidates for key transcription factors of cytokine memory. The essential role of epigenetic modifications is suggested by the requirement of DNA synthesis for the establishment of a cytokine memory in Th lymphocytes. At present the molecular link between transcription factors and epigenetic modifications of cytokine genes in the establishment and maintenance of cytokine memory is not clear. The initial cytokine memory is not stable against adverse differentiation signals, while in repeatedly stimulated lymphocytes it is stabilized by a variety of mechanisms.

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.

Long-term maintenance of virus-specific effector memory CD8+ T cells in the lung airways depends on proliferation

Recent studies have shown that virus-specific effector memory T cells can be recovered from the lung airways long after clearance of a respiratory virus infection. These cells are thought to play an important role in the recall response to secondary viral infection. It is currently unclear whether these cells actually persist at this site or are maintained by continual proliferation and recruitment. In this study, we have analyzed the mechanisms underlying the persistence of memory CD8(+) T cells in the lung airway lumina following recovery from a respiratory virus infection. The data identify two distinct populations of memory cells. First, a large population Ag-specific CD8(+) T cells is deposited in the airways during the acute response to the virus. These cells persist in a functional state for several weeks with minimal further division. Second, a smaller population of Ag-specific CD8(+) T cells is maintained in the lung airways by homeostatic proliferation and migration to lung airways after viral clearance. This rate of proliferation is identical to that observed in the spleen, suggesting that these cells may be recent immigrants from the lymphoid organs. These data have significant implications for vaccines designed to promote cellular immunity at mucosal sites such as the lung.

RANTES production by memory phenotype T cells is controlled by a posttranscriptional, TCR-dependent process

An examination of differences in gene expression between memory and naive phenotype T cells revealed elevated levels of mRNA for several chemokines, especially RANTES, in memory phenotype T cells. Although RANTES mRNA is spliced and cytoplasmic, these cells do not contain or secrete significant amounts of RANTES protein without TCR stimulation. This secretion is independent of transcription, but requires translation. In vivo, CD8+ memory T cells proliferate continuously and slowly in response to IL-15; however, IL-15 does not stimulate RANTES secretion. These results show that memory phenotype CD8+ T cells use preexisting mRNA to produce and secrete RANTES rapidly following TCR stimulation. Such storage of preformed mRNAs for important inflammatory mediators may contribute to the speed of secondary immune responses.

Selective delivery of augmented IL-2 receptor signals to responding CD8+ T cells increases the size of the acute antiviral response and of the resulting memory T cell pool

CD8(+) T cells respond to IL-2 produced both endogenously and by CD4(+) Th during an antiviral response. However, IL-2R signals can potentially promote CD8(+) T cell death as well as proliferation, making it unclear whether IL-2R signals provide a predominantly positive or negative effect upon CD8(+) T cell responses to viral infection. To more precisely define the direct role of IL-2R signaling on CD8(+) T cells during the response to a virus, we examined the effect of delivering augmented IL-2R signals selectively to CD8(+) T cells responding to lymphocytic choriomeningitis virus infection. Although naive CD8(+) T cells are competent to produce IL-2, CD8(+) T cells lose this capacity upon differentiation into effector CD8(+) T cells. However, effector CD8(+) T cells do retain the capacity to produce GM-CSF upon Ag stimulation. Thus, to deliver enhanced autocrine IL-2R signals to CD8(+) T cells, we established a transgenic mouse strain expressing a chimeric GM-CSF/IL-2R (GMIL2R). As GM-CSF production is Ag dependent, the GMIL2R delivers an augmented IL-2R signal exclusively to CD8(+) T cells responding to Ag. Following lymphocytic choriomeningitis virus infection, GMIL2R transgenic mice exhibited an increase in both the peak CD8(+) T cell response achieved and the size of the resulting memory pool established. Upon secondary viral challenge, the GMIL2R also enhanced the proliferative response of memory CD8(+) T cells. Thus, our findings indicate that IL-2 delivery to responding CD8(+) T cells is a limiting factor in both the acute and memory antiviral responses.

Overexpression of interleukin-15 prevents the development of murine retrovirus-induced acquired immunodeficiency syndrome

LP-BM5 murine leukemia virus (MuLV) infection causes murine acquired immunodeficiency syndrome (MAIDS), a disease characterized by varied functional abnormalities of immunocompetent cells. We found that MAIDS progression was severely retarded in IL-15 transgenic (Tg) mice constructed with cDNA encoding secretable IL-15 under the control of an MHC class I promoter. Several immune defects, including impaired natural killer activity, depressed IFN-gamma production by T cells stimulated with anti-T cell receptor cross-linking, and increased susceptibility to Mycobacterium bovis infection, were prevented in IL-15 Tg mice inoculated with LP-BM5 MuLV. Cytotoxic T lymphocyte response to a highly antigenic 10-mer peptide encoded by LP-BM5-defective virus gag p12 gene was detected in the spleen and peritoneal exudate cells from IL-15 Tg mice infected with LP-BM5 MuLV. Intramuscular injection of cDNA encoding secretable IL-15 also prevented the development of MAIDS. These results indicate that IL-15 prevents the progression of MAIDS and may provide insight into an immunotherapeutic approach using the IL-15 gene for controlling retrovirus-induced immunodeficiency.

Systemic administration of IL-15 augments the antigen-specific primary CD8+ T cell response following vaccination with peptide-pulsed dendritic cells

The systemic administration of IL-2 can act as a potent adjuvant for T cell-directed vaccine strategies. However, not only is the administration of IL-2 potentially toxic, but recent evidence suggests that it may also paradoxically limit the duration and magnitude of the cytotoxic T cell response. A recently identified cytokine, IL-15, shares many properties with IL-2 and may provide a preferential means of augmenting T cell-directed vaccine responses. Although well characterized in vitro, there are few data on the ability of IL-15 to augment T cell-mediated responses in vivo. We therefore evaluated the ability of systemic IL-15 to function as a T cell adjuvant in a murine vaccine model. To establish a population of easily identifiable Ag-responsive T cells, naive CD8(+) (OT-1) T cells were first adoptively transferred into mice. Vaccination with peptide-pulsed dendritic cells induced a modest expansion of OT-1 T cells. The addition of systemic IL-15 for 7 days following vaccination resulted in a significant increase in the expansion of responding T cells in the PBL, spleen, and lymph nodes. Importantly, the responding T cells were cytotoxic and maintained a Tc1-biased phenotype. We did not observe either enhanced resistance to activation-induced cell death or preferential generation of memory T cells as a result of treatment with IL-15 compared with IL-2. These studies show for the first time that IL-15 is capable of augmenting the primary CD8(+) T cell response to vaccination and contribute to the basis for future experiments exploring the clinical role of IL-15.

IL-2 negatively regulates IL-7 receptor alpha chain expression in activated T lymphocytes

Interleukin (IL)-2 is a type I four-alpha-helical bundle cytokine that plays vital roles in antigen-mediated proliferation of peripheral blood T cells and also is critical for activation-induced cell death. We now demonstrate that IL-2 potently decreases expression of IL-7 receptor alpha chain (IL-7Ralpha) mRNA and protein. The fact that IL-7Ralpha is a component of the receptors for both IL-7 and thymic stromal lymphopoietin (TSLP) suggests that IL-2 can negatively regulate signals by each of these cytokines. Previously it was known that the IL-2 and IL-7 receptors shared the common cytokine receptor gamma chain, gamma(c), which suggested a possible competition between these cytokines for a receptor component. Our findings now suggest a previously unknown type of cross-talk between IL-2 and IL-7 signaling by showing that IL-2 signaling can diminish IL-7Ralpha expression via a phosphatidylinositol 3-kinase/Akt-dependent mechanism.

Interleukin 15 controls both proliferation and survival of a subset of memory-phenotype CD8(+) T cells

Previous work has shown that memory-phenotype CD44(hi) CD8(+) cells are controlled by a cytokine, interleukin (IL)-15. However, the dependency of CD44(hi) CD8(+) cells on IL-15 is partial rather than complete. Here, evidence is presented that CD44(hi) CD8(+) cells comprise a mixed population of IL-15-dependent and IL-15-independent cells. The major subset of CD122(hi) CD44(hi) CD8(+) cells is heavily dependent on IL-15 by three different parameters, namely (1) "bystander" proliferation induced via IFN-induced stimulation of the innate immune system, (2) normal "background" proliferation, and (3) T cell survival; IL-15 dependency is most extreme for the Ly49(+) subset of CD122(hi) CD44(hi) CD8(+) cells. In contrast to CD122(hi) cells, the CD122(lo) subset of CD44(hi) CD8(+) cells is IL-15 independent; likewise, being CD122(lo), CD44(hi) CD4(+) cells are IL-15 independent. Thus, subsets of memory-phenotype T cells differ radically in their sensitivity to IL-15.

Creating space: an antigen-independent, CpG-induced peripheral expansion of naive and memory T lymphocytes in a full T-cell compartment

Many of the mechanisms that govern T-cell homeostasis remain obscure. Here we report that repeated administration of synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG-ODN) into mice induces a systemic antigen-independent expansion of naive and memory T cells in a full T-cell compartment. Expansion of T cells was observed on both CD4(+) and CD8(+) T-cell subsets and was produced not by inducing the proliferation of the cells but by preventing their death. The antiapoptotic effects of CpG-ODN on T cells were observed against activation-induced death and growth factor withdrawal-mediated death. The ability of CpG-ODN to protect T cells from these forms of death was associated with the up-regulation of antiapoptotic gene products including c-FLIP, bcl-xL, and, to some extent, bcl-2. The effect of CpG-ODN on naive and memory T cells required the expression of CD28 and was not dependent on the presence of B lymphocytes, suggesting that other antigen-presenting cells that respond to CpG-ODN, such as dendritic cells, may provide antiapoptotic signals to T cells in an antigen-independent but CD28/B7-dependent fashion. The present findings suggest that CpG-ODN can disrupt normal T-cell homeostasis not by acting as a mitogen but by preventing T-cell death that normally takes place as a mechanism to maintain steady-state levels of T cells. These findings support a potential means to expeditiously replenish and maintain the peripheral lymphocyte population after severe immunodepletion such as that which occurs in HIV-infected individuals and individuals undergoing cytoablative therapies.

TCR and IL-7 Receptor Signals Can Operate Independently or Synergize to Promote Lymphopenia-Induced Expansion of Naive T Cells

TCR and cytokine signals induce naive T cells to undergo spontaneous divisions as part of a homeostatic response to conditions of T cell deficiency. The conditions under which these signals evoke the homeostatic response and their interaction with each other are poorly understood, and yet are very important clinically in considering strategies for immune reconstitution. Here, we show that p56(lck) (lck)-mediated TCR signals and IL-7R signals are each able to stimulate T cell proliferation in lymphopenic hosts independently of one another, but can also synergize to facilitate proliferation. Furthermore, the relative contribution to the homeostatic response by TCR and cytokine signals is not fixed and critically depends on both the degree of lymphopenia and specific characteristics of individual T cell clones. Finally, we show that only lck and not fyn can mediate the TCR-driven proliferation, while neither lck nor fyn is required for IL-7R-induced proliferation.

Functional impairment of human T-lymphocytes following PHA-induced expansion and retroviral transduction: implications for gene therapy

The immune function of retrovirus-mediated gene modified (GM) T cells is critical for a beneficial effect to follow their adoptive transfer into patients. Recent clinical data show that GM T cells expanded with PHA have reduced function in vivo. However, little functional analysis of PHA stimulation is available. Our results show that expansion of T cells with PHA impairs their ability to respond (proliferation, cytotoxicity and IFN gamma and perforin expression) to allogeneic stimulation or viral antigens in vitro. Conversely, CD3/CD28-based protocols can preserve this immune function. Retroviral transduction did not alter the functional profile induced by polyclonal stimulation. We investigated the mechanisms leading to this functional effect, and identified differential effects of PHA and CD3/CD28 on the distribution of CCR7/CD45RA T cell functional subsets, which may explain the functional differences observed. While CD3/CD28 stimulation parallels the lineage differentiation pattern induced by antigens in physiological conditions, PHA induces a skewed distribution of the CCR7/CD45RA functional T cell subsets, with near disappearance of the subpopulations that display the effector phenotype. Overall, this study demonstrates a functional disadvantage for transduction protocols based on PHA, uncovers mechanisms that may explain this functional effect, and provides us with information to design and select transduction protocols with an improved functional outcome.

TCR signals mediated by Src family kinases are essential for the survival of naive T cells

The role of TCR signals triggered by recognition of self MHCs in maintaining the survival of naive peripheral T cells remains controversial. Here we examine the role of the Src family kinases, p56(lck) (Lck) and p59(fyn) (Fyn), in the survival of naive T cells. We show that long term survival requires a combination of signals transduced by Src family kinases and signals through the IL-7R. In the absence of either one, naive T cells die slowly, but if both signals are removed, cell loss is greatly accelerated. The TCR signal can be mediated by either Fyn or Lck at wild-type levels of expression, but not by Lck alone if expressed suboptimally. The disappearance of T cells in the absence of Fyn and Lck was associated with a complete loss of TCRzeta-chain phosphorylation and down-regulation of CD5, both of which are also MHC contact dependent, indicating that the Src family kinases are critical for transducing a TCR-MHC survival signal.

A subset of CD8 memory T cells from old mice have high levels of CD28 and produce IFN-gamma

Using carboxyfluorescein diacetate succinimidyl ester (CFSE)-tagged cells to measure proliferation in vivo, we found that only memory CD8(+) cells from mice older than 18 months gave measurable levels of proliferation and that the proportion of memory CD8(+) T cells able to proliferate in a nonirradiated recipient increased with age. CD8 cells that had proliferated in vivo contained higher levels of CD28 when compared to CD8 cells that had not divided. Cells with high levels of CD28 were preferentially able to divide in nonirradiated recipients. Using ex vivo intracellular staining analysis, we determined that most of the CD8(+) T cells that were capable of dividing in vivo produced IFN-gamma after isolation from recipient mice or their original host. These studies thus document the presence in aged mice of a population of CD28(hi) CD8(+) cells whose ability to proliferate in vivo without antigenic stimulation and to produce IFN-gamma may be involved in immune regulation.

High numbers of IL-2-producing CD8+ T cells during viral infection: correlation with stable memory development

Using infections with lymphocytic choriomeningitis virus (LCMV) and vesicular stomatitis virus in mice as model systems, we have investigated the ability of antigen-primed CD8+ T cells generated in the context of viral infections to produce IL-2. Our results indicate that acute immunizing infection normally leads to generation of high numbers of IL-2-producing antigen-specific CD8+ T cells. By costaining for IL-2 and IFN-gamma intracellularly, we found that IL-2-producing cells predominantly constitute a subset of cells also producing IFN-gamma. Comparison of the kinetics of generation revealed that IL-2-producing cells appear slightly delayed compared with the majority of IFN-gamma producing cells, and the relative frequency of the IL-2-producing subset increases with transition into the memory phase. In contrast to acute immunizing infection, few IL-2-producing cells are generated during chronic LCMV infection. Furthermore, in MHC class II-deficient mice, which only transiently control LCMV infection, IL-2-producing CD8+ T cells are initially generated, but by 4 weeks after infection this subset has nearly disappeared. Eventually the capacity to produce IFN-gamma also becomes impaired, while cell numbers are maintained at a level similar to those in wild-type mice controlling the infection. Taken together, these findings indicate that phenotyping of T cell populations based on capacity to produce cytokines, and especially IL-2, can provide important information as to the functional status of the analysed cell subset. Specifically, combined analysis of the capacity to produce IL-2 and IFN-gamma can be used as a predictor for loss of function within the CD8+ T cell compartment.

4-1BB-specific monoclonal antibody promotes the generation of tumor-specific immune responses by direct activation of CD8 T cells in a CD40-dependent manner

4-1BB (CD137) is a member of the TNFR superfamily (TNFRSF9). T cell expression of 4-1BB is restricted to activated cells, and cross-linking has been shown to deliver a costimulatory signal. Here we have shown that treatment of tumor-bearing mice with agonistic 4-1BB-specific Abs can lead to T cell-mediated tumor rejection. In vivo mAb depletion experiments demonstrated that this rejection requires CD8(+) cells but not CD4(+) or NK cells. Both IFN-gamma- and CD40-mediated signals were also required, because no benefit was observed on treatment with 4-1BB mAb in mice in which the genes for these molecules had been knocked out. Interestingly, 4-1BB-mediated stimulation of immune responses in CD40L(-/-) mice is effective (although at a reduced level), and may suggest the existence of an alternative ligand for CD40. Additional experiments in IL-15(-/-) mice indicate that IL-15 is not required for either the generation of the primary tumor-specific immune response or the maintenance of the memory immune response. In contrast, the presence of CD4 cells during the primary immune response appears to play a significant role in the maintenance of effective antitumor memory. Finally, in mice in which the number of dendritic cells had been expanded by Fms-like tyrosine kinase3 ligand treatment, the antitumor effects of 4-1BB ligation were enhanced.

Migration and differentiation of CD8+ T cells

Antigen-specific responses by CD8+ T cells require direct cell-cell interactions between T cells and antigen-presenting cells (APC). Initially, naïve T cells must communicate with APC in lymphoid organs. Once stimulated, the resulting effector cells interact with APC in peripheral tissues. To this end, T cells must migrate to discrete sites throughout the body where antigen may be found. Recent progress in the field has revealed that the migratory abilities of T cells are critically dependent on their differentiation state, which is shaped by a multitude of factors. Thus, naïve T cells are normally restricted to recirculate between the blood and secondary lymphoid tissues, although in some autoimmune diseases they may also accumulate in chronically inflamed tissues. When CD8+ T cells encounter antigen and differentiate into short-lived effector CTL, they lose the ability to home to lymph nodes but gain access to peripheral tissues and sites of inflammation. Long-lived memory cells exist in (at least) two flavors: central memory cells that migrate to both lymphoid organs and peripheral sites of inflammation, and effector memory cells that are preferentially localized in non-lymphoid tissues. Our current understanding of the interplay of T cell differentiation and migration has been boosted by the development of T-GFP mice, in which transgenic green fluorescent protein is expressed selectively in naïve and central memory T cells, but not in effector cytotoxic T cells (CTL). This review will focus on recent studies in which T-GFP mice were used to dissect the traffic signals for naïve T cell homing to secondary lymphoid organs, the factors that influence the differentiation of naïve CD8+ T cells into cytotoxic and memory cells, as well as the in vivo trafficking routes of antigen-experienced subsets.

Induction of CD4 T cell changes in murine AIDS is dependent on costimulation and involves a dysregulation of homeostasis

Strong CD4 T cell activation and proliferation are seen in susceptible mice infected with the murine retroviral inoculum, LP-BM5, which produces an immunodeficiency syndrome called murine AIDS (MAIDS). We developed a short term adoptive transfer model of MAIDS to examine the requirements for the CD4 T cell response. Naive CD4 T cells from uninfected donors responded quickly after adoptive transfer into MAIDS-infected hosts, becoming activated and proliferating within several days. Using blocking mAbs to costimulatory ligands and CD4 T cells deficient in expression of their receptors, we found that the CD4 T cell response requires CD28:B7.1/B7.2 interactions, but not CTLA4 or CD40-CD40 ligand interactions. Naive CD4 T cells did not respond in H-2M-deficient mice with MAIDS, suggesting that disease requires recognition of self peptide-MHC complexes. The self MHC-dependent division and accumulation of large numbers of CD4 T cells suggest that MAIDS involves a disruption of the balance of homeostatic signals. Supporting this hypothesis, CD4 T cells from mice with MAIDS failed to regulate the homeostatic division of naive CD4 T cells in a cotransfer model. Thus, a combination of up-regulation of costimulatory ligands and disruption of homeostatic control may be responsible for CD4 lymphoproliferation in MAIDS.

The role of cytokines and their signaling pathways in the regulation of immunity to Toxoplasma gondii

The development of a strong cellular immune response is critical for the control of the intracellular pathogen Toxoplasma gondii. This occurs by activation of a complex, integrated immune response, which utilizes cells of the innate and adaptive immune systems. In the last two decades there have been major advances in our understanding of the role of cytokines in the initiation and maintenance of protective immunity to T. gondii, and IFN-gamma has been identified as the major mediator of resistance to this pathogen. This article provides an overview of the biology of toxoplasmosis and focuses on the pivotal role of cytokines and their signaling pathways during infection. It also addresses the role of cytokines in modulating other immune functions that are critical in determining the balance between a protective and a pathological immune response.

IL-15-dependent activation-induced cell death-resistant Th1 type CD8 alpha beta+NK1.1+ T cells for the development of small intestinal inflammation

To clarify the role of IL-15 at local sites, we engineered a transgenic (Tg) mouse (T3(b)-IL-15 Tg) to overexpress human IL-15 preferentially in intestinal epithelial cells by the use of T3(b)-promoter. Although IL-15 was expressed in the entire small intestine (SI) and large intestines of the Tg mice, localized inflammation developed in the proximal SI only. Histopathologic study revealed reduced villus length, marked infiltration of lymphocytes, and vacuolar degeneration of the villus epithelium, beginning at approximately 3-4 mo of age. The numbers of CD8(+) T cells, especially CD8alphabeta(+) T cells expressing NK1.1, were dramatically increased in the lamina propria of the involved SI. The severity of inflammation corresponded to increased numbers of CD8alphabeta(+)NK1.1(+) T cells and levels of production of the Th1-type cytokines IFN-gamma and TNF-alpha. Locally overexpressed IL-15 was accompanied by increased resistance of CD8alphabeta(+) NK1.1(+) T cells to activation-induced cell death. Our results suggest that chronic inflammation in the SI in this murine model is mediated by dysregulation of epithelial cell-derived IL-15. The model may contribute to understanding the role of CD8(+) T cells in human Crohn's disease involving the SI.

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.

Interleukin (IL)-15 and IL-7 jointly regulate homeostatic proliferation of memory phenotype CD8+ cells but are not required for memory phenotype CD4+ cells

The overall size and composition of the pool of naive and memory T cells are tightly regulated by homeostatic mechanisms. Recent work has shown that homeostasis of naive T cells is controlled by two factors, self-major histocompatibility complex (MHC)/peptide ligands and a cytokine, interleukin (IL)-7. In particular, contact with these two factors is required for naive CD4+ and CD8+ cells to undergo "homeostatic" proliferation, i.e., proliferation induced as a consequence of severe T cell depletion. In contrast to naive T cells, the factors that drive memory T cells to undergo homeostatic proliferation are poorly understood. To address this issue, purified memory phenotype CD4+ and CD8+ cells from normal mice were adoptively transferred into various gene-knockout mice rendered T cell-deficient by sublethal irradiation. Three findings are reported. First, unlike naive T cells, homeostatic proliferation of memory T cells is largely MHC independent. Second, memory CD8+ cells can utilize either IL-7 or IL-15 to undergo homeostatic proliferation; however, in the absence of both IL-7 and IL-15, homeostatic proliferation fails to occur. Third, unlike memory CD8+ cells, homeostatic proliferation of memory CD4+ cells is independent of IL-7 and IL-15 (also IL-4). Thus, the homeostatic proliferation mechanisms that control memory CD8+ cells and memory CD4+ cells are quite distinct.

Overexpression of interleukin (IL)-7 leads to IL-15-independent generation of memory phenotype CD8+ T cells

Transgenic (TG) mice expressing a high copy number of interleukin (IL)-7 cDNA under the control of the major histocomaptability complex (MHC) class II promoter display a 10-20-fold increase in total T cell numbers. Here, we show that the increase in T cell numbers in IL-7 TG mice is most apparent at the level of memory phenotype CD44hi CD122hi CD8+ cells. Based on studies with T cell receptor (TCR) TG mice crossed to IL-7 TG mice, increased levels of IL-7 may provide costimulation for TCR recognition of self-MHC ligands and thus cause naive CD8+ cells to proliferate and differentiate into memory phenotype cells. In addition, a marked increase in CD44hi CD122hi CD8+ cells was found in IL-7 TG IL-15(-) mice. Since these cell are rare in normal IL-15(-) mice, the dependency of memory phenotype CD8+ cells on IL-15 can be overcome by overexpression of IL-7.

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

The generation and efficient maintenance of antigen-specific memory T cells is essential for long-lasting immunological protection. In this study, we examined the role of interleukin (IL)-15 in the generation and maintenance of virus-specific memory CD8 T cells using mice deficient in either IL-15 or the IL-15 receptor alpha chain. Both cytokine- and receptor-deficient mice made potent primary CD8 T cell responses to infection with lymphocytic choriomeningitis virus (LCMV), effectively cleared the virus and generated a pool of antigen-specific memory CD8 T cells that were phenotypically and functionally similar to memory CD8 T cells present in IL-15(+/+) mice. However, longitudinal analysis revealed a slow attrition of virus-specific memory CD8 T cells in the absence of IL-15 signals. This loss of CD8 T cells was due to a severe defect in the proliferative renewal of antigen-specific memory CD8 T cells in IL-15(-/-) mice. Taken together, these results show that IL-15 is not essential for the generation of memory CD8 T cells, but is required for homeostatic proliferation to maintain populations of memory cells over long periods of time.

Cytokine requirements for acute and Basal homeostatic proliferation of naive and memory CD8+ T cells

Both naive and memory T cells undergo antigen-independent proliferation after transfer into a T cell-depleted environment (acute homeostatic proliferation), whereas only memory T cells slowly divide in a full T cell compartment (basal proliferation). We show, first, that naive and memory CD8+ T cells have different cytokine requirements for acute homeostatic proliferation. Interleukin (IL)-7 receptor(R)alpha-mediated signals were obligatory for proliferation of naive T cells in lymphopenic hosts, whereas IL-15 did not influence their division. Memory T cells, on the other hand, could use either IL-7Ralpha- or IL-15-mediated signals for acute homeostatic proliferation: their proliferation was delayed when either IL-7Ralpha was blocked or IL-15 removed, but only when both signals were absent was proliferation ablated. Second, the cytokine requirements for basal and acute homeostatic proliferation of CD8+ memory T cells differ, as basal division of memory T cells was blocked completely in IL-15-deficient hosts. These data suggest a possible mechanism for the dearth of memory CD8+ T cells in IL-15- and IL-15Ralpha-deficient mice is their impaired basal proliferation. Our results show that naive and memory T lymphocytes differ in their cytokine dependence for acute homeostatic proliferation and that memory T lymphocytes have distinct requirements for proliferation in full versus empty compartments.

T cell memory

Typical immune responses lead to prominent clonal expansion of antigen-specific T and B cells followed by differentiation into effector cells. Most effector cells die at the end of the immune response but some of these cells survive and form long-lived memory cells. The factors controlling the formation and survival of memory T cells are reviewed.

Inhibition of the IL-15 pathway in anti-CD25 mAb treated renal allograft recipients

Anti-CD25 mAb's are used for prophylaxis of rejection in allograft transplantation. These agents target the alpha-chain, part of the IL-2Ralphabetagamma complex. The beta- and gamma-chain are signaling components that are not specific for IL-2. The T-cell growth factors IL2, IL-7 and IL-15 utilize the gamma-chain and IL-2 and IL-15 share the beta-chain. We have studied the consequences of targeting the IL-2R alpha-chain with the anti-CD25 mAb basiliximab by measuring the IL-2R alphabetagamma expression levels and the IL-2, IL-7 and IL-15 driven proliferation. By flowcytometry and limiting dilution analysis, the IL-2R complex was analyzed in peripheral blood samples from renal allograft recipients (n = 29) who received basiliximab (20 mg IV bolus on day 0 and 4), cyclosporin and mycophenolate mofetil. In peripheral blood, after induction therapy with basiliximab, no CD25 positive T-cells were detectable for 61 days (median, range 25-93 days). When CD25 cells were covered with basiliximab, the percentage and the mean fluorescence intensity (MFI) of IL-2Rbeta positive T-cells significantly decreased, P = 0.02 and P = 0.013, respectively, whereas the expression level of the IL-2Rgamma was not affected. The inhibition of the expression of the IL-2R alpha- and beta-chain had significant consequences for the function of these cells. Both the IL-2- and the IL-15-dependent proliferation were inhibited, P = 0.007 and P = 0.01, respectively. The control, the IL-7 driven proliferation, was not influenced by basiliximab. In conclusion, therapy with anti-CD25 mAb's affect T-cell-dependent allogeneic immune responses, not only by blocking IL-2 signaling but also by impairing IL-15 signaling through downregulating the IL-2/IL-15 receptor beta-chain.

No one is naive: the significance of heterologous T-cell immunity

Memory T cells that are specific for one virus can become activated during infection with an unrelated heterologous virus, and might have roles in protective immunity and immunopathology. The course of each infection is influenced by the T-cell memory pool that has been laid down by a host's history of previous infections, and with each successive infection, T-cell memory to previously encountered agents is modified. Here, we discuss evidence from studies in mice and humans that shows the importance of this phenomenon in determining the outcome of infection.

Influence of effector molecules on the CD8(+) T cell response to infection

Expansion and contraction of antigen-specific CD8(+) T cells after infection or vaccination results in stable memory. Recent evidence demonstrates that anti-microbial effector molecules such as perforin and IFN-gamma regulate specific aspects of CD8(+) T cell homeostasis by mechanisms that may be dependent or independent of pathogen clearance.

Role for Bcl-6 in the generation and maintenance of memory CD8+ T cells

Naïve T cells proliferate and differentiate into memory cells after antigenic stimulation or in a lymphopenic environment. We showed here transient increases in memory phenotype CD8+ T cell numbers in the lymphopenic environment of spleens of very young mice. The magnitude of the increase correlated with Bcl-6 expression in the T cells. Bcl-6 controlled the generation and maintenance of antigen-specific memory phenotype CD8+ T cells in the spleens of immunized mice. These data suggest that Bcl-6, which is essential for memory B cell development in germinal centers, is a key molecule for the establishment not only of memory T cells but also of the peripheral T cell compartment in infancy.

IFN regulatory factor 4 participates in the human T cell lymphotropic virus type I-mediated activation of the IL-15 receptor alpha promoter

IL-15Ralpha mRNA and protein levels are increased in human T cell lymphotropic virus type-I (HTLV-I)-associated adult T cell leukemia. Previously, we demonstrated that IL-15Ralpha expression was activated by HTLV-I Tax, in part, through the action of NF-kappaB. However, there appeared to be additional motifs within the IL-15Ralpha promoter that were responsive to HTLV-I Tax. In this study, we demonstrated that IL-15Ralpha mRNA expression was activated in human monocytes by IFN treatment, suggesting a role for IFN regulatory factors (IRFs) in IL-15Ralpha transcription. In addition, an IRF element within the Tax-responsive element of the IL-15Ralpha promoter was necessary for maximal Tax-induced activation of this promoter. Furthermore, we demonstrated that IRF-4, a transcription factor known to be elevated in HTLV-I-infected cells, activated the IL-15Ralpha promoter. Inhibition of IRF-4 action lead to reduced Tax-induced activation of the IL-15Ralpha promoter, while inhibition of both IRF-4 and NF-kappaB severely inhibited the Tax-induced activation of this promoter. These findings suggest a role for both NF-kappaB and IRF-4 in the transcriptional regulation of IL-15Ralpha by HTLV-I Tax. It is possible that the HTLV-I Tax-mediated induction of IL-15Ralpha and IL-15 may lead to an autocrine cytokine-mediated stimulatory loop leading to the proliferation of HTLV-I infected cells. This loop of proliferation may facilitate viral propagation and play a role in HTLV-I-mediated disease progression.

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.

Fas-dependent elimination of nonselected CD8 cells and lpr disease

MHC/self peptide interactions with cognate coreceptor/TCR complexes are central to homeostasis of the T cell repertoire. Recent reports have also underlined the critical role of IL-15/IL-2 cytokines in regulating this homeostatic process. In this study, we investigate mechanisms that regulate potentially autoreactive CD8 cells that have escaped intrathymic selection. These cells, upon exit from the thymus, express high levels of CD44, B220, and the IL-15R/IL-2R, and undergo fas-dependent apoptosis. Defects in fas signaling allow increased IL-15/IL-2-dependent survival of these CD44/B220(+) CD8(+) as well as the double-negative T cells characteristic of lpr disease.

Cutting edge: requirement for IL-15 in the generation of primary and memory antigen-specific CD8 T cells

IL-15 and IL-15Ralpha are required for generation of memory-phenotype CD8 T cells in unimmunized mice. However, the role of IL-15 in primary expansion and generation of Ag-specific memory CD8 T cells in vivo has not been investigated. We characterized the CD8 T cell response against vesicular stomatitis virus (VSV) in IL-15(-/-) and IL-15Ralpha(-/-) mice. Surprisingly, IL-15 was required for primary expansion of VSV-specific CD8 T cells. The generation of VSV-specific memory CD8 T cells was also impaired without IL-15 signaling, and this defect correlated with a decrease in memory CD8 T cell turnover. Despite minimal proliferation without IL-15, a subset of memory cells survived long-term. IL-15Ralpha expression was low on naive CD8 T cells, up-regulated on Ag-specific effector cells, and sustained on memory cells. Thus, IL-15 was important for the generation and the subsequent maintenance of antiviral memory CD8 T cells.