IL-7 promotes the transition of CD4 effectors to persistent memory cells

Augmented IL-7 signaling during viral infection drives greater expansion of effector T cells but does not enhance memory

IL-7 signals are crucial for the survival of naive and memory T cells, and the IL-7R is expressed on the surface of these cells. Following viral infection, the IL-7R is expressed on only a subset of effector CD8 T cells, and has been demonstrated to be important for the survival of these memory precursors. IL-7 message levels remain relatively constant during the T cell response to lymphocytic choriomeningitis virus, but a short-lived burst of GM-CSF is observed soon after infection. Retroviral expression of a chimeric GM-CSF/IL-7R, in which binding of GM-CSF by T cells leads to IL-7 signaling, allows for the delivery of an IL-7 signal in all effector T cells expressing the receptor. In mice infected with lymphocytic choriomeningitis virus, CD8 and CD4 T cells transduced with this chimeric receptor underwent an enhanced proliferative response compared with untransduced populations in the same host. Similarly, TCR transgenic CD8 cells expressing the chimeric receptor produced higher effector numbers during the peak of the T cell response to infection. Surprisingly, the enhanced proliferation did not lead to higher memory numbers, as the subsequent contraction phase was more pronounced in the transduced cell populations. These findings demonstrate that artificial IL-7 signaling during an infection leads to significantly increased Ag-specific effector T cell numbers, but does not result in increased numbers of memory progeny. The extent of contraction may be dictated by intrinsic factors related to the number of prior cell divisions.

Cutting edge: IL-7-independent regulation of IL-7 receptor alpha expression and memory CD8 T cell development

Expression of IL-7Ralpha on a subset of Ag-specific effector CD8 T cells is believed to identify memory cell precursors. However, whether IL-7 regulates IL-7Ralpha expression in vivo and is responsible for selective survival of IL-7Ralpha(+) effector cells is unknown. Our results show that in the absence of IL-7, IL-7Ralpha expression was extinguished on the majority of CD8 T cells responding to virus infection, sustained on a subset of effector cells transitioning to memory, and expressed at high levels by memory cells. Additionally, an IL-7-deficient environment was capable of supporting bcl-2 up-regulation and memory cell development in response to virus infection. Thus, IL-7Ralpha regulation occurs independently of IL-7 in responding CD8 T cells, indicating that CD8 memory T cell precursors are not selected by IL-7/IL-7Ralpha interactions.

T-cell homeostasis alteration in HIV-1 infected subjects with low CD4 T-cell count despite undetectable virus load during HAART

OBJECTIVE

To investigate the pathogenesis of low CD4 T-cell count in subjects who are immunological non responders (InR) to HAART.

DESIGN

Thirty-five HIV-positive subjects on HAART for at least 1 year, all with undetectable HIV-1 RNA, were studied. Patients were defined as InR according to a CD4 cell increase < 20% from CD4 cell baseline or CD4 cell count < 200/microl; subjects with a CD4 T-cell increase > 20% from baseline and a CD4 cell count > 200/microl were defined as immunological responders (IR). We performed a comprehensive study to characterize the immune response of InR.

METHODS

The immunological phenotype of peripheral blood mononuclear cells, thymic naive T cells, T-cell receptor Vbeta repertoire, serum concentration of interleukin (IL)-7, the expression of IL-7Ralpha on naive and memory CD4 and CD8 T cells, and regulatory T cells (Treg) were studied.

RESULTS

In InR a significant reduction (P < 0.0001) of naive and thymic naive CD4 T cells was associated with a reduced expression of IL-7Ralpha in both cell subsets, with an increased serum concentration of IL-7 was observed. Furthermore, an increased immune activation with a reduced Treg frequency and increased number of expansions of Vbeta families was observed.

CONCLUSIONS

The reduced expression of IL-7Ralpha associated with the persistent immune activation and the alteration of Treg frequencies in part explains the low level of CD4 T cells observed in InR.

CD4 T cell-mediated protection from lethal influenza: perforin and antibody-mediated mechanisms give a one-two punch

The mechanisms whereby CD4 T cells contribute to the protective response against lethal influenza infection remain poorly characterized. To define the role of CD4 cells in protection against a highly pathogenic strain of influenza, virus-specific TCR transgenic CD4 effectors were generated in vitro and transferred into mice given lethal influenza infection. Primed CD4 effectors conferred protection against lethal infection over a broad range of viral dose. The protection mediated by CD4 effectors did not require IFN-gamma or host T cells, but did result in increased anti-influenza Ab titers compared with untreated controls. Further studies indicated that CD4-mediated protection at high doses of influenza required B cells, and that passive transfer of anti-influenza immune serum was therapeutic in B cell-deficient mice, but only when CD4 effectors were present. Primed CD4 cells also acquired perforin (Pfn)-mediated cytolytic activity during effector generation, suggesting a second mechanism used by CD4 cells to confer protection. Pfn-deficient CD4 effectors were less able to promote survival in intact BALB/c mice and were unable to provide protection in B cell-deficient mice, indicating that Ab-independent protection by CD4 effectors requires Pfn. Therefore, CD4 effectors mediate protection to lethal influenza through at least two mechanisms: Pfn-mediated cytotoxicity early in the response promoted survival independently of Ab production, whereas CD4-driven B cell responses resulted in high titer Abs that neutralized remaining virus.

Decreased CD127 expression on T Cells in HIV-1-infected adults receiving antiretroviral therapy with or without intermittent IL-2 therapy

BACKGROUND

The interleukin-7 (IL-7)/IL-7 receptor alpha (IL-7Ralpha) system is an important regulator of T-cell homeostasis. We evaluated the IL-7/IL-7Ralpha system in a large cohort of HIV-infected patients, including a subset treated with intermittent IL-2.

METHODS

IL-7 serum levels and CD127 (IL-7Ralpha) expression on T cells were evaluated in a cross-sectional study of 36 healthy volunteers, 151 HIV-infected patients, and 83 HIV-infected patients who had received IL-2 therapy. Multivariate regression models were used to determine predictors of CD127 expression.

RESULTS

HIV-infected patients had higher IL-7 levels compared with healthy volunteers (P = 0.022) and IL-2-treated patients (P = 0.012). CD127 expression was significantly lower on CD4 and CD8 T cells of HIV-infected patients compared with healthy volunteers (P = 0.008 and P < 0.001, respectively), and CD127 median fluorescence intensity was lowest on CD4 T cells in IL-2-treated patients (P < 0.001 compared with HIV-infected patients). The proportion of naive and effector memory/effector T cells were significant predictors of CD127 expression on T cells. IL-2 immunotherapy led to the expansion of a CD25/CD127-low subset of CD4 T cells.

CONCLUSIONS

CD127 expression on T cells remains low in HIV-infected patients despite antiretroviral therapy, reflecting persistent aberration in the subset composition of the T-cell pool.

Age-associated changes within CD4+ T cells

As individuals age their ability to respond and clear pathogens declines, leading to a greater incidence and severity of infectious diseases. Additionally, the efficacy of vaccines is frequently decreased in elderly persons. Increased susceptibility to infections and reduced protection after vaccination reflect the impact of age-related changes on the immune system. The immune system undergoes a wide range of changes with increasing age. The aim of this review is to summarize cellular and molecular aspects of aging CD4(+) T cells. CD4(+) T cells play an essential role in mediating both humoral and cellular immune responses. Therefore, age-associated dysfunctions within CD4(+) T cells have a strong clinical impact. Improving our understanding of the aged CD4(+) T cells, in particular but also of the aged immune system in general, is crucial for developing effective prevention and treatment programs which will facilitate healthy aging and improve the quality of life of the elderly population.

Crucial role of MLL for the maintenance of memory T helper type 2 cell responses

The Mixed-Lineage Leukemia (MLL) gene, a mammalian homolog of the Drosophila trithorax, is implicated in regulating the maintenance of Hox gene expression and hematopoiesis. The physiological functions of MLL in the immune system remain largely unknown. Although MLL(+/-) CD4 T cells differentiate normally into antigen-specific effector Th1/Th2 cells in vitro, the ability of memory Th2 cells to produce Th2 cytokines was selectively reduced. Furthermore, histone modifications at the Th2 cytokine gene loci were not properly maintained in MLL(+/-) memory Th2 cells. The reduced expression of MLL in memory Th2 cells resulted in decreased GATA3 expression accompanied with impaired GATA3 locus histone modifications. The direct association of MLL with the GATA3 locus and the Th2 cytokine gene loci was demonstrated. Memory Th2 cell-dependent allergic airway inflammation was decreased in MLL(+/-) Th2 cell-transferred mice. Thus, a crucial role for MLL in the maintenance of memory Th2 cell function is indicated.

Divergent Generation of Heterogeneous Memory CD4 T Cells

Mechanisms for the generation of memory CD4 T cells and their delineation into diverse subsets remain largely unknown. In this study, we demonstrate in two Ag systems, divergent generation of heterogeneous memory CD4 T cells from activated precursors in distinct differentiation stages. Specifically, we show that influenza hemagglutinin- and OVA-specific CD4 T cells activated for 1, 2, and 3 days, respectively, exhibit gradations of differentiation by cell surface phenotype, IFN-gamma production, and proliferation, yet all serve as direct precursors for functional memory CD4 T cells when transferred in vivo into Ag-free mouse hosts. Using a conversion assay to track the immediate fate of activated precursors in vivo, we show that day 1- to 3-activated cells all rapidly convert from an activated phenotype (CD25(high)IL-7R(low)CD44(high)) to a resting memory phenotype (IL-7R(high)CD25(low)CD44(high)) 1 day after antigenic withdrawal. Paradoxically, stable memory subset delineation from undifferentiated (day 1- to 2-activated) precursors was predominantly an effector memory (CD62L(low)) profile, with an increased proportion of central memory (CD62L(high)) T cells arising from more differentiated (day 3-activated) precursors. Our findings support a divergent model for generation of memory CD4 T cells directly from activated precursors in multiple differentiation states, with subset heterogeneity maximized by increased activation and differentiation during priming.

CD127 expression inversely correlates with FoxP3 and suppressive function of human CD4+ T reg cells

Regulatory T (T reg) cells are critical regulators of immune tolerance. Most T reg cells are defined based on expression of CD4, CD25, and the transcription factor, FoxP3. However, these markers have proven problematic for uniquely defining this specialized T cell subset in humans. We found that the IL-7 receptor (CD127) is down-regulated on a subset of CD4⁺ T cells in peripheral blood. We demonstrate that the majority of these cells are FoxP3⁺, including those that express low levels or no CD25. A combination of CD4, CD25, and CD127 resulted in a highly purified population of T reg cells accounting for significantly more cells that previously identified based on other cell surface markers. These cells were highly suppressive in functional suppressor assays. In fact, cells separated based solely on CD4 and CD127 expression were anergic and, although representing at least three times the number of cells (including both CD25⁺CD4⁺ and CD25⁻CD4⁺ T cell subsets), were as suppressive as the “classic” CD4⁺CD25^hi T reg cell subset. Finally, we show that CD127 can be used to quantitate T reg cell subsets in individuals with type 1 diabetes supporting the use of CD127 as a biomarker for human T reg cells.

Bim mediates apoptosis of CD127(lo) effector T cells and limits T cell memory

Following an acute T cell response, most activated effector cells die, while some survive and become memory cells. The pro-apoptotic Bcl-2 family member, Bcl-2 interacting mediator of death (Bim) is critical for eliminating most effector T cells, while expression of CD127 (IL-7Ralpha) has been proposed to mark effector cells destined to become memory cells. Here, we examined the effects of Bim on the death of effector T cells in relationship to CD127 expression and on development of T cell memory following lymphocytic choriomeningitis virus (LCMV) infection. We found that large numbers of CD127(lo) LCMV-specific CD4(+) and CD8(+) T cells were lost in wild-type mice, but were spared in Bim(-/-) mice. Further, while the numbers of CD127(hi) T cells declined only slightly during contraction of the response in wild-type mice, they increased significantly in Bim(-/-) mice due to re-expression of CD127 on CD127(lo) T cells that had avoided apoptosis. Functional memory T cells were significantly increased in Bim(-/-) mice; however, they underwent a slow attrition due to decreased proliferative renewal. Taken together, these data suggest that the absence of Bim-mediated death of LCMV-specific CD4(+) and CD8(+) T cells in vivo can increase T cell memory, but other homeostatic mechanisms control the long-term maintenance of memory cells.

Control of CD4 + T‐cell memory by cytokines and costimulators

During T-cell priming, cytokines and costimulatory molecules provide important signals that determine the magnitude and quality of the response. Although the functions of defined cytokines and costimulators in the primary T-cell response are well characterized, much less is known about how these factors contribute to memory T-cell development and survival. Since memory cells are thought to be long-lived progeny of the primary response, it is conceivable that the same signals shaping initial T-cell expansion and differentiation also contribute to memory generation. Here, we review evidence and show novel data on the role of the cytokines interleukin-2 (IL-2) and IL-7 and the costimulator CD28 in CD4+ memory T-cell development. We emphasize that transient IL-2 and CD28 signals during priming imprint a long-lasting survival advantage in primed T cells, thus contributing to the persistence of a memory population. The requirement for IL-2 and CD28 signals is not linked to promoting T-cell division and expansion but most likely due to their capacity to (i) promote effector cell differentiation; (ii) induce survival proteins, and, as we discuss in more detail; (iii) program expression of receptors for 'memory survival factors' such as IL-7. Studies exploring the therapeutic potential of these insights are also discussed.

Suppressor of cytokine signaling 1 stringently regulates distinct functions of IL-7 and IL-15 in vivo during T lymphocyte development and homeostasis

SOCS1(-/-) mice accumulate within the thymus and periphery CD8(+) lymphocytes that express memory cell markers and display heightened in vitro responses to common gamma-chain cytokines. To investigate whether dysregulated homeostasis of T lymphocytes and acquisition of memory phenotype by CD8(+) cells in SOCS1(-/-) mice were mediated by IL-7 and/or IL-15 in vivo, we have generated SOCS1(-/-)IL-7(-/-), SOCS1(-/-)IL-15(-/-) and SOCS1(-/-)IL-7(-/-)IL-15(-/-) mice. We observed that in mice lacking SOCS1, either IL-7 or IL-15 skewed thymocyte development toward CD8 lineage, whereas IL-15 is the principal mediator of dysregulated homeostasis in the periphery. Homeostatic proliferation of SOCS1(-/-) CD8(+) lymphocytes in Rag1(-/-), Rag1(-/-)IL-7(-/-), Rag1(-/-)IL-15(-/-), and Rag1(-/-)IL-7(-/-)IL-15(-/-) mice showed that SOCS1 deficiency did not overcome the requirement for IL-7 and IL-15 to sustain homeostatic expansion. Differential expression of memory phenotype markers CD44, CD122, and Ly6C by SOCS1(-/-)IL-15(-/-) CD8(+) lymphocytes suggest that multiple signals contributed to the memory cell differentiation program. To address whether increased IL-15 responsiveness of SOCS1(-/-) CD8(+) lymphocytes required prior TCR sensitization, we generated SOCS1(-/-) H-Y TCR transgenic (Tg) mice. Using female SOCS1(-/-) H-Y TCR(tg) mice in Rag1(+/+) and Rag1(-/-) backgrounds, we show that acquisition of the memory phenotype by SOCS1-deficient CD8(+) lymphocytes did not require prior antigenic stimulation, but required the presence of activated T cells. SOCS1 deficiency accelerated the maturation of CD8 single-positive thymocytes expressing Tg TCR, but did not compromise negative selection in HY-TCR(tg) males. Our findings illustrate distinct functions for IL-7 and IL-15 in T lymphocyte development and homeostasis, and stringent regulation of these processes by SOCS1.

IL-7Ralpha expression on CD4+ T lymphocytes decreases with HIV disease progression and inversely correlates with immune activation

Many factors can influence the rate of HIV disease progression, including those that maintain T cell homeostasis. One key homeostatic regulator is the IL-7 receptor (IL-7R). Previous studies have shown IL-7R expression levels decrease in HIV infection, but effects on memory subtypes, CD4(+) T cells, and cell function have not been explored. The present study examined the expression of the IL-7Ralpha chain on naïve and memory T lymphocyte subsets of both HIV-positive and HIV-negative individuals from Nairobi, Kenya to assess the role of IL-7Ralpha in HIV disease. Expression of IL-7Ralpha was significantly reduced in all CD4(+) and CD8(+) T cell subsets in HIV-positive individuals. This reduction was further enhanced in those with advanced HIV progression. Expression of IL-7Ralpha was inversely correlated to immune activation, and apoptosis, and was positively correlated with CD4 count in both bivariate and multivariate analysis. Expression of IL-7Ralpha did not correlate with HIV viral loads, indicating the elevated immune activation seen in HIV-infected individuals may be impacting expression of IL-7Ralpha, independent of viral loads. Signaling via the IL-7R is essential for T cell homeostasis and maintenance of T cell memory. Reduction of this receptor may contribute to the homeostatic disruption seen in HIV.

Diverse functions of IL-2, IL-15, and IL-7 in lymphoid homeostasis

IL-2, IL-15, and IL-7 are cytokines that are critical for regulating lymphoid homeostasis. These cytokines stimulate similar responses from lymphocytes in vitro, but play markedly divergent roles in lymphoid biology in vivo. Their distinct physiological functions can be ascribed to distinct signaling pathways initiated by proprietary cytokine receptor chains, differential expression patterns of the cytokines or their receptor chains, and/or signals occurring in distinct physiological contexts. Recently, the discovery of a novel mechanism of cytokine signaling, trans-presentation, has provided further insights into the different ways these cytokines function. Trans-presentation also raises several novel cell biological and cellular implications concerning how cytokines support lymphoid homeostasis.

CD4 memory T cells survive and proliferate but fail to differentiate in the absence of CD40

Secondary T cell responses are enhanced because of an expansion in numbers of antigen-specific (memory) cells. Using major histocompatibility complex class II tetramers we have tracked peptide-specific endogenous (non–T cell receptor transgenic) CD4 memory T cells in normal and in costimulation-deficient mice. CD4 memory T cells were detectable after immunization for more than 200 days, although decay was apparent. Memory cells generated in CD40 knockout mice by immunization with peptide-pulsed wild-type dendritic cells survived in the absence of CD40 and proliferated when boosted with peptide (plus adjuvant) in a CD40-independent fashion. However, differentiation of the memory cells into cytokine-producing effector cells did not occur in the absence of CD40. The data indicate that memory cells can be generated without passing through the effector cell stage.

Effector T Cell Differentiation and Memory T Cell Maintenance Outside Secondary Lymphoid Organs

Naive T cell circulation is restricted to secondary lymphoid organs. Effector and memory T cells, in contrast, acquire the ability to migrate to nonlymphoid tissues. In this study we examined whether nonlymphoid tissues contribute to the differentiation of effector T cells to memory cells and the long-term maintenance of memory T cells. We found that CD4, but not CD8, effector T cell differentiation to memory cells is impaired in adoptive hosts that lack secondary lymphoid organs. In contrast, established CD4 and CD8 memory T cells underwent basal homeostatic proliferation in the liver, lungs, and bone marrow, were maintained long-term, and functioned in the absence of secondary lymphoid organs. CD8 memory T cells found in nonlymphoid tissues expressed both central and effector memory phenotypes, whereas CD4 memory T cells displayed predominantly an effector memory phenotype. These findings indicate that secondary lymphoid organs are not necessary for the maintenance and function of memory T cell populations, whereas the optimal differentiation of CD4 effectors to memory T cells is dependent on these organs. The ability of memory T cells to persist and respond to foreign Ag independently of secondary lymphoid tissues supports the existence of nonlymphoid memory T cell pools that provide essential immune surveillance in the periphery.

IL-7 promotes T cell proliferation through destabilization of p27Kip1

Interleukin (IL)-7 is required for survival and homeostatic proliferation of T lymphocytes. The survival effect of IL-7 is primarily through regulation of Bcl-2 family members; however, the proliferative mechanism is unclear. It has not been determined whether the IL-7 receptor actually delivers a proliferative signal or whether, by promoting survival, proliferation results from signals other than the IL-7 receptor. We show that in an IL-7–dependent T cell line, cells protected from apoptosis nevertheless underwent cell cycle arrest after IL-7 withdrawal. This arrest was accompanied by up-regulation of the cyclin-dependent kinase inhibitor p27^Kip1 through a posttranslational mechanism. Overexpression of p27^Kip1 induced G1 arrest in the presence of IL-7, whereas knockdown of p27^Kip1 by small interfering RNA promoted S phase entry after IL-7 withdrawal. CD4 or CD8 T cells transferred into IL-7–deficient hosts underwent G1 arrest, whereas 27^Kip1-deficient T cells underwent proliferation. We observed that IL-7 withdrawal activated protein kinase C (PKC)θ and that inhibition of PKCθ with a pharmacological inhibitor completely blocked the rise of p27^Kip1 and rescued cells from G1 arrest. The conventional pathway to breakdown of p27^Kip1 is mediated by S phase kinase-associated protein 2; however, our evidence suggests that PKCθ acts via a distinct, unknown pathway inducing G1 arrest after IL-7 withdrawal from T cells. Hence, IL-7 maintains T cell proliferation through a novel pathway of p27^Kip1 regulation.

Homeostasis of the memory T cell pool

Memory T cells are critical for the establishment of long-term immunity. The number of memory T cells formed at the conclusion of the primary response is strongly influenced by the number of effector T cells generated in the response, but some factors can additionally enhance the efficiency and quality of memory cell recruitment. Homeostasis of the memory T cell pool depends on cytokine-mediated regulation of cell survival and proliferation. This review discusses factors that influence both the development and the maintenance of the memory T cell pool.

Vaccination of human subjects expands both specific and bystander memory T cells but antibody production remains vaccine specific

Human subjects maintain long-term immunologic memory against infective organisms but the mechanism is unclear. CD4+ T-helper memory (Thmem) cells are pivotal in controlling humoral and cellular responses, therefore their longevity and response to vaccination are critical for maintenance of protective immunity. To probe the dynamics of the Thmem-cell response to antigenic challenge, we investigated subjects following a booster injection with tetanus toxoid (TT). Expansion of TT-specific Thmem cells and cytokine production showed complex kinetics. Strikingly, parallel expansion and cytokine production occurred in pre-existing Thmem cells specific for 2 other common antigens: purified protein derivative of tuberculin and Candida albicans. Bystander expansion occurred in Thmem but not in Thnaive cells. Antibody production against TT peaked approximately 2 weeks after vaccination and gradually declined. However, pre-existing antibody against the other antigens did not change. It appears that although all Thmem cells are readily stimulated to expand, antibody responses are controlled by antigen availability. These findings relate to the maintenance of memory and have consequences for assessments of specific T-cell responses to vaccination.

Loss of IL-7Ralpha is associated with CD4 T-cell depletion, high interleukin-7 levels and CD28 down-regulation in HIV infected patients

OBJECTIVE

Elevated levels of interleukin (IL)-7 are present in the blood of HIV-positive patients and it is known that IL-7 receptor (IL-7R)alpha expression decreases on T cells during HIV infection. The subset(s) of T cells with low IL-7Ralpha and the consequence of low IL-7Ralpha expression for T-cell survival are poorly characterized.

DESIGN

The frequency of IL-7Ralpha-negative T cells in HIV-positive patients was studied in relation to CD4 T-cell counts, IL-7 concentration and survival in culture. We analysed IL-7Ralpha expression in different T-cell populations and in relation to Bcl-2 expression.

METHODS

Specimens from 38 HIV-1 patients and 17 controls were examined. IL-7Ralpha and Bcl-2 expression in different T-cell populations was studied by flow cytometry. The influence of IL-7Ralpha expression on T-cell survival was studied by culturing T cells in the presence of IL-7.

RESULTS

Down-regulation of IL-7Ralpha on T cells correlated with depletion of CD4 T cells (P < 0.001) and also with increased concentration of serum IL-7 (P < 0.05). The decreased IL-7Ralpha expression was associated with low Bcl-2 expression and with the reduced survival capacity of T cells in the presence of IL-7 in vitro. Particularly, T cells with memory phenotype showed a decreased IL-7Ralpha expression in association with CD28 down-regulation.

CONCLUSIONS

The positive effects of IL-7 on survival and homeostatic proliferation of T cells might be severely impaired in HIV-infected individuals due to IL-7Ralpha down-regulation. Differentiation towards a CD28-negative memory phenotype in response to chronic activation may lead to an overall decrease of IL-7 mediated survival within the peripheral T-cell pool.

Skin-derived interleukin-7 contributes to the proliferation of lymphocytes in cutaneous T-cell lymphoma

Cutaneous T-cell lymphomas (CTCLs) are malignancies of T cells that have a special affinity for the skin. We have previously reported that much of the T-cell receptor repertoire is altered in CTCL, and both malignant and nonmalignant clones are numerically expanded, presumably in response to T-cell trophic cytokines. We therefore examined levels of the T-cell trophic cytokines IL-2, IL-4, IL-7, IL-12, IL-13, and IL-15 in plasma in 93 CTCL patients and healthy controls. Only IL-7 levels were elevated in CTCL. We next looked at lesional skin from patients with CTCL and found elevated levels of IL-7 mRNA. Explant cultures of normal and lesional CTCL skin biopsies revealed significantly more IL-7 protein production in CTCL skin. Additionally, cultures of CTCL skin released greater numbers of T cells than normal skin; this was blocked by the addition of an IL-7 neutralizing antibody. Finally, these cultures induced proliferation of normal peripheral skin-homing T cells that were added to the cultures. These observations led us to postulate that IL-7 produced by skin cells contributes to the survival and proliferation of T cells within skin lesions and is likely the source of elevated circulating IL-7 in CTCL.

CD28 is required for induction and maintenance of immunological memory in toxin-reactive CD4+ T cells in vivo

We previously reported that Vbeta3+ CD4+ T cells maintained a protracted expansion, with the phenotypes of memory Th2 cells, for 30 days in C57BL/6 (B6) mice implanted with SEA-containing mini-osmotic pumps. In the present study, we followed the fate of Vbeta3+ CD4+ T cells in CD28-/- mice. Vbeta3+ CD4+ T cells increased to a degree similar to that of B6 Vbeta3+ CD4+ T cells until day 10 after implantation, then declined rapidly reaching the control level by 28 days. Remaining Vbeta3+ CD4+ T cells at that time did not exhibit memory phenotypes nor Th2-deviated responses. The rapid drop in Vbeta3+ CD4+ T cells in CD28-/- mice was attributable to upregulated induction of apoptosis owing to marginal inductions of Bcl-2 and Bcl-xL. Collectively, these data indicate CD28 to play critical roles in the generation and maintenance of SEA-reactive CD4+ T cells in vivo.

Peptide-stimulated DO11.10 T cells divide well but accumulate poorly in the absence of TLR agonist treatment

Immunological adjuvants increase the clonal burst size of antigen-specific T cell populations by mechanisms that remain incompletely understood. Using the DO11.10 adoptive transfer system to study peptide-stimulated T cell responses, we found that TLR agonist treatment increased the extent of cellular division undergone by responding T cells, but not by enough to explain the net increases in T cell yield that were achieved. Two novel analyses involving CFSE dye dilution analysis were used to characterize the shortfall, both of which were consistent with the idea that DO11.10 T cells are frequently lost during proliferation unless TLR agonists are present. T cell loss during clonal expansion was correlated with decreased levels of Bcl-2, but TLR agonists did not appear to afford protection by restoring levels of Bcl-2 or of cell surface IL-7Ralpha chain expression. TLR-mediated protection also failed to correlate with increased expression of Bcl-x or decreased expression of pro-apoptotic Bim. Our findings suggest that DO11.10 T cells stimulated by antigenic peptide in vivo divide well, but fail to accumulate efficiently unless TLR agonists are present.

Regulation of CD4 T cell memory by OX40 (CD134)

CD4 memory T cells play a critical role in protection against repeated exposure to infectious agents such as viruses, bacteria, and helminth parasites, yet can also contribute to the aberrant immune responses associated with autoimmune and allergic reactions. Understanding the mechanisms that control effective memory responses has important ramifications for vaccine design and in the management of adverse immune reactions. Recent advances in studies of T cell memory have implicated the tumor-necrosis-factor receptor (TNFR) family member, OX40 (CD134), as a key co-stimulatory molecule involved in the regulation of CD4 memory T cells. In this review we discuss these new developments in the context of past research and current models for the generation, persistence, and re-activation of memory T cells.

Simultaneous Th1-type cytokine expression is a signature of peritoneal CD4+ lymphocytes responding to infection with Listeria monocytogenes

The robust murine response to infection with Listeria monocytogenes makes an excellent model to study the functional development of immune cells. We investigated the cellular immune response to i.p. infection using intracellular cytokine staining to identify Ag-specific lymphocytes. CD4(+) peritoneal exudate cells obtained 10 days postinfection predominantly coexpressed TNF-alpha, IFN-gamma, and IL-2 after polyclonal or Ag stimulation. A population of cells simultaneously making TNF-alpha and IFN-gamma was also detected but at a lower frequency. By following the kinetics of the response to Listeria, we found that CD4(+) lymphocytes coexpressing TNF-alpha and IFN-gamma dominated on day 6 postinfection and then declined. From days 10-27, TNF-alpha(+)IFN-gamma(+)IL-2(+) (triple-positive) was the most prevalent cytokine phenotype, and the frequency steadily declined. These characteristic cytokine expression patterns were observed in both primary and secondary responses to Listeria infection and developed even when infection was terminated with antibiotic treatment. A cytokine-assisted immunization procedure resulted in both double- and triple-positive cells, but the clear predominance of triple-positive cells required Listeria infection. Triple-positive cells were preferentially noted in the peritoneal cavity tissue site; spleen cells displayed a predominant population of double-positive T cells (TNF-alpha(+)IFN-gamma(+)). We speculate that the appearance of triple-positive cells represents a functionally significant subset important in host defense at nonlymphoid tissue sites.

Effects of increasing IL-7 availability on lymphocytes during and after lymphopenia-induced proliferation

IL-7 is critically involved in regulating peripheral T cell homeostasis. To investigate the role of IL-7 on lymphopenia-induced proliferation of polyclonal lymphocytes, we have transferred CFSE-labeled cells into a novel T-lymphopenic, IL-7-transgenic mouse line. Results obtained indicate that T and B cells do not respond in the same way to IL-7-homeostatic signals. Overexpression of IL-7 enhances proliferation of both CD4(+) and CD8(+) T cells but with distinctly temporal effects. Expansion of naturally arising CD4(+)-regulatory T cells was like that of conventional CD4(+) T cells. IL-7 had no effect on B cell proliferation. By immunohistology, transferred T cells homed to T cell areas of spleen lymphoid follicles. Increasing IL-7 availability enhanced T cell recovery by promoting cell proliferation and reducing apoptosis during early stages of lymphopenia-induced proliferation. Taken together, these results provide new insights into the pleiotropic effects of IL-7 on lymphopenia-induced T cell proliferation.

Two sides of a cellular coin: CD4(+)CD3- cells regulate memory responses and lymph-node organization

We propose that CD4(+)CD3(-) cells have two functions: a well-established role in organizing lymphoid tissue during development, and a newly discovered role in supporting T-cell help for B cells both during affinity maturation in germinal centres and for memory antibody responses. As CD4(+)CD3(-) cells express the HIV co-receptors CD4 and CXC-chemokine receptor 4, we think that infection of these cells by HIV, and their subsequent destruction by the host immune system, could help to explain the loss of memory antibody responses and the destruction of lymphoid architecture that occur during disease progression to AIDS.

The many faces of IL-7: from lymphopoiesis to peripheral T cell maintenance

IL-7 is well known as a lymphopoietic cytokine, but recent studies have also identified a critical role for IL-7 in peripheral T cell homeostasis. IL-7 is well poised to serve as a homeostatic cytokine because it is produced by resting stromal cells, the IL-7R is present on most T cells, and IL-7 down-regulates its own receptor. These features allow IL-7 to signal large numbers of resting T cells and to be efficiently used when supplies are limiting. Consistent with this, in normal hosts, IL-7 is required for survival of naive T cell populations, and IL-7 contributes to homeostatic cycling of naive and memory cells. In addition, lymphopenic hosts accumulate increased levels of IL-7, and the supranormal levels are largely responsible for inducing homeostatic peripheral expansion in response to lymphopenia. Thus, IL-7 plays critical and nonredundant roles in both T cell lymphopoiesis and in maintaining and restoring peripheral T cell homeostasis.

Understanding the generation and function of memory T cell subsets

Memory T cells can be broadly divided into central memory and effector memory subsets, which are endowed with different capacities to home to lymphoid or non-lymphoid tissues, to proliferate in response to antigen or cytokines and to perform effector functions. In the past few years progress has been made in understanding the properties of these memory T cell subsets and, in particular, the signals required for their generation and maintenance. Collectively these data point to a critical role of central memory T cells in conferring long-term immunity.

Cutting edge: emergence of CD127high functionally competent memory T cells is compromised by high viral loads and inadequate T cell help

In this report we have inspected whether difficulties in controlling viral infections negatively impacts the generation of CD127(high) memory T cells. Using both MHC class I and II tetramers we reveal that CD127(low) T cells are not necessarily rapidly deleted but can persist in a pseudoeffector state in which they display the hallmarks of activated effector cells but are functionally inferior. CD127(high) cells can, however, emerge if the infection is contained. We also show that in the absence of CD4 T cell help significant populations of CD127(high) CD8 T cells fail to emerge. Analyses of cytokine-producing activities by both mouse and human CD8 T cells further document that the extended maintenance of T cells in a CD127(low) state has functional consequences which manifest as an impairment of IL-2 production.

Effects of lymphocyte isolation and timing of processing on detection of CD127 expression on T cells in human immunodeficiency virus-infected patients

Decreases in the detection of CD127 expression on T cells of human immunodeficiency virus-infected patients by flow cytometry can occur by delayed processing or by peripheral blood mononuclear cell isolation and cryopreservation. These observations should be considered in the interpretation of functional studies and the planning of multicenter clinical trials.

Evolution of diverse antiviral CD8+ T cell populations after murine cytomegalovirus infection

Cytomegalovirus (CMV) is a major human pathogen normally controlled by cellular immune responses. The infection can be modeled in the mouse using murine CMV (MCMV). During the latent phase of infection, two different patterns of CD8(+) T cell responses have been observed: some specificities show increasing frequencies over time ("memory inflation"), while others, which are present acutely, are barely detectable at later time points. This distinction is independent of initial immunodominance. We analyzed the extent to which such responses differ functionally and tracked both their population distribution and their evolution over time. We observed two clear patterns of memory development that diverged early after infection. Acutely, CD8(+) T cells directed against all epitopes showed similar activation, phenotype and distribution. Thereafter, one set of responses ("inflationary") increased in frequency over time, was found in high numbers in non-lymphoid organs and was associated with an activated (CD28(low) CD27(low)CD122(low)) phenotype. In contrast, CD8(+) T cells responses specific for other MCMV epitopes ("non-inflationary") showed a slow reversion to a classical "central" memory phenotype without enrichment in non-lymphoid organs. A simple model to describe the equilibrium state in MCMV is presented, which may point to previously unexplored antiviral populations present after human CMV infection.

T cells in G1 provide a memory-like response to secondary stimulation

The commitment of naive T cells to proliferate is a function of the strength and duration of stimuli mediated by the TCR and coreceptors. Ranges of 2-20 h of stimulation have been reported as necessary in vitro. Whether T cells actually experience uninterrupted stimulation for such long periods under physiological conditions is controversial. Here we ask whether commitment to proliferate requires continuous stimulation, or can T cells integrate intermittent periods of stimulation. T cells were stimulated for two short-term (subthreshold) periods (5-7 h) either sequentially or separated by an interval of rest. Naive lymph node T cells were able to integrate interrupted stimulation, even when the duration of rest was as long as 2 days. Furthermore, when short-term-stimulated T cells were separated by density, three populations were observed: low density blasts, intermediate density G(1) cells, and high density G(0) cells. Low density cells progressed to division without further stimulation, whereas G(0) and G(1) cells remained undivided. However, after a period of rest, a second subthreshold stimulation caused the G(1) but not the G(0) fraction to quickly proceed through the cell cycle. We conclude that noncycling T cells in the G(1) phase of the cell cycle remain in a state of readiness for prolonged periods of time, and may represent a population of memory-like effectors capable of responding rapidly to antigenic challenge.

Mice deficient in OX40 and CD30 signals lack memory antibody responses because of deficient CD4 T cell memory

Recently, we reported that a CD4(+)CD3(-)CD11c(-) accessory cell provided OX40-dependent survival signals to follicular T cells. These accessory cells express both OX40 ligand and CD30 ligand, and the receptors, OX40 and CD30, are both expressed on Th2-primed CD4 T cells. OX40 and CD30 signals share common signaling pathways, suggesting that CD30 signals might substantially compensate in OX40-deficient mice. In this report we have dissected the signaling roles of CD30 alone and in combination with OX40. CD30-deficient mice showed an impaired capacity to sustain follicular germinal center responses, and recall memory Ab responses were substantially reduced. Deficiencies in OX40 and CD30 signals were additive; secondary Ab responses were ablated in double-deficient mice. Although the initial proliferation of OX40/CD30 double-knockout OTII transgenic T cells was comparable to that of their normal counterparts, they failed to survive in vivo, and this was associated with reduced T cell numbers associated with CD4(+)CD3(-) cells in B follicles. Finally, we show that OX40/CD30 double-knockout OTII transgenic T cells fail to survive compared with normal T cells when cocultured with CD4(+)CD3(-) cells in vitro.

Cytokine-driven cell cycling is mediated through Cdc25A

Lymphocytes are the central mediators of the immune response, requiring cytokines for survival and proliferation. Survival signaling targets the Bcl-2 family of apoptotic mediators, however, the pathway for the cytokine-driven proliferation of lymphocytes is poorly understood. Here we show that cytokine-induced cell cycle progression is not solely dependent on the synthesis of cyclin-dependent kinases (Cdks) or cyclins. Rather, we observe that in lymphocyte cell lines dependent on interleukin-3 or interleukin-7, or primary lymphocytes dependent on interleukin 7, the phosphatase Cdc25A is the critical mediator of proliferation. Withdrawal of IL-7 or IL-3 from dependent lymphocytes activates the stress kinase, p38 MAPK, which phosphorylates Cdc25A, inducing its degradation. As a result, Cdk/cyclin complexes remain phosphorylated and inactive and cells arrest before the induction of apoptosis. Inhibiting p38 MAPK or expressing a mutant Cdc25A, in which the two p38 MAPK target sites, S75 and S123, are altered, renders cells resistant to cytokine withdrawal, restoring the activity of Cdk/cyclin complexes and driving the cell cycle independent of a growth stimulus.

Early proliferation of CCR5(+) CD38(+++) antigen-specific CD4(+) Th1 effector cells during primary HIV-1 infection

We investigated whether HIV-1 antigen-specific CD4(+) T cells expressed the viral coreceptor CCR5 during primary HIV-1 infection (PHI). In the peripheral blood of subjects with very early PHI (< 22 days after onset of symptoms), there was a 10- to 20-fold increase in the proportion of highly activated (CD38(+++)) and proliferating (Ki-67(+)) CD4(+) T cells that expressed CCR5(+), and were mostly T-cell intracellular antigen-1 (TIA-1)(+) perforin(+) granzyme B(+). Inthe same patient samples, CD4(+) T cells producing interferon (IFN)-gamma in response to HIV group-specific antigen (Gag) peptides were readily detected (median, 0.58%) by intracellular cytokine assay-these cells were again predominantly CD38(+++), Ki-67(+), and TIA-(++), as well as Bcl-2(low). On average, 20% of the Gag-specific CD4(+) T cells also expressed interleukin-2 (IL-2) and were CD127 (IL-7R)(+). Taken together, these results suggest that Gag-specific T-helper 1 (Th1) effector cells express CCR5 during the primary response and may include precursors of long-term self-renewing memory cells. However, in PHI subjects with later presentation, antigen-specific CD4(+) T cells could not be readily detected (median, 0.08%), coinciding with a 5-fold lower level of the CCR5(+)CD38(+++) CD4(+) T cells. These results suggest that the antiviral response to HIV-1 infection includes highly activated CCR5(+)CD4(+) cytotoxic effector cells, which are susceptible to both apoptosis and cytopathic infection with HIV-1, and rapidly decline.

Chemokines: control of primary and memory T-cell traffic

Motility is a hallmark of leukocytes, and breakdown in the control of migration contributes to many inflammatory diseases. Chemotactic migration of leukocytes largely depends on adhesive interaction with the substratum and recognition of a chemoattractant gradient. Chemokines are secreted proteins and have emerged as key controllers of integrin function and cell locomotion. Numerous distinct chemokines exist that target all types of leukocytes, including hematopoietic precursors, leukocytes of the innate immune system, as well as naive memory, and effector lymphocytes. The combinatorial diversity in responsiveness to chemokines ensures the proper tissue distribution of distinct leukocyte subsets under normal and pathological conditions. Inflammatory chemokines are readily detected in lesional tissue and local cellular infiltrates carry corresponding chemokine receptors. Blocking of inflammatory chemokines represents a promising strategy for the development of novel anti-inflammatory therapeutics. This review focuses on a separate class of chemokines, termed homeostatic chemokines, with steady-state production at diverse sites, including primary and secondary lymphoid tissues as well as peripheral (extralymphoid) tissues. More precisely, we discuss the chemokines involved in T-cell traffic during the initiation of adaptive immunity and compare the distinct migration properties of short-lived effector T cells and long-lived memory T cells. Memory T cells are currently classified according to the presence of the lymph node-homing receptor CCR7 into CCR7+ central memory T (T(CM)) cells and CCR7- effector memory T (T(EM)) cells. For better understanding memory T-cell function, we propose the distinction of a third category, termed peripheral immune surveillance T (T(PS)) cells, which typically reside in healthy peripheral tissues, such as skin, lung, and gastrointestinal tract. Localization and relocation of memory T cells is strictly related to their function in recall responses. Therefore, detailed knowledge of their generation and tissue distribution may help to design better vaccination strategies.

IL-6 Increases Primed Cell Expansion and Survival

Cytochrome c-specific CD4 T cells from transgenic donors transferred to syngeneic B10.A mice expand more vigorously upon immunization if exogenous IL-6 is provided during the initial phase of immunization. The resultant increase in the frequency and number of Ag-specific cells is observed in the blood, lymph nodes, spleen, liver, and lung and persists for at least 3 mo. Treatment of immunized recipients with anti-IL-6 or use of IL-6 knockout recipients reduced the frequency of Ag-specific CD4 T cells during a comparable period, indicating that IL-6 is physiologically involved in the expansion of memory and/or effector cells and thus in the persistence of memory. IL-6 did not alter the duration of Ag-presenting activity. Both CFSE dilution studies and labeling with BrdU indicated that IL-6 does not effect proliferative rates of responding CD4 T cells. By contrast, annexin V staining was diminished in responding cells from the IL-6-treated animals, particularly among those cells that had undergone five or more divisions. These results indicate that IL-6 reduces the level of apoptosis among Ag-stimulated cells; thus, it plays a central role in determining numbers of memory and/or effector CD4 T cells in response to immunization over extended periods.

IL-7 exacerbates chronic colitis with expansion of memory IL-7Rhigh CD4+ mucosal T cells in mice

We have previously demonstrated that mucosal CD4(+) T cells expressing high levels of IL-7 receptor (IL-7R(high)) are pathogenic cells responsible for chronic colitis. Here we investigate whether IL-7 is directly involved in the expansion of IL-7R(high) memory CD4(+) mucosal T cells and the exacerbation of colitis. We first showed that CD4(+) lamina propria lymphocytes (LPLs) from wild-type, T cell receptor-alpha-deficient (TCR-alpha(-/-)), and recombinase-activating gene (RAG)-2(-/-)-transferred mice with or without colitis showed phenotypes of memory cells, but only CD4(+) LPLs from colitic mice showed IL-7R(high). In vitro stimulation by IL-7, but not by IL-15 and thymic stromal lymphopoietin, enhanced significant proliferative responses and survival of colitic CD4(+), but not normal CD4(+) LPLs. Importantly, in vivo administration of IL-7 mice accelerated the expansion of IL-7R(high) memory CD4(+) LPLs and thereby exacerbated chronic colitis in RAG-2(-/-) mice transferred with CD4(+) LPLs from colitic TCR-alpha(-/-) mice. Conversely, the administration of anti-IL-7R monoclonal antibody significantly inhibited the development of TCR-alpha(-/-) colitis with decreased expansion of CD4(+) LPLs. Collectively, the present data indicate that IL-7 is essential for the expansion of pathogenic memory CD4(+) T cells under pathological conditions. Therefore, therapeutic approaches targeting the IL-7R pathway may be feasible in the treatment of human inflammatory bowel disease.

Repeated stimulation of CD4 effector T cells can limit their protective function

Chronic infections often result in CD8 T-cell deletion or functional nonresponsiveness. However, to date no definitive studies have attempted to determine the impact of repeated T cell receptor stimulation on CD4 effector T cell generation. We have determined that when antigen presentation is limited to 2 d, optimum in vitro CD4 effector generation is achieved. Alternatively, repeated stimulation results in decreased CD4 effector expansion, decreased cytokine production, and altered migration. Similarly, functionally impaired effectors develop in vivo when antigen-pulsed antigen-presenting cells are replenished every 24 h during a primary immune response. CD4 effectors that are generated with repeated stimulation provide no protection during influenza infection, and have an impaired ability to provide cognate help to B cells. These results suggest that duration of antigen presentation dictates CD4 effector function, and repeated T cell receptor stimulation in vitro and in vivo that exceeds an optimal threshold results in effectors with impaired function.

IL-7: maintaining T-cell memory and achieving homeostasis

During an immune response, peripheral T-cell populations expand and then contract as the response subsides, thus maintaining a fairly constant number of CD4 and CD8 T cells throughout the life of the individual. The important factors that control this homeostasis are now beginning to be understood. Interleukin-7 (IL-7) has emerged as a central regulator of the survival and homeostasis of CD4 and CD8 T cells. Both naive and memory T-cell populations are highly dependent on the presence of IL-7 for their persistence and survival. In this Review, we discuss the role of IL-7 in the survival and homeostasis of naive and memory T cells and how that role is regulated by other factors.

Dynamic regulation of IL-7 receptor expression is required for normal thymopoiesis

Interleukin-7 receptor (IL-7R) levels are tightly controlled during ontogeny: high on double-negative (DN) cells, absent on double-positive (DP) cells, and high once again on thymocytes undergoing positive selection. To determine if loss of IL-7–mediated survival signals in DP cells is necessary for normal antigen-specific selection, we created T-lineage–specific IL-7R α chain (IL-7Rα) transgenic (Tg) mice in which IL-7R is expressed throughout ontogeny. There was no effect of the IL-7Rα Tg on negative selection. Surprisingly, however, although the thymi of IL-7Rα Tg mice were comparable at birth, there was a decrease in thymocyte number as the mice aged. This was found to be due to competition between DN and IL-7R–expressing DP cells for endogenous IL-7, which resulted in decreased levels of Bcl-2 in DN cells, increased DN apoptosis, and decreased DN cell number. Therefore, the down-regulation of IL-7R on DP cells is an “altruistic” act required for maintaining an adequate supply of local IL-7 for DN cells.

Duration of Infection and Antigen Display Have Minimal Influence on the Kinetics of the CD4+T Cell Response toListeria monocytogenesInfection

The T cell response to infection consists of clonal expansion of effector cells, followed by contraction to memory levels. It was previously thought that the duration of infection determines the magnitude and kinetics of the T cell response. However, recent analysis revealed that transition between the expansion and contraction phases of the Ag-specific CD8+ T cell response is not affected by experimental manipulation in the duration of infection or Ag display. We studied whether the duration of infection and Ag display influenced the kinetics of the Ag-specific CD4+ T cell response to Listeria monocytogenes (LM) infection. We found that truncating infection and Ag display with antibiotic treatment as early as 24 h postinfection had minimal impact on the expansion or contraction of CD4+ T cells; however, the magnitudes of the Ag-specific CD4+ and CD8+ T cell responses were differentially affected by the timing of antibiotic treatment. Treatment of LM-infected mice with antibiotics at 24 h postinfection did not prevent generation of detectable CD4+ and CD8+ memory T cells at 28 days after infection, vigorous secondary expansion of these memory T cells, or protection against a subsequent LM challenge. These results demonstrate that events within the first few days of infection stimulate CD4+ and CD8+ T cell responses that are capable of carrying out the full program of expansion and contraction to functional memory, independently of prolonged infection or Ag display.

CD4 T-cell memory

CD4 T-cell memory is in some ways more enigmatic than CD8 T-cell memory. This is mostly due to the fact that CD4 T cells tend to expand far less in response to antigenic stimuli, thereby thwarting attempts at their detection during the course of an immune response. Nevertheless, there is a wide range of experimental models that have provided information regarding the survival and maintenance of CD4 memory cells, their functional capacities, their differentiation states and program of development following activation. The emerging picture is one of great versatility and functional heterogeneity as befits their central position within the immune system.

IL-15-independent proliferative renewal of memory CD8+ T cells in latent gammaherpesvirus infection

IL-15 is known to be critical in the homeostasis of Ag-specific memory CD8(+) T cells following acute viral infection. However, little is known about the homeostatic requirements of memory CD8(+) T cells during a latent viral infection. We have used the murine gammaherpesvirus-68 (MHV-68) model system to investigate whether IL-15 is necessary for the maintenance of memory CD8(+) T cells during a latent viral infection. IL-15 is not essential either for the initial control of MHV-68 infection or for the maintenance of MHV-68-specific memory CD8(+) T cells. Even at 140 days postinfection, the proportion of CD8(+) T cells recognizing the MHV-68 epitopes were the same as in control mice. The maintenance of these memory CD8(+) T cells was attributable to their ability to turn over in vivo, probably in response to the presence of low levels of Ag. IL-15(-/-) mice had a significantly higher turnover rate within the virus-specific memory CD8(+) T cell population, which was the result of increased levels of viral gene expression rather than an increase in viral load. These cells did not accumulate in the spleens of the IL-15(-/-) mice due to an increased sensitivity to apoptosis as a result of decreased Bcl-2 levels. Intriguingly, memory CD8(+) T cells from latently infected mice failed to undergo homeostatic proliferation in a naive secondary host. These data highlight fundamental differences between memory CD8(+) T cells engaged in active immune surveillance of latent viral infections vs memory CD8(+) T cells found after acute viral infections.

T cell responses to influenza virus infection: effector and memory cells

New approaches to visualizing antigen-specific primary responses to influenza and the development of memory subsets in distinct sites suggest that both CD4 and CD8 T cells play complex roles in primary viral clearance and have the potential to contribute to protection from secondary infection.

Age-associated change in the frequency of memory CD4+ T cells impairs long term CD4+ T cell responses to influenza vaccine

We investigated the relationship of memory CD4+ T cells with the evolution of influenza virus-specific CD4+ T cell responses in healthy young and elderly people. Elderly individuals had a similar frequency of CD69+CD4+ T cells producing IFN-gamma and TNF-alpha at 1 wk, but a lower frequency of these CD4+ T cells at 3 mo after influenza vaccination. Although the elderly had a higher frequency of central memory (CM; CCR7+CD45RA-) CD4+ T cells, they had a significantly lower frequency of effector memory (EM; CCR7-CD45RA-) CD4+ T cells, and the frequency of the latter memory CD4+ T cells positively correlated with the frequency of influenza virus-specific CD69+CD4+ T cells producing IFN-gamma at 3 mo. These findings indicate that the elderly have an altered balance of memory CD4+ T cells, which potentially affects long term CD4+ T cell responses to the influenza vaccine. Compared with the young, the elderly had decreased serum IL-7 levels that positively correlated with the frequency of EM cells, which suggests a relation between IL-7 and decreased EM cells. Thus, although the healthy elderly mount a level of CD4+ T cell responses after vaccination comparable to that observed in younger individuals, they fail to maintain or expand these responses. This failure probably stems from the alteration in the frequency of CM and EM CD4+ T cells in the elderly that is related to alteration in IL-7 levels. These findings raise an important clinical question about whether the vaccination strategy in the elderly should be modified to improve cellular immune responses.

Interleukin (IL)-4-independent maintenance of histone modification of the IL-4 gene loci in memory Th2 cells

Interleukin (IL)-4-induced STAT6 activation and the subsequent up-regulation of GATA3 are crucial for the induction of chromatin remodeling of the Th2 cytokine gene loci as Th2 cells undergo development. This study probes the role of these molecules in the maintenance of memory Th2 cells. IL-4 was not required to maintain the capability for Th2 cytokine production in in vivo generated antigen-specific memory Th2 cells. Histone H3-K9/14 hyperacetylation and intergenic transcripts associated with the IL-4 gene locus were preserved in the absence of IL-4, but those associated with the IL-13 gene were partially IL-4-dependent. Histone H3-K4 methylation of the IL-13 and IL-4 gene loci was fully preserved in memory Th2 cells and accompanied by memory cell-specific accumulation of Pol II complex to highly restricted sites. Thus, memory Th2 cells maintain a unique Th2-specific remodeled chromatin in the IL-4 and IL-13 gene loci by active molecular events that are IL-4-independent.