Homeostasis of memory T cells

Persistent retention of colitogenic CD4+ memory T cells causes inflammatory bowel diseases to become intractable

Despite the advent of an age when "malignant" leukemia is cured by bone marrow transplantation, "benign" inflammatory bowel diseases (IBDs) are still intractable lifelong diseases. Why is it that once an IBD develops it lasts a long time? We propose that, the same as in the response to vaccination, immune memory T cells that remember the disease are formed in IBDs and, perceiving them as "benign T-cell leukemia"-like lifelong pathology that hematogenously spreads throughout the body, we here propose that the bone marrow itself, which produces large amounts of the survival factor IL-7, is the reservoir for colitogenic CD4(+) memory T cells responsible for the intractability of IBDs.

Potential plasma markers of Type 1 and Type 2 leprosy reactions: a preliminary report

Background

The clinical management of leprosy Type 1 (T1R) and Type 2 (T2R) reactions pose challenges mainly because they can cause severe nerve injury and disability. No laboratory test or marker is available for the diagnosis or prognosis of leprosy reactions. This study simultaneously screened plasma factors to identify circulating biomarkers associated with leprosy T1R and T2R among patients recruited in Goiania, Central Brazil.

Methods

A nested case-control study evaluated T1R (n = 10) and TR2 (n = 10) compared to leprosy patients without reactions (n = 29), matched by sex and age-group (+/- 5 years) and histopathological classification. Multiplex bead based technique provided profiles of 27 plasma factors including 16 pro inflammatory cytokines: tumor necrosis factor-α (TNF-α), Interferon-γ (IFN-γ), interleukin (IL)- IL12p70, IL2, IL17, IL1 β, IL6, IL15, IL5, IL8, macrophage inflammatory protein (MIP)-1 alpha (MIP1α), 1 beta (MIP1β), regulated upon activation normal T-cell expressed and secreted (RANTES), monocyte chemoattractrant protein 1 (MCP1), CC-chemokine 11 (CCL11/Eotaxin), CXC-chemokine 10 (CXCL10/IP10); 4 anti inflammatory interleukins: IL4, IL10, IL13, IL1Rα and 7 growth factors: IL7, IL9, granulocyte-colony stimulating factor (G-CSF), granulocyte macrophage-colony stimulating factor (GM-CSF), platelet-derived growth factor BB (PDGF BB), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF).

Results

Elevations of plasma CXCL10 (P = 0.004) and IL6 (p = 0.013) were observed in T1R patients compared to controls without reaction. IL6 (p = 0.05), IL7 (p = 0.039), and PDGF-BB (p = 0.041) were elevated in T2R. RANTES and GMCSF were excluded due to values above and below detection limit respectively in all samples.

Conclusion

Potential biomarkers of T1R identified were CXCL10 and IL6 whereas IL7, PDGF-BB and IL6, may be laboratory markers of TR2. Additional studies on these biomarkers may help understand the immunopathologic mechanisms of leprosy reactions and indicate their usefulness for the diagnosis and for the clinical management of these events.

Early infection termination affects number of CD8+ memory T cells and protective capacities in listeria monocytogenes-infected mice upon rechallenge

Here, we reevaluate the effects of early termination of infection on primary T cell expansion, subsequent memory cell development, and protective immunity. Using a murine Listeria monocytogenes (LM) infection model, we found the primary expansions of both CD4(+) and CD8(+) T cells were affected even when ampicillin was given as late as 60 h postinfection (p.i.). Subsequent development of CD8(+) memory T cells was also impaired, although to a lesser extent, and only mice that received ampicillin at 24 h p.i. revealed a significant decrease in memory CD8(+) T cells. Upon rechallenge with 1 x 10(5) CFU of LM, all ampicillin-treated mice cleared LM as effectively, and they generated similar amounts of Ag-specific CD8(+) T cells as with untreated mice. However, mice that received ampicillin at 24 h p.i. lost their protective abilities when rechallenged with 7.5 x 10(5) CFU of LM. Ampicillin treatment also revealed early down-regulation of B7.1 and B7.2, but not CD40, on dendritic cells 72 h p.i. Our results have several important implications: 1) they argue against the hypothesis that brief exposure of T cells to an Ag is sufficient for full-fledged primary T cell responses and subsequent memory T cell development in vivo; 2) they suggest the existence of a reservoir of memory T cells, more than the immune system can possibly expand during secondary infection; and 3) they suggest that protective capacity is correlated with the number of preexisting memory T cells and that secondary expanding T cells play a limited role, at least in murine LM infection.

Migration, maintenance and recall of memory T cells in peripheral tissues

After the resolution of an immune response, antigen-specific memory T cells persist at many sites in the body. The antigen-specific memory T-cell pool includes memory T cells that preferentially reside in peripheral tissues, such as the skin, gut and lungs, where they provide a first line of defence against secondary pathogen infection. Determining how peripheral memory T cells are regulated is essential for our understanding of host-pathogen interactions and for vaccine development. In this Review, we discuss recent insights into the generation, control and recall of peripheral T-cell memory responses.

Co-immunization with IL-15 enhances cellular immune responses induced by a vif-deleted simian immunodeficiency virus proviral DNA vaccine and confers partial protection against vaginal challenge with SIVmac251

Simian immunodeficiency virus (SIV) infection of rhesus macaques is a valuable animal model for human immunodeficiency virus (HIV)-1 vaccine development. Our laboratory recently described the immunogenicity and limited efficacy of a vif-deleted SIVmac239 proviral DNA (SIV/CMVDelta vif) vaccine. The current report characterizes immunogenicity and efficacy for the SIV/CMVDelta vif proviral DNA vaccine when co-inoculated with an optimized rhesus interleukin (rIL)-15 expression plasmid. Macaques co-inoculated with rIL-15 and SIV/CMVDelta vif proviral plasmids showed significantly improved SIV-specific CD8 T cell immunity characterized by increased IFN-gamma ELISPOT and polyfunctional CD8 T cell responses. Furthermore, these animals demonstrated a sustained suppression of plasma virus loads after multiple low dose vaginal challenges with pathogenic SIVmac251. Importantly, SIV-specific cellular responses were greater in immunized animals compared to unvaccinated controls during the initial 12 weeks after challenge. Taken together, these findings support the use of IL-15 as an adjuvant in prophylactic anti-HIV vaccine strategies.

Stem cell-like plasticity of naïve and distinct memory CD8+ T cell subsets

Most models regarding the 'clonal' origin of CD8(+) T cell effector and memory subset diversification suggest that during the first contact of a naïve T cell with the priming antigen-presenting cell major decisions for subsequent differentiation are made. Data using novel single-cell T cell tracking technologies demonstrate that a single naïve CD8(+) T cell can give rise to virtually all different subtypes of effector and memory T cells, and direct major determinants of subset diversification to the time period beyond the first cell division. Thereby, some 'stem cell-like' characteristics typical for naïve T cells are probably still maintained within distinct subsets of memory T cells. These observations have direct consequences for clinical applications like adoptive T cell therapy.

Differential requirement for CARMA1 in agonist-selected T-cell development

Caspase recruitment domain-containing membrane-associated guanylate kinase protein-1 (CARMA1) is a critical component of the NF-kappaB signaling cascade mediated by TCR engagement. In addition to activation of naïve T cells, TCR signaling is important for the development of agonist-selected T-cell subsets such as Treg, NKT cells, and CD8-alpha alpha T cells. However, little is known about the role of CARMA1 in the development of these lineages. Here we show that CARMA1-deficient mice (CARMA1(-/-)) have altered populations of specific subsets of agonist-selected T cells. Specifically, CARMA1(-/-) mice have impaired natural and adaptive Treg development, whereas NKT cell numbers are normal compared with wild-type mice. Interestingly, CD8-alpha alpha T cells, which may also be able to develop through an extrathymic selection pathway, are enriched in the gut of CARMA1(-/-) mice, whereas memory-phenotype CD4(+) T cells (CD62L(low)/CD44(high)) are present at reduced numbers in the periphery. These results indicate that CARMA1 is essential for Treg development, but is not necessary for the development of other agonist-selected T-cell subsets. Overall, these data reveal an important but differential role for CARMA1-mediated TCR signaling in T-cell development.

MHC-dependent desensitization of intrinsic anti-self reactivity

The survival of naive T cells is compromised in the absence of molecules encoded by the major histocompatibility complex (MHC) while antigen-experienced T cells survive. We hypothesized that survival pressures in an in vivo, MHC-deficient environment would permit enrichment of less frequent antigen-experienced autoreactive cells at the expense of the majority of antigen naive T cells. To test this hypothesis, we generated MHC class I- and class II-deficient mice in NOD and C57Bl/6 (B6) backgrounds, and examined the capacity of adoptively transferred autoimmune-prone NOD T cells, or non-autoimmune prone naive B6 T cells, respectively, to reject transplanted wild-type pancreatic islets or transplantable tumors in the MHC-deficient mice. In the MHC-deficient environment, CD4 T cells acquired self-hostile properties (islet rejection and tumor invasion) that were independent from their genetic propensity for autoreactivity, while CD8 T cells required appropriate prior exposure to antigen in order to survive and function (reject tumor) in this environment; however, disengagement of Tob1, a negative regulator of proliferation, led to a reverse phenotype with regard to persistence of CD4 and CD8 T cells in the MHC-deficient environment. Our data suggest that self-peptide/MHC interactions have dual roles to facilitate survival and restrain autoreactivity, thus acting as integral components of an intrinsic network of negative regulation that maintains tolerance.

CD40 regulates human dendritic cell-derived IL-7 production that, in turn, contributes to CD8(+) T-cell antigen-specific expansion

CD40L (CD154) expressed on activated CD4(+) T cells has been shown to provide CD40(+) dendritic cells (DCs), a critical signal for establishing CD8(+) T-cell immunity. CD40L-CD40 interaction leads to DC maturation with IL-12 production and upregulation of various costimulatory molecules. In this study, we show that CD40 engagement provides a unique maturation signal for human monocyte-derived DCs to upregulate IL-7 production. Other inducers of DC maturation, such as TLR 4 and TLR 7/8 agonist, fail to induce IL-7 upregulation. Neutralization of IL-7 activity in human CD8(+) T-cell cultures stimulated with CMV pp65-NLV peptide-pulsed mature DCs (mDCs) leads to a reduction in antigen-specific CD8(+) T-cell yields suggesting a role for mDC-derived IL-7 during T-cell receptor (TCR) activation. Furthermore, IL-7 signaling requires a temporal coordination with TCR activation for maximal antigen-specific T-cell yields. These results show that CD40 signals regulate DC-derived IL-7 production that, in turn, may instruct CD8(+) T cells at the time of TCR engagement for survival leading to an increased expansion of antigen-specific T cells.

A low interleukin-2 receptor signaling threshold supports the development and homeostasis of T regulatory cells

Interleukin-2 receptor (IL-2R) signaling is essential for T regulatory (Treg) cell development and homeostasis. Here, we show that expression of IL-2Rbeta chains that lack tyrosine residues important for the association of the adaptor Shc and the transcription factor STAT5 in IL-2Rbeta-deficient mice resulted in production of a normal proportion of natural Treg cells that suppressed severe autoimmunity related with deficiency in IL-2 or IL-2R. These mutant IL-2Rbeta chains supported suboptimal and transient STAT5 activation that upregulate the transcription factor Foxp3 to normal amounts in natural, but not induced, Treg cells. Nevertheless, gene expression profiling revealed many targets in peripheral natural Treg cells that were IL-2 dependent and a substantial overlap between the Treg cell IL-2-dependent gene program and the Treg cell transcriptional signature. Collectively, these findings demonstrate that a critical, and perhaps minor, subset of IL-2-dependent targets is indexed to a low IL-2R signaling threshold and that a substantial proportion of the Treg cell gene program is regulated by IL-2.

IL-7 and IL-15 allow the generation of suicide gene–modified alloreactive self-renewing central memory human T lymphocytes

Long-term clinical remissions of leukemia, after allogeneic hematopoietic stem cell transplantation, depend on alloreactive memory T cells able to self-renew and differentiate into antileukemia effectors. This is counterbalanced by detrimental graft-versus-host disease (GVHD). Induction of a selective suicide in donor T cells is a current gene therapy approach to abrogate GVHD. Unfortunately, genetic modification reduces alloreactivity of lymphocytes. This associates with an effector memory (TEM) phenotype of gene-modified lymphocytes and may limit antileukemia effect. We hypothesized that alloreactivity of gene-modified lymphocytes segregates with the central memory (TCM) phenotype. To this, we generated suicide gene–modified TCM lymphocytes with a retroviral vector after CD28 costimulation and culture with IL-2, IL-7, or a combination of IL-7 and IL-15. In vitro, suicide gene–modified TCM cells self-renewed upon alloantigen stimulation and resisted activation-induced cell death. In a humanized mouse model, only suicide gene–modified T cells cultured with IL-7 and IL-15 persisted, differentiated in TEM cells, and were as potent as unmanipulated lymphocytes in causing GVHD. GVHD was halted through the activation of the suicide gene machinery. These results warrant the use of suicide gene–modified TCM cells cultured with IL-7 and IL-15 for the safe exploitation of the alloreactive response against cancer.

Memory T cell RNA rearrangement programmed by heterogeneous nuclear ribonucleoprotein hnRNPLL

Differentiation of memory cells involves DNA-sequence changes in B lymphocytes but is less clearly defined in T cells. RNA rearrangement is identified here as a key event in memory T cell differentiation by analysis of a mouse mutation that altered the proportions of naive and memory T cells and crippled the process of Ptprc exon silencing needed to generate CD45RO in memory T cells. A single substitution in a memory-induced RNA-binding protein, hnRNPLL, destabilized an RNA-recognition domain that bound with micromolar affinity to RNA containing the Ptprc exon-silencing sequence. Hnrpll mutation selectively diminished T cell accumulation in peripheral lymphoid tissues but not proliferation. Exon-array analysis of Hnrpll mutant naive and memory T cells revealed an extensive program of alternative mRNA splicing in memory T cells, coordinated by hnRNPLL. A remarkable overlap with alternative splicing in neural tissues may reflect a co-opted strategy for diversifying memory T cells.

IL-15 independent maintenance of virus-specific CD8(+) T cells in the CNS during chronic infection

The role of IL-15 in T cell survival was examined during chronic CNS coronavirus infection. Similar numbers of virus-specific CD8⁺ T cells were retained in the CNS of IL-15^−/− and wt mice, consistent with loss of IL-2/15 receptor (CD122) expression. IL-15 deficiency also had no affect on IL-7 receptor (CD127) expression, Bcl-2 upregulation, granzyme B expression, or IFN-γ secretion in CNS persisting CD8⁺ T cells. Furthermore, CD8⁺ T cell division in the CNS was reduced compared to spleen. CD8⁺ T cells in the persistently infected CNS are thus characterized by IL-15 independent, low level proliferation and an activated/memory phenotype.

Memory T-lymphocyte survival does not require T-cell receptor expression

The factors controlling memory T (Tm)-cell longevity are still poorly defined, and their identification is pivotal to the design of a vaccine conferring long-term protection against infection. Tm cells have the ability to survive in the absence of the T-cell receptor (TCR)-MHC interaction. This does not exclude a possible role for TCR-intrinsic ligand-independent constitutive signaling in Tm-cell homeostasis. Using a unique TCR tetracycline-inducible expression system, we show that the ablation of TCR expression, which abrogates any possible signaling via the TCR, did not influence the survival and self-renewal of antigen-specific CD8(+) Tm cells even when they have to compete with endogenous T cells for survival factors. Moreover, CD8(+) Tm-cell functionality was not altered even on prolonged maintenance in the absence of TCR-MHC interactions. Furthermore, our results show that a subset of CD4(+) Tm cells can survive in the absence of TCR expression in nonlymphopenic hosts.

Melanoma-specific memory T cells are functionally active in Ret transgenic mice without macroscopic tumors

We previously reported that bone marrows of breast cancer patients contained tumor antigen-specific CD8(+) T cells with central or effector memory phenotype. Using a recently developed ret transgenic mouse melanoma model, we now show that bone marrows and tumors of transgenic mice contain high frequencies of CD8(+) T cells specific for the melanoma antigen tyrosinase-related protein 2 and showing mostly effector memory phenotype. Moreover, increased numbers of bone marrow tyrosinase-related protein-2-specific effector memory CD8(+) T cells are also detected in transgenic animals older than 20 weeks with disseminated melanoma cells in the bone marrow and lymph nodes but showing no visible skin tumors and no further melanoma progression. After a short-term coincubation with dendritic cells generated from the bone marrow and pulsed with melanoma lysates, bone marrow memory T cells from mice without macroscopic melanomas produced IFN-gamma in vitro and exerted antitumor activity in vivo after adoptive transfer into melanoma-bearing mice. Our data indicate that functionally active bone marrow-derived melanoma-specific memory T cells are detectable at the phase of microscopic tumor load, suggesting that thereby they could control disseminated melanoma cells.

Functionally diverse subsets in CD4 T cell responses against influenza

BACKGROUND

Antibody alone cannot provide optimal protection against many infectious diseases impacting global heath. In these cases, our challenge is to develop innovative vaccines that generate protective populations of memory T cells. However, our studies suggest that current paradigms explaining how memory CD4 T cells provide protection are inadequate. This is likely due to both the paucity of and heterogeneity of memory CD4 T cells observed in vivo, which make analysis extremely difficult.

SUMMARY

Here, we discuss new findings that indicate there is extensive functional heterogeneity within effector and memory CD4 T cell populations both in vivo and in vitro. Using influenza as an example, we also discuss the merits of employing reductionist approaches to explore how unique subsets of CD4 T cells are generated, what mechanisms of protection they use, and where they stand on the axes of differentiation that define T cell subsets.

Colitogenic CD4+ effector-memory T cells actively recirculate in chronic colitic mice

BACKGROUND

Although the clinical usefulness of leukocytapheresis for patients with inflammatory bowel disease (IBD) has been reported as a selective removal therapy targeting pathogenic immune cells in blood circulation, it remains unclear whether colitogenic CD4(+) T cells continuously recirculate in peripheral blood during the chronic phase of colitis.

METHODS

To resolve this question we conducted a series of in vivo experiments using a murine chronic colitis model induced by adoptive transfer of CD4(+)CD45RB(high) cells into SCID mice in combination with a parabiosis system.

RESULTS

In colitic SCID recipients, first, almost all CD4(+) CD45RB(high) donor cells were converted to CD4(+)CD44(high)CD62L(-) IL-7Ralpha(high) effector-memory T (T(EM)) cells at 8 weeks after transfer and were distributed throughout the whole body, including colonic lamina propria, mesenteric lymph nodes, thoracic duct, peripheral blood, spleen, and bone marrow. Second, SCID mice retransferred with the colitic peripheral blood CD4(+) T cells developed colitis that is identical to the original colitis. Third, CD4(+) cells in parabionts between established colitic RAG-2(-/-) mice induced by adoptive transfer of Ly5.1(+) or Ly5.2(+) CD4(+)CD45RB(high) T cells were well mixed in almost equal proportions at various sites 2 weeks after parabiosis surgery, and the redistribution of Ly5.1(+) and Ly5.2(+) CD4(+) T cells was significantly suppressed in FTY720-treated parabionts.

CONCLUSIONS

Together, these findings indicate that colitogenic CD4(+) T(EM) cells continuously recirculate in established colitic mice, suggesting that therapeutic approaches targeting systemic CD4(+) T(EM) cells, such as bone marrow transplantation, rather than those targeting only intestinal CD4(+) T cells, may be feasible for the treatment of IBD.

Dissociation of its opposing immunologic effects is critical for the optimization of antitumor CD8+ T-cell responses induced by interleukin 21

PURPOSE

Interleukin 21 (IL-21) is a promising new cytokine, which is undergoing clinical testing as an anticancer agent. Although IL-21 provides potent stimulation of CD8(+) T cells, it has also been suggested that IL-21 is immunosuppressive by counteracting the maturation of dendritic cells. The dissociation of these two opposing effects may enhance the utility of IL-21 as an immunotherapeutic. In this study, we used a cell-based artificial antigen-presenting cell (aAPC) lacking a functional IL-21 receptor (IL-21R) to investigate the immunostimulatory properties of IL-21.

EXPERIMENTAL DESIGN

The immunosuppressive activity of IL-21 was studied using human IL-21R(+) dendritic cells. Antigen-specific CD8(+) T cells stimulated with human cell-based IL-21R(-)aAPC were used to isolate the T-cell immunostimulatory effects of IL-21. The functional outcomes, including phenotype, cytokine production, proliferation, and cytotoxicity were evaluated.

RESULTS

IL-21 limits the immune response by maintaining immunologically immature dendritic cells. However, stimulation of CD8(+) T cells with IL-21R(-) aAPC, which secrete IL-21, results in significant expansion. Although priming in the presence of IL-21 temporarily modulated the T-cell phenotype, chronic stimulation abrogated these differences. Importantly, exposure to IL-21 during restimulation promoted the enrichment and expansion of antigen-specific CD8(+) T cells that maintained IL-2 secretion and gained enhanced IFN-gamma secretion. Tumor antigen-specific CTL generated in the presence of IL-21 recognized tumor cells efficiently, demonstrating potent effector functions.

CONCLUSIONS

IL-21 induces opposing effects on antigen-presenting cells and CD8(+) T cells. Strategic application of IL-21 is required to induce optimal clinical effects and may enable the generation of large numbers of highly avid tumor-specific CTL for adoptive immunotherapy.

Memory CD8 T-cell compartment grows in size with immunological experience

Memory CD8 T cells, generated by natural pathogen exposure or intentional vaccination, protect the host against specific viral infections. It has long been proposed that the number of memory CD8 T cells in the host is inflexible, and that individual cells are constantly competing for limited space. Consequently, vaccines that introduce over-abundant quantities of memory CD8 T cells specific for an agent of interest could have catastrophic consequences for the host by displacing memory CD8 T cells specific for all previous infections. To test this paradigm, we developed a vaccination regimen in mice that introduced as many new long-lived memory CD8 T cells specific for a single vaccine antigen as there were memory CD8 T cells in the host before vaccination. Here we show that, in contrast to expectations, the size of the memory CD8 T-cell compartment doubled to accommodate these new cells, a change due solely to the addition of effector memory CD8 T cells. This increase did not affect the number of CD4 T cells, B cells or naive CD8 T cells, and pre-existing memory CD8 T cells specific for a previously encountered infection were largely preserved. Thus, the number of effector memory CD8 T cells in the mammalian host adapts according to immunological experience. Developing vaccines that abundantly introduce new memory CD8 T cells should not necessarily ablate pre-existing immunity to other infections.

The transcriptional regulator PLZF induces the development of CD44 high memory phenotype T cells

Transcriptional pathways controlling the development of CD44(hi) memory phenotype (MP) T cells with "innate-like" functions are not well understood. Here we show that the BTB (bric-a-brac, tramtrack, broad complex) domain-containing protein promyelocytic leukemia zinc finger (PLZF) is expressed in CD44(hi), but not in CD44(lo), CD4(+) T cells. Transgenic expression of PLZF during T cell development and in CD4(+) and CD8(+) T cells induced a T cell intrinsic program leading to an increase in peripheral CD44(hi) MP CD4(+) and CD8(+) T cells and a corresponding decrease of naïve CD44(lo) T cells. The MP CD4(+) and CD8(+) T cells produced IFNgamma upon PMA/ionomycin stimulation, thus showing innate-like function. Changes in the naïve versus memory-like subset distribution were already evident in single-positive thymocytes, indicating PLZF-induced T cell developmental alterations. In addition, CD1d-restricted natural killer T cells in PLZF transgenic mice showed impaired development and were severely reduced in the periphery. Finally, after anti-CD3/CD28 stimulation, CD4(+) transgenic T cells showed reduced IL-2 and IFNgamma production but increased IL-4 secretion as a result of enhanced IL-4 production of the CD44(hi)CD62L(+) subset. Our data indicate that PLZF is a novel regulator of the development of CD44(hi) MP T cells with a characteristic partial innate-like phenotype.

Negative regulators in homeostasis of naïve peripheral T cells

It is now apparent that naïve peripheral T cells are a dynamic population where active processes prevent inappropriate activation while supporting survival. The process of thymic education makes naïve peripheral T cells dependent on interactions with self-MHC for survival. However, as these signals can potentially result in inappropriate activation, various non-redundant, intrinsic negative regulatory molecules including Tob, Nfatc2, and Smad3 actively enforce T cell quiescence. Interactions among these pathways are only now coming to light and may include positive or negative crosstalk. In the case of positive crosstalk, self-MHC initiated signals and intrinsic negative regulatory factors may cooperate to dampen T cell activation and sustain peripheral tolerance in a binary fashion (on-off). In the case of negative crosstalk, self-MHC signals may promote survival through partial activation while intrinsic negative regulatory factors act as rheostats to restrain cell cycle entry and prevent T cells from crossing a threshold that would break tolerance.

A novel role for CD4+ T cells in the control of cachexia

Cachexia is the dramatic weight loss and muscle atrophy seen in chronic disease states, including autoimmunity, cancer, and infection, and is often associated with lymphopenia. We have previously shown that CD4(+) T cells that express the lowest density of CD44 (CD4(+)CD44(v.low)) are significantly reduced in diabetic NOD mice that are cachexic compared with diabetic mice that are not cachexic. Using this model, and a model of cancer cachexia, we test the hypothesis that CD4(+)CD44(v.low) cells play an active role in protecting the host from cachexia. CD4(+)CD44(v.low) cells, but not CD4(+) cells depleted of CD44(v.low) cells, delay the onset of wasting when infused into either diabetic or prediabetic NOD recipients. However, no significant effect on the severity of diabetes was detected. In a model of cancer cachexia, they significantly reduce muscle atrophy, and inhibit muscle protein loss and DNA loss, even when given after the onset of cachexia. Protection from wasting and muscle atrophy by CD4(+)CD44(v.low) cells is associated with protection from lymphopenia. These data suggest, for the first time, a role for an immune cell subset in protection from cachexia, and further suggest that the mechanism of protection is independent of protection from autoimmunity.

CD4 memory T cells divide poorly in response to antigen because of their cytokine profile

Immunological memory is a hallmark of adaptive immunity, and understanding T cell memory will be central to the development of effective cell-mediated vaccines. The characteristics and functions of CD4 memory cells have not been well defined. Here we demonstrate that the increased size of the secondary response is solely a consequence of the increased antigen-specific precursor frequency within the memory pool. Memory cells proliferated less than primary responding cells, even within the same host. By analyzing the entry of primary and memory cells into the cell cycle, we found that the two populations proliferated similarly until day 5; after this time, fewer of the reactivated memory cells proliferated. At this time, fewer of the reactivated memory cells made IL-2 than primary responding cells, but more made IFNgamma. Both these factors affected the low proliferation of the memory cells, because either exogenous IL-2 or inhibition of IFNgamma increased the proliferation of the memory cells.

Early growth response gene 2 (Egr-2) controls the self-tolerance of T cells and prevents the development of lupuslike autoimmune disease

Maintaining tolerance of T cells to self-antigens is essential to avoid autoimmune disease. How self-reactive T cells are kept functionally inactive is, however, unknown. In this study, we show that early growth response gene 2 (Egr-2), a zinc-finger transcription factor, is expressed in CD44^high T cells and controls their proliferation and activation. In the absence of Egr-2, CD44^high, but not CD44^low T cells, are hyperreactive and hyperproliferative in vivo. The accumulation of activated CD4⁺CD44^high T cells leads to the development of a late onset lupuslike autoimmune disease characterized by the accumulation of interferon (IFN)-γ and interleukin (IL)-17–producing CD4⁺ T cells, loss of tolerance to nuclear antigens, massive infiltration of T cells into multiple organs and glomerulonephritis. We found that the expression of cyclin-dependent kinase inhibitor p21cip1 was impaired in Egr-2–deficient T cells, whereas the expression of IFN-γ and IL-17 in response to T cell receptor ligation was significantly increased, suggesting that Egr-2 activates the expression of genes involved in the negative regulation of T cell proliferation and inflammation. These results demonstrate that Egr-2 is an intrinsic regulator of effector T cells and controls the expansion of self-reactive T cells and development of autoimmune disease.

Identification of two major types of age-associated CD8 clonal expansions with highly divergent properties

CD8 memory T cells are tightly regulated in young, healthy individuals but are often perturbed in aged animals by the appearance of large CD8 T cell clones. These clones are associated with impaired immunity in the aged. The molecular basis of this phenomenon remains unclear. Here, it is shown that the issue is confused by the fact that the clones are heterogeneous. Some clones bear high, and others, low levels of integrin alpha(4) (itgalpha4). These subtypes differ by multiple criteria. They appear in mice of different ages, concentrate in different tissues, and have different stabilities in vivo and responses to stimulation in vitro. itgalpha4(high), but not itgalpha4(low), CD8 clonal expansions have several characteristics consistent with a chronically stimulated phenotype. These properties include lowered levels of CD8, decreased expression of some cytokine receptors, and elevated expression of various inhibitory receptors, including the programmed death-1 (PD1) receptor and the killer cell lectin-like receptor G1 (KLRG1). The characteristics of itgalpha4(high) clonal expansions suggest that they may arise from age-dependent alterations in antigen expression and tolerance. These data redefine CD8 clonal expansions into at least two distinct entities and indicate that there are multiple mechanisms that drive age-related alterations of CD8 T cell homeostasis.

Age-associated T-cell Clonal Expansions (TCE) in vivo—Implications for Pathogen Resistance

Age-related T-cell clonal expansions (TCE) are an incompletely understood disturbance in T-cell homeostasis found frequently in old humans and experimental animals. These accumulations of CD8 T-cells have the potential to distort T-cell population balance and reduce T-cell repertoire diversity above and beyond the changes seen in the aging of T-cell pool in the absence of TCE. This chapter discusses our current knowledge of the role of these expansions in health and disease, with a special focus on their influence upon immune defense against infectious diseases.

CD4 memory T cells on trial: immunological memory without a memory T cell

Immunological memory crucially depends on CD4 T cells. In contrast with B cells, we find no decisive evidence that CD4 T cells are permanently altered by antigen stimulation. We propose that the memory response is derived from an increase in frequency of resting naïve-like CD4 T cells with a half-life of years (or months in rodents), rather than the currently proposed specialized T-cell types that have a known lifespan of days. In addition, residual antigen will significantly influence the longevity of a memory response. Our model offers a new insight into immunological memory that could assist the development of CD4 T cell-based vaccines.

Ageing and life-long maintenance of T-cell subsets in the face of latent persistent infections

A decline in T-cell immunity is one of the most consistent and most profound deficiencies of the elderly. Therapeutic correction of this decline often restores immune responsiveness and immune defence.

T-cell immune decline in the elderly has at least two underpinnings: a drop in the responsiveness of naive T cells to stimulation (cell-autonomous defects) and a reduction in naive T-cell numbers and diversity that leads to a dominant memory T-cell pool (T-cell population imbalance).

This article discusses two key causes of age-related T-cell population imbalance: homeostatic cycling or proliferative expansion in the peripheral T-cell pool, and latent persistent infections, which repeatedly stimulate the T-cell pool over the lifetime of the individual.

The reduction in production of naive T cells by the thymus forces the ageing organism to rely on compensatory homeostatic mechanisms to maintain the balance between naive and memory T-cell pools. Although this may be initially successful, recent evidence suggests that late in life these mechanisms exhaust their usefulness and actually contribute to a further demise of the remaining naive T cells.

Latent persistent infections, particularly with herpesviruses, lead to life-long periodic restimulation of the immune system, here, evidence is presented for the role of viral reactivation in this restimulation using a mouse model of herpesvirus infection and ageing.

Relative roles and the interplay between the homeostatic and viral factors are discussed, with the former having a surprisingly prominent role. Finally, modes of immune rejuvenation and anti-ageing intervention are debated in light of these advances in our knowledge.

A decline in T-cell immunity is a major cause of morbidity and mortality from infectious diseases in the elderly. Janko Nikolich-Žugich weighs up the relative roles of and the interplay between homeostatic factors and persistent viruses in immune senescence.

A diverse and well-balanced repertoire of T cells is thought to be crucial for the efficacious defence against infection with new or re-emerging pathogens throughout life. In the last third of the mammalian lifespan, the maintenance of a balanced T-cell repertoire becomes highly challenging because of the changes in T-cell production and consumption. In this Review, I question whether latent persistent pathogens might be key factors that drive this imbalance and whether they determine the extent of age-associated immune deficiency.

Immunosenescent colitogenic CD4(+) T cells convert to regulatory cells and suppress colitis

Inflammatory bowel diseases progress steadily by the expansion of colitogenic CD4(+) cells. However, it remains unknown whether colitogenic CD4(+) cells are long-living like memory cells or exhausted like effector cells. To assess the longevity of colitogenic lamina propria (LP) CD4(+) cells, we performed sequential transfers of LP CD4(+) cells from colitic CD4(+)CD45RB(high) cell-transferred SCID mice into new SCID mice. Although SCID mice transferred with colitic LP CD4(+) cells stably developed colitis until at least the sixth transfer, the interval to the development of colitis gradually lengthened as the number of transfers increased. The incidence of colitis gradually decreased after the seventh transfer. Furthermore, non-colitic LP CD4(+) cells from mice transferred over seven times expressed significantly higher levels of PD-1 and produced significantly lower amounts of IFN-gamma, TNF-alpha, and IL-17 than colitic LP CD4(+) cells recovered after the first transfer. Most notably, we found that re-transfer of non-colitic LP CD4(+) cells recovered after multiple transfers prevented the development of colitis in SCID mice co-transferred with CD4(+)CD45RB(high) cells. Thus, colitogenic LP CD4(+) cells may be exhausted over time, become non-functional, convert to regulatory cells, and finally suppress colitis in the process of immunosenescence.

MyD88-dependent pathway in T cells directly modulates the expansion of colitogenic CD4+ T cells in chronic colitis

TLRs that mediate the recognition of pathogen-associated molecular patterns are widely expressed on/in cells of the innate immune system. However, recent findings demonstrate that certain TLRs are also expressed in conventional TCRalphabeta(+) T cells that are critically involved in the acquired immune system, suggesting that TLR ligands can directly modulate T cell function in addition to various innate immune cells. In this study, we report that in a murine model of chronic colitis induced in RAG-2(-/-) mice by adoptive transfer of CD4(+)CD45RB(high) T cells, both CD4(+)CD45RB(high) donor cells and the expanding colitogenic lamina propria CD4(+)CD44(high) memory cells expresses a wide variety of TLRs along with MyD88, a key adaptor molecule required for signal transduction through TLRs. Although RAG-2(-/-) mice transferred with MyD88(-/-)CD4(+)CD45RB(high) cells developed colitis, the severity was reduced with the delayed kinetics of clinical course, and the expansion of colitogenic CD4(+) T cells was significantly impaired as compared with control mice transferred with MyD88(+/+)CD4(+)CD45RB(high) cells. When RAG-2(-/-) mice were transferred with the same number of MyD88(+/+) (Ly5.1(+)) and MyD88(-/-) (Ly5.2(+)) CD4(+)CD45RB(high) cells, MyD88(-/-)CD4(+) T cells showed significantly lower proliferative responses assessed by in vivo CFSE division assay, and also lower expression of antiapoptotic Bcl-2/Bcl-x(L) molecules and less production of IFN-gamma and IL-17, compared with the paired MyD88(+/+)CD4(+) T cells. Collectively, the MyD88-dependent pathway that controls TLR signaling in T cells may directly promote the proliferation and survival of colitogenic CD4(+) T cells to sustain chronic colitis.

Immunosuppression in islet transplantation

Islet transplantation can temporarily cure type 1 diabetes mellitus (T1DM) but requires simultaneous immunosuppression to avoid allograft rejection. In this issue of the JCI, Monti et al. report that immune conditioning via use of the Edmonton protocol - a treatment approach in which T1DM patients infused with pancreatic islets from multiple cadaveric donors simultaneously receive immunosuppressive drugs - results in lymphopenia that is associated with elevated serum levels of the homeostatic cytokines IL-7 and IL-15, which causes in vivo expansion of the autoreactive CD8(+) T cell population (see the related article beginning on page 1806). Reemergence of autoreactivity is likely the main culprit underlying long-term islet graft failure, and new strategies will need to be tested to circumvent this homeostatic expansion and recurrent autoreactivity.

Naive and innate memory phenotype CD4+ T cells have different requirements for active Itk for their development

The Tec family kinase Itk regulates the development of conventional and innate CD8(+) T cells. However, little is known about the role of Itk in the development of CD4(+) T cell lineages, although the role of Itk in the T cell activation and function is well defined. We show in this study that Itk null mice have increased percentage of CD62L(low)CD44(high) memory phenotype CD4(+) T cells compared with wild-type mice. These cells arise directly in the thymus, express high levels of transcripts for the T-bet and IFN-gamma and are able to produce IFN-gamma directly ex vivo in response to stimulation. Itk deficiency greatly decreases the number of CD4(+) T cells with CD62L(high)CD44(low) naive phenotype, but has no effect on the number of memory phenotype CD4(+) T cells, indicating that the development of memory phenotype CD4(+) T cells is Itk-independent. We further show that the development of the naive phenotype CD4(+) T cells is dependent on active Itk signals and can be rescued by expression of Itk specifically in T cells. Our data also show that Itk is required for functional TCR signaling in these cells, but not for the innate function in response to IL-12/IL-18 or Listeria monocytogenes stimulation. These results indicate that CD62L(high)CD44(low) "naive" CD4(+) and CD62L(low)CD44(high) "innate memory phenotype" CD4(+) T cells may be independent populations that differ in their requirement for Itk signals for development. Our data also suggest that CD4(+)CD62L(low)CD44(high) memory phenotype T cells have innate immune function.

An IFN-gamma-IL-18 signaling loop accelerates memory CD8+ T cell proliferation

Rapid proliferation is one of the important features of memory CD8⁺ T cells, ensuring rapid clearance of reinfection. Although several cytokines such as IL-15 and IL-7 regulate relatively slow homeostatic proliferation of memory T cells during the maintenance phase, it is unknown how memory T cells can proliferate more quickly than naïve T cells upon antigen stimulation. To examine antigen-specific CD8⁺ T cell proliferation in recall responses in vivo, we targeted a model antigen, ovalbumin(OVA), to DEC-205⁺ dendritic cells (DCs) with a CD40 maturation stimulus. This led to the induction of functional memory CD8⁺ T cells, which showed rapid proliferation and multiple cytokine production (IFN-γ, IL-2, TNF-α) during the secondary challenge to DC-targeted antigen. Upon antigen-presentation, IL-18, an IFN-γ-inducing factor, accumulated at the DC:T cell synapse. Surprisingly, IFN-γ receptors were required to augment IL-18 production from DCs. Mice genetically deficient for IL-18 or IFN-γ-receptor 1 also showed delayed expansion of memory CD8⁺ T cells in vivo. These results indicate that a positive regulatory loop involving IFN-γ and IL-18 signaling contributes to the accelerated memory CD8⁺ T cell proliferation during a recall response to antigen presented by DCs.

Interleukin-15/interleukin-15R alpha complexes promote destruction of established tumors by reviving tumor-resident CD8+ T cells

Tumors often escape immune-mediated destruction by suppressing lymphocyte infiltration or effector function. New approaches are needed that overcome this suppression and thereby augment the tumoricidal capacity of tumor-reactive lymphocytes. The cytokine interleukin-15 (IL-15) promotes proliferation and effector capacity of CD8(+) T cells, natural killer (NK) cells, and NKT cells; however, it has a short half-life and high doses are needed to achieve functional responses in vivo. The biological activity of IL-15 can be dramatically increased by complexing this cytokine to its soluble receptor, IL-15R alpha. Here, we report that in vivo delivery of IL-15/IL-15R alpha complexes triggers rapid and significant regression of established solid tumors in two murine models. Despite a marked expansion of IL-2/IL-15R beta(+) cells in lymphoid organs and peripheral blood following treatment with IL-15/IL-15R alpha complexes, the destruction of solid tumors was orchestrated by tumor-resident rather than newly infiltrating CD8(+) T cells. Our data provide novel insights into the use of IL-15/IL-15R alpha complexes to relieve tumor-resident T cells from functional suppression by the tumor microenvironment and have significant implications for cancer immunotherapy and treatment of chronic infections.

Adoptive immunotherapy for indolent non-Hodgkin lymphoma and mantle cell lymphoma using genetically modified autologous CD20-specific T cells

Adoptive immunotherapy with T cells expressing a tumor-specific chimeric T-cell receptor is a promising approach to cancer therapy that has not previously been explored for the treatment of lymphoma in human subjects. We report the results of a proof-of-concept clinical trial in which patients with relapsed or refractory indolent B-cell lymphoma or mantle cell lymphoma were treated with autologous T cells genetically modified by electroporation with a vector plasmid encoding a CD20-specific chimeric T-cell receptor and neomycin resistance gene. Transfected cells were immunophenotypically similar to CD8(+) effector cells and showed CD20-specific cytotoxicity in vitro. Seven patients received a total of 20 T-cell infusions, with minimal toxicities. Modified T cells persisted in vivo 1 to 3 weeks in the first 3 patients, who received T cells produced by limiting dilution methods, but persisted 5 to 9 weeks in the next 4 patients who received T cells produced in bulk cultures followed by 14 days of low-dose subcutaneous interleukin-2 (IL-2) injections. Of the 7 treated patients, 2 maintained a previous complete response, 1 achieved a partial response, and 4 had stable disease. These results show the safety, feasibility, and potential antitumor activity of adoptive T-cell therapy using this approach. This trial was registered at www.clinicaltrials.gov as #NCT00012207.

Effects of IL-7 on memory CD8 T cell homeostasis are influenced by the timing of therapy in mice

IL-7 is integral to the generation and maintenance of CD8(+) T cell memory, and insufficient IL-7 is believed to limit survival and the persistence of memory CD8(+) T cells. Here, we show that during the mouse T cell response to lymphocytic choriomeningitis virus, IL-7 enhanced the number of memory CD8(+) T cells when its administration was restricted to the contraction phase of the response. Likewise, IL-7 administration during the contraction phase of the mouse T cell response to vaccinia virus or a DNA vaccine potentiated antigen-specific CD8(+) memory T cell proliferation and function. Qualitatively, CD8(+) T cells from IL-7-treated mice exhibited superior recall responses and improved viral control. IL-7 treatment during the memory phase stimulated a marked increase in the number of memory CD8(+) T cells, but the effects were transient. IL-7 therapy during contraction of the secondary CD8(+) T cell response also expanded the pool of memory CD8(+) T cells. Collectively, our studies show differential effects of IL-7 on memory CD8(+) T cell homeostasis and underscore the importance of the timing of IL-7 therapy to effectively improve CD8(+) T cell memory and protective immunity. These findings may have implications in the clinical use of IL-7 as an immunotherapeutic agent to bolster vaccine-induced CD8(+) T cell memory.

Age-related dysregulation of CD8+ T cell memory specific for a persistent virus is independent of viral replication

The immune system devotes substantial resources to the lifelong control of persistent pathogens, which were hypothesized to play an important role in immune aging. Specifically, the presence of latent herpesviruses has been correlated with immune exhaustion and shorter lifespan in octogenarians. But neither the causality nor the mechanistic link(s) were established, and the relative roles of persistent antigenic stimulation and of virus-independent homeostatic disturbances in T cell aging remain unresolved. We longitudinally analyzed expansion, contraction, and long-term maintenance of CD8(+) T cells responding to localized infection with a latent virus, HSV-1. Young mice exhibited the expected expansion and contraction of HSV-1-specific cells and the stable maintenance of memory T cells into advanced adulthood. However, upon entry into senescence, many (>40%) animals exhibited an accumulation in Ag-specific cells (memory inflation) which in some animals was comparable to that observed in acute infection. Inflation occurred to the same extent in control mice and mice continuously treated with the anti-HSV drug famciclovir, which inhibits viral replication and was able to reduce expression of the glycoprotein B. Age-related inflation was also found long after infection with an acute virus. The inflating cells largely maintained Ag-specific function, and exhibited typical central memory phenotype, with no signs of Ag-specific activation. They exhibited increased expression of CD122 and CD127, akin to the Ag-independent T cell clonal expansions found in old specific pathogen-free laboratory mice. This collectively suggests that, in this model, the inflating cells may be selected for high responsiveness to environmental cytokines largely in an Ag-independent manner.

Characterization of a subset of antigen-specific human central memory CD4+ T lymphocytes producing effector cytokines

CCR7(+ )central memory (T(CM)) CD4(+) T cells play a central role in long-term immunological memory. Recent reports indicate that a proportion of CD4(+) T(CM) is able to produce effector cytokines. The phenotype and the role of this subset remain unknown. We characterized cytokine-producing human CD4(+) T(CM) specific for cleared protein and persistent viral Ag. Our results demonstrate that the type of Ag stimulation is a major determinant of CD4(+) T(CM) differentiation. CMV-specific T(CM) were significantly more differentiated than protein Ag-specific T(CM) and included higher proportions of IFN-gamma-producing cells. The expression of killer cell lectin-like receptor G1 (KLRG1) by protein Ag- and CMV-specific T(CM) was associated with increased production of effector cytokines. KLRG1(+) T(CM) expressed high levels of CD127, suggesting that they can survive long term under the influence of IL-7. The induction of KLRG1(+) T(CM) may therefore represent an important target of vaccination against pathogens controlled by cellular immune responses.

Loss of IL-7R and IL-15R expression is associated with disappearance of memory T cells in respiratory tract following influenza infection

Following influenza virus infection, memory CD8 T cells are found in both lymphoid and nonlymphoid organs, where they exhibit striking differences in survival. We have assessed persistence, phenotype, and function of memory CD8 T cells expressing the same TCR in the airways, lung parenchyma, and spleen following influenza virus infection in mice. In contrast to memory CD8 T cells in the spleen, those residing in the airways gradually lost expression of IL-7R and IL-15R, did not respond to IL-7 and/or IL-15, and exhibited poor survival both in vivo and in vitro. Following adoptive transfer into the airways, splenic memory CD8 T cells also down-regulated IL-7R and IL-15R expression and failed to undergo homeostatic proliferation. Thus, although cytokines IL-7 and IL-15 play an essential role in memory CD8 T cell homeostasis in lymphoid organs, the levels of IL-7R and IL-15R expression likely set a threshold for the homeostatic regulation of memory CD8 T cells in the airways. These findings provide a molecular explanation for the gradual loss of airway memory CD8 T cells and heterosubtypic immunity following influenza infection.

Functional and genomic profiling of effector CD8 T cell subsets with distinct memory fates

An important question in memory development is understanding the differences between effector CD8 T cells that die versus effector cells that survive and give rise to memory cells. In this study, we provide a comprehensive phenotypic, functional, and genomic profiling of terminal effectors and memory precursors. Using killer cell lectin-like receptor G1 as a marker to distinguish these effector subsets, we found that despite their diverse cell fates, both subsets possessed remarkably similar gene expression profiles and functioned as equally potent killer cells. However, only the memory precursors were capable of making interleukin (IL) 2, thus defining a novel effector cell that was cytotoxic, expressed granzyme B, and produced inflammatory cytokines in addition to IL-2. This effector population then differentiated into long-lived protective memory T cells capable of self-renewal and rapid recall responses. Experiments to understand the signals that regulate the generation of terminal effectors versus memory precursors showed that cells that continued to receive antigenic stimulation during the later stages of infection were more likely to become terminal effectors. Importantly, curtailing antigenic stimulation toward the tail end of the acute infection enhanced the generation of memory cells. These studies support the decreasing potential model of memory differentiation and show that the duration of antigenic stimulation is a critical regulator of memory formation.