Differential T cell receptor-mediated signaling in naive and memory CD4 T cells

Nicotinic Acid Adenine Dinucleotide Phosphate (NAADP)-Mediated Calcium Signaling Is Active in Memory CD4+ T Cells

Nicotinic acid adenine dinucleotide phosphate (NAADP), identified as one of the most potent calcium-mobilizing second messengers, has been studied in different eukaryotic cell types, including lymphocytes. Although aspects of NAADP-mediated calcium release in lymphocytes are still under debate, the organelles pertaining to NAADP-mediated calcium release are often characterized as acidic and related to lysosomes. Although NAADP-mediated calcium release in different subsets of T cells, including naïve, effector and natural regulatory T cells, has been studied, it has not been widely studied in memory CD4+ T cells, which show a different calcium flux profile. Using a pharmacological approach, the effect of Ned-19, an NAADP pathway antagonist, on the involvement of NAADP in TCR activation in murine memory CD4+ T cells and their downstream effector functions, such as proliferation and cytokine production, was studied. According to this study, Ned-19 inhibited TCR-mediated calcium flux and its downstream effector functions in primary memory CD4+ T cells. The study also revealed that both extracellular and intracellular calcium stores, including endoplasmic reticulum and lysosome-like acidic calcium stores, contribute to the TCR-mediated calcium flux in memory CD4+ T cells. NAADP-AM, a cell permeable analogue of NAADP, was shown to release calcium in memory CD4+ T cells and calcium flux was inhibited by Ned-19.

Negative intracellular regulators of T-cell receptor (TCR) signaling as potential antitumor immunotherapy targets

Immunotherapy strategies aim to mobilize immune defenses against tumor cells by targeting mainly T cells. Co-inhibitory receptors or immune checkpoints (ICPs) (such as PD-1 and CTLA4) can limit T cell receptor (TCR) signal propagation in T cells. Antibody-based blocking of immune checkpoints (immune checkpoint inhibitors, ICIs) enable escape from ICP inhibition of TCR signaling. ICI therapies have significantly impacted the prognosis and survival of patients with cancer. However, many patients remain refractory to these treatments. Thus, alternative approaches for cancer immunotherapy are needed. In addition to membrane-associated inhibitory molecules, a growing number of intracellular molecules may also serve to downregulate signaling cascades triggered by TCR engagement. These molecules are known as intracellular immune checkpoints (iICPs). Blocking the expression or the activity of these intracellular negative signaling molecules is a novel field of action to boost T cell-mediated antitumor responses. This area is rapidly expanding. Indeed, more than 30 different potential iICPs have been identified. Over the past 5 years, several phase I/II clinical trials targeting iICPs in T cells have been registered. In this study, we summarize recent preclinical and clinical data demonstrating that immunotherapies targeting T cell iICPs can mediate regression of solid tumors including (membrane associated) immune-checkpoint inhibitor refractory cancers. Finally, we discuss how these iICPs are targeted and controlled. Thereby, iICP inhibition is a promising strategy opening new avenues for future cancer immunotherapy treatments.

Gut Commensal Segmented Filamentous Bacteria Fine-Tune T Follicular Regulatory Cells to Modify the Severity of Systemic Autoimmune Arthritis

Autoantibodies play a major pathogenic role in rheumatoid arthritis. T follicular helper (Tfh) cells promote germinal center B cell and Ab responses. Excessive Tfh cell responses lead to autoimmunity, and therefore, counterregulation is crucial. T follicular regulatory (Tfr) cells, mainly differentiated from T regulatory cells, can negatively regulate Tfh and germinal center B cells. Dysbiosis is involved in rheumatoid arthritis's pathogenesis. We previously demonstrated that the gut microbiota, segmented filamentous bacteria (SFB), promote autoimmune arthritis by inducing Tfh cells. However, little is known regarding whether gut microbiota influence systemic (nongut) Tfr cells, impacting gut-distal autoimmunity. In this study, using SFB in autoimmune arthritic K/BxN mice, we demonstrated that SFB-induced arthritis is linked to the reduction of Tfr cells' CTLA-4, the key regulatory molecule of Tfr cells. This SFB-mediated CTLA-4 reduction is associated with increased Tfr glycolytic activity, and glycolytic inhibition increases Tfr cells' CTLA-4 levels and reduces arthritis. The surface expression of CTLA-4 is tied to TCR signaling strength, and we discovered that SFB-reduced CTLA-4 is associated with a reduction of Nur77, an indicator of TCR signaling strength. Nur77 is known for repressing glycolytic activity. Using a loss-of-function study, we demonstrated that Nur77+/- haplodeficiency increases glycolysis and reduces CTLA-4 on Tfr cells, which is associated with increased arthritis and anti-glucose-6-phosphate isomerase titers. Tfr-specific deletion (KRN.Foxp3CreBcl-6fl/fl) in autoimmune condition reveals that Tfr cells repress arthritis, Tfh cells, and autoantibody responses and that SFB can mitigate this repression. Overall, these findings demonstrated that gut microbiota distally impact systemic autoimmunity by fine-tuning Tfr cells.

The Innate Biologies of Adaptive Antigen Receptors

Nonclonal innate immune responses mediated by germ line-encoded receptors, such as Toll-like receptors or natural killer receptors, are commonly contrasted with diverse, clonotypic adaptive responses of lymphocyte antigen receptors generated by somatic recombination. However, the Variable (V) regions of antigen receptors include germ line-encoded motifs unaltered by somatic recombination, and theoretically available to mediate nonclonal, innate responses, that are independent of or largely override clonotypic responses. Recent evidence demonstrates that such responses exist, underpinning the associations of particular γδ T cell receptors (TCRs) with specific anatomical sites. Thus, TCRγδ can make innate and adaptive responses with distinct functional outcomes. Given that αβ T cells and B cells can also make nonclonal responses, we consider that innate responses of antigen receptor V-regions may be more widespread, for example, inducing states of preparedness from which adaptive clones are better selected. We likewise consider that potent, nonclonal T cell responses to microbial superantigens may reflect subversion of physiologic innate responses of TCRα/β chains.

Memory T cells: strategies for optimizing tumor immunotherapy

Several studies have demonstrated that memory T cells including stem cell memory (Tscm) T cells and central memory (Tcm) T cells show superior persistence and antitumor immunity compared with effector memory T (Tem) cells and effector T (Teff) cells. Furthermore, the Tcm/Teff ratio has been reported to be a predictive biomarker of immune responses against some tumors. Thus, a system-level understanding of the mechanisms underlying the differentiation of effector and memory T cells is of increasing importance for developing immunological strategies against various tumors. This review focuses on recent advances in efficacy against tumors, the origin, formation mechanisms of memory T cells, and the role of the gut microbiota in memory T cell formation. Furthermore, we summarize strategies to generate memory T cells in (ex) vivo that, might be applicable in clinical practice.

Rapid Recall Ability of Memory T cells is Encoded in their Epigenome

Even though T-cell receptor (TCR) stimulation together with co-stimulation is sufficient for the activation of both naïve and memory T cells, the memory cells are capable of producing lineage specific cytokines much more rapidly than the naïve cells. The mechanisms behind this rapid recall response of the memory cells are still not completely understood. Here, we performed epigenetic profiling of human resting naïve, central and effector memory T cells using ChIP-Seq and found that unlike the naïve cells, the regulatory elements of the cytokine genes in the memory T cells are marked by activating histone modifications even in the resting state. Therefore, the ability to induce expression of rapid recall genes upon activation is associated with the deposition of positive histone modifications during memory T cell differentiation. We propose a model of T cell memory, in which immunological memory state is encoded epigenetically, through poising and transcriptional memory.

Endogenous Nur77 Is a Specific Indicator of Antigen Receptor Signaling in Human T and B Cells

Distinguishing true Ag-stimulated lymphocytes from bystanders activated by the inflammatory milieu has been difficult. Nur77 is an immediate early gene whose expression is rapidly upregulated by TCR signaling in murine T cells and human thymocytes. Nur77-GFP transgenes serve as specific TCR and BCR signaling reporters in murine transgenic models. In this study, we demonstrate that endogenous Nur77 protein expression can serve as a reporter of TCR and BCR specific signaling in human PBMCs. Nur77 protein amounts were assessed by immunofluorescence and flow cytometry in T and B cells isolated from human PBMCs obtained from healthy donors that had been stimulated by their respective Ag receptors. We demonstrate that endogenous Nur77 is a more specific reporter of Ag-specific signaling events than the commonly used CD69 activation marker in both human T and B cells. This is reflective of the disparity in signaling pathways that regulate the expression of Nur77 and CD69. Assessing endogenous Nur77 protein expression has great potential to identify Ag-activated lymphocytes in human disease.

Imbalanced signal transduction in regulatory T cells expressing the transcription factor FoxP3

FoxP3(+) T regulatory (Treg) cells have a fundamental role in immunological tolerance, with transcriptional and functional phenotypes that demarcate them from conventional CD4(+) T cells (Tconv). Differences between these two lineages in the signaling downstream of T-cell receptor-triggered activation have been reported, and there are different requirements for some signaling factors. Seeking a comprehensive view, we found that Treg cells have a broadly dampened activation of several pathways and signaling nodes upon TCR-mediated activation, with low phosphorylation of CD3ζ, SLP76, Erk1/2, AKT, or S6 and lower calcium flux. In contrast, STAT phosphorylation triggered by interferons, IL2 or IL6, showed variations between Treg and Tconv in magnitude or choice of preferential STAT activation but no general Treg signaling defect. Much, but not all, of the Treg/Tconv difference in TCR-triggered responses could be attributed to lower responsiveness of antigen-experienced cells with CD44(hi) or CD62L(lo) phenotypes, which form a greater proportion of the Treg pool. Candidate regulators were tested, but the Treg/Tconv differential could not be explained by overexpression in Treg cells of the signaling modulator CD5, the coinhibitors PD-1 and CTLA4, or the regulatory phosphatase DUSP4. However, transcriptome profiling in Dusp4-deficient mice showed that DUSP4 enhances the expression of a segment of the canonical Treg transcriptional signature, which partially overlaps with the TCR-dependent Treg gene set. Thus, Treg cells, likely because of their intrinsically higher reactivity to self, tune down TCR signals but seem comparatively more attuned to cytokines or other intercellular signals.

Staphylococcal Enterotoxin B (SEB) Induces Memory CD4 T Cell Anergy in vivo and Impairs Recall Immunity to Unrelated Antigens

INTRODUCTION

Naïve and memory T cells can utilize unique regulatory pathways to promote protection but prevent self-reactivity. A bacterial superantigen SEB exploits unique TCR proximal signaling processes in memory CD4 T cells to induce clonal anergy. The aim of this study was to determine if SEB could antagonize memory CD4 T cells in vivo and whether there would be consequences on recall immune responses. We evaluated Ab responses to a T-dependent antigen as a measurement of memory T cell helper function.

METHOD

BALB/c mice were primed with TNP-RGG to elicit memory B cells and also immunized with an ovalbumin peptide to elicit memory helper T cells. Another group of TNP-RGG immunized mice were used as adoptive transfer recipients of exogenous DO11.10 memory T cells. Mice were challenged with TNP-OVA with or without prior administration of SEB. B cells secreting IgM or IgG TNP-specific Ab were enumerated by ELISPOT as indicators of primary versus secondary humoral immunity.

RESULTS

Comparing the SEB and non-SEB-treated groups, the SEB-treated group failed to produce TNP-specific IgG in response to challenge with TNP-OVA, even if they were previously immunized with OVA. All groups produced IgM, indicating that the primary Ab responses and naïve helper T cells were not impacted by SEB. SEB had no negative impact when DO11.10 × Fyn^-/- memory T cells were used as donor cells.

CONCLUSION

The present study indicated that SEB selectively targeted memory CD4 T cells in vivo and prevented helper function. Consequently, recall humoral immunity was lost. The data are most consistent with in vivo T cell anergy as opposed to indirect suppression as elimination of Fyn kinase restored helper function. These data suggest that bacterial superantigens can impair post-vaccination memory cell responses to unrelated antigens via their ability to target Vb families and antagonize memory cell activation.

Memory CD8+ T cells exhibit increased antigen threshold requirements for recall proliferation

Memory CD8⁺ T cells require stronger TCR stimulation than naive cells to enter cell cycle due to reduced Zap70 activation and increased levels of protein tyrosine phosphatases.

A hallmark of immunological memory is the ability of previously primed T cells to undergo rapid recall responses upon antigen reencounter. Classic work has suggested that memory T cells proliferate in response to lower doses of antigen than naive T cells and with reduced requirements for co-stimulation. In contrast to this premise, we observed that naive but not memory T cells proliferate in vivo in response to limited antigen presentation. To reconcile these observations, we tested the antigen threshold requirement for cell cycle entry in naive and central memory CD8⁺ T cells. Although both naive and memory T cells detect low dose antigen, only naive T cells activate cell cycle effectors. Direct comparison of TCR signaling on a single cell basis indicated that central memory T cells do not activate Zap70, induce cMyc expression, or degrade p27 in response to antigen levels that activate these functions in naive T cells. The reduced sensitivity of memory T cells may result from both decreased surface TCR expression and increased expression of protein tyrosine phosphatases as compared with naive T cells. Our data describe a novel aspect of memory T cell antigen threshold sensitivity that may critically regulate recall expansion.

CD4+ T cells recognize unique and conserved 2009 H1N1 influenza hemagglutinin epitopes after natural infection and vaccination

Influenza A/California/4/2009 (H1N1/09) is a recently emerged influenza virus capable of causing serious illness or death in otherwise healthy individuals. Serious outcomes were most common in young adults and children, suggesting that pre-existing heterologous immunity may influence the severity of infection. Using tetramers, we identified CD4(+) T-cell epitopes within H1N1/09 hemagglutinin (HA) that share extensive homology with seasonal influenza and epitopes that are unique to H1N1/09 HA. Ex vivo tetramer staining revealed that T cells specific for conserved epitopes were detectable within the memory compartment, whereas T cells specific for unique epitopes were naive and infrequent prior to infection or vaccination. Following infection, the frequencies of T cells specific for unique epitopes were 11-fold higher, reaching levels comparable to those of T cells specific for immunodominant epitopes. In contrast, the frequencies of T cells specific for conserved epitopes were only 2- to 3-fold higher following infection. In general, H1HA-reactive T cells exhibited a memory phenotype, expressed CXCR3 and secreted IFN-γ, indicating a predominantly Th1-polarized response. A similar Th1 response was seen in vaccinated subjects, but the expansion of T cells specific for HA epitopes was comparatively modest after vaccination. Our findings indicate that CD4(+) T cells recognize both strain-specific and conserved epitopes within the influenza HA protein and suggest that naive T cells specific for HA epitopes undergo significant expansion, whereas memory T cells specific for the conserved epitopes undergo more restrained expansion.

Human CD4(+) effector T lymphocytes generated upon TCR engagement with self-peptides respond defectively to IL-7 in their transition to memory cells

The peripheral repertoire of CD4(+) T lymphocytes contains autoreactive cells that remain tolerant through several mechanisms. However, nonspecific CD4(+) T cells can be activated in physiological conditions as in the course of an ongoing immune response, and their outcome is not yet fully understood. Here, we investigate the fate of human naive CD4(+) lymphocytes activated by dendritic cells (DCs) presenting endogenous self-peptides in comparison with lymphocytes involved in alloresponses. We generated memory cells (Tmem) from primary effectors activated with mature autologous DCs plus interleukin (IL)-2 (Tmauto), simulating the circumstances of an active immune response, or allogeneic DCs (Tmallo). Tmem were generated from effector cells that were rested in the absence of antigenic stimuli, with or without IL-7. Tmem were less activated than effectors (demonstrated by CD25 downregulation) particularly with IL-7, suggesting that this cytokine may favour the transition to quiescence. Tmauto and Tmallo showed an effector memory phenotype, and responded similarly to polyclonal and antigen-specific stimuli. Biochemically, IL-7-treated Tmallo were closely related to conventional memory lymphocytes based on Erk-1/2 activation, whereas Tmauto were more similar to effectors. Autologous effectors exhibited lower responses to IL-7 than allogeneic cells, which were reflected in their reduced proliferation and higher cell death. This was not related to IL-7 receptor expression but rather to signalling deficiencies, according to STAT5 activation These results suggest that ineffective responses to IL-7 could impair the transition to memory cells of naive CD4(+) T lymphocytes recognizing self-peptides in the setting of strong costimulation.

Epigenetic regulation of HIV-1 persistence and evolving strategies for virus eradication

Despite the intense effort put by researchers globally to understand Human Immunodeficiency Virus (HIV-1) pathogenesis since its discovery 30 years ago, the acquired knowledge till date is not good enough to eradicate HIV-1 from an infected individual. HIV-1 infects cells of the human immune system and integrates into the host cell genome thereby leading to persistent infection in these cells. Based on the activation status of the cells, the infection could be productive or result in latent infection. The current regimen used to treat HIV-1 infection in an AIDS patient includes combination of antiretroviral drugs called Highly Active Anti-Retroviral Therapy (HAART). A major challenge for the success of HAART has been these latent reservoirs of HIV which remain hidden and pose major hurdle for the eradication of virus. Combination of HAART therapy with simultaneous activation of latent reservoirs of HIV-1 seems to be the future of anti-retroviral therapy; however, this will require a much better understanding of the mechanisms and regulation of HIV-1 latency. In this chapter, we have tried to elaborate on HIV-1 latency, highlighting the strategies employed by the virus to ensure persistence in the host with specific focus on epigenetic regulation of latency. A complete understanding of HIV-1 latency will be extremely essential for ultimate eradication of HIV-1 from the human host.

Signal transduction via the T cell antigen receptor in naïve and effector/memory T cells

T cells play an indispensable role in immune defense against infectious agents, but can also be pathogenic. These T cells develop in the thymus, are exported into the periphery as naïve cells and participate in immune responses. Upon recognition of antigen, they are activated and differentiate into effector and memory T cells. While effector T cells carry out the function of the immune response, memory T cells can last up to the life time of the individual, and are activated by subsequent antigenic exposure. Throughout this life cycle, the T cell uses the same receptor for antigen, the T cell Receptor, a complex multi-subunit receptor. Recognition of antigen presented by peptide/MHC complexes on antigen presenting cells unleashes signaling pathways that control T cell activation at each stage. In this review, we discuss the signals regulated by the T cell receptor in naïve and effector/memory T cells.

Anergy in CD4 memory T lymphocytes. II. Abrogation of TCR-induced formation of membrane signaling complexes

Memory and naive CD4 T cells have unique regulatory pathways for self/non-self discrimination. A memory cell specific regulatory pathway was revealed using superantigens to trigger the TCR. Upon stimulation by bacterial superantigens, like staphylococcal enterotoxin B (SEB), TCR proximal signaling is impaired leading to clonal tolerance (anergy). In the present report, we show that memory cell anergy results from the sequestration of the protein tyrosine kinase ZAP-70 away from the TCR/CD3ζ chain. During SEB-induced signaling, ZAP-70 is excluded from both detergent-resistant membrane microdomains and the immunological synapse, thus blocking downstream signaling. We also show that the mechanism underlying memory cell anergy must involve Fyn kinase, given that the suppression of Fyn activity restores the movement of ZAP-70 to the immunological synapse, TCR proximal signaling, and cell proliferation. Thus, toleragens, including microbial toxins, may modulate memory responses by targeting the organizational structure of memory cell signaling complexes.

Matching phosphorylation response patterns of antigen-receptor-stimulated T cells via flow cytometry

Background

When flow cytometric data on mixtures of cell populations are collected from samples under different experimental conditions, computational methods are needed (a) to classify the samples into similar groups, and (b) to characterize the changes within the corresponding populations due to the different conditions. Manual inspection has been used in the past to study such changes, but high-dimensional experiments necessitate developing new computational approaches to this problem. A robust solution to this problem is to construct distinct templates to summarize all samples from a class, and then to compare these templates to study the changes across classes or conditions.

Results

We designed a hierarchical algorithm, flowMatch, to first match the corresponding clusters across samples for producing robust meta-clusters, and to then construct a high-dimensional template as a collection of meta-clusters for each class of samples. We applied the algorithm on flow cytometry data obtained from human blood cells before and after stimulation with anti-CD3 monoclonal antibody, which is reported to change phosphorylation responses of memory and naive T cells. The flowMatch algorithm is able to construct representative templates from the samples before and after stimulation, and to match corresponding meta-clusters across templates. The templates of the pre-stimulation and post-stimulation data corresponding to memory and naive T cell populations clearly show, at the level of the meta-clusters, the overall phosphorylation shift due to the stimulation.

Conclusions

We concisely represent each class of samples by a template consisting of a collection of meta-clusters (representative abstract populations). Using flowMatch, the meta-clusters across samples can be matched to assess overall differences among the samples of various phenotypes or time-points.

Superantigen-induced CD4 memory T cell anergy. I. Staphylococcal enterotoxin B induces Fyn-mediated negative signaling

Memory CD4 T cells must provide robust protection for an organism while still maintaining self-tolerance. Superantigens reveal a memory cell-specific regulatory pathway, by which signaling through the TCR can lead to clonal tolerance (anergy). Here we show that the src kinase Fyn is a critical regulator of anergy in murine memory CD4 T cells induced by the bacterial superantigen staphylococcal enterotoxin B (SEB). Exposure to SEB results in impaired TCR signaling due to failed CD3/ZAP-70 complex formation. Further, signal transduction through the TCR remains similarly blocked when anergic memory cells are subsequently exposed to agonist peptide antigen. Pharmacological inhibition or genetic elimination of Fyn kinase reverses memory cell anergy, resulting in SEB-induced cell proliferation. The mechanism underlying impaired TCR signaling and subsequent memory cell anergy must involve a Fyn signaling pathway given that the suppression of Fyn activity restores CD3/ZAP-70 complex formation and TCR proximal signaling.

Consequences of increased CD45RA and RC isoforms for TCR signaling and peripheral T cell deficiency resulting from heterogeneous nuclear ribonucleoprotein L-like mutation

CD45 is the most abundant protein tyrosine phosphatase in the plasma membrane of T cells and serves a critical role in TCR signaling. Different CD45 isoforms are made by alternative mRNA splicing depending on the stage of T cell development and activation, yet their role remains unclear. Expression of CD45RA and RC isoforms is increased 20- to 200-fold on T cells from thunder mice with a loss-of-function mutation in the RNA-binding protein, heterogeneous nuclear ribonucleoprotein L-like (hnRNPLL), although total CD45 expression is unaltered. In this study, we test the hypothesis that this shift in CD45 isoform expression alters TCR signaling, thymic selection, and accumulation of peripheral T cells. There was no discernable effect of the change in CD45 isoform expression upon Lck phosphorylation or T cell positive and negative selection, whereas these indices were strongly affected by a decrease in the overall amount of CD45 in Ptprc mutant animals. The one exception to this conclusion was in thymocytes from Ptprc(loc/loc) animals with 4% of normal CD45 protein levels, where Lck505 phosphorylation was increased 25% in Hnrpll mutant cells, suggesting that high m.w. CD45 isoforms had lower Lck505 phosphatase activity in this context. In T cells with no CD45 protein, hnRNPLL mutation still diminished peripheral T cell accumulation, demonstrating that hnRNPLL regulates T cell longevity independently from its effects on CD45 splicing.

The Sts proteins target tyrosine phosphorylated, ubiquitinated proteins within TCR signaling pathways

The T cell receptor (TCR) detects the presence of infectious pathogens and activates numerous intracellular signaling pathways. Protein tyrosine phosphorylation and ubiquitination serve as key regulatory mechanisms downstream of the TCR. Negative regulation of TCR signaling pathways is important in controlling the immune response, and the Suppressor of TCR Signaling proteins (Sts-1 and Sts-2) have been shown to function as critical negative regulators of TCR signaling. Although their mechanism of action has yet to be fully uncovered, it is known that the Sts proteins possess intrinsic phosphatase activity. Here, we demonstrate that Sts-1 and Sts-2 are instrumental in down-modulating proteins that are dually modified by both protein tyrosine phosphorylation and ubiquitination. Specifically, both naïve and activated T cells derived from genetically engineered mice that lack the Sts proteins display strikingly elevated levels of tyrosine phosphorylated, ubiquitinated proteins following TCR stimulation. The accumulation of the dually modified proteins is transient, and in activated T cells but not naïve T cells is significantly enhanced by co-receptor engagement. Our observations hint at a novel regulatory mechanism downstream of the T cell receptor.

Biochemical signaling pathways for memory T cell recall

Memory T cells exhibit low activation thresholds and rapid effector responses following antigen stimulation, contrasting naive T cells with high activation thresholds and no effector responses. Signaling mechanisms for the distinct properties of naive and memory T cells remain poorly understood. Here, I will discuss new results on signal transduction in naive and memory T cells that suggest proximal control of activation threshold and a distinct biochemical pathway to rapid recall. The signaling and transcriptional pathways controlling immediate effector function in memory T cells closely resemble pathways for rapid effector cytokine production in innate immune cells, suggesting memory T cells use innate pathways for efficacious responses.

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.

Cellular and molecular mechanisms of memory T-cell survival

Long-term maintenance of the memory T-cell response is the hallmark of immune protection and, hence, constitutes one of the most important objectives of vaccine-development strategies. Persistent memory T cells, developed after vaccination or microbial infections, ensure the generation of an antimicrobial response upon re-exposure to the pathogen through rapid clonal proliferation and activation of effector functions. However, in the context of many pathogen infections, these memory T cells fail to persist and die. In this review, we will highlight recent exciting findings in studies of memory T cells, their generation, their lineage relationships and their survival pathways; indeed, survival of memory T cells and maintenance of their functionality are key features of the immune response in its quest to control disease progression and in the development of vaccines to persistent microbial infections.

IFN-lambda1 (IL-29) inhibits GATA3 expression and suppresses Th2 responses in human naive and memory T cells

IFN-lambda1 (IL-29) plays a novel, emerging role in the inhibition of human Th2 responses. Here, we demonstrate that both naive and memory human CD4(+) T cells express mRNA for the IFN-lambda1-specific receptor, IL-28Ralpha, and are responsive to IFN-lambda1. Expression of Th2 cytokines (IL-4 and IL-13) was suppressed in naive and memory CD4(+) T cells by IFN-lambda1, without affecting their proliferation. Further, acquisition of IL-4Ralpha expression after stimulation was inhibited by IFN-lambda1, as was GATA3 expression. Finally, IFN-lambda1 diminished the change in cell-surface phenotype that accompanies differentiation of "central memory" T cells into "effector memory" T cells. Taken together, our data describe unique immunomodulatory effects of IFN-lambda1 and identify novel mechanisms for the reduction of existing Th2 responses and the regulation of new ones, in circulating naive and memory CD4(+) T cells.

Self-antigen prevents CD8 T cell effector differentiation by CD134 and CD137 dual costimulation

We compared how CD4 vs CD8 cells attain the capacity to express the effector cytokine IFN-gamma under both immunogenic and tolerogenic conditions. Although the Ifng gene locus was epigenetically repressed in naive Ag-inexperienced CD4 cells, it had already undergone partial remodeling toward a transcriptionally competent configuration in naive CD8 cells. After TCR stimulation, CD8 cells fully remodeled the Ifng locus and gained the capacity to express high levels of IFN-gamma more rapidly than CD4 cells. Enforced dual costimulation through OX40 and 4-1BB redirected CD8 cells encountering soluble exogenous peptide to expand and differentiate into IFN-gamma and TNF-alpha double-producing effectors rather than becoming tolerant. Despite this and the stronger tendency of CD8 compared with CD4 cells to differentiate into IFN-gamma-expressing effectors, when parenchymal self-Ag was the source of tolerizing Ag, enforced dual costimulation selectively boosted expansion but did not push effector differentiation in CD8 cells while both expansion and effector differentiation were dramatically boosted in CD4 cells. Notably, enforced dual costimulation was able to push effector differentiation in CD8 cells encountering cognate parenchymal self-Ag when CD4 cells were simultaneously engaged. Thus, the ability of enforced OX40 plus 4-1BB dual costimulation to redirect CD8 cells to undergo effector differentiation was unexpectedly influenced by the source of tolerizing Ag and help was selectively required to facilitate CD8 cell effector differentiation when the tolerizing Ag derived from self.

T cell receptor-mediated activation of CD4+CD44hi T cells bypasses Bcl10: an implication of differential NF-kappaB dependence of naïve and memory T cells during T cell receptor-mediated responses

Previous studies have demonstrated that Bcl10 (B-cell leukemia/lymphoma 10) is essential for T cell receptor-mediated NF-kappaB activation and subsequent proliferation and interleukin 2 (IL2) production. However, here we demonstrate that, contrary to expectations, Bcl10 is differentially required for T cell activation, including for both proliferation and cytokine production. When CD4+ and CD8+ T cells were divided based on expression levels of CD44, which distinguishes naïve cells (CD44lo) versus those that are antigen-experienced (CD44hi), IL2 production by and proliferation of CD4+CD44lo naïve cells and both subpopulations of CD8+ T cells were clearly Bcl10-dependent, whereas these same functional properties of CD4+CD44hi T cells occurred largely independent of Bcl10. As with the other subpopulations of T cells, CD4+CD44hi T cells did not activate the NF-kappaB pathway in the absence of Bcl10; nevertheless, these CD4+CD44hi antigen-experienced T cells efficiently secreted IL2 after T cell receptor stimulation. Strikingly, therefore, T cell receptor-mediated IL2 production in these cells is NF-kappaB-independent. Our studies suggest that antigen-experienced CD4+ T cells differ from their naïve counterparts and from CD8+ T cells in their ability to achieve activation independent of the Bcl10/NF-kappaB pathway.

HIV-induced changes in T cell signaling pathways

Infection with HIV usually results in chronic activation of the immune system, with profound quantitative and qualitative changes in the T cell compartment. To better understand the mechanistic basis for T cell dysfunction and to discern whether such mechanisms are reversed after effective antiviral treatment, we analyzed changes in signaling pathways of human CD4(+) and CD8(+) T cells from 57 HIV-infected subjects in varying stages of disease progression and treatment, including long-term nonprogressors, progressors, and chronically infected subjects provided effective antiretroviral therapy (responders). A previously described PhosFlow method was adapted and optimized so that protein phosphorylation could be visualized in phenotypically defined subpopulations of CD4(+) and CD8(+) T cells (naive, memory, and effector) by flow cytometry. T cell signaling induced by TCR cross-linking, IL-2, or PMA/ionomycin was found to be blunted within all T cell subpopulations in those with progressive HIV disease compared with long-term nonprogressors and responders. Although alterations in cellular signaling correlated with levels of basal phosphorylation, viral load, and/or expression of programmed death-1, it was the level of basal phosphorylation that appeared to be the factor most dominantly associated with impaired signaling. Notably, provision of effective antiretroviral therapy was associated with a normalization of both basal phosphorylation levels and T cell signaling. These data, in aggregate, suggest that generalized dysfunction of the T cell compartment during progressive HIV disease may be in part dependent upon an increased basal level of phosphorylation, which itself may be due to the heightened state of immune activation found in advanced disease.

Regulation of T cell survival through coronin-1-mediated generation of inositol-1,4,5-trisphosphate and calcium mobilization after T cell receptor triggering

T cell homeostasis is essential for the functioning of the vertebrate immune system, but the intracellular signals required for T cell homeostasis are largely unknown. We here report that the WD-repeat protein family member coronin-1, encoded by the gene Coro1a, is essential in the mouse for T cell survival through its promotion of Ca2+ mobilization from intracellular stores. Upon T cell receptor triggering, coronin-1 was essential for the generation of inositol-1,4,5-trisphosphate from phosphatidylinositol-4,5-bisphosphate. The absence of coronin-1, although it did not affect T cell development, resulted in a profound defect in Ca2+ mobilization, interleukin-2 production, T cell proliferation and T cell survival. We conclude that coronin-1, through activation of Ca2+ release from intracellular stores, is an essential regulator of peripheral lymphocyte survival.

Allelic variant in CTLA4 alters T cell phosphorylation patterns

Little is known regarding the functional effects of common autoimmune susceptibility variants on human immune cells. The SNP CT60 (rs3087243; A/G) located in the 3' UTR of the CTLA4 gene has been associated with autoimmune diseases. We examined a cohort of healthy individuals stratified by genotypes at CTLA4 to gain insight into the functional effects of allelic variation on T cell signaling. Using phospho-site-specific mAbs, we tested the hypothesis that the CT60 genotype at CTLA4 is associated with altered T cell antigen receptor (TCR) signaling in naive and/or memory T cells. By normalizing for the extent of the initial TCR signaling event at CD3zeta, we observed that the relative responsiveness to TCR stimulation as assessed by phosphorylation levels of downstream signaling molecules was altered in naive (CD4(+)CD45RA(high)) and memory (CD4(+)CD45RA(low)) T cells obtained from individuals with the disease-susceptibility allele at CTLA4. Thus, allelic variation associated with autoimmune disease can alter the signaling threshold of CD4(+) T cells. These experiments provide a rational approach for the dissection of T cell-susceptibility genes in autoimmune diseases.

A Biochemical Signature for Rapid Recall of Memory CD4 T Cells

Mechanisms for the rapid recall response mediated by memory T cells remain unknown. In this study, we present a novel, multiparameter analysis of TCR-coupled signaling and function in resting and activated naive and memory CD4 T cells, revealing a biochemical basis for immunological recall. We identify a striking elevation in expression of the proximal tyrosine kinase Zap70 in resting Ag-specific and polyclonal mouse memory vs naive CD4 T cells that is stably maintained independent of protein synthesis. Elevated Zap70 protein levels control effector function as IFN-gamma production occurs exclusively from the Zap70(high) fraction of activated T cells in vitro and in vivo, and specific down-modulation of Zap70 expression in memory CD4 T cells by small interfering RNA or protein inhibition significantly reduces rapid IFN-gamma production. Downstream of Zap70, we show quantitative differences in distal phosphorylation associated with effector function in naive and memory subsets, with low accumulation of phosphorylation in memory T cells producing IFN-gamma at early time points, contrasting extensive phosphorylation associated with IFN-gamma production following sustained activation of naive T cells. Our results reveal a novel biochemical signature imparted to memory CD4 T cells enabling efficacious responses through increased Zap70 expression and reduced accumulation of downstream signaling events.

Experimental approaches to the study of HIV-1 latency

Viral latency is a reversibly non-productive state of infection that allows some viruses to evade host immune responses. As a consequence of its tropism for activated CD4(+) T cells, HIV-1 can establish latent infection in resting memory CD4(+) T cells, which are generated when activated CD4(+) T cells return to a quiescent state. Latent HIV-1 persists as a stably integrated but transcriptionally silent provirus. In this state, the virus is unaffected by immune responses or antiretroviral drugs, and this latent reservoir in resting CD4(+) T cells is a major barrier to curing the infection. Unfortunately, there is no simple assay to measure the number of latently infected cells in a patient, nor is there an entirely representative in vitro model in which to explore the molecular mechanisms of latency. This Review will consider current approaches to the analysis of HIV-1 latency both in vivo and in vitro.

Defective T cell receptor-mediated signal transduction in memory CD4 T lymphocytes exposed to superantigen or anti-T cell receptor antibodies

Lymphocytes must promote protective immune responses while still maintaining self-tolerance. Stimulation through the T cell receptor (TCR) can lead to distinct responses in naive and memory CD4 T cells. Whereas peptide antigen stimulates both naive and memory T cells, soluble anti-CD3 antibodies and bacterial superantigens stimulate only naive T cells to proliferate and secrete cytokines. Further, superantigens, like staphylococcal enterotoxin B (SEB), cause memory T cells to become anergic while soluble anti-CD3 does not. In the present report, we show that signal transduction through the TCR is impaired in memory cells exposed to either anti-CD3 or SEB. A block in signaling leads to impaired activation of the kinase ZAP-70 so that downstream signals and cell proliferation do not occur. We further show that the signaling defect is unique to each agent. In anti-CD3-treated memory T cells, the src kinase Lck is only transiently activated and does not phosphorylate and activate ZAP-70. In SEB-treated memory T cells, ZAP-70 does not interact with the TCR/CD3 complex to become accessible to Lck. Finally, we provide evidence that alternative signaling pathways are initiated in SEB-treated memory cells. Altered signaling, indicated by an elevation in activity of the src kinase Fyn, may be responsible for memory cell anergy caused by SEB. Thus, differentiation of naive T cells into memory cells is accompanied by alterations in TCR-mediated signaling that can promote heightened recall immunity or specific tolerance.

Differential requirement for Lck during primary and memory CD8+ T cell responses

T cell receptor (TCR) signaling mediates cell fate decisions throughout the life of a T cell. The earliest biochemical events during antigen-stimulated TCR signaling include activation of the Src-family protein tyrosine kinase, p56(Lck) (Lck), which is an integral component of the TCR signaling complex by its association with the cytoplasmic tails of CD8 or CD4. CD8 and Lck are obligatory during thymic selection of CD8+ T cells. What remain unknown are when and with what stringency Lck is required for effective TCR-mediated activation and function throughout the life of a mature CD8+ T cell. Using mice that express an inducible Lck transgene in T cells, we have investigated the temporal importance of Lck-mediated TCR signaling in antigen-specific CD8+ T cell responses during acute viral infections. We show that Lck deficiency induced in naive mice abrogated the antigen-specific activation and clonal expansion of CD8+ T cells during a primary response to acute viral infections. Moreover, the magnitude of primary CD8 T cell expansion depended on the duration of Lck-dependent TCR signaling. Quite unexpectedly, however, Lck was dispensable for enhanced functional avidity, maintenance, and reactivation of memory CD8+ T cells in vitro and in vivo. These observations suggest that the TCR signaling apparatus is rewired from an Lck-dependent state in naive CD8+ T cells to an Lck-independent state in memory CD8+ T cells. Less stringent requirements for antigen-specific TCR signaling to activate memory CD8+ T cells could, in part, account for their unique hyperreactivity to antigen, which contributes to accelerated immune control during secondary infections.

Reshaping the past: Strategies for modulating T-cell memory immune responses

Memory T cells are generated following an initial encounter with antigen, persist over the lifetime of an individual, and mediate rapid and robust functional responses upon antigenic recall. While immune memory is generally associated with protective immune response to pathogens, memory T cells can be generated to diverse types of antigens including autoantigens and alloantigens through homologous or crossreactive priming and comprise the majority of circulating T cells during adulthood. Memory T cells can therefore play critical roles in propagating and perpetuating autoimmune disease and in mediating allograft rejection, although the precise pathways for regulation of memory immune responses remain largely undefined. Moreover, evaluating and designing strategies to modulate memory T-cell responses are challenging given the remarkable heterogeneity of memory T cells, with different subsets predominating in lymphoid versus non-lymphoid tissue sites. In this review, we discuss what is presently known regarding the effect of current immunomodulation strategies on the memory T-cell compartment and potential strategies for controlling immunological recall.

Committed to memory: lineage choices for activated T cells

The mechanisms for the generation of memory T cells and their delineation into heterogeneous subsets remain unknown. The linear model for memory T-cell generation from differentiated effector cells has been favored, although there is evidence that memory T cells can emerge directly from naive T cells undergoing homeostatic expansion and from activated T cells lacking effector functions. Here, we discuss the evidence from diverse studies of memory generation that support a new 'intersecting pathway' model for memory T-cell generation in which antigen-driven effector differentiation and homeostasis-driven memory differentiation follow distinct but analogous pathways. Antigen withdrawal during effector differentiation enables intersection with the memory pathway through a pre-memory intermediate, and memory heterogeneity is influenced by homeostasis, migration and persistence in vivo.

Lysine methylation and ‘signaling memory’

A combination of various post-translational modifications regulates the formation of signaling networks in both the nucleus and the cytosol. Of these, lysine methylation provides a relatively stable marker on histones and contributes to the formation of a distinct pattern of histone-dependent gene regulation ('transcriptional memory'). Thus far, lysine methylation was considered to be nucleus specific; however, recent findings indicate that lysine methylation contributes to receptor-mediated signal transduction in the cytosol.

Signaling control of memory T cell generation and function

Memory T cells exhibit low activation thresholds and mediate rapid effector responses when recalled by antigen; contrasting the higher activation threshold, slower responses and predominant IL-2 production by naive T cells. While the sequence of intracellular events coupling the T cell-receptor (TCR) to naive T cell activation is well characterized, biochemical control of memory T cell differentiation and function remains undefined. In this review, we will discuss recent developments in T cell-receptor signal transduction as they pertain to memory T cells, and will discuss how signal dampening may drive memory generation, and more efficient spatial organization of signaling molecules may promote rapid recall responses.

Differences in signaling molecule organization between naive and memory CD4+ T lymphocytes

The immunological synapse is a highly organized complex formed at the junction between Ag-specific T cells and APCs as a prelude to cell activation. Although its exact role in modulating T cell signaling is unknown, it is commonly believed that the immunological synapse is the site of cross-talk between the T cell and APC (or target). We have examined the synapses formed by naive and memory CD4 cells during Ag-specific cognate interactions with APCs. We show that the mature immunological synapse forms more quickly during memory T cell activation. We further show that the composition of the synapse found in naive or memory cell conjugates with APCs is distinct with the tyrosine phosphatase, CD45, being a more integral component of the mature synapses formed by memory cells. Finally, we show that signaling molecules, including CD45, are preassociated in discrete, lipid-raft microdomains in resting memory cells but not in naive cells. Thus, enhanced memory cell responses may be due to intrinsic properties of signaling molecule organization.

T-cell receptor–induced phosphorylation of the ζ chain is efficiently promoted by ZAP-70 but not Syk

Engagement of the T-cell receptor (TCR) results in the activation of Lck/Fyn and ZAP-70/Syk tyrosine kinases. Lck-mediated tyrosine phosphorylation of signaling motifs (ITAMs) in the CD3-ζ subunits of the TCR is an initial step in the transduction of signaling cascades. However, ζ phosphorylation is also promoted by ZAP-70, as TCR-induced ζ phosphorylation is defective in ZAP-70–deficient T cells. We show that this defect is corrected by stable expression of ZAP-70, but not Syk, in primary and transformed T cells. Indeed, these proteins are differentially coupled to the TCR with a 5- to 10-fold higher association of ZAP-70 with ζ as compared to Syk. Low-level Syk-ζ binding is associated with significantly less Lck coupled to the TCR. Moreover, diminished coupling of Lck to ζ correlates with a poor phosphorylation of the positive regulatory tyr352 residue of Syk. Thus, recruitment of Lck into the TCR complex with subsequent ζ chain phosphorylation is promoted by ZAP-70 but not Syk. Importantly, the presence of ZAP-70 positively regulates the TCR-induced tyrosine phosphorylation of Syk. The interplay between Syk and ZAP-70 in thymocytes, certain T cells, and B-chronic lymphocytic leukemia cells, in which they are coexpressed, will therefore modulate the amplitude of antigen-mediated receptor signaling.

CD45: a critical regulator of signaling thresholds in immune cells

Regulation of tyrosine phosphorylation is a critical control point for integration of environmental signals into cellular responses. This regulation is mediated by the reciprocal actions of protein tyrosine kinases and phosphatases. CD45, the first and prototypic receptor-like protein tyrosine phosphatase, is expressed on all nucleated hematopoietic cells and plays a central role in this process. Studies of CD45 mutant cell lines, CD45-deficient mice, and CD45-deficient humans initially demonstrated the essential role of CD45 in antigen receptor signal transduction and lymphocyte development. It is now known that CD45 also modulates signals emanating from integrin and cytokine receptors. Recent work has focused on regulation of CD45 expression and alternative splicing, isoform-specific differences in signal transduction, and regulation of phosphatase activity. From these studies, a model is emerging in which CD45 affects cellular responses by controlling the relative threshold of sensitivity to external stimuli. Perturbation of this function may contribute to autoimmunity, immunodeficiency, and malignancy. Moreover, recent advances suggest that modulation of CD45 function can have therapeutic benefit in many disease states.

Remembrance of antigens past: new insights into memory T cells

Memory immune responses against foreign antigens protect the host from pathogens previously encountered via illness or vaccination, yet can also contribute to the pathology of autoimmune disease when generated against self-antigens. Memory immune responses are classically attributed to the reactivation of long-lived, antigen-specific T lymphocytes that arise directly from differentiated effector T cells and persist in a uniformly quiescent state. Recent findings in both humans and mice, using new biochemical, molecular and cellular approaches, have identified novel features of memory T cells providing new insight into models for memory cell development and differentiation. Biochemical and molecular studies on memory T cells have identified novel markers for memory T cells that may play integral roles in their generation and maintenance. Recent cellular immunological studies have uncovered remarkable heterogeneity amongst antigen-specific memory T cells. Memory cell heterogeneity in the expression of homing and chemokine receptor delineates functional subsets of memory T cells that differ in their proliferative capacity, differentiation potential, homing properties and protective abilities. These findings suggest that memory T cells with diverse properties residing in both lymphoid and nonlymphoid tissues may be necessary to elicit a rapid and effective protective recall immune response involving both cellular and humoral immunity.

TCR signal transduction in antigen-specific memory CD8 T cells

Memory T cells are more responsive to Ag than naive cells. To determine whether memory T cells also have more efficient TCR signaling, we compared naive, effector, and memory CD8 T cells of the same antigenic specificity. Surprisingly, initial CD3 signaling events are indistinguishable. However, memory T cells have more extensive lipid rafts with higher phosphoprotein content before TCR engagement. Upon activation in vivo, they more efficiently induce phosphorylation of-LAT (linker for activation of T cells), ERK (extracellular signal-regulated kinase), JNK (c-Jun N-terminal kinase), and p38. Thus, memory CD8 T cells do not increase their TCR sensitivity, but are better poised to augment downstream signals. We propose that this regulatory mechanism might increase signal transduction in memory T cells, while limiting TCR cross-reactivity and autoimmunity.

The FcR gamma subunit and Syk kinase replace the CD3 zeta-chain and ZAP-70 kinase in the TCR signaling complex of human effector CD4 T cells

The TCR-mediated signals required to activate resting T cells have been well characterized; however, it is not known how TCR-coupled signals are transduced in differentiated effector T cells that coordinate ongoing immune responses. Here we demonstrate that human effector CD4 T cells up-regulate the expression of the CD3zeta-related FcRgamma signaling subunit that becomes part of an altered TCR/CD3 signaling complex containing CD3epsilon, but not CD3zeta. The TCR/CD3/FcRgamma complex in effector cells recruits and activates the Syk, but not the ZAP-70, tyrosine kinase. This physiologic switch in TCR signaling occurs exclusively in effector, and not naive or memory T cells, suggesting a potential target for manipulation of effector responses in autoimmune, malignant, and infectious diseases.

Staphylococcal enterotoxin B induces anergy to conventional peptide in memory T cells

Microbial superantigens can alter host immunity through aberrant activation and subsequent anergy of responding naive T cells. We show here that the superantigen, staphylococcal enterotoxin B (SEB), directly induces tolerance in memory CD4 T cells. Murine naive and memory CD4(+) T cells were labeled with the fluorescent dye CFSE and the cells were exposed to SEB before they were cultured with specific peptide antigen. Memory, but not naive, T cells became anergic and did not respond to their cognate peptide antigen. The extent and duration of T cell receptor (TCR) clustering was similar to promote naive T cell activation and memory T cell anergy, suggesting similar TCR-SEB interactions led to distinct intracellular signaling processes in the two cell types. Like SEB, soluble anti-CD3 mAb does not stimulate memory cell proliferation. However, unlike SEB, soluble anti-CD3 mAbs did not induce anergy to cognate peptide. Anergy was directly visualized in vivo. CD4(+) memory T cells were identified in mice that had been administered SEB. The cells failed to proliferate in response to subsequent immunization with their cognate recall antigen. Hence, one mode of pathogen survival is the modulation of host immunity through selective elimination of memory T cell responses.

Dysregulated lymphocyte proliferation and differentiation in patients with rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic, inflammatory disease of the synovium of uncertain pathogenesis. A number of phenotypic and functional T-cell defects have been described in RA, including abnormal clonal expansions and suppressed proliferative responses, which suggest a defect in T-cell differentiation. Here, we show that RA patients possess fewer naive CD4(+) T cells than healthy controls. Furthermore, a smaller proportion of these cells contains a T-cell receptor excision circle (TREC). Patients with RA also have unusual populations of T cells. These include immature cells characterized as CD45RB(bright)CD45RA(+)CD62L(-) by flow cytometry and a large population that coexpresses CD45RA and CD45RO. These cells are hyperresponsive to mitogen and TCR stimulation when compared to naive cells. Additionally, an unusual putative central memory subset expressing CD62L, but not CD45RA, appears in RA patients at the expense of more typical cells. Levels of C-reactive protein correlate inversely with the TREC content of naive T cells and positively with the sizes of naive and immature atypical T-cell subsets. These data suggest that inflammation drives proliferation of naive T cells in RA and encourages their differentiation into atypical, hyperresponsive progeny. TREC content of individual naive and atypical T-cell subsets suggests an ontogeny consistent with this hypothesis. These studies provide further evidence of a T-cell differentiation defect in RA, which could explain some of the well-characterized immunologic features of the disease.

Dissecting the complexity of the memory T cell response

Memory immune responses are classically attributed to the reactivation of long-lived, antigen-specific T lymphocytes that persist in a quiescent state. Determining mechanisms for the generation of memory T cells and dissecting the functional nature of the memory T cell pool has been encumbered by an inability to distinguish recently activated effector T cells from memory T cells. We have established new activation and biochemical criteria that distinguish effector and memory T cells and have applied these criteria to follow memory generation from activated cells in vivo. We found that the resultant memory T cell pool is heterogeneous and consists of effector-like and resting memory-like subsets that differ in expression of the homing receptor, CD62L. We discuss these findings in the context of memory T cell heterogeneity identified in human and mouse systems. These results suggest that more than one type of previously activated T cell can mediate recall or memory immune responses and that elucidating the fundamental phenotypic and functional features of memory T cell subsets is therefore critical to deciphering the complex nature of the memory immune response.

Effector CD4 cells are tolerized upon exposure to parenchymal self-antigen

It has long been established that exposure of naive T cells to specific Ag in the absence of adjuvant leads to tolerization. Nonetheless, the potential of effector CD4 cells to be tolerized has been less well characterized. To address this issue, we have used an adoptive transfer system in which naive TCR transgenic hemagglutinin (HA)-specific CD4(+) T cells are initially primed to express effector function upon exposure to an immunogenic recombinant vaccinia virus expressing HA, and then exposed to forms of HA that are tolerogenic for naive CD4 cells. HA-specific effector CD4 cells residing in both the spleen as well as in two separate nonlymphoid tissues were tolerized upon exposure to high doses of exogenous soluble HA peptide. Additionally, tolerance could also be induced by bone marrow-derived APCs that cross-present parenchymally derived self-HA. Thus, effector CD4 cells are susceptible to similar tolerogenic stimuli as are naive CD4 cells.

Genetic analysis of integrin activation in T lymphocytes

Among the myriad receptors expressed by T cells, the sine qua non is the CD3/T cell receptor (CD3/TCR) complex, because it is uniquely capable of translating the presence of a specific antigen into intracellular signals necessary to trigger an immune response against a pathogen or tumor. Much work over the past 2 decades has attempted to define the signaling pathways leading from the CD3/TCR complex that culminate ultimately in the functions necessary for effective T cell immune responses, such as cytokine production. Here, we summarize recent advances in our understanding of the mechanisms by which the CD3/TCR complex controls integrin-mediated T cell adhesion, and discuss new information that suggests that there may be unexpected facets to this pathway that distinguish it from those previously defined.