Ablation of SLP-76 signaling after T cell priming generates memory CD4 T cells impaired in steady-state and cytokine-driven homeostasis

Ibrutinib enhances the bias of T cell responses towards staphylococcal superantigens sustaining inflammation in chronic lymphocytic leukaemia

Chronic lymphocytic leukaemia (CLL) is an uncurable haematological malignancy and is associated with significant infection morbidity. Bruton's tyrosine-kinase inhibitors (e.g., ibrutinib) have improved disease outcomes, but severe infections and poor immunization responses afflict patients. Recently, carriage of the endemic Staphylococcus aureus (SA) was associated with lymphocytosis and decreased survival in CLL patients. We then hypothesized that exposure to staphylococcal superantigens (SAgs), known to promote hyper-inflammatory responses, impairs immunity and increases severe infection risk in CLL patients. Herein, we evaluate the reactivity of T cells and CLL cells to SA SAgs, in cultures derived from ibrutinib-treated and untreated CLL patients. We found that ibrutinib-treated patients had less naive CD8+ T cells (p=0.0348), more checkpoint receptor (TIM-3) expression in memory T cells (p<0.0001), and lower IFNγ/cytokine responses in patient T cells (p≤0.0298). Exposure to SA SAg further increased the accumulation of memory T cells with an exhaustion-phenotype, preferentially in cultures derived from ibrutinib-treated patients (p≤0.0350). Nevertheless, staphylococcal SAgs could not induce regulatory T cells from CLL patients inasmuch as healthy donors (p≤0.0461) and this was associated with accumulation of inflammatory T cells. Significantly, SAg-exposure enhanced inflammatory activation of CLL tumour cells, which acquired CD38, CD40, CD86, while downregulating CD27 (p≤0.005), even in cultures from ibrutinib-treated CLL patients. Thus, we suggest that environmental SAg-exposure promotes the accumulation of pseudo-exhausted T cells, which induce/sustain tumour cell activation, not counteracted by ibrutinib. Our study critically helps understand the chronic inflammatory milieu in CLL patients, with implications for infection morbidity, disease aetiology and future interventions.

A guide to adaptive immune memory

Immune memory - comprising T cells, B cells and plasma cells and their secreted antibodies - is crucial for human survival. It enables the rapid and effective clearance of a pathogen after re-exposure, to minimize damage to the host. When antigen-experienced, memory T cells become activated, they proliferate and produce effector molecules at faster rates and in greater magnitudes than antigen-inexperienced, naive cells. Similarly, memory B cells become activated and differentiate into antibody-secreting cells more rapidly than naive B cells, and they undergo processes that increase their affinity for antigen. The ability of T cells and B cells to form memory cells after antigen exposure is the rationale behind vaccination. Understanding immune memory not only is crucial for the design of more-efficacious vaccines but also has important implications for immunotherapies in infectious disease and cancer. This 'guide to' article provides an overview of the current understanding of the phenotype, function, location, and pathways for the generation, maintenance and protective capacity of memory T cells and memory B cells.

T-cell receptor signal strength and epigenetic control of Bim predict memory CD8+ T-cell fate

Most effector CD8+ T cells die, while some persist and become either "effector" (TEM) or "central" (TCM) memory T cells. Paradoxically, effector CD8+ T cells with greater memory potential have higher levels of the pro-apoptotic molecule Bim. Here, we report, using a novel Bim-mCherry knock-in mouse, that cells with high levels of Bim preferentially develop into TCM cells. Bim levels remained stable and were regulated by DNA methylation at the Bim promoter. Notably, high levels of Bcl-2 were required for Bimhi cells to survive. Using Nur77-GFP mice as an indicator of TCR signal strength, Nur77 levels correlated with Bim expression and Nur77hi cells also selectively developed into TCM cells. Altogether, these data show that Bim levels and TCR signal strength are predictive of TEM- vs. TCM-cell fate. Further, given the many other biologic functions of Bim, these mice will have broad utility beyond CD8+ T-cell fate.

Simulation of Stimulation: Cytokine Dosage and Cell Cycle Crosstalk Driving Timing-Dependent T Cell Differentiation

Triggering an appropriate protective response against invading agents is crucial to the effectiveness of human innate and adaptive immunity. Pathogen recognition and elimination requires integration of a myriad of signals from many different immune cells. For example, T cell functioning is not qualitatively, but quantitatively determined by cellular and humoral signals. Tipping the balance of signals, such that one of these is favored or gains advantage on another one, may impact the plasticity of T cells. This may lead to switching their phenotypes and, ultimately, modulating the balance between proliferating and memory T cells to sustain an appropriate immune response. We hypothesize that, similar to other intracellular processes such as the cell cycle, the process of T cell differentiation is the result of: (i) pleiotropy (pattern) and (ii) magnitude (dosage/concentration) of input signals, as well as (iii) their timing and duration. That is, a flexible, yet robust immune response upon recognition of the pathogen may result from the integration of signals at the right dosage and timing. To investigate and understand how system's properties such as T cell plasticity and T cell-mediated robust response arise from the interplay between these signals, the use of experimental toolboxes that modulate immune proteins may be explored. Currently available methodologies to engineer T cells and a recently devised strategy to measure protein dosage may be employed to precisely determine, for example, the expression of transcription factors responsible for T cell differentiation into various subtypes. Thus, the immune response may be systematically investigated quantitatively. Here, we provide a perspective of how pattern, dosage and timing of specific signals, called interleukins, may influence T cell activation and differentiation during the course of the immune response. We further propose that interleukins alone cannot explain the phenotype variability observed in T cells. Specifically, we provide evidence that the dosage of intercellular components of both the immune system and the cell cycle regulating cell proliferation may contribute to T cell activation, differentiation, as well as T cell memory formation and maintenance. Altogether, we envision that a qualitative (pattern) and quantitative (dosage) crosstalk between the extracellular milieu and intracellular proteins leads to T cell plasticity and robustness. The understanding of this complex interplay is crucial to predict and prevent scenarios where tipping the balance of signals may be compromised, such as in autoimmunity.

Maintenance of pathogenic Th2 cells in allergic disorders

Immunological memory is an important protective mechanism that enables host organisms to respond rapidly and vigorously to pathogens that have been previously encountered. In addition to the protective function, memory CD4⁺ T helper (Th) cells play a central role in the pathogenesis of chronic inflammatory disorders, including asthma. Recently, several investigators have identified phenotypically and functionally distinct memory Th2 cell subsets that produce IL-5. These memory Th2 cell subsets play an important role in the pathology of allergic inflammation and function as memory-type "pathogenic Th2 (Tpath2) cells" both in mice and humans. We review the role of lung Tpath2 cells in the development of allergic inflammation and, in the context of recent findings, propose a mechanism by which Tpath2 cells not only survive but also continue to function at the sites where antigens were encountered. A greater understanding of the functional molecules or signaling pathways that regulate the inflammatory niche for Tpath2 cells may aid in the design of more effective treatments for chronic inflammatory disorders.

Chromatin priming elements establish immunological memory in T cells without activating transcription: T cell memory is maintained by DNA elements which stably prime inducible genes without activating steady state transcription

We have identified a simple epigenetic mechanism underlying the establishment and maintenance of immunological memory in T cells. By studying the transcriptional regulation of inducible genes we found that a single cycle of activation of inducible factors is sufficient to initiate stable binding of pre-existing transcription factors to thousands of newly activated distal regulatory elements within inducible genes. These events lead to the creation of islands of active chromatin encompassing nearby enhancers, thereby supporting the accelerated activation of inducible genes, without changing steady state levels of transcription in memory T cells. These studies also highlighted the need for more sophisticated definitions of gene regulatory elements. The chromatin priming elements defined here are distinct from classical enhancers because they function by maintaining chromatin accessibility rather than directly activating transcription. We propose that these priming elements are members of a wider class of genomic elements that support correct developmentally regulated gene expression.

Th2 Cells in Health and Disease

Helper T (Th) cell subsets direct immune responses by producing signature cytokines. Th2 cells produce IL-4, IL-5, and IL-13, which are important in humoral immunity and protection from helminth infection and are central to the pathogenesis of many allergic inflammatory diseases. Molecular analysis of Th2 cell differentiation and maintenance of function has led to recent discoveries that have refined our understanding of Th2 cell biology. Epigenetic regulation of Gata3 expression by chromatin remodeling complexes such as Polycomb and Trithorax is crucial for maintaining Th2 cell identity. In the context of allergic diseases, memory-type pathogenic Th2 cells have been identified in both mice and humans. To better understand these disease-driving cell populations, we have developed a model called the pathogenic Th population disease induction model. The concept of defined subsets of pathogenic Th cells may spur new, effective strategies for treating intractable chronic inflammatory disorders.

Homeostatic cytokines in immune reconstitution and graft-versus-host disease

For numerous patients, allogeneic stem cell transplantation (SCT) is the only therapeutic option that could potentially cure their disease. Despite significant progress made in clinical management of allogeneic SCT, acute graft-versus-host disease (aGVHD) remains the second cause of death after disease recurrence. aGVHD is highly immunosuppressive and the adverse effect of allogeneic SCT on T cell regeneration is typically more important than the levels of immunosuppression normally seen after autologous SCT. In these patients, immune reconstitution often takes several years to occur and restoring immunocompetence after allogeneic SCT represents an important challenge, principally because clinical options are limited and current methods used to accelerate immune reconstitution are associated with increased GVHD. Interleukin-7 and IL-15 are both under clinical investigation and demonstrate the greatest potential on peripheral T cells regeneration in mice and humans. However, awareness has been raised about the use of IL-7 and IL-15 after allogeneic SCT with regards to potential adverse effects on aGVHD. In this review, we will discuss about recent progress made in lymphocyte regeneration, the critical role played by IL-7 and IL-15 in T cell homeostasis and how these cytokines could be used to improve immune reconstitution after allogeneic SCT.

TCR Signaling in T Cell Memory

T cell memory plays a critical role in our protection against pathogens and tumors. The antigen and its interaction with the T cell receptor (TCR) is one of the initiating elements that shape T cell memory together with inflammation and costimulation. Over the last decade, several transcription factors and signaling pathways that support memory programing have been identified. However, how TCR signals regulate them is still poorly understood. Recent studies have shown that the biochemical rules that govern T cell memory, strikingly, change depending on the TCR signal strength. Furthermore, TCR signal strength regulates the input of cytokine signaling, including pro-inflammatory cytokines. These highlight how tailoring antigenic signals can improve immune therapeutics. In this review, we focus on how TCR signaling regulates T cell memory and how the quantity and quality of TCR–peptide–MHC interactions impact the multiple fates a T cell can adopt in the memory pool.

Regulatory T cells require TCR signaling for their suppressive function

Regulatory T cells (Tregs) are a subset of CD4(+) T cells that maintain immune tolerance in part by their ability to inhibit the proliferation of conventional CD4(+) T cells (Tconvs). The role of the TCR and the downstream signaling pathways required for this suppressive function of Tregs are not fully understood. To yield insight into how TCR-mediated signals influence Treg suppressive function, we assessed the ability of Tregs with altered TCR-mediated signaling capacity to inhibit Tconv proliferation. Mature Tregs deficient in Src homology 2 domain containing leukocyte protein of 76 kDa (SLP-76), an adaptor protein that nucleates the proximal signaling complex downstream of the TCR, were unable to inhibit Tconv proliferation, suggesting that TCR signaling is required for Treg suppressive function. Moreover, Tregs with defective phospholipase C γ (PLCγ) activation due to a Y145F mutation of SLP-76 were also defective in their suppressive function. Conversely, enhancement of diacylglycerol-mediated signaling downstream of PLCγ by genetic ablation of a negative regulator of diacylglycerol kinase ζ increased the suppressive ability of Tregs. Because SLP-76 is also important for integrin activation and signaling, we tested the role of integrin activation in Treg-mediated suppression. Tregs lacking the adaptor proteins adhesion and degranulation promoting adapter protein or CT10 regulator of kinase/CT10 regulator of kinase-like, which are required for TCR-mediated integrin activation, inhibited Tconv proliferation to a similar extent as wild-type Tregs. Together, these data suggest that TCR-mediated PLCγ activation, but not integrin activation, is required for Tregs to inhibit Tconv proliferation.

CD4 T-cell memory generation and maintenance

Immunologic memory is the adaptive immune system's powerful ability to remember a previous antigen encounter and react with accelerated vigor upon antigen re-exposure. It provides durable protection against reinfection with pathogens and is the foundation for vaccine-induced immunity. Unlike the relatively restricted immunologic purview of memory B cells and CD8 T cells, the field of CD4 T-cell memory must account for multiple distinct lineages with diverse effector functions, the issue of lineage commitment and plasticity, and the variable distribution of memory cells within each lineage. Here, we discuss the evidence for lineage-specific CD4 T-cell memory and summarize the known factors contributing to memory-cell generation, plasticity, and long-term maintenance.

Memory CD4 T cells in influenza

Influenza A virus is a significant cause of morbidity and mortality worldwide, particularly among young children and the elderly. Current vaccines induce neutralizing antibody responses directed toward highly variable viral surface proteins, resulting in limited heterosubtypic protection to new viral serotypes. By contrast, memory CD4 T cells recognize conserved viral proteins and are cross-reactive to multiple influenza strains. In humans, virus-specific memory CD4 T cells were found to be the protective correlate in human influenza challenge studies, suggesting their key role in protective immunity. In mouse models, memory CD4 T cells can mediate protective responses to secondary influenza infection independent of B cells or CD8 T cells, and can influence innate immune responses. Importantly, a newly defined, tissue-resident CD4 memory population has been demonstrated to be retained in lung tissue and promote optimal protective responses to an influenza infection. Here, we review the current state of results regarding the generation of memory CD4 T cells following primary influenza infection, mechanisms for their enhanced efficacy in protection from secondary challenge including their phenotype, localization, and function in the context of both mouse models and human infection. We also discuss the generation of memory CD4 T cells in response to influenza vaccines and its future implications for vaccinology.

Human memory T cells: generation, compartmentalization and homeostasis

Memory T cells constitute the most abundant lymphocyte population in the body for the majority of a person's lifetime; however, our understanding of memory T cell generation, function and maintenance mainly derives from mouse studies, which cannot recapitulate the exposure to multiple pathogens that occurs over many decades in humans. In this Review, we discuss studies focused on human memory T cells that reveal key properties of these cells, including subset heterogeneity and diverse tissue residence in multiple mucosal and lymphoid tissue sites. We also review how the function and the adaptability of human memory T cells depend on spatial and temporal compartmentalization.

Homeostatic division is not necessary for antigen-specific CD4+ memory T cell persistence

CD4(+) memory T cells are generated in response to infection or vaccination, provide protection to the host against reinfection, and persist through a combination of enhanced survival and slow homeostatic turnover. We used timed deletion of the TCR-signaling adaptor molecule Src homology 2 domain-containing phosphoprotein of 76 kDa (SLP-76) with MHC:peptide tetramers to study the requirements for tonic TCR signals in the maintenance of polyclonal Ag-specific CD4(+) memory T cells. SLP-76-deficient I-A(b):gp61 cells are unable to rapidly generate effector cytokines or proliferate in response to secondary infection. In mice infected with lymphocytic choriomeningitis virus (LCMV) or Listeria monocytogenes expressing the LCMV gp61-80 peptide, SLP-76-deficient I-A(b):gp61(+) cells exhibit reduced division, similar to that seen in in vitro-generated CD44(hi) and endogenous CD4(+)CD44(hi) cells. Competitive bone marrow chimera experiments demonstrated that the decrease in homeostatic turnover in the absence of SLP-76 is a cell-intrinsic process. Surprisingly, despite the reduction in turnover, I-A(b):gp61(+) Ag-specific memory cells persist in normal numbers for >30 wk after LCMV infection in the absence of SLP-76. These data suggest the independent maintenance of a population of Ag-specific CD4(+) memory T cells in the absence of SLP-76 and normal levels of homeostatic division.

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.

TCR signaling requirements for activating T cells and for generating memory

Over the last two decades the molecular and cellular mechanisms underlying T cell activation, expansion, differentiation, and memory formation have been intensively investigated. These studies revealed that the generation of memory T cells is critically impacted by a number of factors, including the magnitude of the inflammatory response and cytokine production, the type of dendritic cell [DC] that presents the pathogen derived antigen, their maturation status, and the concomitant provision of costimulation. Nevertheless, the primary stimulus leading to T cell activation is generated through the T cell receptor [TCR] following its engagement with a peptide MHC ligand [pMHC]. The purpose of this review is to highlight classical and recent findings on how antigen recognition, the degree of TCR stimulation, and intracellular signal transduction pathways impact the formation of effector and memory T cells.

Conditional deletion of SLP-76 in mature T cells abrogates peripheral immune responses

The adaptor protein Src homology 2 domain-containing leukocyte-specific protein of 76 kDa (SLP-76) is central to the organization of intracellular signaling downstream of the T-cell receptor (TCR). Evaluation of its role in mature, primary T cells has been hampered by developmental defects that occur in the absence of WT SLP-76 protein in thymocytes. Here, we show that following tamoxifen-regulated conditional deletion of SLP-76, mature, antigen-inexperienced T cells maintain normal TCR surface expression but fail to transduce TCR-generated signals. Conditionally deficient T cells fail to proliferate in response to antigenic stimulation or a lymphopenic environment. Mice with induced deletion of SLP-76 are resistant to induction of the CD4+ T-cell-mediated autoimmune disease experimental autoimmune encephalomyelitis. Altogether, our findings demonstrate the critical role of SLP-76-mediated signaling in initiating T-cell-directed immune responses both in vitro and in vivo and highlight the ability to analyze signaling processes in mature T cells in the absence of developmental defects.

Transcriptional control of rapid recall by memory CD4 T cells

Memory T cells are distinguished from naive T cells by their rapid production of effector cytokines, although mechanisms for this recall response remain undefined. In this study, we investigated transcriptional mechanisms for rapid IFN-γ production by Ag-specific memory CD4 T cells. In naive CD4 T cells, IFN-γ production only occurred after sustained Ag activation and was associated with high expression of the T-bet transcription factor required for Th1 differentiation and with T-bet binding to the IFN-γ promoter as assessed by chromatin immunoprecipitation analysis. By contrast, immediate IFN-γ production by Ag-stimulated memory CD4 T cells occurred in the absence of significant nuclear T-bet expression or T-bet engagement on the IFN-γ promoter. We identified rapid induction of NF-κB transcriptional activity and increased engagement of NF-κB on the IFN-γ promoter at rapid times after TCR stimulation of memory compared with naive CD4 T cells. Moreover, pharmacologic inhibition of NF-κB activity or peptide-mediated inhibition of NF-κB p50 translocation abrogated early memory T cell signaling and TCR-mediated effector function. Our results reveal a molecular mechanism for memory T cell recall through enhanced NF-κB p50 activation and promoter engagement, with important implications for memory T cell modulation in vaccines, autoimmunity, and transplantation.

CD4+ memory T cell survival

Memory CD4+ T cells specific for a given antigen are generated during the primary response from the effector lymphoblast progeny of naïve precursors. How memory CD4+ T cells differentiate from the effector population is not understood but new tools to assess transcription factor and cytokine expression are allowing for a more careful assessment of this process. Here we review the factors that allow some effector CD4+ T cells to survive the contraction phase of the primary response and become memory cells, and consider whether parallels can be drawn between T and B cells.

Maintenance of CD4+ T-cell memory and HIV persistence: keeping memory, keeping HIV

PURPOSE OF REVIEW

The present review summarizes the current challenges for the design of new therapeutic strategies toward HIV eradication in individuals receiving suppressive highly active antiretroviral therapy (HAART). We will focus on the experimental evidence suggesting that immunological mechanisms involved in the generation and maintenance of memory CD4+ T cells are also responsible for the establishment and persistence of a stable reservoir for HIV.

RECENT FINDINGS

Recent studies performed on clinical samples obtained from virally suppressed HIV-infected individuals indicate that T-cell survival and homeostatic proliferation, two major mechanisms involved in the maintenance of immunological memory, contribute to the persistence of latently infected memory CD4+ T cells. Thus, the long lifespan characteristic of the HIV reservoir is likely a consequence of the capacity of the immune system to generate and maintain memory CD4+ T cells for a long period.

SUMMARY

These findings suggest that strategies aimed at reducing the pool of latently infected cells should interfere with the survival pathways responsible for the long-term maintenance of memory CD4+ T cells. Because memory CD4+ T cells are critical for appropriate immune defense, targeted approaches are needed to interfere only with the long-term survival of discrete fractions of memory T cells carrying proviral DNA.

The memory phase of the CD4 T-cell response to influenza virus infection maintains its diverse antigen specificity

A major gap in our understanding of the immune response to pathogens and vaccines is how closely the antigen specificity in the memory phase mimics repertoire that is rapidly expanded upon priming. Understanding the diversity of the CD4 T-cell memory compartment after a primary response to pathogens is hampered by the technical challenges of epitope discovery and suitable models to study primary immune responses. Recently, we have used overlapping synthetic peptides to empirically map most of the specificities present in the primary response to live influenza infection. We found that the CD4 T-cell response can be exceptionally diverse, depending on the allele(s) of MHC class II molecules expressed. In the current study, using a mouse model of primary influenza infection and peptide-specific cytokine EliSpots, we have asked how this broad CD4 T-cell immunodominance hierarchy changes as the immune response contracts and memory is established. Our studies revealed that, for the most part, diversity is maintained, and most specificities, including those for relatively minor epitopes, are preserved in the memory CD4 T-cell compartment. A modest, but reproducible shift in specificity toward haemagglutinin-derived epitopes was observed, raising the possibility that protein or peptide persistence might play a role in the evolution of the memory phase of the CD4 T-cell response.

Loss of tonic T-cell receptor signals alters the generation but not the persistence of CD8+ memory T cells

The requirements for tonic T-cell receptor (TCR) signaling in CD8(+) memory T-cell generation and homeostasis are poorly defined. The SRC homology 2 (SH2)-domain-containing leukocyte protein of 76 kDa (SLP-76) is critical for proximal TCR-generated signaling. We used temporally mediated deletion of SLP-76 to interrupt tonic and activating TCR signals after clearance of the lymphocytic choriomeningitis virus (LCMV). SLP-76-dependent signals are required during the contraction phase of the immune response for the normal generation of CD8 memory precursor cells. Conversely, LCMV-specific memory CD8 T cells generated in the presence of SLP-76 and then acutely deprived of TCR-mediated signals persist in vivo in normal numbers for more than 40 weeks. Tonic TCR signals are not required for the transition of the memory pool toward a central memory phenotype, but the absence of SLP-76 during memory homeostasis substantially alters the kinetics. Our data are consistent with a model in which tonic TCR signals are required at multiple stages of differentiation, but are dispensable for memory CD8 T-cell persistence.

T-cell receptor signals direct the composition and function of the memory CD8+ T-cell pool

SH2 domain-containing leukocyte phosphoprotein of 76 kDa (SLP-76) nucleates a signaling complex critical for T-cell receptor (TCR) signal propagation. Mutations in the tyrosines of SLP-76 result in graded defects in TCR-induced signals depending on the tyrosine(s) affected. Here we use 2 strains of genomic knock-in mice expressing tyrosine to phenylalanine mutations to examine the role of TCR signals in the differentiation of effector and memory CD8(+) T cells in response to infection in vivo. Our data support a model in which altered TCR signals can determine the rate of memory versus effector cell differentiation independent of initial T-cell expansion. Furthermore, we show that TCR signals sufficient to promote CD8(+) T-cell differentiation are different from those required to elicit inflammatory cytokine production.

Generation, persistence and plasticity of CD4 T-cell memories

The development of immune memory mediated by T lymphocytes is central to durable, long-lasting protective immunity. A key issue in the field is how to direct the generation and persistence of memory T cells to elicit the appropriate secondary response to provide protection to a specific pathogen. Two prevailing views have emerged; that cellular and molecular regulators control the lineage fate and functional capacities of memory T cells early after priming, or alternatively, that populations of memory T cells are inherently plastic and subject to alterations in function and/or survival at many stages during their long-term maintenance. Here, we will review current findings in CD4 T-cell memory that suggest inherent plasticity in populations of memory CD4 T cells at all stages of their development--originating with their generation from multiple types of primed CD4 T cells, during their persistence and homeostatic turnover in response to T-cell receptor signals, and also following secondary challenge. These multiple aspects of memory CD4 T-cell flexibility contrast the more defined lineages and functions ascribed to memory CD8 T cells, suggesting a dynamic nature to memory CD4 T-cell populations and responses. The flexible attributes of CD4 T-cell memory suggest opportunities and mechanisms for therapeutic manipulation at all phases of immune memory development, maintenance and recall.