A bifurcated role for c-Maf in Th2 and Tfh2 cells during helminth infection.

Differences in transcriptomes, transcription factor usage, and function have identified T follicular helper 2 (Tfh2) cells and T helper 2 (Th2) cells as distinct clusters of differentiation 4+",(CD4) T-cell subsets in settings of type-2 inflammation. Although the transcriptional programs driving Th2 cell differentiation and cytokine production are well defined, dependence on these classical Th2 programs by Tfh2 cells is less clear. Using cytokine reporter mice in combination with transcription factor inference analysis, the b-Zip transcription factor c-Maf and its targets were identified as an important regulon in both Th2 and Tfh2 cells. Conditional deletion of c-Maf in T cells confirmed its importance in type-2 cytokine expression by Th2 and Tfh2 cells. However, while c-Maf was not required for Th2-driven helminth clearance or lung eosinophilia, it was required for Tfh2-driven Immunoglobulin E production and germinal center formation. This differential regulation of cell-mediated and humoral immunity by c-Maf was a result of redundant pathways in Th2 cells that were absent in Tfh2 cells, and c-Maf-specific mechanisms in Tfh2 cells that were absent in Th2 cells. Thus, despite shared expression by Tfh2 and Th2 cells, c-Maf serves as a unique regulator of Tfh2-driven humoral hallmarks during type-2 immunity.

Partial STAT5 signaling is sufficient for CD4+ T cell priming but not memory formation.

Signal transducer and activator of transcription 5 (STAT5) plays an important role in regulating gene expression in response to cytokines of the common (γc) chain family. In this capacity, STAT5 promotes CD8+ effector and memory T cell survival and regulatory T cell development. However, its function in conventional CD4+ T cells is less clear. In this study, the requirement of intact STAT5 signaling for CD4+ effector and memory T cell generation and maintenance was investigated by using DO11.10 TCR transgenic T cells that are genetically deficient in STAT5A or B, as well as by transducing DO11 T cells with a dominant-negative STAT5 to temporally block STAT5 function. We found that the presence of STAT5A or B alone was sufficient for primary CD4+ effector T cell generation, but not for establishing a long-lived memory cell population. Similarly, blocking STAT5 signaling during priming did not prevent initial T cell activation, but inhibited the generation of memory cells. Surprisingly, blocking STAT5 post-priming did not impact the long-term survival of CD4+ memory T cells in vivo. Mechanistically, intact STAT5B, but not STAT5A, was required for IL-7Rα re-expression in activated T cells, which is an important cytokine receptor for CD4+ memory generation. These data show that fully functional STAT5 is essential to deliver an early, non-redundant signal for memory programming during the primary CD4+ T cell response, while partial STAT5 signaling is sufficient for effector differentiation. Our results have implications for the precise use of STAT5 inhibitors to timely inhibit memory T cell responses.

Emerging Therapeutic Strategies to Restore Regulatory T Cell Control of Islet Autoimmunity in Type 1 Diabetes.

Despite many decades of investigation uncovering the autoimmune mechanisms underlying Type 1 Diabetes (T1D), translating these findings into effective therapeutics has proven extremely challenging. T1D is caused by autoreactive T cells that become inappropriately activated and kill the ß cells in the pancreas, resulting in insulin insufficiency and hyperglycemia. A large body of evidence supports the idea that the unchecked activation and expansion of autoreactive T cells in T1D is due to defects in immunosuppressive regulatory T cells (Tregs) that are critical for maintaining peripheral tolerance to islet autoantigens. Hence, repairing these Treg deficiencies is a much sought-after strategy to treat the disease. To accomplish this goal in the most precise, effective and safest way possible, restored Treg functions will need to be targeted towards suppressing the autoantigen-specific immune responses only and/or be localized in the pancreas. Here we review the most recent developments in designing Treg therapies that go beyond broad activation or expansion of non-specific polyclonal Treg populations. We focus on two cutting-edge strategies namely ex vivo generation of optimized Tregs for re-introduction in T1D patients vs direct in situ stimulation and restoration of endogenous Treg function.

Pneumonia recovery reprograms the alveolar macrophage pool.

Community-acquired pneumonia is a widespread disease with significant morbidity and mortality. Alveolar macrophages are tissue-resident lung cells that play a crucial role in innate immunity against bacteria that cause pneumonia. We hypothesized that alveolar macrophages display adaptive characteristics after resolution of bacterial pneumonia. We studied mice 1 to 6 months after self-limiting lung infections with Streptococcus pneumoniae, the most common cause of bacterial pneumonia. Alveolar macrophages, but not other myeloid cells, recovered from the lung showed long-term modifications of their surface marker phenotype. The remodeling of alveolar macrophages was (a) long-lasting (still observed 6 months after infection), (b) regionally localized (observed only in the affected lobe after lobar pneumonia), and (c) associated with macrophage-dependent enhanced protection against another pneumococcal serotype. Metabolomic and transcriptomic profiling revealed that alveolar macrophages of mice that recovered from pneumonia had new baseline activities and altered responses to infection that better resembled those of adult humans. The enhanced lung protection after mild and self-limiting bacterial respiratory infections includes a profound remodeling of the alveolar macrophage pool that is long-lasting; compartmentalized; and manifest across surface receptors, metabolites, and both resting and stimulated transcriptomes.

Combining anti-IL-7Rα antibodies with autoantigen-specific immunotherapy enhances non-specific cytokine production but fails to prevent Type 1 Diabetes.

Autoantigen-specific methods to prevent and treat Type 1 Diabetes (T1D) carry high hopes to permanently cure this disease, but have largely failed in clinical trials. One suggested approach to increase the efficacy of islet antigen-specific vaccination is to combine it with a modulator of the T cell response, with the goal of reducing effector differentiation and promoting regulatory T cells (Tregs). Here we asked if addition of antibodies that block the IL-7/IL-7Rα pathway altered the T cell response to islet antigen vaccination and prevented T1D in non-obese diabetic (NOD) mice. Anti-IL-7Rα monoclonal antibodies (mAbs) reduced the numbers of islet antigen-specific T cells generated after vaccination with islet peptides and alum. However, addition of anti-IL-7Rα antibodies to peptide/alum vaccination unexpectedly increased non-specific IFN-γ, IL-2 and IL-10 cytokine production and did not result in improved prevention of T1D onset. In a second approach, we used a conjugate vaccine to deliver islet autoantigens, using Keyhole Limpet Hemocyanin (KLH) as a carrier. Islet antigen-KLH vaccination led to a significant expansion of antigen-specific Tregs and delayed diabetes onset in NOD mice. These outcomes were not further improved by addition of anti-IL-7Rα antibodies. To the contrary, blocking IL-7Rα during vaccination led to non-specific cytokine production and reduced the efficacy of a KLH-conjugated vaccine to prevent T1D. Our study thus revealed that adding anti-IL-7Rα antibodies during autoantigen immunization did not improve the efficacy of such vaccinations to prevent T1D, despite altering some aspects of the T cell response in a potentially advantageous way. Further refinement of this approach will be required to separate the beneficial from the adverse effects of anti-IL-7Rα antibodies to treat autoimmune disease.

Increased Expression and Modulated Regulatory Activity of Coinhibitory Receptors PD-1, TIGIT, and TIM-3 in Lymphocytes From Patients With Systemic Sclerosis.

OBJECTIVE: Immune dysfunction is an important component of the disease process underlying systemic sclerosis (SSc), but the mechanisms contributing to altered immune cell function in SSc remain poorly defined. This study was undertaken to measure the expression and function of the coinhibitory receptors (co-IRs) programmed cell death 1 (PD-1), T cell immunoglobulin and ITIM domain (TIGIT), T cell immunoglobulin and mucin domain 3 (TIM-3), and lymphocyte activation gene 3 (LAG-3) in lymphocyte subsets from the peripheral blood of patients with SSc. METHODS: Co-IR expression levels on subsets of immune cells were analyzed using a 16-color flow cytometry panel. The functional role of co-IRs was determined by measuring cytokine production after in vitro stimulation of SSc and healthy control peripheral blood mononuclear cells (PBMCs) in the presence of co-IR-blocking antibodies. Supernatants from cultures of stimulated PBMCs were added to SSc fibroblasts, and their impact on fibroblast gene expression was measured. Mathematical modeling was used to reveal differences between co-IR functions in SSc patients and healthy controls. RESULTS: Levels of the co-IRs PD-1 and TIGIT were increased, and each was coexpressed, in distinct T cell subsets from SSc patients compared to healthy controls. Levels of TIM-3 were increased in SSc natural killer cells. PD-1, TIGIT, and TIM-3 antibody blockade revealed patient-specific roles of each of these co-IRs in modulating activation-induced T cell cytokine production. In contrast to healthy subjects, blockade of TIGIT and TIM-3, but not PD-1, failed to reverse inhibited cytokine production in SSc patients, indicating that enhanced T cell exhaustion is present in SSc. Finally, cytokines secreted in anti-TIM-3-treated PBMC cultures distinctly changed the gene expression profile in SSc fibroblasts. CONCLUSION: The altered expression and regulatory capacity of co-IRs in SSc lymphocytes may contribute to disease pathophysiology by modulating the cytokine-mediated cross-talk of immune cells and fibroblasts at sites of inflammation and/or fibrosis.

Type 1 diabetes alters lipid handling and metabolism in human fibroblasts and peripheral blood mononuclear cells.

Triggers of the autoimmune response that leads to type 1 diabetes (T1D) remain poorly understood. A possibility is that parallel changes in both T cells and target cells provoke autoimmune attack. We previously documented greater Ca2+ transients in fibroblasts from T1D subjects than non-T1D after exposure to fatty acids (FA) and tumor necrosis factor α (TNFα). These data indicate that metabolic and signal transduction defects present in T1D can be elicited ex vivo in isolated cells. Changes that precede T1D, including inflammation, may activate atypical responses in people that are genetically predisposed to T1D. To identify such cellular differences in T1D, we quantified a panel of metabolic responses in fibroblasts and peripheral blood cells (PBMCs) from age-matched T1D and non-T1D subjects, as models for non-immune and immune cells, respectively. Fibroblasts from T1D subjects accumulated more lipid, had higher LC-CoA levels and converted more FA to CO2, with less mitochondrial proton leak in response to oleate alone or with TNFα, using the latter as a model of inflammation. T1D-PBMCs contained and also accumulated more lipid following FA exposure. In addition, they formed more peroxidized lipid than controls following FA exposure. We conclude that both immune and non-immune cells in T1D subjects differ from controls in terms of responses to FA and TNFα. Our results suggest a differential sensitivity to inflammatory insults and FA that may precede and contribute to T1D by priming both immune cells and their targets for autoimmune reactions.

Broad induction of immunoregulatory mechanisms after a short course of anti-IL-7Rα antibodies in NOD mice.

BACKGROUND: Type 1 diabetes is an autoimmune disease caused by T cell-mediated destruction of the insulin-producing ß-cells in the pancreas. Therefore, approaches that effectively halt the pathogenic T cell response are predicted to have preventive or therapeutic benefit for type 1 diabetes patients. We previously demonstrated that long-term blocking of IL-7 signaling, which is critical for the survival and function of T cells, prevented and reversed type 1 diabetes in non-obese diabetic mice. However, such persistent inhibition of T cell responses raises concerns about causing immunodeficiency. Here, we asked whether a reduced duration of the treatment with anti-IL-7Rα antibodies retained efficacy in preventing diabetes. Moreover, we sought to identify immunoregulatory mechanisms induced by anti-IL-7Rα administration. RESULTS: Anti-IL-7Rα antibodies were administered to prediabetic NOD mice for 3 weeks and blood samples were taken at the end of treatment and 2 weeks later to analyze changes in T cell phenotypes in response to IL-7Rα blockade. We found that the co-inhibitory receptors LAG-3, Tim-3 and PD-1 were increased on peripheral blood CD4+ and CD8+ T cells from anti-IL-7Rα-treated mice. Expression of these receptors contributed to reduced T cell cytokine production in response to TCR stimulation. In addition, the frequency of Tregs within the circulating CD4+ T cells was increased at the end of anti-IL-7Rα antibody treatment and these Tregs showed a more activated phenotype. In vitro restimulation assays revealed that effector T cells from anti-IL-7Rα-treated mice were more sensitive to co-inhibitory receptor induction after TCR stimulation. Importantly, these changes were accompanied by delayed type 1 diabetes disease kinetics. CONCLUSIONS: Together, our data show that short-term blockade of IL-7Rα induces detectable changes in co-inhibitory receptor expression and Treg frequencies in peripheral blood of NOD mice. These changes appear to have long-lasting effects by delaying or preventing type 1 diabetes incidence. Hence, our study provides further support for using anti-IL-7Rα antibodies to modulate autoreactive T cell responses.

Advances in the quantification of mitochondrial function in primary human immune cells through extracellular flux analysis.

Numerous studies show that mitochondrial energy generation determines the effectiveness of immune responses. Furthermore, changes in mitochondrial function may regulate lymphocyte function in inflammatory diseases like type 2 diabetes. Analysis of lymphocyte mitochondrial function has been facilitated by introduction of 96-well format extracellular flux (XF96) analyzers, but the technology remains imperfect for analysis of human lymphocytes. Limitations in XF technology include the lack of practical protocols for analysis of archived human cells, and inadequate data analysis tools that require manual quality checks. Current analysis tools for XF outcomes are also unable to automatically assess data quality and delete untenable data from the relatively high number of biological replicates needed to power complex human cell studies. The objectives of work presented herein are to test the impact of common cellular manipulations on XF outcomes, and to develop and validate a new automated tool that objectively analyzes a virtually unlimited number of samples to quantitate mitochondrial function in immune cells. We present significant improvements on previous XF analyses of primary human cells that will be absolutely essential to test the prediction that changes in immune cell mitochondrial function and fuel sources support immune dysfunction in chronic inflammatory diseases like type 2 diabetes.

Interleukin-7: Fuel for the autoimmune attack.

Interleukin-7 (IL-7) is a critical survival factor for lymphocytes and recent studies suggest targeting the IL-7/IL-7Rα pathway holds promise for the treatment of autoimmune diseases. Several lines of evidence, genetic as well as functional, indicate an important role for this cytokine in autoimmune inflammation: polymorphisms in the IL-7Rα have been associated with increased risk for autoimmune disease and blocking IL-7/IL-7Rα with antibodies showed therapeutic efficacy in several autoimmune mouse models. Insights are starting to emerge about the mechanisms underlying IL-7's role in autoimmunity and tolerance, revealing surprising novel functions beyond its traditional activity as a T cell survival factor. In the first part of this review, the functions of IL-7 in the immune system are concisely described, providing a basis for understanding their potential role in promoting autoimmune responses. In the second part, current knowledge about the role of IL-7 in various autoimmune conditions is reviewed.

A converse 4-1BB and CD40 ligand expression pattern delineates activated regulatory T cells (Treg) and conventional T cells enabling direct isolation of alloantigen-reactive natural Foxp3+ Treg.

Natural regulatory T cells (nTreg) play a central role in the induction and maintenance of immunological tolerance. Experimental transplant models and recent clinical trials demonstrate that nTreg can control alloreactivity. To upgrade Treg-based cell therapies to a selective suppression of undesired immune reactions, only the transfer of Ag-specific nTreg represents the appropriate therapeutic option. However, Ag-specific nTreg are present at extremely low frequencies in the periphery, and so far appropriate surface markers for their precise identification are missing. In this study, we demonstrate that activated nTreg and activated conventional T cells differ in their 4-1BB and CD40 ligand (CD40L) expression signatures, allowing a clear dissection from each other. Based on the expression of 4-1BB and absence of CD40L expression, human alloantigen-reactive Foxp3(+) nTreg can be directly isolated from MLR cultures with high purity. Alloantigen-reactive 4-1BB(+)CD40L(-) nTreg were characterized by a completely demethylated Treg-specific demethylated region and showed alloantigen-specific suppressive properties superior to polyclonal Treg. Importantly, isolated 4-1BB(+)CD40L(-) nTreg maintain the nTreg phenotype and alloantigen-reactivity after in vitro expansion. Our results offer the possibility to simultaneously analyze Ag-specific nTreg and conventional T cells, and to establish cellular therapies with Ag-specific nTreg aiming at a specific inhibition of unwanted immunity.

IL-15 augments TCR-induced CD4+ T cell expansion in vitro by inhibiting the suppressive function of CD25 High CD4+ T cells.

Due to its critical role in NK cell differentiation and CD8(+) T cell homeostasis, the importance of IL-15 is more firmly established for cytolytic effectors of the immune system than for CD4(+) T cells. The increased levels of IL-15 found in several CD4(+) T cell-driven (auto-) immune diseases prompted us to examine how IL-15 influences murine CD4(+) T cell responses to low dose TCR-stimulation in vitro. We show that IL-15 exerts growth factor activity on both CD4(+) and CD8(+) T cells in a TCR-dependent and Cyclosporin A-sensitive manner. In CD4(+) T cells, IL-15 augmented initial IL-2-dependent expansion and once IL-15Rα was upregulated, IL-15 sustained the TCR-induced expression of IL-2/15Rß, supporting proliferation independently of secreted IL-2. Moreover, IL-15 counteracts CD4(+) T cell suppression by a gradually expanding CD25(High)CD4(+) T cell subset that expresses Foxp3 and originates from CD4(+)CD25(+) Tregs. These in vitro data suggest that IL-15 may dramatically strengthen the T cell response to suboptimal TCR-triggering by overcoming an activation threshold set by Treg that might create a risk for autoimmune pathology.

IL-7 receptor blockade reverses autoimmune diabetes by promoting inhibition of effector/memory T cells.

To protect the organism against autoimmunity, self-reactive effector/memory T cells (T(E/M)) are controlled by cell-intrinsic and -extrinsic regulatory mechanisms. However, how some T(E/M) cells escape regulation and cause autoimmune disease is currently not understood. Here we show that blocking IL-7 receptor-α (IL-7Rα) with monoclonal antibodies in nonobese diabetic (NOD) mice prevented autoimmune diabetes and, importantly, reversed disease in new-onset diabetic mice. Surprisingly, IL-7-deprived diabetogenic T(E/M) cells remained present in the treated animals but showed increased expression of the inhibitory receptor Programmed Death 1 (PD-1) and reduced IFN-γ production. Conversely, IL-7 suppressed PD-1 expression on activated T cells in vitro. Adoptive transfer experiments revealed that T(E/M) cells from anti-IL-7Rα-treated mice had lost their pathogenic potential, indicating that absence of IL-7 signals induces cell-intrinsic tolerance. In addition to this mechanism, IL-7Rα blockade altered the balance of regulatory T cells and T(E/M) cells, hence promoting cell-extrinsic regulation and further increasing the threshold for diabetogenic T-cell activation. Our data demonstrate that IL-7 contributes to the pathogenesis of autoimmune diabetes by enabling T(E/M) cells to remain in a functionally competent state and suggest IL-7Rα blockade as a therapy for established T-cell-dependent autoimmune diseases.

Opposing functions of IL-2 and IL-7 in the regulation of immune responses.

Regulation of the magnitude and quality of immune responses is dependent on the integration of multiple signals which typically operate through positive and negative feedback loops. Cytokines that promote or limit T cell expansion and differentiation are often both present in the complex lymphoid environment where antigen-initiated T cell responses take place. The nature and strength of the cytokine signal received by the responding cell, as well as by surrounding regulatory cells, will determine the extent of clonal expansion and the progression towards effector and memory cell differentiation. The mechanisms that determine how much cytokine is produced and how cytokine activities are controlled by receptor expression and intracellular regulators of signaling are not fully understood. Here we discuss the opposing functions of two members of the common receptor gamma chain (γc) cytokines, IL-2 and IL-7 in the generation and regulation of immune responses in vivo.

Cutting edge: mechanisms of IL-2-dependent maintenance of functional regulatory T cells.

IL-2 controls the survival of regulatory T cells (Tregs), but it is unclear whether IL-2 also directly affects Treg suppressive capacity in vivo. We have found that eliminating Bim-dependent apoptosis in IL-2- and CD25-deficient mice restored Treg numbers but failed to cure their lethal autoimmune disease, demonstrating that IL-2-dependent survival and suppressive activity can be uncoupled in Tregs. Treatment with IL-2-anti-IL-2-Ab complexes enhanced the numbers and suppressive capacity of IL-2-deprived Tregs with striking increases in CD25, CTLA-4, and CD39/CD73 expression. Although cytokine treatment induced these suppressive mechanisms in both IL-2(-/-) and IL-2(-/-)Bim(-/-) mice, it only reversed autoimmune disease in the latter. Our results suggest that successful IL-2 therapy of established autoimmune diseases will require a threshold quantity of Tregs present at the start of treatment and show that the suppressive capacity of Tregs critically depends on IL-2 even when Treg survival is independent of this cytokine.

Revisiting the role of IL-2 in autoimmunity.

IL-2 was discovered as a T-cell growth factor that promoted T-cell-dependent immune responses; however, more recent studies suggest that the essential role of IL-2 is to maintain functional Treg and thus control immune responses. These results are leading to new ideas about the potential of IL-2 as a therapeutic strategy in autoimmune diseases. In this issue of the European Journal of Immunology, a study further examines the role of IL-2 in immune regulation and shows for the first time that IL-2 complexes can ameliorate autoantibody-mediated autoimmunity. This commentary examines the current findings in relation to what we already know about IL-2 complexes.

The initial phase of an immune response functions to activate regulatory T cells.

An early reaction of CD4(+) T lymphocytes to Ag is the production of cytokines, notably IL-2. To detect cytokine-dependent responses, naive Ag-specific T cells were stimulated in vivo and the presence of phosphorylated STAT5 molecules was used to identify the cell populations responding to IL-2. Within hours of T cell priming, IL-2-dependent STAT5 phosphorylation occurred primarily in Foxp3(+) regulatory T cells. In contrast, the Ag-specific T cells received STAT5 signals only after repeated Ag exposure or memory differentiation. Regulatory T cells receiving IL-2 signals proliferated and developed enhanced suppressive activity. These results indicate that one of the earliest events in a T cell response is the activation of endogenous regulatory cells, potentially to prevent autoimmunity.

Interleukin-2 in the development and control of inflammatory disease.

Interleukin-2 (IL-2) has multiple, sometimes opposing, functions during an inflammatory response. It is a potent inducer of T-cell proliferation and T-helper 1 (Th1) and Th2 effector T-cell differentiation and provides T cells with a long-lasting competitive advantage resulting in the optimal survival and function of memory cells. In a regulatory role, IL-2 is important for the development, survival, and function of regulatory T cells, it enhances Fas-mediated activation-induced cell death, and it inhibits the development of inflammatory Th17 cells. Thus, in its dual and contrasting functions, IL-2 contributes to both the induction and the termination of inflammatory immune responses.

Targeting T cell-specific costimulators and growth factors in a model of autoimmune hemolytic anemia.

Although it is established that failure of regulatory mechanisms underlies many autoimmune diseases, the stimuli that activate autoreactive lymphocytes remain poorly understood. Defining these stimuli will lead to therapeutic strategies for autoimmune diseases. IL-2-deficient mice develop spontaneous autoimmunity, because of a deficiency of regulatory T cells, and on the BALB/c background, they rapidly die from autoimmune hemolytic anemia. To define the importance of costimulatory pathways in various components of this autoimmune disorder, we first intercrossed IL-2-deficient mice with mice lacking CD28 or CD40L. Elimination of CD28 reduced the activation of autoreactive T cells and lymphoproliferation as well as production of autoantibodies, whereas elimination of CD40L reduced autoantibody production without affecting T cell expansion and accumulation. To examine the role of IL-7, we blocked IL-7R signaling with neutralizing Abs. This treatment inhibited the production of autoantibodies and the development of autoimmune hemolytic anemia. Together, these data indicate that specific costimulatory and cytokine signals are critical for the spontaneous autoantibody-mediated disease that develops in IL-2-deficient mice.

Interleukin-2 enhances CD4+ T cell memory by promoting the generation of IL-7R alpha-expressing cells.

The common gamma chain cytokines interleukin (IL)-2 and IL-7 are important regulators of T cell homeostasis. Although IL-2 is implicated in the acute phase of the T cell response, IL-7 is important for memory T cell survival. We asked whether regulated responsiveness to these growth factors is determined by temporal expression of the cytokine-specific IL-2 receptor (R) alpha and IL-7Ralpha chains. We demonstrate that IL-2Ralpha is expressed early after priming in T cell receptor-transgenic CD4(+) T cells, whereas IL-7Ralpha expression is lost. In the later stage of the response, IL-7Ralpha is reexpressed while IL-2Ralpha expression is silenced. This reciprocal pattern of IL-2Ralpha/IL-7Ralpha expression is disturbed when CD4(+) T cells are primed in the absence of IL-2 signals. Primed IL-2(-/-) or CD25(-/-) (IL-2Ralpha(-/-)) CD4(+) T cells, despite showing normal induction of activation markers and cell division, fail to reexpress IL-7Ralpha late in the response. Because the generation of CD4(+) memory T cells is dependent on IL-7-IL-7Ralpha interactions, primed IL-2(-/-) or CD25(-/-) CD4(+) T cells develop poorly into long-lived memory cells. Retrovirus-mediated expression of IL-7Ralpha in IL-2(-/-) T cells restores their capacity for long-term survival. These results identify IL-2 as a factor regulating IL-7Ralpha expression and, consequently, memory T cell homeostasis in vivo.