IL-7 promotes the survival and maturation but not differentiation of human post-thymic CD4+ T cells

A simultaneous knockout knockin genome editing strategy in HSPCs potently inhibits CCR5- and CXCR4-tropic HIV-1 infection

Allogeneic hematopoietic stem and progenitor cell transplant (HSCT) of CCR5 null (CCR5Δ32) cells can be curative for HIV-1-infected patients. However, because allogeneic HSCT poses significant risk, CCR5Δ32 matched bone marrow donors are rare, and CCR5Δ32 transplant does not confer resistance to the CXCR4-tropic virus, it is not a viable option for most patients. We describe a targeted Cas9/AAV6-based genome editing strategy for autologous HSCT resulting in both CCR5- and CXCR4-tropic HIV-1 resistance. Edited human hematopoietic stem and progenitor cells (HSPCs) maintain multi-lineage repopulation capacity in vivo, and edited primary human T cells potently inhibit infection by both CCR5-tropic and CXCR4-tropic HIV-1. Modification rates facilitated complete loss of CCR5-tropic replication and up to a 2,000-fold decrease in CXCR4-tropic replication without CXCR4 locus disruption. This multi-factor editing strategy in HSPCs could provide a broad approach for autologous HSCT as a functional cure for both CCR5-tropic and CXCR4-tropic HIV-1 infections.

BCG vaccination induces enhanced frequencies of memory T cells and altered plasma levels of common γc cytokines in elderly individuals

BCG vaccination is known to induce innate immune memory, which confers protection against heterologous infections. However, the effect of BCG vaccination on the conventional adaptive immune cells subsets is not well characterized. We investigated the impact of BCG vaccination on the frequencies of T cell subsets and common gamma c (γc) cytokines in a group of healthy elderly individuals (age 60–80 years) at one month post vaccination as part of our clinical study to examine the effect of BCG on COVID-19. Our results demonstrate that BCG vaccination induced enhanced frequencies of central (p<0.0001) and effector memory (p<0.0001) CD4+ T cells and diminished frequencies of naïve (p<0.0001), transitional memory (p<0.0001), stem cell memory (p = 0.0001) CD4+ T cells and regulatory T cells. In addition, BCG vaccination induced enhanced frequencies of central (p = 0.0008), effector (p<0.0001) and terminal effector memory (p<0.0001) CD8+ T cells and diminished frequencies of naïve (p<0.0001), transitional memory (p<0.0001) and stem cell memory (p = 0.0034) CD8+T cells. BCG vaccination also induced enhanced plasma levels of IL-7 (p<0.0001) and IL-15 (p = 0.0020) but diminished levels of IL-2 (p = 0.0033) and IL-21 (p = 0.0020). Thus, BCG vaccination was associated with enhanced memory T cell subsets as well as memory enhancing γc cytokines in elderly individuals, suggesting its ability to induce non-specific adaptive immune responses.

A role for metabolism in determining neonatal immune function

Immune responses of neonates differ markedly to those of adults, with skewed cytokine phenotypes, reduced inflammatory properties and drastically diminished memory function. Recent research efforts have started to unravel the role of cellular metabolism in determining immune cell fate and function. For studies in humans, much of the work on metabolic mechanisms underpinning innate and adaptive immune responses by different haematopoietic cell types is in adults. Studies investigating the contribution of metabolic adaptation in the unique setting of early life are just emerging, and much more work is needed to elucidate the contribution of metabolism to neonatal immune responses. Here, we discuss our current understanding of neonatal immune responses, examine some of the latest developments in neonatal immunometabolism and consider the possible role of altered metabolism to the distinctive immune phenotype of the neonate. Understanding the role of metabolism in regulating immune function at this critical stage in life has direct benefit for the child by affording opportunities to maximize immediate and long-term health. Additionally, gaining insight into the diversity of human immune function and naturally evolved immunometabolic strategies that modulate immune function could be harnessed for a wide range of opportunities including new therapeutic approaches.

Modeling cell-specific dynamics and regulation of the common gamma chain cytokines

The γ-chain receptor dimerizes with complexes of the cytokines interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21 and their corresponding "private" receptors. These cytokines have existing uses and future potential as immune therapies because of their ability to regulate the abundance and function of specific immune cell populations. Here, we build a binding reaction model for the ligand-receptor interactions of common γ-chain cytokines, which includes receptor trafficking dynamics, enabling quantitative predictions of cell-type-specific response to natural and engineered cytokines. We then show that tensor factorization is a powerful tool to visualize changes in the input-output behavior of the family across time, cell types, ligands, and concentrations. These results present a more accurate model of ligand response validated across a panel of immune cell types as well as a general approach for generating interpretable guidelines for manipulation of cell-type-specific targeting of engineered ligands.

Lymphopenia in primary Sjögren's syndrome is associated with premature aging of naïve CD4+ T cells

OBJECTIVE

To investigate peripheral lymphopenia, a frequent finding in primary Sjögren's syndrome (pSS) associated with higher disease activity and increased mortality.

METHODS

Prospective, cross-sectional study of consecutive patients with pSS (n = 66) and healthy controls (n = 181). Lymphocyte subsets were analysed by flow cytometry, naïve (CD45RA+) and memory (CD45RO+) CD4+ T cells were purified by MACS technology. In vitro proliferation and senescence-associated β-galactosidase (SABG) were assessed by flow cytometry. Telomere length and TCR excision circles (TREC) were measured by real-time PCR. Telomerase activity was analysed according to the telomeric repeat amplification protocols (TRAP).

RESULTS

In pSS, lymphopenia mainly affected naïve CD4+ T cells. We noted a lower frequency of proliferating naïve CD4+ T cells ex vivo and decreased homeostatic proliferation in response to IL-7 stimulation in vitro. Furthermore, naïve CD4+ T cells exhibited signs of immune cell aging including shortened telomeres, a reduction in IL-7R expression and accumulation of SABG. The senescent phenotype could be explained by telomerase insufficiency and drastically reduced levels of T-cell receptor excision circles (TRECs), indicating a history of extensive post-thymic cell division. TRECs correlated with the number of naïve CD4+ T cells linking the extend of earlier proliferation to the inability to sustain normal cell numbers.

CONCLUSION

In pSS, evidence for increased proliferation of naïve CD4+ T cells earlier in life is associated with a senescent phenotype unable to sustain homeostasis. The lack of naïve CD4+ T cells forms the basis of lymphopenia frequently observed in pSS.

A new perspective on mesenchymal-immune interactions in adipose tissue

The mammalian immune system has crucial homeostatic functions in different adipose depots. However, white adipose tissue (WAT) inflammation is a hallmark of obesity and can contribute to type 2 diabetes mellitus (T2DM). Recently, mesenchymal cells were identified as highly heterogenous populations displaying specialized immune functions in immune cell migration, activation, survival, and overall lymphoid tissue organization in several tissues. How they regulate the inflammatory milieu within different adipose depots remains unknown. Using recently published single-cell RNA-sequencing (scRNAseq) data sets, we analyze cytokine and chemokine expression of mouse WAT mesenchymal cell subpopulations to highlight potential immunological heterogeneity and specialization, hypothesizing on their immunological functions. This new perspective on immune-mesenchymal cell interactions in adipose tissue may promote studies that heighten our understanding of immune cell processes within WAT during health and obesity. We hope that these studies redefine our knowledge of the roles of mesenchymal cells in regulating adipose tissue inflammation and physiology.

T Cell Subsets During Early Life and Their Implication in the Treatment of Childhood Acute Lymphoblastic Leukemia

The immune system plays a major role in recognizing and eliminating malignant cells, and this has been exploited in the development of immunotherapies aimed at either activating or reactivating the anti-tumor activity of a patient's immune system. A wide range of therapeutic approaches involving T lymphocytes, such as programmed cell death protein ligand-1 (PDL-1) inhibitors, cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) blockers, and CD19-targeted T-cell therapy through chimeric antigen receptor (CAR)-T cells or CD19/CD3 bi-specific T-cell engagers, have been introduced to the field of oncology, leading to significant improvements in overall survival of adult cancer patients. During the past few years, the availability and approval of T-cell based immunotherapies have become a reality also for the treatment of childhood cancers. However, the distribution, ratio of regulatory to effector cells and the quality of T-cell responses early in life are distinct from those during adolescence and adulthood, raising the possibility that these differences impact the efficacy of immunotherapy. Herein we provide a brief overview of the properties of conventional T cell subsets during early life. Focusing on the most common cancer type during childhood, acute lymphoblastic leukemia (ALL), we describe how current conventional therapies used against ALL influence the T-cell compartment of small children. We describe early life T-cell responses in relation to immunotherapies engaging T-cell anticancer reactivity and present our opinion that it is not only immaturity of the adaptive immune system, but also the impact of an immunosuppressive environment that may prove disadvantageous in the setting of immunotherapies targeting pediatric cancer cells.

Decrease in naïve T cell production due to HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) development

Human T-lymphocytic virus 1 (HTLV-1) is mainly associated with adult T-cell leukemia/lymphoma (ATLL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Patients with HAM/TSP exhibit significant changes in their immune response, and HTLV-1 infection can interfere in cytokine production and perhaps in T cell production. The aims of this study were to evaluate thymic function in HAM/TSP patients and HTLV-1 healthy carriers (HCs) and correlate it to age and interleukin 7 (IL-7) gene expression. Thymic function in 21 HAM/TSP patients and 12 HCs was evaluated by quantifying T cell receptor rearrangement excision circle (TREC) particles and IL-7 gene expression, both measured by quantitative polymerase chain reaction. HAM/TSP patients presented lower TREC particle counts (p = 0.0112) and lower IL-7 expression (p = 0.0102) than HCs. Both TREC particles and IL-7 gene expression were separately analyzed in two age groups: ≤ 59 years and ≥60 years, The ≤59-year-old HAM/TSP patients had a lower TREC count compared with the ≤59-year-old HCs (p = 0.0476). In conclusion, HAM/TSP development could interfere with thymic function because the results showed TREC particle reduction in HAM/TSP patients in relation to HCs, and it could be associated with a concomitant reduction in IL-7 expression.

IL-7/αIL-7 mAb M25 immunocomplexes expand CD8+ T cells but paradoxically abrogate the antitumor activity of CTLA-4 and PD-1 blockage

Supraphysiological levels of IL-7 induce increase counts of pre-B cells, naive T cells and memory phenotype CD8⁺ T cells. Immunocomplexes of IL-7 and αIL-7 mAb M25 (IL-7/M25) were described as IL-7 superagonist in vivo. Thus, treatment of mice with IL-7/M25 remarkably increases the size of the T cell pool. We decided to use IL-7/M25 in order to expand the T cell population prior to the administration of αCTLA-4 and αPD-1 mAbs in tumor-bearing mice and in turn boost the immunotherapy based on a combination of CTLA-4 and PD-1 blockage. We found that just four doses of IL-7/M25 increased the absolute numbers of splenocytes approximately fivefold and significantly shifted the CD4⁺:CD8⁺ T cell ratio in favor of CD8⁺ T cells. There was also a substantive increase in relative counts of memory phenotype CD8⁺ T cells (approximately threefold) within CD8⁺ T cells but a significant decrease (approximately 30%) in relative counts of Treg cells within CD4⁺ T cells. All these data suggest that IL-7/M25 offer a suitable approach to potentiate tumor immunotherapy through CTLA-4 and PD-1 blockage. Unexpectedly, IL-7/M25 significantly abrogated the antitumor activity of αCTLA-4 plus αPD-1 mAbs in the following mouse tumor models: MC-38 and CT26 colon carcinoma and B16F10 melanoma. This paradoxical effect of IL-7/M25 on the antitumor activity of CTLA-4 and PD-1 blockage was not mediated via either increased levels of IL-10 or TGF-β in the sera or increased counts of IL-10-producing B or T cells in the spleen of mice injected with IL-7/M25. Thus, our work shows that caution should be exercised when combining two immunotherapy approaches together.

Influence of Inflammation in the Process of T Lymphocyte Differentiation: Proliferative, Metabolic, and Oxidative Changes

T lymphocytes, from their first encounter with their specific antigen as naïve cell until the last stages of their differentiation, in a replicative state of senescence, go through a series of phases. In several of these stages, T lymphocytes are subjected to exponential growth in successive encounters with the same antigen. This entire process occurs throughout the life of a human individual and, earlier, in patients with chronic infections/pathologies through inflammatory mediators, first acutely and later in a chronic form. This process plays a fundamental role in amplifying the activating signals on T lymphocytes and directing their clonal proliferation. The mechanisms that control cell growth are high levels of telomerase activity and maintenance of telomeric length that are far superior to other cell types, as well as metabolic adaptation and redox control. Large numbers of highly differentiated memory cells are accumulated in the immunological niches where they will contribute in a significant way to increase the levels of inflammatory mediators that will perpetuate the new state at the systemic level. These levels of inflammation greatly influence the process of T lymphocyte differentiation from naïve T lymphocyte, even before, until the arrival of exhaustion or cell death. The changes observed during lymphocyte differentiation are correlated with changes in cellular metabolism and these in turn are influenced by the inflammatory state of the environment where the cell is located. Reactive oxygen species (ROS) exert a dual action in the population of T lymphocytes. Exposure to high levels of ROS decreases the capacity of activation and T lymphocyte proliferation; however, intermediate levels of oxidation are necessary for the lymphocyte activation, differentiation, and effector functions. In conclusion, we can affirm that the inflammatory levels in the environment greatly influence the differentiation and activity of T lymphocyte populations. However, little is known about the mechanisms involved in these processes. The elucidation of these mechanisms would be of great help in the advance of improvements in pathologies with a large inflammatory base such as rheumatoid arthritis, intestinal inflammatory diseases, several infectious diseases and even, cancerous processes.

Tissue compartmentalization of T cell responses during early life

The immune system in early life is tasked with transitioning from a relatively protected environment to one in which it encounters a wide variety of innocuous antigens and dangerous pathogens. The immaturity of the developing immune system, and particularly the distinct functionality of T lymphocytes in early life, has been implicated in increased susceptibility to infection. Previous work has demonstrated that immune responses in early life are skewed toward limited inflammation and atopy; however, there is mounting evidence that such responses are context- and tissue-dependent. The regulation, differentiation, and maintenance of infant T cell responses, particularly as it relates to tissue compartmentalization, remains poorly understood. How the tissue environment impacts early-life immune responses and whether the development of localized protective immune memory cell subsets are established is an emerging area of research. As infectious diseases affecting the respiratory and digestive tracts are a leading cause of morbidity and mortality worldwide in infants and young children, a deeper understanding of site-specific immunity is essential to addressing these challenges. Here, we review the current paradigms of T cell responses during infancy as they relate to tissue localization and discuss implications for the development of vaccines and therapeutics.

IL-7-Induced Proliferation of Human Naive CD4 T-Cells Relies on Continued Thymic Activity

Naive CD4 T-cell maintenance is critical for immune competence. We investigated here the fine-tuning of homeostatic mechanisms of the naive compartment to counteract the loss of de novo CD4 T-cell generation. Adults thymectomized in early childhood during corrective cardiac surgery were grouped based on presence or absence of thymopoiesis and compared with age-matched controls. We found that the preservation of the CD31^- subset was independent of the thymus and that its size is tightly controlled by peripheral mechanisms, including prolonged cell survival as attested by Bcl-2 levels. Conversely, a significant contraction of the CD31⁺ naive subset was observed in the absence of thymic activity. This was associated with impaired responses of purified naive CD4 T-cells to IL-7, namely, in vitro proliferation and upregulation of CD31 expression, which likely potentiated the decline in recent thymic emigrants. Additionally, we found no apparent constraint in the differentiation of naive cells into the memory compartment in individuals completely lacking thymic activity despite upregulation of DUSP6, a phosphatase associated with increased TCR threshold. Of note, thymectomized individuals featuring some degree of thymopoiesis were able to preserve the size and diversity of the naive CD4 compartment, further arguing against complete thymectomy in infancy. Overall, our data suggest that robust peripheral mechanisms ensure the homeostasis of CD31^- naive CD4 pool and point to the requirement of continuous thymic activity to the maintenance of IL-7-driven homeostatic proliferation of CD31⁺ naive CD4 T-cells, which is essential to secure T-cell diversity throughout life.

HIV-1 Vpr- and Reverse Transcription-Induced Apoptosis in Resting Peripheral Blood CD4 T Cells and Protection by Common Gamma-Chain Cytokines

HIV-1 infection leads to the progressive depletion of the CD4 T cell compartment by various known and unknown mechanisms. In vivo, HIV-1 infects both activated and resting CD4 T cells, but in vitro, in the absence of any stimuli, resting CD4 T cells from peripheral blood are resistant to infection. This resistance is generally attributed to an intracellular environment that does not efficiently support processes such as reverse transcription (RT), resulting in abortive infection. Here, we show that in vitro HIV-1 infection of resting CD4 T cells induces substantial cell death, leading to abortive infection. In vivo, however, various microenvironmental stimuli in lymphoid and mucosal tissues provide support for HIV-1 replication. For example, common gamma-chain cytokines (CGCC), such as interleukin-7 (IL-7), render resting CD4 T cells permissible to HIV-1 infection without inducing T cell activation. Here, we find that CGCC primarily allow productive infection by preventing HIV-1 triggering of apoptosis, as evidenced by early release of cytochrome c and caspase 3/7 activation. Cell death is triggered both by products of reverse transcription and by virion-borne Vpr protein, and CGCC block both mechanisms. When HIV-1 RT efficiency was enhanced by SIVmac239 Vpx protein, cell death was still observed, indicating that the speed of reverse transcription and the efficiency of its completion contributed little to HIV-1-induced cell death in this system. These results show that a major restriction on HIV-1 infection in resting CD4 T cells resides in the capacity of these cells to survive the early steps of HIV-1 infection.

IMPORTANCE

A major consequence of HIV-1 infection is the destruction of CD4 T cells. Here, we show that delivery of virion-associated Vpr protein and the process of reverse transcription are each sufficient to trigger apoptosis of resting CD4 T cells isolated from peripheral blood. While these 2 mechanisms have been previously described in various cell types, we show for the first time their concerted effect in inducing resting CD4 T cell depletion. Importantly, we found that cytokines such as IL-7 and IL-4, which are particularly active in sites of HIV-1 replication, protect resting CD4 T cells from these cytopathic effects and, primarily through this protection, rather than through enhancement of specific replicative steps, they promote productive infection. This study provides important new insights for the understanding of the early steps of HIV-1 infection and T cell depletion.

Interferon-α inhibits CD4 T cell responses to interleukin-7 and interleukin-2 and selectively interferes with Akt signaling

Persistent type I IFN production occurs during chronic viral infections, such as HIV disease. As type I IFNs have antiproliferative activity, it is possible that chronic exposure to these cytokines could adversely affect T cell homeostasis. We investigated the capacity of IFN-α to impair T cell proliferation induced by the homeostatic cytokine, IL-7, or another common γ-chain cytokine, IL-2, in cells from healthy human donors. We found that IL-7- or IL-2-induced proliferation of CD4(+) T cells was partially inhibited in the presence of IFN-α. The CD4(+) T cells that were exposed to IFN-α also displayed attenuated induction of IL-2 and CD40L following TCR stimulation. Analyses of signaling pathways indicated that IL-7 and IL-2 induced a delayed and sustained P-Akt signal that lasted for several days and was partially inhibited by IFN-α. In contrast, IL-7-induced P-STAT5 was not affected by IFN-α. Furthermore, IFN-α had no detectable effect on P-Akt that was induced by the chemokine SDF-1. Both inhibitors of P-Akt and P-STAT5 blocked IL-7-induced T cell proliferation, confirming that both signaling pathways are important for IL-7-induced T cell proliferation. These results demonstrate that IFN-α can selectively inhibit cytokine-induced P-Akt as a potential mechanism to disrupt homeostasis of T lymphocytes.

A low effective dose of interleukin-7 is sufficient to maintain cord blood T cells alive without potentiating allo-immune responses

Slow reconstitution of T cell immunity remains a critical issue after umbilical cord blood (CB) transplantation. Although this may be a consequence of the low cell dose, it may also reflect the propensity of naïve T cells, which predominate in CB, to undergo apoptotic cell death. Exogenous interleukin 7 (IL-7) can prevent apoptosis of naïve T cells, but at high concentrations, IL-7 may also expand alloreactive T cells, thereby aggravating the risk of graft-versus-host disease. We evaluated the survival of CB T cells from 34 healthy full-term pregnancies, and we found wide interdonor variation, from 17.4% to 79.7%, of CB T cells that were still alive after being rested for 4 days in culture medium without cytokine supplementation. The viability of CB T cells was negatively correlated to infant birth weight (Spearman's ρ = .376; P = .031) and positively correlated to venous CB pH (ρ = .397; P = .027); both associations were confirmed by multivariate analysis (P = .023 and P = .005, respectively). A low supplemental concentration (100 pg/mL) of recombinant human IL-7 was sufficient to maintain the viability of cryopreserved/thawed CB T cells, with most (>80%) cells remaining in a quiescent state and without significant changes in their CD4/CD8 ratio and the proportion of CD4(+) CD31(+) PTK7(+) recent thymic emigrants. IL-7 at 100 pg/mL did not lead to any significant enhancement of the alloreactive response of CB T cells, as evaluated by proliferation rates (thymidine incorporation and carboxyfluorescein diacetate succinimidyl ester dilution) and interferon-gamma production (ELISPOT). This effective concentration of IL-7 is far lower than that obtained in vivo after pharmacological administration of the cytokine. This study suggests that administration of lower doses of recombinant human IL-7 than used in previous clinical trials may be sufficient to sustain the viability of infused CB T cells and, thus, help to accelerate naïve T cell reconstitution without potentiating their alloreactivity.

Human neonatal naive CD4+ T cells have enhanced activation-dependent signaling regulated by the microRNA miR-181a

Compared with older children and adults, human neonates have reduced and delayed CD4(+) T cell immunity to certain pathogens, but the mechanisms for these developmental differences in immune function remain poorly understood. We investigated the hypothesis that impaired human neonatal CD4(+) T cell immunity was due to reduced signaling by naive CD4(+) T cells following engagement of the αβ-TCR/CD3 complex and CD28. Surprisingly, calcium flux following engagement of CD3 was significantly higher in neonatal naive CD4(+) T cells from umbilical cord blood (CB) compared with naive CD4(+) T cells from adult peripheral blood. Enhanced calcium flux was also observed in adult CD4(+) recent thymic emigrants. Neonatal naive CD4(+) T cells also had higher activation-induced Erk phosphorylation. The microRNA miR-181a, which enhances activation-induced calcium flux in murine thymocytes, was expressed at significantly higher levels in CB naive CD4(+) T cells compared with adult cells. Overexpression of miR-181a in adult naive CD4(+) T cells increased activation-induced calcium flux, implying that the increased miR-181a levels of CB naive CD4(+) T cells contributed to their enhanced signaling. In contrast, AP-1-dependent transcription, which is downstream of Erk and required for full T cell activation, was decreased in CB naive CD4(+) T cells compared with adult cells. Thus, CB naive CD4(+) T cells have enhanced activation-dependent calcium flux, indicative of the retention of a thymocyte-like phenotype. Enhanced calcium signaling and Erk phosphorylation are decoupled from downstream AP-1-dependent transcription, which is reduced and likely contributes to limitations of human fetal and neonatal CD4(+) T cell immunity.

CD38 identifies a hypo-proliferative IL-13-secreting CD4+ T-cell subset that does not fit into existing naive and memory phenotype paradigms

CD38 is commonly regarded as an activation marker for human T cells. Herein, we show that CD38 expression identifies a hypo-proliferative CD4(+) T-cell subset that, following TCR stimulation, retains expression of naive cell surface markers including CD45RA, CD62L and CCR7. Hypo-proliferation was mediated by reduced CD25 up-regulation upon TCR stimulation compared to CD4(+) CD38(-) cells and lack of responsiveness to exogenous IL-2. Instead, CD4(+) CD38(+) T cells expressed CD127, and hypo-proliferation was reversed by addition of IL-7, further associated with increased STAT5 phosphorylation. This phenotype was exacerbated by addition of an agonistic CD38-binding antibody, suggesting that signaling through CD38 promotes this cell profile. Activated CD4(+) CD38(+) cells had a bias towards IL-13 secretion, but not other Th2 cytokines such as IL-4 or IL-5. In comparison, the CD4(+) CD38(-) cells had a clear bias towards secretion of Th1-associated cytokines IFN-γ and TNF. The existence of such CD4(+) CD38(+) T cells may play an important role in pathologies such as asthma, which are associated with IL-13, but not IL-4 and IL-5. Coupled with responsiveness to IL-7 but not IL-2, and the involvement of CD38 ligation, our results highlight a unique T-cell subpopulation that does not fit into existing naive and memory cell paradigms.

Neonatal CD8+ T-cell differentiation is dependent on interleukin-12

Neonatal CD8(+) T-cell activation is significantly impaired compared with that in adults. Recent studies have demonstrated that interleukin (IL)-12 is necessary as a third signal, in addition to antigen and co-stimulation, to authorize the differentiation of naive CD8(+) T cells. We examined whether human neonatal CD8(+) T cells, which possess an exclusively naive T-cell phenotype, required a third signal to authorize a productive T-cell response. IL-12 enhanced activated naive CD8(+) T-cell survival, expansion, CD25 expression, and IL-2 production. Activated CD8(+) T cells produced interferon-γ and intracellular granzyme B and were cytotoxic only in the presence of IL-12. Sustained IL-12 signaling for 72 hours was required for optimal interferon-γ production. IL-12, in concert with T cell receptor (TCR) stimulation, sustained late-stage (48-72 hours) intracellular phosphorylation and particularly total protein levels of the proximal TCR components, Lck, and CD3ξ. The requirement for a third signal for productive human neonatal CD8(+) T-cell differentiation may have implications for neonatal vaccination strategies.

Naive T cell homeostasis: from awareness of space to a sense of place

The peripheral naive T cell pool is fairly stable in number, diversity and functional competence in the absence of vigorous immune responses. However, this apparent tranquility is not an intrinsic property of T cells but involves continuous tuning of the T cell pool composition by homeostatic signals. In the past decade, studies have revealed that naive T cells rely on combinatorial signals from self-peptide-MHC complexes and interleukin-7 for their physical and functional maintenance. Competition for these factors dictates T cell 'space'. In addition, recent studies show that these and other homeostatic factors are offered to T cells on stromal cell networks, which also serve to guide T cell trafficking in secondary lymphoid organs. Such findings suggest the importance of 'place' in the perception and integration of homeostatic cues for the maintenance and functional tuning of the naive T cell pool.

Antiretroviral therapy initiation during primary HIV infection enhances both CD127 expression and the proliferative capacity of HIV-specific CD8+ T cells

OBJECTIVES

HIV-specific CD8+ T cells from patients with primary HIV infection (PHI) and after antiretroviral therapy initiation were evaluated for CD127 expression and proliferative capacity and were compared with cells from chronically-infected patients, including long-term nonprogressors and HIV controllers.

METHODS

We studied 30 patients with PHI (from the Agence Nationale de Recherche sur le SIDA Primo-infection Cohort) and 33 patients with chronic HIV infection (including nonprogressor patients from the Agence Nationale de Recherche sur le SIDA ALT Cohort and the Agence Nationale de Recherche sur le SIDA HIV Controllers Study Group). HIV-specific CD8+ T cells were identified by costaining with HIV human leukocyte antigen class I pentamers. CD127 expression was assessed by flow cytometry and cell proliferation by carboxyfluorescein succinimidyl ester labeling.

RESULTS

During PHI, most HIV-specific CD8+ T cells coexpressed CD27 and CD45RO, were highly activated, and showed weak Bcl-2 expression. Their CD127 expression was very low and correlated negatively both with HIV RNA and DNA levels and with expression of the activation marker CD38. CD127 expression correlated positively with CD4 cell count, Bcl-2 expression and proliferative capacity. Strong CD127 expression was observed in the two groups of chronically-infected nonprogressors. CD127 expression on HIV-specific CD8+ T cells increased in early-treated PHI patients, reaching levels similar to those observed in nonprogressors. In parallel, these cells acquired strong proliferative capacity. No change in CD127 expression or proliferative potential was observed in untreated patients.

CONCLUSION

Early antiretroviral therapy initiation enhances CD127 expression on HIV-specific CD8+ T cells, reaching levels similar to those observed in aviremic nonprogressors, and restores their proliferative capacity.

T lymphocyte maturation is impaired in healthy young individuals carrying trisomy 21 (Down syndrome)

Cytokine production, immune activation, T lymphocytes maturation, and serum IL-7 concentration were examined in 24 youngsters with Down syndrome and no acquired diseases (healthy Down syndrome [12 prepubertal, 13 pubertal]) and 42 age- and gender-matched controls (20 prepubertal, 22 pubertal). Results showed that a complex immune and impairment is present in healthy individuals with Down syndrome in whom interferon gamma, interleukin (IL) IL-10 production, as well as serum IL-7 concentrations and activation markers-bearing T lymphocytes were significantly augmented. Additionally, a complex skewing of post-thymic lymphocyte maturation pathways was observed in patients: significant reduction of CD4+ and CD8+ naive (RA+CCR7+) lymphocytes, significant increase of CD4+ and CD8+ central memory (RA-CCR7+), and terminally differentiated (TD) (RA+CCR7-) lymphocytes.

IL-7 sustains CD31 expression in human naive CD4+ T cells and preferentially expands the CD31+ subset in a PI3K-dependent manner

The CD31(+) subset of human naive CD4(+) T cells is thought to contain the population of cells that have recently emigrated from the thymus, while their CD31(-) counterparts have been proposed to originate from CD31(+) cells after homeostatic cell division. Naive T-cell maintenance is known to involve homeostatic cytokines such as interleukin-7 (IL-7). It remains to be investigated what role this cytokine has in the homeostasis of naive CD4(+) T-cell subsets defined by CD31 expression. We provide evidence that IL-7 exerts a preferential proliferative effect on CD31(+) naive CD4(+) T cells from adult peripheral blood compared with the CD31(-) subset. IL-7-driven proliferation did not result in loss of CD31 expression, suggesting that CD31(+) naive CD4(+) T cells can undergo cytokine-driven homeostatic proliferation while preserving CD31. Furthermore, IL-7 sustained or increased CD31 expression even in nonproliferating cells. Both proliferation and CD31 maintenance were dependent on the activation of phosphoinositide 3-kinase (PI3K) signaling. Taken together, our data suggest that during adulthood CD31(+) naive CD4(+) T cells are maintained by IL-7 and that IL-7-based therapies may exert a preferential effect on this population.

Bim and Bcl-2 mutually affect the expression of the other in T cells

The life and death of T cells is controlled to a large extent by the relative amounts of Bcl-2-related proteins they contain. The antiapoptotic protein Bcl-2 and the proapoptotic protein Bim are particularly important in this process with the amount of Bcl-2 per cell dropping by about one-half when T cells prepare to die. In this study we show that Bcl-2 and Bim each control the expression of the other. Absence of Bim leads to a drop in the amount of intracellular Bcl-2 protein, while having no effect on the amounts of mRNA for Bcl-2. Conversely, high amounts of Bcl-2 per cell allow high amounts of Bim, although in this case the effect involves increases in Bim mRNA. These mutual effects occur even if Bcl-2 is induced acutely. Thus these two proteins control the expression of the other, at either the protein or mRNA level.

Thymic output, ageing and zinc

The role of the thymus is vital for orchestration of T-cell development and maturation. With increasing age the thymus undergoes a process of involution which results in a reduction in thymic size, function and output. Until relatively recent it was not feasible to accurately measure the magnitude of age-related loss of thymic function. With the discovery of T-cell receptor excision circles (TRECs), which are the stable by-products of the newly generated T-cells, it is now possible to quantitatively measure the extent of thymic output. This review examines the available data on immune function and zinc deficiency and places them in the context of the aims of the ZINCAGE project which include the evaluation of the role played by zinc in maintaining thymic output in healthy elderly individuals.

Heterogeneity in the CD4 T Cell Compartment and the Variability of Neonatal Immune Responsiveness

Over the past decade, it has become clear that T cell immune responses in both murine and human neonates are very heterogeneous, running the gamut from poor or deviant responsiveness to mature, adult-like inflammatory function. How this variability arises is not well understood but there is now a great deal of information suggesting that differences in the T cell compartments in neonates vs adults play important roles. A number of cell types or processes are qualitatively or quantitatively different in the neonate. These include (a) alternate epigenetic programs at the Th2 cytokine locus, (b) enhanced homeostatic proliferation, (c) a relative abundance of fetal-origin cells, (d) a greater representation of recent thymic emigrants, (e) high proportions of potentially self-reactive cells, (f) a developmental delay in the production of regulatory T cells, and (g) cells bearing TCR with limited N region diversity. Different conditions of antigen exposure may lead to different environmental signals that promote the selective responsiveness of one or more of these populations. Therefore, the variability of neonatal responses may be a function of the heterogeneous nature of the responding T cell population. In this review, we will describe these various subpopulations in detail and speculate as to the manner in which they could contribute to the heterogeneity of neonatal immune responses.

Bim/Bcl-2 balance is critical for maintaining naive and memory T cell homeostasis

We examined the role of the antiapoptotic molecule Bcl-2 in combating the proapoptotic molecule Bim in control of naive and memory T cell homeostasis using Bcl-2^−/− mice that were additionally deficient in one or both alleles of Bim. Naive T cells were significantly decreased in Bim^+/−Bcl-2^−/− mice, but were largely restored in Bim^−/−Bcl-2^−/− mice. Similarly, a synthetic Bcl-2 inhibitor killed wild-type, but not Bim^−/−, T cells. Further, T cells from Bim^+/−Bcl-2^−/− mice died rapidly ex vivo and were refractory to cytokine-driven survival in vitro. In vivo, naive CD8⁺ T cells required Bcl-2 to combat Bim to maintain peripheral survival, whereas naive CD4⁺ T cells did not. In contrast, Bim^+/−Bcl-2^−/− mice generated relatively normal numbers of memory T cells after lymphocytic choriomeningitis virus infection. Accumulation of memory T cells in Bim^+/−Bcl-2^−/− mice was likely caused by their increased proliferative renewal because of the lymphopenic environment of the mice. Collectively, these data demonstrate a critical role for a balance between Bim and Bcl-2 in controlling homeostasis of naive and memory T cells.

Interleukin-7 induces HIV replication in primary naive T cells through a nuclear factor of activated T cell (NFAT)-dependent pathway

Interleukin (IL)-7 plays several roles critical to T cell maturation, survival, and homeostasis. Because of these functions, IL-7 is under investigation as an immune-modulator for therapeutic use in lymphopenic clinical conditions, including HIV. We reported that naive T cells, typically not permissive to HIV, can be productively infected when pre-treated with IL-7. We evaluated the mechanism by which IL-7-mediates this effect. IL-7 potently up-regulated the transcriptional factor NFAT, but had no effect on NFkappaB. Blocking NFAT activity using a number of reagents, such as Cyclosporin A, FK-506, or the NFAT-specific inhibitor known as VIVIT peptide, all markedly reduced IL-7-mediated induction of HIV replication in naive T cells. Additional neutralization of cytokines present in IL-7-treated cultures and/or those that have NFAT-binding sequences within their promotors indicated that IL-10, IL-4, and most significantly IFNgamma, all contribute to IL-7-induction of HIV productive replication in naive T cells. These data clarify the mechanism by which IL-7 can overcome the block to HIV productive infection in naive T cells, despite their quiescent cell status. These findings are relevant to the treatment of HIV disease and understanding HIV pathogenesis in the naive CD4+ T cell compartment, especially in light of the vigorous pursuit of IL-7 as an in vivo immune modulator.

Immunomodulants for the treatment of HIV infection: the search goes on

The natural history of HIV infection has been greatly modified by the introduction of powerful antiretroviral agents that act on multiple steps of HIV replication. Thus, antiretroviral therapy (ART) has prolonged the life of HIV-infected individuals, significantly impacting on the progression to AIDS. It was assumed that ART-induced suppression of HIV would have resulted in a degree of immune recovery sufficient enough to allow immune control over HIV replication independently of the use of drugs. Unfortunately, interruption of therapy, even after long periods of full suppression of viral replication, is almost inevitably associated with a prompt rebound of HIV viraemia. The outcome of this observation is that ART has to be considered as a lifelong therapy, with the associated resulting problems of the emergence of multi-drug resistant viral strains, toxic effects, costs and compliance. The use of immunomodulants in association with ART could achieve the goal of boosting the immune response to a threshold, permitting the immune response to indefinitely suppress HIV replication.

Thymic output and functionality of the IL-7/IL-7 receptor system in centenarians: implications for the neolymphogenesis at the limit of human life

During aging, the thymus undergoes a marked involution that is responsible for profound changes in the T-cell compartment. To investigate the capacity of the thymus to produce new cells at the limit of human lifespan, we analyzed some basic mechanisms responsible for the renewal and maintenance of peripheral T lymphocytes in 44 centenarians. Thymic functionality was analyzed by the quantification of cells presenting the T-cell receptor rearrangement excision circles (TREC). A new method based upon real-time PCR was used, and we found that most centenarians (84%) had undetectable levels of TREC+ cells. Six-color cytofluorimetric analysis revealed that centenarians had an extremely low number of naïve T cells; central memory and effector memory T cells were greatly increased, while terminally differentiated cells were as numerous as in young (aged 20-45) or middle-aged (aged 58-62) donors. Interleukin (IL)-7 and IL-7 receptor alpha-chain (CD127) levels were the same at all ages, as shown by ELISA, flow cytometry and real-time PCR. However, IL-7 plasma levels were higher in centenarian females than males. The presence of TREC+ cells and of very few naïve T lymphocytes suggests that in centenarians such cells could either derive from residues of thymic lymphopoietic islets, or even represent long-living lymphocytes that have not yet encountered their antigen. IL-7 could be one of the components responsible, among others, for the higher probability of reaching extreme ages typical of females.

IL-7 induces immunological improvement in SIV-infected rhesus macaques under antiviral therapy

Despite efficient antiretroviral therapy (ART), CD4+ T cell counts often remain low in HIV-1-infected patients. This has led to IL-7, a crucial cytokine involved in both thymopoiesis and peripheral T cell homeostasis, being suggested as an additional therapeutic strategy. We investigated whether recombinant simian IL-7-treatment enhanced the T cell renewal initiated by ART in rhesus macaques chronically infected with SIVmac251. Six macaques in the early chronic phase of SIV infection received antiretroviral treatment. Four macaques also received a 3-wk course of IL-7 injections. Viral load was unaffected by IL-7 treatment. IL-7 treatment increased the number of circulating CD4+ and CD8+ memory T cells expressing activation (HLA-DR+, CD25+) and proliferation (Ki-67+) markers. It also increased naive (CD45RAbrightCD62L+) T cell counts by peripheral proliferation and enhanced de novo thymic production. The studied parameters returned to pretreatment values by day 29 after the initiation of treatment, concomitantly to the appearance of anti-IL-7 neutralizing Abs, supporting the need for a nonimmunogenic molecule for human treatment. Thus, IL-7, which increases T cell memory and de novo renewal of naive T cells may have additional benefits in HIV-infected patients receiving ART.

IL-7 promotes T cell proliferation through destabilization of p27Kip1

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

The adaptive immune response in neonatal cerebral white matter damage

Hypotheses that inflammation contributes to neonatal cerebral white matter damage have evolved over the last three decades. The latest, expanded here, suggests that the adaptive immune system contributes to the intensity and duration of the processes that result in damage to cerebral white matter in the fetus and newborn. We propose several mechanisms by which fetal T lymphocytes could be activated during fetal exposure to infection. These include specific recognition of bacterial antigens, specific recognition of autoantigens, polyclonal activation by Toll-like receptors, and bystander activation by cytokines.

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

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

Uncoupled anti-HIV and immune-enhancing effects when combining IFN-alpha and IL-7

Cytokine-based therapies have been examined for purging viral reservoirs and immunomodulation in HIV infection. However, single cytokines did not result in either HIV eradication or an efficient HIV-specific immune response. We hypothesize that cytokines with distinct biologic effects need to be combined for immunotherapy of HIV infection. In this study, we investigated the anti-HIV activity and immune-enhancing effects of the combination of IFN-alpha and IL-7. In human lymphocyte aggregate cultures infected ex vivo with the X4 HIV strain NL4-3, IFN-alpha/IL-7 potently inhibited HIV replication and preserved CD4(+) T cells, probably by up-regulating Bcl-2. IFN-alpha/IL-7 also strongly inhibited R5 HIV replication. Furthermore, in allogeneic MLRs, IFN-alpha/IL-7 increased T cell proliferation and IFN-gamma production. IFN-alpha alone also had strong anti-HIV activity, but neither preserved CD4(+) T cells nor increased T cell responses in MLRs. IL-7 alone maintained T cells and enhanced T cell activation in MLRs, but only moderately inhibited or increased HIV replication. Thus, coadministration of IFN-alpha/IL-7 combines the potent anti-HIV activity of IFN-alpha with the beneficial effects of IL-7 on T cell survival and function. We speculate that IFN-alpha will block viral replication, activate APCs, and up-regulate MHC molecules, thus allowing IL-7 to display its effects for generating an efficient immune response. In this scenario, the known reactivation of latent HIV by IL-7 may be advantageous.

T cell-mediated immune responses in human newborns: ready to learn?

Infections with intracellular pathogens are often more severe or more prolonged in young infants suggesting that T cell-mediated immune responses are different in early life. Whereas neonatal immune responses have been quite extensively studied in murine models, studies of T cell-mediated immunity in human newborns and infants are scarce. Qualitative and quantitative differences when compared with adult immune responses have been observed but on the other hand mature responses to certain vaccines and infectious pathogens were demonstrated during the postnatal period and even during foetal life. Herein, we review the evidence suggesting that under appropriate conditions of stimulation, protective T cell-mediated immune responses could be induced by vaccines in early life.

Interleukin-7 signalling is sufficient to phenotypically and functionally prime human CD4 naive T cells

Interleukin-7 (IL-7) is produced by bone marrow and lymphoid stromal cells and is involved in the synthesis, survival and homeostasis of T cells. These attributes are the basis for current strategies to utilize IL-7 as an immune modulator for several clinical conditions to replenish depleted T-cell numbers. Because we had previously determined that IL-7 can induce potent human immunodeficiency virus replication in the otherwise non-permissive CD4(+) naive T-cell compartment, we evaluated here the impact of IL-7 on the phenotype and functional potential of naive CD4(+) T cells in an attempt to understand the mechanism of this induction. We demonstrate that IL-7 mediated the up-regulation of CD25, CD95 and human leucocyte antigen-DR, while it did not alter the expression of CD45RO, CD69, CD40, or CD154. Examination of the cytokine profile of IL-7-treated naive T cells using a Type1/Type2 Proteome Array indicated a remarkable IL-7-mediated induction of interferon-gamma production, while the other cytokines evaluated (IL-2, IL-12, tumour necrosis factor-alpha, IL-4, IL-5, IL-10 and IL-13) were not affected. Intracellular staining of IL-7-treated naive T cells for interferon-gamma verified the Proteome data. IL-7 did not induce cell cycle proliferation of naive CD4(+) T cells, as evaluated by 7-AAD/pyronin immunostaining and carboxyfluorescein diacetate succinimidyl ester dye tracking. IL-7 treatment of naive CD4(+) T cells induced their ability to prime monocytes, as was indicated by induction of CD80 and CD86 expression on monocytes cocultured with IL-7-treated naive CD4(+) T cells. Collectively, these data indicate that IL-7 signalling is sufficient to phenotypically and functionally prime human CD4(+) naive T cells independent of antigen stimulation.

Growth hormone in T-lymphocyte thymic and postthymic development: a study in HIV-infected children

OBJECTIVES

Growth hormone (GH) plays a role in thymic function, and recombinant GH may stimulate thymopoiesis in HIV-infected individuals. We performed immunologic analyses in 26 antiretroviral-treated children matched for age, pubertal status, clinical parameters, and antiretroviral exposure who did or did not show an impaired response to GH-release stimulation tests with arginine + GH-releasing hormone.

RESULTS

The following abnormalities were found in GH-deficient compared with GH-nondeficient children after >4 years of therapy: CD4 count ( P = .02) and percentage ( P = .03), CD4 as percentage of normal cells for age ( P = .003), serum interleukin-7 concentration ( P = .02), and thymic volume ( P = .01). Naive CD4 (4+62+RA+ and 4+CCR7+RA+) and CD8 (8+CCR7+RA+) lymphocytes were lower in GH-deficient children ( P = .003; P = .007; and P = .02, respectively). Postthymic pathways were also impaired in GH-deficient children. Thus, central memory (4+CCR7+RA-) CD4+ cells were reduced ( P = .006), whereas effector memory (4+CCR7-RA-) CD4+ cells ( P = .002) and late effector CD8+ lymphocytes (8+CCR7-RA+ and 8+27-28-) ( P = .009 and P = .002, respectively) were increased in these children.

CONCLUSIONS

Growth hormone plays a role in thymic and postthymic pathways, and defective GH production may be associated with incomplete immunoreconstitution. Immunomodulant agents (including GH) could be useful in patients with defective GH production.

Will telomere erosion lead to a loss of T-cell memory?

Evidence is accumulating that elderly individuals are more susceptible to infection with organisms to which they were previously immune. This indicates that there might be a limit to the persistence of immune memory. This fact is particularly disturbing because the average life expectancy of humans has almost doubled in the past 200 years and is still increasing. We discuss mechanisms that might constrain the persistence of memory T cells and consider whether humans will suffer from memory T-cell exhaustion as life expectancy increases.

Exogenous IL-7 increases recent thymic emigrants in peripheral lymphoid tissue without enhanced thymic function

Interleukin 7 (IL-7) is critical in maintaining thymic-dependent and thymic-independent pathways of T-cell homeostasis. T-cell receptor (TCR) rearrangement excision circles (TRECs) have been used as markers for recent thymic emigrants (RTEs) in assessing human thymic function. To study the thymic and peripheral effects of IL-7 on RTEs, we measured TREC content and peripheral naive T-cell subsets and turnover in IL-7-treated mice. Short-term administration of IL-7 into thymus-intact mice resulted in increased total TREC numbers, consistent with RTE accumulation. Decreases in TREC frequency were attributable to dilution secondary to increased cell turnover. Significantly, IL-7 administration into thymectomized mice resulted in patterns of decreased TREC frequency and increased total TREC number similar to those in IL-7-treated thymus-intact mice. Distinct patterns of naive cell and RTE distribution among peripheral immune organs and altered expression of CD11a were observed following IL-7 treatment in thymus-intact and thymectomized mice. These results demonstrate (1) that total TREC number and not TREC frequency accurately reflects quantitative changes in RTEs; (2) that short-term IL-7 administration results in preferential accumulations of RTEs among peripheral immune organs, accounting for the increase in TRECs in the total peripheral lymphoid pool; and (3) no evidence for regulation of thymic function by short-term IL-7 administration. (Blood. 2004;104:1110-1119)

Human thymic stromal lymphopoietin promotes dendritic cell-mediated CD4+ T cell homeostatic expansion

T cell homeostasis is a self-regulating process for maintaining the size of the peripheral T cell pool. Although dendritic cells (DCs) seem to be important in T cell homeostasis, the molecular regulation of DC-mediated T cell homeostasis is unknown. We show that human DCs activated by thymic stromal lymphopoietin (TSLP) induced a robust expansion of autologous CD4(+) T cell populations, which depended on self peptide-major histocompatibility complex. The proliferating T cells adopted and maintained a central memory polyclonal phenotype and could differentiate into T helper type 1 or type 2 effector cells. These results, together with findings of TSLP expression in epithelial cells of mucosal lymphoid tissues and thymus, indicate that TSLP is involved in DC-mediated CD4(+) T cell homeostasis.

Naive T cells are maintained by thymic output in early ages but by proliferation without phenotypic change after age twenty

Analysing T-cell receptor excision circle numbers in healthy individuals we find a marked change in the source of naive T cells before and after 20 years of age. The bulk of the naive T cell pool is sustained primarily from thymic output for individuals younger than 20 years of age whereas proliferation within the naive phenotype is dominant for older individuals. Over 90% of phenotypically naive T cells in middle age are not of direct thymic origin. Moreover, this change in source of naive T cells is accompanied either by an increased death rate of T cells from the thymus or reduced thymic export. Modelling of these processes shows that new naive T cells of a thymic origin have a half-life of approximately 50 days before this change occurs, and that either the life-span of recent thymic emigrants (but not necessarily of all naive cells) decreases approximately threefold in middle age, or thymic production drops by this same amount. The decay rate of T-cell receptor excision circle levels for individuals over 20 years of age is consistent with the decay rate of the productive thymus. Our modelling suggests that at age 25, thymic export is responsible for 20% of naive T-cell production and that this percentage decreases with the 15.7 year half-life of the productive thymus so that by age 55 only 5% of naive production arises from thymic export.

IL-7 Stimulates T Cell Renewal Without Increasing Viral Replication in Simian Immunodeficiency Virus-Infected Macaques

The main failure of antiretroviral therapy is the lack of restoration of HIV-specific CD4(+) T cells. IL-7, which has been shown to be a crucial cytokine for thymopoiesis, has been envisaged as an additive therapeutic strategy. However, in vitro studies suggest that IL-7 might sustain HIV replication in thymocytes and T lymphocytes. Therefore, in the present study, we evaluated the effect of IL-7 on both T cell renewal and viral load in SIVmac-infected young macaques in the absence of antiretroviral therapy. This evaluation was conducted during the asymptomatic phase in view of a potential treatment of HIV patients. We show that IL-7 induces both a central renewal and a peripheral expansion of T lymphocytes associated with cell activation. No alarming modulation of the other hemopoietic cells was observed. No increase in the viral load was shown in blood or lymph nodes. These data strengthen the rationale for the use of IL-7 as an efficient immunotherapy in AIDS.

IL-15 drives neonatal T cells to acquire CD56 and become activated effector cells

Expression of one or more natural killer (NK) receptors on T cells may correlate with effector function. This study investigated the frequency of neonatal NK receptor-positive (NKR+) T cells and their expansionary properties with interleukin-2 (IL-2), IL-7, or IL-15. While cord blood contains significantly decreased frequencies of NKR+ T cells compared with adult blood, newborn CD56+CD3+ cells could be expanded 200-fold during culture with IL-15. By depleting CD56+ cells, we were able to determine that this expansion was due to a subpopulation of T cells acquiring CD56 expression. Moreover, CD56 acquisition was associated with a distinct CD8+CD25+ interferon gamma-positive (IFN-gamma+) phenotype. This property could therefore be exploited during bone marrow reconstitution and may partially account for the resilience of the newborn to infection.

Murine lymph node-derived stromal cells effectively support survival but induce no activation/proliferation of peripheral resting T cells in vitro

Little is known about the homeostatic mechanisms by which the levels of peripheral lymphocytes are maintained. The survival of naïve T cells in vivo must be maintained by some factors that have not been characterized in an in vitro culture system. In this study, we established a culture system of stromal cells derived from murine lymph nodes and investigated the action of the stromal cells in supporting the survival of resting T cells in vitro. Most of the T cells cocultured with the stromal cells did not die, and the supernatant of cultured stromal cells increase the viability of T cells. This T-cell survival-supporting activity was maintained for more than 7 days. Although interleukin (IL)-4, IL-6, IL-7, and interferon-beta also rescued peripheral T cells from spontaneous cell death, medium-soluble and heat-sensitive factor(s) derived from the stromal cells supported the survival of T cells more effectively and for a longer time than did these cytokines. T cells maintained in the culture system with the stromal cells appeared to remain in a resting G0/G1 state and did not show remarkable DNA synthesis. From these results, it is presumed that some soluble factor(s) other than the tested cytokines that have been identified as supporting T-cell survival are produced from lymph node stromal cells. These factor(s) play an important role in maintenance of resting T cells.

Homeostasis of naive and memory CD4+ T cells: IL-2 and IL-7 differentially regulate the balance between proliferation and Fas-mediated apoptosis

Cytokines play a crucial role in the maintenance of polyclonal naive and memory T cell populations. It has previously been shown that ex vivo, the IL-7 cytokine induces the proliferation of naive recent thymic emigrants (RTE) isolated from umbilical cord blood but not mature adult-derived naive and memory human CD4(+) T cells. We find that the combination of IL-2 and IL-7 strongly promotes the proliferation of RTE, whereas adult CD4(+) T cells remain relatively unresponsive. Immunological activity is controlled by a balance between proliferation and apoptotic cell death. However, the relative contributions of IL-2 and IL-7 in regulating these processes in the absence of MHC/peptide signals are not known. Following exposure to either IL-2 or IL-7 alone, RTE, as well as mature naive and memory CD4(+) T cells, are rendered only minimally sensitive to Fas-mediated cell death. However, in the presence of the two cytokines, Fas engagement results in a high level of caspase-dependent apoptosis in both RTE as well as naive adult CD4(+) T cells. In contrast, equivalently treated memory CD4(+) T cells are significantly less sensitive to Fas-induced cell death. The increased susceptibility of RTE and naive CD4(+) T cells to Fas-induced apoptosis correlates with a significantly higher IL-2/IL-7-induced Fas expression on these T cell subsets than on memory CD4(+) T cells. Thus, IL-2 and IL-7 regulate homeostasis by modulating the equilibrium between proliferation and apoptotic cell death in RTE and mature naive and memory T cell subsets.

IL-7-regulated homeostatic maintenance of recent thymic emigrants in association with caspase-mediated cell proliferation and apoptotic cell death

Homeostasis of T cells is essential to the maintenance of the T cell pool and TCR diversity. In this study, mechanisms involved in the regulation of cytokine-mediated expansion of naive T cells in the absence of Ag, in particular the role of caspase activation and susceptibility to apoptosis of recent thymic emigrants (RTEs), were examined. Low level caspase-8 and caspase-3 activation was detected in proliferating IL-7-treated cells in the absence of cell death during the first days of culture. Caspase inhibitors suppressed IL-7-induced expansion of RTEs. Low level expression of CD95 and blocking Ab experiments indicated that this early caspase activation was CD95 independent. However, CD95 levels subsequently became dramatically up-regulated on proliferating naive T cells, and these cells became susceptible to CD95 ligation, resulting in high level caspase activation and apoptotic cell death. These results show a dual role for caspases in proliferation and in CD95-induced cell death during Ag-independent expansion of RTEs. This method of cell death in IL-7-expanded RTEs is a previously unrecognized mechanism for the homeostatic control of expanded T cells.

Migration of primary and memory CD8 T cells

Our results indicate that a substantial proportion of the antimicrobial CD8 and CD4 T cell response is focused in non-lymphoid tissues. This finding makes teleological sense since maximum protection against infection is better served by the widespread presence of effector and memory cells. In the case of CD8 T cells, it appears that irrespective of the site at which initial activation of naive cells occurs, the end result is production of effector cells with broad migratory capabilities. Memory T cells perhaps have more restricted migratory abilities as compared to effector cells, although this needs to be tested definitively. Our results suggest that memory CD8 T cells in the intestinal LP may not be part of the recirculating pool of memory cells, though our data does not preclude the possibility that migrants from outside the mucosa contribute to the LP memory pool. Our data also demonstrates that CD8 memory T cells in non-lymphoid tissue exhibit heightened effector function as compared to their splenic counterparts. Whether these findings indicate the existence of distinct lineages of memory cells remains to be seen. The functional abilities of migrating memory cells could be modulated by the migration process and/or by the environmental milieu of a particular tissue. Although the development of CD8 memory T cells is a complex process requiring multiple signalling pathways, we identified IL-7 as an important player in memory generation. Much further work is needed to decipher the cellular and molecular mechanisms of memory induction as well as to learn the in vivo functional significance of memory cell subsets.