Overexpression of IL-21 promotes massive CD8+ memory T cell accumulation

The ability of IL-21 to promote in vitro T cell survival led us to investigate its biological activity in vivo. We report that overexpression of IL-21 in transgenic mice drives CD8(+) memory T cell accumulation with a concomitant reduction in naive T cell numbers. These memory T cells are functional, given their ability to rapidly produce IFN-gamma and proliferate following stimulation. Since the homeostasis of naive and memory T cells is controlled by cytokines, we evaluated whether IL-21 influences cytokine receptor expression. We show that IL-21 inhibits IL-7R expression on naive T cells in vitro, suggesting impaired IL-7-mediated naive T cell survival in IL-21-transgenic mice. In contrast, IL-7R expression on CD4(+) memory T cells is not affected, allowing their IL-7-dependent survival in IL-21-transgenic mice. Although IL-21 decreases IL-7R expression on CD8(+) memory T cells, this has no impact on their survival since their maintenance in the T cell pool is IL-7-independent. Rather, we demonstrate that CD8(+) memory T cells are receptive to IL-21 survival signals allowing for their accumulation in IL-21-transgenic mice. This study identifies new roles for IL-21 in T cell homeostasis and in the regulation of T cell responses to cytokines.     

IL-15 induces type 1 and type 2 CD4+ and CD8+ T cells proliferation but is unable to drive cytokine production in the absence of TCR activation or IL-12 / IL-4 stimulation in vitro.

Interleukin 15 (IL-15) is a pleiotropic cytokine produced principally by monocytes and affects both innate and acquired immunity. It has been shown that IL-15 is essential for the proliferation and maintenance of CD8+ memory cells but has little or no effect on naive CD8+ cells or CD4+ T cells. We report here, using an in vitro culture system of antigen-specific OVA TCR transgenic T cells as well as normal mouse T cell activated with anti-CD3 antibody that IL-15, at high concentrations, induced proliferation of both naive and memory CD4+ and CD8+ cells. IL-15 also enhanced the differentiation of type 1 (IFN-gamma-producing) and type 2 (IL-5-producing) CD4+ and CD8+ T cells under IL-12 and IL-4 driving conditions, respectively. However, IL-15 alone was not efficient in stimulating cytokine production of these cells in the absence of T cell subset driving cytokines (IL-12 or IL-4) and / or simultaneous TCR activation. Together, these results demonstrate that IL-15, at high dose, is a pan-T cell growth factor. The apparent requirement of IL-15 for the maintenance of memory CD8+ cell in vivo may reflect the exceptionally restricted nature of this subpopulation of cells for IL-15. The inability of IL-15 alone to stimulate cytokine synthesis also suggests that IL-15 on its own does not drive antigen-specific T cells to exhaustion. The levels of these cells are maintained by IL-15 and they are only mobilized to carry out effector functions when subsequently confronted with specific pathogens.     

Human CD4+ central and effector memory T cells produce IL-21: effect on cytokine-driven proliferation of CD4+ T cell subsets

IL-21 regulates certain functions of T cells, B cells, NK cells and dendritic cells. Although activated CD4(+) T cells produce IL-21, data identifying the specific CD4(+) T cell subsets that produce IL-21 are conflicting. In a previous study, mouse IL-21 message was detected in T(H)2, whereas human IL-21 (hIL-21) message was found in both T(H)1 and follicular helper T cells. To identify the IL-21-secreting cell populations in human, we established a hybridoma cell line producing an anti-hIL-21 mAb. Intracellular hIL-21-staining experiments showed that hIL-21 was mainly expressed in activated CD4(+) central memory T cells and in activated CD4(+) effector memory T cells, but not in activated CD4(+) naive T cells. Moreover, IL-21 was produced upon activation by some IFN-gamma-producing T(H)1-polarized cells and some IL-17-producing T(H)17-polarized cells, but not by IL-4-producing T(H)2-polarized cells. These results suggest that specific CD4(+) T cell populations produce IL-21. In the functional analysis, we found that IL-21 significantly enhanced the cytokine-driven proliferation of CD4(+) helper T cells synergistically with IL-7 and IL-15 without T cell activation stimuli. Taken together, IL-21 produced from CD4(+) memory T cells may have a supportive role in the maintenance of CD4(+) T cell subsets.     

Persistent human immunodeficiency virus-1 antigenaemia affects the expression of interleukin-7Ralpha on central and effector memory CD4+ and CD8+ T cell subsets

Interleukin (IL)-7 and its receptor (IL-7Ralpha) play important roles in regulating lymphopoiesis. Previous studies have reported that human immunodeficiency virus-1 (HIV-1) viraemia affects the expression of IL-7Ralpha, but its effects on CD4+ and CD8+ T cell memory subsets have not been studied. Using eight-colour flow cytometry, we compared the immunophenotypic patterns of CD4+ and CD8+ T cell subsets expressing IL-7Ralpha and activation markers, as well as circulating IL-7 levels, in three well-defined groups of HIV-1-infected subjects: successfully treated, viraemic and long-term non-progressor (LTNP). Compared with successfully treated and LTNP subjects, viraemic patients had reduced expression of IL-7Ralpha on both CD4+ and CD8+ T cells, particularly on central and effector memory T cell compartments, and substantially elevated expression of activation markers on CD8+ T cell subsets. Circulating IL-7 levels were correlated negatively with the number of CD4+ and CD8+ T cell subsets expressing IL-7Ralpha; these associations were stronger with CD4+ T cell subsets and mainly with central and effector memory cells. The expression of activation markers on CD4+ and CD8+ cell T subsets was not related to circulating IL-7 levels. A strong negative correlation was observed between central memory CD4+ or CD8+ T cells expressing IL-7Ralpha and those expressing activation markers, independently of IL-7 levels. Collectively, these results provide further insight on the role of unsuppressed viral load in disrupting the IL-7/IL-7Ralpha system and contributing to HIV-1 disease progression.     

Interleukin-7 mediates the homeostasis of naïve and memory CD8 T cells in vivo

The na?ve and memory T lymphocyte pools are maintained through poorly understood homeostatic mechanisms that may include signaling via cytokine receptors. We show that interleukin-7 (IL-7) plays multiple roles in regulating homeostasis of CD8+ T cells. We found that IL-7 was required for homeostatic expansion of na?ve CD8+ and CD4+ T cells in lymphopenic hosts and for CD8+ T cell survival in normal hosts. In contrast, IL-7 was not necessary for growth of CD8+ T cells in response to a virus infection but was critical for generating T cell memory. Up-regulation of Bcl-2 in the absence of IL-7 signaling was impaired after activation in vivo. Homeostatic proliferation of memory cells was also partially dependent on IL-7. These results point to IL-7 as a pivotal cytokine in T cell homeostasis.     

Functional and phenotypic characterization of tetanus toxoid-specific human CD4+ T cells following re-immunization

The formation of immunological memory is a hallmark of adaptive immunity and the goal of vaccination. For CD8(+ )T cells, successful generation of memory cells has been linked to IL-7 receptor alpha (IL-7Ralpha) expression, suggesting a role for IL-7 signaling, which in turn is important for preventing T cell apoptosis. We thus investigated the kinetics and changes of IL-7Ralpha and anti-apoptotic protein Bcl-2 expression levels in tetanus toxoid (TT)-specific CD4(+ )T cells at different time points prior and after TT re-immunization of TT-immune individuals. Prior to re-immunization, most TT-specific CD4(+ )T cells were high IL-2 producers, CD45RA(-)CCR7(+), IL-7Ralpha(high)Bcl-2(high) cells, resembling typical long-lived central memory cells. Already 5 days, and more importantly at the peak of the response, after TT re-immunization, a substantial fraction of these cells secreted also IFN-gamma, down-regulated CCR7, IL-7Ralpha and Bcl-2 and became Ki67 positive, resembling effector memory cells. In contrast, TT-specific CD4(+ )T cells found 60 days or later after re-immunization were again as baseline. Interestingly, a significant fraction of IL-7Ralpha(high)Bcl-2(high) TT-specific CD4(+ )T cells, i.e. the proposed memory cell precursors, remained stable at any time point upon re-immunization. Together, these results suggest that IL-7Ralpha expression levels might be a useful marker for identifying long-lived Ag-specific CD4(+ )T cells in memory T cells.     

TNF family ligands define niches for T cell memory

Immunological memory is a critical feature of the adaptive immune system and the underlying principal behind vaccination. The mechanisms that maintain memory T cell survival between the initial and subsequent encounter with antigen remain incompletely defined. Although the cytokines IL-15 and IL-7 are important in memory T cell homeostasis, additional signals by way of TNFR family members are required for maximal maintenance of T cell memory. Here we propose a unifying model in which subsets of TNF family ligands distinguish the competitive niches for maintenance of CD4 versus CD8 T cell memory. Understanding the unique 'memory niches' defined by TNF family ligand expression will provide new insights into the mechanisms of memory T cell maintenance.     

Phenotypic heterogeneity of antigen-specific CD4 T cells under different conditions of antigen persistence and antigen load

The factors responsible for the phenotypic heterogeneity of memory CD4 T cells are unclear. In the present study, we have identified a third population of memory CD4 T cells characterized as CD45RA(+)CCR7(-) that, based on its replication history and the homeostatic proliferative capacity, was at an advanced stage of differentiation. Three different phenotypic patterns of memory CD4 T cell responses were delineated under different conditions of antigen (Ag) persistence and load using CD45RA and CCR7 as markers of memory T cells. Mono-phenotypic CD45RA(-)CCR7(+) or CD45RA(-)CCR7(-) CD4 T cell responses were associated with conditions of Ag clearance (tetanus toxoid-specific CD4 T cell response) or Ag persistence and high load (chronic HIV-1 and primary CMV infections), respectively. Multi-phenotypic CD45RA(-)CCR7(+), CD45RA(-)CCR7(-) and CD45RA(+)CCR7(-) CD4 T cell responses were associated with protracted Ag exposure and low load (chronic CMV, EBV and HSV infections and HIV-1 infection in long-term nonprogressors). The mono-phenotypic CD45RA(-)CCR7(+) response was typical of central memory (T(CM)) IL-2-secreting CD4 T cells, the mono-phenotypic CD45RA(-)CCR7(-) response of effector memory (T(EM)) IFN-gamma-secreting CD4 T cells and the multi-phenotypic response of both IL-2- and IFN-gamma-secreting cells. The present results indicate that the heterogeneity of different Ag-specific CD4 T cell responses is regulated by Ag exposure and Ag load.     

The role of cytokines in T-cell memory in health and disease

Upon stimulation with their cognate antigen, naive T cells undergo proliferation and differentiation into effector cells, followed by apoptosis or survival as precursors of long-lived memory cells. These phases of a T-cell response and the ensuing maintenance of memory T cells are shaped by cytokines, most notably interleukin-2 (IL-2), IL-7, and IL-15 that share the common γ chain (γ_c) cytokine receptor. Steady-state production of IL-7 and IL-15 is necessary for background proliferation and homeostatic survival of CD4⁺ and CD8⁺ memory T cells. During immune responses, augmented levels of IL-2, IL-15, IL-21, IL-12, IL-18, and type-I interferons determine the memory potential of antigen-specific effector CD8⁺ cells, while increased IL-2 and IL-15 cause bystander proliferation of heterologous CD4⁺ and CD8⁺ memory T cells. Limiting availability of γ_c cytokines, reduction in regulatory T cells or IL-10, and persistence of inflammation or cognate antigen can result in memory T cells, which fail to become cytokine-dependent long-lived cells. Conversely, increased IL-7 and IL-15 can expand memory T cells, including pathogenic tissue-resident memory T cells, as seen in lymphopenia and certain chronic-inflammatory disorders and malignancies. These abovementioned factors impact immunotherapy and vaccines directed at memory T cells in cancer and chronic infection.     

IL-7 modulates B cells survival and activation by inducing BAFF and CD70 expression in T cells

Interleukin-7 (IL-7) promotes the maintenance and activation of peripheral T cells, whereas it does not act directly on mature B cells due to the lack of IL-7Rα expression on these. We report here that, in spite of the insensitivity of B cells to IL-7, high concentration of IL-7 can lead to increased B cell survival and antibody production in the presence of T cells, without the use of any further B cell stimulatory signal. IL-7 promoted B cell activation through inducing CD70 expression on resting T cells, particularly on CD4+ memory cells. The interaction of CD70 molecules with the B cell costimulatory receptor CD27 led to B cell proliferation, the accumulation of CD38 + CD20- plasmablasts and antibody production. In addition, IL-7 treatment induced BAFF secretion from resting peripheral T cells thereby promoting B cell survival. IL-7 levels can increase in lymphopenic conditions, in autoimmune diseases or in patients receiving T cell regenerative IL-7 therapy. Based on our findings high IL-7 levels can lead to increased B cell activation by inducing the B cell regulatory proteins CD70 and BAFF in resting T cells. Such activity might be beneficial in short term immune-stimulatory IL-7 therapies; permanently increased IL-7 levels, on the other hand, can contribute to impaired B cell tolerance.     

Blood monocytes, myeloid dendritic cells and the cytokines interleukin (IL)-7 and IL-15 maintain human CD4+ T memory cells with mixed helper/regulatory function

The number and function of human T cells in the periphery are regulated by homeostatic signals received from antigen-presenting cells (APCs) and the common gamma chain (gammac) cytokines interleukin (IL)-7 and IL-15. We found that, in the absence of introduced antigen, blood monocytes or myeloid dendritic cells (MDCs) in the presence of IL-7 and IL-15 (IL-7/IL-15) can regulate CD4(+) T memory (Tm) cell numbers by polyclonal cell proliferation. The dynamics of CD4(+) Tm cell proliferation, in the presence of IL-7/IL-15, was dependent on contact with MDCs and to a lesser extent on contact with monocytes. IL-7/IL-15 either alone or combined with monocytes or MDCs enhanced the proportion of CD4(+) Tm cells with activated and effector phenotype and diminished the helper function of CD4(+) Tm cells. These CD4(+) Tm cells, preconditioned with IL-7/IL-15 alone or with monocytes or MDCs and IL-7/IL-15, reduced T cell-dependent immunoglobulin M (IgM) and IgG responses. This appeared to be a contact-dependent effect involving a reduction in antibody-producing CD27(+) B memory cells, but contact-independent suppression by soluble factors also contributed to the antibody-producing capacity of CD27(+) B memory cells. These results indicate that blood monocytes, MDCs and the cytokines IL-7/IL-15 contribute to homeostasis of CD4(+) Tm cells by regulating their number, activation state and helper/suppressor (regulatory) function. In healthy individuals, this mode of regulating CD4(+) Tm cell homeostasis may provide a basis for the control of autoimmune responses.     

Non-redundant role for IL-7R signaling for the survival of CD8+ memory T cells

IL-7 and IL-15 are important cytokines for CD8 memory T cells. However, the extent that IL-7 is essential for CD8 T cell memory remains unclear because blocking IL-7 in vivo results in near complete inhibition of T cell development with the few mature T cells exhibiting functional abnormalities. To bypass this complication, CD8 memory development was examined utilizing a mouse model where transgenic IL-7Ralpha was selectively expressed in the thymus of IL-7Ralpha(-/-) mice. T cell development was corrected but the resulting peripheral T cells were essentially IL-7 non-responsive. Activation of IL-7R-defective OT-I CD8(+) T cells with OVA(257-264) and IL-2 readily yielded CTL. Upon further culture with IL-15, these CTL expressed phenotypic and functional properties of central memory-like cells. Thus, IL-7R-defective CD8(+) T cells do not exhibit intrinsic defects in effector or memory development. When IL-7R-defective OT-I CTL were adoptively transferred into normal or IL-15(-/-) recipient mice in a non-inflammatory setting, they converted into memory-like cells, but did not persist, which was even more striking in IL-15(-/-) recipients. This poor persistence was rescued after expression of transgenic Bcl-2 in IL-7R-defective OT-I T cells. Collectively, these data indicate that IL-7 is non-redundantly required for the survival of CD8 memory T cells.     

Gamma chain required for naïve CD4+ T cell survival but not for antigen proliferation

Lymphoid homeostasis is required to ensure immune responsiveness and to prevent immunodeficiency. As such, the immune system must maintain distinct populations of na?ve T cells that are able to respond to new antigens as well as memory T cells specific to those antigens it has already encountered. Though both na?ve and memory T cells reside in and traffic through secondary lymphoid organs, there is growing evidence that the two populations may be regulated differently. We show here that na?ve T cell survival and memory T cell survival have different requirements for cytokines (including the interleukins IL-2, IL-4, IL-7, IL-9 and IL-15) that use the common cytokine receptor gamma chain (gamma c). Using monoclonal populations of antigen-specific CD4+ T cells, we found that na?ve T cells cannot survive without gamma c, whereas memory T cells show no such requirement. In contrast, neither na?ve nor gamma c-deficient memory T cells were impaired in their ability to proliferate and produce cytokines in response to in vivo antigenic stimulation. These data call into question the physiological role of gamma c-dependent cytokines as T cell growth factors and show that na?ve and memory CD4+ T cell survival is maintained by distinct mechanisms.     

Regulation of mature T cell homeostasis

The overall size and the composition of the mature T cell pool are regulated by homeostatic mechanisms. Recent work has revealed that homeostatic signals are received from contact with two members of the common gamma chain family of cytokines, IL-7 and IL-15, and from self-MHC/peptide ligands. In essence, homeostasis of na?ve T cells is regulated by IL-7 and self-MHC/peptide ligands and homeostasis of memory CD8 cells is controlled by IL-7 and IL-15. All of these signals also appear to be important to a varying degree for homeostasis of memory CD4 cells, but the details are less well understood than for other cell type.     

Cytokine-driven proliferation and differentiation of human naïve,central memory and effector memory CD4+ T cells

Memory T lymphocytes divide in vivo in the absence of antigen maintaining a pool of central memory (T(CM)) and effector memory cells (T(EM)) with distinct effector function and homing capacity. We compared human CD4+ na?ve T, T(CM) and T(EM) cells for their capacity to proliferate in response to cytokines, which have been implicated in T cell homeostasis. Interleukin (IL)-7 and IL-15 expanded with very high efficiency T(EM), while T(CM) were less responsive and na?ve T cells did not respond at all. Dendritic cell (DC)-derived cytokines allowed na?ve T cells to respond selectively to IL-4 and potently boosted the response of T(CM) to IL-7 and IL-15 by increasing the expression of the IL-2/IL-15Rss and the common gamma chain (gamma(c)). The ERK and the p38 MAP kinases were selectively required for TCR and cytokine-driven proliferation, respectively. Importantly, in cytokine-driven cultures T(CM) proliferated and some of the proliferating cells acquired effector function and non-lymphoid tissue homing capacity. Ex vivo BrdU incorporation experiments showed that both T(CM) and T(EM) proliferated under steady state conditions in vivo. Altogether these results provide a plausible mechanism for the maintenance of a polyclonal and functionally diverse repertoire of human CD4+ memory T cells and for a sustained antigen-independent generation of T(EM) from a pool of T(CM) cells.     

Bim mediates apoptosis of CD127(lo) effector T cells and limits T cell memory

Following an acute T cell response, most activated effector cells die, while some survive and become memory cells. The pro-apoptotic Bcl-2 family member, Bcl-2 interacting mediator of death (Bim) is critical for eliminating most effector T cells, while expression of CD127 (IL-7Ralpha) has been proposed to mark effector cells destined to become memory cells. Here, we examined the effects of Bim on the death of effector T cells in relationship to CD127 expression and on development of T cell memory following lymphocytic choriomeningitis virus (LCMV) infection. We found that large numbers of CD127(lo) LCMV-specific CD4(+) and CD8(+) T cells were lost in wild-type mice, but were spared in Bim(-/-) mice. Further, while the numbers of CD127(hi) T cells declined only slightly during contraction of the response in wild-type mice, they increased significantly in Bim(-/-) mice due to re-expression of CD127 on CD127(lo) T cells that had avoided apoptosis. Functional memory T cells were significantly increased in Bim(-/-) mice; however, they underwent a slow attrition due to decreased proliferative renewal. Taken together, these data suggest that the absence of Bim-mediated death of LCMV-specific CD4(+) and CD8(+) T cells in vivo can increase T cell memory, but other homeostatic mechanisms control the long-term maintenance of memory cells.     

Differential role of IL-2R signaling for CD8+ T cell responses in acute and chronic viral infections

IL-2 is a cytokine with multiple and even divergent functions; it has been described as a key cytokine for in vitro T cell proliferation but is also essential for down-regulating T cell responses by inducing activation-induced cell death as well as regulatory T cells. The in vivo analysis of IL-2 function in regulating specific T cell responses has been hampered by the fact that mice deficient in IL-2 or its receptors develop lymphoproliferative diseases and/or autoimmunity. Here we generated chimeric mice harboring both IL-2R-competent and IL-2R-deficient T cells and assessed CD8+ T cell induction, function and maintenance after acute or persistent viral infections. Induction and maintenance of CD8+ T cells were relatively independent of IL-2R signaling during acute/resolved viral infection. In marked contrast, IL-2 was crucial for secondary expansion of memory CD8+ T cells and for the maintenance of virus-specific CD8+ T cells during persistent viral infections. Thus, depending on the chronicity of antigen exposure, IL-2R signaling is either essential or largely dispensable for induction and maintenance of virus-specific CD8+ T cell responses.     

Differential expression of the cytokine receptors for human interleukin (IL)-12 and IL-18 on lymphocytes of both CD45RA and CD45RO phenotype from tonsils, cord and adult peripheral blood

The objective of this study was to demonstrate the variable expression of cytokine receptors on naive versus memory human CD4+ T cell subpopulations in tonsillar tissue, cord blood and adult blood. We prove that the receptors for both interleukin (IL)-12 and IL-18 are expressed exclusively on memory T cells. This observation was seen not only on the CD45RO+ memory T cells but also on a significant percentage of the CD45RA+, CD62L-, CD27- and CCR7- populations. Furthermore, CD45RA+ CD62L+, CD27+ or CCR7+ CD4+ T cells that expressed IL-12Rbeta1 and IL-18Ralpha did not express CD31, a marker for recent thymic emigrants. We reveal that cord blood lymphocytes do not express IL-12Rbeta1 whereas IL-18Ralpha expression was detected at low levels. Importantly, the IL-12Rbeta2 signalling chain, which is absent in all resting T cells, was up-regulated in both CD45RA+ and CD45RO+ T cells as a result of stimulation with anti-CD3 and anti-CD28 in vitro. This observed up-regulation was, however, restricted to 80% of the total CD4+ population. Finally, a very small proportion of the CD4+ CD45RO+ tonsillar T cells expressed the IL-12 and IL-18 receptors, thereby establishing the differential expression of these receptors between peripheral and tonsillar memory T cell subpopulations.