Negative regulators in homeostasis of naïve peripheral T cells

It is now apparent that naïve peripheral T cells are a dynamic population where active processes prevent inappropriate activation while supporting survival. The process of thymic education makes naïve peripheral T cells dependent on interactions with self-MHC for survival. However, as these signals can potentially result in inappropriate activation, various non-redundant, intrinsic negative regulatory molecules including Tob, Nfatc2, and Smad3 actively enforce T cell quiescence. Interactions among these pathways are only now coming to light and may include positive or negative crosstalk. In the case of positive crosstalk, self-MHC initiated signals and intrinsic negative regulatory factors may cooperate to dampen T cell activation and sustain peripheral tolerance in a binary fashion (on–off). In the case of negative crosstalk, self-MHC signals may promote survival through partial activation while intrinsic negative regulatory factors act as rheostats to restrain cell cycle entry and prevent T cells from crossing a threshold that would break tolerance.     

ERK5 regulation in naïve T-cell activation and survival

ERK5 has been implicated in regulating the MEF2-dependent genes Klf2 and nur77 downstream of the TCR and the maintenance of expression of CD62L on peripheral T cells. Based on this data, knockout of ERK5 would be predicted to compromise T-cell development and the maintenance of T cells in the periphery. Using an ERK5 conditional knockout, driven by CD4-CRE or Vav-CRE transgenes resulting in the loss of ERK5 in T cells, we have found that ERK5 is not required for T-cell development. In addition, normal numbers of T cells were found in the spleens and lymph nodes of these mice. We also find that TCR stimulation is not a strong signal for ERK5 activation in primary murine T cells. ERK5 was found to contribute to the induction of Klf2 but not nur77 mRNA following TCR activation. Despite the reduction in Klf2 mRNA, no effect was seen in ERK5 knockouts on either the mRNA levels for the Klf2 target genes CD62L, CCR7 and S1P, or the cell surface expression of CD62L. These results suggest that while ERK5 does contribute to Klf2 regulation in T cells, it is not essential for the expression of CD62L or T-cell survival.     

Stimulation of naïve T-cell adhesion and immunological synapse formation by chemokine-dependent and -independent mechanisms

Chemokines adsorbed to the cell surface play an important role in the initial interactions of T cells with endothelial cells, and may also have a role in T‐cell interactions with dendritic cells. Therefore, we examined the effect of surface‐adsorbed chemokines on the interaction of naïve murine splenic T cells with supported bilayers containing intercellular adhesion molecule (ICAM)‐1, or with bone marrow‐derived cultured dendritic cells in the presence and absence of relevant MHC–peptide complexes. Naïve T cells formed immunological synapses, defined as a ring of lymphocyte function associated (LFA)‐1–ICAM‐1 interactions surrounding a central cluster of MHC–peptide complexes, on supported planar bilayers containing ICAM‐1 and relevant MHC–peptide complexes. Chemokines stimulated an increase in the percentage of naïve cells that adhered to ICAM‐1, but did not increase the average number of LFA‐1–ICAM‐1 interactions in the contact area. In contrast, relevant MHC–peptide complexes resulted in a small increase in the proportion of interacting T cells, but stimulated an 8‐fold increase in the number of LFA‐1–ICAM‐1 interactions in each contact formed. Naïve T cells displayed a significant basal adhesion to bone marrow dendritic cells that was further increased when relevant chemokines were adsorbed to the dendritic cell surface. However, basal and antigen‐stimulated T‐cell adhesion to dendritic cells was not sensitive to pertussis toxin. Thus, there are chemokine‐independent mechanisms that initiate adhesion between T cells and dendritic cells.     

Isolation and characterization of naïve follicular dendritic cells

Follicular dendritic cells (FDC) are specialized antigen-presenting cells to cognate B cells in the follicle of the lymphoid tissues. FDC also support survival and proliferation of the B cells, leading to the germinal center formation. FDC therefore play a central role in humoral immune responses. However, molecular and functional characteristics of FDC are largely unknown, because it is difficult to isolate and analyze FDC due to a very small number of FDC in the lymphoid tissues and the fragility by mechanical and chemical stresses in vitro. In this report, we established a novel method for FDC isolation from the spleen of na?ve mice by flow cytometry and analyzed the phenotypical and functional characteristics. The isolated FDC, which accounted for ~0.2% of the spleen cells of na?ve mice, were CD45(-), FDC-M2(+), and ICAM-1(+), and supported the survival and LPS-induced proliferation of B cells. We also showed that a neutralizing antibody against B cell activating factor TNF family (BAFF) suppressed FDC-dependent B cell proliferation in the presence of LPS, but not survival, demonstrating the evidence that FDC-derived BAFF is involved in B cell proliferation.     

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.     

Age-related loss of naïve T cells and dysregulation of T-cell/B-cell interactions in human lymph nodes

In this study we analysed the effects of age on T and B lymphocytes in human lymph nodes by comparing lymphocyte subsets in paraffin sections from lymph node tissue taken from healthy young and elderly people. We demonstrate that the relative number of CD8(+) T cells decreases with age but that the relative number of CD4(+) T cells does not. There is also a very pronounced age-dependent loss of CD45RA(+) naive T cells. The number and size of follicles and the relative number of CD20(+) B cells are similar in young and elderly donors. For polymerase chain reaction analysis of the T-cell receptor (TCR) repertoire the TCR-gamma gene rearrangements were used as a marker of clonality. This is a reliable tool to detect not only clonal TCR-gammadelta populations but also TCR-alphabeta populations. Young donors with clonal T-cell expansions in their lymph node tissue do, however, have a higher number of CD20(+) B cells, a higher relative size of germinal centres compared to the follicle mantles and a higher number of immunoglobulin M-expressing cells than young donors without evidence of clonal T-cell expansions. Corresponding changes are not observed in elderly donors with clonal T-cell expansions in their lymph node tissue. In summary our findings demonstrate characteristic effects of aging on human lymph node tissue, the most striking feature being the depletion of naive T cells and the apparent dysregulation of T-cell/B-cell interactions in old age.     

Transfer of macrophage-derived mycobacterial antigens to dendritic cells can induce naïve T-cell activation

Mycobacteria are capable of surviving and replicating in host macrophages, where they can release antigenic material into the environment. However, unlike dendritic cells (DCs), macrophages do not appear to be capable of activating naïve T cells. Therefore, this work investigated antigen transfer between macrophages and DCs. We generated culture supernatants from bacille Calmette–Guérin (BCG)-infected and uninfected macrophages and then determined whether DCs could present these extracellular mycobacterial antigens to T cells. Here, we show that DCs pulsed with antigens released from BCG-infected macrophages can stimulate primed T cells in vitro and initiate naïve T-cell responses in vivo. These results suggest that antigen transfer can occur between macrophages and DCs.     

Chemokine regulation of naïve T cell traffic in health and disease

A central feature of the immune response is the precise spatio-temporal convergence of T cells and antigen presenting cells (APC) in particular microenvironments within secondary lymphoid organs (SLO). CCR7 and its ligands CCL19 and CCL21 have been identified as the gatekeepers for both na?ve T lymphocytes and dendritic cells (DC) to these defined anatomical compartments. A new perception on the regulation of lymphocyte traffic in lymph nodes (LN) has come from observations that sphingosine-1-phosphate (S1P) receptor agonists affect T cell entry and exit from these organs. Recent developments in intravital microscopy (IVM) techniques reveal unexpected autonomous random motion of lymphocytes within secondary lymphoid tissues, and provoke questions about the mechanisms that guide their compartmental navigation.     

Homeostasis of naïve, effector and memory CD8 T cells

Homeostatic control of CD8 T cell populations is essential for defense against infectious pathogens. Our understanding of the mechanisms that control na?ve, effector and memory T cell populations in the intact animal has increased significantly over the last several years. There have been some surprises. For example, peripheral tissues have been found to harbor unexpectedly large numbers of effector memory T cells. Also unexpected was the finding of programmed T cell proliferation following very brief exposure to antigen. These and other recent advances are summarized in the following review.     

Rapid static adhesion of human naïve neutrophil to naïve xenoendothelium under physiologic flow is independent of Galalpha1,3-gal structures

Adhesion interactions under flow have long been known to depend on applied wall shear stress. We investigated the ability of human na?ve neutrophils to adhere to xenogeneic endothelial cells under static and flow conditions. We demonstrate that human na?ve neutrophils bind to xenogeneic endothelial cells under flow conditions. This binding is dependent on the applied stress and is independent of Galalpha1,3-gal structures, ICAM-1, or its counter ligands LFA-1alpha and Mac-1. The binding was rapid and is characterized by stationary attachment with no obvious rolling or change in morphology. This binding leads to a transient increase in intracellular-free calcium levels in xenogeneic but not allogeneic-endothelial cells with occasional oscillations that persist long after the initial contact between the two cell types. Previous activation of xenoendothelium by autologous serum or human TNF-alpha augments binding of human na?ve neutrophils to the endothelial cells. Our data suggest novel interaction sites between the xenogeneic endothelial cells and human na?ve neutrophils.     

Familial hypogammaglobulinemia with high RTE and naïve T lymphocytes

Inflammation Research - Most of primary immunodeficiencies with hypogammaglobulinemia are associated with reduced memory B cells. T cell development may be interesting as well, but increased recent...     

Molecular mechanisms of TNF-alpha-induced apoptosis in naïve and memory T cell subsets

Aging in humans is associated with progressive decline in T cell function, hyperimmunoglobulinemia, increased prevalence of autoantibodies and decline in na?ve CD8(+) T cells and accumulation of memory T cells, which appears to be oligoclonal and display feature of senescence, that is, decreased replication, short telomere length and resistance to apoptosis. Recently memory T cells have been further subdivided into central and effector memory T cells, based upon their migratory and homing properties. They are identified by a number of cell surface makers. In this brief review we will discuss molecular mechanisms of apoptosis in na?ve and various types of memory T cells to possibly explain the changes observed in aging, which are very similar to certain autoimmune diseases.     

Expansion of activated human naïve T-cells precedes effector function

Naïve T‐cells divide and mature, both functionally and phenotypically, upon stimulation through the T‐cell receptor. Although much is known about the overall changes that occur in naïve cells upon TCR stimulation, and the different memory/effector populations that arise following stimulation, the relationship between cell division and functional and phenotypical changes that occur after activation is poorly understood. Here, we examine the early stages of human naïve and antigen‐experienced T‐cell activation, and the relationship between cell division and acquisition of effector function during the transition from resting antigen‐experienced or naïve T‐cells into effector cells. Stimulated naïve T‐cells proliferate prior to acquisition of effector function, as measured by cytokine production and expression of effector‐associated cell surface molecules. Additionally, we show that interlukin‐7 (IL‐7) can drive proliferation of naïve T‐cells without TCR:MHC peptide interactions. IL‐7 alone does not, however, drive the proliferation of antigen‐experienced T‐cells. Memory T‐cells will divide in response to exogenous IL‐7 but only in the presence of naïve T‐cells and IL‐2. This study contributes to the current understanding of the mechanistic differences between naïve and memory T‐cell responses by defining the functional and phenotypic changes that occur to T‐cells after stimulation.     

Anti-ergotypic T cells in naïve rats

T regulatory cells play an important role in regulating T cell responses. Anti-ergotypic T cells are a subset of regulatory T cells that proliferate in response to activation markers, ergotopes, expressed on activated, and not on resting syngeneic T cells. Here we report the presence of anti-ergotypic T cells in lymph nodes, spleens and thymuses of naive rats. The development of anti-ergotypic T cells appeared to be independent of antigen priming, as thymocytes from one-day old rats exhibited significant anti-ergotypic proliferative responses. The anti-ergotypic T cells were found to be of the CD8+ phenotype, and included both TCRalpha/beta+ and TCRgamma/delta+ T cells. The TCRgamma/delta+ anti-ergotypic T cells secreted IFNgamma and TNFalpha in response to activated T cells; the TCRalpha/beta+ T cells proliferated but did not secret detectable cytokines. We found that the interaction between the anti-ergotypic T cells and stimulator T cells required cell-to-cell contact between the T cells. Professional APCs were not needed. The response of the TCRalpha/beta+CD8+ anti-ergotypic T cells was MHC-I restricted and B7-CD28 dependent; the response of the TCRgamma/delta+ anti-ergotypic T cells was B7-CD28 dependent, but was not inhibited by antibodies to classical MHC-I or MHC-II molecules. The existence of anti-ergotypic T cells in naive animals suggests that these cells might have a role in the regulation and maintenance of the immune system.     

Susceptibility of naïve and subsets of memory T cells to apoptosis via multiple signaling pathways

Apoptosis is mediated via death receptor, the mitochondrial, and the endoplasmic reticulum pathway. Following activation of na?ve T cells with antigens, different subsets of memory T cells are generated. In this review we have discussed relative sensitivity/resistance of na?ve and different subsets of memory T cells to apoptosis via different signaling pathways. Molecular basis for differential sensitivity to apoptosis is discussed.     

Improved protocol for the isolation of naïve follicular dendritic cells

Follicular dendritic cells (FDCs) in lymphoid organs play an important role in the humoral immune response. However, because the isolation of FDCs is difficult due to their very small population size and fragility under mechanical and chemical stresses, the genetic and biochemical characteristics of FDCs remain unclear. Previously, we identified FDCs as ICAM-1⁺ cells in the CD45⁻ non-hematopoietic cell fraction from naïve mouse spleen after cell separation by means of digestion with a combination of enzymes. In the present study, using a new combination of enzymes, we found that FDCs are highly enriched in the CD45⁻ICAM-1⁺CD21/35⁺ cell fraction. CD45⁻ICAM-1⁺CD21/35⁺ cells in the mouse spleen retained an antigen administered in vivo for more than 7 days. Moreover, CD45⁻ICAM-1⁺CD21/35⁺ cells isolated from the spleen of mice administered with a cognate antigen enhanced the survival and proliferation of antigen-specific B cells in vitro. Our improved protocol for the isolation of naïve FDCs will be useful for the analysis of FDCs in vitro and in vivo.     

A three-cell model for activation of naïve T helper cells

It is generally assumed that the activation of naïve T helper (Th) cells is the result of a two‐cell interaction between the Th cell and a dendritic cell (DC) and that three signals are required. Signal one or stimulation is the recognition by the T‐cell receptor (TCR) of antigenic peptides presented by major histocompatibility complex (MHC) class II molecules. Signal two or co‐stimulation is mainly provided by the triggering of CD28 on the T cell by CD80 and CD86 molecules on the DC. Signal three or polarization directs T‐cell differentiation into various effector phenotypes such as Th1 and Th2. Both signals, two and three, are often assumed to result from the binding of microbial products or endogenous molecular danger signals to germline‐encoded receptors such as toll‐like receptors (TLR) on the DC. However, recent data challenge this two‐cell model by revealing that Th1 polarization requires the presence of interferon‐γ (IFN‐γ) provided by a third cell. I propose here a three‐cell model for naïve Th‐cell activation. In this model, delivery of signal three by the DC is dependent on help provided by other innate immune cells such as NK cells, NK T cells, γδ T cells, mast cells, eosinophils and basophils. The rationale behind this model is that the innate immune system has been designed by evolution to select an appropriate class of immune response to protect the host.