Notch信号通路在T淋巴细胞发育和功能中的作用

T淋巴细胞的发育是由严格的遗传程序调控的精细过程,而Notch信号通路是这一高度复杂遗传程序中最为关键的一环.Notch还与T细胞的激活和功能密切相关.此外,大量的临床和实验室研究证实,Notch信号途径的关键分子是多种人类遗传病的致病基因.Notch1受体基因的染色体易位或点突变是大多数成人T淋巴母细胞白血病的原因.该文将结合此研究室的工作,综述这一领域的相关进展.     

EphrinB1-EphB signaling regulates thymocyte-epithelium interactions involved in functional T cell development

The Eph and ephrin families are involved in numerous developmental processes. Recently, an increasing body of evidence has related these families with some aspects of T cell development. In the present study, we show that the addition of either EphB2-Fc or ephrinB1-Fc fusion proteins to fetal thymus organ cultures established from 17-day-old fetal mice decreases the numbers of both double-positive (CD4(+)CD8(+)) and single-positive (both CD4(+)CD8(-) and CD4(-)CD8(+)) thymocytes, in correlation with increased apoptosis. By using reaggregate thymus organ cultures formed by fetal thymic epithelial cells (TEC) and CD4(+)CD8(+) thymocytes, we have also demonstrated that ephrinB1-Fc proteins are able to disorganize the three-dimensional epithelial network that in vivo supports the T cell maturation, and to alter the thymocyte interactions. In addition, in an in vitro model, Eph/ephrinB-Fc treatment also decreases the formation of cell conjugates by CD4(+)CD8(+) thymocytes and TEC as well as the TCR-dependent signaling between both cell types. Finally, immobilized EphB2-Fc and ephrinB1-Fc modulate the anti-CD3 antibody-induced apoptosis of CD4(+)CD8(+) thymocytes in a process dependent on concentration. These results therefore support a role for Eph/ephrinB in the processes of development and selection of thymocytes as well as in the establishment of the three-dimensional organization of TEC.     

The role of ICOS in the development of CD4 T cell help and the reactivation of memory T cells

We have addressed the role of the inducible costimulator (ICOS) in the development of T cell help for B cells and in the generation, survival and reactivation of memory CD4 T cells and B cells. We find that while T cell help for all antibody isotypes (including IgG2c) is impaired in ICOS knockout (ICOS-KO) mice, the IFN-gamma response is little affected, indicating a defect in helper function that is unrelated to cytokine production. In addition, the ICOS-negative T cells do not accumulate in B cell follicles. Secondary (memory), but not primary, clonal proliferation of antigen-specific B cells is impaired in ICOS-KO mice, as is the generation of secondary antibody-secreting cells. Analysis of endogenous CD4 memory cells in ICOS-KO mice, using MHC class II tetramers, reveals normal primary clonal expansion, formation of memory clones and long-term (10 wk) survival of memory cells, but defective expansion upon reactivation in vivo. The data point to a role of ICOS in supporting secondary, memory and effector T cell responses, possibly by influencing cell survival. The data also highlight differences in ICOS dependency of endogenous T cell proliferation in vivo compared to that of adoptively transferred TCR-transgenic T cells.     

Differential T‐cell receptor signals for T helper cell programming

Upon encounter with their cognate antigen, naive CD4 T cells become activated and are induced to differentiate into several possible T helper (Th) cell subsets. This differentiation depends on a number of factors including antigen‐presenting cells, cytokines and co‐stimulatory molecules. The strength of the T‐cell receptor (TCR) signal, related to the affinity of TCR for antigen and antigen dose, has emerged as a dominant factor in determining Th cell fate. Recent studies have revealed that TCR signals of high or low strength do not simply induce quantitatively different signals in the T cells, but rather qualitatively distinct pathways can be induced based on TCR signal strength. This review examines the recent literature in this area and highlights important new developments in our understanding of Th cell differentiation and TCR signal strength.     

The strength of BCR signaling shapes terminal development of follicular helper T cells in mice

Antibody production is key for effective immune response and relies on follicular helper T (Tfh) cells. B cell‐Tfh cell interactions result either in an extra‐follicular low affinity B‐cell response or in germinal center reactions producing high‐affinity memory B cells and long‐lived plasma cells. As Tfh cells influence B‐cell commitment, it also became clear that B cells influence these interactions in ways that still remain unresolved. We observed that strong BCR signals decreased Tfh‐cell differentiation in vitro, which correlated with decreased expression of ICOS‐L at the surface of stimulated B cells. Further, we comprehensively demonstrated that ICOS‐L expression correlated with the level of Tfh differentiation irrespective of antigen presentation at the surface of activated B cells. Our in vivo experiments could show that immunization with a high‐affinity antigen for B cells resulted in much less Tfh development than immunization with low‐affinity antigen. Furthermore, blocking ICOS‐L in vivo inhibited Tfh development when using low‐affinity antigen. Altogether, these results indicate that BCR affinity shapes Tfh‐cell development in part through ICOS/ICOS‐L interactions. Ultimately, we reveal new depths in the B cell‐Tfh cell crosstalk that could eventually result in better vaccine protocols.     

Pathways of memory CD8+ T‐cell development

CD8⁺ T‐cell responses must have at least two components, a replicative cell type that proliferates in the secondary lymphoid tissue and that is responsible for clonal expansion, and cytotoxic cells with effector functions that mediate the resolution of the infection in the peripheral tissues. To confer memory, the response must also generate replication‐competent T cells that persist in the absence of antigen after the primary infection is cleared. The current models of memory differentiation differ in regards to whether or not memory CD8⁺ T cells acquire effector functions during their development. In this review we discuss the existing models for memory development and the consequences that the recent finding that memory CD8⁺ T cells may express granzyme B during their development has for them. We propose that memory CD8⁺ T cells represent a self‐renewing population of T cells that may acquire effector functions but that do not lose the naïve‐like attributes of lymphoid homing, antigen‐independent persistence or the capacity for self‐renewal.     

Once a killer,always a killer: from cytotoxic T cell to memory cell

Summary: The control of the differentiation pathways followed by responding CD8⁺ T cells to produce protective memory cells has been intensely studied. Recent developments have identified heterogeneity at the effector cytotoxic T-lymphocyte level within which a bona fide memory cell precursor has emerged. The challenge now is to identify the cellular and molecular factors that control this developmental pathway. This review considers aspects of the regulation of the induction of effectors, the transition of effectors to memory cells, and the dynamics of the memory population.     

Distinct T helper cell dependence of memory B‐cell proliferation versus plasma cell differentiation

Several memory B‐cell subclasses with distinct functions have been described, of which the most effective is the class‐switched (CS) memory B‐cell population. We have previously shown, using virus‐like particles (VLPs), that the proliferative potential of these CS memory B cells is limited and they fail to re‐enter germinal centres (GCs). However, VLP‐specific memory B cells quickly differentiated into secondary plasma cells (PCs) with the virtue of elevated antibody production compared with primary PCs. Whereas the induction of VLP⁺ memory B cells was strongly dependent on T helper cells, we were wondering whether re‐stimulation of VLP⁺ memory B cells and their differentiation into secondary PCs would also require T helper cells. Global absence of T helper cells led to strongly impaired memory B cell proliferation and PC differentiation. In contrast, lack of interleukin‐21 receptor‐dependent follicular T helper cells or CD40 ligand signalling strongly affected proliferation of memory B cells, but differentiation into mature secondary PCs exhibiting increased antibody production was essentially normal. This contrasts with primary B‐cell responses, where a strong dependence on CD40 ligand but limited importance of interleukin‐21 receptor was seen. Hence, T helper cell dependence differs between primary and secondary B‐cell responses as well as between memory B‐cell proliferation and PC differentiation.     

The tyrosine phosphatase SHP-2 regulates differentiation and apoptosis of individual primary T lymphocytes

Although phosphatases are key players of intracellular processes, not much is known about the phosphatase SHP-2 during T cell differentiation. Here we show that ectopic over-expression of SHP-2 in primary T helper cells directly reduced the frequency of individual lymphocytes expressing pro-inflammatory cytokines after antigen-specific stimulation by a mechanism impairing activation of protein kinase C. In addition we demonstrate that SHP-2 mediates enhanced migration upon CXCR4 signaling in a G-protein-dependent manner. Most strikingly, SHP-2 mediated a dramatic increase in apoptosis by highly enhanced activation of caspases. Co-immunoprecipitations of SHP-2 and c-Cbl from primary T helper cells demonstrated that SHP-2 strongly interacts with the ubiquitin ligase c-Cbl, indicating that c-Cbl could mediate the negative signals of SHP-2. Our results show that SHP-2 signal transduction regulates central checkpoints of T cell differentiation by the activation of distinct signaling cascades.     

CD38 cross-linking enhances TLR-induced B cell proliferation but decreases IgM plasma cell differentiation

It is becoming increasingly clear that the regulation of proliferation and differentiation of B cells to plasma cells involves the integration of a variety of intracellular signals provided by receptors of both the adaptive and innate immune system. The cross-linking of the surface molecule CD38 induces calcium mobilization, protein phosphorylation and NF-kappaB translocation into the nucleus, ultimately leading to proliferation and isotype switching toward IgG1. Here we describe (a) the effect on B cell activation of stimulating through both CD38 and Toll-like receptors 4, 7 and 9; and (b) that CD38 cross-linking increases the number of proliferating cells and the rate of proliferation in LPS-stimulated B cells by a Bruton's tyrosine kinase- and protein kinase C-dependent mechanism. In contrast, CD38 cross-linking reduces the number of cells committed to IgM plasma cell differentiation as measured by the number of CD138+ cells, antibody secretion, and the expression of PAX5, Bcl6 and Blimp-1. Since a putative ligand for CD38 is expressed by germinal center follicular dendritic cells, and CD38 expression is down-regulated in germinal center B cells, we speculate that CD38 might participate in the outcome of post-germinal center antibody responses.     

In vivo treatment with anti B-220 monoclonal antibody affects T and B cell differentiation.

The B220 cell marker is expressed on B cells and on T cell precursors. In order to determine the involvement of the B220 antigen on murine lymphoid differentiation, we treated 5-10-week-old mice periodically with a specific anti-B220 antibody, RA3-6B2, a non-cytolytic IgG2b. After the third injection, a significant reduction (P less than 0.02) in the number of thymocytes and less dramatically in the number of splenocytes was observed. This reduction was predominantly due to a decrease of cells carrying the following markers: Thy-1.2+, Lyt-1+, Lyt-2.3+, L3T4+, and asGM1+. Mitogenic response to concanavalin A, phytohaemagglutinin and lipopolysaccharide, mixed lymphocyte reaction, cytotoxic T cell activity, and plaque-forming cell generation were significantly decreased after the treatment (P less than 0.01). These results show that the in vivo treatment with anti-B220 monoclonal antibody reduced the number of T and B cells and modified their functional activity. This suggests that the B220 antigen is involved in the maturation of both T and B cells.     

T cell binding to B lymphoid cell lines in humans: a marker for T-B cell interaction?

Binding of human circulating T cells to established normal and malignant B cell lines results in rosette formation. The percentage of B cells, circulating T cells, and thymocytes able to bind to the B-LCL Raji were 0%, 59 +/- 4% and 61 +/- 6%, respectively. The percentage of rosettes formed between Raji cells and circulating mononuclear cells from 92 normal individuals was 27.8 +/- 5.3%, and remained stable over several months. This phenomenon seems to involve relatively mature B cells, and a T cell marker which appears early in T cell ontogeny. In the peripheral blood, most of the B-LCL binding T cells exhibit a 'helper-inducer' phenotype, as determined with the monoclonal antibodies Leu 3a and OKT4. However, a significant percentage of T cells with so-called 'cytotoxic-suppressor' markers (Leu 2a and OKT8) also bind to B-LCL. The T cells involved in this morphological interactive reaction with B cells might conceivably be specifically involved in regulating B cell functions. Enumeration of this particular subset may be useful in conditions where abnormal T-B cell interactions are suspected.     

T follicular helper cell heterogeneity: Time,space, and function

T follicular helper (Tfh) cells play a crucial role in orchestrating the humoral arm of adaptive immune responses. Mature Tfh cells localize to follicles in secondary lymphoid organs (SLOs) where they provide help to B cells in germinal centers (GCs) to facilitate immunoglobulin affinity maturation, class‐switch recombination, and generation of long‐lived plasma cells and memory B cells. Beyond the canonical GC Tfh cells, it has been increasingly appreciated that the Tfh phenotype is highly diverse and dynamic. As naive CD4⁺ T cells progressively differentiate into Tfh cells, they migrate through a variety of microanatomical locations to obtain signals from other cell types, which in turn alters their phenotypic and functional profiles. We herein review the heterogeneity of Tfh cells marked by the dynamic phenotypic changes accompanying their developmental program. Focusing on the various locations where Tfh and Tfh‐like cells are found, we highlight their diverse states of differentiation. Recognition of Tfh cell heterogeneity has important implications for understanding the nature of T helper cell identity specification, especially the plasticity of the Tfh cells and their ontogeny as related to conventional T helper subsets.     

Inducible turnover of optineurin regulates T cell activation

Optineurin (Optn) is an adaptor protein with homology to NF-κB essential modulator (NEMO), the regulatory subunit of the IκB kinase (IKK) complex. Dysregulation of Optn has been linked to neurodegenerative, autoimmune and bone diseases. Optn shares a high degree of homology with NEMO, but is not part of the same high-molecular weight complex containing IKKα and IKKβ. Despite its homology with NEMO and the fact that it has been the subject of extensive study in several cell types, there are no published studies addressing the role of Optn during T cell activation. Here we demonstrate that ectopic expression of Optn down-regulates TCR-induced NF-κB activation and TNF-α production, in a manner dependent on ubiquitin-binding. Conversely, knock-down of Optn enhances NF-κB activation and the production of TNF-α. Consistent with a negative regulatory role for this protein, we observed transient loss of Optn after TCR stimulation in both cell lines and in primary murine T cells. The acute loss of Optn appears to be due to both protein degradation and exocytosis, the latter via activation-induced exosomes. This study therefore provides novel information regarding the role of Optn during TCR activation, suggesting the possible importance of Optn during inflammation and/or autoimmune diseases.     

Persistence of memory B cells in mice deprived of T cell help

The influence of T cell help on the induction, maintenance, and recall of B cell memory to a T cell-dependent antigen was studied in mice. The antigen was a hapten coupled to a protein carrier, and helper T cells were eliminated before, during, or after immunization by treatment of the animals with anti-CD4 antibodies. B cell memory was quantified in an adoptive cell transfer system, transferring B cells from the primed animals together with carrier-primed syngeneic T cells. Anti-CD4 treatment completely inhibited the induction of primary and secondary responses and of B cell memory. In contrast, it did not affect established B cell memory over a period of at least 6 weeks. Consequently, this suggests that within this time range B cell memory to a T cell-dependent antigen does not rely on the continuous recruitment of naive B cells into the memory compartment, and memory B cells need little or no (antigen-mediated) T cell help in order to persist.     

P21‐activated kinase 2 is essential in maintenance of peripheral Foxp3+ regulatory T cells

The p21‐activated kinase 2 (Pak2), an effector molecule of the Rho family GTPases Rac and Cdc42, regulates diverse functions of T cells. Previously, we showed that Pak2 is required for development and maturation of T cells in the thymus, including thymus‐derived regulatory T (Treg) cells. However, whether Pak2 is required for the functions of various subsets of peripheral T cells, such as naive CD4 and helper T‐cell subsets including Foxp3⁺ Treg cells, is unknown. To determine the role of Pak2 in CD4 T cells in the periphery, we generated inducible Pak2 knockout (KO) mice, in which Pak2 was deleted in CD4 T cells acutely by administration of tamoxifen. Temporal deletion of Pak2 greatly reduced the number of Foxp3⁺ Treg cells, while minimally affecting the homeostasis of naive CD4 T cells. Pak2 was required for proliferation and Foxp3 expression of Foxp3⁺ Treg cells upon T‐cell receptor and interleukin‐2 stimulation, differentiation of in vitro induced Treg cells, and activation of naive CD4 T cells. Together, Pak2 is essential in maintaining the peripheral Treg cell pool by providing proliferation and maintenance signals to Foxp3⁺ Treg cells.