Rose Zamoyska

Interleukin 7 and T cell receptor signals regulate homeostasis of CD4 memory cells

Immunological memory depends on the long-term maintenance of memory T cells. Although the factors that maintain CD8 T cell memory are well understood, those responsible for CD4 memory are not well defined. We have shown here that interleukin 7 (IL-7) was an important survival factor for CD4 memory T cells that together with T cell receptor (TCR) signals regulated homeostasis of the CD4 memory population in lymphopenic conditions and in the intact immune system. Thus, IL-7 contributes to the maintenance of all naive and memory T cell subsets, and therefore controls the overall size of the T cell pool.

A Requirement for the Rho-Family GTP Exchange Factor Vav in Positive and Negative Selection of Thymocytes

The T cell repertoire is shaped by positive and negative selection of thymocytes that express low levels of T cell receptor (TCR) and both CD4 and CD8. TCR-mediated signals that determine these selection processes are only partly understood. Vav, a GDP-GTP exchange factor for Rho-family proteins, is tyrosine phosphorylated following TCR stimulation, suggesting that it may transduce TCR signals. We now demonstrate that mice lacking Vav are viable and display a profound defect in the positive selection of both class I- and class II-restricted T cells. In contrast, Vav is not essential for negative selection, though in its absence negative selection is much less effective. Vav may influence the efficiency of TCR-induced selection events by regulating the intracellular calcium flux of thymocytes.

T-cell receptor proximal signaling via the Src-family kinases, Lck and Fyn, influences T-cell activation, differentiation, and tolerance

Summary:  T‐cell development in the thymus and activation of mature T cells in secondary lymphoid organs requires the ability of cells to respond appropriately to environmental signals at multiple stages of their development. The process of thymocyte selection insures a functional T‐cell repertoire, while activation of naive peripheral T cells induces proliferation, gain of effector function, and, ultimately, long‐lived T‐cell memory. The T‐cell immune response is initiated upon engagement of the T‐cell receptor (TCR) and coreceptor, CD4 or CD8, by cognate antigen/major histocompatibility complexes presented by antigen‐presenting cells. TCR/coreceptor engagement induces the activation of biochemical signaling pathways that, in combination with signals from costimulator molecules and cytokine receptors, direct the outcome of the response. Activation of the src‐family kinases p56lck (Lck) and p59fyn (Fyn) is central to the initiation of TCR signaling pathways. This review focuses on our current understanding of the mechanisms by which these two proteins orchestrate T‐cell function.

T cell receptor signalling networks: branched, diversified and bounded

Engagement of antigen-specific T cell receptors (TCRs) is a prerequisite for T cell activation. Acquisition of appropriate effector T cell function requires the participation of multiple signals from the T cell microenvironment. Trying to understand how these signals integrate to achieve specific functional outcomes while maintaining tolerance to self is a major challenge in lymphocyte biology. Several recent publications have provided important insights into how dysregulation of T cell signalling and the development of autoreactivity can result if the branching and integration of signalling pathways are perturbed. We discuss how these findings highlight the importance of spatial segregation of individual signalling components as a way of regulating T cell responsiveness and immune tolerance.

The influence of the src-family kinases, Lck and Fyn, on T cell differentiation, survival and activation

The src‐family kinases p56lck (Lck) and p59fyn (Fyn) are expressed in T cells and are among the first signaling molecules to be activated downstream of the T cell receptor (TCR). Evidence is emerging that although closely related, these signaling molecules have discrete functions during development, maintenance and activation of peripheral T cells. For example, during thymopoiesis Lck is uniquely able to provide all the signals required for pre‐TCRβ selection, although Fyn can substitute for a subset of these. Positive selection of CD4 single‐positive (SP) cells is also critically dependent on the expression of Lck but not Fyn, while differentiation of CD8 SP cells proceeds relatively efficiently in the absence of Lck. In naïve peripheral T cells either Lck or Fyn can transmit TCR‐mediated survival signals, and yet only Lck is able to trigger TCR‐mediated expansion signals under conditions of lymphopenia. Stimulation of naïve T cells by antigenic stimuli is also severely compromised in the absence of Lck, but more subtly impaired by the absence of Fyn. We discuss recent experiments addressing how these two src‐kinase family members interface with downstream signaling pathways to regulate these diverse aspects of T cell behavior.

Involvement of Avidity for Major Histocompatibility Complex in Homeostasis of Naive and Memory T Cells

The requirements for survival and self-renewal of peripheral T cells and the nature of mechanisms controlling the size of the naive and memory pool are not completely understood. Here, we examine the involvement of the major histocompatibility complex (MHC) in survival and homeostatic expansion of naive and memory T cells. We show that the homeostatic behavior of naive T cell receptor (TCR)-transgenic T cells can be deduced by the expression levels of TCR and CD5, a negative regulator of TCR signaling. Both these factors determine the strength of TCR stimulation by MHC-derived signals. We further show that, similarly to naive T cells, MHC-derived signals influence the homeostatic expansion capacity of memory T cells under lymphopenic conditions. In contrast to naive T cells, however, memory T cells can reach a homeostatic equilibrium, in which survival/self-renewal of each clone is dissociated from their avidity for MHC-derived signals.

TCR and IL-7 Receptor Signals Can Operate Independently or Synergize to Promote Lymphopenia-Induced Expansion of Naive T Cells

TCR and cytokine signals induce naive T cells to undergo spontaneous divisions as part of a homeostatic response to conditions of T cell deficiency. The conditions under which these signals evoke the homeostatic response and their interaction with each other are poorly understood, and yet are very important clinically in considering strategies for immune reconstitution. Here, we show that p56lck (lck)-mediated TCR signals and IL-7R signals are each able to stimulate T cell proliferation in lymphopenic hosts independently of one another, but can also synergize to facilitate proliferation. Furthermore, the relative contribution to the homeostatic response by TCR and cytokine signals is not fixed and critically depends on both the degree of lymphopenia and specific characteristics of individual T cell clones. Finally, we show that only lck and not fyn can mediate the TCR-driven proliferation, while neither lck nor fyn is required for IL-7R-induced proliferation.

Inducible Expression of a p56Lck Transgene Reveals a Central Role for Lck in the Differentiation of CD4 SP Thymocytes

The T lymphocyte-specific protein tyrosine kinase p56lck (Lck) is an essential component of the TCR-mediated signal transduction complex. Lck knockout mice have reduced numbers of double-positive thymocytes and very few mature single-positive cells, particularly of the CD4 lineage. Here we demonstrate the ability of a tetracycline-based tissue-specific inducible Lck transgene to restore expansion of early thymocytes and maturation of single-positive cells in Lckneg mice upon induction with doxycycline. Restoration of Lck expression is particularly important for positive selection to the CD4+ lineage but has a lesser impact on selection to the CD8+ lineage, suggesting activation of Lck is an important component of the signals involved in lineage choice during thymic differentiation.

TCR Signals Mediated by Src Family Kinases Are Essential for the Survival of Naive T Cells

The role of TCR signals triggered by recognition of self MHCs in maintaining the survival of naive peripheral T cells remains controversial. Here we examine the role of the Src family kinases, p56lck (Lck) and p59fyn (Fyn), in the survival of naive T cells. We show that long term survival requires a combination of signals transduced by Src family kinases and signals through the IL-7R. In the absence of either one, naive T cells die slowly, but if both signals are removed, cell loss is greatly accelerated. The TCR signal can be mediated by either Fyn or Lck at wild-type levels of expression, but not by Lck alone if expressed suboptimally. The disappearance of T cells in the absence of Fyn and Lck was associated with a complete loss of TCRζ-chain phosphorylation and down-regulation of CD5, both of which are also MHC contact dependent, indicating that the Src family kinases are critical for transducing a TCR-MHC survival signal.

CD4 and CD8: modulators of T-cell receptor recognition of antigen and of immune responses?

The response of T cells to antigen involves the participation of a number of distinct receptor-ligand engagements. The major players in the recognition of complexes of major histocompatibility complex molecules and peptide antigens are the T-cell receptors and the co-receptors CD4 and CD8. Progress in understanding the physical structures of these molecules, and how complexes between them are formed, is helping our understanding of how they participate in regulating the signals transduced to T cells.