Abul K. Abbas

Natural versus adaptive regulatory T cells

The regulation of immune responses to self-antigens is a complex process that involves maintaining self-tolerance while retaining the capacity to mount robust immune responses against invading microorganisms. Over the past few years, many new insights into this process have been gained, leading to the re-emergence of the idea that regulatory T (TReg) cells are a central mechanism of immune regulation. These insights have raised fundamental questions concerning what constitutes a TReg cell, where they develop and what signals maintain TReg-cell populations in a functional state. Here, we propose the existence of two subsets of CD4+ TReg cells — natural and adaptive — that differ in terms of their development, specificity, mechanism of action and dependence on T-cell receptor and co-stimulatory signalling.

Induction of peripheral T cell tolerance in vivo requires CTLA-4 engagement

Studies of T cell anergy in vitro have led to the widely accepted view that anergy is induced by T cell antigen recognition without costimulation. We show that the induction of T cell anergy in vivo is due to an abortive T cell response that requires recognition of B7 molecules, since blocking B7 maintains T cells in an unactivated but functionally competent state. Furthermore, the induction of anergy is prevented by blocking CTLA-4, the inhibitory T cell receptor for B7 molecules. Thus, in vivo T cell anergy may be induced not because of a lack of costimulation, but as a result of specific recognition of B7 molecules by CTLA-4. In contrast, blocking CD28 on T cells prevents priming but not the induction of tolerance. Therefore, the outcome of antigen recognition by T cells is determined by the interaction of CD28 or CTLA-4 on the T cells with B7 molecules.

The fas antigen is involved in peripheral but not thymic deletion of T lymphocytes in T cell receptor transgenic mice

The role of a cell death-associated gene, fas, in T lymphocyte development and responses to antigen has been analyzed by breeding a transgenic T cell receptor specific for the 81-104 peptide of pigeon cytochrome c into fas-defective MRL-lpr/lpr and control MRL+/+ mice. Transgene-expressing T cells mature normally in both strains and populate peripheral lymphoid tissues in normal numbers. Mature CD4+ T cells from the lpr/lpr mice are resistant to suppression by high doses of antigen and to apoptotic cell death. In vivo administration of peptide antigen causes deletion of thymic T cells in both MRL-lpr/lpr and MRL+/+ strains. By contrast, antigen-induced deletion of peripheral T cells occurs in the MRL+/+ but not in the MRL-lpr/lpr strain. Therefore, the fas gene plays an essential role in activation-induced cell death in mature T lymphocytes, but not in the negative selection of immature cells in the thymus.

Opinion-regulatory lymphocytes: Natural versus adaptive regulatory T cells

The regulation of immune responses to self-antigens is a complex process that involves maintaining self-tolerance while retaining the capacity to mount robust immune responses against invading microorganisms. Over the past few years, many new insights into this process have been gained, leading to the re-emergence of the idea that regulatory T (TReg) cells are a central mechanism of immune regulation. These insights have raised fundamental questions concerning what constitutes a TReg cell, where they develop and what signals maintain TReg-cell populations in a functional state. Here, we propose the existence of two subsets of CD4+ TReg cells — natural and adaptive — that differ in terms of their development, specificity, mechanism of action and dependence on T-cell receptor and co-stimulatory signalling.

Antigen-dependent proliferation of CD4+ CD25+ regulatory T cells in vivo.

The failure of CD25+ regulatory T cells (Tregs) to proliferate after T cell receptor (TCR) stimulation in vitro has lead to their classification as naturally anergic. Here we use Tregs expressing a transgenic TCR to show that despite anergy in vitro, Tregs proliferate in response to immunization in vivo. Tregs also proliferate and accumulate locally in response to transgenically expressed tissue antigen whereas their CD25− counterparts are depleted at such sites. Collectively, these data suggest that the anergic state that characterizes CD25+ Tregs in vitro may not accurately reflect their responsiveness in vivo. These observations support a model in which Treg population dynamics are shaped by the local antigenic environment.

Biochemical Mechanisms of IL-2–Regulated Fas-Mediated T Cell Apoptosis

Activation-induced cell death (AICD) of lymphocytes is an important mechanism of self-tolerance. In CD4+ T cells, AICD is mediated by the Fas pathway and is enhanced by IL-2. To define the mechanisms of this pro-apoptotic action of IL-2, we analyzed CD4+ T cells from wild-type and IL-2-/- mice expressing a transgenic T cell receptor. T cells become sensitive to AICD after activation by antigen and IL-2. IL-2 increases transcription and surface expression of Fas ligand (FasL) and suppresses transcription and expression of FLIP, the inhibitor of apoptosis. The ability of IL-2 to enhance expression of a pro-apoptotic molecule, FasL, and to suppress an inhibitor of Fas signaling, FLIP, likely accounts for the role of this cytokine in potentiating T cell apoptosis.

Uncoupling IL-2 Signals that Regulate T Cell Proliferation, Survival, and Fas-Mediated Activation-Induced Cell Death

IL-2 is an important growth and survival factor for T lymphocytes but also sensitizes these cells to Fas-mediated activation-induced cell death (AICD). The molecular basis of these different effects of IL-2 was studied by introducing wild-type and mutant forms of the IL-2 receptor beta (IL-2Rbeta) chain that lacked specific signaling capacities into receptor-deficient T cells by retroviral gene transfer. Activation of Stat5 by IL-2 was found to be involved in T cell proliferation and promoted Fas ligand (FasL) expression and AICD. T cell survival was dependent on a receptor region that activated Akt and the expression of Bcl-2. Thus, distinct IL-2Rbeta chain signaling modules regulate T cell fate by stimulating growth and survival or by promoting apoptosis.

IL-4 induces allergic-like inflammatory disease and alters T cell development in transgenic mice

We have assessed the biologic role of IL-4 by fusing its gene to an immunoglobulin promoter/enhancer and introducing it into transgenic mice. By attenuating the transgene promoter through the insertion of E. coli lac operator sequences, we have created a series of animals that constitutively express varying amounts of IL-4. Overexpression of IL-4 results in a marked increase in serum IgE levels and the appearance of an inflammatory ocular lesion (blepharitis) with characteristic histopathologic features seen in allergic reactions. In addition, expression of the IL-4 transgene in the thymus perturbs T cell maturation, reducing the population of immature CD4+CD8+ thymocytes and peripheral T cells while increasing the population of mature CD8+ thymocytes. These results demonstrate that deregulation of a single cytokine gene in vivo can induce a complex inflammatory reaction resembling that observed in human allergic disease.

CTLA-4 Regulates Induction of Anergy In Vivo

The requirement for CTLA-4 during the induction of peripheral T cell tolerance in vivo was investigated using naive TCR transgenic T cells lacking CTLA-4. CTLA-4(-/-) T cells are resistant to tolerance induction, as demonstrated by their proliferative responses, IL-2 production, and progression into the cell cycle. Following exposure to a tolerogenic stimulus in vivo and restimulation in vitro, wild-type T cells are blocked at the late G1 to S restriction point of the cell cycle. In contrast, CTLA-4(-/-) T cells enter into the S phase of the cell cycle, as shown by downregulation of p27(kip1), elevated cdk2 kinase activity, and Rb hyperphosphorylation. Thus, CTLA-4 has an essential role in determining the outcome of T cell encounter with a tolerogenic stimulus.

The Roles of Costimulation and Fas in T Cell Apoptosis and Peripheral Tolerance

Using cells from TCR transgenic mice that do or do not express Fas, we show that there are two mechanistically distinct forms of apoptosis in CD4+ T cells. Naive T cells undergo apoptosis if cultured in the absence of antigen or costimulation. This form of programmed cell death (PCD) is not dependent on Fas, and is prevented by CD28-mediated signals, which lead to the secretion of growth factors and the expression of survival genes, such as bcl-xL. Recently activated T cells undergo apoptotic death upon repeated stimulation. This activation-induced cell death (AICD) is mediated by Fas, but is independent of costimulation and is not prevented by IL-2 or bcl-xL. Finally, we show that peripheral tolerance may be induced in vivo independent of Fas-mediated cell death.