Cytotoxic T lymphocyte-associated antigen 4 plays an essential role in the function of CD25(+)CD4(+) regulatory cells that control intestinal inflammation

CTLA-4 up-regulation plays a role in tolerance mediated by CD45

Cytolytic T lymphocyte-associated antigen 4 (CTLA-4) is a critical down-regulatory molecule in T cells that plays a major role in peripheral tolerance. Although the CD45 protein tyrosine phosphatase is a potent immunomodulatory target, the mechanisms by which antibody against CD45RB isoforms (anti-CD45RB) induces allograft tolerance remain unclear. We show here that anti-CD45RB treatment alters CD45 isoform expression on T cells, which is associated with rapid up-regulation of CTLA-4 expression. These effects appear specific and occur without up-regulation of other activation markers. Administration of a blocking monoclonal antibody to CTLA-4 at the time of transplantation prevents anti-CD45RB therapy from prolonging islet allograft survival. In addition, treatment with cyclosporin A blocks anti-CD45RB-induced CTLA-4 expression and promotes acute rejection. These data suggest that anti-CD45RB acts through mechanisms that include CTLA-4 up-regulation and demonstrate a link between CD45 and CTLA-4 that depends on calcineurin-mediated signaling. They demonstrate also that CTLA-4 expression may be specifically targeted to enhance allograft acceptance.

The role of different subsets of T regulatory cells in controlling autoimmunity

T regulatory cells-in addition to clonal deletion and anergy-are essential for the downregulation of T cell responses to both foreign and self antigens, and for the prevention of autoimmunity. Recent progress has been made in characterising the different subsets of T regulatory cells, the factors that drive their differentiation, and their mode of action. The resolution of these mechanisms will make it possible to use T regulatory cells therapeutically in human autoimmune diseases.

Animal models of autoimmunity and their relevance to human diseases

T cells mediate various autoimmune diseases. Pathologic autoimmunity can be induced by manipulating thymic or peripheral control of self-reactive T cells. There is, for example, accumulating evidence that elimination or dysfunction of regulatory T cells can elicit T cell mediated, destructive autoimmune disease in otherwise normal animals and enhance autoimmunity in spontaneous models.

Induction of interleukin 10-producing, nonproliferating CD4(+) T cells with regulatory properties by repetitive stimulation with allogeneic immature human dendritic cells

The functional properties of dendritic cells (DCs) are strictly dependent on their maturational state. To analyze the influence of the maturational state of DCs on priming and differentiation of T cells, immature CD83(-) and mature CD83(+) human DCs were used for stimulation of naive, allogeneic CD4(+) T cells. Repetitive stimulation with mature DCs resulted in a strong expansion of alloreactive T cells and the exclusive development of T helper type 1 (Th1) cells. In contrast, after repetitive stimulation with immature DCs the alloreactive T cells showed an irreversibly inhibited proliferation that could not be restored by restimulation with mature DCs or peripheral blood mononuclear cells, or by the addition of interleukin (IL)-2. Only stimulation of T cells with mature DCs resulted in an upregulation of CD154, CD69, and CD70, whereas T cells activated with immature DCs showed an early upregulation of the negative regulator cytotoxic T lymphocyte-associated molecule 4 (CTLA-4). These T cells lost their ability to produce interferon gamma, IL-2, or IL-4 after several stimulations with immature DCs and differentiated into nonproliferating, IL-10-producing T cells. Furthermore, in coculture experiments these T cells inhibited the antigen-driven proliferation of Th1 cells in a contact- and dose-dependent, but antigen-nonspecific manner. These data show that immature and mature DCs induce different types of T cell responses: inflammatory Th1 cells are induced by mature DCs, and IL-10-producing T cell regulatory 1-like cells by immature DCs.

Review article: molecular signals and genetic reprogramming in peripheral T-cell differentiation

Rearrangement of gene segments occurs in T lymphocytes during thymic development as the T-cell receptor (TCR) is first expressed, allowing T cells to become central regulators of antigen specificity in the acquired immune system. However, further development of T cells occurs after population of peripheral lymphoid tissues, which can result in T-cell expansion and differentiation into effectors of various immune function, or progression to memory T cells, anergic cells or death by apoptosis. This review focuses on more recent developments concerning the choices that peripheral T cells make between first encountering antigen through TCR recognition and death. These decisions are associated with a process of genetic reprogramming that alters the behaviour of cells so that immune responses are appropriately regulated.

Dynamics of T lymphocyte responses: intermediates, effectors, and memory cells

The immune response is initiated in organized lymphoid tissues where antigen-loaded dendritic cells (DCs) encounter antigen-specific T cells. DCs function as packets of information that must be decoded by the T cell before an appropriate immune response can be mounted. We discuss how the dynamics of DC-T cell encounter and the mechanism of T cell differentiation make the decoding of this information stochastic rather than determinate. This results in the generation of both terminally differentiated effector cells and intermediates that play distinctive roles in protection, immunoregulation, and immunological memory.

Immunologic self-tolerance maintained by CD25(+)CD4(+) regulatory T cells constitutively expressing cytotoxic T lymphocyte-associated antigen 4

This report shows that cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) plays a key role in T cell-mediated dominant immunologic self-tolerance. In vivo blockade of CTLA-4 for a limited period in normal mice leads to spontaneous development of chronic organ-specific autoimmune diseases, which are immunopathologically similar to human counterparts. In normal naive mice, CTLA-4 is constitutively expressed on CD25(+)CD4(+) T cells, which constitute 5-10% of peripheral CD4(+) T cells. When the CD25(+)CD4(+) T cells are stimulated via the T cell receptor in vitro, they potently suppress antigen-specific and polyclonal activation and proliferation of other T cells, including CTLA-4-deficient T cells, and blockade of CTLA-4 abrogates the suppression. CD28-deficient CD25(+)CD4(+) T cells can also suppress normal T cells, indicating that CD28 is dispensable for activation of the regulatory T cells. Thus, the CD25(+)CD4(+) regulatory T cell population engaged in dominant self-tolerance may require CTLA-4 but not CD28 as a costimulatory molecule for its functional activation. Furthermore, interference with this role of CTLA-4 suffices to elicit autoimmune disease in otherwise normal animals, presumably through affecting CD25(+)CD4(+) T cell-mediated control of self-reactive T cells. This unique function of CTLA-4 could be exploited to potentiate T cell-mediated immunoregulation, and thereby to induce immunologic tolerance or to control autoimmunity.