Ronald H. Schwartz

Selective, stable demethylation of the interleukin-2 gene enhances transcription by an active process

A role for DNA demethylation in transcriptional regulation of genes expressed in differentiated somatic cells remains controversial. Here, we define a small region in the promoter-enhancer of the interleukin-2 (Il2) gene that demethylates in T lymphocytes following activation, and remains demethylated thereafter. This epigenetic change was necessary and sufficient to enhance transcription in reporter plasmids. The demethylation process started as early as 20 minutes after stimulation and was not prevented by a G1 to S phase cell cycle inhibitor that blocks DNA replication. These results imply that this demethylation process proceeds by an active enzymatic mechanism.

Egr-2 and Egr-3 are negative regulators of T cell activation.

T cell receptor engagement in the absence of proper accessory signals leads to T cell anergy. E3 ligases are involved in maintaining the anergic state. However, the specific molecules responsible for the induction of anergy have yet to be elucidated. Using microarray analysis we have identified here early growth response gene 2 (Egr-2) and Egr-3 as key negative regulators of T cell activation. Overexpression of Egr2 and Egr3 was associated with an increase in the E3 ubiquitin ligase Cbl-b and inhibition of T cell activation. Conversely, T cells from Egr3−/− mice had lower expression of Cbl-b and were resistant to in vivo peptide-induced tolerance. These data support the idea that Egr-2 and Egr-3 are involved in promoting a T cell receptor–induced negative regulatory genetic program.

Transactivation by AP-1 Is a Molecular Target of T Cell Clonal Anergy

Anergy is a mechanism of T lymphocyte tolerance induced by antigen receptor stimulation in the absence of co-stimulation. Anergic T cells were shown to have a defect in antigen-induced transcription of the interleukin-2 gene. Analysis of the promoter indicated that the transcription factor AP-1 and its corresponding cis element were specifically down-regulated. Exposure of anergic T cells to interleukin-2 restored both antigen responsiveness and activity of the AP-1 element.

Differential expression of two distinct T-cell receptors during thymocyte development

The product of the T-cell receptor (TCR) γ-gene1 has recently been found to be expressed on a subset of both peripheral cells2 and thymocytes3,4. As an initial approach to understanding the role of this γ-chain of TCR (TCRγ) in T-cell development, we have studied the ontogeny of TCR expression at the protein level in the developing murine thymus. We show here that the first T3-associated TCR to be expressed in the developing thymus is a disulphide-linked heterodimer composed of a γ-chain of relative molecular mass 35,000 (Mr 35K) and a 45K partner (termed TCR δ). This TCR γδ is first detected approximately two days before the appearance of cell-surface TCR αβ heterodimers. We report that N-glycosidase digestions reveal that all of the γ-protein expressed on fetal thymocytes, as in adult CD4−8− (L3T4−, Lyt2−) thymocytes4, bear N-linked carbohydrate side chains. The major γ-gene transcribed in mature, αβ-bearing T cells ( Vγ1.2 Cγ2) encodes no N-linked glycosylation site1 so these results suggest that the fetal γδ receptor defines a distinct T-cell lineage whose development in the thymus precedes classical αβ -bearing cells.

T CELL CLONAL ANERGY

Recent experiments have elucidated two molecular mechanisms that may account for the failure of anergic T cell clones to initiate IL-2 gene transcription following TCR stimulation. First, a block has been identified in the ERK and JNK mitogen-activated protein kinase pathways; the block results from a failure to activate p21ras. It leads to reduced induction of c-Fos and JunB proteins and to a failure to form and phosphorylate the activator protein (AP)-1 heterodimers required for IL-2 gene transcriptional activation. Second, repressor molecules (Nil-2-a and a molecule related to AP-1) have been characterized that dominantly inhibit IL-2 gene transcription.

Interaction between antigen-presenting cells and primed t lymphocytes. An assessment of ir gene expression in the antigen-presenting cell.

It is now well recognized that specific immune response (/r) genes play a major role in thymus-dependent immtme responses and that they are intimately involved in the regulation of cellular interactions of thymus-dependent (T) lymphocytes (Paul & Benacerraf 1977). A particular focus of research by our colleagues and by us has been the role of Ir gene products in the control of the interactions of antigen-presenting cells and specific T lymphocytes. The concept that such interactions might be an important site for the action of Ir gene products draws partictilar support from the initial description in guinea pig systems of the inhibition of Ir gene controlled responses by antisera specific for I-region gene products (Shevach et al. 1972) and from the finding that collaboration between antigen-presenting parental macrophages and responding Fi T lymphocytes reqtiires that the donor of the macrophage possess an Ir gene allowing it to respond to the antigen being tested (Shevach & Rosenthal 1973).

Natural regulatory T cells and self-tolerance.

The adaptive immune system allows individual organisms to mount defensive reactions against unanticipated pathogens by developmentally creating a diverse repertoire of clonally distributed receptors capable of recognizing a multitude of antigens and then expanding as effector cell populations those that can recognize molecules from the pathogens. To function properly, the system must deal with the problem of randomly generated receptors that can recognize self components. Most solutions to this self-tolerance problem are cell intrinsic and involve the deletion or inactivation of autoreactive cells. However, an extrinsic form of dominant tolerance has been demonstrated that takes the form of CD4+ regulatory T cells. This perspective discusses why such a mechanism might have evolved and the problems it presents for self–non-self discrimination.

The Fine Specificity of Antigen and la Determinant Recognition by T Cell Hybridoma Clones Specific for Pigeon Cytochrome c

The activation of proliferative T lymphocytes normally involves the simultaneous recognition of a particular foreign antigen and a particular Ia molecule on the surface of antigen-presenting cells, the phenomenon of major histocompatibility complex (MHC) restriction. An analysis of T cell clones specific for pigeon cytochrome c, from B10.A and B10.S(9R) strains of mice, revealed the unusual finding that several of the clones could respond to antigen in association with Ia molecules from either strain. Using these cross-reactive clones, we performed experiments which demonstrated that both the Ia molecule and the T cell receptor contribute to the specificity of antigen recognition; however, MHC-linked low responsiveness to tuna cytochrome c (an immune response gene defect) could not be attributed solely to the efficacy with which the Ia molecules associated with the antigen. These results imply that antigen and Ia molecules are not recognized independently, but must interact at least during the process of T cell activation.

Molecular regulation of interleukin-2 expression by CD28 co-stimulation and anergy

Summary: The consequences of T‐cell receptor engagement (signal I) are profoundly affected by the presence or absence of co‐stimulation (signal 2). T‐cell receptor (TCR) stimulation in the absence of CD 28‐mediated co‐stimulation not only results in little interleukin (IL)‐2 production, but induces a long lasting hyporesponsive state known as T‐cell clonal anergy The addition of CD 28 ligation to signal 1, on the other hand, results in the production of copious amounts of IL‐2. Our laboratory has utilized CD4+ Th1 clones in an effort to understand the molecular events resulting in enhanced IL‐2 production by co‐stimulation and the inhibition of IL‐ 2 production in anergy Our current studies have focused on defining the post‐transcriptional effects of CD28‐enhanced IL‐2 production The data suggest that a major component of CD28′s ability to regulate IL‐2 production occurs at the level of message stability and involves the 3′‐untranslated region of the message. In terms of anergy, our recent studies support the notion that it is not the result of TCR engagement in the absence of co‐stimulation. but rather signal 1 in the absence of IL‐2 receptor signaling and proliferation. Furthermore, T‐cell anergy appears to be an active negative state in which IL‐2 production is inhibited both at the level of signal transduction and by cis‐dominant repression at the level of the IL‐2 promoter.

Immune Response(Ir)Genes of the Murine Major Histocompatibility Complex

Publisher Summary This chapter opens up with a detailed analysis of the early experiments of these scientists, culminating in the discovery by McDevitt that these genes were linked to the major histocompatibility complex (MHC). One of the major unifying threads in cellular immunology that ended in the identification of MHC-encoded molecules as the Ir gene products and the merging of the two phenomena: Ir gene control and MHC restriction (the requirement for T cells to recognize an MHC-encoded molecule in addition to the antigen in order to become activated). The recent developments in cellular and molecular cloning have moved the problem of Ir gene function to the biochemical level. Ir genes are genetically linked to the MHC. A formal proof of the genetic linkage of Ir-genes to the MHC was performed using the segregating population in a backcross to the low responder parent.