John B. Imboden

Cell surface molecules and early events involved in human T lymphocyte activation

The physiologic activation of human T cells by antigen involves events that occur between ligands and receptors at the interface of the T cell and antigen-presenting cell (or target cell). These events have been examined by identifying the cell surface receptors involved in such interactions using mAb. Whereas the T3/T cell antigen receptor plays a central role in such interactions, other T cell receptors have been identified which may also contribute to T cell activation in providing primary activation signals or by functioning as accessory molecules. Although the ligands of these other receptors are currently unknown or ill defined, it is likely that this will provide a fruitful area of investigation. The use of mAb as probes to mimic these putative ligands has facilitated the study of the requirements for activation and the biochemical events initiated by the receptors involved. The T cell receptor, a multisubunit complex, has been most intensively studied. Ligands that bind to T3/Ti cannot initiate activation by themselves and require the participation of accessory molecules. Stimulation of T3/Ti results in the formation of at least two potent intracellular second messengers, IP3 and DG, through the hydrolysis of PIP2. These second messengers, in turn, induce an increase in [Ca2+]i and the activation of pkC. These two events appear to be essential in the transcriptional activation of certain targeted genes through ill-defined pathways leading to the manifestations of T cell activation.

CD28 Mediates Transcriptional Upregulation of the Interleukin-2 (IL-2) Promoter through a Composite Element Containing the CD28RE and NF-IL-2B AP-1 Sites

Mutagenesis studies have demonstrated the requirement for the CD28-responsive element (CD28RE) within the interleukin-2 (IL-2) promoter for transcriptional upregulation by CD28. Here, we demonstrate that CD28 responsiveness is conferred by a composite element containing both the CD28RE and the NF-IL-2B AP-1 sites (RE/AP). Mutations at either site within the RE/AP composite element abolish activity. The RE/AP composite element is a site for signal integration within the IL-2 promoter, since its activation is dependent on at least two separate signalling pathways being activated, through the T-cell receptor, CD28, and/or phorbol myristate acetate. Activation is maximal when all three signals occur simultaneously. By using a panel of CD28 cytoplasmic domain mutants, it was found that the transcriptional activation of the RE/AP composite element correlates exactly with the pattern of IL-2 secretion induced by these mutants upon stimulation. Similar to the upregulation of IL-2 secretion, the transcriptional upregulation of the RE/AP composite element by CD28 is FK506 insensitive. The pattern of activation of the RE/AP composite element is different from that observed for either an NFAT or consensus AP-1 site, implying that RE/AP represents a unique element. Using gel shift analysis, we demonstrate that stimulation by CD28 induces the association of the NF-kappaB family member c-Rel to the CD28RE within the RE/AP composite element. The transcriptional upregulation of IL-2 by CD28 appears, therefore, to be mediated through the RE/AP composite element, involving the association of c-Rel with the CD28RE.

The Immunopathogenesis of Rheumatoid Arthritis

Rheumatoid arthritis is a chronic inflammatory polyarthritis whose etiology remains uncertain. Recently we have learned that autoimmunity to citrullinated protein antigens has specificity for rheumatoid arthritis and defines a clinically and genetically distinct form of the disease. Multiple genes contribute to disease susceptibility, with the HLA locus accounting for 30% to 50% of overall genetic risk. Five risk loci have been identified and validated: HLA-DRB1, PTPN22, STAT4, a region in 6q23, and the TRAF1/C5 locus. Also, there is renewed interest in the contribution of T cells to ongoing inflammation in rheumatoid arthritis. Autoantibodies to citrullinated protein epitopes are specific for rheumatoid arthritis, are associated with a more aggressive disease course, and are pathogenic in an animal model of autoimmune arthritis. There is a strong association between shared-epitope-expressing HLA-DRB1 alleles and the development of rheumatoid arthritis associated with autoimmunity to citrullinated protein antigens.

Negative Regulation of T Cell Receptor-Lipid Raft Interaction by Cytotoxic T Lymphocyte-associated Antigen 4

Cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) is an essential negative regulator of T cell activation. Recent evidence suggests that CTLA-4 association with the immunological synapse during contact with antigen-presenting cells is important for its inhibitory function. In the present study, we observed a direct interaction of CTLA-4 with the phosphorylated form of T cell receptor (TCR)ζ within the glycolipid-enriched microdomains associated with the T cell signaling complex. In this setting, CTLA-4 regulated the accumulation/retention of TCRζ in the signaling complex, as the lipid raft fractions from CTLA-4KO T cells contained significantly higher amounts of the TCR components when compared with wild-type littermates. In contrast, coligation of CTLA-4 with the TCR during T cell activation selectively decreased the amount of TCRζ that accumulated in the rafts. These results suggest that CTLA-4 functions to regulate T cell signaling by controlling TCR accumulation and/or retention within this a critical component of the immunological synapse.

Socioeconomic determinants of disability and depression in patients with rheumatoid arthritis

To examine the relationship between functional limitation, socioeconomic inequality, and depression in a diverse cohort of patients with rheumatoid arthritis (RA).

Patient-physician discordance in assessments of global disease severity in rheumatoid arthritis.

To determine the degree of discordance between patient and physician assessment of disease severity in a multiethnic cohort of adults with rheumatoid arthritis (RA), to explore predictors of discordance, and to examine the impact of discordance on the Disease Activity Score in 28 joints (DAS28).

Transmembrane signalling by the T3-antigen receptor complex.

Monoclonal antibodies to the T-cell antigen receptor can mimic the effects of antigen and induce cellular activation. In this article john Imboden and his colleagues report on the use of these antibodies as probes to study the mechanism by which the antigen receptor initiates activation. They also describe the critical role of cytoplasmic free calcium in transmembrane signal transduction by the T-cell antigen receptor.

Association of Src-like protein tyrosine kinases with the CD2 cell surface molecule in rat T lymphocytes and natural killer cells.

The cell surface molecule CD2 has a signaling role in the activation of T lymphocytes and natural killer cells. Because perturbation of CD2 leads to the appearance of tyrosine-phosphorylated proteins, we investigated the possibility that CD2 associates with cytoplasmic protein tyrosine kinases. As determined by in vitro kinase assays and phosphoamino acid analysis, protein tyrosine kinase activity coprecipitated with CD2 from rat T lymphocytes, T lymphoblasts, thymocytes, interleukin-2-activated natural killer cells, and RNK-16 cells (a rat natural killer cell line). In each case, both p56lck and p59fyn were identified in the CD2 immunoprecipitate. In the thymus, the association between CD2 and these kinases occurred predominately in a small subset of thymocytes that had the cell surface phenotype of mature T cells, indicating that the association is a regulated event and occurs late in T-cell ontogeny. The finding that CD2 is associated with p56lck and p59fyn in detergent lysates suggests that interactions with these Src-like protein kinases play a critical role in CD2-mediated signal transduction.

Distinct regions in the CD28 cytoplasmic domain are required for T helper type 2 differentiation.

CD28 costimulation is essential for CD4+ T cell proliferation, survival, interleukin 2 (IL-2) production and T helper type 2 development. To define the nature of the signals that may drive different T cell responses, we have done a structure-function analysis of the CD28 cytoplasmic tail in primary T cells. CD28-mediated T cell proliferation and IL-2 production did not require a particular cytoplasmic domain. In contrast, IL-4 production was driven by the cooperative activity of specific motifs within the CD28 cytoplasmic tail. Using a gene-complementation approach, we provide evidence that one component of this T helper type 2 differentiation signal was mediated by 3-phosphoinositide-dependent protein kinase 1. Thus, different mechanisms underlie the induction of distinct T cell functional responses by CD28.

The CD28-B7 costimulatory pathway and its role in autoimmune disease.

The activation of naive CD4+ T cells requires two discrete signals: a signal delivered by the T cell receptor following recognition of antigen and an accessory signal transduced when costimulatory receptors interact with their ligands. Particularly important in the development of an immune response to foreign antigens is the T cell molecule CD28, which delivers a potent costimulus when engaged by ligands, B7‐1 and B7‐2, on antigen‐presenting cells. It is interesting that blockade of B7 molecules, which disrupts interactions with CD28 and prevents delivery of the CD28 costimulus, also alters the immune responses to self antigens and prevents the development of clinical disease in murine models of systemic and organ‐specific autoimunity. Herein we review the roles of CD28 and its B7 ligands in the pathogenesis of autoimmunity, discuss efforts to treat animal models of autoimmunity by modifying the CD28 signal, and consider the mechanisms by which manipulation of the CD28 signal alters the course of experimental autoimmune disease. J. Leukoc. Biol. 62: 156–162; 1997.