Despite extensive study, several of the major components involved in T cell receptor-mediated signaling remain unidentified. Here we report the cloning of the cDNA for a highly tyrosine-phosphorylated 36-38 kDa protein, previously characterized by its association with Grb2, phospholipase C-gamma1, and the p85 subunit of phosphoinositide 3-kinase. Deduced amino acid sequence identifies a novel integral membrane protein containing multiple potential tyrosine phosphorylation sites. We show that this protein is phosphorylated by ZAP-70/Syk protein tyrosine kinases leading to recruitment of multiple signaling molecules. Its function is demonstrated by inhibition of T cell activation following overexpression of a mutant form lacking critical tyrosine residues. Therefore, we propose to name the molecule LAT-linker for activation of T cells.

The elucidation of the phenomena of T cell antagonism and partial activation by altered peptide ligands has necessitated a revision in the traditional concepts of TCR recognition of antigen and subsequent signal transduction. Whereas previous models supported a single ligand specificity for any particular T cell, many studies using analogs of immunogenic peptides have now demonstrated a flexibility in this recognition. Moreover, interaction with such altered peptide ligands can result in dramatically different phenotypes of the T cells, ranging from inducing selective stimulatory functions to completely turning off their functional capacity. Investigations of the biochemical basis leading to these phenotypes have shown that altered peptide ligands can induce a qualitatively different pattern of signal transduction events than does any concentration of the native ligand. Such observations imply that several signaling modules are directly linked to the TCR/CD3 complex and that they can be dissociated from each other as a direct result of the nature of the ligand bound. Interestingly, many in vivo models of T cell activation are compatible with a selective signaling model, and several studies have shown that peptide analogs can play a role in various T cell biologic phenomena. These data strongly suggest that naturally occurring altered peptide ligands for any TCR exist in the repertoire of self-peptides or, in nature, derived from pathogens, and recent reports provide compelling evidence that this is indeed the case. The concept of altered peptide ligands, their effects on T cell signaling, the hypothesized mechanisms by which they exert their effects, and their possible roles in shaping the T cell immune response are the scope of this review.

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Activation of CD4+ T helper cells results from the occupancy of the T-cell receptor (TCR) by immunogenic peptide bound to a class II major histocompatibility complex (MHC) molecule, together with a co-stimulatory signal from the antigen-presenting cell (APC). This activation leads to proliferation, cytokine production (Th1 or Th2 profile) and cytolysis. Engagement of the TCR in the absence of co-stimulation causes Th1 cells to become unresponsive to subsequent antigenic stimulation. We have previously demonstrated that analogues of an immunogenic peptide could stimulate Th1 and Th2 cells to carry out some effector functions without inducing proliferation, a phenomenon we term partial activation. Here we study the consequences of such partial activation through the TCR of two Th1 clones using peptide analogues presented by a live APC. A peptide analogue that is unable to stimulate clonal proliferation or production of cytokine or inositol phosphate can induce the T cells to become profoundly unresponsive to subsequent stimulation with the immunogenic peptide. Thus, altering the ligand of the TCR by using a peptide analogue on a functional APC sends a signal to Th1 clones that results in anergy.

Objective. We undertook this study to evaluate safety, tolerability, pharmacokinetics, pharmacodynamics, and efficacy of LY2439821, a humanized antiinterleukin-17 (anti-IL-17) monoclonal antibody, in a first in-human trial in rheumatoid arthritis (RA) patients taking oral disease-modifying antirheumatic drugs (DMARDs).Methods. This randomized, double-blind, placebo- Results. Baseline characteristics were similar across all groups. Changes in the DAS28 were significantly greater in the 0.2 mg/kg, 2.0 mg/kg, and all-LY2439821-combined groups (؊2.3, ؊2.4, and ؊2.3, respectively) than in the placebo group (؊1.7) at week 10 (P < 0.05), and these differences were significant as early as week 1. Percentages of ACR20, ACR50, and ACR70 responses as well as improvements in the ACR core set of measures were greater in LY2439821-treated patients than in placebo-treated patients at multiple time points. There was no apparent dose-response relationship in treatment-emergent adverse events.Conclusion. LY2439821 added to oral DMARDs improved signs and symptoms of RA, with no strong adverse safety signal noted. This first evaluation of LY2439821 supports neutralization of IL-17 as a potential novel goal for the treatment of RA.