Bryan A. Irving

The cytoplasmic domain of the T cell receptor ζ chain is sufficient to couple to receptor-associated signal transduction pathways

The function of the T cell antigen receptor (TCR) invariant chains, CD3 gamma, delta, epsilon, and zeta, is poorly understood. Evidence suggests that CD3 couples receptor ligand binding to intracellular signaling events. To examine the role of the CD3 zeta chain in TCR-mediated signal transduction, a chimeric protein linking the extracellular and transmembrane domains of CD8 to the cytoplasmic domain of the zeta chain was constructed. The CD8/zeta chimera is expressed independently of the TCR and is capable of transducing signals that, by criteria of early and late activation, are indistinguishable from those generated by the intact TCR. These data indicate that CD8/zeta can activate the appropriate signal transduction pathways in the absence of CD3 gamma, delta, and epsilon, and suggest that the role of CD3 zeta is to couple the TCR to intracellular signal transduction mechanisms.

Interaction of the unique N-terminal region of tyrosine kinase p56lck with cytoplasmic domains of CD4 and CD8 is mediated by cysteine motifs

p56lck, a lymphocyte-specific member of the src family of cytoplasmic protein-tyrosine kinases, is associated noncovalently with the cell surface glycoproteins CD4 and CD8, which are expressed on functionally distinct subpopulations of T cells. Using transient coexpression of p56lck with CD4 or CD8 alpha in COS-7 cells, we show that the unique N-terminal region of p56lck binds to the membrane-proximal 10 and 28 cytoplasmic residues of CD8 alpha and CD4, respectively. Two cysteine residues in each of the critical sequences in CD4, CD8 alpha, and p56lck are required for association. Our results suggest a novel role for cysteine-mediated interactions between unrelated proteins and provide a model for the association of other src-like cytoplasmic kinases with transmembrane proteins.

New insights into T-cell antigen receptor structure and signal transduction

Recent studies have provided insights into how the complex structure of the T-cell antigen receptor relates to its signal transduction function. Both the CD3 and zeta subunits contain functioning signaling modules that regulate the activation of tyrosine kinases and phosphorylation of cellular substrates.

Signaling checkpoints during the development of T lymphocytes

Two major lineage decisions face immature T cells as they develop in the thymus. At an early stage in their development, they must first commit to either the gamma delta or alpha beta lineages. If they opt for the alpha beta lineage, then at a later stage they must also choose between a CD4+ or CD8+ fate before they can pass through the thymic medulla and exit to the periphery. Thymocyte survival at key developmental checkpoints is determined by signaling from cytokine receptors and the T-cell receptor. Recent advances have been made in contemporary understanding of the signals that regulate thymocyte survival, proliferation and lineage decisions.

Molecular and Genetic Insights Into T Cell Antigen Receptor Signal Transduction

T lymphocytes are relatively quiescent cells, poised in the G0 stage of the cell cycle, until they encounter antigen. The initiation of an immune response by T cells requires the recognition of antigen by the T cell antigen receptor (TCR) and the conversion of this recognition event into biochemical signals that can induce a cellular response. This cellular response leads to the clonal expansion of antigen-specific differentiated effector T cells.

Surface chimeric receptors as tools in study of lymphocyte activation.

In this chapter we have described a powerful technology that has allowed the functional dissection of individual subunits from oligomeric receptors. We have focused primarily on chimeras derived from antigen receptors or their downstream signaling components to illustrate the wide utility of the approach; however, the technology has been applied to numerous multimeric receptors of the immune system including cytokine receptors, Fc receptors, and natural killer (NK) cell inhibitory receptors. Although the significance of the structural complexity of oligomeric receptors is by no means understood, it is certain that valuable benefits must be derived from the integrated function of their subunits. In the case of antigen receptors, the multiplicity of ITAMs likely allows the cell to distinguish subtle variations in ligand affinities with exquisite sensitivity. Clearly, an isolated subunit that is ligated with antibodies cannot confer such complex function. For instance, it cannot reveal the subtle changes in signal transduction that likely occur on stimulation with altered antigenic peptide ligands or during a complex cell-cell interaction. However, before the intricacies of integrated receptor function can be appreciated, the potential or unique functional properties contributed by each individual receptor component must first be understood. Providing a tool to acquire this kind of understanding has been the greatest asset of this technology. Acknowledging its limitations, the use of surface chimeric receptors remains an invaluable approach toward our understanding the complex function of oligomeric receptors.

A Clue to Antigen Receptor Tails

In science, context is essential for interpreting and integrating new observations into current paradigms. Without sufficient context, the significance of an important or insightful discovery may go unrealized until additional information is gleaned that brings its value to light. One such example

Molecular Dissection of T-Cell Antigen Receptor Signal Transduction

Antigen recognition by mature and immature T-cells induces a cascade of events which ultimately leads to a variety of cellular responses (Weiss 1991). One of the major features of this signal transduction pathway is the tyrosine phosphorylation of several cytoplasmic and membrane proteins. Both kinetic studies and analyses using tyrosine kinase inhibitors showed that one of the earliest events in T-cell activation involves tyrosine kinases (June et al. 1990 a, b). PLC-γ1 is one of the substrates of this tyrosine kinase activity induced by T cell antigen receptor (TCR) stimulation (Weiss et al 1991b) This phosphorylation activates the enzyme and induces the catalysis of phosphatidylinositol 4,5-bisphosphate (PIP2) into diacylglycerol and inositol 1,4,5-trisphosphate (IP3), which activate PKC and mobilize intracellular calcium, respectively (Rhee et al 1989; Nishibe S et al 1990).