Differential clustering of CD4 and CD3zeta during T cell recognition

Role of the actin cytoskeleton in T cell absorption and internalization of ligands from APC

A feature of T-APC interaction is that, via either TCR or CD28, T cells can absorb molecules from APC on to the cell surface and then internalize these molecules. Here, using both normal and TCR-transgenic T cells, we investigated the mechanism of T cell absorption of molecules from APC and the role of the cytoskeleton. The results show that although activated T cells could absorb APC molecules in the form of cell fragments, uptake of molecules by resting T cells required direct T-APC interaction. Based on studies with latrunculin B, surface absorption of molecules by resting T cells was crucially dependent upon the actin cytoskeleton for both CD28- and TCR-mediated absorption. Significantly, however, TCR-mediated absorption became strongly resistant to latrunculin B when the concentration of MHC-bound peptide on APC was raised to a high level, implying that the actin cytoskeleton is only important for absorption when the density of receptor/ligand interaction is low. By contrast, in all situations tested, the actin cytoskeleton played a decisive role in controlling T cell internalization of ligands from the cell surface.

Changes in the T cell receptor macromolecular signaling complex and membrane microdomains during T cell development and activation

Initiation and propagation of T cell receptor signaling pathways involves the mobilization and aggregation of a variety of signaling intermediates with the T cell receptor and associated molecules into specialized signaling complexes. Accumulating evidence suggests that differential regulation of the formation and composition of the T cell receptor macromolecular signaling complex may affect the different biological consequences of T cell activation. The regulatory mechanisms involved in the assembly of these complexes remains poorly understood, but in part is affected by the avidity of the T cell receptor ligand, co-stimulatory signals, and by the differentiation state of the T cell.

Single-cell analyses reveal two defects in peptide-specific activation of naive T cells from aged mice

Confocal fluorescent microscopy was used to study redistribution of membrane-associated proteins in naive T cells from young and old mice from a transgenic stock whose T cells express a TCR specific for a peptide derived from pigeon cytochrome C. About 50% of the T cells from young mice that formed conjugates with peptide-pulsed APC were found to form complexes, at the site of binding to the APC, containing CD3epsilon, linker for activation of T cells (LAT), and Zap-70 in a central area and c-Cbl, p95(vav), Grb-2, PLC gamma, Fyn, and Lck distributed more uniformly across the interface area. Two-color staining showed that those cells that were able to relocalize c-Cbl, LAT, CD3epsilon, or PLC gamma typically relocalized all four of these components of the activation complex. About 75% of conjugates that rearranged LAT, c-Cbl, or PLC gamma also exhibited cytoplasmic NF-AT migration to the T cell nucleus. Aging had two effects. First, it led to a diminution of approximately 2-fold in the proportion of T cell/APC conjugates that could relocalize any of the nine tested proteins to the immune synapse. Second, aging diminished by approximately 2-fold the frequency of cytoplasmic NF-AT migration among cells that could generate immune synapses containing LAT, c-Cbl, or PLC gamma. Thus naive CD4 T cells from old mice exhibit at least two separable defects in the earliest stages of activation induced by peptide/MHC complexes.

T Cell Receptor Binding to a pMHCII Ligand Is Kinetically Distinct from and Independent of CD4

Immune recognition of pMHCII ligands by a helper T lymphocyte involves its antigen-specific T cell receptor (TCR) and CD4 coreceptor. We have characterized the binding of both molecules to the same pMHCII. The D10 alphabeta TCR heterodimer binds to conalbumin/I-A(k) with virtually identical kinetics and affinity as the single chain ValphaVbeta domain module (scD10) (Kd = 6-8 microm). The CD4 ectodomain does not alter either interaction. Moreover, CD4 alone demonstrates weak pMHCII binding (Kd = 200 microm), with no discernable affinity for the alphabeta TCR heterodimer. Hence, rather than providing a major contribution to binding energy, the critical role for the coreceptor in antigen-specific activation likely results from transient inducible recruitment of the CD4 cytoplasmic tail-associated lck tyrosine kinase to the pMHCII-ligated TCR complex.

Information transfer at the immunological synapse

Antigen-specific activation of T lymphocytes requires the interaction of their clonally distributed T-cell receptors with plasma membrane ligands composed of foreign peptide antigens bound to major histocompatibility complex molecules. For proliferation and differentiation to ensue, a variety of other adhesive and accessory proteins must also interact with their counter-receptors on the antigen-presenting cell to facilitate and complement the T-cell receptor-antigen recognition event. Recent studies have revealed that these various proteins show an unexpected degree of spatial organization in the zone of cell-cell contact. This region of membrane approximation is now referred to as the "immunological synapse" because of its functional analogy to the site of intercellular information transfer between neurons. Here, we review the evidence for signaling-dependent control of the dynamic changes in protein distribution that gives rise to the synapse and try to relate the emerging spatio-temporal information on synapse formation to T-cell biology.

Thirty-six views of T-cell recognition

While much is known about the signalling pathways within lymphocytes that are triggered during activation, much less is known about how the various cell surface molecules on T cells initiate these events. To address this, we have focused on the primary interaction that drives T-cell activation, namely the binding of a particular T-cell receptor (TCR) to peptide-MHC ligands, and find a close correlation between biological activity and off-rate; that is, the most stimulatory TCR ligands have the slowest dissociation rates. In general, TCRs from multiple histocompatibility complex (MHC) class-II-restricted T cells have half-lives of 1-11s at 25 degrees C, a much narrower range than found with antibodies and suggesting a strong selection for an optimum dissociation rate. TCR ligands with even faster dissociation rates tend to be antagonists. To observe the effects of these different ligands in their physiological setting, we made gene fusions of various molecules with green fluorescent protein (GFP), transfected them into the relevant lymphocytes, and observed their movements during T-cell recognition using multicolour video microscopy. We find that clustering of CD3zeta-GFP and CD4-GFP on the Tcell occurs concomitantly or slightly before the first rise in calcium by the T cell, and that various GFP-labelled molecules on the B-cell side cluster shortly thereafter (ICAM-1, class II MHC, CD48), apparently driven byT-cell molecules. Most of this movement towards the interface is mediated by signals through the co-stimulatory receptors, CD28 and LFA-1, and involves myosin motors and the cortical actin cytoskeleton. Thus, we have proposed that the principal mechanism by which co-stimulation enhances T-cell responsiveness is by increasing the local density of T-cell activation molecules, their ligands and their attendant signalling apparatus. In collaboration with Michael Dustin and colleagues, we have also found that the formation and stability of the TCR-peptide-MHC cluster at the centre of the interaction cap between T and B cells is highly dependent on the dissociation rate of the TCR and its ligand. Thus, we are able to link this kinetic parameter to the formation of a cell surface structure that is linked to and probably causal with respect to T-cell activation.