Molecular dynamics in the membranes of helper T cells.

Abstract

We provide evidence that redistributions and interactions of integral proteins in the fluid membranes of helper T (Th) cells may play important roles in Th-cell activation. A particular monoclonal antibody, 3D3, directed to a clonotypic determinant on the T-cell receptor (TCR) of the cloned Th-cell line D10, had previously been shown to be distinctively capable of directly activating D10 cells at low concentrations. We demonstrate here by immunofluorescence experiments that it is also distinctively able itself to produce a clustering (capping) of the TCRs on the D10 cell surface. Simultaneously, by means of double-immunofluorescence experiments, we find that the 3D3-induced clustering of the TCRs distinctively produces a co-clustering of the accessory molecule CD4 with the TCR clusters, although the CD4 and TCR molecules are normally independent of one another in the D10 cell membrane. These results, and related ones previously obtained from studies of the interactions of D10 Th cells with antigen-presenting cells, are analyzed to suggest that the membrane clustering of TCRs and the induced TCR-CD4 interactions are critical to the signaling events in Th-cell activation.The direct interaction of helper T (Th) cells with specific antigen-presenting B cells (APCs) has been implicated in the proliferation and ultimate differentiation of APCs into antibody-secreting plasmacytes as well as in the proliferation of, and induction of lymphokine secretion by, Th cells. That such a direct cell-cell interaction occurs was previously inferred from the fact that a Th cell expresses on its surface a clonotypic T-cell receptor (TCR) with specific affinity for a unique ligand on the surface of an APC, a ligand consisting of a fragment of an antigen molecule attached to a molecule of the polymorphic class II major histocompatibility complex (MHC), and was supported by studies of specific binding of Th hybridoma cells to monolayers of APCs (1). That specific binary Th-APC interactions indeed occur has been demonstrated by immunofluorescence microscopy experiments with individual cell-cell couples formed between cloned Th cells and APCs (2-4), and also by specific cell-cell binding observations (5). Two phenomena were shown to identify a specific Th-APC couple and distinguish it from a nonspecific cell couple: (i) massive reorganizations of cytoplasmic organelles and of cytoskeletal proteins occur inside the Th cell (2, 3), and (ii) the TCRs become concentrated into the region of the Th cell surface that is in contact with the APC (4).In addition to the TCR, the accessory molecules CD4 and lymphocyte function-associated antigen 1 (LFA-1) on the Th cell are thought to play important roles in the Th-APC interaction (see Discussion), but the precise nature of those roles is not yet known. Of considerable interest, therefore, are the observations that both CD4 (4) and LFA-1 molecules (A.K., S. L. Swain, and S.J.S., unpublished experiments) are also clustered with the TCR into the region of the Th-cell membrane that is in contac...

Molecular Basis of T Cell Inactivation by CTLA-4

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“…To study whether receptor accumulation at the T cell-APC interface could be actively driven by the T cell cytoskeleton, we monitored the general movement of the cortical actin cytoskeleton and linked receptors (11) using the classical technique of attaching large beads to the surfaces of antigen-specific T cells. We coated 4.5-m beads with an antibody to the T cell surface antigen ICAM-1 (12).…”

“…T cell activation, initiated by TCR ligation and CD28 costimulation, results in the recruitment of p56 lck to the TCR cap and phosphorylation of multiple substrates including TCR, ZAP-70, and LAT. CTLA-4, newly expressed (1) or preexisting in resting cells (10), is exported to the cell surface and binds to B7 molecules present in antigen presenting cells (11). CTLA-4 membrane localization may be facilitated by colocalization to the site of TCR engagement through direct interaction with TCR and p56 lck -induced tyrosine phosphorylation of CTLA-4 (12).…”

Live-Cell Dynamics and the Role of Costimulation in Immunological Synapse Formation

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Effect of T-cell receptor antagonism on interaction between T cells and antigen-presenting cells and on T-cell signaling events.

“…First, the low-affinity interaction between antagonists and TCRs could, because of a rapid off-rate, prevent critical clustering of the receptors required for signaling. Alternatively, it could be speculated that the multiplicity of steps involved in T-cell signaling reflect conformational changes in the TCR induced by the bound peptide/MHC complexes (25,26). It was suggested previously that both crosslinking and conformational changes in the TCR upon antigen recognition may be required for optimal T-cell activation (27).…”