Article Update

Friday, November 6, 2020



As we have described above, successful TCR triggering involves a multitude of signal transduction events that culminate in Tcell activation. But what is the probability of this occurring in an in vivo setting? Tcells need to be highly efficient at finding their cognate antigen and discriminating between activating and non-activating peptideMHC complexes for several reasons.

First, the numbers of Tcells bearing the correct TCRs for productive engagement with peptide–MHC are typically small; 1 in 100 000 or fewer cells being capable of responding to a particular peptide–MHC combination is not unusual. Therefore, Tcells need to be able to efficiently recognize the correct peptide–MHC combination in a veritable sea of self peptide–MHC and nonactivating peptideMHC molecules. Because of the need to search for the correct peptide–MHC combination, naive Tcells are in continual motion within a lymph node, scanning at speeds that enable them to visit up to 5000 DCs in 1 hour; quite the social networkers! Because of this ferocious rate of movement, TCR–peptide–MHC inter­ actions are very fleeting, as cells brush past each other at high speed. When an activating TCR–peptide–MHC interaction does occur, as few as 10 peptide–MHC complexes can persuade a Tcell to stop and linger, forming a more stable interaction that leads to the productive assembly of the immunological synapse (described below). Of course, the reader will under­ stand that the Tcell needs to be pretty certain that this is the correct peptide–MHC complex to respond to, for if an error is made, the consequences are potentially calamitous and can result in autoimmunity.

The behavior of a Tcell within a lymph node as it searches for the correct peptide–MHC combination can be divided into several phases. Whereas Tcell movement is rapid during the seeking phase (phase I), encounter with agonistic peptide–MHC leads to stable TcellDC interactions lasting approximately 12 hours (phase II), during which cytokines such as IL2 are produced. This is followed by a return to rapid movement involving further transient DC interactions (phase III), during which the Tcell divides a number of times and exits the lymph node.


A serial TCR engagement model for Tcell activation

We have already commented that the major docking forces that conjugate the APC and its Tlymphocyte counterpart must come from the complementary accessory molecules such as ICAM1/LFA1 and LFA3/CD2, rather than through the relatively lowaffinity TCRMHC plus peptide links (Figure 7.3). Nonetheless, cognate antigen recognition by the TCR remains a sine qua non for Tcell activation. Fine, but how can as few as 10 MHC–peptide complexes on a DC, through their lowaffinity complexing with TCRs, effect the Herculean task of sustaining a raised intracellular calcium flux for the 60 minutes required for full cell activation? Any fall in calcium flux, as may be occasioned by adding an antibody to the MHC, and NFATc dutifully returns from the nucleus to its cytoplasmic location, so aborting the activation process.

Serial triggering model of T‐cell receptor (TCR) activation

Figure 7.19 Serial triggering model of Tcell receptor (TCR) activation. Intermediateaffinity complexes between MHC–peptide and TCR survive long enough for a successful activation signal to be transduced by the TCR, and the MHC–peptide dissociates and fruitfully engages another TCR. A sustained high rate of formation of successful complexes is required for full Tcell activation. Lowaffinity complexes have a short halflife that either has no effect on the TCR or produces inactivation, perhaps through partial phosphorylation of ζ chains. Green dots denote successful TCR activation; red dots denote TCR inactivation; and a dash denotes no effect. The length of the horizontal bar indicates the lifetime of that complex. Being of low affinity, they recycle rapidly and engage and inactivate a large number of TCRs. Highaffinity complexes have such a long lifetime before dissociation that insufficient numbers of successful triggering events occur. Thus modified peptide ligands of either low or high affinity can act as antagonists by denying the agonist access to adequate numbers of vacant TCRs. Some modified peptides act as partial agonists in that they produce differential effects on the outcomes of Tcell activation. For example, a single residue change in a hemoglobin peptide reduced IL4 secretion 10fold but completely knocked out Tcell proliferation. The mechanism presumably involves incomplete or inadequately transduced phosphorylation events occurring through a truncated halflife of TCR engagement, allosteric effects on the MHC–TCR partners, or orientational misalignment of the peptide within the complex.

Surprisingly, Salvatore Valitutti and Antonio Lanzavecchia have shown that as few as 100 MHC–peptide complexes on an APC can downregulate 18 000 TCRs on its cognate Tlymphocyte partner. They suggest that each MHC–peptide complex can serially engage up to 200 TCRs. In their model, conjugation of a MHC–peptide dimer with two TCRs activates signal trans­ duction, phosphorylation of the CD3associated ζ chains with subsequent downstream events, and then downregulation of those TCRs. Intermediate affinity binding favors dissociation of the MHC–peptide, freeing it to engage and trigger another TCR, so sustaining the required intracellular activation events. The model for agonist action would also explain why peptides giving interactions of lower or higher affinity than the optimum could behave as antagonists (Figure 7.19). The important phenomenon of modified peptides behaving as partial agonists, with differential effects on the outcome of Tcell activation, is addressed in the legend to Figure 7.19.


The immunological synapse

Figure 7.20 The immunological synapse. (a) The formation of the immunological synapse. Tcells were brought into contact with planar lipid bilayers and the positions of engaged MHC–peptide (green) and engaged ICAM1 (red) at the indicated times after initial contact are shown. Reproduced with permission of AAAS.). (b) Diagrammatic representation of the resolved synapse in which the adhesion molecule pairs CD2/LFA3 and LFA1/ICAM1, which were originally in the center, have moved to the outside and now encircle the antigen recognition and signaling interaction between TCR and MHC–peptide and the costimulatory interaction between CD28 and B7. The CD43 molecule has been reported to bind to ICAM1 and Eselectin, and upon ligation is able to induce IL2 mRNA, CD69, and CD154 (CD40L) expression and activate the DNAbinding activity of the AP1, NFkB, and NFAT transcription factors.

The immunological synapse

Experiments using peptide–MHC and ICAM1 molecules labeled with different fluorochromes and inserted into a planar lipid bilayer on a glass support have provided evidence for the idea that Tcell activation occurs in the context of an immunological synapse. These and other imaging studies have revealed that the immunological synapse between the Tcell and the DC has a “bull’s eye” pattern with a central cluster of TCR– peptide–MHC, known as the cSMAC (central supramolecular activation complex), surrounded by a ring of the integrin LFA1 interacting with its cognate ligand ICAM1 on the DC (Figure 7.20). Initially unstable TCR–MHC interactions occur outside of the integrin ring, followed by transit of the peptide–MHC molecules into the cSMAC, changing places with the adhesion molecules that now form the outer ring (Figure 7.20). It has been suggested that the generation of the immunological synapse only occurs after a certain initial threshold level of TCR triggering has been achieved, its formation being dependent upon cytoskeletal reorganization and leading to potentiation of the signal. LFA1 engagement with ICAM1 is essential for formation of immunological synapses for a number of reasons. In the early stages of synapse formation, these molecules serve in a predominantly adhesive capacity to tether the opposing cells to facilitate TCR and peptide–MHC interactions, thereby allowing the Tcell to scan the peptide–MHC complex on offer.

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