T-cell triggering thresholds are modulated by the number of antigen within individual T-cell receptor clusters

Howard Hughes Medical Institute, Department of Chemistry, and Biophysics Graduate Group, University of California, Berkeley, CA 94720, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 05/2011; 108(22):9089-94. DOI: 10.1073/pnas.1018771108
Source: PubMed


T cells react to extremely small numbers of activating agonist peptides. Spatial organization of T-cell receptors (TCR) and their peptide-major histocompatibility complex (pMHC) ligands into microclusters is correlated with T-cell activation. Here we have designed an experimental strategy that enables control over the number of agonist peptides per TCR cluster, without altering the total number engaged by the cell. Supported membranes, partitioned with grids of barriers to lateral mobility, provide an effective way of limiting the total number of pMHC ligands that may be assembled within a single TCR cluster. Observations directly reveal that restriction of pMHC content within individual TCR clusters can decrease T-cell sensitivity for triggering initial calcium flux at fixed total pMHC density. Further analysis suggests that triggering thresholds are determined by the number of activating ligands available to individual TCR clusters, not by the total number encountered by the cell. Results from a series of experiments in which the overall agonist density and the maximum number of agonist per TCR cluster are independently varied in primary T cells indicate that the most probable minimal triggering unit for calcium signaling is at least four pMHC in a single cluster for this system. This threshold is unchanged by inclusion of coagonist pMHC, but costimulation of CD28 by CD80 can modulate the threshold lower.

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    • "Just after the contact of a T cell with an APC, the formation of microclusters which are clusters of TCRs and other molecules is induced on the cell surface [41] [42] [43]. Manz et al. reported that the minimal triggering unit for the T cell activation (calcium signaling) is probably at least four pMHCs in a single cluster [44]. Huang et al. also showed that a single TCR microcluster contains about 100 TCRs within 2 μm 2 around a single pMHC, 1 minute after the antigen presentation [35]. "
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    • "Thus, the combination of short pMHC binding half-lives with a TCR and TCR nanoclusters may facilitate serial engagement and/or multiple rounds of rebinding[1] [22]. It has also been suggested that multivalent arrays of two or more pMHC molecules need to encounter TCR clusters for calcium mobilization[23]. In addition, confinement of TCRs in nanoclusters may create a close contact zone between the T cell and APC surface that leads to the exclusion of phosphatases with large ectodomains, leading to prolonged TCR phosphorylation[24] [25]. "
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