Research ArticleTCR Signaling

Mapping the stochastic sequence of individual ligand-receptor binding events to cellular activation: T cells act on the rare events

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Science Signaling  15 Jan 2019:
Vol. 12, Issue 564, eaat8715
DOI: 10.1126/scisignal.aat8715

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Linked events activate T cells

How T cells discriminate between T cell receptor (TCR) ligands that do or do not stimulate cellular activation remains unclear. Lin et al mapped both the TCR input and NFAT activation response in single live T cells using simultaneous single-molecule and epifluorescence microscopy. By varying both the density and TCR affinity of the peptide-MHC input, they found that neither the total number nor the duration of input binding events fully explained T cell activation to all stimuli. Instead, correlation analysis indicated that T cells were activated by inputs coupled in space and time. These data clarify how T cells respond to low-affinity ligands, which may inform the development and use of cancer immunotherapies.

Abstract

T cell receptor (TCR) binding to agonist peptide major histocompatibility complex (pMHC) triggers signaling events that initiate T cell responses. This system is remarkably sensitive, requiring only a few binding events to successfully activate a cellular response. On average, activating pMHC ligands exhibit mean dwell times of at least a few seconds when bound to the TCR. However, a T cell accumulates pMHC-TCR interactions as a stochastic series of discrete, single-molecule binding events whose individual dwell times are broadly distributed. With activation occurring in response to only a handful of such binding events, individual cells are unlikely to experience the average binding time. Here, we mapped the ensemble of pMHC-TCR binding events in space and time while simultaneously monitoring cellular activation. Our findings revealed that T cell activation hinges on rare, long–dwell time binding events that are an order of magnitude longer than the average agonist pMHC-TCR dwell time. Furthermore, we observed that short pMHC-TCR binding events that were spatially correlated and temporally sequential led to cellular activation. These observations indicate that T cell antigen discrimination likely occurs by sensing the tail end of the pMHC-TCR binding dwell time distribution rather than its average properties.

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