UC Berkeley

Cells integrate signals from environmental inputs to regulate cellular programming, such as protein production, differentiation, and proliferation. This cellular decision making process occurs across multiple time and length scales, during which a cascade of biochemical reactions transduces signal from inputs to a functional outcome. T cells are a prototypical example of cells that make activation decisions based on discriminating a low level of agonist ligands, which is a peptide fragment presented on Major Histocompatibility Complex (pMHC). In recent years it has become evident that only a handful of ligands are sufficient to activate T cells to secrete cytokines or cytotoxic granules. In this low density pMHC regime, the sequence of T cell receptor (TCR) binding events that a T cell collects is highly stochastic, and the physical mechanism of input integration that overcomes the noise and accurately translates the signal remains unknown.

This work investigates the molecular processes underlying how T cell input integration results in a cellular activation outcome. First, an imaging assay based on a live cell-supported membrane platform is introduced to directly observe the sequence of single molecule pMHC:TCR bindings (inputs) and the presence of ligand:receptor:kinase complexes. Titrations of pMHC densities are performed to study the effect of initial binding events on subsequent binding events. Similar measurements were extended to auto-reactive human T cells to compare their binding affinities to normal human T cells. We further map the sequence of pMHC:TCR binding events to early T cell activation, as read out by the translocation of the transcription factor NFAT. Results from the input-outcome assay indicates that T cells may coordinate input accumulation. We then investigate protein assemblies as a potential mechanism of coordinated input accumulation. This work highlights the common feature of spatiotemporally coordinated input accumulation as a general design principle in cellular decision making.