Human CD8+ T Cells Exhibit a Shared Antigen Threshold for Different Effector Responses.

T cells recognizing cognate pMHC Ags become activated to elicit a myriad of cellular responses, such as target cell killing and the secretion of different cytokines, that collectively contribute to adaptive immunity. These effector responses have been hypothesized to exhibit different Ag dose and affinity thresholds, suggesting that pathogen-specific information may be encoded within the nature of the Ag. In this study, using systematic experiments in a reductionist system, in which primary human CD8+ T cell blasts are stimulated by recombinant peptides presented on MHC Ag alone, we show that different inflammatory cytokines have comparable Ag dose thresholds across a 25,000-fold variation in affinity. Although costimulation by CD28, CD2, and CD27 increased cytokine production in this system, the Ag threshold remained comparable across different cytokines. When using primary human memory CD8+ T cells responding to autologous APCs, equivalent thresholds were also observed for different cytokines and killing. These findings imply a simple phenotypic model of TCR signaling in which multiple T cell responses share a common rate-limiting threshold and a conceptually simple model of CD8+ T cell Ag recognition, in which Ag dose and affinity do not provide any additional response-specific information.

Architecture of a minimal signaling pathway explains the T-cell response to a 1 million-fold variation in antigen affinity and dose.

T cells must respond differently to antigens of varying affinity presented at different doses. Previous attempts to map peptide MHC (pMHC) affinity onto T-cell responses have produced inconsistent patterns of responses, preventing formulations of canonical models of T-cell signaling. Here, a systematic analysis of T-cell responses to 1 million-fold variations in both pMHC affinity and dose produced bell-shaped dose-response curves and different optimal pMHC affinities at different pMHC doses. Using sequential model rejection/identification algorithms, we identified a unique, minimal model of cellular signaling incorporating kinetic proofreading with limited signaling coupled to an incoherent feed-forward loop (KPL-IFF) that reproduces these observations. We show that the KPL-IFF model correctly predicts the T-cell response to antigen copresentation. Our work offers a general approach for studying cellular signaling that does not require full details of biochemical pathways.

Cooperative Clustering Digitizes Biochemical Signaling and Enhances its Fidelity.

Many membrane-bound molecules in cells form small clusters. It has been hypothesized that these clusters convert an analog extracellular signal into a digital intracellular signal and that this conversion increases signaling fidelity. However, the mechanism by which clusters digitize a signal and the subsequent effects on fidelity remain poorly understood. Here we demonstrate using a stochastic model of cooperative cluster formation that sufficient cooperation leads to digital signaling. We show that despite reducing the number of output states, which decreases fidelity, digitization also reduces noise in the system, which increases fidelity. The tradeoff between these effects leads to an optimal cluster size that agrees with experimental measurements.

Perfect adaptation of CD8+ T cell responses to constant antigen input over a wide range of affinities is overcome by costimulation.

Reduced T cell responses by contrast antigen stimulation can be rescued by signals from costimulatory receptors.

Dynamic regulation of CD28 conformation and signaling by charged lipids and ions.

CD28 provides an essential costimulatory signal for T cell activation, and its function is critical in antitumor immunity. However, the molecular mechanism of CD28 transmembrane signaling remains elusive. Here we show that the conformation and signaling of CD28 are regulated by two counteractive charged factors, acidic phospholipids and Ca2+ ions. NMR spectroscopy analyses showed that acidic phospholipids can sequester CD28 signaling motifs within the membrane, thereby limiting CD28 basal signaling. T cell receptor (TCR) activation induced an increase in the local Ca2+ concentration around CD28, and Ca2+ directly disrupted CD28-lipid interaction, leading to opening and signaling of CD28. We observed that the TCR, Ca2+, and CD28 together form a dual-positive-feedback circuit that substantially amplifies T cell signaling and thus increases antigen sensitivity. This work unravels a new regulatory mechanism for CD28 signaling and thus contributes to the understanding of the dependence of costimulation signaling on TCR signaling and the high sensitivity of T cells.