Dissecting common γ chain cytokine family signaling in T cells using cell-to-cell variability analysis.

Natural variability in abundance of signaling regulators can lead to divergence in cell fate, even within genetically identical cells sharing a common differentiation state. To leverage this observation, we introduce cell-to-cell variability analysis (CCVA), an experimental and computational methodology to quantify the correlation between variability in signaling regulator abundance and variation in sensitivity to stimuli. Here, we apply CCVA to investigate the unexpected effects of the interleukin 2 (IL-2) receptor α chain (IL-2Rα) on the sensitivity to common-gamma chain (γc) cytokines in primary T lymphocytes. Our work demonstrates that increased IL-2Rα abundance decreases the concentration of IL-2 but increases the concentrations of IL-7 and IL-15 required for a half-maximal activation (EC50) of downstream signal transducer and activator of transcription 5 (STAT5), without affecting the EC50 of other γc cytokines. To probe the mechanism of IL-2Rα's effect on γc family cytokine EC50s, we introduce a Bayesian-inference computational framework that models the formation of receptor signaling complexes using prior biophysical measurements. Using this framework, we demonstrate that a model in which IL-2Rα drives γc depletion through pre-assembly of complete IL-2 receptors is consistent with both CCVA data and prior measurements. The combination of CCVA and computational modeling yields quantitative understanding of the crosstalk of γc cytokine signaling in T lymphocytes.