Initial TCR Signal Strength Imprints GATA3 and Tbet Expression Driving T-helper Cell Fate Decisions

With the exception of the T-helper 2 (Th2) subset, cytokine driven pathways provide a robust mechanistic explanation for the observed outcomes of CD4+ T-cell differentiation. Using a quantitative model of activation, we studied the integration of TCR-signal-strength with cytokine signalling during Th2 differentiation. Upon the initial activation of Th-naive cells, TCR signalling was found to set early expression levels for the master regulators of differentiation Tbet and GATA3, independent of the presence of polarizing cytokines. Subsequently cytokine stimuli modulated transcription factor (TF) expression levels to tune the outcome of differentiation. Here, weak TCR signalling was sufficient to drive the early upregulation of GATA3 and induce Th2 differentiation, in an IL-4 independent manner. Th1 differentiation was however shown to require additional cytokine signalling input, either in the form of autocrine IFN{gamma} or exogenous IL-12. Using mathematical modelling we demonstrate that T-helper differentiation occurs along a continuum of states. Set by the relative co-expression of regulatory TFs, where effector cytokine production is controlled in a probabilistic manner determined by the relative levels GATA3 and Tbet expressed. Together, our data indicate TCR signalling inputs drive an early bifurcation in the T-helper differentiation pathway. Together, the integration of TCR signal strength with cytokine inputs act as a mechanism for the detection of immuno-evasive parasitic infections, whilst providing an additional checkpoint to prevent aberrant Th1 associated immunopathology.