The overall molecular and biochemical mechanisms of interferon signalling are relatively well understood. Biochemical descriptions, however, do not directly lead to understanding how different interferon concentrations are translated into distinct responses in the same cell. Moreover, conventional approaches have measured the averaged response, which hides the variability of individual cells, and therefore misleadingly suggests precise input-output relationships.
To understand to what extend the dose determines response to interferons in individual cells we quantified abundance and localisation of p-YSTAT1, STAT1 and IRF1 in single cells over time using confocal imaging and immunocytofluorescence, upon stimulation with different concentrations of IFN-β and IFN-γ. Integration of experimental data with computational modelling has revealed that in a basal state cell-to-cell variability upon IFN stimulation enables cells only to discrimination between presence and absence of IFN stimulation. Information about the presence and absence of stimulation is first encoded in the abundance of nuclear p-YSTAT and subsequently remembered in the overall level of STAT1. Surprisingly, we have found that one of the effects of remembering the prior exposure is a reduced overall variability of p-YSTAT1 in response to a subsequent exposure, which leads to a changed expression pattern of IRF1.
These results demonstrate that individual cells have a rather limited capacity to recognise concentrations of interferons, which however can be remembered over a period of time. Moreover, additional concentrations can be recognised and translated into distinct responses in emergency situations like inflammation.