(*)?=?IKK isoform-dependent phosphorylation sites: p-IKK Ser176/180, p-IKK Ser177/181. in the Supplementary Materials; the remaining data are available from the corresponding author on request. Abstract The innate immune system processes pathogen-induced signals into cell fate decisions. How information is turned to decision remains unknown. By combining stochastic mathematical modelling and experimentation, we demonstrate that feedback interactions between the IRF3, NF-B and STAT pathways lead to switch-like responses to a viral analogue, poly(I:C), in contrast to pulse-like responses to bacterial LPS. Poly(I:C) activates both IRF3 and NF-B, a requirement for induction of IFN expression. Autocrine IFN initiates a JAK/STAT-mediated positive-feedback stabilising nuclear IRF3 and NF-B in first responder cells. Paracrine IFN, in turn, sensitises second responder cells through a JAK/STAT-mediated positive feedforward pathway that upregulates the positive-feedback components: RIG-I, PKR and OAS1A. In these sensitised cells, the live-or-die decision phase following poly(I:C) exposure is shorterthey rapidly produce antiviral responses and commit to apoptosis. The interlinked positive feedback and feedforward signalling is key for coordinating cell fate decisions in cellular populations restricting pathogen spread. Introduction Molecular networks process analogue signals into discrete cell fate decisions1. Information processing employs regulatory elements Aceclofenac such as gene switches, logic gates, or feedback/feedforward loops2. In the NF-B pathway, negative feedbacks mediated by NF-B inhibitors, IB Aceclofenac and A20, transform tonic TNF3,4, IL15 or LPS6C8 signals into oscillatory or pulse-like responses. Positive feedbacks may lead to bi- or multistability allowing cells to assume one of mutually exclusive states depending on the strength and/or duration of stimuli9,10. Interlinked negative and positive feedbacks may lead to a more elaborate behaviour, that combines oscillatory responses with binary switches11. Pathways that evolved to respond to stress are governed by systems of coupled feedbacks12 that may also involve cell-to-cell communication13. The question is how the specific topologies of these networks enable cell fate decisions. Here, to address this query we combine mathematical modelling and experimental validation, and analyse how feedbacks coupling NF-B, IRF3 and STAT pathways govern the innate immune system and travel cells into the antiviral state and apoptosis. Even though bacterial LPS and a viral nucleic acid analogue, poly(I:C), activate the same innate immunity pathways, the response characteristics are stimulus-dependent14. LPS elicits transient or oscillatory activation of NF-B, terminated by synthesis of IB and A206C8. The response to poly(I:C) offers different dynamics. Most cells are inert, but a portion respond by stable activation of IRF3, NF-B and STAT1/2, and eventually commit to apoptosis. Cell fate is not determined exclusively from the stimuli but also depends on the initial state of the cell (extrinsic noise) and stochasticity in transmission processing (intrinsic noise)15,16. Higher organisms with intercellular signalling may benefit from stochasticity by keeping only a subpopulation of cells sensitive to particular stimuli. Recent research shown the part of stochasticity-driven human population heterogeneity and paracrine transmission propagation in shaping the antiviral response of cell human population17C19. Here we investigate the interconnections Aceclofenac of the major signalling arms of the innate immune response to viral patterns schematically demonstrated in (Fig.?1a). We determine autocrine and paracrine feedbacks coupling the IRF3, NF-B and STAT1/2 pathways, that allow for proportionate cell Aceclofenac fate decisions coordinated across heterogeneous populations. Our data suggest that a small human population of the sensitive cells form the first line of defence and sensitise additional cells by secreting IFN. The IFN-primed cells have increased levels of positive-feedback parts, which allows them MYO9B to shorten the live-or-die decision phase and increase their apoptotic rate after a subsequent poly(I:C) activation. In the following, we discuss the data gathered to derive the mathematical model of innate immune reactions. For brevity, actually before showing the model, we juxtapose experimental and.