Role of Cell-to-Cell Variability in Activating a Positive Feedback Antiviral Response in Human Dendritic Cells Jianzhong Hu1, German Nudelman2,3, Yishai Shimoni2,3, Madhu Kumar1, Yaomei Ding1, Carolina Lo´ pez1, Fernand Hayot2,3, James G. Wetmur1,3*, Stuart C. Sealfon1,2 1 Department of Microbiology, Mount Sinai School of Medicine, New York, New York, United States of America, 2 Department of Neurology, Mount Sinai School of Medicine, New York, New York, United States of America, 3 Center for Translational Systems Biology, Mount Sinai School of Medicine, New York, New York, United States of America Abstract In the first few hours following Newcastle disease viral infection of human monocyte-derived dendritic cells, the induction of IFNB1 is extremely low and the secreted type I interferon response is below the limits of ELISA assay. However, many interferon-induced genes are activated at this time, for example DDX58 (RIGI), which in response to viral RNA induces IFNB1. We investigated whether the early induction of IFNBI in only a small percentage of infected cells leads to low level IFN secretion that then induces IFN-responsive genes in all cells. We developed an agent-based mathematical model to explore the IFNBI and DDX58 temporal dynamics. Simulations showed that a small number of early responder cells provide a mechanism for efficient and controlled activation of the DDX58-IFNBI positive feedback loop. The model predicted distributions of single cell responses that were confirmed by single cell mRNA measurements. The results suggest that large cell-to-cell variation plays an important role in the early innate immune response, and that the variability is essential for the efficient activation of the IFNB1 based feedback loop. Citation: Hu J, Nudelman G, Shimoni Y, Kumar M, Ding Y, et al. (2011) Role of Cell-to-Cell Variability in Activating a Positive Feedback Antiviral Response in Human Dendritic Cells. PLoS ONE 6(2): e16614. doi:10.1371/journal.pone.0016614 Editor: Patricia Aguilar, University of Texas Medical Branch, United States of America Received September 7, 2010; Accepted January 3, 2011; Published February 8, 2011 Copyright: 2011 Hu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by contract HHSN266200500021C and grant U19 AI06231 from the National Institute of Allergy and Infectious Diseases. The funders have no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: james.wetmur@mssm.edu Introduction The innate immune response to viral infection is essential in fighting infections, and the dynamics of the response can determine whether the infection evolves into pathology [1]. Dendritic cells (DCs) are the primary response cells mediating the progression from innate to adaptive immunity and the induction of self-tolerance [2,3]. It has recently been recognized that understanding the internal dynamics of DCs following viral infection can help elucidate the function of the immune system and help lead to better vaccination protocols [4,5,6,7]. Many viruses have evolved immune antagonists that subvert the innate immune response and facilitate their replication. Newcastle disease virus (NDV), however, is an avian virus that lacks a functioning immune antagonist for human cells [7,8,9]. Therefore it efficiently stimulates the innate immune response and represents an ideal stimulus with which to probe the activation dynamics of human DCs. Upon viral infection of DCs, constitutive RIG-I proteins (coded from the gene DDX58) can be activated upon detecting evidence of negative stranded viruses such as NDV in the cytoplasm, leading to IFNB1 induction through a signaling cascade. IFNB1 encodes IFNb, which is a type 1 interferon (IFN) that is secreted into the extracellular medium, where it binds to IFN cell surface receptors on the secreting cell and on neighboring cells. This binding activates a gene program that plays an essential role in both the DC antiviral response [10] and in DC maturation [9,11], and causes the up-regulation of many genes [12]. In particular, IFNb binding activates the Jak/Stat pathway, inducing the DDX58 gene and leads to RIG-I production. In infected cells, the activation of newly induced RIG-I, as well as of other genes involved in interferon induction such as IRF7 [13], leads to additional IFNB1 production, thus completing an IFNB1-DDX58 positive feedback loop. Previous single cell measurements of IFNB1 induction in NDV infected human monocyte-derived dendritic cells (MDDCs) showed large cell-to-cell variation ranging over 3–4 orders of magnitude at 8–10 hours after infection. These results, comple- mented by simultaneous measurements of DDX58 (coding for RIG-I) production at 6 and 10 hours, are confirmed by the experiments presented here. Taking as a starting point the observed stochasticity of the cellular IFNB1 response, we developed a stochastic agent-based model (ABM) that, after a fit to the later time data points, allowed extrapolation to early times after infection, where direct measurements are very difficult due to sampling limitations. Model simulations suggest that there is a small subset of early responder cells responsible for propagating cellular resistance to viral infection through the efficient activation of the IFNB1-DDX58 feedback loop. The existence of such a subset resolves an apparent paradox of average cellular response, namely the fact that contrary to the temporal order of the feedback loop, IFNB1 production remains near the limit of detection until 6 hours after infection, whereas that of DDX58, induced by IFNb through the feedback loop, reaches a significant level of expression several PLoS ONE | www.plosone.org 1 February 2011 | Volume 6 | Issue 2 | e16614