Dynamics of viral infection

Division Of Basic Sciences - Nci

Investigators

Abstract

Our recent projects pivoted from immune responses to coronaviruses to the more fundamental aspects of host/pathogen interactions namely the tug-of-war between viral replication and innate immune response at the individual cell level. Cytosolic viral sensors (CyVSP) like PKR, RIG-I, and MDA5 initiate innate immune responses upon detecting viral RNA. However, many viruses evade these defenses and replicate efficiently. In our project, we discovered that successful replication requires surpassing a viral input threshold that activates negative feedback regulators to suppress innate signaling. We termed this concept the virus input threshold for replication (VITREP). Using multiple types of viral infections, in diverse human cell types, we demonstrated that only supra-unitary levels of viral RNA-more than one viral genome per cell-could trigger activation of key suppressors. These molecules were rapidly upregulated in a manner that was dependent on the amount of viral genome, but independent of viral protein synthesis or replication, indicating that sensing occurred at the input stage. We tackled these experimental findings by retrofitting our adaptive kinetic proofreading model initially developed to understand the sharpness of ligand discrimination by T cells. Indeed, CyVSP must discriminate between a large quantity of self mRNA and a small quantity of viral (non-self) RNAs. The biophysics of RNA/CyVSP interactions did not add up so we explored how proximal negative feedback regulators might be used to enhance the sharpness of CyVSP discrimination of RNA moieties. Our modeling effort (supported by quantitative measurements) made pointed predictions about VITREP that we could validate experimentally. Our findings identify VITREP as a conserved and tunable checkpoint in viral pathogenesis. By uncovering how viral input quantity governs innate immune evasion, we provided a new framework for understanding viral replication dynamics and suggested novel strategies for antiviral interventions that raise VITREP to restrict infection.