Activation of innate immune responses by foreign RNAs

Double-stranded RNA (dsRNA) is a potent activator of multiple cellular responses in mammalian cells. While some dsRNAs activate innate immune responses resulting in interferon (IFN) production and action, and apoptosis, others mediate gene specific silencing through RNA interference (RNAi). It is important to understand the characteristics of dsRNAs that result in the activation of these disparate pathways, especially considering the utility of the RNAi pathway for research into gene function and the potential development of novel therapeutics for a variety of human diseases. The ability of dsRNAs to activate the innate immune system was recognized over forty years ago, but it was only recently that the RNA sensors that detect them have been characterized. The cytosolic RNA sensor pathway that utilizes the interferon promoter stimulator gene 1 (IPS-1) adaptor protein results in the induction of type I IFNs. These IFNs bind to cell surface receptors and the resulting signaling induces IFN-stimulated genes including the dsRNA-activated protein kinase PKR. Through the use of mammalian cell lines, I examined the characteristics of mediators of RNAi and longer dsRNAs that result in the activation of innate immune responses. In addition, I attempted to delineate the cytosolic RNA signaling pathways that these RNAs activated and the mechanism by which specific proteins including IPS-1 and PKR contribute to this activation. My results demonstrate that both IPS-1 and PKR are important mediators of IFN-beta induction and apoptosis with IPS-1 activating signaling pathways directly and PKR indirectly through translation inhibition. As an extension of this research, I worked with my laboratory colleagues and observed that these same proteins mediate the innate immune response to infection by measles viruses lacking C and V accessory proteins. Finally, as part of a collaborative research project with the Safinya laboratory in the Materials Research Laboratory, I studied the structural and functional characteristics of cationic lipids that deliver small interfering RNA mediators of RNAi into cells. We were able to determine the structural composition of these delivery complexes and the optimum lipid compositions required to achieve maximal delivery of these RNAs with the lowest level of toxicity to cells.