Immunostimulatory function of the defective interfering RNA of Sendai virus

Abstract

The Cantell strain of Sendai virus (SeV-C) represents a typical laboratory attenuated virus which is less virulent and able to induce large amount of interferon in infected cells. This strain has widely been used in the laboratory for immunological studies due to its extraordinary ability to stimulate type I interferon production. Nevertheless, the underlying mechanism by which SeV-C is sensed by immune receptors as an invading microorganism is still largely unknown.

Meanwhile, during the course of infection, Sendai virus is known to generate substantial amount of non-infectious viral particles consisting of defective interfering RNAs (DI RNAs) from replication errors. It was known that one major form of DI RNAs generated by some negative stranded RNA viruses may adopt a stem-loop panhandle structure with a relatively long double stranded region. Therefore, we hypothesized that some SeV-C DI RNAs may bear structures similar to intermediate-length dsRNA recognized by cytosolic immune receptor RIG-I, thus triggering interferon production. In this study, three DI RNAs were successfully isolated from SeV-C infected cells. Particularly, one of them designated T4 was found to contain a double stranded region of 93 base pair, and it was capable of stimulating interferon β production when transfected to reporter cells. The immunostimulatory activity of T4 alone was as potent as that of SeV-C, suggesting that T4 would be the major component in attenuated virus SeV-C that activates type I interferon production. Furthermore, cellular dsRNA binding protein PACT was shown to play a role in T4 recognition by RIG-I. T4 binds to PACT and potently stimulates PACT-induced activation of RIG-I. The identification and characterization of T4 reveals the major immunostimulatory component in SeV-C. Our work defines a RIG-I agonist naturally produced during the course of viral infection and provides a new mechanism to explain virus attenuation. We also demonstrate the role of PACT in immune sensing of a viral RNA. In addition, our findings also provide new strategies for the design and development of vaccines, vaccine adjuvants and other immunostimulatory agents.