Novel Mutations in nsP2 Abolish Chikungunya Virus-Induced Transcriptional Shutoff and Make the Virus Less Cytopathic without Affecting Its Replication Rates.

Alphavirus infections are characterized by global inhibition of cellular transcription and rapid induction of a cytopathic effect (CPE) in cells of vertebrate origin. Transcriptional shutoff impedes the cellular response to alphavirus replication and prevents establishment of an antiviral state. Chikungunya virus (CHIKV) is a highly pathogenic alphavirus representative, and its nonstructural protein 2 (nsP2) plays critical roles in both inhibition of transcription and CPE development. Previously, we have identified a small peptide in Sindbis virus (SINV) nsP2 (VLoop) that determined the protein's transcriptional inhibition function. It is located in the surface-exposed loop of the carboxy-terminal domain of nsP2 and exhibits high variability between members of different alphavirus serocomplexes. In this study, we found that SINV-specific mutations could not be directly applied to CHIKV. However, by using a new selection approach, we identified a variety of new VLoop variants that made CHIKV and its replicons incapable of inhibiting cellular transcription and dramatically less cytopathic. Importantly, the mutations had no negative effect on RNA and viral replication rates. In contrast to parental CHIKV, the developed VLoop mutants were unable to block induction of type I interferon. Consequently, they were cleared from interferon (IFN)-competent cells without CPE development. Alternatively, in murine cells that have defects in type I IFN production or signaling, the VLoop mutants established persistent, noncytopathic replication. The mutations in nsP2 VLoop may be used for development of new vaccine candidates against alphavirus infections and vectors for expression of heterologous proteins.IMPORTANCE Chikungunya virus is an important human pathogen which now circulates in both the Old and New Worlds. As in the case of other Old World alphaviruses, CHIKV nsP2 not only has enzymatic functions in viral RNA replication but also is a critical inhibitor of the antiviral response and one of the determinants of CHIKV pathogenesis. In this study, we have applied a new strategy to select a variety of CHIKV nsP2 mutants that no longer exhibited transcription-inhibitory functions. The designed CHIKV variants became potent type I interferon inducers and acquired a less cytopathic phenotype. Importantly, they demonstrated the same replication rates as the parental CHIKV. Mutations in the same identified peptide of nsP2 proteins derived from other Old World alphaviruses also abolished their nuclear functions. Such mutations can be further exploited for development of new attenuated alphaviruses.