Effects of an In-Frame Deletion of the 6k Gene Locus from the Genome of Ross River Virus

The alphaviral 6k gene region encodes the two structural proteins 6K protein and, due to a ribosomal frameshift event, the transframe protein (TF). Here, we characterized the role of the 6k proteins in the arthritogenic alphavirus Ross River virus (RRV) in infected cells and in mice, using a novel 6k in-frame deletion mutant. Comprehensive microscopic analysis revealed that the 6k proteins were predominantly localized at the endoplasmic reticulum of RRV-infected cells. RRV virions that lack the 6k proteins 6K and TF [RRV-(Delta 6K)] were more vulnerable to changes in pH, and the corresponding virus had increased sensitivity to a higher temperature. While the 6k deletion did not reduce RRV particle production in BHK-21 cells, it affected virion release from the host cell. Subsequent in vivo studies demonstrated that RRV-(Delta 6K) caused a milder disease than wild-type virus, with viral titers being reduced in infected mice. Immunization of mice with RRV-(Delta 6K) resulted in a reduced viral load and accelerated viral elimination upon secondary infection with wild-type RRV or another alphavirus, chikungunya virus (CHIKV). Our results show that the 6k proteins may contribute to alphaviral disease manifestations and suggest that manipulation of the 6k gene may be a potential strategy to facilitate viral vaccine development. IMPORTANCE Arthritogenic alphaviruses, such as chikungunya virus (CHIKV) and Ross River virus (RRV), cause epidemics of debilitating rheumatic disease in areas where they are endemic and can emerge in new regions worldwide. RRV is of considerable medical significance in Australia, where it is the leading cause of arboviral disease. The mechanisms by which alphaviruses persist and cause disease in the host are ill defined. This paper describes the phenotypic properties of an RRV 6k deletion mutant. The absence of the 6k gene reduced virion release from infected cells and also reduced the severity of disease and viral titers in infected mice. Immunization with the mutant virus protected mice against viremia not only upon exposure to RRV but also upon challenge with CHIKV. These findings could lead to the development of safer and more immunogenic alphavirus vectors for vaccine delivery.