Construction and In Vitro Properties of a Series of Attenuated Simian Immunodeficiency Viruses with All Accessory Genes Deleted

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Abstract
We have generated simplified simian immunodeficiency virus (SIV) constructs lacking the nef , vpr , vpx , vif , tat , and rev genes (Δ6 viruses). To accomplish this, we began with an infectious molecular clone of SIV, i.e. SIVmac239, and replaced the deleted segments with three alternate elements: (i) a constitutive transport element (CTE) derived from simian retrovirus type 1 to replace the Rev/Rev-responsive element (RRE) posttranscriptional regulation system, (ii) a chimeric SIV long terminal repeat (LTR) containing a cytomegalovirus (CMV) promoter to augment transcription and virus production, and (iii) an internal ribosome entry site (IRES) upstream of the env gene to ensure expression of envelope proteins. This simplified construct (Δ6CCI) efficiently produced all viral structural proteins, and mature virions possessed morphology typical of wild-type virus. It was also observed that deletion of the six accessory genes dramatically affected both the specificity and efficiency of packaging of SIV genomic RNA into virions. However, the presence of both the CTE and the chimeric CMV promoter increased the specificity of viral genomic RNA packaging, while the presence of the IRES augmented packaging efficiency. The Δ6CCI virus was extremely attenuated in replication capacity yet retained infectiousness for CEMx174 and MT4 cells. We also generated constructs that retained either the rev gene or both the rev and vif genes and showed that these viruses, when complemented by the CMV promoter, i.e., Δ5-CMV and Δ4-CMV, were able to replicate in MT4 cells with moderate and high-level efficiency, respectively. Long-term culture of each of these constructs over 6 months revealed no potential for reversion. We hope to shortly evaluate these simplified constructs in rhesus macaques to determine their long-term safety as well as ability to induce protective immune responsiveness as proviral DNA vaccines.