König RR, Flory E, Steidl S, Neumann J, Coulibaly C, Holznagel E, Holzammer S, Norley S, Cichutek K (2002): Engineered CD4- and CXCR4-Using Simian Immunodeficiency Virus from African Green Monkeys Is Neutralization Sensitive and Replicates in Nonstimulated Lymphocytes
J. Virol. 76 (21): 10627-10636.

During human immunodeficiency virus type 1 (HIV-1) infection, disease progression correlates with the occurrence of variants using the coreceptor CXCR4 for cell entry. In contrast, apathogenic simian immunodeficiency virus (SIV) from African green monkeys (SIV_agm), specifically the molecular virus clone SIV_agm3mc, uses CCR5, Bob, and Bonzo as coreceptors throughout the course of infection. The influence of an altered coreceptor usage on SIV_agm3mc replication was studied in vitro and in vivo. The putative coreceptor binding domain, the V3 region of the surface envelope (SU) glycoprotein, was replaced by the V3 loop of a CD4- and CXCR4-tropic HIV-1 strain. The resulting virus, termed SIV_agm3-X4mc, exclusively used CD4 and CXCR4 for cell entry. Consequently, its in vitro replication was inhibited by SDF-1, the natural ligand of CXCR4. Surprisingly, SIV_agm3-X4mc was able to replicate in vitro not only in interleukin-2- and phytohemagglutinin-stimulated but also in nonstimulated peripheral blood mononuclear cells (PBMCs) from nonhuman primates. After experimental infection of two pig-tailed macaques with either SIV_agm3-X4mc or SIV_agm3mc, the coreceptor usage was maintained during in vivo replication. Cell-associated and plasma viral loads, as well as viral DNA copy numbers, were found to be comparable between SIV_agm3mc and SIV_{agm 3-X4mc} infections, and no pathological changes were observed up to 14 months postinfection. Interestingly, the V3 loop exchange rendered SIV_agm3-X4mc susceptible to neutralizing antibodies present in the sera of SIV_agm3-X4mc- and SIV_agm3mc-infected pig-tailed macaques. Our study describes for the first time a successful exchange of a V3 loop in nonpathogenic SIV_agm resulting in CD4 and CXCR4 usage and modulation of virus replication in nonstimulated PBMCs as well as sensitivity toward neutralization.