Nonpathogenic CCR2-tropic SIVrcm after serial passage and its effect on SIVmac infection of Indian rhesus macaques

The natural host of SIVrcm is the red-capped mangabey (Cercocebus torquatus torquatus). Although this virus infects macaques and human PBMCs, its pathogenic potential is unknown. We serially passaged SIVrcm through 9 rhesus macaques to assess its potential for virulence. SIVrcm infected all macaques with peak viremia 2 weeks postinfection yet viral loads decreased to undetectable levels about one month after inoculation. Remarkably, SIVrcm replication and virulence did not increase following 7 serial passages. While CD4+ T cells in the gut were decreased in early infection, proportions of memory CD4+CCR5+ T cells were not affected. Three SIVrcm-infected macaques were subsequently challenged with SIVmac251 to assess the potential for superinfection. Interestingly, animals previously infected with SIVrcm had 100 fold lower levels of SIVmac251 in plasma compared to naive animals inoculated with SIVmac251. These results suggest that SIVrcm is nonpathogenic and may be useful for examining effective immune responses in SIV infection.