When inoculated peripherally, the most commonly used strains of mouse scrapie have disease incubation times in excess of 6 months. Mo-MuLV is a highly pathogenic mouse retrovirus which causes a fatal Calycosin T-cell lymphoma 3�C5 months after inoculation into susceptible newborn mice. Thus, co-infected mice might succumb to T-cell lymphoma before they became clinically ill with scrapie. To address the issue of whether or not retroviral infection of a mouse would alter prion pathogenesis, murine Friend leukemia retrovirus was substituted for Mo-MuLV. FMuLV and spleen focus-forming virus together form the pathogenic Friend virus complex which infects adult immunocompetent mice. Co-infection with mouse scrapie and FV has been done before using mice highly susceptible to Friend virus-induced leukemia with no effect on scrapie. However, this lack of effect might have been due to lack of virus early during prion infection since the mice were infected with FV months after being infected with murine scrapie. In order to ensure that actively replicating virus would be present for the entire course of prion infection, we used a mouse strain, F1, that becomes highly viremic during the first weeks of acute FV infection but which then mounts strong immune responses and recovers without the development of leukemia. Importantly, following the immune-mediated resolution of the acute FV infection, infectious virus still persists at detectable levels in the spleen for the lifetime of the mouse. Thus, unlike the previous study, where peripheral prion replication and spread can be critical for efficient neuroinvasion as well as at low levels in the spleen throughout the rest of the scrapie incubation period. In the current study we show that, similarly to Mo-MuLV, coinfection of mouse scrapie-infected tissue cells with F-MuLV leads to an increase in exosomes, prion protein and scrapie Chlorzoxazone infectivity in vitro. However, peripheral co-infection of mice with 22L mouse scrapie and FV did not lead to a decrease in scrapie disease incubation times or any significant alterations in 22L scrapie pathogenesis. Our results show that retroviral co-infection does not necessarily alter prion disease pathogenesis in vivo. Although the scrapie incubation period was not affected, it remained possible that co-infection altered the level of PrPSc accumulation. The amount of PrPSc present in the brains and spleens of co-infected mice was compared to the amount of PrPSc in the brains and spleens of mice infected with 22L alone. As shown in Figure 3A, the amount of PrPSc present in the spleen varied within each group, with some mice having higher levels of PrPSc in the spleen at late-stage clinical disease than others.