Clonal expansion of SFV infected cells in Papio anubis

Non-human primates (NHPs) are natural hosts for exogenous retroviruses such as STLV-1 (Simian T-cell Leukemia Virus type 1) and SFV (Simian Foamy Virus), and natural cases of co-infections have been reported (SFV/STLV-1). As described for HTLV-1 in humans, STLV-1 is the etiologic agent of ATL (Adult T cell Leukemia) in NHPs, but the majority of infected animals remain asymptomatic. Furthermore, as for HTLV-1, which is able to replicate through clonal expansion of infected cells, the quantification of the proviral load together with the identification of proviral integration sites in STLV-1-infected NHPs have highlighted the clonal expansion of STLV-1 in vivo (Turpin et al 2017). On the other hand, SFV is considered as non-pathogenic and mechanisms involved in co-infections to regulate SFV replication is unknown. The aim of the project is to evaluate the influence of STLV-1 on the proviral load and the retroviral clonality of SFV in naturally infected baboons. First, we have reported that the SFV proviral load increases in the blood of baboons naturally co-infected with SFV and STLV-1 (Alais et al 2018). We next demonstrate for the first time that SFV is able to replicate by clonal expansion of infected cells in vivo, as described for HTLV-1 and STLV-1. Our study also shows that SFV proviruses preferentially integrate within repeated regions in the baboon's genome. Moreover, STLV-1 does not seem to promote the propagation of SFV in vivo. Altogether, this work sheds light on the replication of SFV in naturally infected NHPs, and emphasizes the fact that co-infection with STLV-1 has no impact on SFV replication in vivo.