## 4 Discussion
In our study, tolerance to EBOV could only be observed for primary cells from _M. condylurus_ and no noticeable cell damage or destruction could be detected in most of these cells. Although higher EBOV RNA titers were observed for some primary cell isolates from _M. condylurus_ (MoSk Prim or MoTes Prim, 96 hpi), cells showed high viability and could be passaged several times. Remarkably, MoLu Prim_EBOV cells tolerated high amounts of virus in the supernatant and intracellularly for 150 days without any signs of cytopathology. Tolerance to EBOV at the cellular level suggest a similar outcome in the corresponding organs in vivo, which would result in a lower degree of cell damage and therefore contribute to asymptomatic infections in _M. condylurus_. The importance of tolerance for asymptomatic infections of this potential reservoir host has to be determined in further in vivo experiments. To investigate the intrinsic ability of _M. condylurus_ primary cells to support EBOV persistence, cells were passaged and cultivated for several weeks post-infection. In contrast to primate cells, all examined primary cells from _M. condylurus_ showed characteristics that support tolerance to EBOV and establishment of persistent infections. Only infected lung primary cells reached confluency, were passaged weekly, and showed a permanent persistence of EBOV for five months, after which the experiment was stopped. The virus and the host cells seemed to be in equilibrium, such that virus replication and high virus titers were permanently tolerated without cell damage or impairment of cell division and viability. The persistence of EBOV in MoLu Prim cells was recapitulated with EBOV-GFP in an independent experiment, indicating a reliable outcome for this cell isolate. Conversely, no permanent persistence could be observed in _M. condylurus_ primary cells derived from immune privileged organs (brain and testicles), in which EBOV was reported to persist in humans [59; 60; 61; 95]. However, the special characteristics of these organs such as the blood-brain and blood-testis barriers, or the suppression of proinflammatory immune responses, cannot be simulated in cell culture, so that the validity of our results concerning virus persistence in these organs of the potential reservoir host is rather limited. During the persistent EBOV infection in MoLu Prim cells, only a small proportion of cells in the monolayer became infected, although NPC1 receptor expression levels were already markedly increased. A relatively high virus titer of 6 \(\times\) 10\({}^{5}\) TCID\({}_{50}\)/mL on day 150 post-infection resulted from efficient virus replication in the relatively few infected cells, indicating that high virus titers in the supernatant do not necessarily reflect the overall permissiveness of a cell culture. For the establishment of virus persistence, a repression of virus replication in at least some infected cells or a slow spread from cell-to-cell are potential mechanisms [37]. Although the specific mechanism of EBOV persistence in vitro and in vivo cannot be determined based on our data, they indicate virus replication occurs in individual cells of certain organs only. While the virus spreads slowly from cell-to-cell, the majority of cells remain uninfected and can presumably fulfil their specific functions in the organ, which may also facilitate an asymptomatic infection in the reservoir host. [PERSON] et al. showed, that respiratory syncytial virus (RSV) may persist for months in lungs and airways following acute infection, although the mechanisms of persistence are poorly understood [96]. The recent discovery of RNA from a previously unknown filovirus in lungs of _Rousettus_ and _Eonyoteris_ fruit bats in China [97; 98], repeated detection of Bombai virus RNA in lungs of _M. condylurus_ bats [24; 26] and repeated establishment of EBOV persistence in lung primary cells from _M. condylurus_ indicates that ebolaviruses may persist in vivo in lungs of this bat species. Detection of EBOV in lungs of _M.