## 4 Discussion
Our data clearly demonstrate that TIM-1, Axl, Mer, and DC-SIGN have important yet redundant roles during filovirus entry. This is in line with published studies supporting the function of C-type lectins, TAM tyrosine kinases, and TIM proteins in cell entry [23; 24; 27; 30; 31; 69; 79; 80; 81]. Prospective further attachment factor protein expression profiling among susceptible and resistant cell lines could provide even a clearer picture on the cellular requirements for productive filovirus cell entry. In sum, expression of TIM-1, Axl, Mer, or DC-SIGN is an important determinant of susceptibility to filoviruses. Finally, the required intracellular endosomal proteins were expressed and functional in all cell lines studied here including the resistant SH-SYSY cells, irrespective of their susceptibility to EBOV infection, suggesting that they are not the determinants of resistance in SH-SYSY cells. A similar scenario has been observed in resistant lymphocytic cell lines, which clearly express NPC1 yet they are resistant to infection [68]. In fact, the Jurkat cells analyzed here had higher _NPC1_ mRNA levels than primary human hepatocytes (432 over 308 RLU) (Table S2) and expressed NPC1 protein (Figure 5B). Thus, resistance to filoviral cell entry is more common than previously thought and not restricted to either nonadherent or lymphocytic cells or to the presence of intracellular host factors. The disparity between susceptibility to infection and surface protein expression could be attributed to different reasons. First, resistant cell lines might express a dominant restriction factor that blocks productive internalization or viral fusion (reviewed in [61]). However, this was not the case for the SH-SYSY cells reported here, as heterokaryons of susceptible HEK293T-H6 and SH-SYSY cells were also susceptible to filovirus GP-driven transduction. Second, filoviruses might rely on different cofactors in different cell types. In Jurkat cells, this might be conceivable. An overexpression of DC-SIGN but not of TIM-1 renders otherwise resistant Jurkat cells susceptible to EBOV-GP virus-like particles [76]. Moreover, EBOV virions bind to resistant Jurkat and activated CD4+ T-cells in a TIM-1 dependent-manner without productive infection [78]. However, ectopic expression of Axl, an inhibitor of the type I interferon signaling [69], renders Jurkat cells susceptible. This suggests that Jurkat cells might be able to resist filovirus infection because they have an intrinsic antiviral innate immune mechanism activated. This hypothesis could be tested in the future by determining whether heterokaryons of resistant Jurkats with susceptible cells will be susceptible or resistant to filovirus infection. In SH-SYSY cells, however, an ectopic expression of any potential attachment-promoting plasma membrane protein of filoviruses, such as DC-SIGN, Axl, Mer, or TIM-1, conferred susceptibility to filovirus GP-driven transduction or infection. Interestingly, the enabling potency was different among attachment factors. These differences may be related to the mechanism by which the attachment factors promote susceptibility in SH-SY5Y cells. For instance, TAM receptors may confer susceptibility by enhancing micropinocytosis, whereas TIM-1 or DC-SIGN could bind to the virus GP [26; 30; 77]. Further mechanistic studies on the role of the studied attachment factors could shed light on the differences observed in GP-mediated filovirus susceptibility in SH-SY5Y cells. Nevertheless, these data highlight the concept that virus-receptor interactions at the cell surface are required for productive entry but several alternative host cell factors can be utilized. To our knowledge, this is the first study which directly shows that several unrelated proteins can be utilized alternatively to initiate filoviral cell entry in one cellular context. These findings may help to explain the broad cell tropism of filoviruses.