## 4 Discussion  
The large number of host proteins reported to promote filoviral internalization and thus infection prompted us to analyze systematically the cellular requirements for filovirus susceptibility. We tested a panel of immortalized cell lines for EBOV and MARV GP-mediated transduction to correlate susceptibility to the presence of plasma membrane proteins or endosomal proteins required during the early phases of infection. As previously reported, fibroblast, epithelial, and endothelial cells were susceptible to filovirus GP-driven transduction, whereas the lymphocytic cell line Jurkat was fully resistant to pseudoparticle mediated reporter expression [16; 22]. However, we identified two neuroblastoma cell lines, SK-N-MC and SH-SYSY, which were resistant to filovirus GP-driven transduction. This was unexpected, as most human cell lines previously described to be resistant to filovirus infection are nonadherent or lymphocytic cell lines [14; 16; 19]. SK-N-MC was isolated from metastatic neuroblastoma tissue of a 14 years old Caucasian female [56]. These cells were not susceptible to EBOV-GP transduction or EBOV infection but supported a low level of MARV-GP transduction and MARV infection. This disparity might be due to different receptor usage among filoviruses, to different internalization mechanisms, or even indicate a host factor specifically required for EBOV but not for MARV [17]. On the other hand, SH-SYSY cells, a thrice cloned subline of the neuroblastoma SK-N-SH cell line [56; 70], were highly resistant to both EBOV and MARV by all assays (Figures 1 and 2). These cells are susceptible to chikungunya virus, dengue virus, varicella-zoster virus, and neurovirulent poliovirus [71; 72; 73; 74]. Our study revealed that SH-SYSY cells are also susceptible to transduction driven by the G protein of rabies virus (Figure2B). Together, these results suggest that the entry defect observed is largely filovirus specific. According to the current model on filovirus cell entry, one or several proteins on the cell surface facilitate infection in addition to the essential intracellular endosomal proteins. At the plasma membrane, Ca\({}^{2+}\)-dependent carbohydrate binding C-type lectins interact with N-glycans attached to filoviral GP [75] and function as attachment factors [34]. Furthermore, members of the TAM (Tyro3, Axl, Mer) and TIM-1 PtdSer binding protein families enhance filovirus cell entry, TAMs likely by enhancing macropinocytosis [37] and TIM-1 by a process depending on GP or PtdSer [30; 76]. Finally, the intracellular proteins cathepsin B and cathepsin L, TPC1 and TPC2, and the bona fide receptor NPC1 are essential for productive filovirus infection in vitro and in vivo [38; 40; 41]. Our panel of cell lines was largely lacking expression of lectins at the mRNA level with the exception of ASGR1 and ASGR2, which were expressed on Huh-7.5 cells, as reported previously [28]. The absence of expression of other entry promoting lectins is in line with the tissue distribution of DC-SIGN, hMGL (dendritic cells and macrophages), L-SIGN, and LSEctin (endothelial cells of the liver and lymph nodes) [24; 25; 27]. Therefore, although lectins might contribute to the cell tropism of filoviruses [20], clearly they are not essential for filovirus infection. In line with previous studies [30; 31; 36; 68; 77], TAM and TIM-1 mRNA expression was heterogeneous among the cell lines studied here, suggesting that they are not essential but may facilitate filovirus attachment and internalization. Indeed, studies on the role of Axl in filovirus cell entry have shown a cell-type-specific effect with antibodies against Axl reducing Axl-dependent filoviral GP-driven transduction in some Axl-positive cell lines but not in others [77]. Similarly, overexpression of DC-SIGN but not of TIM-1 renders Jurkat cells susceptible to filovirus virus-like particles (VLP) [76] and infection studies with TIM-1-\({}^{\prime}\)- mice also suggest a dispensable role of TIM-1 since EBOV viral loads are similar in KO and WT mice [78].